Toe Changes for Track Use

[260DET]

"Optimumise the end of the car that slides first, then move to the opposite end of the car. For example, if the car understeers when cornering, optimumise the front susprension alignment, wheel width, tire size and tire pressure". Tire Rack expert advice, posted because I agree and the words of an expert carry more weight

 

Ackermann. There are more geometry changes involved I believe than just plain simple slip angle changes, where modern cars with say ~9 degrees caster are involved anyway. Actually I ran this this question here a while ago and got no response but from my observations there is a lot going on when when you turn the steering wheel of a good handling strut suspended modern car. Strut suspension is open to all sorts of design tweaks, make a good 'ask the expert' topic.

Edited December 24, 2010 by 260DET

[Leon]

I was on vacation for a few days and decided to take a look through Milliken & Milliken to see what they say about Ackermann as it pertains to this discussion.

 

Here is a direct quote from the Milliken book.

 

For low lateral acceleration usage (street cars) it is common to use Ackermann geometry. This geometry ensures that all the wheels roll freely with no slip angles because the wheels are steered to track a common turn center. Note that at low speed all wheels are on a significantly different radius, the inside front wheel must steer more than the outer front wheel.

 

High lateral accelerations change the picture considerably. Now the tires all operate at significant slip angles and the loads on the inside track are much less than on the outside track. Looking back to the tire performance curves, it is seen that less slip angle is required at lighter loads to reach the peak of the cornering force curve. If the car has low speed geometry (Ackermann), the inside front tire is forced to a higher slip angle than required for maximum side force. Dragging the inside tire along at high slip angles (above the peak lateral force) raises tire temperature and slows the car down due to slip angle induced drag. For racing, it is common to use parallel steering or even reverse Ackermann.

Milliken & Milliken, RCVD, p. 713-715 in Ackermann Steering Geometry, section 19.2. Some words are bold in the book and others I made bold for emphasis.

 

 

The Millikens' perspective echoes exactly what I've been trying to explain.

 

As the Millikens point out, when you're racing the turns are relatively large radius, thus Ackermann effects are minimized. Here is where your static alignment settings come into play.

 

What does this mean and how does it tie back into our toe settings? The Z has parallel steer, meaning that if you draw a line along the longitudinal center-line of the tires while they are steering, those lines will never intersect (theoretically). In other words, the relative angle between the wheels does not change as you turn the steering wheel. When you make static toe changes you are changing the relative angle between the steered wheels.

 

For example, let's say we set the front wheels with static toe-out. We are on a road course, so it's safe to assume large radius turns. Now when you turn the steering wheel, the inside wheel will always be at a higher steering angle than the outside wheel, meaning that the inside slip angle is going to be higher than the outside slip angle (unless the car tears in half). Think of this as psuedo-Ackermann steering. This is the opposite of what we want in a high speed corner. In reality, whether you're on a road course or an autoX run, there's more to handling than steady-state corners. I am not saying that certain, or any, suspension settings are always good or always bad. What I am saying, and have been trying to explain, is that setting static toe-out will decrease the capability of the front of the car to generate lateral force (in steady corners) because of the aforementioned phenomena, and thus introduce more understeer than with static toe at zero or slight toe-in.

 

To sum this up, this is a discussion purely about understanding the effects of steering geometry and toe-changes on handling. A properly set up suspension will factor in all facets of wheel alignment (toe, camber, caster) and suspension geometry (camber curves, bump steer, ad nauseum) along with the desired needs from the vehicle in order to get satisfactory performance. However, a more informed person will be better and quicker at making the right decisions.

Edited December 30, 2010 by Leon

[Leon]

I'd like to also add that the single biggest element in vehicle handling is the tires. Without proper tire data, it is much more difficult to predict vehicle behavior. What works for one tire may not work for another. Tire characteristics are what ultimately drive suspension setup. In order to keep it apples-to-apples the tires on our imaginary vehicles must be the same, otherwise the entire discussion is moot.

[gnosez]

Okay, there are only a few more hours left in 2010 so I using them to ask a question or more that I'm likely to get flamed for.

 

What exactly can be done to add/reduce Ackermann in an S30? What parts need to be purchased, made or removed to do this? Price/time? And just how much of a faster lap time would it result in? Since there are so many variables going on as the car transitions from straight to cornering I wonder if driver input has more of an impact than Ackermann (pos/anti) ever will.

