Car Physics: camber thrust in sims and real life

I've been playing with the camber (and Pacejka coefficients) in Ruud's Racer in an attempt to identify the bugs that are currently making the handling "interesting"..... but I've kind of run out of real-life knowledge. Help! :0) How does camber thrust work? If I have a real life car with negative camber on all 4 wheels ....... /----x----\ ....... and it's parked on a flat surface, then it appears to me that the tyres shouldn't generate any lateral force at all. I've seen some posts where people talk about the "camber thrust on the left tyres generating inward-pointing lateral forces that exactly counter the lateral forces generated by the right hand tyres" but that doesn't make sense.... if that *was* correct then a car with super grippy tyres on the right and with super-slippy tyres on the left (eg. if the left tyres were resting on a patch of ice on the road) would crab sideways when parked..... So assuming there's no camber thrust when parked, is there also no camber thrust when driving steadily in a straight line? And as for cornering..... if I think about a car cornering steadily, then the car is "leaning" on the outside tyres, which are generating a lateral force to turn the car (yes, the inside wheels are too, but I'm just considering the outsides at the moment). In this case I figure that camber thrust will be generating centripetal lateral force, which "helps the car turn". It's as if the camber thrust only appears when needed...... if you lean on the outside tyres then they will push back at you, but if you don't lean on them, they won't push (kinda like brakes..... max brake pedal when parked on the flat gives zero braking torque, but max brake pedal at 100 mph generates lots of brake torque...... so the brakes only generate as much torque as is "needed"). Racer has a quirk whereby stationary cars with high negative camber can scoot sideways across the tarmac..... it's as if the camber thrust is being generated all the time, and not only when needed. (this would be like your brakes *always* generating a braking torque so your car would reverse when you brake when parked). Can someone help? I'm not sure how much I'm getting right and how much is complete rubbish :0) Cheers Jim

Jim Seamus <[email hidden]> wrote in message news:QELf9.2226$[email hidden]...

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> How does camber thrust work? If I have a real life car with negative camber > on all 4 wheels ....... /----x----\ ....... and it's parked on a flat > surface, then it appears to me that the tyres shouldn't generate any lateral > force at all. > > I've seen some posts where people talk about the "camber thrust on the left > tyres generating inward-pointing lateral forces that exactly counter the > lateral forces generated by the right hand tyres" but that doesn't make > sense.... if that *was* correct then a car with super grippy tyres on the > right and with super-slippy tyres on the left (eg. if the left tyres were > resting on a patch of ice on the road) would crab sideways when parked..... > > So assuming there's no camber thrust when parked, is there also no camber > thrust when driving steadily in a straight line?

As I understand it, it would cause the tyres to behave as if slightly conical - that would steer the contact patch into the direction of camber, which if the tyre is rolling will then generate an effective slip angle. Yeah, that sounds plausible... ;-) > And as for cornering..... if I think about a car cornering steadily, then > the car is "leaning" on the outside tyres, which are generating a lateral > force to turn the car (yes, the inside wheels are too, but I'm just > considering the outsides at the moment). In this case I figure that camber > thrust will be generating centripetal lateral force, which "helps the car > turn". It's as if the camber thrust only appears when needed...... if you Hmm... I'd guess it's possibly load sensitive (a lightly-loaded tyre will have a smaller contact patch, so less room for conicity and slip angle to develop?) which again would seem to fit the real-world evidence. Must admit it's not something I've modelled, though, and my camber adjustment on my model racers is limited to keeping the tyres wearing flat :-) Jonny

"Jonny Hodgson" <[email hidden]> wrote in message news:ZXNf9.2434$[email hidden]... > > As I understand it, it would cause the tyres to behave as if slightly > conical - that would steer the contact patch into the direction of > camber, which if the tyre is rolling will then generate an effective > slip angle. Yeah, that sounds plausible... ;-) Sounds good to me. If that's the case then, we'd not get any camber thrust when the car is parked (because the cone isn't rolling), so I can only assume the magnitude of the thrust will increase as speed rises. Pacejka's formulae don't accept speed as an input, so I guess this would have to be something that Ruud would need to add in alongside the Pacejka....? I also assume that the magnitude of the camber thrust force will increase with tyre load, and hence when cornering the outside tyres will generate more thrust than the inside tyres and so there'll be a net increase in grip due to camber thrust? I wonder how camber thrust varies with camber angle.

