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Disc vs. Rim Brakes - Torque on Spokes?

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Disc vs. Rim Brakes - Torque on Spokes?

Old 09-15-07, 08:44 AM
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Disc vs. Rim Brakes - Torque on Spokes?

Okay, I guess I don't have the mechanical knowledge to wrap my poor brain around this question. I'm trying to figure out if disc brakes put more torque on the spokes than rim brakes. I mean, if you stop your bike by clamping down on the wheel near the hub (disc brakes), are you more likely to break spokes than if you clamp down on the rims?

I'm a Clydesdale tourer, and breaking spokes is a problem I've encountered to my chagrin, and something that is always in the back of my mind when I'm considering what equipment to use. My gut feeling is that disc brakes would put the spokes under more stress, leading to eventual spoke failure quicker than with rim brakes. But I don't know if I'm right or full of beans.

Any mechanical engineers or physicists (or just smarties) out there who can figure this out and explain it to me?

Thanks in advance.
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Old 09-15-07, 08:52 AM
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I don't fit into any of your 3 catagories, but I still have an opinion.

A wheel has 4 components - hub, spokes, rim and assembly quality. Of the four, the last is the most important.

Spokes, even skinny little spokes, have plenty of tensile strength for handling the brakeing loads. When they fail it's generally either at the bend due to inadequate tension or at the nipple due to a bad angle.
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Old 09-15-07, 09:14 AM
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Yes, hub brakes of any sort put more stress on the spokes. Braking forces are transferred to the tire (where the action is) through the spokes. No such transfer of torgue or stress is involved with rim brakes. This is one reason tandems with rim brakes use lots of spokes.
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Old 09-15-07, 09:37 AM
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When a wheel is build with the disk they are laced different to make up for that torque and making them stronger. In this case I would think it's a 3/ cross. Also a lot of bikes today come with carbon forks and when they do, the forks for disk brakes are made for more torque as well.
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Old 09-15-07, 11:06 AM
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Originally Posted by Sluggo
No such transfer of torgue or stress is involved with rim brakes. This is one reason tandems with rim brakes use lots of spokes.
**********??
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Old 09-15-07, 12:33 PM
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Originally Posted by Sluggo
Yes, hub brakes of any sort put more stress on the spokes. Braking forces are transferred to the tire (where the action is) through the spokes. No such transfer of torgue or stress is involved with rim brakes. This is one reason tandems with rim brakes use lots of spokes.
Tandems use lots of spokes because the wheel has to support twice the mass of one-rider bikes.

I think you're wrong across the board. Braking must dissipate the kinetic energy of the vehicle, 98% or so of which is non-rotating. If braking force is applied at the rim, all that energy is transferred through the spokes. If applied at the hub, only the remaining 2% ((mass of spokes+rim+tire)/total mass) is transferred through the spokes.


Edit: I'm not real happy with this answer. It doesn't explain the tendency to skid under braking. If the force at the tire is the same, then the force on the spokes is the same. But I'm having a tough time 'splainin' it to myself.

In any event, rim braking does not exert more force on the spokes.
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Old 09-15-07, 12:57 PM
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Well, I did a little mathematical analysis of this problem, and my conclusion is: rim brakes put exactly the same amount of stress on spokes as disk brakes.

The proof of this theorem is left as an exercise to the reader.
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Old 09-15-07, 01:42 PM
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Originally Posted by Retro Grouch
Originally Posted by Sluggo
No such transfer of torgue or stress is involved with rim brakes. This is one reason tandems with rim brakes use lots of spokes.
**********??
There's actually torque due to rim-brakes as well. However, the moment-arm length are fairly equal with rim-brakes. The distance between the hub and brake-pads is roughly equal to the distance between the hub and tyre's contact patch. When the brakes are applied, the pads pull on the trailing spokes while at the same time, the contact-patch pulls back on the pushing spokes. The net result is that the two torques cancel each other out, but the spokes are still being stressed, just evenly.