 

Why when I ask about Ackermann at race shops that built and ran S30 cars back in the day do they look at me like I have two heads (do I)?

 

And a few for BJ - would you care to share your spring/shock combo setting for your rigid race car? Do you run a rear sway bar?

 

I just guessing here but I can see this discussion lasting through 2011...

 

Happy New Year's

[JMortensen]

For example' date=' let's say we set the front wheels with static toe-out. We are on a road course, so it's safe to assume large radius turns. Now when you turn the steering wheel, the inside wheel will always be at a higher steering angle than the outside wheel, meaning that the inside slip angle is going to be [i']higher[/i] than the outside slip angle (unless the car tears in half). Think of this as psuedo-Ackermann steering. This is the opposite of what we want in a high speed corner. In reality, whether you're on a road course or an autoX run, there's more to handling than steady-state corners. I am not saying that certain, or any, suspension settings are always good or always bad. What I am saying, and have been trying to explain, is that setting static toe-out will decrease the capability of the front of the car to generate lateral force (in steady corners) because of the aforementioned phenomena, and thus introduce more understeer than with static toe at zero or slight toe-in.

Assuming that you started with the tire that produced the chart that you posted, I'm with you right up to the last sentence here. If you were racing on a super speedway, I'd be with you all the way, because the turns are so large that a slip angle that would produce the lateral acceleration desired on the inside wheel might very well require toe in. In reality though, Z racing doesn't usually involve really long steady state cornering, at least at the tracks that I've been to, and certainly not at autocrosses. For that kind of racing, the slip angle still has to be found relative to the individual wheel's intended path, and that path is going to require a lot more toe out on the inside wheel for a lot of those turns. Running a lot of toe out is a stop gap measure for those that don't or can't make the Ackerman changes required. It would also be possible, assuming that one could get a lot of Ackerman in a Z (I don't think my rack modification got me 100% Ackerman) that one could then run toe in, so that on those long sweepers you might get a result closer to what the tire needs for optimal traction, but when you get to the hairpin the inside tire turns a lot sharper as it needs to do.

 

If you use the chart from the Smithee's website then I think Ackerman or toe out would still be a better way to go, assuming the drag increase didn't slow the car more than the lateral accel improved it (something relevant to both the pro and anti-Ackerman cases).

 

 

 

John, Ackerman can be added by moving the steering rack back towards the crossmember, or by lengthening the steer knuckles, or by cutting and welding or bending the steer knuckles out towards the rotors. Toe out is free, Ackerman is hard to do, that's why most every Z racer runs static toe out.

Edited December 31, 2010 by JMortensen

[Leon]

You missed some important points in my post. I'll quote them from the previous posts for clarification. Italics are my posts.

 

Assuming that you started with the tire that produced the chart that you posted, I'm with you right up to the last sentence here. If you were racing on a super speedway, I'd be with you all the way, because the turns are so large that a slip angle that would produce the lateral acceleration desired on the inside wheel might very well require toe in. In reality though, Z racing doesn't usually involve really long steady state cornering, at least at the tracks that I've been to, and certainly not at autocrosses. For that kind of racing, the slip angle still has to be found relative to the individual wheel's intended path, and that path is going to require a lot more toe out on the inside wheel for a lot of those turns. Running a lot of toe out is a stop gap measure for those that don't or can't make the Ackerman changes required. It would also be possible, assuming that one could get a lot of Ackerman in a Z (I don't think my rack modification got me 100% Ackerman) that one could then run toe in, so that on those long sweepers you might get a result closer to what the tire needs for optimal traction, but when you get to the hairpin the inside tire turns a lot sharper as it needs to do.

For example, let's say we set the front wheels with static toe-out. We are on a road course, so it's safe to assume large radius turns. Now when you turn the steering wheel, the inside wheel will always be at a higher steering angle than the outside wheel, meaning that the inside slip angle is going to be higher than the outside slip angle (unless the car tears in half). Think of this as psuedo-Ackermann steering. This is the opposite of what we want in a high speed corner. In reality, whether you're on a road course or an autoX run, there's more to handling than steady-state corners. I am not saying that certain, or any, suspension settings are always good or always bad. What I am saying, and have been trying to explain, is that setting static toe-out will decrease the capability of the front of the car to generate lateral force (in steady corners) because of the aforementioned phenomena, and thus introduce more understeer than with static toe at zero or slight toe-in.

