Toe Changes for Track Use

[EMWHYR0HEN]

Cameron

 

The way you describe how you car handles seems similar to mine. I'm actually surprised that your not having much understeer problems with a big front bar, no rear bar, and stiffer front springs.

 

I think stiffer rear springs will help balance out the car a little better and reduce squat under acceleration. I would try just bumping the rear springs up first and feeling that out. I don't know if you've installed droop limiters on you car but, I've noticed a reduced amount of roll during cornering after I installed some. The car feels much flatter throughout the entire turn.

 

Have you considered a bigger rear wing or wider rear tires to help corner exit speed? What speeds are we talking about here?

[heavy85]

 

I think stiffer rear springs will help balance out the car a little better and reduce squat under acceleration. I would try just bumping the rear springs up first and feeling that out. I don't know if you've installed droop limiters on you car but, I've noticed a reduced amount of roll during cornering after I installed some. The car feels much flatter throughout the entire turn.

 

Have you considered a bigger rear wing or wider rear tires to help corner exit speed? What speeds are we talking about here?

 

Wider rear tires are not in the works at least for now (running the Rota group buy rims) but I'm working on adding a spoiler below the rear wing and some additional front end aero work. Since I have 375 up front I was thinking of swapping those to the rear but then have to decide on rate to replace the front with. Removing the rear bar helped so I dont want to upset the balance too far but I think I really need better front grip. Maybe I'll get excel working and try the magic number spreadsheet ... maybe. I keep thinking about droop limiters and even bought cables last year just haven't had time to try. You have any pics of your droop limiter set-up?

 

As far as the slip angle debate goes I think JohnC is right - it depends. To the best of my reasoning standing in the shower last night I think there are four primary contributors to the 'depends'.

 

1. Track width. The wider the more ackerman could help do to the smaller 'ideal' diameter the inside tire has to travel. This is probably insignificant in my guess.

 

2. Radius of the turn. Tighter the radius the more ackerman will help since the relative angle of the inside tire will be more than a larger radius turn.

 

3. Front end load transfer. The more the load transfer the more the outside tire will be at a higher ideal slip angle than the inside tire favoring less ackerman.

 

4. Tire's ideal slip angle. Higher ideal slip angle favors less ackerman similar to #3. This may partially explain Caroll Smith change from anti-ackerman to ackerman as bias tires (older technology) favor higher slip angles where radials like a tidier smaller slip angle. Were those Benneton cars on bias or radials back then?

 

Cameron

[JMortensen]

2. Radius of the turn. Tighter the radius the more ackerman will help since the relative angle of the inside tire will be more than a larger radius turn.

This is one of the things that is so cool about Ackerman vs toe out. Don't turn the wheel as much, and you have less Ackerman. Run a lot of toe out, and you have the wandering front end regardless of whether you need it on a given part of the track.

 

3. Front end load transfer. The more the load transfer the more the outside tire will be at a higher ideal slip angle than the inside tire favoring less ackerman.

It is always the name of the game to try and reduce weight transfer. Lower cg, wider track, whatever can be done should be done. But this slip angle on the inside tire argument still doesn't take into account where the tire is pointed. In RCVD they literally took an old plane and removed the landing gear and bolted it to the back of a truck. Then they put the tire to be tested on, and turned the tire away from straight at various angles, put a load on it with a nitrogen cylinder and DRAGGED it behind the truck and used force gauges to measure the lateral force generated, and that's how they came up with the ideal slip angle for a vertical load. When you look at the graph that Leon posted, it only tells you what angle and what load produces the highest amount of force. It does not say that the highest load slip angle should be coupled with the lowest load slip angle to produce the best cornering speed. It would be more accurate to say that at low speed with little downforce, the slip angles for maximum cornering traction will be lower than at high speed with lots of downforce.

 

Graph 2.46 has forces that are more reasonable for comparing to a Z, and it shows a 2 degree difference between the tire loaded at 800 lbs vs the one loaded at 400 lbs. Just as running 2 degrees of toe in would be insane and stupid, likewise trying to get the inside tire to point 2 degrees towards the outside in an attempt to maximize it's traction doesn't make any sense at all.