With hub-brakes, due to the different lengths of the moment-arms, the disc has to apply 4x as much torque on the hub as with rim-brakes. This uneven difference in torque would tend to stress the trailing spokes more.
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Old 09-15-07, 01:51 PM
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With a rim brake, deceleration forces go through the spokes, but not as torque.

If there were torque acting on spokes from a rim brake, you could not use radial spokes, even on a front wheel. Just try using a hub brake and radial spokes.
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Old 09-15-07, 04:02 PM
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How about a few quotes from Jobst Brandt's "the Bicycle Wheel". My copy was printed in 1984 and there may have been an update, but the physics have not changed.

"Radial loads are caused by your weight on the bicycle..." (page 19); "Braking forces from caliper brakes cause small but measurable radial loads..." (my emphasis) (page 20).

"When you pedal or use a hub brake torque results, which enters the wheel through the hub... to the rim through the spokes" (page 26).

Back to original post: hub brakes, whether coaster, drum or disk, load the spokes more than rim brakes and will contribute to the eventual fatigue of the spokes. If a large and/or powerfull rider wants to use hub brakes, he/she might want to go to 40 or even 48 spokes, depending on how large and/or powerfull she/he is how long a life is expected from the wheel.
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Old 09-15-07, 04:19 PM
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Originally Posted by Sluggo
"Radial loads are caused by your weight on the bicycle..." (page 19); "Braking forces from caliper brakes cause small but measurable radial loads..." (my emphasis) (page 20).
The radial loads are caused by two equal and opposing torques, so the net force is radial. The vector is actually aimed backwards at the hub. So the loading is as if you're loading the front wheel with the bike vertical.

And it's not a "small" load either. Consider that without braking, your wheels are roughly splitting your weight 50/50. So for a 150-lb rider, that's 75-lbs on the front wheel. Then the weight-transfer under braking moves ALL of that rear-weight to the front-wheel, so now it's got 75+75=150-lbs of load on the front. And it's possible to brake at 1g, so you're adding an additional 150-lbs on the front-wheel, 150-lbs weight + 150-lbs deceleration = 300 lbs force on the front-wheel under maximum-braking. That's 4-times more load than just riding along.

Now if you've got 1g of braking-force on both rim vs. disc-brakes, the total additional loads on the spokes may be the same (150-lbs deceleration). With rim-brakes, this force is loaded evenly between ALL of the front-wheel spokes. So on 32-hole wheel, you've got an extra 150/32= +4.7 lbs per spoke.

However, due to the uneven torque on the pulling-spokes with hub-brakes, this extra load is +1.9 lbs on the pushing spokes and +7.5 lbs on the pulling spokes. So yes, disc-brake do stress some of the spokes more for the same braking-force than with rim-brakes.

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Old 09-15-07, 07:57 PM
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I know this is not intuitively obvious folks, but the simple fact is that THERE IS NO TORQUE TRANSMITTED THROUGH THE SPOKES DURING BRAKING FROM A RIM BRAKE. It is not about resultant vectors. The forces created by applying the brake are transmitted to the road through the rim (compression on the back and tension on the front). Deceleration forces of the bike and rider are transmitted by strictly radial forces through the spokes. And Brandt measured them as 5-10 kg, which are pretty small.

Furthermore, a standard bike will not generate 1 g deceleration by use of the brakes. The back wheel lifts off the ground at about 0.5 g (according to Whitt and Wilson, p. 160); harder braking means you land on your face. (Which does mean that all the rider's (and bike's) weight is on the front wheel during hard braking, but it is still supported radially, which is why radial spokes work on the front.)

And by the way, two equal and opposite torques do not create a radial force -- they add up to zero.
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Old 09-15-07, 10:22 PM
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Maybe you should define the axis of the torque you're talking about.
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Old 09-16-07, 12:57 AM
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Originally Posted by DannoXYZ
There's actually torque due to rim-brakes as well. However, the moment-arm length are fairly equal with rim-brakes. The distance between the hub and brake-pads is roughly equal to the distance between the hub and tyre's contact patch. When the brakes are applied, the pads pull on the trailing spokes while at the same time, the contact-patch pulls back on the pushing spokes. The net result is that the two torques cancel each other out, but the spokes are still being stressed, just evenly.

