Disc rotor - why not larger?
 

I ride a cyclocross bike to work at the moment with a disc fork/brake installed. BB7 road version and Campy 2nd gen ergos (rounded tops, not pointy, not new 11s shape). The front wheel is a 700c Mavic T520 (top of line touring rim) laced with 32 db spokes to a Schmidt 20R disc brake hub. Canti in the rear.

I commute in London. Actual collisions are infrequent but emergency stops aren't.

I have to replace my rotor pretty soon and I'm considering going bigger than the 160mm I currently use. This won't be very expensive (and anyway, money isn't a big concern) - rotor and adaptor: perhaps £25 - £50.

I'm thinking why not have the extra power of a larger rotor? Weight isn't much of a concern on my commuter bike. I'm happy with the brakes are they are now - they stop GOOD! Still, I don't see how more could be a bad thing.

I can't lock up my front wheel now on dry roads. At least, I have never done so. I guess it is possible that I might be able to pull the lever REALLY hard but I'm not so sure. This suggests to me more braking force is possible without the tyre losing traction. This is true even given that I rock climb and I have strong hands (I can do fingertip pull ups). The brake is set up properly - it works like a dream.

A 203mm rotor only produces a quarter more torque than a 160mm one. A 622 wheel pulls with about a tenth more torque than a 559 one already. A 180mm on a "29er" is roughly proportional to a 160mm on a "26er".

So... any reason not to go to 180/203?

Are you sure front wheel skidding your limiter.

Larger rotors do not increase stopping power. It puts more stress on the caliper. The caliper will have to grip harder to stop the bike. Ask your LBS, likely they will say the same thing.

It should be less hard. The closer to the center you're trying to grab to stop, the more linear pressure you have to generate to make the same amount of stopping torque.

Now, given that the pad grips the rotor with X amount of force, a bigger rotor will generate more torque, and that will be harder on the spokes and probably the hub too.

If you can't brake hard enough to spectacularly wreck with a front disc, something is wrong that's not the disc size.

I'm not seeing that a larger rotor will be better. The caliper will have to cover more area to stop the bike efficiently. When I had my Ellsworth built, I asked the same question, and all three wrenches in the shop said the six inch rotor is more efficient.

Oh well.

Rotor size does very little for stopping power on a road bike.

The reason you see 203s on tandems isn't for more stopping power, but for the heat dissipation. 203s offer a much greater surface for heat dissipation. 160s will stop a tandem just fine, but 203s will stop it repeatedly better because the pads and rotor stay cooler. This is especially important on tandems in hilly or mountainous regions.

I have 160s on my road bike and the only time in 18,000 miles that I ever needed a bigger rotor was on a 14-mile descent in the Rocky Mountains, where I was riding the brake for the better part of 45 minutes.

I don't think larger disc brakes is what you need. Rather for better stopping power, get good brake pads. Large discs are to help dissipate heat. Bicycle discs are exposed so heat escapes much easier than that of an automobiles. So repeat stops on a bike should not hurt braking performance too much let alone warp the discs. Unlike a sports car that travels many times faster than a bicycle, large discs will just add to the rotational unsprung weight of your bike. It may hurt handling capabilities. Besides So bottom line is you want better pads.

Please correct me if my understanding of the physics is faulty but my understanding is that all else being equal, larger rotors should provide more braking torque, not just greater heat storage/disipation.

This is because braking "forces" are actually torques (torque being the rotational equivalent of force). Torque is the product of force and distance from pivot. Therefore, braking torque increases linearly with rotor size for a given clamping force, pad type etc. 182 is about 25% more than 160, hence about 25% more torque for the same clamping pressure at the caliper.

Given this, less force at the lever will be needed for the same braking torque. Or, the same force at the lever for greater braking torque. However, the gain is not so huge as to be unreasonable. It is in fact less than the difference in leverage at the lever between riding on the hoods and in the drops.

I'm using the stock Avid pads that came with the BB7. Would be happy to hear suggestions for better pads. However, even with better pads I see no reason not to go for bigger rotors.

I'm not too bothered about more rotational weight, especially this close to the hub. The front hub is already 400g with no impact on handling.

Perhaps so, but the limiting factor in bicycle braking isn't in the brake, it's in the tire's contact patch. A larger rotor does nothing to increase the size or available traction of the contact patch.

When front braking forces exceed available traction, the wheel locks up--it won't skid like the rear (or like a car). So yes, with a larger rotor you'll be able to lock the wheel and launch yourself over the bars more easily, but is that your intended outcome?

This is why in bicycling--or at least in road cycling--larger rotors are used only for their greater heat dissipation.

Going over the bars isn't to do with lack of road/tyre traction, it is because the forward force of the rider due to decelleration is greater than your arms are able to counter. Going over the bars is nothing to do with a wheel locking up, which is what happens because of insuficient friction between the tyre and the road.

Yes, I suppose the front wheel would probably stop spinning when it no longer had your weight on it because the friction is now much less, and I suppose you could call that "locking up", but everybody's limit of what forces their upper body can resist is different. For me, my upper body is very strong due to rock climbing, hence I can probably stop quicker without going OTB than most other cyclists.

By the way, this backs up the above quite well:

http://www.sheldonbrown.com/brandt/over-the-bars.html

The fact is you never want to skid your front wheel; if your front wheel loses traction, you're going to hit the deck. Thanks to the weight transfer that takes place when stopping, I'm not convinced this can actually happen absent some abnormal surface conditions. Normally one of two things will happen before the front skids: the bike stops or you go over the bars.

