Tirade on road discs
Last Post 11/18/2014 03:56 PM by Cosmic Kid. 81 Replies.
Author Messages
6ix

Posts:133

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07/21/2014 12:58 PM
While riding my mountain-bike yesterday on some local fire roads, my front brake started rubbing a little. Bike hasn't been used much. 2013 Scott Scale 910 with Fox 32 fork, thru-axle and XT discs. With the thru-axle, there is basically zero guess-work when installing the wheel. First time I've ever had the rotor rub and groan. I understand the tolerances for discs have to be very, very precise in order to work properly. With rim calipers, there is a lot of wiggle room. Let's face it...crap happens while riding. In a closed environment, I'm sure hydraulic discs are fantastic but they sure do suck when something goes amiss. In contrast to being easy to dial in a cable-actuated brake with some pliers and a hex-key, hydraulics aren't user-friendly in the slightest. I'm a somewhat decent mechanic and don't know how to fix this rotor rub. That's frustrating. My main point is that for road disc to be really successful (for the end consumer, not the market and sales guys), they need to design in a release for the pistons to open up the clearance for situations just like this. Thru-axle is absolutely mandatory for these system to come close to working properly. I'm just not convinced yet. Here is one other thing I've noticed; if you get more power as the rotor grows in diameter, then wouldn't logic show that braking at the RIM be the most powerful? After all, it's still two pads clamping down on a rotating surface, right? Carbon braking surfaces suck but the move to hydraulic discs isn't the answer. The answer is to NOT use carbon braking surfaces!
jmdirt

Posts:755

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07/21/2014 01:52 PM
A larger rotor does not create more power. A larger rotor cools slightly better, reducing fade, but increased contact surface area (more friction) is what creates more power. Hydros have more power than cable because there is less energy lost in flex (cable, brake arm...). It would be difficult to crate as much braking surface on a rim as you have with discs.

As to the issue you had yesterday: as great as hydros have become, they still have a weak link, the seal around the pistons. As that gets contaminated, the piston retract less than perfectly. If a piston gets stuck out an extra few mm it will likely rub, or if one piston gets stuck in, the other piston may compensate and it will rub. Sometimes, just opening your skewer, and wiggling the wheel side to side will free up a slightly stuck piston. If not, when you get home drop the wheel out and use a cotton swab to clean the exposed part of the pistons, and the seals. Then using a brake spatula, retract both pistons completely. Put your wheel back in and grab a hand full of brake several times to get the pads reset. Also be sure that your rotor is true.
79pmooney

Posts:1192

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07/21/2014 03:57 PM
jm, simple physics suggest the larger diameter disc do generate more power. Power is energy per unit time. The time here is stopping time. If we say all brakes are required to stop the bike in the same distance, ie time, time is constant and can be ignored. Energy is the change of the square of the speed. A larger diameter disc has a higher speed at the rotor, hence more energy change as the rotor is slowed. Now, for our constant time analysis, this is offset by the lesser rotor force required. But use the same rotor force, the energy change is larger, the deceleration is more and therefore the time is less. Since time is in the denominator, this adds up to considerably more braking power. (Assuming tires, etc. can handle it.)

Ben
longslowdistance

Posts:744

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07/21/2014 04:55 PM
To sum up Ben's post in one word: leverage. I don't know if it's true. Any physicists or engineers out there?
Ride On

Posts:454

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07/21/2014 05:32 PM
You said it exactly right.

Carbon braking surfaces suck but the move to hydraulic discs isn't the answer. The answer is to NOT use carbon braking surfaces!
jmdirt

Posts:755

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07/21/2014 10:13 PM
Ben, I can't argue the simple physics, but my assertion comes from Ducati Superbike and GP testing (unfortunately not by me). The one sentence summary of their extensive testing : A few more mm of pad/rotor contact created more power than rotors that were nearly the size of the rim. They were able to decrease the size of the rotor (better handling) and increase braking power with slightly more contact area.

I don't know if discs will be better on the road, but on the dirt, I will take my hydro discs over canties any day even if a few days a year that means a bit of rub.

Disclaimer: I'm not directly comparing moto GP brakes to XT brakes. Materials, weight, temperature, force...everything is different, but doesn't it seem like this aspect is transferable?
longslowdistance

Posts:744

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07/21/2014 10:46 PM
So the 8" rotors on downhill and free ride bicycles vs. the 6" rotors on CX bikes is about more contact area and better heat dissipation only?
jmdirt

Posts:755

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07/22/2014 12:13 AM
lsd, there isn't more contact area if you only enlarge the rotor. You have to increase pad size to increase contact area. Six inch rotors have plenty of power for a DH rig so I assume that they are going by the basic physics that Ben described above plus better heat dissipation. Ducati reducing rotor size while increasing pad size to get more power seems pretty compelling to me. I'd be surprised if Shimano hasn't done a lot of testing as well. Maybe they put bigger rotors on DH rigs because people are convinced that they need them for more power? Actually Shimano did increase pad size ever so slightly, and add 'ice' fins to all but their XC race stuff.

EDIT: How much more stopping power is there from 6 to 8" rotor vs. adding 2 mm surface area to the pad? Is friction a more important factor or time?
Ride On

Posts:454

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07/22/2014 06:00 AM
The coefficient of friction of the pad to rotor surface is a function of the interface surface area as well as the type of material. So you can change the surface area but you could also change the type of material of the pad or rotor to change the braking power. With a rim brake you can do the same thing, change the pad to rim contact area or change the materials used and you change the braking power.
Yo Mike

Posts:279

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07/22/2014 09:53 AM
I don't have discs on any of my bicycles; not sure I want them, but I suspect for my needs mechanicals may be sufficient, if I ever get them.