With hub-brakes, due to the different lengths of the moment-arms, the disc has to apply 4x as much torque on the hub as with rim-brakes. This uneven difference in torque would tend to stress the trailing spokes more.
Amen!
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Old 09-16-07, 06:42 AM
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Originally Posted by Retro Grouch
**********??
I was struck by the contradiction in this statement, also, Retro. I'm no mechanic, but, in my mind, I'm thinking that the only possible difference in stress between hub based brakes and rim based brakes would be that the direction of the stopping force upon the spokes would be reversed - but you would think that wheels would be designed to handle that force direction change (if, indeed, there is any difference between wheel designs that makes coping with the change of direction of force a challenge in the first place).

The other distance, not related to the point at which the stopping force is applied, is how much force one braking system can generated as compared to the other.

I have owned good examples of both disc and rim based systems, and can report that, in my first hand experience, the disc system definitely was capable of developing more force - enough force that you could lock up either or both wheels at any speed if you chose. That's almost impossible for me to do with my rim based system even when the pads are new, cables recently adjusted.

Of course, all that torque is of little use to a cyclist most of the time. Usually, the bike looses contact with the road which is a very bad thing.

As for breaking spokes, I'm guessing again that in most situations, the cyclist won't break that hard, and, even if he does, the tension and force on the wheel will be relieved through some other avenue besides the spokes (again, the tires will likely lose contact with the road before the tension has a chance to break well-designed and adjusted spokes).

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Old 09-16-07, 02:23 PM
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I can understand why youall do not take my word on this since you don't know me and I am obviously not making myself clear. But if you don't believe Jobst Brandt or Whitt and Wilson (for rim brakes, "...the braking torque does not have to be transmitted through the hub and spokes...", page 154), I do not know how to convince you of this fundamental and simple fact.

But I will say it again anyway: when you apply a rim brake it does not twist the rim relative to the hub, so the spokes do not carry a torsional load. All load on the spokes from braking using rim brakes is radial. On the other hand, hub brakes work by twisting the hub relative to the rim. I understood this when I was a working bike mechanic from reading the above authors and from observation. Engineering school reinforced my understanding of it.

How important the differences are between rim and hub brakes is something reasonable people can disagree about. I have seen shortened spoke life from hub brakes in heavy-duty applications, such as tandems. Perhaps others have had different experiences.
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Old 09-16-07, 03:35 PM
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Originally Posted by Sluggo
Yes, hub brakes of any sort put more stress on the spokes. Braking forces are transferred to the tire (where the action is) through the spokes. No such transfer of torgue or stress is involved with rim brakes. This is one reason tandems with rim brakes use lots of spokes.
i think the last sentence, and how it relates to the rest of the post is what most people are having trouble with. if rim brakes don't put as much stress on the spokes as hub brakes, then why would a tandem equipped with rim brakes need more spokes?

being a mechanic, and having dealt with many people who fall into the "clydesdale" category, i'd say the ones who have the most trouble with broken spokes are the ones with wheels that were poorly built with low quality components, or those who simply don't know how to avoid a pothole, curb, large rock, or tree root. we put a lot of heavier riders on bikes with disc brakes because they tend to be more reliable, and have more stopping power with the same, or less input as a rim brake. i've seen a lot of rims fail before their time from brake track wear when the rider was 250lbs+, or in tandem applications. i can only guess that it's a combination of needing to hold the brakes for longer periods of time to stop, and also having to squeeze the brakes harder to achieve the desired effect.

for the disc vs. rim brake argument, i think that the initial torque on the spokes from a disc brake is going to be higher, but once the bike is in the process of slowing down, it will even out. if it were possible to have a bicycle with a rider on it decelerate from say, 15mph to 0mph in under 1 second without the bike flipping over, it would put enough of a load on the spokes to cause breakage. since that is not the case, and there are thousands of larger riders on disc brake equipped bikes out there having zero problems with it, this is a case of over-thinking a problem that doesn't exist.

