3

I am hoping that a mechanical engineer can give an educated answer to my question here. I am concerned about this because I have always had some trouble with spokes pulling through the rim on the rear wheel of my touring bike. (I use a 36 spoke wheel.)

Consider this: In general, a rear tire will wear at least 3 times faster than the front because of the forces applied to the rear wheel from pedaling. And for this same reason, we have far more problems with rims and spokes on the rear wheel than the front.

I am trying to make the point that the relatively small forces associated with pedaling, all of which are transmitted through the spokes, already cause the rear wheel and tire to fail much faster the front wheel and tire.

Now consider that the rear brake alone can cause the bike to decelerate at a rate of about 5 times that which we can accelerate with the pedals, implying about 5 times the force to the wheel.

When a traditional brake is applied, the forces are transmitted from the brake to the ground, primarily through the circumference of the rim. While there will certainly be some additional forces on the spokes, I don't believe it is substantial.

With a disc brake however, all the braking forces must be transmitted through the spokes. Isn't it logical then to assume that disc brakes will cause even more problems with the rear wheel and spokes than a traditional braking system?

I asked a bike dealer about this, and he insisted that this wasn't a problem. He then went on to show me the new Surly Disc Long Haul Trucker. Curiously, this bike has mounting brackets for carrying 2 spare spokes.

I would suggest that this is either a really dumb marketing gimick, or Surly has a significant amount of trouble with broken spokes on this bike and does this to assuage customer complaints. In my mind, one is just as likely as the other.

To repeat, do disc brakes aggravate the problem with broken spokes or spokes pulling through the rim?

Edit: After reading the comments, I would like to explain that I am trying to understand the differences between a disc brake and a traditional braking system in the way the braking forces are transmitted from the ground to the bike frame.

I tried to analyze the problem myself using force vectors, but lack the ability to reason through the problem. I suspect that disc brakes cause a great deal of additional stress on the spokes and rim, but simply do not know. That is why why I posted the question.

2nd Edit I didn't intend to imply that I have a significant amount of wheel problems. I've been riding for over 30 years and at one time or another, I have been stranded at the side of the road for busting virtually every part on the bike, except the wheels. (Seat rails twice, seat post, frame, rear axle, bottom bracket, chain, pedals twice). I was only asking whether disc brakes aggravate wheel problems, of which we all have some degree of trouble with.

Improve this question
12
  • 1
    @user5108_Dan: difficult, but not impossible, and it could be necessary if you are a long way from any town. Bicycles have braze-ons for frame pumps too, despite people being able to easily tape those to the frame.
    – whatsisname
    Commented Mar 17, 2015 at 21:28
  • 1
    You have some logic problems and physics problems. The regular truckers also spoke holders. There are some very knowledgeable bicycle people trying to help you and just want dismiss based on your misinformed perceptions.
    – paparazzo
    Commented Mar 17, 2015 at 21:55
  • 3
    Really you think there is no braking force on the spoke with a rim brake? Just how do you think the force actually stops the bike and person attached?
    – paparazzo
    Commented Mar 17, 2015 at 22:09
  • 1
    Yes, disk brakes place more stress on spokes than rim brakes (which place very little stress on the spokes). But broken spokes on a road bike are pretty rare, if the spokes are good quality, and most such failures are due to fatigue. The spoke holder on the Surly is there because it's a good idea for tourists to carry spare spokes (and a lot of tourists carry them inside their panniers).
    – Daniel R Hicks
    Commented Mar 18, 2015 at 12:03
  • 2
    (And if spokes are pulling through the rim, vs breaking along their length, then it's a lousy rim or the nipples are the wrong size.)
    – Daniel R Hicks
    Commented Mar 18, 2015 at 12:06

6 Answers 6

Reset to default
5

The rear wheel doesn't see more abuse solely due to forces from pedaling. It also supports much more weight, and they are dished (except single-speed wheels), so spokes on one side bear significantly more load than the other. As Sheldon Brown says: "If you have the same number of spokes front and rear, either the front wheel is heavier than it needs to be, or the rear wheel is weaker than it should be. "

Surprisingly, a disc wheel can actually be stronger than a non-disc wheel, because it will have less dish.

