Brake bias under trail braking

[raymondu999]

Given that trail braking immediately helps to move weight over the front axle - and thus, off the rear axle - obviously the front will have more bite to the tarmac. However under trail braking, you're also asking for more of the front tyres (compared to the rears) are you not? The fronts have to do braking + turning, while the rears are just braking, and holding on in terms of keeping the car laterally stable.

What kind of brake bias should you use in this scenario? Forwards? Rearwards? Neutral? I suspect a neutral would be the best, but I'm not entirely certain. Would a rearward brake bias really be beneficial? You'd be asking the rears (which, remember, have the load taken off them) to brake more, despite giving them less vertical loading.

Obviously the picture gets more complicated when a driver is in the picture, as the driver would probably have a certain handling preference under braking, but theoretically speaking, if we had an infinitely capable driver comfortable with whatever the balance under braking - what is the quickest?

Anyone?

[Greg Locock]

Think about the friction circle for each tire. Suppose you were applying zero steer, then obviously you want the brake balance to suit those friction circles in the longitudinal direction . Now start steering, generating a side force (initially) at the front wheels only. This will use up some of their available braking friction to apply sidethrust, so ideally you'd move the brake balance rearward. You'll also reduce the weight transfer forwards, so that makes the front circle even smaller.

Given that all this is transient i doubt it is a big deal.

[raymondu999]

Does the forward weight transfer really reduce? I've read before (somewhere, can't remember - I think it was Ciro that posted it somewhere) but the differences in weight transfer, along with the changes done in brake bias is insignificant, and the bigger difference comes from how hard you brake.

[Tommy Cookers]

Does the forward weight transfer really reduce? I've read before (somewhere, can't remember - I think it was Ciro that posted it somewhere) but the differences in weight transfer, along with the changes done in brake bias is insignificant, and the bigger difference comes from how hard you brake.

agreed !
anyway trail braking is much more extensive than just at that tiny moment when the front wheels are developing a cornering force but the back wheels aren't (yet)
trail braking is for any time the line is tightening

BTW do drivers manage brake bias corner-by corner ?? (as they manage the diff setting)
(my Lotus 18 mentality is no help here)

[raymondu999]

I believe they do. I've seen quite a few drivers changing the lever corner by corner. Schumacher does this a lot.

I've even seen Rosberg either this year or last year changing the bias mid corner in Turn 12 Barcelona - though I'm not sure if he was still braking .

[Jersey Tom]

The fronts have to do braking + turning, while the rears are just braking, and holding on in terms of keeping the car laterally stable.

Not true. Both the fronts and rears are braking and "turning" (generating lateral force). F = ma holds just as true at the rear axle as it does the front.

[Tim.Wright]

I agree with JT, the front and rear axle do the same job from a cornering force point of view. However the delay in force between the front and the rear axles is what does the "turning".

In turn entry, the front axle has more force which turns the car into the corner
In turn exit, the rear axle has more force which turns the car out of the corner

In anycase, the "optimum" brake balance for a cornering condition often has as much to do with stability as it does for overall braking performance.

Tim

[Tommy Cookers]

do drivers change brake bias within the braking-trail braking-cornering sequence ?
(this would be needed to fit the subject of this thread ?)

there's a lot going on, diff settings/behaviour, braking (incl KERS), then trail braking (left foot) and (some?) throttle pickup
all contributing to car behaviour/balance/lap time

[Sevach]

Not true. Both the fronts and rears are braking and "turning" (generating lateral force). F = ma holds just as true at the rear axle as it does the front.

Yes, while the rears don't turn like the fronts they are still subject to the lateral stress of the corner whether you are on the brakes or not.

The main objective of trail braking is keeping the fronts loaded for improved grip.
Obviously, you don't wanna overstress them (the fronts), because that would go against your objective of a better front end grip.
This means translates that the usual is moving the brake bias a bit backwards in relation to a straight line braking minded setting.

It might still have more than 50% on the fronts, just less than you would use while straight line braking.

[Tommy Cookers]

I think that the main objective of trail braking is to slow the car as the corner tightens whilst not being slower any earlier than necessary in the less tight part of the corner (ie to minimise lap time on a track where trail braking necessary to fill the 'friction circle')
(if you are approaching a 30 mph apex via a 70 mph bend you do not slow to 30 for the 70 bend, you trail brake it so that you are only down to 30 at the apex)

if the track allows a line entirely of straights and constant radius corners the benefits of trail braking would be less, and not obvious (on such a track you can fill the 'friction circle' without trail braking)

are you saying that drivers do adjust the brake bias after straight-line braking, ie as they commence trail braking ??