Camber thrust is a non-existant force, kind of like centrifugal force and coriolis force. It's the appearance of a force and it's a measurement that you can make under a certain test setup, nothing more. Basically camber thrust is derived by putting a tire on a rolls (tire test dyno) and adding camber to it. Of course, when you bend the tire carcass sideways it puts a stress on it that results in a lateral force acting on the wheel. Since the tire is fixed on a test stand that force also reacts on the contact patch. But the important thing to understand is that the force has been developed between the tire carcass and the wheel, not at the contact patch. The contact patch force is just a reaction force. Camber thrust does not magically add to the lateral force that can be sustained at the contact patch. Now take this and put it on a stationary car with assymetrical tires as you described. The tire carcasses will flex as necessary until they reach an equilibrium, and then nothing further happens. So why do racers use negative camber? Imagine a tire with zero camber going through a corner. You put a lateral force on it, and the carcass bends inward. The outside sidewall gets pulled further under the wheel, and the inside sidewall gets pulled inward so that it is no longer under the wheel. As a result the outside of the contact patch ends up with a lot of pressure on it, and the inside ends up with very little. This results in less effective use of the contact patch and a drop in lateral force capability. Adding negative camber to the tire pulls the contact patch back under the wheel and evens out the pressure, potentially regaining the full use of the contact patch. It just so happens that the force difference experienced by doing this is close enough, within the large range of experimental error inherent in tire testing, that it appears similar to the camber thrust measurements made on a test stand. Therefore people assume that they are getting camber thrust and don't bother to look any deeper. But that's not really what it is. And that's probably why the camber thrust models used in most sims aren't very convincing. I think you would pretty much need an FEA model of the tire to get it right. "Jim Seamus" <[email hidden]> wrote in message news:QELf9.2226$[email hidden]... > > How does camber thrust work?

I have always believed that racing motorbikes corner almost totally due to camber thrust. OK so they have tyres with rounded cross sections that make camber thrust much more significant than you'd find with a car's tyres.... but the camber thrust still exists.... otherwise how are motorbikes generating the centripetal force? "Haqsau" <[email hidden]> wrote in message news:alomd5$1rvq10$[email hidden]...

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> Camber thrust is a non-existant force, kind of like centrifugal force > and coriolis force. It's the appearance of a force and it's a > measurement that you can make under a certain test setup, nothing more. > Basically camber thrust is derived by putting a tire on a rolls (tire > test dyno) and adding camber to it. Of course, when you bend the tire > carcass sideways it puts a stress on it that results in a lateral force > acting on the wheel. Since the tire is fixed on a test stand that force > also reacts on the contact patch. But the important thing to understand > is that the force has been developed between the tire carcass and the > wheel, not at the contact patch. The contact patch force is just a > reaction force. Camber thrust does not magically add to the lateral > force that can be sustained at the contact patch. > > Now take this and put it on a stationary car with assymetrical tires as > you described. The tire carcasses will flex as necessary until they > reach an equilibrium, and then nothing further happens. > > So why do racers use negative camber? Imagine a tire with zero camber > going through a corner. You put a lateral force on it, and the carcass > bends inward. The outside sidewall gets pulled further under the wheel, > and the inside sidewall gets pulled inward so that it is no longer under > the wheel. As a result the outside of the contact patch ends up with a > lot of pressure on it, and the inside ends up with very little. This > results in less effective use of the contact patch and a drop in lateral > force capability. Adding negative camber to the tire pulls the contact > patch back under the wheel and evens out the pressure, potentially > regaining the full use of the contact patch. > > It just so happens that the force difference experienced by doing this > is close enough, within the large range of experimental error inherent > in tire testing, that it appears similar to the camber thrust > measurements made on a test stand. Therefore people assume that they > are getting camber thrust and don't bother to look any deeper. But > that's not really what it is. And that's probably why the camber thrust > models used in most sims aren't very convincing. I think you would > pretty much need an FEA model of the tire to get it right. > > > > "Jim Seamus" <[email hidden]> wrote in message > news:QELf9.2226$[email hidden]... > > > > How does camber thrust work? >

