Do bikes with disc brakes break more spokes?

[Globalti]

Yes, I sold my Tricross to a colleague for commuting (admittedly he is quite beefy) and on the very first morning he broke a front spoke by braking hard while hitting a pothole. I fixed the wheel for him at lunch time and warned him that it wasn't a mountain bike. Curiously the only other time a front spoke broke was in the mile or so that my cycling buddy jumped on the bike to try it.

 

[I like Skol]

Let's look at it a different way. I'm pretty confident that I can apply similar or greater torque to the rear hub by pedalling than the front disk brakes will generate during a good, hard stop.

 

[Ajax Bay]

I'm pretty confident that I can apply similar or greater torque to the rear hub by pedalling than the front disk brakes will generate

So you're saying you can accelerate (rate of change of speed - let's stay scalar here) with a magnitude 'greater' than you can brake? Torque to the rear hub will accelerate you. Application of the disc brake will apply torque to the (front) hub slowing you down.
The AASHTO Guide for the Development of Bicycle Facilities identifies a bicycle acceleration rate of 1.4 m/sec2 in its equation to determine the minimum green time. And uses 3.3m/sec2 as the deceleration rate and allows 7m of braking distance from 22kph (6m/sec). Would you be up to 22kph within 7m from a standing start?
Awesome power. But not as awesome as effective disc braking.

 

[I like Skol]

So you're saying you can accelerate (rate of change of speed - let's stay scalar here) with a magnitude 'greater' than you can brake?

No.

I am saying that I can probably apply a momentary torque at the rear hub (dry conditions, correct gear, body weight forward trying to prevent front wheel lifting, cranks in the correct position, etc) equal or close to the figure that can be generated at the front brake in 'normal' hard braking. I am not debating that this won't be for more than a couple of pedal strokes and only at certain points of the pedal stroke. The braking force that can be exerted is limited by the point at which the bike/rider Centre of Gravity(CoG) will be pitched over the front wheel contact point. Assuming a CoG that is midway between the wheels (and this is difficult because irrespective of bike geometry the rider can do a lot to move the CoG backwards/forwards & up/down) it will require the same force to lift the front wheel under acceleration as it will to lift a back wheel when braking and it is this tipping point that limits the force exerted at the hubs.

I don't understand why you think traffic light timings have anything to do with this? Impressive of you to quote some figures, but also entirely meaningless without knowing what they relate to. Are they meant to represent the maximum acceleration/deceleration that a fit, skilled cyclist can generate, or more likely, a rough estimated figure derived from a few sessions with a stopwatch watching everyday cyclists set off from the lights outside the office and a few quick braking tests done in the carpark with a tape measure, stopwatch and a bike borrowed from one of the team's sons as he is a pretty big lad and it's almost an adult sized bike? If you can clear up what these figures are then you 'might' be able to justify working them into the discussion, but even then I doubt they would be significant in this case as the linear acceleration figure does not realistically represent the very non-linear application of pedal power required to achieve that acceleration.

Getting back to the OP, the rear wheel will most likely experience more torque from pedalling than from braking, due to the limited braking force that can be generated at the back wheel. A well built disc braked wheel will not break spokes any more than a rim braked wheel in the same conditions.

 

[iandg]

Don't remember the last time I broke a spoke - always use 36 spoke wheels (since stopping racing in 1984), never owned a disc brake bike - I couldn't comment

 

[XC26]

Tricky subject - one needs a PhD in applied physics, mechanics and maths - and maybe someone here has and can further expound. Anyway, as I couldn’t keep out of the debate, I just wanted to mention that one is advised not to use a radially spoked wheel with disc/drum brakes or as the driving (mostly rear) wheel. One is advised to use only crossed spoke lacing patterns because they are stronger and better able to handle the higher torque placed upon the hubs of such wheels. This implies that radially spoked wheels are only suitable for rim braked front wheels.

