Brake bias on downhill

[Just_a_fan]

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.

As the car brakes, the load transfer not only pushes the front tyres in to the track surface more but it will also make the front end settle closer to the ground. The rear will rise - even F1 cars with their stiff suspension have to deal with this effect. The forward pitching will lead to the front wing being closer to the track surface. At this point it will generate more downforce and the rear, conversely, will generate less downforce - i.e. the CoP migrates forwards under braking. So in order to prevent the rear wheels from locking you move the brake balance forward to make use of the added downforce-assisted grip at the front and lessen the chances of locked rears. Going downhill, the car will already have a slightly more forward weight balance which will add to the overall effect. Sure, it might be a small difference but these cars live and die on fractions of a percent changes...

I think Hill's comments were just typical media-style hyperbole intended to get the idea across to a non-technical audience.

[Jersey Tom]

I think Hill's comments were just typical media-style hyperbole intended to get the idea across to a non-technical audience.

This is what it really boils down to. Which is almost always the case with announcers...

[Nando]

If anyone wants a more extreme case look at your normal F1 race.
Some corners are so different that you have to run different BB in order to optimize the braking.
It´s not always for changing track conditions or for the KERS harvesting.

When at the absolute limit on braking, a couple of percent plus or minus can decide if you lock up or not.
Small nuances in the track elevation makes a difference in such stiff cars.

[olefud]

Hill's comment is a bit fuzzy in my mnd. But he seems to be saying the car lacks stability, perhaps due to perceived rear unweighting. If there's such instability, it make sense to have front brake bias. Front brake locking leads to understeer, which is generally better than rear-lock oversteer. And with even a small weight transfer forward, it would be the indicated move.

[peanutaxis]

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.

The load transfer downhill is exactly the same as when on the flat or braking uphill because the CoG is still moving parallel to the road (not horizontally as you suggested earlier).

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.

An interesting result of this is that because the normal force (friction force) reduces on both down AND up hill, there is LESS braking ability going uphill as well (although gravity still helps to slow you down).

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.

MMnnn. Yeap, you're right here. It's only on the flat that if the CoM was at the height of the axle tat you would get zero weight transfer.

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

Sorry, who is Hill? EDIT: Oh right, Damon Hill I mentioned in the OP! Er......well he's talking nonsense about the engine trying to vault over him - it's not doing this any more than it is on the flat - again because the engine is always traveling parallel to the road, or rather, the rest of the car. But the CoM being above the road, yes, you're right.

[peanutaxis]

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.

Exactly, but as rifraf has pointed out the CoM height is more important than I first though because it matters how high it is above the road, not the axles, as i first thought.

[peanutaxis]

Hill's comment is a bit fuzzy in my mnd. But he seems to be saying the car lacks stability, perhaps due to perceived rear unweighting. If there's such instability, it make sense to have front brake bias. Front brake locking leads to understeer, which is generally better than rear-lock oversteer. And with even a small weight transfer forward, it would be the indicated move.

I think that Hill believes his mistaken explanation.

But I think the main reason for changing the brake bias is probably merely that the consequences of losing the rear on downhill are more serious than losing them on uphill because the action of gravity on the uphill will keep slowing you down whereas gravity will keep accelerating you on downhill!

[riff_raff]

Exactly, but as rifraf has pointed out the CoM height is more important than I first though because it matters how high it is above the road, not the axles, as i first thought.

Another factor to consider, even though it is not as significant, is the effect of suspension geometry squat/dive during braking and how it changes F/R load distribution.

[Jersey Tom]

Another factor to consider, even though it is not as significant, is the effect of suspension geometry squat/dive during braking and how it changes F/R load distribution.

Hmm. Not sure I'd go so far to consider that. Only significant effect that would have is through changing body attitude and in doing so, aerodynamic force. But if we were to do that we might as well talk about springs and such too.

[peanutaxis]

Another factor to consider, even though it is not as significant, is the effect of suspension geometry squat/dive during braking and how it changes F/R load distribution.

Hmm. Not sure I'd go so far to consider that. Only significant effect that would have is through changing body attitude and in doing so, aerodynamic force. But if we were to do that we might as well talk about springs and such too.