On Thu, 12 Sep 2002 03:37:09 +0100, Jim Seamus wrote: > I have always believed that racing motorbikes corner almost totally due to > camber thrust. OK so they have tyres with rounded cross sections that make > camber thrust much more significant than you'd find with a car's tyres.... > but the camber thrust still exists.... otherwise how are motorbikes > generating the centripetal force? > In the light of losing my hard earned degree in physics here, just call me "Rehoov" for now. ;-) I thought motorcycles corner because when you try to "tip" the cycle onto its side while going straight, you exert a torque / angular momentum force (damn, what's the english term for that? "Drehmoment" in German) on the rotating wheels. As the resulting force is actually the cross product of the two force vectors the resulting force acting on the cycle makes it turn and not just fall over. All of this might be massively wrong, but that's the process generating the "original" force. Cheers, rehoov. -- mail replies to Uwe at schuerkamp dot de ( yahoo address is spambox) Uwe Schuerkamp //////////////////////////// http://www.schuerkamp.de/ Herford, Germany \\\\\\\\\\\\\\\\\\\\\\\\\\ (52.0N/8.5E) GPG Fingerprint: 2E 13 20 22 9A 3F 63 7F 67 6F E9 B1 A8 36 A4 61

If a "perfect" tire is forced to roll straight along a road with no steer angle (and thus no slip angle), then there is no side force. If the wheel is then leaned to one side (inclination in tire axis system) a lateral force is produced. Or, if the path is not constrained to be straight, then the tire will roll off at an angle. If the tire is then halted, the lateral force will remain unless the tire if free to move sideways a small amount (relax the stress in sidewalls). See tire chapter in RCVD for lots more detail. Also, I think there is more in Tony Foale's new book on motorcycle dynamics (recommended!) -- Doug Milliken (on holiday with poor email connections) www.millikenresearch.com On Wed, 11 Sep 2002, Haqsau wrote:

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> Camber thrust is a non-existant force, kind of like centrifugal force > and coriolis force. It's the appearance of a force and it's a > measurement that you can make under a certain test setup, nothing more. > Basically camber thrust is derived by putting a tire on a rolls (tire > test dyno) and adding camber to it. Of course, when you bend the tire > carcass sideways it puts a stress on it that results in a lateral force > acting on the wheel. Since the tire is fixed on a test stand that force > also reacts on the contact patch. But the important thing to understand > is that the force has been developed between the tire carcass and the > wheel, not at the contact patch. The contact patch force is just a > reaction force. Camber thrust does not magically add to the lateral > force that can be sustained at the contact patch. > > Now take this and put it on a stationary car with assymetrical tires as > you described. The tire carcasses will flex as necessary until they > reach an equilibrium, and then nothing further happens. > > So why do racers use negative camber? Imagine a tire with zero camber > going through a corner. You put a lateral force on it, and the carcass > bends inward. The outside sidewall gets pulled further under the wheel, > and the inside sidewall gets pulled inward so that it is no longer under > the wheel. As a result the outside of the contact patch ends up with a > lot of pressure on it, and the inside ends up with very little. This > results in less effective use of the contact patch and a drop in lateral > force capability. Adding negative camber to the tire pulls the contact > patch back under the wheel and evens out the pressure, potentially > regaining the full use of the contact patch. > > It just so happens that the force difference experienced by doing this > is close enough, within the large range of experimental error inherent > in tire testing, that it appears similar to the camber thrust > measurements made on a test stand. Therefore people assume that they > are getting camber thrust and don't bother to look any deeper. But > that's not really what it is. And that's probably why the camber thrust > models used in most sims aren't very convincing. I think you would > pretty much need an FEA model of the tire to get it right. > > > > "Jim Seamus" <[email hidden]> wrote in message > news:QELf9.2226$[email hidden]... > > > > How does camber thrust work? > > >