 

The slip angle is the angle that an individual tire travels relative to its own intended path, not related to the other tire or the set divided by 2 (fig 2.33 on p.62 makes this clear). Pointing the inside tire towards the outside of the turn you're trying to make is almost never a good idea. If you have the outside tire determining the radius of the turn you're making, you really do not want the inside tire making its most efficient force while pointing to the outside of that radius. You're going to lose some grip that way.

 

4. Tire's ideal slip angle. Higher ideal slip angle favors less ackerman similar to #3. This may partially explain Caroll Smith change from anti-ackerman to ackerman as bias tires (older technology) favor higher slip angles where radials like a tidier smaller slip angle. Were those Benneton cars on bias or radials back then?

I don't think that bias ply tires favor anti-ackerman. Certainly wasn't the case on that 510.

[bjhines]

I dunno JM..

 

I see your TOE-OUT thinking here... You are thinking the inside tire is scrabbling to pull the car around the turn. Since it doesn't do much anyway, then why not have it pulling inward in the turn.

 

But Leon has a point you should try to get your head around. The outside tire is working BEST at a fairly high slip angle. The inside tire is working best at a reduced slip angle in the same highly loaded turn. That would indicate slight toe-in.

 

But I agree that there are so many variables that can all affect everything else that it is hard to know why some things are working for most people. If you can get the thing sorted and fast you might end up with some odd settings that work for your car.

 

I built a super stiff chassis, and I sprung it lightly. I am going to see what works this season.

Edited December 22, 2010 by bjhines

[Leon]

Alright JM, believe what you want to believe, I'll talk to my F1 guy and see what he thinks. I'm sure he'll have some good insight.

 

1. Track width. The wider the more ackerman could help do to the smaller 'ideal' diameter the inside tire has to travel. This is probably insignificant in my guess.

 

2. Radius of the turn. Tighter the radius the more ackerman will help since the relative angle of the inside tire will be more than a larger radius turn.

 

3. Front end load transfer. The more the load transfer the more the outside tire will be at a higher ideal slip angle than the inside tire favoring less ackerman.

 

4. Tire's ideal slip angle. Higher ideal slip angle favors less ackerman similar to #3. This may partially explain Caroll Smith change from anti-ackerman to ackerman as bias tires (older technology) favor higher slip angles where radials like a tidier smaller slip angle. Were those Benneton cars on bias or radials back then?

 

Cameron, I think you have a decent idea of what's going on.

 

I'm not sure if the Benetton cars were running bias, but he was there around '00-'01 or so, I don't remember exactly.

[Leon]

I dunno JM..

 

I see your TOE-OUT thinking here... You are thinking the inside tire is scrabbling to pull the car around the turn. Since it doesn't do much anyway, then why not have it pulling inward in the turn.

 

But Leon has a point you should try to get your head around. The outside tire is working BEST at a fairly high slip angle. The inside tire is working best at a reduced slip angle in the same highly loaded turn. That would indicate slight toe-in.

 

But I agree that there are so many variables that can all affect everything else that it is hard to know why some things are working for most people. If you can get the thing sorted and fast you might end up with some odd settings that work for your car.

 

I built a super stiff chassis, and I sprung it lightly. I am going to see what works this season.

 

Thanks BJ, I got tired of repeating myself.

 

I also agree that there are plenty of things to play with with regards to suspension setup, but ideal slip angles are what you are ultimately trying to achieve with a given setup. JM, how can you say that the ideal slip angle wont create more lateral force? That is the whole point of optimizing slip angle!

[JMortensen]

Thanks BJ, I got tired of repeating myself.

 

I also agree that there are plenty of things to play with with regards to suspension setup, but ideal slip angles are what you are ultimately trying to achieve with a given setup. JM, how can you say that the ideal slip angle wont create more lateral force? That is the whole point of optimizing slip angle!

OK, I think I really nailed it this time. Read carefully and I think you'll get the picture.

 

The ideal slip angle will give more lateral force, what you've suggested won't because the lateral force is reduced, and what is there comes at the expense of the tires fighting each other. Each wheel has it's own slip angle as shown in 2.33, and a tire's slip angle is relative to the path of its own travel, not the other tire. As I said, in the other graph which has vertical loads more like what a Z would see, the optimal slip angle comes at 2 degrees less for the less loaded tire. So if you ran parallel steer and .412" toe in (this would give you 2 degrees of toe in on a 225/50/15 wheel), then you would have your theoretical maximum grip when the outside tire hit its perfect slip angle according to your theory. Nobody runs that. Why not?