Disc braking does create some forces that require some consideration when building a wheel, but that is solved by proper selection of spoke gauges.

Disc brakes can offer a significant advantage over rim brakes for touring, as you can have a pretty badly damaged, out of true wheel, but still be able to brake reasonably well, as the wobbly wheel won't affect your braking, which can be the difference between riding vs walking to the next town.

There are many ways wheels can fail in the wild, but spokes pulling through the rim on high-quality rims should not be one of them. What kind of rim does your touring bike have?

So ultimately the answer is no, not necessarily. Like all bike part selections, they have upsides and downsides, and whether it's a net positive or negative depends on application.

Lastly, spoke holders on bikes have been around for far longer than disc brakes.

Improve this answer
7
  • Thank you, but I was hoping for a more analytical answer based on force vectors. In other words, does the analysis show a significant increase in strain on the spokes with disc brakes. Marketing people can and will say anything.
    – user5108_Dan
    Commented Mar 17, 2015 at 21:34
  • Well you are expecting something that is mostly irrelevant. People have been using hub brakes for eons. It's not as big a deal as you think it is.
    – whatsisname
    Commented Mar 17, 2015 at 21:39
  • 2
    There will definitely be more strain on the NDS braking spokes, yes, but it shouldn't be problematic if the wheel is built correctly. Just to reiterate, something is terribly wrong if you routinely have spokes pull through a rim, and while you should fix whatever is causing that, it's not a good foundation for reasoning about other wheels.
    – Useless
    Commented Mar 17, 2015 at 21:49
  • My concerns may be over blown, I am not sure. Regarding hub brakes however, those are more typically found on childrens bikes, not quality tourng bikes.
    – user5108_Dan
    Commented Mar 17, 2015 at 22:07
  • 1
    In the nicest possible way, you asked a question and got an answer. This isn't really the right format for long conversations; it works better to ask follow-up questions as self-contained new questions on the site.
    – Useless
    Commented Mar 17, 2015 at 22:43
3

A hub brake (any kind, coaster brake, drum brake, disk brake) exerts a torque at the hub which is transmitted to the rim through all the spokes, or if the hub barrel is flexible, mostly through all on the side with the brake: leading spokes loosening, trailing spokes tightening -- opposite the load from pedaling. Because this load is transmitted through many spokes, its effect on the tension of any of them is relatively small as long as the wheel has a near-tangential cross pattern and large enough hub flange spoking diameter. The force on any particular spoke is less than that of the weight load, which is carried by reduction of tension of only a few spokes at the bottom of the rim. It would help to read Jobst Brandt's book "The Bicycle Wheel" to understand this in more detail.

Improve this answer
3
  • I'm with you until you say that the force due to braking is less than the weight load. I haven't worked it out, but it seems to me that the short lever arm represented by the hub could theoretically create substantial force.
    – Daniel R Hicks
    Commented May 4, 2015 at 3:04
  • 1
    I venture that that is actually incorrect. A thought experiment: a bicycle in motion with the rear wheel more heavily loaded than the front. The force on the front wheel is less than half the total weight of the bike. Under sudden braking the rear wheel lifts off the ground, meaning that all weight is transferred to the front wheel. Thus the force under braking must be greater than the weight load (whether on one or many spokes, the total is greater)
    – Móż
    Commented Nov 30, 2015 at 3:11
  • This answer is wrong. You can't compare the weight load on spokes with the torque (rotational stress) on the spokes - completely different types of forces. The spokes are mainly designed for weight load, they're not good with torque, which moves them sideways. Also, the weight of the bike is always carried on the TOP spokes, never those on bottom of the rim.
    – Edward
    Commented Nov 29, 2021 at 2:24
1

You are overthinking the problem.

Ultimately the stopping force needs to be transmitted to the center of mass.