Looking at the physics of the matter, per Ben's post, is instructive. I recall that in my vintage (70s and 80s) motorcycle magazines, they would mention Brake Swept Area. Does this term ever come up with bicycle disc brakes?

Motos with dual discs up front would usually - not always - have smaller rotors than a moto with a single disc, and if the moto had a rear disc - that disc would typically be smaller than the front disc(s).

How about dual front mechanical discs for a bicycle? ;-)

jmdirt

Posts:755

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07/22/2014 10:46 AM
Ride on, in the Ducati testing they decided on rotor and pad composition first, and kept that constant when testing rotor and pad size.

I wish I wouldn't have started with: "A larger rotor does not create more power. " Before I try to reinvent physics, a larger rotor AND larger contact area obviously has more power. But what we are looking for is the best balance of performance. If 2 mm of pad surface has the same improvement as 50 mm of rotor size the obvious choice is 2 mm of pad surface. I should have started with "contact area has a greater effect on power than diameter..." or something like that.

Ride on brought up a big factor involved here: Coefficient of friction. One of the limiting factors in brake power with bikes is the coefficient of friction of the tires. What hydro discs provide is less effort at the lever for the same power.

Mike, even on DH rigs they try to keep things light so if they add anything I would bet on thicker rotors before going to two.
Orange Crush

Posts:1251

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07/22/2014 11:45 AM
Posted By Evan Solida on 07/21/2014 12:58 PM
Here is one other thing I've noticed; if you get more power as the rotor grows in diameter, then wouldn't logic show that braking at the RIM be the most powerful? After all, it's still two pads clamping down on a rotating surface, right?
6ix - the one thing you are overlooking is the difference in caliper mechanisms between a standard rim brake and a disc brake.

The disc brake system is far more efficient in terms of generating brake torque for a certain amount of hand torque applied at the brake lever than a rim brake caliper. This translates into greater stopping power and greater ability to modulate.

Those are the upsides but yeah there's still a lot of downsides.
jmdirt

Posts:755

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07/22/2014 01:59 PM

So, I consulted a friend of mine who has made a living out of being a bike industry bum.
Me: Tell me about brake power.
D: What do you mean?
Me: I don't want to lead your answer.
D: Most people want to talk about absolute text book power. If that's all there is to it, let toss two rim size rotors on each wheel and we can skid around the trails all day long. Bike brakes aren't just about power, they are about effort at the lever, mod, and control. (OC and I said that too). How about bigger, cooler pads? Better pad materials? More oil volume?

Notes: He talked for about 30 minutes, and made little quote signs with his fingers each time he said "power". The above paraphrase supported what I was poorly saying, but he also talked about perceptions due to people wanting apply text book physics.

D: The SLX XC brakes are good for World Cup DH racing, but all of those guys and their mechanics think that they need a huge rotor. Yah in 1999 we needed a big rotor to make up for crappy pads and cals. Have they run the numbers lately? How about better tire selection? I wish the Honda guys would have stayed in DH because they used real data.

Anyway, like I said, I wish I would have phased my first assertion better.

OC, for dirt use, there are no down sides IMO.

Oldfart

Posts:490

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07/22/2014 02:09 PM
I rarely get brake rub on either of my mountain bikes. One XT the other is XTR. Rub was a problem when I had cheaper hubs with standard QR's. I think it was due to the end caps or locknuts not being perfectly square. I found if I removed a wheel for one reason or another, the disc rubbed when I put the wheel back in until I started to put a mark on the lock nut so that the hub went back in the same orientation. Problem solved. Got DT hubs, problem solve better because no matter which way the axle was oriented there was no brake rub.

But most of the time off road there is too much noise from things to hear it well if it is present. Bigger rotors tend to rub more often so if road bikes use those 140 mm rotors it might not be an issue.
Oldfart

Posts:490

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07/22/2014 02:16 PM
Posted By Ride On on 07/21/2014 05:32 PM
You said it exactly right.

Carbon braking surfaces suck but the move to hydraulic discs isn't the answer. The answer is to NOT use carbon braking surfaces!

Except that super cars, F1 cars and the like use carbon rotors? I have a pair of carbon rotors for my mountain bike made by Kettle. They lack proper grip though so they had to come off. Kind of sketchy riding the North Shore with a death grip on the brake levers and still not slowing enough.

So maybe what would be better than discs for carbon rims is just a better brake bad material? maybe that would also mean a different and very costly rim material though? Those Kettle rotors are not made like a rim as far as I know and they have addressed the crappy performance with a different process and different pads.

http://kettlecycles.com/product/siccc-sfl-bicycle-brake-rotors/


dkri

Posts:91

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07/23/2014 09:58 AM
I wouldn't ride a mountain bike without them. Now, against my preference, I get to race cx on a disc bike this season. I hate them. They are a nightmare pain in the ass to set up, the rotors need to be precisely true not to rub, and the braking advantages are nil. I have BB7s (road) paired with Force levers.

Why anyone would want to subject himself to this on the road I don't know. After what I'm doing at A2 wind tunnel in NC next week CK will probably put a poster of me on his bedroom wall (and you will all have a chance to read about it in sort of a big way - I'm psyched about this), but I can tell you aero implications aside, I hate them.

