Toe Changes for Track Use

[heavy85]

I'm actually fairly happy with how my car behaves now. It's relatively fast and very controllable at the limit. However in the quest for lower lap times I want to make a few changes over the winter break. Already planning some aero mods and time permitting duall masters so now it's time to figure out the rest of the setup. I'm running 375/300 springs big 1 1/8 ish (need to measure) front bar and no rear bar (removing rear bar was a good improvement). Koni 'race' single adjustables and a bunch of other stuff. LS1 powered approx 2600 lb race weight and approx 50/50 dist. The front end pushes at the limit but the back end has a habit of getting squirely putting the power down on track out. Pics show the back end squats at launch and since the rear bar was removed I get quite a bit of rear travel. So I'm thinking of swapping the 375s to the rear and going 424ish in the front. The rear increase to help grip under power by limiting travel and the front increase basically to keep up. My '88 factory LSD is also dead. It still drives fine but is LOUD and goes crunch when I spin he diff. I think it's just the side bearings not the LSD itself but well see. So I'm also considering diff changes while I'm in there. Any opinions on if a gear type (read OBX I'm too cheap for a quaif) diff would help put power down better? Any thought would be good. This is for a track only car. Mostly 2 miles tracks but an occasional trip to Road America.

 

Thanks

Cameron

[JMortensen]

I'll take that old, beat up LSD off your hands (seriously). Also seriously, It may be that the pinion bearing is bad on the diff. That is very common. The LSD is probably fine if you haven't stripped the tabs off of the clutches.

[h4nsm0l3m4n]

If you dont mind, what size wheels/tires and what alignment specs are you running. I'm trying to get a good baseline for an LS1 z car from which I can adjust to my own preferences. Also, if you go OBX please post your results. I'm weighing buying quaife myself but am still having a hard time dropping that much money on it.

Edited December 19, 2010 by h4nsm0l3m4n

[johnc]

I second the above question about alignment. Also, trying to solve a rear traction issue with a fF***ed up LSD is foolish. With the suspension setup you have an OBX should work ok.

[heavy85]

The LSD seems fine. I just took it out and it's the pinion bearings that are fubar. So I need to replace the bearings for sure but which diff would be the best choice - the factory clutch or the OBX for this applicaion?

 

Also appreciate any thoughts on the spring choices.

 

I run either 245/45-17 RE01 street tires on the Rota group by rims or 15x10 with 23x9.5-15 slicks. Street tires I'm running 2.5 camber front and about 1 rear. Slicks being bias need much less camber about 1.5 / .75. Both were tuned by tire temps and wear. 1/8 toe out front and 1/8 toe in rear. Caster I don't know but I added ~1/2" spacer in the T/C rod to effectively lengthen it and add some caster. 3/4" 'bumpsteer' spacers. Front LCA pivot moved up 5/8-3/4" don't remember exactly. Bumpsteer adjustable tie-rod ends. Some chassis reinforcement (see my pics).

 

I wonder if I can get more front traction that will help as well. Theory is if the oversteer is setup earlier mid turn when the front is pushing then as I add throttle the rear looses side traction (friction circle) and snaps out. It's all bench racing for now as the season in us colder climates doesn't start until April or May. Shoot some ideas at me.

[JMortensen]

I am biased towards the CLSD, but the helical will give less understeer as it doesn't do as much on turn in. It will also be more likely to spin one under accel, but if you're running a stiffer front spring type of setup that shouldn't be as much of a problem.

[Leon]

I run either 245/45-17 RE01 street tires on the Rota group by rims or 15x10 with 23x9.5-15 slicks. Street tires I'm running 2.5 camber front and about 1 rear. Slicks being bias need much less camber about 1.5 / .75. Both were tuned by tire temps and wear. 1/8 toe out front and 1/8 toe in rear. Caster I don't know but I added ~1/2" spacer in the T/C rod to effectively lengthen it and add some caster. 3/4" 'bumpsteer' spacers. Front LCA pivot moved up 5/8-3/4" don't remember exactly. Bumpsteer adjustable tie-rod ends. Some chassis reinforcement (see my pics).

 

I thought that bias ply tires like more camber than their radial counterparts?

 

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.