On Thu, 12 Sep 2002 03:37:09 +0100, "Jim Seamus" <[email hidden]> wrote: >I have always believed that racing motorbikes corner almost totally due to >camber thrust. OK so they have tyres with rounded cross sections that make >camber thrust much more significant than you'd find with a car's tyres.... >but the camber thrust still exists.... otherwise how are motorbikes >generating the centripetal force? As Haqsau said, camber thrust doesn't exist in itself. It's a name for an effect. In the same way, you have d'Alembert forces; the reverse of the acceleration of a body (F=m*a, so you could see acceleration as a force; is useful for free-body diagrams it seems). I regard camber thrust as follows: say you have a car with cambered wheels only on the right side. It's driving forward. Now it tends to turn right (or left, depending on the direction of camber). Imagine a kart that just doesn't want to turn in. This often is just a result of one side being so much off-camber. If it doesn't want to turn left, it probably wants to turn right twice as good. So you're driving forward, the cambered wheels pull the car to a side (just like indeed a conically shaped object would). That force is the camber thrust. In Pacejka, it's added into the calculations next to pure slip angle. Problem with Pacejka that it indeed doesn't have speed as an input, which is just plain wrong at low speed. Therefore, at low speed you can use relaxation lengths (which are physically very correct, as they give you a spring effect). In Pacejka, with camber at speed=0, you STILL get a force, much like there is a Sv and Sh variable which directly shifts (hence the 'S') the graph. So at 0 slip angle you'd still get a force (normally used for conical effects and the other one, which I forgot, lol). At low speed, what you should probably do is cut out camber effects and any Sv/Sh movements. Well, what *I* should do. ;-) Racer's Pacejka graphs seemingly currently all go through the origin nicely, otherwise (if Sh!=0 or Sv!=0) you'd have seen this a long time ago (Ashley mentioned this a year or so ago while playing with Pacejka curves). Now this doesn't mean the *force* should end up at 0. As a car standing sideways on a hill must still have lateral force buildup, otherwise it would slide slowly downwards, since the gravity force pulls the car down the slope, and the lateral tire forces must fight that. But that should be doable with just the relaxation slip angle, which should reach an equilibrium. You could see Pacejka's lateral force as: Fy=f(slipAngle+slipAngleEffective) where slipAngleEffective=camber*magicFactor to see the effect of camber as some sort of slip angle (though it isn't). But slipAngle is relaxed (in beta6) while camber isn't. To avoid having to do both (and get a mess) it's easier to throw out camber at low speed, where you'll hardly notice it anyway). And keep the car sideways on the hill using a tricky slipAngle equilibrium. You'll quickly be able to test if that works in beta6. :) Hope this clarifies instead of mystifies. ;-) Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

nice - I also just came to this group wanting to post a question on camber thrust. I have recently added Anti-Roll bars to my simulation. When playing around with the new car setups, I noticed there wasn't really any noticable difference in cornering with or without ARBs. Sure, the cars don't lean into the curves as before, but they seem to take corners just as good (or bad) as before. So i went to think what difference less roll would have in real life. The conclusion i came to was that it would have to do with camber thrust (which i currently ignore in my engine). if cars lean outward in curves, camber thrust would generate a lateral force "pulling" the car out of the curve. Is this correct? or might this rather be the effect that Haqsau mentioned in this thread (that tires have less grip when tilted, due to a smaller contact patch). Or is the idea behind Anti-Roll something entierly different? Thanks for any suggestions, jonas "Jim Seamus" <[email hidden]> wrote in message news:<QELf9.2226$[email hidden]>...

