Brake bias on downhill

[peanutaxis]

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.

Mmmm, I think they will. The tire does not differentiate between being pushed upwards by a bump (or undulating road) and being pushed downwards by the wings - they are the same to the tire. And if the radius is larger at high speed as you say, (I agree) then the tires are necessarily less compliant under downforce from the wings, and therefore also to bumps.

[GSpeedR]

There will certainly be a difference in frequency content between aero downforce and a rough (undulating) road input. All tires exhibit at least some frequency dependence, some more than others, so I would claim that a tire can indeed differentiate between the two inputs. A tire will behave the same when experiencing the same load from different sources (spring, shock, etc.), maybe that was the point you were making.

Many tires will show competing effects with higher speeds. Internal pressure (from heat) will often increase with speed which tends to decrease compliance, while inertial effects ('centrifugal forces') of the tire will often increase compliance. The end result depends heavily on the tire.

[munks]

And if the radius is larger at high speed as you say, (I agree) then the tires are necessarily less compliant under downforce from the wings, and therefore also to bumps.

I guess I'm still confused by what you're saying here. Compliance (m/N) is simply the reciprocal of the spring rate (N/m), and is different than deflection (m) or radius (m). How does the rotational speed of the tires "necessarily" increase the spring rate (i.e. decrease compliance)?

[olefud]

And if the radius is larger at high speed as you say, (I agree) then the tires are necessarily less compliant under downforce from the wings, and therefore also to bumps.

I guess I'm still confused by what you're saying here. Compliance (m/N) is simply the reciprocal of the spring rate (N/m), and is different than deflection (m) or radius (m). How does the rotational speed of the tires "necessarily" increase the spring rate (i.e. decrease compliance)?

Maybe a better term would be preload as a result of the centrifugal force –though I doubt that tire obey Hooke’s law and could well have a higher spring rate under centrifugal preload.

[lolzi]

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

I have no physics education at all (except high school level), but I have seen the F1 cars literally bouncing on the pit speed limiter, especially if they are in a high gear. I've no idea why this is, but it seems that the engine power is "cut" for too long, which means they will accelerate, stop (decelerate from air/rolling resistance) and repeat doing that. It looks very uncomfortable for the drivers, their heads are thrown back and forth.
Also, isn't it true that there's more energy (transfer) going on when doing 4G cornering in a longer corner (say, Istanbul T8) then in a shorter corner?

[peanutaxis]

I guess I'm still confused by what you're saying here. Compliance (m/N) is simply the reciprocal of the spring rate (N/m), and is different than deflection (m) or radius (m). How does the rotational speed of the tires "necessarily" increase the spring rate (i.e. decrease compliance)?

Perhaps I have misunderstood your comment. As I understand it by 'compliance' you mean how easy it is to compress a tire (from a bump or downforce mainly). If you were to stand by a stationary, non-spinning tire and poke it on it''s tread with 10N, it would comply by let's say 10mm. Now imagine the tire is spinning and you could spin around with it so that the surface of the tire looks stationary to you (just like the tire tread looks stationary to the road). Now if you poke it with 10N it will comply less than 10mm because you have to overcome the centrifugal forces which are forcing all parts of the tire outwards (while you are trying to move it inwards). Hence, it is less compliant when it is rotating.

[peanutaxis]

I have no physics education at all (except high school level), but I have seen the F1 cars literally bouncing on the pit speed limiter, especially if they are in a high gear. I've no idea why this is, but it seems that the engine power is "cut" for too long, which means they will accelerate, stop (decelerate from air/rolling resistance) and repeat doing that. It looks very uncomfortable for the drivers, their heads are thrown back and forth.
Also, isn't it true that there's more energy (transfer) going on when doing 4G cornering in a longer corner (say, Istanbul T8) then in a shorter corner?

Perhaps it is bumps you are seeing moving their heads. Here's a video: http://www.youtube.com/watch?v=-9Q4yoD9 ... re=related
When on the pit limiter all seems very calm. No bouncing noise from the engine, that's for sure. Even his head seems no more bouncy than when he's on the track.