On the carbon braking thing, I did some field testing in VT last week. Descended the top of Smuggler's Notch (11% avg grade, 19% max, 1k long, all switchbacks) at 10 mph constant speed on carbon clinchers. First run at my body weight (158), then with a 25# kettle bell in a backpack, then added 15# kettle bell to that. Had a thermal scanner in my pocket. In the worst case, I was 150* away from the lab-verified heat tolerance temp of our rims. Absolute braking power is fine, and I much prefer the way carbon braking modulates in road riding situations. Rain braking with rim brakes ain't so hot, and it's less hot with carbon, but for the 5 rides a year when that matters, I'll live with it.

I will be doing more testing in NC next week. We're trying hard to solve the issue of having a lab-verified tolerance temp but no one's really given riders any clue of how that relates to the temps they see while riding. Most of the previous gen rims and a broad array of current-gen rims would have failed in my VT test, so diligence is required, but again, I was trying as hard as possible to cause a problem.

I have no dog in the fight, our disc-brake wheel lineup is excellent and we happily sell lots of them.
formerly dkri
jmdirt

Posts:755

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07/23/2014 10:11 AM
gkri, what if you had XTR discs on a CX bike (I know there isn't a lever yet, but pretend there is).?

Like you, I love hydro disks, but absolutely hated the BB cable discs that came on my WUSS. Cable discs aren't even part of the discussion in my mind.
gabbard

Posts:27

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07/23/2014 11:56 AM
Said a different way - power dissipated is equal to torque X RPM.  RPM is wheel rotational speed, and for a given bike speed and wheel diameter, doesn't change.  Torque on the wheel is a function of frictional force and distance from the center of the rim, with frictional force being related to pad material, rotor material, pad surface area (although the simplest frictional force equation doesn't include area) and force pressing in on the pads.  

Bigger rotors stop faster with all things being equal, and more force on the pads stop faster.  Pretty obvious, but since rim brakes typically don't have as much stopping power as disc brakes, this at least tells me that the force at the rim is about 3X less than the force on a disc, since the rim is about 3X larger diameter.  Could be a function of anodized rims and rubber pads versus steel rotors and sintered pads.

Regarding disc brake rubbing - this is just a function of travel of the pads that will clear bent rotors.  I have always wondered why they don't increase the reservoir size slightly, increase the fluid moved per brake lever travel, and have the pads retract more.  The current clearance is less than a mm on each side of the rotor - seems to tight to me.
Orange Crush

Posts:1251

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07/23/2014 12:39 PM
Posted By Dave Kirkpatrick on 07/23/2014 09:58 AM
Rain braking with rim brakes ain't so hot, and it's less hot with carbon, but for the 5 rides a year when that matters, I'll live with it.
For someone like myself living on the wet coast, it's more like 20 training rides a year and probably on the order of 50 or more commutes in the rain; its a significant percentage of overall number of rides. This is the one situation where discs (on my commuter) really stand out from rim brakes (road bikes).

Downsides are the zing zing of grit stuck in the brakes, which translates into excessive wear of the pads and probably having to replace expensive disc pads about 2-3 times as often as rim brake pads. Upsides are confident braking in sketchy situations even with the low end cable actuated disc brake system I have on my commuter; OK, except the one time someone forgot to properly secure the cotter pin
dkri

Posts:91

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07/23/2014 09:30 PM
jmdirt - It would certainly be better than what I've got now! I like a soft front brake for cx. The worst brake thing that can happen in a cx race is to over brake during a dismount. Broken face time. And the rotors on my mtb still need truing all the time. Creek crossing at the bottom of a descent? I cringe - certain rotor warpage.

Gabbard - With mechs it's not possible, cable pull is super limited. Hydros should be able to get more throw, you gotta think, right?

OC - If I had a commuter it would have discs. When I still had a day job I commuted on my race bike. Now I commute in my bunny slippers, but a commute is the only time I'd tolerate a lame tire like an Armadillo, and if my rotors are a little janky, so what. Reliability you can take for granted is what you want there. But on my cx bike I'm totally geeking out on making it hyper efficient, and disc zing... nails on a chalk board.
formerly dkri
Orange Crush

Posts:1251

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07/23/2014 10:15 PM
Exactly DKRI, reliability is the key word. Not just for commuting but also winter road riding. I don't want to have to fix anything under those conditions and that is the determining factor for how my winter bike is equiped. Bit of extra effort to keep it rolling, who cares. I just know that by the time winter is dlnemy condition is better than most (an advantage that I lose by June). FWIW; I am considering a CX bike as my next winter bike, no discs. Its possibly the best middle ground.
jacques_anquetil

Posts:234

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07/24/2014 08:10 AM
this is a GREAT thread, thank you!
6ix

Posts:133

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07/24/2014 08:27 AM
Just wanted to thank you guys for the great conversation and suggestions on how to remedy my little issue. I found a helpful Park Tool video that detailed how to reset the brakes. Everything works fine now but it was still incredibly frustrating. Still seems to me to be more trouble than they are worth.