 

Now if you could steer (and even better camber) each tire independently, then it would be possible to do what you want to do and get the maximum out of each tire around every corner. But since the radius of every turn is different, the toe requirement on the inside tire relative to the outside is going to be different on every turn. The outside tire determines the path of travel for the most part as stated before due to its taking the weight transfer. The inside, in order to hit this perfect amount of grip, would then have to somehow sense the radius of the turn and the slip angle of the inside tire, and then point 2 degrees less on ITS OWN path, not relative to the outside tire, to maximize traction. If it were possible, you might even be able to turn the inside tire first to get the Ackerman effect on corner entry, and then bleed off the toe out until you got to the optimal angle. That would be seriously cool. Unfortunately this is currently un-possible.

 

One last example of why it is not going to happen, then I'll call it quits and let you guys figure it out for yourselves. Staniforth has a graph on p70 of turn radius and degree of toe change in order to maintain perfect Ackerman steering. A turn of 40 foot radius has a toe difference of 1 degree for the tires to run the appropriate paths around the turn with no scrubbing. A turn with a 15 ft radius has a 6.5 degree difference if the wheels just follow the curve with no scrubbing. So let's say you want to set toe to optimize lateral acceleration on both tires for both corners.

 

On the large radius turn, if you run the prescribed .412" inches of toe in that you get from misreading the chart in RCVD your results are not too terrible. The outside tire is running 6 degrees of slip angle and defines the corner's arc with its maximum traction at the perfect slip angle. The inside tire needs 2 degrees less slip angle, so it needs 2 degrees less turning angle relative to the true path of the wheel, which was supposed to be 1 degree sharper to begin with. So it requires really one degree of toe in to get 2 degrees less slip angle. Not so bad.

 

Continuing on, you're out of that corner and down the straight and into the hairpin with the 15 ft radius. You cruise into this corner with your .412" of toe in and parallel steer in your Z. Let's see what happens... You need 6.5 degrees more toe out on the inside of this corner, but you have 2 degrees of toe in, because you misread the chart. This time, you're FUBAR. Now with your goal of 4 degrees of scrub on the inside you would need 4.5 degrees of toe out relative to your outside tire to hit your max lateral acceleration mark on the inside tire. But you don't have anything like it. Your car understeers off the side of the track and into a badly positioned corner workers' stand, and is a complete write off. Oh, that sucks!

 

So you can see from this example when static toe in on a parallel steer car works; when the radius of the corner is VERY LARGE. It's going to work well on those super speedways that you always see Z cars running on, which coincidentally, is where you see real racers using AA or toe in.

 

This example disregards the effect of Ackerman at corner entry, which Rouelle would argue is vastly more important than optimizing the toe setting for max lateral acceleration on the inside tire in mid corner.

 

Don't like my explanation? Fine. Don't listen to me. Try it and see how bad it sucks. Toe settings cost nothing but about 5 minutes of your time.

Edited December 22, 2010 by JMortensen

[Leon]

OK, I think I really nailed it this time. Read carefully and I think you'll get the picture.

 

The ideal slip angle will give more lateral force, what you've suggested won't because the lateral force is reduced, and what is there comes at the expense of the tires fighting each other. Each wheel has it's own slip angle as shown in 2.33, and a tire's slip angle is relative to the path of its own travel, not the other tire. As I said, in the other graph which has vertical loads more like what a Z would see, the optimal slip angle comes at 2 degrees less for the less loaded tire. So if you ran parallel steer and .412" toe in (this would give you 2 degrees of toe in on a 225/50/15 wheel), then you would have your theoretical maximum grip when the outside tire hit its perfect slip angle according to your theory. Nobody runs that. Why not?

 

But the two wheels are solidly connected, how in any way can the slip angle of the two front wheels be independent as you imply? I completely understand that each tire's slip angle is defined by the difference between wheel centerline and actual path, but that in no way means that the slip angles left to right are independent. There could be myriad reasons why nobody runs that, such as it compromises other aspects of handling (which any suspension setup does). I am not arguing this, what I've been trying to get through to you is that the ideal toe angles in high speed corners are counter to what most people believe. I think I've gotten you to understand this, maybe somewhat.

