Brake bias on downhill

[peanutaxis]

At the Monaco Grand Prix this year Damon Hill commented that when coming out of the tunnel and braking down the hill towards the chicane the brake bias is often wound forward because ~"the engine behind [the driver], being a great weight, wants to go straight on and over the driver".

Well, first of all this demonstrates the terrible un-understanding that many/most/all drivers have of physics. One can easily imagine that many a budding formula racer is focused on anything but his school work! (Hill would be perfectly correct if the engine were travelling in a horizontal direction down(!?!) the hill after the tunnel. Unfortunately - or should I say fortunately the engine is travelling in the same direction as the car, and as the driver, which ensures that the engine is certainly NOT trying to vault the driver, but is, in fact, 'pushing' against the drivers back in the very same direction that the car is travelling. And it does this whether the car is going uphill, downhill, or on the flat. Or, to put it another way, if the engine is trying to vault over the driver, it is equally trying to vault over the driver whether he is on the flat or on a hill.)

But it got me thinking about brake bias tweaks going downhill and uphill and, despite having a degree in the subject, I really can't understand why twiddling the brake bias makes any difference. Assuming that the hill (not Hill) - upward or downward - is relatively straight i.e. not undulating, then the car is going in a perpendicular direction to the downforce of the wings. The centre of mass of the car hovers somewhere just behind the driver (no doubt they ensure that it shares a centre of mass with the fuel [tank] so that as the tank empties/fills the car's CoM doesn't change). If you look at the cars these days, they sit so low that I would not be surprised if the CoM was at around about the height of the centre of the wheels, which would make any brake bias adjustment completely pointless.

But even if the centre of mass is slightly higher, the grade of the road is never going to be more than about 10%. A 25% hill is a very steep hill, and the steepest street in the world, here in my country, is only 35% (http://en.wikipedia.org/wiki/Baldwin_Street,_Dunedin). So the centre of mass is only going to move forward by a centimetre or so.

Does anyone have any (physically justified) ideas as to why they tinker with their brake bias? I'd be interested because I'm at a loss!


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[gato azul]

this may help

Forward Vehicle Dynamics

in simple terms, a proportion of the force due to gravity (F=m*a), will accelerate the car downhill, and at the same time
will have an effect on the vertical load of the tires.

[Jersey Tom]

I'd bias the brakes forward just by virtue of the fact it's a high speed braking zone. Lot of forward load transfer.

[peanutaxis]

this may help

Forward Vehicle Dynamics

in simple terms, a proportion of the force due to gravity (F=m*a), will accelerate the car downhill, and at the same time
will have an effect on the vertical load of the tires.

Hmmmn. Part of the force due to gravity will work to accelerate the car, which will mean that the load through the tires will lessen. But the braking forces from the brakes will work parallel to the road surface, so I still don't see any change in balance from front to rear.

[olefud]

Parked on a hill nose downward, there will be more weight on the front tires than when parked on the level. The gravity force vector through the CoG will intersect the ground closer to the front wheels. Maybe that's what the driver feel through the seat of his pants.

[riff_raff]

With a chassis that is aerodynamically very pitch sensitive like an F1 car, changes in the F/R aero CoP under braking are probably of more concern than F/R weight transfer on a slight downhill slope.

[peanutaxis]

Parked on a hill nose downward, there will be more weight on the front tires than when parked on the level. The gravity force vector through the CoG will intersect the ground closer to the front wheels. Maybe that's what the driver feel through the seat of his pants.

Yep, certainly. But I'd be surprised if it were more than few centimeters, and considering the wheelbases are over 3m, it's be but a percent of two.

[you_nes]

Don't have a reliable data source but I kind of agree with riff_raff.
I think you will find that any car travelling down hill at that speed will "go light" this will raise ride hight which, since the diffuser is the device most sensitive to ride hight, will probably push CoP forward.

Also, I don't think its a perfectly straight breaking zone out of the tunnel so they might just be doing it for stability reasons?

[Nando]

When you are going straight, let´s take Austin GP track turn 1.

As you go up, there´s added force on the car because the road wants the car to go up while the car would rather just go horizontal.
So when applying the brakes with the same BB as on a straight way it means you will load the front tires more which means they most definitely will lock at some point during the braking phase, also depends on the type of car.

So what you want to do is to move brake bias backwards when braking in uphills and vice versa when going downhill, because the front end gets light.

[peanutaxis]

As you go up, there´s added force on the car because the road wants the car to go up while the car would rather just go horizontal.

Absolute nonsense. Draw yourself some force diagrams and you'll see.

[GSpeedR]

Nando may be referring to the rearward pitch acceleration as the car engages the hill (going up in his example) causing front loads to momentarily increase, however this a transient event and reverses as pitch acceleration reverses. In the steady-state the opposite will be true which can be easily seen in a FBD.

[rifrafs2kees]

As you go up, there´s added force on the car because the road wants the car to go up while the car would rather just go horizontal.

Absolute nonsense. Draw yourself some force diagrams and you'll see.

We come here to discuss and improve our knowledge of vehicle dynamics and this is the tone in which you respond to a post? While braking downhill, you have your obvious load transfer to the front of the vehicle and this gives the front tires more traction. Secondly more of the vehicles static weight, however slight you may consider it, is supported by the front tires in a downhill situation. For these two reasons, the front tires are in a position to do more to slow the vehicle down. Remember, static friction(and not kinetic friction because the vehicle isn't skidding) is directly proportional to the normal force exerted by the road on the tires.

Also, the load transfer has nothing to do with the center of the wheel. It has to do with the height of the center of mass from the point of contact of the load...which is where the tires and the road meet.
Lets assume you move the brake bias in the other direction...with the back end getting light under braking, you're more likely to lock them up and spin. Unless for whatever reason, you're trying to induce oversteer and are confident you can handle it.

I would also assume that Hill has first hand experience in this...he may not know just how to explain it...but who chooses the right words all the time

[Lycoming]

As I understand it, the OP is saying that load transfer occurs due to effects of gravity on uphill/downhill braking, but because of the long wheelbase and low CG height, this is negligible and should not require an adjustment in brake bias.

[rifrafs2kees]

Even if the wheelbase is long and the CG, very low, I think we need to remember these guys are braking close to or on the limit. I suppose they trail brake into that corner also. So whatever adjustments a driver can make to gain an advantage, no matter how minute such gains might be, would be important. Last thing you want is the back stepping out on you into that downhill turn.

[Jersey Tom]

Even if the wheelbase is long and the CG, very low, I think we need to remember these guys are braking close to or on the limit. I suppose they trail brake into that corner also. So whatever adjustments a driver can make to gain an advantage, no matter how minute such gains might be, would be important. Last thing you want is the back stepping out on you into that downhill turn.

You could make the argument (a bit hand waving) that with a downward incline you move your nose weight percentage forward and you'd maybe have to pull brake bias rear to free the car up to make up for that.

Still, the fact that it's a high speed braking zone I think blows away anything else. High speed = high downforce = high braking G's = high forward load transfer.

Consider 4-5G of braking and the effect that has... how much load transfer.. versus some fraction of a percentage of nose weight.