But it doesn't really matter. There probably aren't downsides to going with a bigger rotor. Some may say you will have less "modulation," but that's the sort of thing you get used to. I still say that if a good caliper with a 160mm rotor isn't "enough" for your commuting purposes (and you're within a reasonable weight range), then you're either doing something wrong or something is maladjusted, but if you want a bigger rotor, then go for it.

Larger rotors will displace heat better and you will be able to modulate the brake a little better.Larger rotors will give you more braking leverage(less pull on the lever for the same amount of stopping power). It has nothing to do with stopping power.If you can lock the wheel,that's as good as it get's.

If you want a bigger rotor,go for it.No down side except a tiny bit of weight.The rotor will last longer,stay truer longer and you will have slightly less pull on the brake handle.

I have an urban bike, originally 1x9 gearing, which I converted to a drop bar commuter. 180mm front disk. No issues, looks fantastic. Complete overkill, only thing I worry about is grabbing a handful in a panic situation and locking up the front. Hasn't happened yet...

Ok, I ordered a big Shimano Deore rotor, 203mm. With the adaptor was about £15 from ChainReactionCycles here in the UK.

It arrived and this thing looks HUGE! No time to set it up now, will post back with how it goes. Anticipating braking that is noticeably more powerful - enough to be worthwhile but not massively so.

Will have to post some pics of my commuter bike soon too, it really is a sweet ride.

Will any frame/fork designed for disc brake be able to support larger rotors? I'm thinking about putting larger rotors on my Kona Sutra touring bike.

Adam

I'm not sure about any but I should think with most the larger rotor would physically fit. In my case, I just checked visually checked that an extra inch on the radius of the rotor wouldn't touch the fork leg. I also checked the existing brake cable would be long enough if the brake were placed an inch further back.

The brake itself usually attaches to the fork via an adaptor, you'll need to change that adaptor but these are cheap. In my case, I bought an IS 203mm Avid adaptor.

I guess there's also the question of if everything can take the extra braking forces. I'm sure my rigid alu fork will be fine though and the wheels are well built to a high tension with strong hub and rim.

Yeah, that's why DH bikes come with 203's.... They don't do a thing for stopping over 160's. Right.

Any LBS that would tell me that would see me walk out their door one last time. That's pure crap.

Yes, there is more stress on components. But that stress comes from MORE BRAKING POWER.

Personal experience: had a 203front/185rear setup on my bike two years ago. The 203 would lock up the front wheel at 15 mph. Due to a hard time affording parts, I was forced to reduce to 185/160 (after recovering from the shattered c-bone that ensued from the front wheel lockup). Never locked a wheel again. Now, I'm back to 185/185, and my rear wheel will skid readily if I squeeze too hard, but I have the braking power I want. That 203 rotor is now wall art.

A larger rotor puts more leverage on the wheel, which means more stopping power. Clamping force is the same. Yes, there is more force applied to hubs/spokes -- because they're being required to stop more forcefully. If you think that's a problem, realize that the makers of premium bike parts do testing on new tech before they offer it to the market (Santa Cruz and Intense, for example, worked with VPP for 2-3 years before introducing it, and have since refined it, as well; they did so because faulty execution would have killed the product in one year.) So disc hubs have been made to handle the extra torque, and a well-made wheel will take it, anyway -- and hey, who can show me a disc-equipped wheel with radial lacing? No? Because radial laced wheels won't take the torque.

So, again -- if you think that's a problem, don't run discs. Mine have been faultless, and I have every confidence they will continue to be.

Gonna have to be a physics nazi here and say not true. We are essentially dealing with two lever arms here...... the first is the radius of the wheel/tire (tyre to keep things "proper" in the context of the OP) acting on the axle. The second is the extension from the axle to the disc mounting radius, extended out to the friction plane where the calipers meet the rotor.
The interface at the caliper/rotor has to overcome the torque being generated by the larger lever arm of the wheel. A larger rotor WILL offer better counter-torque for a given amount of friction from the caliper. Please note that this WILL take LESS force from the caliper to attain when compared to a smaller rotor.
HOWEVER, as others have said, this added "power" is absorbed elsewhere in the system (increased stresses closer to the axle).

I personally feel that, if your setup is sensitive enough, the larger rotor could give you a greater range of modulation, and,yes, better ultimate stopping power.

Finally, as it has been mentioned, there might be some gremlins in the works if you can't lock up your discs, even with the 160's.

This. Either the brakes aren't set up/adjusted properly,or you need to work on your braking technique. It's pretty common in the motorcycling world for people not to use the front brakes as hard as they could for fear of locking the wheel or going over the bars. I have two bikes with BB7's and drop bars,I can lock up the front on either with two fingers.

Disc brakes can vary in sensitivity and brake lever force due to manufacturing decisions too. My Swobo Dixon came with Avid BB7 Road calipers but Swobo has matched them with linear brake levers. This decreases the braking force for a given lever force applied. This may well be due to the expected skill level of the customers they have aimed the bike at.

tat, when you say 'linear brake levers', are you referring to V-brake-style levers? Or the older-style canti? The canti's will work well with BB& road models, V's won't; V's will MULTIPLY the force applied.

Also, the type of brake pad you use will have significant effect; when I switched from the factory pads to ceramics, I saw an improvement in both modulation and power. The whole thing just worked BETTER! Stock BB7 pads are kinda "on/off"....

Yours should have mountain models(like mine). The short-throw road models would be overly sensitive with long-pull levers.

+10 - I ride a Surly Big Dummy cargo bike with me and my GF on board [300lbs + 40lbs of bike] I can stop easily with Avid BB7's and 160mm rotors. Surly notes that going to larger rotors is unnecessary and if you apply to much braking force can damage your frame/fork.

If that was my bike I'd sort out what was wrong with my brakes as they are rather than spending $$$/time upgrading the rotor which may well not solve your problem.