GPL: Braking question

On Ricardo Nunnini's site (http://website.lineone.net/~richardn/ I read this about braking in GPL: ------------------------ PHASE 0 threshold, straight line braking. Start by braking hard (to get the tires up to temp.) then modulate back to squeal; PHASE 1 initial turn in. The front of the car usually needs to be loaded to get it to turn in, which is accomplished by braking. Relax the braking effort (to use some traction budget for turning), and turn in, aiming to keep the squeal constant. ------------------------ I allways thought the optimal braking point was just before the tires start to squeel, instead of having the tires squeeling? Andre

I think you're right about the optimal braking point. But, if there's no sound coming from the front wheels, how can you tell if you're braking hard enough? I should have said, "brake until the squeal just starts and then play around with the brake pedal to keep the squeal on the threshold". Sent via Deja.com http://www.deja.com/ Before you buy.

The tyres don't start to squeel at a set point, the part of the tyre that starts to slip gets bigger and bigger across the footprint of the tyre until the whole footprint is slipping and you have total lock-up. I think for dry conditions, you need about 15% of the footprint slipping for optimal braking. I don't know how this translates to car simulators, but GPL is pretty realistic, so I expect they have it modelled well. You should probably be aiming for a quieter squeel as possible, but as long as you can still hear it. If you can't hear any squeeling, you aren't braking hard enough. Scott

Your explanation sounds very convincing, but contradictory to what other people stated. It's difficult to prove who's right, since it's so difficult to test in gpl at which point braking is best: little squeel or just before the squeel... Andre

Hello Andre & Scott; I don't usually reply to many posts but whether your braking, turning, four wheel drifting or whatever. Each driver has their own style and {as me} the best way to find the optimal spot or zone is the same way to Carnegie Hall. Practice, etc, etc. Now I'm not laying down a vague smug answer/opinion but if you keep going into a set or just a 'single' turn to brake or find your best line or whatever you'll find you personal way of doing it correctly! I'm certainly no expert but everything is subjective anyway!! Good Luck.. Thom_j.

I agree that you can archieve it with practice, but information could help you understand and improve it faster. there are facts or 'rules' in racing, which apply to every driver regardless of his driving style. and I personally tried different braking techniques without really finding out the 'perfect one'. therefore I'd like to hear about the experiences of other drivers. as far as I observed it, the best braking is with the tires squeeling just a little bit as ricardo and scott mentioned before. Lio

When the tyres are squeeling slightly they're just indicating that you're coming closer to their traction (grip) limit. My tyres squeel quite often when I drive hard my Honda Civic and they're not locking up, neither they lost the grip (still plenty left). Tadej

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> On Ricardo Nunnini's site (http://website.lineone.net/~richardn/) I > read this about braking in GPL: > ------------------------ > PHASE 0 threshold, straight line braking. Start by braking hard (to > get the tires up to temp.) then modulate back to squeal; > PHASE 1 initial turn in. The front of the car usually needs to be > loaded to get it to turn in, which is accomplished by braking. Relax > the braking effort (to use some traction budget for turning), and turn > in, aiming to keep the squeal constant. > ------------------------ > I allways thought the optimal braking point was just before the tires > start to squeel, instead of having the tires squeeling? > Andre

First thing of course is to get the brake balance correct. Too far forward and the front tyres will be locking up too early causing sliding. Too far back and the rear end will start moving about as the back end locks up. Well, I've just done a GPL session to figure out what I do during braking - it's on such a subconscious level that I wasn't quite sure just thinking about it off the top of my head - this is what I've discovered. I find that I slam on the anchors hard during each gear change and then ease off slightly whilst between gear changes. So it goes something like this. Slam on anchors and start to change down. Once back in gear ease off from hard braking and then back on again during next downshift and so on. There are odd times that I recall changing down 2 gears at one time, but these are rare and would only happen when going from 5th to 3rd. Using this technique my braking is pretty good and it even allows me to be competitive under braking into the Parabolica using GPL 1.0. -- Peter Ives