I'd agree. They run third dampers for ride height (and I wouldn't be surprised if they run third springs) so when they jam on the brakes ther'll be very little dip due to the third damper (or it'll be smooth); and of course the springs are very stiff. The tires aren't though - the tires will not compress much at high speed due to centripetal forces, but will at low speed. And then there are the forces through the wings - I think I read somewhere here that the front and rear wings have different Force vs. Speed curves. But I doubt they go into all that much detail. I mean, I'm sure that they design the suspension with braking in mind but I wouldn't be surprised if they DON'T go so far as considering separately the up and downhill effect of braking.

[cheapracer]

The load transfer downhill is exactly the same as when on the flat or braking uphill .

It is most certainly not.

I think that Hill believes his mistaken explanation.

Just to get some balance here since you're taking on a World Champion, could you please offer up your driving experience/results?

I actually side with Hill, not just because he's a World Champion with a long and successful driving history but also because of my (more humble) seat time and understanding of rudimentry physics.

The key element is gravity and I'm surprised also there's not a few here with dirt bike riding experience mentioning you might be less than correct.

[munks]

... the tires will not compress much at high speed due to centripetal forces, but will at low speed.

Maybe I'm misunderstanding you, but the rotational speed of the tires won't affect their vertical compliance directly. Yes the radius is slightly larger at high speed, meaning the shape of the tire would change, but whether that would increase or decrease the compression for a given load depends on the construction, pressure, etc.

In other words, it seems more likely to me that the tread area is more rounded at high speed (looking at it from a cross-sectional viewpoint) which would usually increase compliance (i.e. decrease vertical spring rate).

[hollus]

From the OP:

...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...

I just went to see the onboard video in the official formula1 site. It's Webber's lap.
He starts braking in a relatively modest downhill slope. After he starts braking, the slope increases significantly. From then on, the slope of the tarmac stays more or less constant, but he starts turning while still braking, and continues to brake well into the first corner of the chicane, by which point he is moving at an angle to the actual slope of the tarmac, which means the car sees a smaller slope (as well as lateral loads).
Why are we discussing this in terms of a constant slope? The slope both increases and then decreases again within the braking area.

[Greg Locock]

did anybody try to draw a freebody diagram in side view to see if his comment makes sense? It would reduce the amount of handwaving by 98%

Of course, you might enjoy hand waving, it is a good way to dry nail polish, so i've heard.

[peanutaxis]

The load transfer downhill is exactly the same as when on the flat or braking uphill .

It is most certainly not.

For sure there is more weight on the front axle when going downhill because the CoM moves towards the front axle - this is in a static sense (i.e. not a transfer) - you can see this by drawing a static diagram.
But the transfer is the same: If you consider only the front axle and the CoM under braking, the CoM is trying to keep moving forwards. But it is above the front axle and so a torque will exist. But that torque does not depend at all on whether we are decelerating downhill, uphill, or on flat. All that matters is the orientation of the force vectors of the engine trying to keep moving forward, and the axle which is pushing back under braking. These force vectors are always parallel (and opposing), no matter the gradient.

I think that Hill believes his mistaken explanation.

Just to get some balance here since you're taking on a World Champion, could you please offer up your driving experience/results?

Being a world champion is completely irrelevant, or do you seriously believe that a person who understands physics better is more likely to be a world champion! =D>

Here's another example: Martin Brundle, as insanely good a commentator that he is, irks me a bit with at least two of his comments, which speak to a misunderstanding of technical aspects/physics. He often talks about cars "bouncing on the limiter". There is no bouncing here at all. Cars reach the rev limit and they sit there. To be fair, though, he may well know this but still use the phrase.
The other thing he sometimes says when he talks about g-forces etc. - that there is "a lot of energy going on". No one with physics knowledge would use this phrase. Energy can only be stored or transferred and, in fact, neither of these two 'properties' of energy are even taking place when g-forces are considered . Again, I don't blame him for this, but it shows that drivers don't need to - and very often don't - understand physics.

p.s. Here's another one that annoys me: viewtopic.php?f=6&t=13124&p=359127#p359127