Auto-X tuning ideas, want feedback

[wheelman]

This last winter I made major upgrades to the suspension of the Z and now it's developed a mild case of understeer.

I want to run some ideas for tweaks past you guys, see what you think.

 

First, here is the current setup.

350lbs/in springs front

325lbs/in springs rear

Koni Yellows, at softest rebound dampening front and rear

MSA anti-roll bar set 1" front, 7/8" rear.

Energy suspension poly bushings every where there is a bushing

R200, 3:54 CLSD

1" roll center spacers

-2 degrees front camber (have to use adjustable LCAs to get more)

-1.5 degrees rear camber

+3.5 degrees caster

~0 toe in front and close to stock in the rear.

Stock control arms (for now)

ride height about 4" at frame rail, basically level front to rear, at least as much I can tell.

sectioned strut tubes (not hitting bump stops)

New Kumho V710s, 265/45-16, 22psi front, 20psi rear

 

The car's weight distribution is about 52/48.

 

The goal is to get the car to a more neutral/free setup

This is for auto-x, not high speed, lots of quick transitions

Here are my ideas

1. Move the heavier springs to the rear.

2. Raise the rear ride height (already raised it 1/4" which helped some)

3. Run more rebound dampening in the rear

4. Use adjustable LCAs in the front to get more negative camber

5. Remove camber from the rear

6. Play with tire pressures

I don't really want to as the pressures I'm running are to get the tires to roll over all the way to the edge of the tread, big tires on a light car. Tried going lower in the front but turn-in became vague.

 

The understeer is not horrible, this is more of a fine tuning exercise than anything. The car puts power down very well exiting elements but will push if I apply too much mid-corner, I'd rather it rotate a bit instead.

This is why I'm thinking the heavier springs may work better in the rear, there's not a lot of difference between them, it would soften the front a little and at the same time stiffen the rear.

I also have a set of 250lbs/in and 225lbs/in springs I took off.

 

Feel free to ask questions and make suggestions.

Future plans are to add adjustable control arms to the front and rear, a splitter and most likely a wing for higher speed stability.

[seattlejester]

I'll take a shot

as a rule of thumb since the stiffest end looses traction first...

 

I think increasing the tire pressure would be the cheapest fix, higher rear pressure would make the car more likely to break loose in the rear. But good tread roll over means excellent tire usage/longevity so I can see the hesitation.

 

Raising the rear dampening should stiffen up the rear, especially if you are running on the softest setting.

Swapping the springs should also have the rear stiffer promoting it to scoot out.

 

Now this is from experience in a fwd car, but I believe it should be applicable, albeit silly in some ways:

Run a thinner or stock front sway bar or a thicker/adjustable rear sway bar

Add more weight past the rear shock towers

For those that really wanted the rear to whip around we would even run thinner tires at high pressures in the rear to promote over steer.

 

now this is heresay which i hate to do, but for the vague feeling in steering maybe consider switching the outer tension rod with a ball and socket type fitting instead of the urethane bushing?

[johnc]

Its all about the alignment.

 

1. Camber plates - you need at least -3 in front a -2.5 in back. Don't use adjustable LCAs to get more camber.

2. Increase front caster to +5.

3. Raise the ride height 1" in front and 1.5" in back.

4. Front toe should be 1/8" out. Rear toe should be 1/16" in.

 

Also, tire pressures should probably be 26psi F and 24psi R cold.

[JMortensen]

I think Ken is measuring ride height to the frame rail, not the rocker, so that might make a difference in terms of ride height. I agree with John's other recommendations. I might be inclined to go a little more on the toe out, especially for autox, and I'd be inclined to keep the tire pressures low to get some heat in them. I'd also suggest a pyrometer to eliminate some of the guessing if you don't already have one.

Edited July 19, 2011 by JMortensen

[RebekahsZ]

John C- (not arguing here, just curious) what does it matter whether you use camber plate or adjustable LCAs in order to get the camber?

[johnc]

Increasing LCA length tends to run the tie rods out of adjustment. Since you need toe out you need all the adjustment you can get out of the tie rods. Also, increasing LCA length increases scrub. Ultimately we do have to run a lot of scrub in front to get these cars to handle, but its something you should minimize as much as possible. And with adjustable camber plates you can add caster while also increasing negative camber.

 

I also tried lower pressures when I ran V710s but they tended to get laggy and imprecise on turn in. I ran the pressures I listed above and the car was more responsive and the course times were equal or better.

[wheelman]

Increasing LCA length tends to run the tie rods out of adjustment. Since you need toe out you need all the adjustment you can get out of the tie rods. Also, increasing LCA length increases scrub. Ultimately we do have to run a lot of scrub in front to get these cars to handle, but its something you should minimize as much as possible. And with adjustable camber plates you can add caster while also increasing negative camber.

