September 23, 2014 Login  


Tirade on road discs
Last Post 08/02/2014 09:59 PM by Dale Dale. 46 Replies.
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dkri

Posts:80

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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:700

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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:1179

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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:80

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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:1179

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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:217

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

Posts:120

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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:666

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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:700

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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:80

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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:487

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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:700

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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).
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