DKRI - interested in knowing more about your time scheduled at A2. Are you in engineering?

steelbikerider

Posts:44

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07/24/2014 08:40 AM
Here is what DKRI is testing:
http://www.novemberbicycles.com/blog/2014/6/16/the-wind-cries-tunnel.html
designer, product developer, salesman, racer, beer taster - he does it all.
He even made a liar out of me when I bought a carbon bike from him.


longslowdistance

Posts:744

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07/24/2014 09:32 AM
Not sure if this is the best place to bring this up but here's a medium term follow up report on TRP cable actuated hydro road disc:
Using with SRAM brifters and 160 mm shimano ice tech rotor, compared with a bb7 and Hayes CX used with their own steel rotors.
In a word: Great!
More power and better feel than the mechs. Have not experienced any fade, but haven't tried on a truly long brake drag type descent, so that critical issue remains a question mark. Also need longer term experience regarding pad wear.
jmdirt

Posts:755

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07/24/2014 10:02 AM
dkri, what rotors are you using that require frequent truing? I had some Stan's rotors about eight years ago that required weekly truing, but with Shimano its maybe yearly. The disclaimer here though is that I don't have too many creek crossings here, but even in the spring when there is some water in the drainage I don't have rotor issues.

6ix, I have a sticky piston two or three times per year (out of about 250 days on the dirt) so they are easily worth it.
dkri

Posts:91

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07/24/2014 09:09 PM
jmdirt - I've used a lot. My big upcoming switch is to centerlocks. Off top of head, every SRAM rotor made, Ashima... Put a brand new SRAM G3 on this morning, and it was maybe .4mm out of straight, which is too much.

I may have to try the cable/hydro setup.

As to A2, we're testing a raft of stuff. Some alloy wheels, carbon wheels, disc brake wheels versus their rim brake counterparts, disc bikes with disc wheels vs rim bikes with rim wheels. A lot. The corporate checkbook is cowering in the drawer.

The more interesting part of the trip will be the practical counterpart to our brake heat lab test. Knowing the number they can deal with is all well and good but what does it mean on the bike? So I don a fat suit to turn my 160 pound self into a 200 pound guy and do repeats down Mt Mitchell with a GoPro and thermal scanner. Piece of cake, right?

Thanks, Steel. How did the BB work out?
formerly dkri
Dale

Posts:524

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07/24/2014 09:37 PM
Posted By Dave Kirkpatrick on 07/24/2014 09:09 PM
...So I don a fat suit to turn my 160 pound self into a 200 pound guy and do repeats down Mt Mitchell with a GoPro and thermal scanner. Piece of cake, right?



"Let me turn on my GoPro" is the Gen X version of "Here, hold my beer and watch this!"

All in the name of science, Dave?
steelbikerider

Posts:44

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07/24/2014 09:45 PM
BB is fine. Everything was torqued to spec but I checked anyway. It is either seat post clamp, saddle clamp or saddle rails. Can't wait the see the test results. How about a Wheelhouse frame against a well known aero frame?
jmdirt

Posts:755

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07/25/2014 03:54 PM
dkri, Serriously, SRAM/Avid has never caught up to Shimano. I prefer centerlock rotors (the only down side is that they require removal for hub maintenance/adjustment).
dkri

Posts:91

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07/26/2014 08:53 AM
Dale - All in the name of science, indeed. Also, body armor.

Steel - We aren't able to source the Wheelhouse anymore (supplier went insane) so that's a dead end for us. Wheelhouse MkII (sex bomb, hand made in Italy) isn't ready yet, so no testing of our frames. Check your skewers.

jmdirt - I hear you, I just always had 6 bolt hubs. Now WI has nice centerlocks, and I'm dying to build a set of wheels on DT240 straight pull center locks.

formerly dkri
Keith Richards

Posts:759

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07/28/2014 11:19 AM
dkri--just got back from Stowe, VT yesterday.

Smugglers Notch is crazy man! I also did Weeks Hill and Stagecoach Rd. The descent off of Stagecoach into Morristown is NUTS!

Sorry fro the threadjack....I approve of your testing protocol!
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
longslowdistance

Posts:744

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07/28/2014 05:02 PM
Your rode parts of a classic two lap 86 mile race. One of my all time favorites.
Keith Richards

Posts:759

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07/28/2014 05:43 PM
Mahn....the terrain out there is technical. Between the severe climbs and the "all hands on deck" descending it was a serious solo 40 miles. I was totally unprepared with a 42/26 on my training bike. My family drove Stagecoach to meet me for lunch at Morristown and when they got out of the car they just shook their heads, 'you rode a bike here on THAT road?!?!?"
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
79pmooney

Posts:1192

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07/28/2014 06:15 PM
Keith, did you do up from Stowe and over? The old race? That descent was one of the fastest I've ever done. The word was out that you only needed brakes for the first two corners. Got it. First time it was drizzling at the top. Couldn't see through my glasses. Just followed the yellow line staying a couple of feet off it. Didn't see the big frost heave until it was about 20' away. Just enough time to grip the bars. Hit it hard! Got kicked a couple of feet into the air. Bike (my brand new Fuji Pro) came through perfect, except the cheap Hupel Rider seatpost was bent! I won a Zeus post that race so all was good.

Oh, and gearing? I was a young stud. 42-23. Seat and post neve3r got used for the climb!

And to get a little on track. Dia Comp Grand Compe sidepulls. Good brakes.

Ben
SideBySide

Posts:191

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07/28/2014 06:21 PM
Posted By Keith Jackson on 07/28/2014 05:43 PM
they just shook their heads
But you probably get that a lot. 