[heavy85]

I thought that bias ply tires like more camber than their radial counterparts?

 

Not in my experience.

 

It also looks like you set up your front end to understeer ... Your car must push quite a bit.

 

Do tell. Rear toe is to help calm the back end down during heavy braking which it does quite well. The front end, well to be honest I've never realized much difference with front toe. It's there due to copycat more than anything. Maybe it's but insensitive butt-o-meter but I've never really found much use for it one way or another. I'm interested to hear in more detail your comments.

 

Thanks

Cameron

[Leon]

Not in my experience.

 

 

 

Do tell. Rear toe is to help calm the back end down during heavy braking which it does quite well. The front end, well to be honest I've never realized much difference with front toe. It's there due to copycat more than anything. Maybe it's but insensitive butt-o-meter but I've never really found much use for it one way or another. I'm interested to hear in more detail your comments.

 

Thanks

Cameron

 

Like I explained in the prior post, running toe-out up front will tend to understeer when in steady-state cornering, with the positive being that it makes turn-in a little quicker before much weight transfer happens. Before turn-in, the inside wheel already has a slip angle before you even turn the wheel so you produce lateral force quicker initially. However, after weight transfers and you reach steady state, the outside tire is loaded much more than the inside. Thus, your outside tire is producing the majority of the lateral force when compared to the inside. With static toe-out, the outside tire will need to be turned more than if you have zero static toe, or "understeer."

 

In high-speed corners, you want the inside tire toed-in more than the outside (see "reverse Ackerman") since the outside tire is more loaded, and thus requires a higher slip angle.

 

In the rear, in order for the car to rotate, the slip angle points outside of the body, tending to yaw the car. With the rear tires pointed in, initially the slip angle points inside of the car. A similar effect to the front occurs, where initially the inside rear lets the car rotate. But, once you reach steady state the loaded outside wheel has a smaller slip angle than in the zero toe situation, therefore the car does not rotate as much.

 

Therefore, both your front and rear suspensions are set up for understeer. If the rear is unsettled during braking, I would adjust f/r brake balance before fiddling with the alignment. You need much more braking up front than the rear, and this is exacerbated with higher grip (slicks) as it causes more weight transfer. You may need as much as 80-85% front brake distribution.

 

Here is a little article I found on google, just search here and on google with keyword combos like slip angle+handling, etc. I'll also refer you to Milliken & Milliken's Vehicle Dynamics book.

[Leon]

Not in my experience.

 

I guess I mixed up camber angle and slip angle in my head.

[JMortensen]

Static toe out is a way to mimic Ackerman on a car that has parallel steer like the Z. The toe out helps the initial turn in, and since as you said the inside tire isn't as heavily loaded when in the middle of the turn, it doesn't have as much effect then. If you want more steering angle, just turn the wheel more. Lots of info on Ackerman out there. I think the Millikens went over it, but Competition Car Suspension by Staniforth has a lot of info on running Ackerman and so does the tech page of www.woodwardsteering.com.

[Leon]

Jon, I'm not sure if you're arguing here or what you're trying to say, so sorry if misinterpreted your post.

 

I'll point out once again that static toe-out induces steady state understeer. You're right about the inside tire being less loaded during a turn but its effect is still prominent enough, and if you're looking to squeeze out all of the car's cornering capability, you want the inside wheel toed-in relative to the outside. The outside wheel has a larger effect, and since it was statically toed-out, when loaded it will be at a disadvantageous slip angle when compared to zero toe.

 

Yes, static toe-out will quicken the transient turn-in, but if your car is stiffly sprung (Cameron's car) then weight transfer will happen quicker. It's all about compromises, are you willing to compromise steady state cornering prowess for quick turn in? Maybe for autocross, but definitely not for road racing. Cameron speaks of lap times, so I'm trying to discuss what is pertinent to his situation.

 

Let's also discuss Ackerman steering. Ackerman steering is only advantageous in low speed turning, with no weight transfer and no slip angles, i.e. a parking lot. It tries to accomplish the perfect geometric turn, with no tire scrub. During high speed cornering with weight transfer and high slip angles, you're in a completely different ballgame, so to say.

 

I don't want to be argumentative, I just want to provide helpful information to the discussion.