In either rim or disk brakes the force is transmitted through the spokes to the hub and via the hub connection to the frame to the center of mass of bike and rider.

It doesn't really matter if the rim is decelerating wrt to the hub ( rim brake ) or the hub is decelerating wrt to the rim ( hub/disk brake ), the force is exactly the same just in the opposite direction.

However in either case, the force is far less than the force due to gravity. If the wheel is strong enough to support your weight, it's strong enough to stand up to braking. The tire is going to skid long before the deceleration rate gets anywhere near the acceleration due to gravity.

You put at least an order of magnitude greater force on a wheel riding through a pothole, than you ever will using your brakes.

Improve this answer
2
  • ah, but fatigue can have different effects depending on the braking type. I just had some disc wheels built, and I can rock the wheel slightly back and forth with the disc brake fully engaged, resulting in some small spoke movement. This causes different stresses for a spoke than a rim brake (rotational instead of the vector on the hub from the bike), but perhaps it is below the steel fatigue limit.
    – daaxix
    Commented Oct 8, 2015 at 18:48
  • The assertions that "the force is far less than the force due to gravity" and "the tire is going to skid long before the deceleration rate gets anywhere near the acceleration due to gravity" could do with references. Specifically "far less than" when bicycles can exceed 0.6G under braking is IMO risky.
    – Móż
    Commented Nov 30, 2015 at 3:15
1

I think this is a great question and a lot of people here are arguing complete nonsense.

As you mentioned, the braking is applied from the disc THROUGH the spokes and to the rim/tire whereas on rim brakes, the braking does not go through the spokes.

It's obvious disc brakes involve the spokes more, and Giant agrees - if you look at the 2018 TCR disc and non-disc variants, you will see the disc variant has far more spokes - these are the exact same frames:

Rim brake enter image description here

Also read more here:

In addition, a minimum of two-cross lacing is required to contend with the forces of braking as well. Front wheels will suffer the biggest weight penalty due to heavier hubs and more spokes. Thus, disc-equipped wheelsets will always be heavier than traditional wheels in the same way that clincher wheelsets will always be heavier than tubulars.

Improve this answer
0

Slowing the wheel and stopping the bike are two different forces
Clearly it takes less force to stop the wheel when you are riding as opposed to bike upside down and wheel just spinning

What stops the bike is the contact of tire with the ground
That stopping force is transmitted to bike through the spokes
That force is shared across all the spokes because the rim is solid

The brake is a separate and equal and opposite force
The only difference is which end the force came from
On a rim brake the tensile force on the spoke comes from rim side
On a disc brake the tensile force on the spoke comes from hub side
Either way it is the same tensile force and the spoke does not really know the difference

Pretend a magic wheel with two spokes
The exact same forces
enter image description here

It is as simple as the spokes are between the bike and the ground.
The force has to go through the spokes

Improve this answer
1
  • Ah, but what happens when the disc version has inherent rotation to wind up tension through the hub, it isn't a static system...
    – daaxix
    Commented Oct 8, 2015 at 18:53
0

I have had two rear rims fail on a tandem, where I accept the loads on the rear wheel are greater than a solo. The rear braking system is a disc brake. With a disc brake I would expect that the torsional forces in the hub (is that the right term?) are greater than on a rim brake where the stopping motion is applies to both sides of the wheel. With a disc brake there must be greater stresses set up on the side of the disc. I am not sufficiently competent enough to do the maths so I can't say whether this is significant. The only reason I can come up with for the failures is that we live in a hilly area and use the rear disc a lot during a ride. The current rear rim (our third in 4 years) is, we've been told, a more robust rim. Time will tell.

Improve this answer
1
  • 2
    Hi and welcome to SE bicycles. While your answer is interesting, it also doesn't really answer the question. Remember the front wheel does 90% of the braking, so your rear rim failures are probably related to something else. Do consider asking about the failures as a separate question though, that could be worth exploring.
    – Criggie ♦
    Commented Apr 24, 2017 at 22:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.