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> I've been playing with the camber (and Pacejka coefficients) in Ruud's Racer > in an attempt to identify the bugs that are currently making the handling > "interesting"..... but I've kind of run out of real-life knowledge. Help! > :0) > > How does camber thrust work? If I have a real life car with negative camber > on all 4 wheels ....... /----x----\ ....... and it's parked on a flat > surface, then it appears to me that the tyres shouldn't generate any lateral > force at all.

This might just be semantics, i.e. maybe I am not interpreting you correctly, but no, they corner due to traction, just like any other wheeled vehicle. Leaning the bike causes the front wheel to start to turn in due to caster, and initially camber thrust, or rather sidewall distortion, helps the motorcycle change direction and develop a lateral force, but the lateral force still has to be supported by the contact patch, which is friction, or as us old fart engineers prefer to call it, traction. "Jim Seamus" <[email hidden]> wrote in message news:p5Tf9.3498$[email hidden]... > I have always believed that racing motorbikes corner almost totallydue to > camber thrust. OK so they have tyres with rounded cross sections thatmake > camber thrust much more significant than you'd find with a car'styres.... > but the camber thrust still exists.... otherwise how are motorbikes > generating the centripetal force? >

Just to clarify since I probably didn't explain it very well, what I was trying to point out was that the phenomenon that is normally referred to as camber thrust is a force developed by elastic tire carcass distortion as it is rolled through the contact patch. This happens because, and only when, the tire is constrained to roll under the cambered wheel, rather than assume its natural shape. You don't get camber thrust out of a stationary tire unless you constrain it to act that way. Grab a bike tire and lean it over - you can stick your hand under the rim and support it totally with a vertical force, it doesn't push sideways unless you support it from the side. And similarly you won't get it when rolling a cambered tire if the tire is not constrained in some way. The tire rolling at an angle when not constrained is due to it distorting into a conical shape in the contact patch, not due to camber thrust. I know this sounds nitpicky, but I feel like I hear the term misused a lot and that's why I am trying to define it a little more precisely. But mostly I wanted to point out that camber thrust does not add to an existing cornering force, because it is a force in the sidewall, not in the contact patch. Cambering a tire does affect the contact patch and consequently cornering force, but for a different reason, and that is important to know for anyone trying to set up a car or trying to write a simulation. Regards, Hal Raymond DaimlerChrysler Corporation <still waiting for somebody to try an FEA tire model in a sim> "Doug Milliken" <[email hidden]> wrote in message news:[email hidden]... > > If a "perfect" tire is forced to roll straight along a road with nosteer > angle (and thus no slip angle), then there is no side force. If thewheel > is then leaned to one side (inclination in tire axis system) a lateral > force is produced. Or, if the path is not constrained to be straight,then

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> the tire will roll off at an angle. If the tire is then halted, the > lateral force will remain unless the tire if free to move sideways > a small amount (relax the stress in sidewalls). > > See tire chapter in RCVD for lots more detail. Also, I think there is > more in Tony Foale's new book on motorcycle dynamics (recommended!) > > -- Doug Milliken (on holiday with poor email connections) > www.millikenresearch.com > >

As I understand it, when you stiffen the front ARB and corner hard left, the body rolls to the right and this roll is resisted by the springs and ARBs. If the front ARB is stiffer than the rear one then there will be more lateral weight transfer at the front than at the rear of the car (because the car is rolling, say, 4 degrees, and that 4 degrees then twists the front and rear ARBs by approximately the same amount, and the front one is stiffer so the force required to twist it that amount is higher than for the rear bar). So stiffen the front ARB and you get the same overall lateral weight transfer as before but you get more weight on the outside front and less on the outside rear (and less on the inside front). [1] Tyre load sensitivity does the rest.... that's the thing whereby doubling the load on a tyre doesn't double the max force that the tyre can generate - instead, the max force increases by less than a factor of 2. So to optimise grip you want equal weights per wheel. So stiffer ARBs mean more weight transfer at the front and so the load sensitivity reduces relative front grip. And you get understeer. Don't take this as gospel because I'm not 100% sure about this. Jim [1] According to GPL Foolishness the weight on the inside rear is unchanged - can anyone tell me why?? "jonas echterhoff" <[email hidden]> wrote in message news:[email hidden]...