 

I also tried lower pressures when I ran V710s but they tended to get laggy and imprecise on turn in. I ran the pressures I listed above and the car was more responsive and the course times were equal or better.

 

Thanks for the advice John, I'll try running a little higher tire pressures and see what happens. At first I was running pressures closer to those you listed but the grip is better at the lower pressures. The turn in is laggy as you put it but like I said I wanted the tires to roll all the way to the edge of the tread, maybe with the V710s rounded shoulder that shouldn't be as much of a consideration.

 

I was thinking of using adjustable arms to get more camber because I'm using biscuit type plates and am at the max adjustment with them, guess it's time to switch to weld in plates and possibly modify the front strut towers in order to get as much camber as possible.

 

Whats the best way to get caster adjustment? I don't like the Techno Toy adjustable TC arms because they move the pivot point, is that really a big deal, especially if the suspension range is reduced with stiffer springs?

 

I tried running 1/8" toe out and felt like the inner tire was scrubbing real bad in tight turns, maybe I'm suffering from bump steer in the system caused by the power steering swap I did. I really need to measure that and see just how bad it is. Anyway I set the toe back to about 0 and it worked better, something else to play with.

Toe in the rear is about what you stated, close to stock.

 

From your comment about the ride height I get the impression there should be a slight rake front to back, ~1/2" higher in the rear. I'm close to 1/4" right now, I'll add another 1/4" before the next event and see if Ican even feel a difference.

 

Any suggestions on how to set the rebound dampening on the Konis? They are at full soft right now.

 

Just an added piece of information, the surface I race on most frequently is relatively low grip. It's an old roundy-round tri-oval that used polished river rock in the asphalt when it was laid. This rock now makes up a large portion of the surface as the tar component has partly worn away.

 

I'll make sure to only change 1 variable at a time when I do adjustments.

[h4nsm0l3m4n]

Please correct me if I'm wrong but the impression I got from your original post was that you liked the way your car is behaving overall but wanted to do some "fine tuning" to get the car behaving exactly as you like. When it comes to fine tuning tire pressure is the easiest thing to work on and costs you nothing. If you have adjustable shocks, these are also an easy thing to play with. Most cars that I've seen run heavier springs in the rear. Switching them around is also easy and costs nothing. I feel that between these 3 variables and a lot of testing you should be able to find an optimum setting that you are happy with and with little expense. Part of the problem though is that in autocross there is little actual time to make adjustments to the car and since the course changes every event it can be difficult to figure out if the changes you made are positive. Unfortunately this means things will take longer, but that's part of the fun!

 

Spending a lot of money and making large changes to the suspension (adjustable LCAs, camber plates, etc) might be more fun, but ultimately will lead you down the same road - you will still have to do a lot of testing to find the optimum settings for all your adjustments.

 

What I'm jealous of is your power steering setup, I bet that works great for autocross

Edited July 20, 2011 by h4nsm0l3m4n

[260DET]

On rear toe in, what are the results if you run more than 1/16" on a S30? Just curious because with my rear multilink suspension I tried 10mm (not a typo) toe in once and the effect on slower circuit corners was quite noticeable, like the car had rear wheel steer. I don't do autocross but have always wondered if such a characteristic would be handy there.

[johnc]

With the tires pressures I mention above, I start cold at those pressures and bleed back down to those pressures as the tire warms up.

 

The biscuit style camber plates should give you at least 3 degrees negative. Try ovaling the two mounting holes in the strut tower.

 

I prefer Brian's TC rods:

 

http://www.zccjdm.com/catalog.php/azcarbum/dt93255/pd1987788/ADJ._TENSION_CONTROL_RODS__ZCAR__510__

 

If the rebound on you Koni's are on full soft, add in two turns of rebound before doing anything else to correct your understeer issue.

[JMortensen]

FWIW Richard, I always ran more than 1/16" rear toe in and I felt that it made the car easier to handle on hard braking and easier to catch if the rear end got loose.

[wheelman]

If the rebound on you Koni's are on full soft, add in two turns of rebound before doing anything else to correct your understeer issue.

 

Is this all 4, just the fronts or the rears?

 

Thanks for the link to those TC rods, I really like the looks of them.

[260DET]

FWIW Richard, I always ran more than 1/16" rear toe in and I felt that it made the car easier to handle on hard braking and easier to catch if the rear end got loose.

 

Fairly sure that Neil Roberts in his totally excellent book Think Fast discusses rear toe in detail Jon.

