Disc Brakes
 

Although I ride regularly, I don't read cycling magazines or follow equipment trends online. This forum is new to me having just found it by accident a few weeks ago. Was wondering if what I've read recently about disc brakes putting undue stress on spokes (and breaking them) is true. The two bikes I've purchased recently both have discs and I like them. What gives? Any real-life spoke breaking going on with discs?

Old Town

I am pretty sure any bike fitted with disc brakes will have spokes that are designed to handle the load, as well as the forks. Okay, who knows what the score is with cheap low end bikes. My MTB has a front disc and in four years I have had no spokes let go, and my MTB is a real cheap bike. I have hammered the brakes on some of my outings, and no issues apart from pad wear.

Cadfael: Good to know. Thanks.

Old Town

Disc brakes under maximum-braking actually don't put any more stress on the spokes than your weight, 1G. Hitting bumps, pot-holes, jumping kerbs will actually stress the wheels much more than that, about 3-4 Gs. Even more important is the rate of building up those Gs and there's no way you can grab the brake and build up forces as quick as hitting a speed-bump in a parking lot.

Basically, nothing to worry about with disc brakes.

I have been thinking about this...

If you had the same braking power on the rims as you do on a disc closer to the hub... then you may have issues, yes? If someone invented a rim brake that was as effective as a disc, then spokes would give?

or am I over-thnking?

google "ztl buell" and read up. not really applicable to bicycles, though. even a 160 will lock the front wheel, so a 26" brake rotor would be overkill.

Locking up the wheel is about as effective as you can get, and rim brakes were/are able to do that.

Well, the actual maximum braking force from disc-brakes is exactly the same as rim-brakes, about 0.80-0.83g of deceleration. That limit is actually based upon how much traction there is between the tyre and the ground. It's useless for the brake to apply more clamping force than that because it would just over come tyre-traction and/or throw you over the bars. So disc brakes do not brake any faster than rim-brakes.

The only times disc-brakes are better is for repeated braking efforts where heat cannot be shed fast enough by rim-brakes and the pads cannot tolerate the higher temperatures. Such as downhills on heavy bikes, such as tandems. This is more a durability issue, not maximum-braking force and the spokes won't experience any higher loads than with regular rims.

The other time disc-brakes have an advantage is rain or muddy conditions which reduces the friction between the rim and pads. The discs with its harder pads and higher clamping force (to overcome the lower mechanical advantage) can force the water from between the disc and pads better. And discs are further away from the ground, so they don't get as muddy in off-road conditions.

So... the premise that disc-brakes generate higher braking-forces is mistaken. That limit is based purely on the tyre-traction at the contact-patch. And that limit can be reach and overcome just as easily with rim-brakes as it can with disc-brakes.

Nope. Disc brakes, no matter the rotor size, still transmit power through the hub. Rim brakes can run on radial-laced wheels, where the spokes cannot handle any significant torque.

another advantage is that for me anyway, dics are way easier on the hands. I live in the rockies, grabbing a handful cantilever brake on my touring rig (especially while carrying a 50-60 pound load) was giving me repetitive stress injury to the hands. I am a guitarist, can't have that.

For a typical road bike the limit is closer to 0.5G and the limiting factor is rear wheel lift, not tire (tyre?) traction. Once the rear wheel comes off the ground, any harder braking will just rotate the bike over the front wheel and you get an over-the-bars crash.

Actually "disc brakes" have been used on bikes nearly forever. Up until recently, however, the "disc" was 26" or 27" in diameter and rim mounted. :)

Try some brake-testing yourself and plug into the equations for deceleration-G. Very easy to measure braking-distances (real empirical test data), and then work backwards to calculate braking-G. Measure the starting velocity (V) when braking starts and braking-distance (d) and plug into these two equations (remember to use SI units):

V=at and D=1/2at^2

V=at --> t = V/a ; substitue this for t in your second equation:

D=1/2at^2
D =1/2a(V/a)^2
D =1/2a(V^2/a^2)
D=1/2(V^2/a)

And rearrange to:

a=1/2(V^2/D)

You'll find that in real-world actuality, you can easily exceed 0.5g of braking. The 0.5g assumption was due to faulty logic and calculations. Just because the rear-wheel was supporting 0.5g of your weight, doesn't mean that it takes 0.5g of deceleration to lift it off the ground. Deceleration-G is not a 1:1 relationship with weight-transfer. There's an angular-pivot around the front contact-patch and the ratio of COG-height and wheelbase actually determined weight-shift to the front-tyre. By juggling these two variables, you change the ratio of weight-transfer to deceleration-G.

If you're sitting high on a hybrid with upright bars, it's possible to have all weight-transfer to front-tyre and go over the bars at less than 0.5g of deceleration. And it's actually possible with a long-wheelbase bike, like an MTB, with skillful laying of your belly on the saddle and the jewels skimming the rear-tyre, to actually lock up the front-tyre at 0.83-0.85g without lifting the rear wheel.