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> nice - I also just came to this group wanting to post a question on > camber thrust. I have recently added Anti-Roll bars to my simulation. > When playing around with the new car setups, I noticed there wasn't > really any noticable difference in cornering with or without ARBs. > Sure, the cars don't lean into the curves as before, but they seem to > take corners just as good (or bad) as before. So i went to think what > difference less roll would have in real life. The conclusion i came to > was that it would have to do with camber thrust (which i currently > ignore in my engine). if cars lean outward in curves, camber thrust > would generate a lateral force "pulling" the car out of the curve. > Is this correct? or might this rather be the effect that Haqsau > mentioned in this thread (that tires have less grip when tilted, due > to a smaller contact patch). Or is the idea behind Anti-Roll something > entierly different? > > Thanks for any suggestions, > jonas > > "Jim Seamus" <[email hidden]> wrote in message

news:<QELf9.2226$[email hidden]>... > > I've been playing with the camber (and Pacejka coefficients) in Ruud'sRacer > > in an attempt to identify the bugs that are currently making thehandling > > "interesting"..... but I've kind of run out of real-life knowledge.Help! > > :0) > > > > How does camber thrust work? If I have a real life car with negativecamber > > on all 4 wheels ....... /----x----\ ....... and it's parked on a flat > > surface, then it appears to me that the tyres shouldn't generate anylateral > > force at all.

Um..... yes ok everything tyre-related works due to traction. I think we're going to need to break it down and get a little more detail in there in order to simulate things though....? "Haqsau" <[email hidden]> wrote in message news:alra75$g29u$[email hidden]...

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> This might just be semantics, i.e. maybe I am not interpreting you > correctly, but no, they corner due to traction, just like any other > wheeled vehicle. Leaning the bike causes the front wheel to start to > turn in due to caster, and initially camber thrust, or rather sidewall > distortion, helps the motorcycle change direction and develop a lateral > force, but the lateral force still has to be supported by the contact > patch, which is friction, or as us old fart engineers prefer to call it, > traction. > > "Jim Seamus" <[email hidden]> wrote in message > news:p5Tf9.3498$[email hidden]... > > I have always believed that racing motorbikes corner almost totally > due to > > camber thrust. OK so they have tyres with rounded cross sections that > make > > camber thrust much more significant than you'd find with a car's > tyres.... > > but the camber thrust still exists.... otherwise how are motorbikes > > generating the centripetal force? > > > >

"Uwe Schuerkamp" <[email hidden]> wrote in message news:[email hidden]...

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> I thought motorcycles corner because when you try to "tip" the > cycle onto its side while going straight, you exert a torque / > angular momentum force (damn, what's the english term for that? > "Drehmoment" in German) on the rotating wheels. > > As the resulting force is actually the cross product of the two > force vectors the resulting force acting on the cycle makes it > turn and not just fall over. > > All of this might be massively wrong, but that's the process > generating the "original" force.

Ooh yeah the gyroscopic effect..... precession is it?

As I said, this is probably just semantics. You said "racing motorbikes corner almost totally due to camber thrust" and I took that to mean that you thought that they were using camber thrust to generate the entire cornering force, which is not true. In a sense though it is camber thrust which is transmitting the contact patch force (traction) to the wheel, but that's a little different. And again, due to the way tires are constrained when testing camber thrust the measurements would correspond to some degree. But it's not quite the same thing - camber thrust is a measurement of the response to leaning a tire over, whereas in cornering on a bike you are applying a force to it. In the latter case you would get carcass distortion and lateral thrust even if the tire were vertical (although not for long :>). I'm just trying to get people past the point of thinking of camber thrust as a force which acts on the tire - it is not, it is a force developed within the tire. And I had hoped that that would clear up some of the confusion about what camber thrust does and does not do in a sim. "Jim Seamus" <[email hidden]> wrote in message news:Xccg9.6458$[email hidden]... > Um..... yes ok everything tyre-related works due to traction. I thinkwe're > going to need to break it down and get a little more detail in therein > order to simulate things though....? >