[JMortensen]

Jon, I'm not sure if you're arguing here or what you're trying to say, so sorry if misinterpreted your post.

Make no mistake, I am arguing, because I think your point is wrong. Nothing personal.

 

I'll point out once again that static toe-out induces steady state understeer. You're right about the inside tire being less loaded during a turn but its effect is still prominent enough, and if you're looking to squeeze out all of the car's cornering capability, you want the inside wheel toed-in relative to the outside. The outside wheel has a larger effect, and since it was statically toed-out, when loaded it will be at a disadvantageous slip angle when compared to zero toe.

So if the outside wheel has the larger effect, why can you not just turn the wheel a little more to get the steering angle you want? There is something inconsistent about saying that Ackerman is only in play at very slow speeds, and then saying that static toe out causes understeer at high speeds, because they both essentially do the same thing, which is to turn the outside tire less than the inside tire. I know you're going to have a problem with that statement, but keep reading.

 

Yes, static toe-out will quicken the transient turn-in, but if your car is stiffly sprung (Cameron's car) then weight transfer will happen quicker. It's all about compromises, are you willing to compromise steady state cornering prowess for quick turn in? Maybe for autocross, but definitely not for road racing. Cameron speaks of lap times, so I'm trying to discuss what is pertinent to his situation.

 

Let's also discuss Ackerman steering. Ackerman steering is only advantageous in low speed turning, with no weight transfer and no slip angles, i.e. a parking lot. It tries to accomplish the perfect geometric turn, with no tire scrub. During high speed cornering with weight transfer and high slip angles, you're in a completely different ballgame, so to say.

OK, follow me here. If you have parallel steer like a stock Z or worse yet anti-Ackerman and you are in a turn, the inside wheel will not follow the track that it needs to. Although it is not as heavily loaded it will still be steering to the OUTSIDE of the turn. I think your problem is you're looking at the toe as if it stays the same regardless of which way the wheel is pointed. Toe is relative. Example: what happens if you take one tie rod, loosen the jam nuts and adjust it 6 turns (either direction), and lock it back down and drive the car? The wheels may be pointed with the right wheel straight and the left wheel toed out 3/8". Regardless, the car will drive STRAIGHT. The wheels are equally loaded, so they don't turn one way or the other because of the toe change. They just split the toe difference and head straight down the road. The steering wheel will be way off center, but the car will drive so that the tires are equally toed out or in, whichever way you adjusted the wheel. Similarly when you are in that high g turn, the outside loaded wheel is the one that "points the car" for the most part and the inside is largely just along for the ride, at least to the extent that weight is transferred off of the inside and that reduces grip. However, instead of putting a smaller thrust vector towards the outside of the turn as it would with anti-Ackerman or parallel steer, it will point that smaller amount of force towards the inside of the turn. Ackermann is most useful at slower speeds, but that's because it does it's thing to a greater extent at a greater steering angle. And here's how it works (this works for Ackermann and toe out): Going down the straight the tires are equally loaded, so the car goes straight. Turn the wheel, and the inside tire turns more sharply than you would have with anti or parallel steer. This puts a yaw force onto the inside tire that swings the car into the corner harder than it would with a parallel steer. Once into the corner that inside tire drives with whatever cornering force it has left after the weight transfers towards the inside of the corner, in the case of a lot of Ackerman or a high degree of slip angle the inside tire actually scrubs, which is almost like the old roundy-round trick of having more brake pressure on the LF caliper. It really helps the car to pivot. 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.

 

I don't want to be argumentative, I just want to provide helpful information to the discussion.

I am also interesting in providing helpful information, and I hope you take this as a friendly debate, because that's how it is intended. Static toe out is a nearly universal setting on a race Z, and Z's are pretty well known to understeer out of the box. I've driven a 510 before and after the steer knuckles were modified for Ackerman. The difference was obvious, and I didn't move my rack closer to the crossmember specifically to get some Ackerman because I wanted less grip in the front end. It really does help, and it's not uncommon to do in all kinds of racing. That said there are some situations where Anti-Ackerman would be called for, but those are not usually the types of situations that you can put a Z car in. You're talking about things like high speed ovals at 220 mph, stability is much more important than ultimate grip as those cars have grip to spare. For the racing that we do, unless you're land speed racing or drag racing, Ackerman (or barring that, some static toe out) helps.

