Braking forces

[3spd]

I have been advised that it would not be a good idea to fix disc brakes to my non disc brake forks due to the higher forces.
I presently run HS33's, hydraulic rim brakes, which in the dry have sufficient power to lift the back wheel when the bike is fully loaded for touring.
I am probably wrong but I'm thinking the leverage is applied from where the tyre touches the ground and acts against the forks at the bottom headset bearing. So how does a disc brake apply greater force? Wouldn't the front wheel skid or rotate me into a face plant if too much power is applied?

[hayers]

The load is the same in terms of the total moment at the steerer BUT with a disc it's presented between the dropouts and a point on the blade a few inches above, rather than between dropouts and the crown on a rim braked fork, so stresses the fork in a different way, potentially overstressing the caliper mounting region on a non disc-specific fork.

[NATURAL ANKLING]

Hi,
On rim brakes some of the reaction force is at the crown of the fork.
On disc the force is close to wheel spindle, so the forks are seeing most of the reaction force, therefore the forks have to be stronger.

[andrew_s]

If you are braking hard enough to be on the verge of lifting the back wheel, the tyre will be exerting a force of about 50 kgf, acting about 340 mm from the axle.

With caliper brakes, the reaction force is exerted at the fork crown, 380 mm from the axle, and hence will be about 45 kgf (= 50*340/380).

With V/canti brakes the force is exerted by the bosses, about 280 mm from the axle, so that's about 60 kfg, shared between 2 bosses, or 30 kgf each side.

With a disc brake, the top of the attachment is about 90 mm from the axle, so the reaction force is 50*340/90 = 188 kgf, acting on a single fork blade.
A non-disc fork will be thin-walled, and may be only 12 mm in diameter, and may not be up to the job.

(Your weight and wheel/tyre sizes may vary the actual numbers)

[Brucey]

Suppose that you need 100kgf tangential thrust at the tyre for maximum brake effort; taking moments about the front hub means that there is a similar force at the rim brake (assuming the tyre height to be small) and summing thrust loads on the wheel means there is a thrust on the axle that is approximately 200kgf, shared between both bearings in the hub, i.e. about 100kgf for each bearing, more than normal. There is a small bending stress in the fork tip, relatively speaking.

With disc brakes, (assuming the disc is 160mm with a track width of 20mm, on a 700C wheel) and taking moments about the hub axle again gives a torque around the axle of 100kgf x 0.35m = 35kgf.m. This is resisted by the brake which is operating on a mean radius of 0.07m, meaning that the thrust on the brake pads (and the caliper mounts) is 35/0.07 = 500kgf.
Summing thrust loads on the wheel shows that the hub bearings see a radial thrust load of ~600kgf (more than normal) but this time (because the brake is to one side of the wheel) the bulk of this load (say approximately 80%) is on the LH hub bearing and dropout. [The loads trying to move the hub in the dropouts are at least as large as the loads on the rear wheel that (for example) cause QR wheels to move around in slotted dropouts eg when low gear ratios are used.]

If you assume the caliper mounts are a similar distance away from the axle as the brake pads, and take moments about the caliper mount position, under braking there is an extra bending load in the fork tip of the disc brake of 500kgf x 0.07m = 35kgf.m.
A similar calculation for the rim brake shows a bending load of 100kgf x 0.07m = 7kgf.m in the same part of the fork tip . In other words the tip of a disc fork sees about x5 more bending stress under braking than a rim brake fork. In addition there is a larger bending load near the crown in the left fork blade than the right.

Doubtless there are some non-disc forks which are (accidentally) built strong enough that you could just nail a disc mount on them. There are a few that are deliberately like that, where if you get a non-disc fork today, it is like the 'equivalent' disc fork (i.e. built with heavy tubes and heavier and stiffer overall than a non-disc fork should be).

But by and large tube-makers have spent decades refining their designs so that fork blades are tapered, in view of the (normally low ) bending stresses near the fork tip. Just nailing a disc mount to the fork risks breakage. Some manufacturers have not done their sums right, and in fact have produced disc forks that are not strong enough for a disc mounts....

If you want to (safely) convert your bike to a hydraulic front disc brake you will need a brake caliper/hose, a MC, a wheel, a fork.

My suggestion is that it probably isn't worth it. Not all disc brakes have brilliant 'initial bite' in very wet conditions, so arguably you are really addressing only a small proportion of the duty cycle; when it is wet enough to make the rims wet, but not so wet that it makes brake discs wet.

cheers

[LuckyLuke]

Interesting stuff.
Does anyone know the forces involved with roller brakes and drum brakes?
Sheldon's site recommends using a beefier fork, though the internet features several examples of ordinary forks using retrofitted, clamp on tabs for the reaction arms.
I've done the same in the past for a roller brakes on 1" forks, designed for canti brakes.
I'm planning a winter commutter build with the 90mm Sturmey drum brake dynohub. Would this brake be too strong for, say, a 531 1" fork?
Thanks in advance for any advice.
Best wishes,
Luke

[Brucey]

a 70mm brake will usually be OK on that fork, but a 90mm one is too much IMHO. You can do stoppies very easily with a 90mm brake, so a stronger fork is a good idea.

cheers

[LuckyLuke]

Hi Brucey, most helpful, ta v. much.

Best wishes,

Luke