Jim Seamus <[email hidden]> wrote in message news:mQbg9.6453$[email hidden]... > So stiffen the front ARB and you get the same overall lateral weight > transfer as before but you get more weight on the outside front and less on > the outside rear (and less on the inside front). [1] Pretty much what I was going to say. AFAIK there's little if any difference, on a smooth track, between a roll couple distribution due to a certain spring combination, and the same RCD arrived at by a combination of springs and ARBs. What ARBs allow you to do is vary the RCD independently of the springs, in situations where there are other factors (such as keeping the front up during braking, or increasing compliance to cope with bumps) which affect the preferred spring rates. I believe that, as a generalisation, rally cars run soft springs and heavy ARBs, using the ARBs to limit roll; whilst touring cars run stiffer springs and fairly light ARBs, using the ARBs as fine- -tuning devices. > [1] According to GPL Foolishness the weight on the inside rear is > unchanged - can anyone tell me why?? <goes away to re-read GPLF...> Hmm, can't immediately find that bit - but it doesn't make sense to me: If adding a front 'bar increases the front load transfer, then the rear weight transfer must be reduced because the total load transfer is only a function of CG height, track width and lateral acceleration. Even if it makes the car roll less, that should actually *reduce* the total load transfer... Jonny

Hi

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> Just to clarify since I probably didn't explain it very well, what I was > trying to point out was that the phenomenon that is normally referred to > as camber thrust is a force developed by elastic tire carcass distortion > as it is rolled through the contact patch. This happens because, and > only when, the tire is constrained to roll under the cambered wheel, > rather than assume its natural shape. You don't get camber thrust out > of a stationary tire unless you constrain it to act that way. Grab a > bike tire and lean it over - you can stick your hand under the rim and > support it totally with a vertical force, it doesn't push sideways > unless you support it from the side. And similarly you won't get it > when rolling a cambered tire if the tire is not constrained in some way. > The tire rolling at an angle when not constrained is due to it > distorting into a conical shape in the contact patch, not due to camber > thrust. I know this sounds nitpicky, but I feel like I hear the term > misused a lot and that's why I am trying to define it a little more > precisely. But mostly I wanted to point out that camber thrust does not > add to an existing cornering force, because it is a force in the > sidewall, not in the contact patch. Cambering a tire does affect the > contact patch and consequently cornering force, but for a different > reason, and that is important to know for anyone trying to set up a car > or trying to write a simulation.

This all sounds entirely logical (and very handy from an implementation point of view :) ) but it does seem to differ from what Doug Milliken says so I'm not quite sure what to think. I've also read the bit in RCVD about this and again it seems imcompatible with what you have said. Am I somehow not understanding what Doug says and you do agree?

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> "Doug Milliken" <[email hidden]> wrote in message > news:[email hidden]... > > > > If a "perfect" tire is forced to roll straight along a road with no > steer > > angle (and thus no slip angle), then there is no side force. If the > wheel > > is then leaned to one side (inclination in tire axis system) a lateral > > force is produced. Or, if the path is not constrained to be straight, > then > > the tire will roll off at an angle. If the tire is then halted, the

An angle, you mean it rolls off in a straight line? or in a circle? I had kind of assumed from what I had read in RCVD that it would be a circle but angle implies straight line? > > lateral force will remain unless the tire if free to move sideways > > a small amount (relax the stress in sidewalls). I do like the sound of what Haqsau says here, sounds very easy to implement! Paul

