Brake bias- tubular long chassis

[Eric & Julie]

Hey all
So, I've got a strange problem with my outfit- A Derbyshire long chassis with a 1100 GSXR air oil.
In short, I can't get a descent brake balance between front and rear.. either I have too much ot the front, and lock the front under hard braking, or if I give more to the rear, the rear brakes lock on..
The setup is simple- front wheel has 2 x 235mm disks, with AP racing 2 piston calipers, both linked to a single girling 16mm master cylinder. Rear and chair have 235mm disks each with one AP racing twin piston caliper each, again driven buy a single 16mm Girling master cylinder. Both master cylinders are actuated on the pedal from a common pivot point.
So after again having been stuck twice with the rear/chair brake locked on, I pulled down both master cylinders, cleaned them refitted them and bled the crap out of them power bleeder, calipers off to have the bleed screw at the highest point, then reverse bled them to be sure). and this is what I have found now-
- With both master cyl needs to return full travel to the end of stroke for the return to reservoir to open. If it doesn't, the port is blocked, and after a few pumps the brakes lock on. This explains my rear brake locking problem.
- BUT, with both circuits fully bled, the 2 circuits don't behave the same piston travel. When measured, the front brake circuit needs 5mm of piston travel from 0 to the pedal goes hard and front wheel is locked. The rear cylinder needs 8mm, and after 5mm it has almost no resistance..
I thought maybe it is a history of air in the circuit, but I switched the 2 master cyl and then the rear circuit was more responsive, and the front would not lock, so the fault followed the part, eliminating an air problem.

So, I am a little stumped. If I try and adjust the actuator rod for the rear circuit so it contacts earlier, I block the return circuit with the pedal released.. if I set them up equal, I lock the front and the rear does almost nothing.. Of course the front wheel has twice the braking force so it should grip harder, but the piston stroke should be equal as both circuits are identical..
So I am open to any and all ideas.. I have thought about off-setting the cylinders to they are not on equal pedal stroke, but my fear there is that I will finish with a much higher (30%) circuit pressure for the front than rear on the same pedal force.
I have also toyed with the idea of linking one front disk with the rear and the other front with the chair, which will equalize the brake force a little, but then it becomes impossible to adjust the brake force front/rear..

HELP!

[steve-e]

Moved this to the section of the forum here as we are just about to start moving more sections of the forum together now that it's not so hectic as it used to be
Hopefully this will help

[G JONES]

Not sure if it's any help - but we have an in-built front bias by using different size M'Cyls 5/8" to the front pair of calipers & 0.7mm for the rear / chair - works well...
It would be interesting also to see your pedal to m/cyl setup linkage etc (pictures)
(AP Racing CP2623 M/Cyls )

[ada]

are you saying both master cylinders are the same bore? our bike uses .625 front and .7 for the rear master clyinders and dont have issues adjusting bias

[Eric & Julie]

Thanks for the replies
Yes, both front and rear are indeed the same master cylinders, 5/8" bore.
I've got some pics from when we bought it, pre rebuild but show the bits and bobs..

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[Eric & Julie]

Here is the front end

front end

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front end

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[Eric & Julie]

And the master cyls and outfit

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[steve-e]

do you have photo of the bias adjuster?
It sounds (from a passenger so not at all technical ) like the adjuster is more of a 100% bias switch than adjuster.

[Eric & Julie]

here is the master/pedal setup

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[steve-e]

I don't see a bias adjuster?

[steve-e]

I see the problem now, you don't have a brake bias adjuster in there. You have both onto a common point (I know you typed that but I didn't imagine it being that common til the photo) and no real adjustment, so I guess your adjustment is just being done on rod length.

Erm.. no. I'll try to find you some pics of bias adjusters, or find some people who can post some of theirs as I don't have a sidecar. Your problem is right there.

[Eric & Julie]

Steve, all I can do bias wise is give priority to the front or rear. By that I mean I can make the front or rear begin to recieve pressure before the other (and ultimately get less at full stroke)

[steve-e]

this is a brake balance bar. . each end goes into a master cylinder, the ball in the middle is fed from the brake lever - that's what we need to find you photos of.

as you screw the bar, the centre pivot moves, adjusting the pressure from one end to the other. This gives you a ratio of pressure between the two.

We need to find you photos from racers set ups..

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[steve-e]

From Paul Lowther, this might help you see what you need.

Thanks Paul!

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[duncan]

Here's my set up on my P5 Sidecar.