 

Here is a quote from Woodward's tech info: http://woodwardsteer...ion%20guide.pdf

"To sum up, Ackermann geometry will make corner entry more positive on both dirt and pavement, and, on dirt, will make the car easier to

control in a slide. It is entirely possible that a comparison of these diagrams with the front end layout of your car will suggest certain physical

improvements. If so, they are worth doing. Getting rid of reverse Ackermann is one of the most dramatic and instantly rewarding changes you can

make to a race car."

 

Competition Car Suspension has a full chapter on the subject and the author describes looking at a Dallara F3 car and seeing huge amounts of Ackermann built into it and then deciding to mess around with it on his own hillclimb car. He opens the chapter by saying he is a "total believer and convert" to Ackerman, and calls it one of the few things that is self adjusting to the precise demand of various circumstances (meaning you need less at high speeds when your steering inputs are smaller, and more at lower speeds where steering inputs are larger). He describes trying 100% Ackermann and then 200%, and says "what had seemed decent turn-in had improved in startling fashion, with the sharper and slower the corner the better even to the extent of a tailhappy tendency." That matches the 510 experience I have, although he went from whatever a 510 has from the factory to 100% Ackerman.

 

BTW both spellings of Ackerman are correct from what I can tell, so I use both so that people searching will find Ackermann too.

Edited December 20, 2010 by JMortensen

[johnc]

Just a thought...

 

One thing that I see happen in threads discussing suspension setup is a focus on one issue as if that issue works in isolation. I'm as guilty of it as the next guy.

 

Just because the one "thing" (caster, spring rate, weight distribution, Ackerman angle, roll centers, etc.) we're focusing on induces some behavior in the car doesn't mean that's how the car is actually behaving. Front wheel toe-out may induce a particular behavior but spring rate balance front-to-back my produce the opposite behavior thus canceling each other out and making the car feel neutral at the limit.

[Leon]

JM, I honestly don't have the energy for a long discussion after 12 hours of work. I will say that you are not considering (understanding?) slip angles in this discussion at all, and those are very important pieces to the puzzle.

 

FWIW, I have had long discussions with a former Gurney's Eagles and Benneton-Renault F1 engineer and he would tend to agree with me, at least in a general sense. I don't have the experience of playing with Z-car geometry, but there are physical laws that govern vehicle dynamics which come into play no matter what vehicle you're driving. I just don't feel like dissecting your long, largely unspaced post right now.

[JMortensen]

Come back to it when you have the time and we'll hash it out some more if you like. Conventional thinking in the late 70's and early 80's was that anti-Ackerman was the way to go. Anti-Ackerman is in Smith's book Tune to Win, which is probably the most common source for the idea nowadays, but thinking has moved on and now lots of Ackerman is the prevailing theory. Even Smith reversed himself in his later book Engineer to WIn: http://books.google....epage&q&f=false

 

Ackerman effect and static toe out are cumulative in effect. What I do now is put in enough Ackerman that the car begins to become unstable, back off from that point a bit and call that my optimum Ackerman for the course. When we get to the street courses, like Trois Rivieres and Long Beach, I add static toe out until the car and driver are happy. I carry about three alternative Ackerman setups around with me.

 

Here's another quote from the same page that deals with slip angles and Ackermann:

We learned in Tune to Win that a tire's ability to generate cornering force decreases as the load on it is reduced and that, up to a limiting angle, its cornering force will increase with increased slip angle. I feel that the improvement in initial point is just as simple as it seems but the rest of the answer has to do with a complex trade off between decreasing vertical load and increased slip angle due to increased steer angle. Somewhere in the equation, reduced inside tire drag at today's lower slip angles may come in as well.

 

I am certain that you are now as unsure of WHY increased Ackerman steering geometry works as I am. I can only apologize; that's the best I can do at the moment. As with so many things, I wish that I could do some meaningful testing here. There just HAS to be a lot to be gained, especially on the ovals. Maybe someday...