To state this even simpler, you don't have to be on a super speedway to have large radius turns as you assume. Any corner where you're going "fast," turn the wheel, weight transfers (transient response), and you hold the wheel as the car turns is a steady state corner. This will happen on any track that has sweeping turns, and most road courses do. In this case, with a typical tire (as shown in Milliken), the car will understeer more. How dramatic of an effect is it? Probably not very, but for someone squeezing out every last hundredth it might just matter.

 

Notice I say nothing else. I'm not talking about whether a certain setting will make the car slower or faster around the track as a whole or how it will drive in an autoX. This is a cold hard fact and what it should be used for is for someone to be able to make a more well informed decision, e.g. you're at a track with a bunch of sweepers, how do you set up your suspension, etc.

 

The second bold statement is self-explanatory. Seems like you missed it.

 

If you use the chart from the Smithee's website then I think Ackerman or toe out would still be a better way to go, assuming the drag increase didn't slow the car more than the lateral accel improved it (something relevant to both the pro and anti-Ackerman cases).

 

Let me also repeat my last post.

In order to keep it apples-to-apples the tires on our imaginary vehicles must be the same, otherwise the entire discussion is moot.

 

There are a lot of different types of tires out there.

 

To sum it up (again), here is another part of my last post:

 

What works for one tire may not work for another. Tire characteristics are what ultimately drive suspension setup.

[JMortensen]

So for an imaginary track with no sharp turns or mostly sweepers, not considering turn in or other handling effects, and using the tire constraints from the graph you posted, you would like to posit that toe in would generate more grip. All right then, I concede this point to you. Let's try to put this back in the real world again, shall we?

 

I wish I knew the real steering ratio of the Z car in terms of steering wheel degrees to tire degrees. I really don't know what the ratio is in a Z, but I'd guess that most cars are in the 15 to one range. Here are various in car camera clips from various Z's at various tracks. I watched them and only noted how many times the wheel crossed 90 degrees in a lap by watching the spokes or the driver's hands (rudimentary, but it's what I have to work with). If you had a 90 degree turn at the steering wheel, that's going to be 6 degrees at the tire with the assumed 15:1 ratio. 100% Ackerman gives 6 degrees on the inside and 5 on the outside. The outside is the important one, so if my assumption of 15:1 is right in all of these cases the car would have more than a degree difference in the tires if it were running 100% Ackerman. Again, using a 225/50/15 tire, that's .206" of toe out (note how John Coffey somewhat coincidentally suggests .125 to .1875 toe out in his alignment sticky). This is a very significant amount of toe that is definitely going to affect handling. I wish I could have caught exactly what the steering wheel angle was, because of course the toe changes more radically as the wheel turns farther, but I think this makes the point. On a road course, the turns really are tight enough for Ackerman to make a difference, and I believe that's why so many Z racers run toe out on road courses.

 

Datsun 240Z at Laguna Seca has the wheel turning past 90 6 times in a lap:

 

Datsun 240Z at Streets of Willow - 5 times:

 

Datsun 240Z at Firebird Track West - 9 times:

 

Datsun 240Z at some track in Phoenix - 10 times:

 

Datsun 240Z at VIR - 4 times:

 

Datsun 240Z at Heartland Park Topeka - 8 times:

 

Datsun 240Z at an autox - 18 times (counted 19 but discounted one because I think it was a counter steer, hard to watch and count at the same time):

Edited December 31, 2010 by JMortensen

[Leon]

So for an imaginary track with no sharp turns or mostly sweepers, not considering turn in or other handling effects, and using the tire constraints from the graph you posted, you would like to posit that toe in would generate more grip. All right then, I concede this point to you. Let's try to put this back in the real world again, shall we?