Thanks Peter, I've never tried this technique, I'll try it! Andre

I'm confused, let me know if I'm thinking about this right. When tires squeal, what's producing the sound? It has to be *** sliding across asphalt, no? As long as no sliding is occurring, 100% of the tire's normal contact patch is gripping the road. When you're cornering and the tires start to "squeal a little", some portion of the contact patch (say 10%) must be sliding while the remainder of the contact patch (90%) is still maintaining grip. Then when the percentage gripping falls below some critical point, the tire loses grip completely and you slide. It would seem to be a little different with straight-line braking. Here the tire is trying to slide straight ahead instead of sideways. There's always another contact patch right next to the current one. In order to make the tire squeal, you would have to either lock the wheel, or reduce the size of the contact patch. The only way to reduce the size of the contact patch is to reduce the weight on the tire. Whenever you're breaking, you're putting more weight on the front tires and less on the rears. This could cause the rears to "squeal a little" without locking the fronts or having the car slide noticeably. Does any of this sound right?

Richard S Beckett wrote ... I'm thinking that with a certain amount of weight on the tire, it maintains traction. As you lighten the weight, it has less traction, until it gets to some point where it's slipping. When you're cornering to the right, you would have greater weight on the left side of each tire than on the right. The right edge of each tread could be slipping a bit while the left edge is maintaining grip. In other words, the right edge is pregnant and the left edge is just drunk without protection ;-)

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> Richard S Beckett wrote ... > > If you assume that the sound is caused by the tyre sliding across the > > road, then how can you explain that only a part of the tyre is sliding? > > Surely the tyre is either sliding, or not sliding - you can't be a > > little bit pregnant! :-) > I'm thinking that with a certain amount of weight on the tire, it maintains > traction. As you lighten the weight, it has less traction, until it gets to > some point where it's slipping. When you're cornering to the right, you > would have greater weight on the left side of each tire than on the right. > The right edge of each tread could be slipping a bit while the left edge is > maintaining grip. > In other words, the right edge is pregnant and the left edge is just drunk > without protection ;-)

I think I know what you're saying, and I think you're on the right track. Have a look at this site, It may shed some light on the subject.. http://members.home.net/rck/phor/ -- ========================================= Mike Barlow of Barlow Racing? ========================================= http://members.xoom.com/BarlowRacing/ Racing online with the help of...... Race Communications Association Holodyne Engineering Mystic Music (have Your !!Name/Address!! placed here)

Sorry guys, this makes no sense to me. If part of the tire is slipping across the surface of the road, and part is gripping the surface of the road, then the tire is coming apart or is being compressed. Just my $.02 Gene > > Richard S Beckett wrote ... > > > If you assume that the sound is caused by the tyre sliding across the > > > road, then how can you explain that only a part of the tyre is sliding? > > > Surely the tyre is either sliding, or not sliding - you can't be a > > > little bit pregnant! :-) > > I'm thinking that with a certain amount of weight on the tire, it maintains > > traction. As you lighten the weight, it has less traction, until it gets to > > some point where it's slipping. When you're cornering to the right, you > > would have greater weight on the left side of each tire than on the right. > > The right edge of each tread could be slipping a bit while the left edge is > > maintaining grip. > > In other words, the right edge is pregnant and the left edge is just drunk

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> > without protection ;-) > I think I know what you're saying, and I think you're on the right > track. Have a look at this site, It may shed some light on the > subject.. http://members.home.net/rck/phor/ > -- > ========================================= > Mike Barlow of Barlow Racing? > ========================================= > http://members.xoom.com/BarlowRacing/ > Racing online with the help of...... > Race Communications Association > Holodyne Engineering > Mystic Music > (have Your !!Name/Address!! placed here)

Both. I think they call it tire "squirm" Mike Barlow posted a link to some racing physics pages, great reading. Check out chapter 10.