Keith Richards

Posts:759

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07/28/2014 08:41 PM
Ben, I was in staying in Stowe proper, right off of 108. And WORD on climbing in the 42. You will stand and gut it out...it is the only way to get up those climbs. Character building. No, character revealing. I forgot how much I enjoy the outer limits of road cycling.
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
longslowdistance

Posts:744

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07/28/2014 09:37 PM
Back in the day, one maniac named Scott Dorwart (high level XC skier, real hammerhead type) used a 42x18 low gear.
42x23 seemed just right back then, but I always saw stars by the top of the notch. 60 mph descent, then the hot sunny slog east on rt. 15.
It was the west hill road climb connecting laps one and two that really hurt. The race always lit up there.
Keith Richards

Posts:759

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07/29/2014 10:23 AM
Dude, Weeks and West Hill are BEASTS.
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
79pmooney

Posts:1192

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07/29/2014 11:51 AM
lsd, David Lamb rode the juniors race'77 on a Florida straight block, 14-18 I believe.

Ben
Ride On

Posts:454

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07/29/2014 05:39 PM
Dave I've always been interested in seeing how tire pressure affects braking. My thought was that high tire pressures lead to lower friction/grip on the road which affected braking performance. If you lower the tire pressure do you get better grip and then not have to brake as often or as hard and therefore reduce the rim temps ? The type of rubber used to make the tire would also have an effect. A ton of variables to play with Discs just seem like trying to solve the problem with a sledge hammer.
longslowdistance

Posts:744

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07/29/2014 07:15 PM
Yes lower pressure = better braking
dkri

Posts:91

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08/01/2014 07:37 PM
Sorry, been in the midst of a crazy crazy week. Left Saturday for DC (I am once again really dkri, not dkdc as I was for a long time), did a bunch of stuff with my business partner, rode the 10am for the first time in about a year with the ex (? I'm sure we have no idea what to call each other, we're still really close), got a quality dose of hate from the Muscle (I have no idea why he seems to dislike me so much, it's super weird - but he's slowed down a ton), and arrived in Blowing Rock, NC on Sunday. Monday I did a few repeats down Beech Mountain, first with 40 pounds in a bag, then with 15 pounds in a bag. Which meant I climbed Beech with 15 pounds in a bag. Not that fun. Then I climbed Beech again, to ride down with no weight. The video of that is here https://www.youtube.com/watch?v=VyzL8iHJ33s Got stuck behind cars but still made it down about 5 minutes faster (accounting for the time I spent stopped to check temp on the "slow" runs) than I had when I was mimicking "timid descender man." Finally, I had to climb up Beech again to get the car.  That was a lot of climbing that day. 

The first two runs down I stopped several times to shoot the rim temps. 175# and really bad technique gets rims hotter than 200# and moderately bad technique. 160 pounds going nuts to the wall doesn't do anything to the rim temps - even stuck behind the cars, it's no issue.

There was a freshly painted stencil in the road that said "Viva Lance."  Lance has some history in the area.  A photo of me riding past that is on our FB page, it's pretty funny. 

Then it was off to Mooresville, NC - the beating heart of NASCAR and home to the A2 wind tunnel. There, we tested most of the wheels we make (if we tested all of our alloy variants, we'd have gone bankrupt), as well as the disc variants of our carbon wheels, the validity of the shallow front/deep back setup, different tire widths on different wheels, wheels in bikes, and finally we left some wheels down there for a test that our former forum host will be publishing.

Ever notice how differently some tires inflate on some rims? That turns out to be a huge thing.

Ride On - I totally agree with lsd. Traction is traction, whether for turning or braking, and with the low psi you run on cx and mtb tires, it's like being glued to the dirt. Plus your tires are in more regular contact with the ground so you aren't doing that "micro-skidding" thing.

Ricco's an idiot, Vaughters gets pretty big props for how he's handled the Dennis thing.
formerly dkri
jmdirt

Posts:755

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08/01/2014 10:02 PM
PSI: I run 25 psi on the dirt (tubeless) (I would run less in a wider rim), and 90 on the road (tube) (same) ...great traction (propulsion and braking) and a nice ride. On the dirt I have 2.25 training tires, and 2.1 racing tires (Rocket Ron), but the same psi works well in both even though the volume is different.

dkri, it sounds like yo are a busy man!

Ride on, high psi makes for a rough ride too.

Cosmic Kid

Posts:1192

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08/02/2014 07:07 PM
Dkri - what did you see re: the aero penalty (or lack thereof) for discs on the road?
Just say "NO!" to WCP!!!!
dkri

Posts:91

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08/02/2014 09:13 PM
Can't tell you yet. That story is going to run on the other VN shortly.

Sorry.
formerly dkri
Dale

Posts:524

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08/02/2014 09:59 PM
Cool! That will be interesting to read
Ride On

Posts:454

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11/08/2014 07:17 AM
I was thinking about disc brakes and the peleton again last night.

Say you are in a group traveling at 40 ft/sec ( 25 mph = 37 ft/sec ) and you can stop in 20 ft ( avg stoping distance of a bike traveling at 25 mph is 24 ft ) you will stop in 2 sec. Assuming a constant deceleration rate you will be slowing down at 2 ft/sec.

Ok so I'm behind you by 1/2 ft and we are bothe traveling at the 40 ft/sec. All is good. You spot something and hit the brakes. Because you are decelerating at 2 ft/sec I close that 1/2ft gap in .25 sec. If I now hit my brakes and decelerate at the same rate as you all will be good when we both stop. My front wheel will be right up on your back wheel but not to bad.

