Brake bias question, difference between 2wd and 4wd

[351ciofgrunt]

Hey I was just thinking, with regards to my GQ patrol, when in 2wd the braking effect to the front wheels is more than to the rear, as per all cars. But when in 4wd because of the transfer case meaning the front driveshaft turns at the same speed as the rear driveshaft does that in turn mean that the braking effect is 50/50 between front and rear wheels?

[DamTriton]

Weight shift from deceleration will alter the bias to the front.

[351ciofgrunt]

But the driveshafts are turning at the same speed? The weight sensing rear brake bias has nothing to do with it once you're in 4wd?

What I'm trying to say is that if you had a wilwood style brake proportioning valve on the rear circuit, this would affect the braking dynamics dramatically in 2wd but would it have any difference in 4wd? Because now the front and rear wheels are forced to turn at the same speed by the transfer case

[DamTriton]

But the driveshafts are turning at the same speed? The weight sensing rear brake bias has nothing to do with it once you're in 4wd?

The front will still dig in with the weight shift and the rerar will still lift off the road a bit. Both of these will affect the coefficient of friction between the tyres and the road (ignoring the "constants" of the locked driveshafts and braking). The front will still be doing 70-80% of the braking.

[351ciofgrunt]

So once you're in 4wd a brake proportioning valve will have no effect, correct?

[DamTriton]

BPV will have an effect if the vehicle has enough fore/aft pitching on deceleration. It is the amount of fore/aft pitching that may be reduced in 4WD, not the effect of the BV. BPV ratio is usually related to the absolute height of the rear.

Pretty arbitrary to be a major discussion point unless you are doing 150km/h around corners anyway. In most 4WD applications near enough is good enough

[oldmate]

So once you're in 4wd a brake proportioning valve will have no effect, correct?

No the proportioning valve will still work as it does. It works by connecting the axle to the chassis, and works on the height difference between the chassis and rear axle, and the tendancy for the suspension to sag under a load.


When you say the braking force on the front is less in 4wd than 2wd, that is not actually true. The braking force created by the caliper is idential in both modes, however the resistance of the front wheel to not stop spinning against the force of the caliper is greater due to the link to the rear wheels.

[351ciofgrunt]

So if you replaced the factory rear proportioning valve with an adjustable willwood type proportioning valve, would you notice any difference when adjusting the settings in high speed applications, seen as the front and rear diffs are forced to turn at the same speed in 4wd?

[DamTriton]

So if you replaced the factory rear proportioning valve with an adjustable willwood type proportioning valve, would you notice any difference when adjusting the settings in high speed applications, seen as the front and rear diffs are forced to turn at the same speed in 4wd?

On dirt more so than on bitumen, yes. This would be because of the lower friction that dirt offers.

I would suggest contacting some rallying forums that may have more experience in the field

[oldmate]

So if you replaced the factory rear proportioning valve with an adjustable willwood type proportioning valve, would you notice any difference when adjusting the settings in high speed applications, seen as the front and rear diffs are forced to turn at the same speed in 4wd?

There will be more braking effect on the rear brakes. Which due to the transfer link affects all 4 wheels. However will you 'notice' it?, I dunno. If you're going fast on dirt, and can lock all 4 wheels now, there would be no point to adding such a proportioning valve.

[-Scott-]

I've always considered the brake proporitioning valve is there to stop the rear wheels locking under heavy braking. In 4wd, with the front and rear diffs locked together, it would be impossible to lock a rear wheel without locking a front wheel, so from that perspective the BPV is probably reducing overall braking force (by not allowing the rear brakes to contribute as much retardation as they otherwise could).

I guess if you disabled it in 4wd, and hit the brakes hard in a bend you would run the risk of locking both inside wheels? Would this matter, if the outside wheels are doing most of the cornering work anyway?

[uninformed]

ummm.....isnt brake bias also effected by piston area??? like having more braking area piston/pad up front than rear....

[DamTriton]

ummm.....isnt brake bias also effected by piston area??? like having more braking area piston/pad up front than rear....

No. Bias would be reflected in brake line pressure difference front to rear. More piston area will give you more friction in your front brakes over your rear brakes FOR THE SAME LINE PRESSURE. As noted above the front brakes do 70-80% of the braking due to the effective weight transfer to the front when braking, makes sense to have bigger more effective brakes where they are needed.

[oldmate]

The proportioning valve is there to control the fluid pressure to the rear brakes, depending on the load on the rear axle. So the more weight you carry, the more the rear brakes work.

The fronts are designed to lock up before the rears. That's because if the rears lock then the vehicle will oversteer. But that is not a function of the proportioning valve so much as it is the master cylinder and caliper sizes in most cars.

[digsy]

351ciofgrunt - I think I know what you are saying and I think you are correct.

In my Jeep that I race at LCMP (when I can) it has nearly no brakes on the back - but in 4x4 (thru transfer case) if you can lock the front wheels it will lock the back wheels up too because they are mechanically interlocked.

So with this thinking and a locked or tight LSD rear diff I believe an aftermarket brake bias valve would be not very effective.
However if you are ever in the situation where you choose to run 2wd at events for whatever reason, you would definately see a gain in braking performance once "tuned" to your driving.

What everyone else is saying IS indeed correct. The bias valve still operates etc - but mechanically interlocked...
The only advantage that may be worth considering is that you can set the manual valve to 50/50 whilst in 4x4 and then all the brakes are working at their maximum
ie even if a rear wheel is off the ground - thru the diff/tailshaft/etc - it is still working with the front wheels to brake the vehicle - thus spreading the heat between the 4 rotors? is this a sound theory?

[351ciofgrunt]

Cheers digsy

thats pretty much what I was trying to get at