 

Now if you could steer (and even better camber) each tire independently, then it would be possible to do what you want to do and get the maximum out of each tire around every corner. But since the radius of every turn is different, the toe requirement on the inside tire relative to the outside is going to be different on every turn. The outside tire determines the path of travel for the most part as stated before due to its taking the weight transfer. The inside, in order to hit this perfect amount of grip, would then have to somehow sense the radius of the turn and the slip angle of the inside tire, and then point 2 degrees less on ITS OWN path, not relative to the outside tire, to maximize traction. If it were possible, you might even be able to turn the inside tire first to get the Ackerman effect on corner entry, and then bleed off the toe out until you got to the optimal angle. That would be seriously cool. Unfortunately this is currently un-possible.

 

That would be cool, and wouldn't you know it, I have done a bunch of research and a paper on the topic.

 

One last example of why it is not going to happen, then I'll call it quits and let you guys figure it out for yourselves. Staniforth has a graph on p70 of turn radius and degree of toe change in order to maintain perfect Ackerman steering. A turn of 40 foot radius has a toe difference of 1 degree for the tires to run the appropriate paths around the turn with no scrubbing. A turn with a 15 ft radius has a 6.5 degree difference if the wheels just follow the curve with no scrubbing. So let's say you want to set toe to optimize lateral acceleration on both tires for both corners.

 

On the large radius turn, if you run the proscribed .412" inches of toe in that you get from misreading the chart in RCVD your results are not too terrible. The outside tire is running 6 degrees of slip angle and defines the corner's arc with its maximum traction at the perfect slip angle. The inside tire needs 2 degrees less slip angle, so it needs 2 degrees less turning angle relative to the true path of the wheel, which was supposed to be 1 degree sharper to begin with. So it requires really one degree of toe in to get 2 degrees less slip angle. Not so bad.

 

Continuing on, you're out of that corner and down the straight and into the hairpin with the 15 ft radius. You cruise into this corner with your .412" of toe in and parallel steer in your Z. Let's see what happens... You need 6.5 degrees more toe out on the inside of this corner, but you have 2 degrees of toe in, because you misread the chart. This time, you're FUBAR. Now with your goal of 4 degrees of scrub on the inside you would need 4.5 degrees of toe out to hit your max lateral acceleration mark on the inside tire. But you don't have anything like it. Your car understeers off the side of the track and into a badly positioned corner workers' stand, and is a complete write off. Oh, that sucks!

 

This portion of your argument is invalid for a reason that I already mentioned. The definition of perfect Ackerman steering is when your wheels follow a path that produces no tire scrub, with neglected lateral forces and slip angles. It's purely, solely geometric steering. Imagine solidly connecting two bicycles and finding the individual steering angles that create no scrub at low speed. These angles are going to be different, even on the same corner when you increase velocity. All this stuff about Ackerman steering is valid for low speed turning.

 

It is a very typical mistake of assuming "Ackerman steering" (not Ackerman steering angle) has anything to do with high speed driving.

 

Now you are correct that different corners need different toe angles because lateral acceleration (a=v²/r) is different, therefore lateral force is different, ergo load transfer is different.

 

So you can see from this example when static toe in on a parallel steer car works; when the radius of the corner is VERY LARGE. It's going to work well on those super speedways that you always see Z cars running on, which coincidentally, is where you see real racers using AA or toe in.

 

Yes, it works on the super speedway, but how about a long sweeper or a high speed kink, etcetera? Everything is a trade-off, it's up to the user to decide what is best for each sitation. I'm just providing the facts.

 

This example disregards the effect of Ackerman at corner entry, which Rouelle would argue is vastly more important than optimizing the toe setting for max lateral acceleration on the inside tire in mid corner.

 

Don't like my explanation? Fine. Don't listen to me. Try it and see how bad it sucks. Toe settings cost nothing but about 5 minutes of your time.

 

No problem, I'll report the results if I'm able to make up a proper, valid test for it.

 

Again, suspension setup is a series of compromises involving multiple variables, as other posters have alluded to. What will work for one track will not work for another, let alone one corner to another. Take my information for what it's worth, a small explanation of vehicle dynamics so that hopefully someone can take advantage of the knowledge and have a better understanding of what they're doing. I hate relying just on anecdotal evidence espectially the stuff that's been through many mediums. It becomes a game of broken telephone (or whatever you call it) where information is taken to be gospel without actually asking the last guy what the heck he's talking about. It happens all the time, and the guys that have been "around it for 60 years," while usually full of good advice, tend to be the worst at spreading false truths as well since they are never questioned on the reasons behind their statements.

