October 30, 2014 Login  


Tirade on road discs
Last Post 08/02/2014 09:59 PM by Dale Dale. 46 Replies.
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6ix

Posts:127

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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:711

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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:1173

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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:701

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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:442

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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:711

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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:701

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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:711

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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:442

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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:269

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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:711

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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:1216

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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:711

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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:485

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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:485

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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/


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