Brake Bias

[Grrrrrrr!]

Okay, so after replacing the warped rotors I now have good functional brakes.. but I'm not entirely happy with them. The braking performance of the car is not as good as I would expect. It seems the fronts always lock, I cant remember ever locking a rear brake, so I'm wondering about moving the brake bias rearward, and what is the best way to go about it.
Car has almost 60R:40F static weight distribution, so the rear brakes should be able to do a decent amount of work.
As I see it the options are:

1) Change the rear brake pads to something with a higher friction rating - Will probably do this anyway since its only got decent road pads in the back.
2) Gut/bypass the proportioning valve so the rears get full line pressure all the time. A bit dubious about this since there must be a reason for the valve, wouldn't want to shift it too far back and end up always locking the rears.
3) Change the rear brake setup, thicker/bigger rotors and more piston area to increase braking force = $$$$ (parts + cert).
4) Dual master cylinder pedal box = $$$$ (Parts + cert)

Are there any other options? Preferably cheap, effective and not requiring a cert...

[jacobrjett]

i too am interested in this, though sometimes i wonder if i dont notice the rear brakes locking much because i try to brake in only a straight line when braking hard and all the weight is on the front

[Grrrrrrr!]

Found one possible solution Will hunt one down and see what the price is going to be (looks reasonable @US$45). Might pair it up with a hydraulic handbrake setup.

[jacobrjett]

am i reading this right? it just backs off an adjustable amount of pressure to the rear brakes.

IE a line lock...

[Grrrrrrr!]

No, its an adjustable version of what is there already. It lets you adjust the pressure to the back brakes as a ratio of input pressure. A line lock shuts off the rear brakes completely.

Have just run some numbers on the brake forces on my car. With full line pressure to the back brakes it looks like about 42%R:58%F braking torque balance. Doesn't seem excessively rearward biased with a 60% rear static weight bias.

And running a guesstimated COG height of 500mm, that would be perfectly balanced for 1.2g of braking. Hmmmm..

[KinLoud]

Use same brake pads front and rear....

Cars are set up so front will always lock before rear
It's a bit stressful/entertains if rears lock first.... play with your handbrake next time you're approaching a slow corner to remind yourself why!

Also - delay going for brake bias changes until all 4 tyres are the same
Grippy rear tyres on the rear and normal tyres on the front will also give you early front wheel lock

[Grrrrrrr!]

Cheers Ken.

Has Near New Toyo T1Rs on the rear, and unknown age GSD3s on the front at the moment, so probably a bit imbalanced. In future chances are it will get more rear tyre and unfortunately more rearward weight bias too.
It was hilarious/scary bad when I threw the 195/50-15 China specials on the front the other day for testing.

Pads are currently the same all round, since I swapped the race pads out of the front.

[Lloyd]

Bear in mind that your static weight distribution is just that, static. When you're hard on the brakes you're going to have the majority of the weight on the front wheels and very little over the back (straight line anyway)

And those Wilwood bias valves work great. Usually easiest to mount them by the handbrake if you want the adjustment while you're driving.

[Adoom]

And those Wilwood bias valves work great. Usually easiest to mount them by the handbrake if you want the adjustment while you're driving.

I use one. With the combination of parts I had, there was too much pressure to the rear brakes and they were locking up well early.
I set the bias by driving about 80kph on a rural straight road and jumping hard on the brakes, I kept winding the rear pressure down until the rear no longer locked up.

[DexGT]

Static isn't any use to you for brake set up , I have done all the calcs for my starlet which has 62% front weight static and on semi slicks ends up at around 92% dynamic.
I actually weighed the car level and with the back a metre in the air to work out the height of the COG and sure enough it ended up at almost the top of the tyres .
I think I used this site http://www.longacreracing.com/articles/art.asp?ARTID=22

[Grrrrrrr!]

Made a slight miscalc , with a 500mm COG height the brake balance isnt perfect for 1.2g... more like 0.75G. At 1g the weight balance is still 37% rear. 1.2 G is 33% rear. So it wont need a lot of forward bias, unless the real COG height is in SUV/people mover territory.

[KinLoud]

OK
Done a bit more thinking...

Corner weights - check for a big split (say more than 10%)
Maybe mismatched/sagging springs or bent swaybar is causing a weight bias, letting one front to lockup early.

Brake bias valve - will be setup for diagonal brake circuit? i.e. left front, right rear
So aftermarket bias valve won't work for you unless you redo brakes to a front/rear system instead of diagonal system
Or use 2 bias valves and try to set them up to be equal bias reduction

Rekit/overhaul/replace brake master, brake calipers, brake bias valve to make sure everything is 100%

Bigger/wider tyres won't give more brake to that end of the car

MR2 already has way more brake calliper piston area and brake disk diameter/leverage to make allowance for weight bias (dynamic)
That's why the rear disks are ventilated (SW20, ZZW30)

Still not convinced you need aftermarket bias valve

[Grrrrrrr!]

Brakes are front/rear split already. Waiting for a response from wilwood on some details of the bias valve output and range. Will definately get one as car will need different setup for road tyres and semislicks.

Took car for a decent fang yesterday and seems a bit better after adjusting tyre pressures and getting out on some back roads, maybe the rear rotors were a bit glazed up or the roads I was testing the brakes on the other day are just slimey.

[DexGT]

What sort of car is it and are brakes standard or has it been modified ? cause I can't think of anything vaguely modern that isn't split diagonal .
That's why I changed my race car to a pedal box with dual masters and front/rear split .

[Grrrrrrr!]

Bog standard sw20 brakes. Non-ABS cars (early ones at least) just have a single hardline to the rear firewall, and it tees off there to go to the back calipers.

[DRFTIN]

if i read this right the bias valve wont do what you want as you need more pressure to the rear, not less

[Grrrrrrr!]

Removing the factory fixed proportioning valve and fitting an adjustable one will give me the ability to change it.
By raising the knee point pressure up a couple of hundred psi it will get more pressure to the back under heavy braking and make no difference under light braking when line pressure is below the knee pressure.
Just waiting to hear what the rate after the knee is on the Wilwood valve, the Tilton is 3:1.