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> Sorry guys, this makes no sense to me. If part of the tire is slipping > across the surface of the road, and part is gripping the surface of the > road, then the tire is coming apart or is being compressed. > Just my $.02 > Gene > > > Richard S Beckett wrote ... > > > > If you assume that the sound is caused by the tyre sliding across

the

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> > > > road, then how can you explain that only a part of the tyre is > sliding? > > > > Surely the tyre is either sliding, or not sliding - you can't be a > > > > little bit pregnant! :-) > > > I'm thinking that with a certain amount of weight on the tire, it > maintains > > > traction. As you lighten the weight, it has less traction, until it > gets to > > > some point where it's slipping. When you're cornering to the right,

you > > > would have greater weight on the left side of each tire than on the > right. > > > The right edge of each tread could be slipping a bit while the left edge

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> is > > > maintaining grip. > > > In other words, the right edge is pregnant and the left edge is just > drunk > > > without protection ;-) > > I think I know what you're saying, and I think you're on the right > > track. Have a look at this site, It may shed some light on the > > subject.. http://members.home.net/rck/phor/ > > -- > > ========================================= > > Mike Barlow of Barlow Racing? > > ========================================= > > http://members.xoom.com/BarlowRacing/ > > Racing online with the help of...... > > Race Communications Association > > Holodyne Engineering > > Mystic Music > > (have Your !!Name/Address!! placed here)

You are absolutely correct. Assume you are braking in a straight line. (This might be for acceleration, I can't remember which way around it is) The middle part of the contact patch is compressed and as that part of the tire moves away from the middle of the contact patch, it starts to slip. If you are only braking lightly, it will only start to slip when it has more or less left the road, and you won't hear anything. Under heavy braking, but not lock-up, the tyre starts to slip earlier and earlier, causing more sound, until the whole contact-patch is slipping. Someone must have an animation of this somewhere, as I'm sure I'm not the only one who can't visualise it. Scott

Scott. this may illustrate this example better. Think of a Top-fuel dragster slick. As it grows and balloons on acceleration the sides of the foot-print leave the surface. The phenomenon of this thread happens in sort of reverse order of the slick. If you've ever been close enough to listen as a drag slick regains it's traction after a burnout. You will hear a sharp "chirp" as the traction coefficient increases as the tire shrinks and the inertia subsides to flatten out or widen the foot-print and the tire regains it's grip. Hope this helps!! DBH

Hi, guys! I've read this thread for a couple of days now with great interest, because I also wanted to know, why a non-locked tire sqeals and what it means that a part of the contact patch is sliding while another part still has grip. But unfortunately your answers didn't really enlighten my mind, so I've put together my own theory. <guess mode on> You say, when the tire is near its limit a part of the contact patch loses its grip. This part will be the one, which has the biggest friction force and the least normal force exerted on it (which part it will be depends, if it's while braking, cornering or accelerating or any combination of the three). OK, but what happens then? This part of the contact patch will begin to slide, i.e. to move relative to the track (sliding with reduced coeff.of friction), but it will also move relative to the rest of the tire (and the rest of the contact patch, which still has grip). Obviously this moving relative to the rest of the tire can't go on forever (not even the few seconds you spend braking for Parabolica ;-) otherwise the tire should be ripped apart. The rest of the tire will exert a force onto the slipping part of the contact patch, which wants to move it back in place (and which will get bigger, the further the part has moved). Now this force should be directed more or less against the friction force exerted by the track onto the sliding part of the contact patch, thus reducing it maybe enough that the sliding part will regain grip again. Anyway, as long as the whole tire isn't locked it will force the part of *** which 'locked' to turn with it. Of course, now the tire has turned a little and the contact patch (partly) is made up of another piece of ***. The force, which wanted to push the sliding part of *** back in place a few fractions of a second ago will be more or less gone, so a part of the current contact patch will start to slide again. And the whole procedure will repeat itself at a quite high frequency. The whole tire will begin to oscillate at that frequency and this oscillation will produce the sqealing sound. <guess mode off> What do you think? Reasonable? Complete nonsense? Please let me know! Peter Vienna, Austria > I'm confused, let me know if I'm thinking about this right. When tires > squeal, what's producing the sound? It has to be *** sliding across > asphalt, no? As long as no sliding is occurring, 100% of the tire's normal > contact patch is gripping the road. When you're cornering and the tires > start to "squeal a little", some portion of the contact patch (say 10%) must > be sliding while the remainder of the contact patch (90%) is still > maintaining grip. Then when the percentage gripping falls below some > critical point, the tire loses grip completely and you slide. > It would seem to be a little different with straight-line braking. Here the > tire is trying to slide straight ahead instead of sideways. There's always > another contact patch right next to the current one. In order to make the > tire squeal, you would have to either lock the wheel, or reduce the size of > the contact patch. The only way to reduce the size of the contact patch is > to reduce the weight on the tire. Whenever you're breaking, you're putting > more weight on the front tires and less on the rears. This could cause the