(throttle-off (in cornering) is a weaker equivalent of trail braking
any torque about any axle(s) generates a related reaction in wheel loads ie contact forces
trail braking at normal brake bias (like throtle-off) changes handling balance (reduces rear grip, increases front)
ideal trail braking bias would be less front, more rear ??
nobody bothered about trail braking when race cars had a basic oversteer ......... just thinking out loud)

[olefud]

I think that the main objective of trail braking is to slow the car as the corner tightens whilst not being slower any earlier than necessary in the less tight part of the corner (ie to minimise lap time on a track where trail braking necessary to fill the 'friction circle')

if the track allows a line entirely of straights and constant radius corners the benefits of trail braking would be less, and not obvious (on such a track you can fill the 'friction circle' without trail braking)

are you saying that drivers adjust the brake bias after straight-line braking, ie as they commence trail braking ??

In practice brake bias is adjusted for straight line braking with the front locking a bit before the rear. This provides stability in that a bit of front lock results in stable understeer while the driver backs off. Rear bias can be unstable as the greater braking force is forward of the CoG.

Trailing brake into the corner is more a brake modulation to utilize unused tire traction oval cornering thrust during turn in. It allows for later braking since the car is still braking while turning. Maximum braking is usually not workable. Mark Donahue’s book Unfair Advantage has a pretty good discussion of this.

In short, adjust bias for max straight-line braking and leave the trail brake modulation to the driver use of the brake pedal (though F-1 grade drivers may adjust bias for different portions of the track or to diminish handling problems).

[Sevach]

I think that the main objective of trail braking is to slow the car as the corner tightens whilst not being slower any earlier than necessary in the less tight part of the corner (ie to minimise lap time on a track where trail braking necessary to fill the 'friction circle')
(if you are approaching a 30 mph apex via a 70 mph bend you do not slow to 30 for the 70 bend, you trail brake it so that you are only down to 30 at the apex)

if the track allows a line entirely of straights and constant radius corners the benefits of trail braking would be less, and not obvious (on such a track you can fill the 'friction circle' without trail braking)

are you saying that drivers do adjust the brake bias after straight-line braking, ie as they commence trail braking ??

(throttle-off (in cornering) is a weaker equivalent of trail braking
any torque about any axle(s) generates a related reaction in wheel loads ie contact forces
trail braking at normal brake bias (like throtle-off) changes handling balance (reduces rear grip, increases front)
ideal trail braking bias would be less front, more rear ??
nobody bothered about trail braking when race cars had a basic oversteer ......... just thinking out loud)

The car will naturally scrub some speed off as you enter the corner (engine braking, sliding of tires...) so you don't need to be sitting at 30 as you enter the corner in your proposed scenario.

I think that the amount of braking you can do while cornering is marginal, it doesn't shorten braking distances all that much, so to me trail braking is about keeping the weight on the front of the car and getting a better turn radius, imo it's a lot more about how you want to position the car in the corner than shortening braking distances.

I doubt any driver changes the brake settings while they are braking, mainly they look at how they intend to take the next corner and move the bias before hand.

[Belatti]

What have worked the best for me in Formula Renault type cars is to set brake balance in the optimum straight line braking. No matter if you trailbrake or not, the laptime and a low tyre degradation depends much on the fact of stopping the car right in the first part, where you have got some aero downforce. For that you need optimum straight line balance. For trailbraking, then you have to work in drivers braking profile using data acquisition.

[olefud]

Yet another POV that pretty much agrees with all the above. I’ve always used trail braking with late apex cornering. With constant radius cornering braking is essentially complete when entering the corner hopefully at maximum cornering speed. Early apex means you have overcooked the corner entry.

With late apex the entry speed is greater than constant radius with a lower speed at the tighter apex portion of the turn. This allows a hot entry with continued trail braking as allowed by the traction oval. With normal forward brake bias for straight line braking, the modulation of the brake is ideally a function of the traction oval of the outside front. If the traction oval is exceeded, the resulting push can be corrected by backing off the brake and/or line. Not good, but better than looping.