[JMortensen]

I need to read that book again. I got a bit turned off by the style of the book and I think I didn't comprehend or retain a lot of his messages because of the way it is written. Everything is rules of thumb, little or no principles to explain them. I checked and did find two very useful tidbits on toe, one that a rear toe in setting helps to get drive out of the inside tire on corner exit, and the other that the heavier you go with the front toe out and the rear toe in, the better set up the car will be for tight corners.

 

I also totally didn't understand and completely glossed over his instructions on setting brake bias until he commented on it elsewhere in more detail. I reposted that info here previously and I think that was a really great bit of insight. Here it is again: http://forums.hybridz.org/index.php/topic/99500-tail-wagging-under-hard-braking/page__view__findpost__p__932930

[johnc]

Is this all 4, just the fronts or the rears?

 

Thanks for the link to those TC rods, I really like the looks of them.

 

 

Yes, start at two turns from full soft on all 4. Then adjust accordingly.

[tube80z]

Just an added piece of information, the surface I race on most frequently is relatively low grip. It's an old roundy-round tri-oval that used polished river rock in the asphalt when it was laid. This rock now makes up a large portion of the surface as the tar component has partly worn away.

 

I'll make sure to only change 1 variable at a time when I do adjustments.

 

You've already gotten a lot of good info so far. One thing I found is that my car worked much better when running stiffer springs and less bar on surfaces that had less grip and/or when it was damp. I would be looking at 100 to 150 more in spring rate and a much smaller front bar and possibly no rear bar. You can then tune it using rake (lower the end for more grip/raise the end to take more roll stiffness). That's a better way to balance the car than reducing traction on one end.

 

For autox the quicker the car will transition the faster it will generally be. I'd look at how much droop you have with the spring seated and see about removing some of that with a limiter. I'm guess you have somewhere around 1.75 inches of droop presently. Try removing 3/4 of an inch and see what you think. It should be an easy and cheap thing to try.

 

And another things most people don't understand when changing only one thing at a time is a change in springs requires a change in ride height or you have not changed one thing at a time.

 

BTW, to go faster you really need to start looking at reducing friction on the suspension. That means getting rid of poly and any bind you may have. That's where the next big gains will come from.

 

Cary

[wheelman]

It's been while since this conversation took place, figured I'd update the forum on how things have progressed.

 

Here are the changes I've made in sequence and how the handling changed each time.

All adjustments were run at the same track (Tri-City raceway, but different course configurations and weather conditions)

Generally the track is a low grip surface, the weather ranged from warm to hot but always dry, Kumho V710 tires.

 

1. Cranked the shock settings to 2 turns from full soft. (This is almost full hard on these Konis)

Handling didn't change significantly but the car did feel skittery and disconnected from the surface.

 

2. Increased tire pressures to John's recommended levels (26 front, 24 rear)

The car was horrible with this setup, very loose, both front and rear.

 

3. Dropped pressures back to where I normally run them ( 22 front, 20 rear) and softened shocks to 1 turn front 1.5 turns rear

Car back to how it handled before, slight understeer on sweepers, have to be careful on turn in of tight corners as it will push pretty bad.

 

4. Purchased and installed a set of adjustable TC rods from Brian (excellent design and quality, I highly recommend these). Adjusted to produce ~6* caster. (had to trim the front of the wheel wells to clear my tires.

Huge difference on turn in of tight corners. Pivot cones are now my friend. Rear of car is a bit loose.

 

5. Adjusted front camber to as close to -3* as I can get, left is ~3*, right is closer to -2.8*, did not adjust rear camber.

Front now sticks very well, understeer is gone but the rear is VERY loose. Spun off track into the dirt 3 times the day after making this change.

 

6. Adjusted rear camber to -2.5*.

The rear is still a little loose but controllable, agressive throttle input will spin the car unless it's pointed straight. Turn in on tight corners is still great, no understeer detectable on sweepers.

 

7. Loosened rear sway bar.

Car is very neutral with slight understeer in sweepers, tight corners are still very good.

 

The car is easy to drive the way it is right now but turn in on all but very tight turns is a bit vague.

I think I'll try switching to Hoosiers next year, I've heard they require less heat and I have a big problem getting enough heat into the Kumhos. I believe this is why the lower tire pressures seem to work better for me.

 

I'm also going to continue experimenting with the rear sway bar and build a set of adjustable rear control arms so I can get the car squared up. There are still some lingering effects from when I wrecked it in 2007 that make it impossible to get it aligned exactly right.

 

As Cary suggested I'll also be eliminating all poly bushings except inboard on the rear control arms.

 

Hope this helps someone else get their suspension tuned.