to the OP: if you're already a heavier rider and are doing a lot of heavily loaded touring, possibly the best solution would be to get a high quality wheelset, with a 36, or even 40 hole rear wheel, built up with a disc hub, and a rim that will allow you to use a rim brake, and a bike that will accommodate both brake setups. use a cable-actuated disc brake (BB-7) with as large a rotor as your bike can fit, hooked up to a bar end shifter to use as a drag brake for long descents, and then rim brakes as your primary stoppers. you could also use a drum brake as a drag brake, as they handle the heat better, and are easier to attenuate than a disc, but they tend to weigh more. or if you're not carrying a lot of stuff, but aren't satisfied with the performance of your rim brakes, disc brakes in the front and rear might be a viable option, and shouldn't cause you or your spokes any undue grief.

also, to the poster who assumed that the weight distribution between the wheels of a bicycle was "roughly 50%", that is wrong. look at a bike sometime and notice where the seat is in relation to the wheels. most of the load is going into the rear wheel, i would guess that it's closer to 70%. try this: deflate the tires on your bike to about 20psi if you have a larger volume tire, or 60psi on a smaller volume tire and sit on the seat. check out how much each tire sags, the rear should sag more because more of your weight is over the rear tire. or take two bathroom scales and put one under each tire then sit on it and read each scale.
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Old 09-16-07, 05:05 PM
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I buy what you say Sluggo. To speak of spokes taking torsional loads, is stictly speaking wrong. If it were correct it would mean that the spokes are being twisted. Spokes only take loads in tension. That is my opinion.
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Old 09-16-07, 05:27 PM
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[LewisBlackRant=ON]

I had a nice long post composed and BF started to F*K with me again!!!

[LewisBlackRant=OFF]

Lost it, of course.

I figured out the error in my original post. But before I try to reply AGAIN I'll have to read all the new erudition and blather. Bottom line, I think if we fuse Danno and Sluggo, we get close to the right answer.

Oh, and the OP was wrong in applying the notion of torque to spokes. Applying it to wheel assemblies, otoh, is a different matter.
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Old 09-16-07, 05:36 PM
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Here's one, truth or fiction: all other things being equal, wheels with disc brakes are more prone to coming out of true than wheels with rim brakes.

I have heard that disc wheels are indeed more prone to coming out of true than rim brake wheels, but I rode a disc equipped mountain bike off and on for about six months awhile back, the wheels never needed truing. When I say "all other things being equal," let's say the wheels are Mavic X717 rims/XT hubs/32 14/15/14 spokes w/v-brakes vs. Mavic X717disc rims/XT disc hubs/32 14/15/14 spokes w/disc brakes. Meticulously built wheels, both cases. Same bike other than the wheels/brakes, same rider, same terrain, etc. etc. What do you all think? Again, I've heard it's true that the disc wheels would be inclined to come out of true more often, but I don't have enough first hand experience with discs to have a strong opinion about it, and the experience I do have was that disc wheels didn't need to be trued often, despite agressive XC mountain biking on them-

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Old 09-16-07, 06:59 PM
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Okay, so discs place more stress on spokes by pulling on them between the hub and the rim. But does this stress really amount to anything significant, or does the weight of the bike and rider and road bumps, etc. amount to way more stress on the spokes. If that's the case, and I believe it is, the whole disc thing doesn't really mean a lot.
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Old 09-17-07, 04:49 AM
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If you accept the fact that torque applies to rear wheels while pedaling and rear wheels are built with the proper trailing spoke geometry (ie at least one side of the rear wheel does not have radial spokes) then you may be able to view the front wheel with disk brakes as the same situation with the trailing spokes in the opposit direction. Disc brakes apply torque to the wheel in the same manner as the chain but in the opposit direction.
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