Sprinter brake bias issues

**Sam_Q** · 02-11-2006, 08:13 PM

I in my sprinter I have ma61 256mm front rotors and calipers and at the back I have 280mm rotors with ae-82 calipers. I used to have so little brake bias after this conversion that my machining marks didnt even wear off the rear rotors. I gutted the brake proportioning valve and now I have decent braking all round. I would like to have even more rear brake bias, I bought a PBR P8573 valve assy but the PBR sales guy said that I in no way should fit this to the front.

Is this PBR guy full of it?

is there another option for myself?

**psychofox** · 02-11-2006, 09:18 PM

Front brakes do about 80% of the work of your braking system.

you should NEVER install a proportioning valve on the fronts as you are limiting the amount of force you can exert on the pistons on the front calipers. This is REALLY REALLY bad.

A problem you may have with your brakes is that you may have not designed it correctly taking into account the loading on the fronts vs the backs and selecting appropriate rotor/caliper sizing. Having significantly larger rear rotors is going to induce heavy rear bias. Proportioning valves aren't going to do much when you have such a large imbalance in the braking power front/rear. What size master are you running? What are the piston sizes front and rear?

When you say you had 'so little brake bias' - what do you mean? were you locking the fronts before the rears? or vice versa? You should always have a little bit of front bias - ie fronts locking first as rears locking first is quite dangerous.

**oldcorollas** · 02-11-2006, 09:40 PM

Originally Posted by Sam_Q

I in my sprinter I have ma61 256mm front rotors and calipers and at the back I have 280mm rotors with ae-82 calipers. I used to have so little brake bias after this conversion that my machining marks didnt even wear off the rear rotors. I gutted the brake proportioning valve and now I have decent braking all round. I would like to have even more rear brake bias, I bought a PBR P8573 valve assy but the PBR sales guy said that I in no way should fit this to the front.

Is this PBR guy full of it?

is there another option for myself?

as said, never ever fit to front brakes....

why is there so much front bias????

what are the piston sizes for the calipers?
roughly, piston area multiplied by the disc radius, and then working out proportion front to rear should give rough idea of what it will be now....

with such large discs on the rear, it seems strange to have no rear bias... are the calipers rooted?

**RobertoX** · 02-11-2006, 09:55 PM

Besides getting a twin MC setup with bias bar (if its a track car then I would suggest looking into this),
you could source a different rear calliper with a larger piston area or fronts with smaller (you should consider the pedal feel of your setup currently). It would be good to get it so that there is slightly more rear bias than needed and then use the proportioning valve to tune it to how you need it.

Just reading a small excerpt in "the braking handbook" by Fred Puhn on proportioning valves (I've never really known their characteristics before

) it seems that if you install one on the front you will be giving your brakes progressively less pressure as you brake harder. This is bad as with forward weight transfer when you brake at very low speeds you want brake bias (force not hydraulic pressure) to be approximately 1:1 and as braking force increases you want the bias to move towards the front.

With a proportioning valve, for low pressures you will have the same line pressure front/rear and as it increases the valve 'kicks in' reducing proportionate pressure in the line with the valve fitted. If you have it in the front line it will be changing pressures in the opposite direction to what they should. (ie giving progressively more bias to the rear as braking force increases).

Hope that makes sense...

**takai** · 02-11-2006, 09:58 PM

If you are going to go unboosted then a good little master cylinder is teh Datto 240K ones, cheap and cheerful. Easy to mount up too.

**allencr** · 03-11-2006, 12:40 AM

The PBR guy is trying to save you, & others from you.
Softer rear pads and/or harder front pads.
I wouldn't want to be anywhere near you on the road when it's raining!

**Sam_Q** · 03-11-2006, 09:24 PM

ok heres the deal:

The MA61 calipers I fitted have a huge diameter, by my hazed memory its something like 50mm. When I fitted them I found that the pedle went down a fair way, so I would safely say thaty they used a considerable amount more fluid. I also would run out of vacume after a bit of hard driving.

I dont know if I had trouble with my rear bias with drums they were too crap to tell. Now I upgraded the rears and I barely had them working at all. I have changed one of the calipers at one point and the wear between disks were even so its not the calipers.

I gutted my proptioning valve and the difference was really dramatic. The car stayed level more and it pulled up hard. I just thought it would of been good to try and see if I could get a little more braking power to the rear to pull up even harder. But I know I have it close to right right now and I am just being a perfectionist.

Also in the middle I upgraded my master cylender from 13/16 to 15/16 and it had no effect other than peddle travel.