Brake bias options
Discussion
I have a 1987 mk1 MR2, with about 44/55 front/rear weight bias.
The problem is that the standard brakes have too much front bias, so the front tyre(s) lock before the rears are fully working.
I have fitted the later rear disks, which have a slightly larger diameter, but it's still not ideal.
It was clearly a design decision by Toyota, but I want better.
Options :
Stiffen up the rear bias valve spring to raise the rear pressure knee point (may compromise wet braking?)
Remove rear bias valve altogether (wet braking again an issue)
Run softer front pads
Try and find some calipers with larger (front) or smaller (rear) pistons (no easy swaps that I have found yet).
Any other suggestions?
The car is driven daily, but I also do the odd hillclimb etc; currently at 160 hp, so outright brake performance is not the problem, just front/rear balance.
The problem is that the standard brakes have too much front bias, so the front tyre(s) lock before the rears are fully working.
I have fitted the later rear disks, which have a slightly larger diameter, but it's still not ideal.
It was clearly a design decision by Toyota, but I want better.
Options :
Stiffen up the rear bias valve spring to raise the rear pressure knee point (may compromise wet braking?)
Remove rear bias valve altogether (wet braking again an issue)
Run softer front pads
Try and find some calipers with larger (front) or smaller (rear) pistons (no easy swaps that I have found yet).
Any other suggestions?
The car is driven daily, but I also do the odd hillclimb etc; currently at 160 hp, so outright brake performance is not the problem, just front/rear balance.
My research says the supercharged model (heavier at the rear) had more rear bias.
I believe that the only difference is the spring in the bias valve - if that is the case I can probably order one new from Toyota
.
It will cost peanuts, but take weeks to get here
. The local dealerships also do everything by VIN, which makes ordering parts for a different model a pain.
I think fitting a different spring is a simple thing to try, but I don't know how much difference it will make.
ps Everything I have read about braking systems says "don't put a bias valve on the front brakes", and I don't like taking chances with brakes.
I believe that the only difference is the spring in the bias valve - if that is the case I can probably order one new from Toyota
.It will cost peanuts, but take weeks to get here
. The local dealerships also do everything by VIN, which makes ordering parts for a different model a pain.I think fitting a different spring is a simple thing to try, but I don't know how much difference it will make.
ps Everything I have read about braking systems says "don't put a bias valve on the front brakes", and I don't like taking chances with brakes.
AW111 said:
My research says the supercharged model (heavier at the rear) had more rear bias.
I believe that the only difference is the spring in the bias valve - if that is the case I can probably order one new from Toyota.
It will cost peanuts, but take weeks to get here. The local dealerships also do everything by VIN, which makes ordering parts for a different model a pain.
I think fitting a different spring is a simple thing to try, but I don't know how much difference it will make.
ps Everything I have read about braking systems says "don't put a bias valve on the front brakes", and I don't like taking chances with brakes.
Does your car not have a chassis number ? That would be rather strange.I believe that the only difference is the spring in the bias valve - if that is the case I can probably order one new from Toyota
.It will cost peanuts, but take weeks to get here
. The local dealerships also do everything by VIN, which makes ordering parts for a different model a pain.I think fitting a different spring is a simple thing to try, but I don't know how much difference it will make.
ps Everything I have read about braking systems says "don't put a bias valve on the front brakes", and I don't like taking chances with brakes.
Have you removed the spring to see what it looks like ?
I agree, fitting a bias valve to the front would be very odd indeed, but then the braking problem also sounds odd.
depends how much it really is biased towards the front, or if there is another problem...or it is working exactly as intended for safety reasons.
The chassis number is an issue if I want to buy a part for another model, eg. a supercharged bias valve, because they want to know the VIN for that car, not mine. Even when I quote the correct Toyota part number, it just seems to confuse them 
As far as I can tell, Toyota were playing it very safe : it was their first mid-engined car, pre-ABS, targeted largely at the US. I can see why they made sure that locking the rear brakes was well-nigh impossible.
In fact I suspect that even with bald rears and new fronts, the fronts will still lock first.
It's not a major issue, but I am exploring ways to improve it a little.
As far as I can tell, Toyota were playing it very safe : it was their first mid-engined car, pre-ABS, targeted largely at the US. I can see why they made sure that locking the rear brakes was well-nigh impossible.
In fact I suspect that even with bald rears and new fronts, the fronts will still lock first.
It's not a major issue, but I am exploring ways to improve it a little.
I have the same car with the same problem.Playing with the pad material made no difference to a design fault, or an american market safety feature. Celica front brakes will fit but means running 17" wheels.I seem to remember Lancia Beta`s had the same problem which they "fixed" by servo-ing the rears not the fronts, worth exploring? Anybody got a better idea?
old traveller said:
I have the same car with the same problem.Playing with the pad material made no difference to a design fault, or an american market safety feature. Celica front brakes will fit but means running 17" wheels.I seem to remember Lancia Beta`s had the same problem which they "fixed" by servo-ing the rears not the fronts, worth exploring? Anybody got a better idea?
The rear calipers from a late mk2 (sw20) can be made to fit, but if you don't change the fronts, the balance is now too far to the rear, although that can be sorted with an adjustable bias valve.The best I can come up with as bolt-on solution is
Mk1a front disks (smallest rotor)
Mk1b rear disks
Mk1b supercharged bias valve/spring (as the s/c engine & gearbox were heavier, it had a bit more rear bias)
I am still looking for a better solution.
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