 

FWIW,

Leon

[JMortensen]

But the two wheels are solidly connected, how in any way can the slip angle of the two front wheels be independent as you imply? I completely understand that each tire's slip angle is defined by the difference between wheel centerline and actual path, but that in no way means that the slip angles left to right are independent.

I really feel like we're getting somewhere here, so I'm back to drive the point home.

 

The slip angle can be different on each side of the car because the steering angles can change relative to each other on each side of the car. If you're looking at the slip angle of the car as a whole, you're right but that's a pretty simplistic way of viewing slip angle. Why not look at the tires individually? The loaded side dominates, and determines the path of the car as a whole. Call it 0 toe, because where it points is where the car goes. I don't mean that literally, but even when you're sliding, it's the loaded tire and the amount of steering input that is going to determine the path of the car. If you come at it with this realization that the outside tire is at 0 toe, you can see how the inner wheel can be toed in or out independently using a static toe setting, bumpsteer, Ackerman, or all 3. The inside tire is the "victim" of the outside tire, kind of like being dragged behind the truck on the landing gear, but it is going to do its own thing and its individual slip angle will be a result of the loads, its toe in relation to the inside tire, and the direction the car is traveling (dragging it at). I think this gets us back to the static vision of toe that you seem to have. The inside tire can have toe in or toe out relative to the outside tire, and the effect could be a higher or lower slip angle.

 

The slip angles have to be able to be independent. If they aren't then your idea of running toe in to change the slip angle of the inner tire doesn't make any sense, right?

Edited December 22, 2010 by JMortensen

[thehelix112]

That would be cool, and wouldn't you know it, I have done a bunch of research and a paper on the topic.

 

Citation please. Not that I'm likely to be able to understand it, but someone might be able to.

 

Dave

 

PS. Incidentally, if I ever get my car going and visit Laguna Seca I'll be inviting both Leon and Jon to come help. Oh and bringing popcorn.

Edited December 22, 2010 by thehelix112

[thehelix112]

...The front end pushes at the limit but the back end has a habit of getting squirely putting the power down on track out...

 

Maybe some more information on when the front end pushes would help the conversation along. Is it most prominent during turn-in, steady state, corner exit? All corner? Slow, medium, fast corners? On corners that are part of a sequence more so than ones that are at the end of a straight?

 

WRT putting the power down, when does it happen? Immediately? After the car has squatted? After the car has rolled (transferred weight)?

 

Note I dunno what to do with the answers to these questions, but someone with more experience likely will.

 

Dave

[redneck1545]

id put a quote box around everything that has been said but whats the point in quoting two pages???

some really GREAT info on this page...MAYBE if we get to where we can agree on things it could be a sticky in the making

 

in the mean time all i have to say is, this is worse than physics class...

 

 

[johnc]

MAYBE if we get to where we can agree on things it could be a sticky in the making

.

 

Never happen. Each person needs to gather the information and make their own decision.

[EMWHYR0HEN]

This is a super long debate over static toe in/out settings. We can argue over what works best theoretically but, I say play with it for your self and see what works best for your car and on different tracks.

 

Cameron, I haven't taken any pics of my droop limier setup. The mounting points are similar to JM's but I used SS cable to limit the droop. I'll be sure to take some pics after the Holidays or when it stops raining.

Edited December 23, 2010 by EMWHYR0HEN

[Leon]

I really feel like we're getting somewhere here, so I'm back to drive the point home.

 

The slip angle can be different on each side of the car because the steering angles can change relative to each other on each side of the car. If you're looking at the slip angle of the car as a whole, you're right but that's a pretty simplistic way of viewing slip angle. Why not look at the tires individually? The loaded side dominates, and determines the path of the car as a whole. Call it 0 toe, because where it points is where the car goes. I don't mean that literally, but even when you're sliding, it's the loaded tire and the amount of steering input that is going to determine the path of the car. If you come at it with this realization that the outside tire is at 0 toe, you can see how the inner wheel can be toed in or out independently using a static toe setting, bumpsteer, Ackerman, or all 3. The inside tire is the "victim" of the outside tire, kind of like being dragged behind the truck on the landing gear, but it is going to do its own thing and its individual slip angle will be a result of the loads, its toe in relation to the inside tire, and the direction the car is traveling (dragging it at). I think this gets us back to the static vision of toe that you seem to have. The inside tire can have toe in or toe out relative to the outside tire, and the effect could be a higher or lower slip angle.