 

I wish I knew the real steering ratio of the Z car in terms of steering wheel degrees to tire degrees. I really don't know what the ratio is in a Z, but I'd guess that most cars are in the 15 to one range. Here are various in car camera clips from various Z's at various tracks. I watched them and only noted how many times the wheel crossed 90 degrees in a lap by watching the spokes or the driver's hands (rudimentary, but it's what I have to work with). If you had a 90 degree turn at the steering wheel, that's going to be 6 degrees at the tire with the assumed 15:1 ratio. 100% Ackerman gives 6 degrees on the inside and 5 on the outside. The outside is the important one, so if my assumption of 15:1 is right in all of these cases the car would have more than a degree difference in the tires if it were running 100% Ackerman. Again, using a 225/50/15 tire, that's .206" of toe out (note how John Coffey somewhat coincidentally suggests .125 to .1875 toe out in his alignment sticky). This is a very significant amount of toe that is definitely going to affect handling. I wish I could have caught exactly what the steering wheel angle was, because of course the toe changes more radically as the wheel turns farther, but I think this makes the point. On a road course, the turns really are tight enough for Ackerman to make a difference, and I believe that's why so many Z racers run toe out on road courses.

 

Datsun 240Z at Laguna Seca has the wheel turning past 90 6 times in a lap:

 

Datsun 240Z at Streets of Willow - 5 times:

 

Datsun 240Z at Firebird Track West - 9 times:

 

Datsun 240Z at some track in Phoenix - 10 times:

 

Datsun 240Z at VIR - 4 times:

 

Datsun 240Z at Heartland Park Topeka - 8 times:

 

Datsun 240Z at an autox - 18 times (counted 19 but discounted one because I think it was a counter steer, hard to watch and count at the same time):

 

Once again, all I'm saying is that in a long sweeper the outside tire should be turned in more than the inside. That's it.

 

There are myriad other things to worry about when racing around a track, as I've been saying. If the car is in a corner where one tire is more loaded than the other, then it might benefit from toe-in, and I saw that happen quite a bit in every video. One big reason for setting static toe-out is to compensate for the effects of negative camber, and the Z sure has a bit of that when it's lowered.

 

By the way, the ratio of the steering wheel turning to the road wheel turning involves more than the steering box, you also have to factor in knuckle length.

[JMortensen]

Once again, all I'm saying is that in a long sweeper the outside tire should be turned in more than the inside. That's it.

Really? If that were the case, I wouldn't have continued arguing the point. Let me remind you what you started out saying:

 

It also looks like you set up your front end to understeer. As the outside tire gets loaded, it wants higher and higher slip angle to generate lateral force. With toe-out, more steering input would be required to generate a certain lateral force when compared to zero toe. I believe your rear end is also set for SS understeer. Your car must push quite a bit.

Maybe your position has changed since we started. If you'd like to limit it to "in a long sweeper the outside tire should be turned in more than the inside tire" than I think we can agree and end the argument.

 

By the way, the ratio of the steering wheel turning to the road wheel turning involves more than the steering box, you also have to factor in knuckle length.

Right. I was talking about steering wheel turns to tire angle change. The rack ratio is 1.8125 inches per turn. That means nothing with regards to the angularity of the wheel in and of itself.

[Leon]

Nothing about my argument has changed and what I said still holds true. It will understeer in steady state corners, exactly as I said originally in the post you quoted.

[gnosez]

So, is that any real data showing the results of adding/reducing Ackermann in an S30 body? If it hasn't been done it would need to limit as many other variables that are possible to control:

 

1) driver

2) tires

3) alignment (static)

4) track

5) temperature

6) weight

7) speed (corner entry)

8) position on the track

 

Run a car, record data, change the Ackermann, and run it again.

 

If, and this is a big if, you could duplicate your first set of test laps, the driver's input and lap times should answer the differences in opinion on this matter.

 

Until there's data (and data in my line of work is information that has been verified) this is, to me, beating a point to death. Theory is nice, civil discussion on points of contention is the reason Hybridz works, but for me I want to know if we're talking a minimal increase in performance or an increase that would warrant time and money to redo my Z.

[JMortensen]

I don't think you're going to get such a nicely refined test, because any test would require quite a bit of work between runs. Probably the least labor intensive would be to have two sets of steer knuckles, and swap between runs. You wouldn't want to use longer steer knuckles to test because that would slow the steering and that would change the feel to the point where it might hide the effect of the Ackerman, so it would have to be the cut and welded or the bent knuckles.

 

The proof to me is the ubiquitous toe out recommendation (which I have tried and found works in back to back runs) and is a rough simulation of the effect of adding Ackermann. If toe out works, Ackerman should work, because the effect is basically the same in toeing out the inside wheel. Ackermann has an advantage in that the tighter the course the more toe out you would need, and with the Ackerman just turning the wheel farther gets you more toe change.