Disagree is probably a bit too strong a word. Doug is working with traditional ideas derived from traditional tire testing, and within that context he is perfectly correct. What I am trying to say is that those traditional models are inadequate and have led to false assumptions and poorly implemented tire models in sims. Unfortunately I don't have the necessary data to propose an adequate alternative. I'm really trying to encourage programmers and any lurking engineers alike to think outside the box and come up with something new. A wheel is basically a rim with a couple of highly damped springs under it connected to a contact patch whose size varies with pressure. Anything that strays far from that concept is probably going to give you the wrong answer at some point. What I have seen of Pacejka (sp?) seems to ignore this and instead just try to model existing tire data. That seems fine on the surface, except that as I have tried to point out existing data is not formulated in a very practical way. Camber thrust, such as it is, could easily be explained by the wheel, spring and contact patch model, and looking at it that way it should also be clear what the limitations of camber thrust are. Sorry, I guess I am not really giving you a very clear answer, I'm just trying to stir the pot a little, get people to throw away the old models, and see what new things they can come up with. Again, I would love to see somebody take on a real time FEA tire model. I know non-real-time FEA models are being done in the industry and at universities, and I personally believe that we now have enough computing power to do a simplified (i.e. small number of elements) FEA model in real time in a computer game. Codemasters is already doing real time FEA damage simulation in Pro Race Driver, for example. Not being very helpful, am I? ;o) Regards, Hal "Paul Laidlaw" <[email hidden]> wrote in message news:am5itq$2sdu0$[email hidden]... > Hi > > This all sounds entirely logical (and very handy from animplementation > point of view :) ) > but it does seem to differ from what Doug Milliken says so I'm notquite > sure what to think. > I've also read the bit in RCVD about this and again it seemsimcompatible > with what you > have said. Am I somehow not understanding what Doug says and you doagree? > >

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> Disagree is probably a bit too strong a word. Doug is working with > traditional ideas derived from traditional tire testing, and within that > context he is perfectly correct. What I am trying to say is that those > traditional models are inadequate and have led to false assumptions and > poorly implemented tire models in sims. Unfortunately I don't have the > necessary data to propose an adequate alternative. I'm really trying to > encourage programmers and any lurking engineers alike to think outside > the box and come up with something new. A wheel is basically a rim with > a couple of highly damped springs under it connected to a contact patch > whose size varies with pressure. Anything that strays far from that > concept is probably going to give you the wrong answer at some point. > What I have seen of Pacejka (sp?) seems to ignore this and instead just > try to model existing tire data. That seems fine on the surface, except > that as I have tried to point out existing data is not formulated in a > very practical way. Camber thrust, such as it is, could easily be > explained by the wheel, spring and contact patch model, and looking at > it that way it should also be clear what the limitations of camber > thrust are. Sorry, I guess I am not really giving you a very clear > answer, I'm just trying to stir the pot a little, get people to throw > away the old models, and see what new things they can come up with. > Again, I would love to see somebody take on a real time FEA tire model. > I know non-real-time FEA models are being done in the industry and at > universities, and I personally believe that we now have enough computing > power to do a simplified (i.e. small number of elements) FEA model in > real time in a computer game. Codemasters is already doing real time > FEA damage simulation in Pro Race Driver, for example. Not being very > helpful, am I? ;o)

Oh I don't know, u have made me think a bit. Like I said, the info you posted seemed entirely logical to me. I've never hear of the an FEA model though what am I missing? (although I have found a couple of PDFs to read about it) Thanks. Paul

FEA stands for Finite Element Analysis, which you have undoubtedly heard of. I guess FEM (Finite Element Method) is the more common abbreviation. Anyway I was not referring to any specific model, although I have seen papers on some models somewhere (SAE?) but I was referring in general to the idea that the tire model ought to be a problem that could be solved through application of FEA/FEM. I don't even think the whole tire has to be modeled, just a sector from slightly in front of the contact patch to slightly behind, and then put appropriate boundary conditions on that sector based on more traditional models. Plus with racing slicks you wouldn't have to model the tread, so you could conceivably get away with fewer elements. Just thinking out loud though, I have never actually tried to apply FEA to the tire model myself. "Paul Laidlaw" <[email hidden]> wrote in message news:[email hidden]...

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> > Oh I don't know, u have made me think a bit. Like I said, the info you > posted > seemed entirely logical to me. > > I've never hear of the an FEA model though what am I missing? > (although I have found a couple of PDFs to read about it) > > Thanks. > > Paul > >