That there are forces doesn't mean that there is energy transfer. There are forces acting when you are just standing there, doing nothing. It's the same with a corner in a car. There are forces trying to push the car out (and lose traction) and there are friction forces from the tires, keeping it from losing traction, but there is no energy transfer going on. (Except a little in friction from the tires turning into heat). I guess it seems as though there should be energy transfer because the driver experiences such violent forces but, for instance, if you stood on a planet that was twice the mass of earth, you would feel a great force trying to push you into the ground. Just to stand up would be hard, but there is not energy transfer going on (if you are not moving, and just standing there). This same thing is going on when you stand on earth, but it feels like nothing is happening because our muscles are so used to doing it that it's easy (unlike four G's in a corner!)

[p

[munks]

Maybe a better term would be preload as a result of the centrifugal force –though I doubt that tire obey Hooke’s law and could well have a higher spring rate under centrifugal preload.

That occurred to me which is why I have continued asking ... whether there was a deeper down technical explanation or whether peanutaxis was using the simple but flawed explanation (see below). So yes, I agree that a tire is unlikely to obey Hooke's law, the rubber parts in particular. Now whether this counteracts the rounder shape I talked about (which would seem to be easier to compress), I'm not sure ... GSpeedR suggests that the overall effect is towards higher compliance, though.

Now if you poke it with 10N it will comply less than 10mm because you have to overcome the centrifugal forces which are forcing all parts of the tire outwards (while you are trying to move it inwards).

Here's where I think you are incorrect. The centrifugal forces have already been used up expanding the tire's radius, you don't have to overcome them. Here's where I think your confusion comes from: your starting point is different. When the tire wasn't rotating, the undeflected radius was 300mm and you pushed it in 10mm (axle height would be 300-10=290). But when the tire *is* rotating, the undeflected radius might be 305mm but there's nothing stopping you from pushing it in 10mm again (approximately, it may depend on the nonlinearities discussed above). The second axle height is 295, which when compared to the first value of 290 may trick you into thinking it was more difficult to deflect, but it should be pretty much the same.

[peanutaxis]

Here's where I think you are incorrect. The centrifugal forces have already been used up expanding the tire's radius, you don't have to overcome them. Here's where I think your confusion comes from: your starting point is different. When the tire wasn't rotating, the undeflected radius was 300mm and you pushed it in 10mm (axle height would be 300-10=290). But when the tire *is* rotating, the undeflected radius might be 305mm but there's nothing stopping you from pushing it in 10mm again (approximately, it may depend on the nonlinearities discussed above). The second axle height is 295, which when compared to the first value of 290 may trick you into thinking it was more difficult to deflect, but it should be pretty much the same.

MMnnn, that's a good point. If you are correct, we should be able to check: when at full speed, the 'flat' spot where the tire touches the ground should be larger, shouldn't it?

[andylaurence]

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.

A picture speaks a thousand words. Here's a sample from my data logger where I hit the rev limiter. Try and work out at what point I hit the rev limiter.

I'd say it's the point where the line starts bouncing...

[peanutaxis]

A picture speaks a thousand words. Here's a sample from my data logger where I hit the rev limiter. Try and work out at what point I hit the rev limiter.

I'd say it's the point where the line starts bouncing...

And what brand of Formula One car do you own? =D>

[Jersey Tom]

A picture speaks a thousand words. Here's a sample from my data logger where I hit the rev limiter. Try and work out at what point I hit the rev limiter.

I'd say it's the point where the line starts bouncing...

And what brand of Formula One car do you own? =D>

I can't think of a single chip-limited motor I've heard that doesn't bounce off the limiter.

[peanutaxis]

Si why don't F1 cars warble like road cars?

[cheapracer]

And what brand of Formula One car do you own?

A rev limiter hits it's ceiling then cuts the engine which then drops in revs, it then senses it is below the rev limit and starts the engine back up again (in simplistic terms), hits the limiter gain, cuts it again, starts it back up again etc, etc,...

The constant cut then start, cut then start, etc. is what is termed as "bouncing" between the 2 - simple systems that drop say 100rpm are clear to hear, very complicated systems that need to drop as few RPM's as possible to maximise performance (such as F1 needs to use) can be difficult to hear by the human ear but are easy to see even with a 50 year old osiliscope. Teams actually take recordings of other team's cars and check them out this way.

Martin Brundle is correct, as he should be for all his experience and it's amazing to see an individual dis both a World Champion and now one of the better, most experienced drivers the world has seen.

A picture speaks a thousand words.
I'd say it's the point where the line starts bouncing...

indeed, thanks for posting it.

[cheapracer]

Si why don't F1 cars warble like road cars?

When they aren't going through the TV networks sound filters they do ...

http://www.youtube.com/watch?v=ZW2VSt_VfGM