Edited December 21, 2010 by JMortensen

[Leon]

I'll add that it doesn't make sense to turn the less-loaded tire more. A less-loaded tire will perform optimally with less slip angle than the higher-loaded tire at the front wheels, and front slip angle is driven by steering angle. With toe-out, the wheels are inherently in the wrong position for this. Therein lies the core of this argument.

 

Here is a reference photo from Milliken & Milliken:

 

http://features.evolutionm.net/imageview.php?image=1538

Edited December 21, 2010 by Leon

[JMortensen]

I'll add that it doesn't make sense to turn the less-loaded tire more. A less-loaded tire will perform optimally with less slip angle than the higher-loaded tire at the front wheels, and front slip angle is driven by steering angle. With toe-out, the wheels are inherently in the wrong position for this. Therein lies the core of this argument.

 

Here is a reference photo from Milliken & Milliken:

 

http://features.evol....php?image=1538

It certainly doesn't make any sense to turn the inside tire less. The outside wheel is the one that controls where the car goes. If you want to turn harder, turn the wheel more. The inside is largely along for the ride, so you can choose to make it turn less or more than the outside wheel. What advantage do you see in turning it less or the same as the outside wheel? Since the inside tire has to turn a sharper radius, even having the wheels parallel would mean that the inside tire is trying to drive a wider arc than would be idea to follow the track of the outside tire, which is again the one that determines the radius of the turn for the most part.

Edited December 21, 2010 by JMortensen

[Leon]

It certainly doesn't make any sense to turn the inside tire less. The outside wheel is the one that controls where the car goes. If you want to turn harder, turn the wheel more. The inside is largely along for the ride, so you can choose to make it turn less or more than the outside wheel. What advantage do you see in turning it less or the same as the outside wheel? Since the inside tire has to turn a sharper radius, even having the wheels parallel would mean that the inside tire is trying to drive a wider arc than would be idea to follow the track of the outside tire, which is again the one that determines the radius of the turn for the most part.

 

I understand this may be "mind blowing" but yes it does make sense to turn the inside tire less once you realize the physics involved. You are still thinking completely geometrically and not considering slip angles at all and how they pertain to vehicle dynamics. You are either not listening to what I'm saying or you don't understand the concept of slip angles.

 

Slip angles dictate the relationship between the tire and the road when cornering at high speeds. Stop thinking about where the centerline of the tire points and start thinking in terms of what are the optimal slip angles given a normal load and lateral acceleration. Look at the Milliken figure I linked earlier and the answer will be staring right at you. A less loaded tire needs less slip angle to generate its maximum lateral force, therefore you want the inside tire at a smaller slip angle than the outside. It's simple, but you have to understand slip angles first and I'm not convinced you do judging from this discussion.

[JMortensen]

Stop thinking about where the centerline of the tire points and start thinking in terms of what are the optimal slip angles given a normal load and lateral acceleration.

You're getting two issues confused. One is the slip angle at which you obtain maximum grip. The other is whether obtaining the maximum grip with the wheel pointed in the direction that obtains that grip maximizes the corner speed. I don't see how you can get around the idea that maximum grip on the inside tire with a thrust vector towards the outside of the turn is going to tend to give you understeer. Even if the grip level is slightly reduced by adding Ackermann, having the grip be pointed in the direction you want the car to go is better. Again, Smith was mystified by the fact that the cars with heavy downforce turned so much better with Ackerman than without, probably for the same reasons that you're having trouble with it. The simple reason is that the tires are not fighting each other, they are both trying to turn a corner of the same radius.

 

Above and beyond that, steady state maximum cornering power is not necessarily where the best lap times come from. Although I haven't taken his seminar, I have read where Claude Rouelle says that 80% of your corner speed comes from the first 10% of the corner. The importance of getting the front end to respond to that initial steering input is paramount, and that is why Ackerman is so popular with racers and static toe out is popular where Ackermann isn't easy to come by. The benefit is obvious when you've driven the same car with and without Ackerman. You can test the same thing on a stock Z by taking the recommendation of just about every Z racer and running some toe out. It won't cost you a penny, and if you have a course where you know what the car feels like with your normal toe setting, I think you'll feel the difference immediately. It will seem really twitchy, because the front end is suddenly more apt to change direction. Turn the wheel sharply, it reacts better, and there's your first 10% of the corner.