Russ Roth

I've been following this question and love the way it has continued. Looking at the problem I would have to conclude that discs would increase load on a rim's spoke bed even if only momentarily. Sheldon isn't wrong about the idea that a hub is free to rotate when the brake is used, when using a rim brake the spokes are actually resisting the attempted free movement of the hub which is transferring the force of the bike and ride through the spokes to the rim. But in doing so it is loading half the spokes all that the same time assuming a properly built evenly tensioned system. The rim is stopping and the hub is attempting to rotate against all the pulling spokes simultaneously. With a disc brake the hub is attempting to transfer the force to all the pulling spokes at the same time while first imparting the initial load to the disc side first. Even arguing against transfer through the spokes, since both braking systems are attempting to stop the same mass at the same velocity, properly setup hydraulic disc can stop a bike faster then a rim brake assuming perfect conditions of road and tire and perfect braking technique. That shorter stopping, even if only by inches requires that greater braking forces have to be transferred to the tires which will go through the rim. Take those higher forces of faster stopping and match it up with the hubs ability to resist twisting and you may find that momentarily 1/4 of the spokes receive a noticeably higher torque. As to the hub's ability to resist twist, something like an I9 or an XTR might be highly resistant and succeeds in having all the torque transferred to both sides simultaneously where an altus or cheap Chinese hub might make the situation worse, I'd suspect thru-axles would increase resistance while a QR hub has more flex in the design. Not knowing what their rim brakes hubs are built to a company could understandably slap a sticker on to cover themselves.

Bike Gremlin

I agree that hub is free to turn with rim brakes. Not transferring any braking torque (as in twisting torque).

However, say we have a braking force of 500 N.
That is what spokes need to take - no matter which braking system is used.

With rim brakes, the force is taken by a few front, and a few rear spokes (front ones loosing some, rear ones gaining some tension).

With disc brakes, that force is taken by all the spokes in the wheel. Leading by gaining some, trailing by loosing some tension.
If a hub is not very stiff, then yes, half the spokes take the load, with the opposite side half taking very little.

Russ Roth

With a rim brake it isn't only a few, looking at a front wheel, the torque of the hub will transfer that torque through its momentum to all the pulling spokes at once, unless you're arguing that the rim is deformed while riding bit then we would have to look at how that deformation changes as braking changes the dynamics of the system also recognizing that a disc wheel is under similar deformation. The trailing spokes are of course slightly unloadeunloaded.
But with a disc brake, using a cheap underbuilt one which is easy to grab off ebay, the initial force, even though it is truly only momentarily, the force is directed through the braking side pulling spokes only while the trailing spokes will be weakened. Either way, discs stopping the force in a shorter time span results in a greater transfer through the spokes and into the rim.

Het Volk

I am struggling with the concept that only a portion of the spokes are taking the brake load. When I apply rim brakes to the rim, the entire rim is taking the initial load and transferring that load through all of the spokes. Even the ones at the bottom of wheel are transferring the rim's want to slow down through to the hub.

And again, I still wonder, if because the rim is wider than the hub, is there more stress on the spoke trying to stop the larger rim (disc) or the smaller hub (rim brake)? Centrifugal force is greater when trying to ask the spoke to stop the rim, no?

Russ Roth

There are two sets of spokes on a wheel, the pulling spokes and the trailing spokes, the video further up covers the idea that under load the pulling spokes have an increase in tension as the force is pulled through them while the trailing spokes experience a reduction in tension at the same time, that's why there really is only a portion of spokes taking the load. The trailing spoke are taking some of the load but only after the pulling spokes have increased tension. The question about rim strength is really only focusing on the stresses on the rim under braking. I'm really not certain that its the rim transferring the desire to slow down through the spokes to the hub as much as it is the hub's desire to continue in motion even as the rim is stopping and transferring the velocity of the bike and rider through the hub and spokes and into the rim. I suspect a real engineer or physicist would be needed to determine that.

Bike Gremlin

My thoughts:

With disc brakes (and riding torque) - spokes carry the load by both tightening, and loosening (just depends on whether a spoke is a leading one, or a trailing one).
Neither of the two should be underestimated. Spokes loosing tension (unless they go completely slack, and the wheel buckles) are also doing the work. Related to that:

When rider's weight is carried, only the bottom spokes loose an amount of tension (OK, those right next to those gain very little tension). This is because the rim deforms, no matter how slightly.
Think it proves two things:
1) the rim deforms. Even very thick, rigid, few-spoke wheel rims deform - having only bottom spokes loose tension when rider sits on the bike.
2) spokes can carry / transfer a load by loosing some tension. Wheel is a pre-tensioned structure.

When applying rim brakes, what happens?
Ground and rim brake push towards the rear, effectively.
There is no torque transferred to the hub - because the hub is free to turn (hub's inertia si really so tiny in comparison that I think it can be disregarded). Just like when a rider just sits on the bike and coasts. It takes pedalling, or a disc brake (both affecting the hub) to create torque load (with the ground, using the rim/tyre as a lever opposing that force).
Force of the braking is taken by the few spokes at 3, and 9 o'clock, as the front facing ones loose some tension, and the rear facing ones gain some tension. Suppose the rim gets very slightly deformed, from the force of rim brakes, and the ground, pushing it towards the rear (at the top and the bottom).

Is braking stronger with disc brakes?
Well, with any decent brakes, the weak link is the contact patch, that is the tyre's grip. I'm able to lock up the wheels with both disc, and rim brakes, even on good quality pavement. Main advantage of disc brakes is very good modulation, and better performance in the wet/muddy/freezing conditions (roller-hub brakes excel in the latter, but severely lack the former). People with poorly set up brakes, poor choice of pads, brakes, rims, and weak hands, are the ones complaining how rim brakes are not strong enough.