Now we both get disc brakes. Things are better we both now can stop from the 40 ft/sec in 15 ft instead of 20 ft. Great. Ok not so great. Your and mine deceleration rate is now 2.7 ft/sec. Remember that 1/2 ft gap I had behind you ? I now close that gap in .185 sec not .25 sec. My reaction time from when I see you slow to when I apply my brakes has just shrunk

Given that human reaction times are some where between .18 to .5 sec with the avg right around .25 sec I'm most likely going to run into the back of you because you are declerating at a faster rate and my distance behind you didn't cahnge. .065 sec doesn't sound like much but in terms of reaction times it is pretty big.

So disc brakes are great when you are on your own. Stopping sooner / faster is great. In a group if I slow down to fast it decrease the reaction time for the guy behind me which could results in something not so good for the both of us.
jmdirt

Posts:755

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11/08/2014 01:45 PM
Ride On, there was an interview with Tyler F. and he said the same thing without the math so the pros are thinking about it. He, and others, have said that either everyone in the platoon uses discs or no one uses discs.
jacques_anquetil

Posts:234

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11/08/2014 07:52 PM
breaking at two speeds.

imagine a wet criterium with sharp, downhill corners. it's gonna be utter pandemonium as slippery-carbon rim-brake'd riders pile into the back of the panic-grabby disc-brake'd racers.

isn't the UCI suggesting that the 2015 Paris Roubaix be run as a disc brake-only test event?
Orange Crush

Posts:1251

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11/08/2014 09:04 PM
The governing body was originally shooting for 2015, but Campagnolo’s slow movement toward discs and SRAM’s recent recall moved that optimistic timeline back. Even 2016 may be overly optimistic. “If the answers aren’t here today, I don’t think the UCI is a year or two away,” Meyer said.

The UCI, and in particular its new technology officer Dimitris Katsanis, has made it clear that it believes that a wholesale swap is the safest route, when the technology is ready. http://velonews.competitor.com/2014/08/bikes-and-tech/stopping-progress-debate-hydraulic-disc-brakes-pro-peloton_340905
Ride On

Posts:454

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11/08/2014 09:04 PM
I guess I explained it wrong.

You close on the rider in front of you faster because he is decelerating quicker because of the disc brakes he has. It maters not what brakes I have if I never get a chance to apply them
Orange Crush

Posts:1251

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11/08/2014 09:30 PM
You explained it well; I got it. But I don't think it matters

1. You could make that argument for any speed sport, F1, moto etc. 2. You can only brake so hard with skinny tires and can already lock up standard brakes 3. This could not be any worse than braking on carbon rims.

I'm only cynical on this cause a problem was created (braking on carbon rims) that subsequently had to be "solved". Money + money equals more money to be made.
Ride On

Posts:454

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11/09/2014 06:01 AM
This also does bring up the issue with non pro bike races. Sure the UCI can get all the riders to change at once, but what about you and I in our local races. Some guys change over others don't. I will less safe if I'm lined up with 20 guys on disc brake bikes and 20 guys on rim brakes. None of us are safer. Local races should ban them until everyone changes. NEVER.

This whole idea that disc brakes will make group riding safer just is so stupid. Great idea for me by myself , terrible idea for bike racing in a group.
longslowdistance

Posts:744

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11/09/2014 10:05 AM
Just chewing the fat here, but an argument for more powerful brakes is that some accidents would never happen if everyone had them. A problem such as an unexpected suddenly appearing road hazard could be avoided by everyone.
If you want to ban something, ban carbon rims.
Ride On

Posts:454

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11/09/2014 11:08 AM
Just hammering home the point, if you are by yourself more powerful brakes should help you stop sonner. More powerful brakes in a group increases your deceleration rate which in turn decreases the reaction time of the rider behind you from rear ending you. In theory it can lead to MORE crashes not less.
Oldfart

Posts:490

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11/09/2014 11:46 AM
Silly argument. Racing in a peloton we all ride way way too close. Follow that close on the highway in your car where we all have good brakes and see what happens. Don't care what kind of brakes anyone has, when rider you are following 30 cm back hammers on his brakes you're hitting him. This happens now.
Master50

Posts:252

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11/09/2014 05:10 PM
Posted By Ride On on 11/09/2014 11:08 AM
Just hammering home the point, if you are by yourself more powerful brakes should help you stop sonner. More powerful brakes in a group increases your deceleration rate which in turn decreases the reaction time of the rider behind you from rear ending you. In theory it can lead to MORE crashes not less.


Just to pile on this idea. I agree that in a pack the emergency grab a handful will result in crashes now. I don't see disks changing anything here. it is for the fast decent on a wet Carbon rim and a guy on a disk brake bike. The carbon wheel and rim brake guy starts braking a week sooner and slows in an interminably long time. the disk brake guy passes him before the corner Brakes hard and late and drops the other guy every corner. they can adjust instantly just like we could with cantilevers, V brakes, U brakes and disk brakes all intermixed on MTB rides. Disk brakes for the road are not about ultimate stopping power but predictability and modulation are big benefits of disks that rim brakes do not deliver. Sure aluminum rims are pretty good when wet but disks are still a lot better.
Ride On

Posts:454

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11/10/2014 06:45 AM
Posted By Andy Eunson on 11/09/2014 11:46 AM
Silly argument. Racing in a peloton we all ride way way too close. Follow that close on the highway in your car where we all have good brakes and see what happens. Don't care what kind of brakes anyone has, when rider you are following 30 cm back hammers on his brakes you're hitting him. This happens now.