 

I don't think you'll find a more tested opinion than Keith Thomas's when it comes to Z cars. Here is what he has to say on the subject of toe out:

http://www.improvedtouring.com/forums/showthread.php?t=16613

Here's my preference for things not so varied. Rear toe: I believe in the low drag setup' date=' zero or at most 1/32 total toe in. I refuse to correct bad handling at the back by dragging the tires around. At the front I used to like 1/32 to 1/16 in, but to overcome persistent low speed corner entry problems we eventually settled on 1/8 total toe out. If I can get away with less for some tracks I do (again, low drag), and I begin to squeal when I start needing over 3/32 total out to get it to turn in.[/quote']

 

Again, if Keith squeals when he needs a lot of toe out, that to me means a couple of things: 1. Toe out works and 2. He wouldn't have to run as much toe out if he had Ackerman, so Ackerman>toe out by itself in the case of the Z car.

[Zmanco]

This is a fantastic discussion and has really helped me to better visualize how various suspension settings affect handling. I am a bit surprised that there hasn't been much discussion of caster. Wouldn't adding caster be another way to improve turn-in and increase front grip?

[gnosez]

Well, since I run both the race car and my street car with zero toe in the rear and just under an 1/8 total in the front I guess all I can do now is change to a different set of knuckles or move the rack back.

 

I did run my street 240 with a 1/8 rear toe but after J.R. Mitchell (GMT Racing and former BSR crew member) drove the car he suggested zero rear toe. They both handle real well.

[JMortensen]

Why not just try toe out in the front? I used to have a reference line on the inner tie rod and two on the outer so I could run toe in on the street and toe out at the track. I'd drive to the track and make the change from one mark to the other, only takes a minute, and then when I was done I'd put it back to toe in for street use so I didn't chew up the front tires. The only downside was I had the wheel slightly off center in both toe settings, but it wasn't so far off that it was bothersome to me.

 

EDIT--Also, I don't think you'd get even close to 100% Ackerman with just modding the steer knuckles to get them closer to the rotor. You'd need the line drawn through the tie rod and ball joint to intersect at the rear axle line, which I don't think is possible on the Z. I don't think you can get 100% by moving the rack back either. I think with moving the rack I'll get something closer to 100% than the steer knuckle mod alone, but I'm doubtful that both together would actually get 100%.

 

Caster or other suspension settings that help front end grip are also relevant, but that's a subject for another thread and it's been talked about quite a bit. Let's keep this one isolated on toe setting.

Edited January 2, 2011 by JMortensen

[jt1]

Are we considering any bump/roll steer in all of this? You know, the outer tire in bump and the inner in rebound?

 

 

jt

[JMortensen]

Are we considering any bump/roll steer in all of this? You know, the outer tire in bump and the inner in rebound?

Pay attention!

 

What hurts is a lot of bumpsteer. This dynamic toe out of the outside wheel under bump does cause understeer, not because of the relationship between the two wheels but because of the relationship of the heavily loaded wheel in comparison to the direction you want to go. When the wheel you're leaning on toes out after being loaded, you get the effect of understeer.

Actually I didn't address the toe in on rebound, but I think the key there is to minimize bumpsteer as much as possible. If you do that then the effect is pretty negligible. At least it was on my car after slotting the crossmember and adjusting it out that way.

[JMortensen]

Just remembered this other thread, because I recalled that Richard had cut and welded the steer knuckles on his previous car. Two people who have modded for Ackerman posted. Guess what they both thought?

 

http://forums.hybridz.org/index.php/topic/39229-ackermann-article/page__p__317252__hl__%2B260det+%2Backerman__fromsearch__1#entry317252

[gnosez]

Jon - my response was misleading and/or poorly worded. The front toe for both the race car and my street car is set at 1/8-inch OUT. I could care less about tire wear.

[johnc]

Just remembered this other thread, because I recalled that Richard had cut and welded the steer knuckles on his previous car. Two people who have modded for Ackerman posted. Guess what they both thought?

 

http://forums.hybridz.org/index.php/topic/39229-ackermann-article/page__p__317252__hl__%2B260det+%2Backerman__fromsearch__1#entry317252

 

Forgot about that thread. And I still stand by my basic conclusion: The ROI isn't there for Ackerman on a production based race car.