Suspension tuning advice needed

[Zmanco]

Last weekend I took the Z out to High Plains Raceway for the first time since the turbo swap. My primary goal was to test/tune the engine and make sure that nothing overheated and that it all stayed together. The good news is that it ran great and for the first time in my driving experience, I have enough power to throttle steer the car in 3rd gear at 80+ mph. This completely changed my thoughts on how to approach higher speed sweepers

 

I also did a few ride alongs and in particular, in a stock NISMO 350z and also a mostly stock Z32 2+2 NA (only upgrade was stiffer struts). I noticed that both cars were far more forgiving of fast transitions, such as going from throttle-on through a turn into hard braking for the next turn in the other direction. Both cars kept the rear end planted. In my 73 (more on the setup in a moment) doing that at the same speeds would almost certainly result in the rear coming around and spinning. I'd guess that my entry speeds were 5+ mph slower because of this. Here's the track map: http://www.highplainsraceway.com/images/track-map-large3.gif The two turns where this was the biggest issue were 5 and 10. Both are downhill approaches and the initial turn in speed is ~70 mph (or more).

 

So I'm looking for some advice on how to start sorting out the suspension.

 

Current setup: 73 240Z

- Tokico springs and Illumina struts

- 1" bump steer spacers

- MSA camber bushings up front

- 1" front, 7/8" rear anti roll bars

- 16 x 7 Panasports with Dunlop Direzza Sport Z1 Star Spec 225/50-16

- MSA strut bars

- Powerbrute CLSD

- Tire pressures: 36/30

- Illuminas F/R: 4/5, later in the day 2/5

 

I'm attaching the last alignment sheet when we installed the camber bushings.

 

I started the day out with the struts set at 4 front, 5 rear which is how I've run in the past. As the day progressed I gradually softened the fronts down to 2 (where I run on the street) and left the rear at 5. This seemed to help with the initial turn in, but didn't make much difference that I could tell with keeping the rear planted.

 

My primary use of the car is street, but now that the engine is mostly sorted, I hope to do maybe half a dozen track days next season. The point is that I want to improve track performance, but not to the point that the car is miserable to drive on the street.

 

In hind sight I'm wondering if I was going in the wrong direction softening up the front. During these transitions it seems like the weight is transferring both rear to front, and also side to side. It seems to be the combination of both at the same time that upsets the car. Perhaps I should have been softening the rear to give it more grip?

 

Before the track day I had been thinking of swapping to custom LCA and TC rods in the front next season to allow more camber and caster. But as above, I expect those mods would give me MORE grip up front, and that probably would make this transition issue worse, no?

 

So I'm looking for some advice on how to attack this. If possible I'd like to keep these springs and struts and solve this with suspension settings. My instincts tell me this would result in a setup that would still be streetable, and would make the track handling more forgiving.

 

Thoughts/Suggestions?

Alignment 2008-06-24.pdf

[johnc]

Drop the rear Illumina settings to 3.

 

Your car as setup will be more neutral then the cars you compared yourself to. You will drive it differently to get faster lap times and focus more on corner exit speed then corner entry speed. Focus on smooth braking, steering, and throttle inputs to keep from upsetting the car in transitions.

 

If you're coming out of the corner 5 mph faster then the 350Z that's much more important then entering the corner 5 mph faster.

 

Try to get more negative camber in front and back. See my suggestions in another post. Also, more spring on the car will help. If you're running the 240Z Tokico HP springs try upgrading to the 280Z Tokico HP springs. They have a 185F and a 200R rate which will work better.

 

I would also swap out the rear anti-roll bar for the Suspension techniques 3/4" bar that mounts behind the diff.

[JMortensen]

If you just want to look at the oversteer problem, I'd focus on the rear bar. Mine was an MSA bar and it was bound up when the suspension went through it's travel. Take the rear springs off and push the suspension through its motion by hand and see if the bar hits anything. I think your 73 has factory sway bar mounts, and mine had the bar bolted through the floor with the supplied brackets, so it may be different, but it was a problem for me.

 

My car plowed really bad on most corners, and then occasionally would whip the back end around on corner entry when I first started autoxing. Sound familiar? I worked on the front end and added caster and camber and that helped a lot to get the front end working better. Even right at the end before I moved to WA after years of screwing with it (albeit with a VERY limited budget), I was still fighting plow and oversteer. The car was a lot faster than when I started, but the problems were the same (tough to see the understeer, but there is quite a bit on the slower corners):

http://videos.streetfire.net/video/2000-autox-indisde-and-outside-I-think-my_8051.htm

 

I didn't do too much with the rear, just added stickier tires and those oversteer problems mostly went away because of that, but then when I took the car down I realized what a problem that rear bar was. Stickier tires will hide a lot of bad handling traits.

 

With those soft springs I wouldn't put the Illuminas any higher than 3, maybe 4 at the most. Even with 250 in/lb rear springs I never ran them on 5. It was just ridiculously stiff. If you want stiffer, get stiffer springs and run dampers to match. I think you could run springs in the mid 200's low 300's and get a reasonably comfortable ride and better performance with a better set of struts.

 

I would do the following:

 

Caster and LCA - this will require getting rid of the camber bushings

MSA rear sway bar - I'd check it for binding. I bet it does, and if so replace with the rear mounted Susp Techniques bar.

More neg camber - Plates are the way to go IMO, since you can easily move the plate for more camber at the track, and then back after you're done for the day.