Yes and disc brakes make this worse. Safe follow distance is directly related to deceleration rate ( quality of the brakes ) . The faster you decelerate the less reaction time I get to apply my brakes. Give us both crappy brakes and I can follow much closer behind you since you can not slow down quickly ( neither can I ) but we are a connected system. It is a very flawed conclusion that better brakes makes us riding together safer.
Master50

Posts:252

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11/10/2014 10:07 AM
Posted By Ride On on 11/10/2014 06:45 AM
Posted By Andy Eunson on 11/09/2014 11:46 AM
Silly argument. Racing in a peloton we all ride way way too close. Follow that close on the highway in your car where we all have good brakes and see what happens. Don't care what kind of brakes anyone has, when rider you are following 30 cm back hammers on his brakes you're hitting him. This happens now.


Yes and disc brakes make this worse. Safe follow distance is directly related to deceleration rate ( quality of the brakes ) . The faster you decelerate the less reaction time I get to apply my brakes. Give us both crappy brakes and I can follow much closer behind you since you can not slow down quickly ( neither can I ) but we are a connected system. It is a very flawed conclusion that better brakes makes us riding together safer.


This is already a problem. mix 20 types of carbon an aluminum brake tracks under rim brakes I recall a new set of wheels I had never used wet yet. A race in the rain coming to a fast corner and I am the only one not slowing down. Thankfully my guy to the left saw that he was going to hit me if he tried to corner so we both went straight through and u turned to chase back on. I appreciate the disparity between different wheels and combinations. A wholesale change would equalize all the braking but as it is there is a huge disparity between the wheels in use now I don't think your concern is a show stopper. If anything disks would reduce this disparity a lot. Standardization is what I think will be the biggest deal. all wheels fit all frames and all use the same retention method whether thru axles or what ever.
ChinookPass

Posts:494

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11/11/2014 01:34 PM
This thread has a lot of opinions, a few anecdotes, and even some nifty calculations. But there must be some data out there from races over the past year. I'm betting someone has been in a mass-start road race with other riders using discs in which there has been an opportunity to explore their effects and whether they were an adverse factor in a pileup or not.

On a different note, regarding commuting, I'm riding my "D" bike this week while waiting for parts to rebuild my hubs, and am missing the discs in the wet. This bike has the worlds worst p.o.s Cane Creek cantis. I'm really having to think about braking this week.
longslowdistance

Posts:744

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11/11/2014 02:24 PM
Hey CP,
To answer your excellent question would require that discs be allowed in road racing. Catch 22. It might be dangerous so it can't be tried in real use. The only we would allow it is if we know it's proved to be safe in real use. Got that Capt. Yossarian?
Maybe someone with skill has been using discs in fast group rides, that would be similar.

I surely agree 100% that commuter and adventure bikes are a perfect application for discs.
I'm loving my TRPs on my gravel road bike. I still haven't tested them on a high speed paved fast-slow-fast-slow descent to see if they fade. No problems with drag braking down gravel descents, but I don't think that's similar.
longslowdistance

Posts:744

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11/11/2014 02:25 PM
PS: to Capt. Yossarian and all of the others: Thank You on this Veteran's Day.
ChinookPass

Posts:494

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11/11/2014 03:22 PM
I know they are banned by the UCI but are they also banned from USAC races? OBRA? The Tour of Podunk USA Crit?
jacques_anquetil

Posts:234

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11/12/2014 09:58 AM
CP has a good point. If we're participating in local-level USCF, CCA or whatever affiliation race, doesn't the highest level of UCI equipment qualification count? like, you can't race your 20" bmx in a crit or not have brakes installed. that said, unsanctioned races and events might be a wild-west free for all.

one criticism from a pro that might be valid is what happens if you crash in the pack after a hard descent, and fall on a red-hot calliper. that'd leave a mark.
Dale

Posts:524

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11/12/2014 11:37 AM
Posted By jacques anquetil on 11/12/2014 09:58 AM

what happens if you crash in the pack after a hard descent, and fall on a red-hot calliper. that'd leave a mark.


Look on the bright side, if you get slashed by the red hot, razor sharp rotor the wound will be instantly cauterized and you won't bleed much. You guys are such Debbie-downers
Cosmic Kid

Posts:1192

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11/12/2014 10:32 PM
Posted By jacques anquetil on 11/12/2014 09:58 AM
CP has a good point. If we're participating in local-level USCF, CCA or whatever affiliation race, doesn't the highest level of UCI equipment qualification count? like, you can't race your 20" bmx in a crit or not have brakes installed. that said, unsanctioned races and events might be a wild-west free for all.

one criticism from a pro that might be valid is what happens if you crash in the pack after a hard descent, and fall on a red-hot calliper. that'd leave a mark.


USAC races never check bike weights and routinely let non-UCI compliant tri bikes race in TT's unless it is a UCI race.
Just say "NO!" to WCP!!!!
Keith Richards

Posts:759

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11/13/2014 02:35 PM
I don't think it will make nearly the difference people think it will make due to the limiting factor of the tires.

When I first got dual caliper brakes, I almost went down the first time I used them such was the difference in brake lever feel. Brake lever feel is what will change a lot, braking distances not so much I think.
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
79pmooney

Posts:1192

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11/13/2014 03:00 PM
Keith, I paired up dual pivots with V-brake levers on my new fixie. Love the combo. Takes a solid squeeze to stop hard, but they will. That panic grab? No lock-up. Perfect for fast descents on rural pavement. Only drawback is less than full power braking over the tops. But for that bike and HBs, I prefer riding the drops anyway.