 

The slip angles have to be able to be independent. If they aren't then your idea of running toe in to change the slip angle of the inner tire doesn't make any sense, right?

 

Let me sum my answer up for this one. Clearly, the inside tire can have toe in or out relative to the outside tire, that's what this whole discussion is about.

 

However, you don't seem to understand what independent means, maybe this is a semantic issue. Yes, the wheels each have individual slip angles that may or may not be equal. Notice I used individual and not independent, those are two completely different things.

 

The slip angles are dependent on one another because the left and right wheels are connected by links. I can write you an equation that relates left slip angle to right in a certain situation, e.g. given velocity and turn radius. What does that mean? It means that one is dependent on the other.

 

Here's an example, say your car goes around the same exact corner, at the same exact speed, with the wheels traveling over exactly the same spots over and over. Given the slip angle that is produced on the outside wheel, is the inside wheel going to have different slip angles every time (or anytime?). Can you do anything, given these parameters, to change the inside tire's slip angle without changing the outside tire? The answer is no to both, and that means they're dependent.

 

I think we should have that out of the way now.

 

I'm curious if you have any response on the more pertinent parts of your argument?

[Leon]

Citation please. Not that I'm likely to be able to understand it, but someone might be able to.

 

Dave

 

I can't do that, at least not now. I'd need consent from some people.

 

PS. Incidentally, if I ever get my car going and visit Laguna Seca I'll be inviting both Leon and Jon to come help. Oh and bringing popcorn.

 

Well, Laguna is less than 2 hours from me. I'm sure I'll be doing track days there when my Z is finally sorted.

[heavy85]

I have been thinking more precisely how the car behaves plus watched in-car videos. Here is goes. Typical turn starts by braking hard then ease off while turning in (I tend to trail brake a lot). As soon as it takes a set start to ease on the throttle. Keep easing on the throttle to try to hit the track out point without moving the steering wheel. I can easily modulate the throttle to move the front end around. More throttle and more the front end washes out, less throttle the less it washes out. I typically cant hit WOT until I'm nearly straight. This is if all goes well, if I apply too much throttle too fast the rear steps out dorifto style.

 

In general this is fine and controllable but I think I should be able to hit WOT without having to wait for the wheel to be nearly straight. I dont have THAT much power as it's just a completely stock LS1. This is where I'm thinking more front grip and better diff may help. So any more thoughts on the factory LSD vs the OBX for this application? Obviously throttle will transfer weight off the front tires thus giving them less grip but I think there is a set-up that could perform better in this scenario than what I have now if I could find it.

 

Cameron

 

PS on the debate: back in FSAE in college the rule of thumb was that smaller rims and taller sidewall produced a relatively flat and wide slip angle curve so was very forgiving while bigger rims with shorter sidewall produced more peak grip but with a very narrow range of usable slip angles. We used the small rim combo since we weren't pro drivers. This may also explain some of the reversion in Carrol Smith from the anti-ackerman to ackerman set-up as cars have migrated to larger and larger diameter rims over the years.

[JMortensen]

I think we should have that out of the way now.

Yep. That was a semantics issue for sure.

 

I don't have anything more really that I haven't already said. I did find the site below which has a bunch of stuff supporting what I have said, and also has some interesting views on anti-Ackerman and definitely supports using both in different situations. Unfortunately it quotes Carroll Smith before he retracts his view supporting anti-Ackermann and quotes Staniforth in what must have been an earlier version of Competition Car Suspension where he is not yet totally convinced about the benefits of Ackerman. According to the page AA is more useful than I gave it credit for, but if you look, the places where they suggest it to be used are in very high speed cars, NASCAR and Australian V8 Supercars, etc. and very fast corners, which are both places where you want stability and don't want the drag that might be associated with a lot of Ackerman or toe out. You might note that there are some tire traction graphs there that curve the opposite way from the one you posted, suggesting for the tire tested Ackerman would be helpful even by your standards because the inside would need a higher slip angle to achieve maximum grip. After reading the page I wish I had those RCE magazines from 2001. Looks like Erik Zapletal took the time to prove the "LF brake on the roundy round car" theory, which in and of itself might be enough to justify using Ackerman in a Z which IME has a pretty strong tendency towards understeering.

