Setting brake bias for dummies

[Fman]

Hello all,

Looking for some guidance to get some clarification on how I should be setting my front/rear brake bias for general street driving, I have mustang brakes. I have always aimed for a target of 60% front and 40% rear on street cars. I have read the Mk4 is rear weight biased so trying to clarify what exactly this means. Do you not want the front brakes locking up before the rear in a hard stop? what is the best front and rear % bias you are trying to achieve on the Mk4? I currently have it set for the front wheels to lock up before the rear, probably close to a 60/40 setup but definitely hard to even totally confirm this ratio all I know if I slam the brakes on the fronts will lock up first. I know there is really no way to get it 100% exact but want to also make sure I am in the general ballpark to have safe stopping in a hard stop situation.

Any help on this much appreciated... thanks:)

[mike223]

You never want the rear locking before the front in a panic stop.

It's pretty ticky in these short wheelbase cars because the weight transfers either direction very quickly.


Work up cautiously - in a safe place with plenty of room to spin.


For final development I recommend the autocross test which is:

Blast up to about 60mph as fast as you can and pile off the throttle and onto the brakes hard (in a straight line, in a safe place - preferably an autocross course). The car is going to tend to want to spin anyway, just from second gear engine braking + weight transfer (even with 0% bias to the rear brakes).

If you detect any early rear lockup during this sort of maneuver - you need less bias to the rear.


IMHO, it's critical to test + set up in a manner that accounts for worst case engine braking + panic braking + weight transfer.


By the time you get there you'll probably be using a lot less rear bias than you might currently think you need - many variables involved - especially caliper piston sizes, effective friction of the pads (which is subject to change with time + conditions), effective friction from the road surface + tires (always subject to temperature + other variables).


Another huge variable is how the operator is putting pressure on the brake pedal - how sensitive are you to the weight transferring to the front axle, and are you modulating brake pressure appropriately? Will you do that exactly the same way in a panic stop?

[swwebb]

See this recent thread on the other forum.

https://www.ffcars.com/threads/adjusting-the-willwood-balance-bar.281269/

[Fman]

You never want the rear locking before the front in a panic stop.

It's pretty ticky in these short wheelbase cars because the weight transfers either direction very quickly.


Work up cautiously - in a safe place with plenty of room to spin.


For final development I recommend the autocross test which is:

Blast up to about 60mph as fast as you can and pile off the throttle and onto the brakes hard (in a straight line, in a safe place - preferably an autocross course). The car is going to tend to want to spin anyway, just from second gear engine braking + weight transfer (even with 0% bias to the rear brakes).

If you detect any early rear lockup during this sort of maneuver - you need less bias to the rear.


IMHO, it's critical to test + set up in a manner that accounts for worst case engine braking + panic braking + weight transfer.


By the time you get there you'll probably be using a lot less rear bias than you might currently think you need - many variables involved - especially caliper piston sizes, effective friction of the pads (which is subject to change with time + conditions), effective friction from the road surface + tires (always subject to temperature + other variables).


Another huge variable is how the operator is putting pressure on the brake pedal - how sensitive are you to the weight transferring to the front axle, and are you modulating brake pressure appropriately? Will you do that exactly the same way in a panic stop?

Thank you, I appreciate all this info. I have only done a couple hard lock ups at 40 mph. I feel like I have a good baseline but definitely need to continue tweaking as I begin to drive the car.

[CraigS]

Thank you, I appreciate all this info. I have only done a couple hard lock ups at 40 mph. I feel like I have a good baseline but definitely need to continue tweaking as I begin to drive the car.
That is exactly how I do it, at about 40. You absolutely have to adjust until you get rear wheel lockup and then back off a little. You do NOT want to lock the rears at much over 40 as it will get to pucker factor quickly. You want to do your testing under ideal road conditions. The rest will take care of itself. Here is why. Say you get the perfect 'fronts barely lock first' adjustment in good conditions. When conditions are less than ideal you will have less powerful deceleration, so you will have less weight transfer to the front wheels, so the fronts will tend to lock earlier. Exactly what you want.

[Avalanche325]

You do NOT want to lock the rears at much over 40 as it will get to pucker factor quickly. ABSOLUTLEY!!! 110% agree.
I see way too many people that set up for locking up all four at the same time or for the fronts to lock before the rear, but the rears still locking up. In an ideal world the rears never ever lock up. If that sounds crazy, here is why.

Panic stopping, the fronts are locked, the rears are still rolling. You have basically lost front grip and front directional stability, but the rolling rears are acting like rudders and at least the car keeps going in a straight line. The second that the rears lock, you now have lost front and rear grip. The rudders are gone. You now have no directional stability. The car can go wherever the crown in the road takes it, which is usually into the curb or ditch. If your front wheels are not straight, that can induce a spin.

If the rears lock first, imagine getting on a surfboard backwards.

[mike223]

Here is why. Say you get the perfect 'fronts barely lock first' adjustment in good conditions. When conditions are less than ideal you will have less powerful deceleration, so you will have less weight transfer to the front wheels, so the fronts will tend to lock earlier. Exactly what you want.

Absolutely true + to the point.