Another plus - wonderful, huge lever hoods, longset I have ever seen.

Ben
longslowdistance

Posts:744

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11/13/2014 07:53 PM
Good point KR. This would be another nudge towards wider tires with lower pressures in some settings, i.e. to take advantage of better brakes.
Gonzo Cyclist

Posts:238

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11/15/2014 02:23 PM
as i saw this post, my 1st question was, "What kind of disk brakes do you have?"
Avid/SRAM are terrible at best, Shimano has one of the best, and my favs are Maguras, I won a pair of their new NEXT bakes at Interbike, can't wait to get them set and use them, but mt MT's are working great.
Rubbing does happen on the trail at time, ding a rotor, sometimes they warp from high heat, or if you glaze the pads, then they squeal like holy heck.
As OC states, on a commuter, in the rain, they are the best, coolest thing was no black mess all over your rims, bikes, etc....
I have a pair of Shimano CX77's on my bad weather/commuter bike, these put Avid mechanicals to shame, in performance, and set up
as for rotor size, I do believe that you get more power, and better modulation, never thought about the cooling effect, but that makes sense
Keith Richards

Posts:759

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11/17/2014 12:50 PM
In the car world we tell people all the time, if you want to stop get sticky tires. Big brakes won't help if the road/tire contact point is middling.

The irony of it is that the thing that they use to market cycling tires, low rolling resistance, is exactly the OPPOSITE of what sells performance tires for cars. Low rolling resistance tires are for high MPG uses (Prius is a good example of a car with low rolling resistance tires to aid in efficiency). Performance tires for cars are super sticky (some of them cannot even be stored, much less used, below 40 deg!) to aid in braking and cornering.

Just throwing some stuff out there to think about.
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
79pmooney

Posts:1192

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11/17/2014 02:39 PM
Keith, a funny. A few years ago, the Vittoria winter tire came out. Heavy, sticky tread, a stopper. Bought a pair for my winter fixie from the shop that was well known in Portland as the fix gear place, (Owner sponsored and coached a velodrome team.)

Went in a few weeks later to report on the tires. Owner said that he was going to stop selling them; the fixie riders were bringing them back because they were hurting themselves trying to stop. Pulled muscles and the like because they wouldn't skid!

Ben

Keith Richards

Posts:759

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11/17/2014 05:06 PM
lol! I have been buying these mid range Schwalbe tires for a while now, Luganos.

They wear out fast and I can FEEL the extra rolling resistance from them but DAMN if they are not the best cornering tire I have ever used. I can actually feel the tires load up in a corner.
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
Cosmic Kid

Posts:1192

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11/17/2014 05:50 PM
Rolling resistance be a bigger time / performance suck than aerodynamics.

Just say "NO!" to WCP!!!!
79pmooney

Posts:1192

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11/17/2014 07:01 PM
CK, except: when that additional rolling resistance means that you show up for the race with bike, not with your arm in a sling.

Ben
Cosmic Kid

Posts:1192

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11/17/2014 10:50 PM
Plenty of great tires with excellent Crr and cornering. No need to chose one over another.

Oh, and run latex tubes.
Just say "NO!" to WCP!!!!
Keith Richards

Posts:759

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11/18/2014 01:33 PM
Posted By Cosmic Kid on 11/17/2014 05:50 PM
Rolling resistance be a bigger time / performance suck than aerodynamics.


Since I am sure you have them, mind point me to some data?
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
Cosmic Kid

Posts:1192

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11/18/2014 02:06 PM
Gotta dig it out....had it bookmarked, but just switched laptops.

but if you run something like Conti Gatorskins, they can cost you MINUTES over the course of an IM bike leg, and over a minute on a 40K TT.

Seriously...Crr can be REALLY bad for some tires.
Just say "NO!" to WCP!!!!
Cosmic Kid

Posts:1192

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11/18/2014 02:22 PM
This wasn't the chart I was thinking of, but it is pretty good. Note the gap between some of these tires at similar pressure can be nearly 7 watts.....and these are all pretty good, performance based tires. A gatorskin isn't and its Crr is horrific, so the difference between a gatorskin and a 4000 IIs is gonna be pretty damn big.

But I also like running gatorskins for my training tires.....then when I get on my race wheels w/ good rubber, I feel like I am flying for a the same effort.

http://www.bicyclerollingresistance.com/road-bike-reviews
Just say "NO!" to WCP!!!!
Keith Richards

Posts:759

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11/18/2014 02:57 PM
I remember that study. But again, my car autocrossing background raises its head. Tire pressure is a very Big Deal. While I think using the same tire pressure for the testing makes sense, it also makes me wonder what the Crr of these tires would be if each tire was used at its optimal pressure.
----- It is his word versus ours. We like our word. We like where we stand and we like our credibility."--Lance Armstrong.
Cosmic Kid

Posts:1192

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11/18/2014 03:56 PM
Oh yeah...part of the problem here is that the testing device is perfectly smooth and you get a nice decrease in Crr as tire pressure increases. But we also know that Crr will increase with higher pressure on rougher roads.

But when you get a really poor Crr tire like a Gatorskin, the performance gains are obvious and noticeable. But for tires like a Conti 4000IIs vs. a Vittoria Open CX III, the "real world" difference may not be a factor.

But, there is no reason to trade off Crr for better cornering.....all of those tires with low Crr are great race tires.
Just say "NO!" to WCP!!!!


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