Edit: I know some of you will say “just use a rim brake you’re overthinking it” but just hear me out okay. I have my reasons for preferring disc brakes.
So I love disc brakes and steel forks but there’s one thing that bothers me about the steel disc forks that I have and that is that they are pretty heavy and don’t offer much in terms of compliance because they need to be strong enough to deal with the reaction forces of the brake.
The way all the forks work that I’ve seen is that the brake caliper sits about as closely to the back of the fork leg as possible. In that configuration, the caliper is pushed into the fork blade and the balancing force is pulling back at the axle, which puts a massive twisting force on the lower section of the fork blade.
But today I thought: what if there’s a smarter way to direct the forces of the disc brake?
What if we move the caliper lower down on the rotor, and direct the forces up into the fork blade? If we can do that, and have a brake tab with a flexible lower stay and a big strong upper strut, I think it might be possible to use a lighter, more flexible blade without running into issues where the lower section of the fork is twisted to oblivion.
Here’s a little doodle made with my best fusion and MS paint skills:
Red is where the pistons currently grab and where the forces are directed.
Green is what I imagine could significantly reduce stresses on the lower section of the fork leg.
It’s drawn on a 180mm rotor because that’s what I have but it would be much better with a little 160mm rotor I think.
It would also need to be TA so the wheel doesn’t get ejected if the axle isn’t tight enough.
Is anyone brave enough to build something and put my theory to the test?
Genius idea. I think it works. Instead of bending the fork blade you’re pushing up its length, where it’s already very strong and stiff (especially if you use a straight blade).
You would want to use a thru-axle as the other side of the torque couple will be trying to pop the wheel out even more than usual. But people mostly use thru-axles these days anyway.
Edit: actually not sure it does work… If the two forces in the couple are both inline with the blade then you won’t have any torque and that can’t work. If they aren’t you will still be trying to bend the blade.
For example, suppose you do manage to get one of the forces to be straight up the blade. If the other force is vertically downwards, then you’re still trying to twist the end of the blade.
Since you need a torque to oppose the disk there will always be a torque trying to bend the blade whatever you do.
Yeah you see I’m also confused about whether it’s possible to translate the torque of the brake into a force that more or less tries to pull the blade apart rather than twisting it.
I might just need to get some 2-3mm thin flexible steel flat stock to make up a little test setup and see what does what, good opportunity to test my flux core welder as well. (Which I won’t be using on real proper frames, no worries)
Edit: okay after making some really barebones sketches with the load paths, I can see that yes there will always be that torque on the fork blade… damn of course it was too good to be true. I’ll have to look further to find ways of making disc forks more comfortable.
I suppose if you put a disk and a caliper on both sides you’d only have half the torque on each blade But it adds weight.
Even if the caliper was only attached to the dropout and not to the blade at all you would still have a torque on the blade.
I use these dropouts below for the rear caliper. The caliper mounts are entirely within the casting. But when you put the brake on, something has to stop that whole dropout rotating, and that will be a pull on one stay and a push on the other.
But at the front you only have one stay: the fork leg.
the friction between the tire and the floor puts the torque at the end of the brake mount, a longer one can actually be weaker if you’re using butted blades since the torque would be in/closer to the thin part of the tube.
this old garelli motorcycle had its caliper mounted on a mount that floats on the axle and then had a rod that went up the fork to the crown, transfering all of the torque into the crown and not on the fork legs at all. you could explore something like that maybe.
That’s anti-dive mechanism. The torque from braking pushes the crown upward resisting the dive when under braking. This allows suspension to be softer for light bikes and riders.
But it adds weight.
