Brainstorm: Rear Brake stay relocation?

[DirtyErnie]

Wondering if there would be a benefit to bracing the rear caliper (disk brakes) to the frame rather than the swingarm.
One would probably have to make a custom brace and add a pivot point to the frame somehow. Anyone know what sort of effect geometry that's non-parallel with the swingarm would have?

I have the idea that bracing to the swingarm puts the brake torque into the swingarm and makes brake action try to compress the shocks, taking weight off the rear of the bike. If you put that force into the frame, or could harness it to extend the shocks/ put weight on the ground while breaking could it make your brakes more effective?

 

[osteoderm]

A floating rear caliper can be tuned by the angle and attachment point of the brake stay to the frame to affect several suspension characteristics under rear braking. The primary effect is to separate braking forces from suspension forces, keeping the rear suspension supple and active even when the rear brake is applied. However, you could also tune the linkage to fight "brake-jack"; in effect, letting the rear end squat a little under heavy braking.

I took a good look at getting an active braking pivot worked into the rear suspension of my bike build, and saw that it was (for me, at least) too much complication and effort for very little effective gain.

The mountain bike guys have been all over this issue. Do a search for <floating brake jack> and you'll get all sorts of info.

 

[DirtyErnie]

Mine's an '80 Special, rear disk. the caliper bracket is rather free to rotate around the axle with the stock rear brake set-up. The only concern I have would be how to keep that area lubricated, since aluminum isn't real nice about that sort of motion.

 

[osteoderm]

I'm not 100% familiar with the stock rear disc, but I thought that the caliper mount was part of the spacer stack, ie., that with the axle bolted tight, the caliper bracket is clamped in place. Moreover, in the stock configuration (of the stay to the swingarm), the bracket is designed to remain motionless relative to the swingarm.

To get a true floating brake, the caliper bracket needs to rotate as smoothly and freely as possible, even with the axle bolted tight. If the stock caliper bracket has enough meat (which I think it does), you could bore it out and face it down thinner in way of the axle spacers to accept a pair of flanged bronze bushings, which in turn would ride on a compression sleeve. If the face-to-face thickness of the bracket was just a few thousandths less than the distance between the outboard face of the inner bearing spacer and the inboard face of the swingarm, it would move freely enough with a little lube.

Long story short, you need some sort of compression tube to transmit the clamping force of the axle to the inner bearing race(s), and you need some sort of bearing/bushing for the caliper bracket to allow free rotation with an absolute minimum of radial play or run-out.

It's a neat idea for sure, and one which I took some time to really research and plan out. In the end however, I decided that I could get far better results with not appreciatively more machine work by going to an entirely different rear brake.

 

[DirtyErnie]

Maybe I'll get bored and pull that bolt between the caliper bracket and stay. I've never made a deliberate attempt to see if the bracket is free floating or if it's clamped as part of the rear axle stack. I do know it was flopping around back there during assembly and it was a pain in the ass for a little while. If it's truely free, the only issue I would see as far as the bracket is concerned is how to handle friction between the aluminum bracket and the steel axle. I assume I could have it sleeved with something, a bronze bushing or hard steel or some such.
Then there's that pesky 'geometry' and 'force vector' issue...