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> rears to "squeal a little" without locking the fronts or having the car > slide noticeably. > Does any of this sound right? > > x-no-archive: yes > > I think most people will agree that when cornering hard, tyres will > > start to squeal a little before any noticeable sliding ocurrs, so this > > _must_ apply to braking, too. > > R.

>Hi, guys! >I've read this thread for a couple of days now with great interest, because >I also wanted to know, why a non-locked tire sqeals and what it means that a >part of the contact patch is sliding while another part still has grip. But >unfortunately your answers didn't really enlighten my mind, so I've put >together my own theory. There's a BIG difference between a "locked" tire and a "slipping" tire. A tire doesn't have to be locked to be squealing or sliding. Trips

Peter Prochazka wrote, among other very technical-sounding things... Okay, you lost me completely ;-) I guess I'm saying that in cornering, one side of the contact patch could move faster or slower than the other side. It wouldn't have to go on forever, only for the split-second that it IS part of the contact patch. During the other 300+ degrees of the tire's rotation, both sides of the tread would be rotating at exactly the same speed.

Well, OK... maybe I dealt with the word 'locked' a little bit too sloppy. What I meant with a 'non-locked tire' is a tire, which is slipping (and sqealing), but not has totally lost its grip (i.e. is sliding as a whole and would be 'locked' while braking in most cases). I've also used the term 'locked' later in my post wrongly. Replace that with 'slipping' or 'sliding'. BTW is there any difference between the terms 'slipping' and 'sliding' in 'racing-English'? My Cassell's translates them almost the same... Peter Vienna, Austria Trips schrieb > >Hi, guys! > >I've read this thread for a couple of days now with great interest, because > >I also wanted to know, why a non-locked tire sqeals and what it means that > a > >part of the contact patch is sliding while another part still has grip. But

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> >unfortunately your answers didn't really enlighten my mind, so I've put > >together my own theory. > There's a BIG difference between a "locked" tire and a "slipping" tire. A > tire doesn't have to be locked to be squealing or sliding. > Trips

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> <guess mode on> > You say, when the tire is near its limit a part of the contact patch loses > its grip. This part will be the one, which has the biggest friction force > and the least normal force exerted on it (which part it will be depends, if > it's while braking, cornering or accelerating or any combination of the > three). OK, but what happens then? This part of the contact patch will begin > to slide, i.e. to move relative to the track (sliding with reduced coeff.of > friction), but it will also move relative to the rest of the tire (and the > rest of the contact patch, which still has grip). Obviously this moving > relative to the rest of the tire can't go on forever (not even the few > seconds you spend braking for Parabolica ;-) otherwise the tire should be > ripped apart.

It can go on "forever"as long as the wheel is rotating. Even when a tire is not at it's limit there are always speed differences in the contact patch while cornering (the outside of the tire has to move faster than the inside of the tire). This generates what is called camber thrust which tries to rotate the contact patch (and tire and vehicle) and heats the tire. The tire returns to it's natural shape after leaving the contact patch, cools off and gets ready to do it all over again.