 

http://www.smithees-racetech.com.au/ackerman.html

 

Here is the meaty part:

When Would You Use Ackerman (or Anti-Ackerman)?

 

• When you set the negative camber, based on the tyre temperature readings for instance, you are maximising outside tyre grip, at the expense of inside tyre grip. Toe out helps to compensate for negative camber on the inside tyre. This indicates pro-Ackerman might be usefull for cars carrying a lot of negative camber.
• In using Ackerman steering we hope to be able to influence the slip angle on the inside tyre to our advantage. There will be a range of slip angles where the inside tyre will be producing near maximum grip (Figure 3). So we have a degree of flexibility in how much Ackerman we use.
• To rotate the car on corner entry we are talking about creating increasing drag at the inside tyre. As the cornering force builds the inside tyre must at some point reach it's optimum lateral grip. We then use Ackerman to toe the tyre out further - say increase the slip angle a couple of degrees. The tyre grip doesn't change that much but the longitudinal component of tyre grip, the tyre drag, does increase in line with the increased slip angle. For this to work we would need to know that we have sufficient steering angle to generate the Ackerman needed.
• If in the process above, we started to loose outside tyre grip, and the driver wound on some more lock, we would have increased drag at the outside tyre. We would then loose the effect. The oversteer torque we were looking for would be overcome by the larger understeer torque.
• The above indicates that pro-Ackerman would probably not work with low powered cars in fast corners. It might also be a problem generally with heavy cars with spool or locker diffs that might want to push a bit, such as V8 Supercars.
• With pro-Ackerman, the higher slip angle on the inside tyre will put more heat into the tyre. This will help bring the tyre up to temperature, but could overheat the tyre on a longer run.
• If our race car is faster with toe in, we will use anti-Ackerman. This implies a tyre curve where the lightly loaded inside tyre has maximum grip at a lesser slip angle (Figure 6a)
• Sprint cars and similar speedway, dirt short circuits, can make a lot of use of varying degrees of pro-Ackerman. With dirt tyres we expect very large slip angles. Nascars and similar will use anti-Ackerman (Figure 6a).
• With low profile tyres the slip angles will be a lot less. The tyre drag will be less. The slip angle on the inside tyre will have a smaller drag component (Figure 8). So it may be more difficult to use pro-Ackerman to create the oversteer torque. The toe out from the slip angles will be less. The slip angle variation from outside to inside tyre will be a smaller number, requiring different Ackerman to achieve what we want.
• We will probably use initial toe out to help turn in. The idea is to get the inside tyre working as discussed earlier. Other settings you would use to help initial turn in are stiffer front shocks, and higher front roll centre height. By delaying the roll we help to keep the weight on the inside, to again keep the inside tyre working.
• We make the assumption that the outside wheel will always have the ideal trajectory, with all the toe out being seen at the inside wheel. This may not always be the case. For instance, if the car has a lot of caster and/or caster trail this might have the effect of splitting some of the toe to the outside wheel. If the outside wheel does take on some of the toe out, this will decrease slip angle and the outside wheel will loose grip.

 

I also found this little video, but I think it would be better if they started with no slip angle graphs, and then turned the wheels, added the graphs, added the Ackerman, changed the graphs, etc. As it is, I don't think it shows much of anything, but it does seem to suggest that the Ackerman relationship still exists after slip starts, because the lines still point to the instant center after the slip is accounted for, and they still wouldn't if you made the same movement with the parallel steer. It's more of a "what could have been", but I thought you might like to see it.

 

http://www.racecartuner.com/03/207.html

[johnc]

Changed the topic title for you guys.

[tube80z]

From what I remember of the Roulle seminar the ackerman versus anti-ackerman argument depended on the tire construction. It had to do with how the peaks of force versus slip angle changed. So what works on one set of tires may not necessarily work or be right for the next set. Neil Roberts didn't think it was too important for road course work but also mentioned you might as well set it at 100% to make pushing the car in the pits easier. My personal feel is this will be more important for tracks with slower corners and more steering angle (more autox like).