Regarding anecdotal incidences of broken spokes with disc vs rim brakes, as already mentioned I think it really depends on design, quality and number of spokes. Disc hubs appear to be beefier and thus stronger than non-disc hubs. Again, this would imply they encounter more stress.

At least it keeps our brains tuned up!

 

[Ajax Bay]

Getting back to the OP, the rear wheel will most likely experience more torque from pedalling than from braking, due to the limited braking force that can be generated at the back wheel. A well built disc braked wheel will not break spokes any more than a rim braked wheel in the same conditions.

I agree with the first element (though this is not your original assertion that I queried as you have changed the braking to the back wheel); and thank you for agreeing to my proposition (and others', and most recently @XC26's ) that "this is more about the quality of wheel building".

Trying to address your over-generous yet dismissive "impressive but meaningless" assessment, in the spirit of (Cycle) Chat.
Braking from 22kph (6m/sec) in 7m (I'm sure you could do better than that) implies decelerating the wheel rim by 4.5m/sec2 and this suggests applied torque (disc or rim) of 150Nm (rider and bike = 100kg). Please correct my maths if I've got this wrong.
"Chris Hoy could produce an explosive finish with power figures around 2,500w" Source Work he could apply to the pedal - 516 lb-ft of torque can only maintain it for a second (?two revolutions at 120 cadence) - 700Nm.
So if your peak torque capability is 1/4 Hoy's that'd be 175Nm.
Looks like you might be able to "apply a momentary torque at the rear hub (dry conditions, correct gear, body weight forward trying to prevent front wheel lifting, cranks in the correct position, etc) equal or close to the figure that can be generated at the front brake in 'normal' hard braking." But what are you saying "equal or close to" or "similar or greater"? Original comment:

I'm pretty confident that I can apply similar or greater torque to the rear hub by pedalling than the front disk brakes will generate

the linear acceleration figure does not realistically represent the very non-linear application of pedal power required to achieve that acceleration.

Why not? Power in at the crank: rider and bike acceleration is the result (without wheel spin or a wheelie), and these are directly proportional.

 

[Ian H]

Braking at the hub does impose more strain on the spokes than braking at the rim, but a properly-built wheel shouldn't fail. Similarly, the forks should be stronger (thus more rigid) for discs (and the drop-outs arranged to avoid the wheel creeping out under braking).

 

[Globalti]

Blimey there's been some waffle in this thread!

 

[Ajax Bay]

Blimey there's been some chat in this thread!

Corrected that for you. I thought your earlier post was very much to the point and answered the OP's request. Bravo!

 

[pjd57]

Every bike I've ever had :- no broken spokes.

 

[andrew_s]

I had a rush of broken spokes with a disc brake wheel - 4 in one ride.

 

[hoopdriver]

It comes down to how well your wheel is built and trued, and in the case of touring using wheels with an appropriate number of spokes for the loads you expect to carry. I do not have any bikes with disc brakes, but I have done a great deal of touring and cannot recall the last time I broke a spoke - would have to have been at last 25 years.

 

[SkipdiverJohn]

I would put multiple breakages in one incident down to the fact that once one goes ping, all the remaining ones are both more heavily, and more unevenly, loaded. Having been away from cycling for a good few years, I'm shocked at how poor and flimsy the engineering on many modern bikes has become. The spoke counts seem to have reached silly low figures with very little inbuilt safety margin to cope with the loss of one or two. A lot of cyclists now seem to have become so obsessed about shaving a few extra ounces of weight off their bike that they are compromising durability and reliability. I can't ever recall breaking a single spoke on any of my 1980's vintage all-steel machines - and I used to dish out some pretty harsh treatment to them at times playing silly buggers jumping off wooden ramps etc.

 

[Ajax Bay]

Have not thought about before

Perhaps the OP (who has implied that 20 spokes is 'normal' for a rim braked front wheel) could share his experience of breaking spokes (disc or rim braked)?