crankholio

I understand "for disc brakes only" or "compatible with rim brakes", but I'm not understanding what property a rim would have that would make it incompatible with disc brakes. Seems like if you've got a hub that supports disc brakes, there shouldn't be an issue. What am I missing?

dsaul

Maybe the spoke bed is not thick enough to prevent the nipples from pulling out of the rim under braking.

With a rim brake, the spokes are not loaded during braking, since you are stopping the rim from rotating and the hub and spokes are free to rotate. In that scenario, the load from braking is applied to the rim/tire bead interface.

With a disc brake, you are stopping the hub from rotating and the spokes are now a part of the load path that is resisting the forward momentum of the bike. If the rim was designed to only have the spoke bed be strong enough to resist the spoke tension, the additional force of disc braking could pull the nipples from the rim.

Moe Zhoost

I disagree that the spokes are not loaded during rim braking. How do you think the stopping force is transferred to the bike and cargo if not through the spokes. If the transferred forces are significantly greater with disk brakes, then your theory is a plausible possibility; however I've not seen any discussion of this actually happening.

cyccommute 

That sounds more like lawyering than engineering. I can’t see any reason why rims for rim brakes can’t be used for discs. Rim brake rims look a little goofy with disc hubs...kind of a belts and suspenders approach...but functionally a rim is just a rim.

I fully agree. Both braking systems are the same and the spoke loading should be equivalent.

dsbrantjr

But they do give the ability to swap the wheels between rim brake and disc brake bikes.

dsaul

You are correct that the spokes are loaded from resisting the forward momentum of the bike and rider, but I don't believe the loading is the same as a disc brake, where the load is directly pulling the spoke in a linear path from the elbow to the nipple. The force applied is the same, but it is distributed differently. There is a reason why we don't radially lace disc brake wheels. It would be interesting to see some data with strain sensors on the spokes of both types of wheels.

cyccommute 

Which is also goofy looking.

Pop N Wood

Can you give us a rim model that says that?

Maybe something to do with spoke offset's not being compatible with narrower disc hubs?

cyccommute 

That shouldn’t matter. Rims don’t come in front and rear version and the difference in the hub size is quite large. There is a 35mm difference between the front and rear on a 100mm/135mm hubset, for example. It doesn’t matter what rim you use, only the spokes length is adjusted.

BCDrums

I thought that rims for disc brakes didn’t have brake tracks at the rim.

70sSanO

If there really is a rim with that sticker/warning it is probably on there because it is only sold as a wheelset.

I “thought” there were some Shimano wheelsets that were marketed with a “wheelset” model number. They obviously didn’t have any rim brake only stickers on them.

John

frankenmike 

Not a physics major, but isn’t the difference in forces involved sort of like different locations of a fulcrum on a lever system? I.E. the brake pads acting further or closer from the outside of the wheel, where the friction with the ground is? If only Jobst were still around lol!

abourgault

With rim brakes, the spokes have their initial tension plus the load applied by the bike and cargo while braking. With disc brakes you have those 2 loads plus the tension generate in the spoke by the hub pulling on them to decelerate the rotation.

70sSanO

For all the analysis, has anyone ever seen a rim brake rim fail when set up for disc brakes?

It is probably to keep someone from sticking the wheel on a bike with disc brakes and then rolling over a cliff.

Probably for those who survived quick release skewers.

John

cyccommute 

No. The load on the spokes would be the same. Nothing has changed. The hub pulls the spoke no matter where the spokes are attached.

royphotog

Just my thoughts, I am no physicist, but The actual contact point is where the rubber meets the road, thus don't rim brakes transmit the force to the rim directly and thus to the tire and the road surface, where disks must go through the spokes to reach the rim and tire.

cyccommute 

They both go through the spokes to the contact patch. The hub isn’t the only part that is spinning. The whole wheel is a unit that spins. A rim brake and a hub mounted disc are the same thing...a spinning disc of metal to which friction is applied. They work in the same way. Only the applied force matters, not the manner or location to which it is applied.

Think of it this way, if the force were doubled using a hub mounted disc, the spokes would be under more stress and more likely to break. That’s just not something that happens with hub mounted disc. Additionally, If the force were doubled, either the bike would stop in half the time (not in my experience) or the brakes would require 1/2 the force to send the rider over the bars. Braking deceleration on a bicycle is limited by the location of the center of gravity. Too much braking doesn’t result in skidding of the front wheel...we can’t overcome the friction between the tire and the road...but by the rider rotating around the hub (not precisely accurate but close enough). Double the force on the brakes and you need 1/2 as much force to rotate the rider over the hub and into the ground.

Finally, I’m not sure that radial lacing would be an issue. The wheel is under tension and, again, a unit. Push in one place to decrease the tension and the tension will increase in another place. The hub can’t rotate under braking because the hub can’t rotate, period.

Fissile

Why would a rim says, "for disc brakes only"? For the same reason vinyl-clad, particle board specials slapped together on drained swampland are called, "Balmoral Estates"...marketing. Srsly, lots of things are just marketing BS. Many people are cutting the cord now and putting up antennas. Lately I've been seeing a lot of TV antennas marketed as 'digital' antennas. Guess what? There is no such thing as a 'digital' TV antenna. Antennas are tuned to pick up certain frequencies, the antenna doesn't care if what's coming over on that frequency is analog or digital. Like B.T. Barnum used to say, "There's a sucker born every minute."

abourgault

When the brakes are not applied, with rim brakes or disk brakes you have exactly the same loads on the spokes. Now consider the brakes are applied on the disk brakes. You're saying that the loads on the spokes will not change? So how the braking moment create by the disk brake is transfered to the rim?

Skulking

The weight of the bike and rider is all carried on the hub and transferred to the wheel through the spokes. It doesn't matter where the braking force is applied to the wheel, if you are slowing down over the same distance then you are slowing down with the same braking force and the same braking load will be experienced by the spokes.

holytrousers

i think you didn't take into account how braking forces are transmitted to the rim: with disc brakes, they are transmitted to the rim through the holed part of rim.. with rim brakes, they are located on rim walls.
So it seems ok to accept that some designs are not compatible with rim brakes.

cyccommute 

There is no rim design that I can think of that would be incompatible with a hub mounted disc brake. The forces on the rim are small. The spokes do most of the work. There are legitimate reasons for not using a disc rim with a rim brake...the sidewalls have been reduced and there is no brake track...but the opposite isn’t true of rim brake rims. Rims for rim brakes tend to be heavier because the brake track adds to the weight but, if anything, that would add strength to the rim, not take it away.

cyccommute 

Couldn’t have said it better. And I wouldn’t have been nearly as succinct

HTupolev

No, it will differ. There are aftward forces transmitted from rim to hub to bike in both cases, but disc brakes also apply torsion from the hub to the rim through the spokes. That's why many rim-brake front wheels are laced completely radially, while disc-brake wheels always have some leading and trailing spokes in the lacing pattern.

phughes

From Sheldon: https://www.sheldonbrown.com/torque-spoking.html

An excerpt: " A rim brake transmits all torque to the frame or fork directly -- none through the spokes: the hub is free to turn, except for the tiny amount of torque due to rolling resistance of the hub's bearings. For this reason, a wheel with a rim brake can be spoked radially, as long as the hub's flanges can withstand the direct outward pull of the spokes. .With a hub brake -- drum, disc, coaster -- on the other hand, all of the torque from braking is transmitted through the spokes, and so the spokes must be laced in a cross pattern."

And there is more, it is a great article.