Stiffer springs (coilovers preferably so you can swap springs cheaply)

Section struts

Reduce rear toe - possibly G machine bushings if you don't want to do control arms

[Zmanco]

John and Jon, thanks so much for the responses - I was hoping you would both weigh in. A few thoughts/clarifications:

 

- I'll check the rear bar for binding. The good news is that it's rather dirty under there so I imagine if it has been binding that I should be able to see some marks.

 

- I thought the springs felt a little soft. After the holidays I'll do the swap to the 280z tokicos. John, just to confirm, are those linear springs? And should I plan to cut a turn or so off to keep the height the same as with the 240z tokicos?

 

- The driver of the 300zx rode with me and made the same comment that with the power of my car he thought the faster approach would be slow in / fast out. He was running a NA and the track is 5k feet elevation so he was more focused on momentum.

 

- John, is your suggestion to go with the 3/4" rear bar to avoid binding by the way it mounts, or because you think the rear should have a smaller bar? It pushes more on high speed entry I suspect due to the CLSD locking up, so I have been reluctant to soften the rear.

 

- How much camber/caster can be gained with plates and the stock strut assemblies? I like the idea of changing the settings for the track and then back for street, but I thought the stock spring perch would hit the inside of the strut tower? That's why I went with the camber bushings, but which only added about 3/4 degree. In a perfect world, I'd be happy with the 1 degree neg on the street I currently have, and the ability to add maybe another degree for the track. Is that achievable with camber plates and stock strut assemblies?

 

- Jon, you mentioned sectioning the struts. I thought that was only done as part of converting to coil overs. Were you suggesting to do this with the stock strut assemblies and Tokico 280z springs?

 

- What is the effect of reducing rear toe? Would it help reduce understeer? How does it affect the suddenness of the transition from understeer to oversteer?

 

As I'm asking all these questions I'm wondering if there is a good book on suspension tuning principles? Perhaps an equivalent to Maximum Boost for turbo charging?

[johnc]

- I thought the springs felt a little soft. After the holidays I'll do the swap to the 280z tokicos. John, just to confirm, are those linear springs? And should I plan to cut a turn or so off to keep the height the same as with the 240z tokicos?

 

You will need to assemble them, set the car on the ground, measure, and then determine if you need to cut to get the ride height you want.

- John, is your suggestion to go with the 3/4" rear bar to avoid binding by the way it mounts, or because you think the rear should have a smaller bar? It pushes more on high speed entry I suspect due to the CLSD locking up, so I have been reluctant to soften the rear.

 

Yes. With the springs you've chosen there isn't much you can do about the CLSD push except to: 1) enter the corner slower and use more throttle to get the car to rotate or 2) get really good at left foot trail braking, getting the back end to come around, and then controlling it with the throttle.

 

- How much camber/caster can be gained with plates and the stock strut assemblies? I like the idea of changing the settings for the track and then back for street, but I thought the stock spring perch would hit the inside of the strut tower? That's why I went with the camber bushings, but which only added about 3/4 degree. In a perfect world, I'd be happy with the 1 degree neg on the street I currently have, and the ability to add maybe another degree for the track. Is that achievable with camber plates and stock strut assemblies?

 

Camber plates with stock OD springs don't get you much negative camber.

 

- What is the effect of reducing rear toe? Would it help reduce understeer? How does it affect the suddenness of the transition from understeer to oversteer?

 

When we're talking about understeer/oversteer we're mostly talking about steady state behavior. Toe settings have more to do with transitory behavior. Less rear toe in will make the car rotate better on corner entry but make it a little more unstable under power on corner exit. With the CLSD behavior that you've alluded to those toe changes would help the car get into the corner better.

[JMortensen]

- How much camber/caster can be gained with plates and the stock strut assemblies? I like the idea of changing the settings for the track and then back for street, but I thought the stock spring perch would hit the inside of the strut tower? That's why I went with the camber bushings, but which only added about 3/4 degree. In a perfect world, I'd be happy with the 1 degree neg on the street I currently have, and the ability to add maybe another degree for the track. Is that achievable with camber plates and stock strut assemblies?

I'm not sure how much movement you can get with camber plates and stock springs. If you do camber plates, it would really make sense to go with coilovers too. I don't think it makes much sense to do one without the other. Coilovers give you a lot more freedom to try different ride heights and roll centers, and change spring rates cheaply. It's the way to go if you can afford it.

 

- Jon, you mentioned sectioning the struts. I thought that was only done as part of converting to coil overs. Were you suggesting to do this with the stock strut assemblies and Tokico 280z springs?

You can section the struts and use any spring you want, you just have to find a spring that fits. I had friends with 510s who would section their struts and cut the spring perches off of 240 struts and weld them on, then use 240 springs, for example.

 

I'm not sure if you could use the 280Z Tokicos with sectioned struts. From the pictures I've seen, I think the 240 springs would DEFINITELY coil bind. You need springs that have enough travel if you section to hit the bumpstop before they coilbind, but the advantage is that you would be able to run the car at the same height or lower with more suspension travel.

 

- What is the effect of reducing rear toe? Would it help reduce understeer? How does it affect the suddenness of the transition from understeer to oversteer?

Reducing the rear toe would decrease understeer and increase oversteer as a general rule. I just used your conversion calculator that you had put in the download section to put the alignment into inches, and in doing so it looks fine to me, so I would retract my statement. Looks like you have .145" total, which is not excessive in my opinion. Probably is excessive in others' opinions, but that worked OK for me.

 

There isn't a good book on tuning that I've found. You'll find lots of books on theory, but they don't really say "Try X and if that doesn't work, try Y". I think you'll get better info on a forum like this one than you will from a book. Only problem is you're following the leader for the most part, so you need to know when to listen to me or John and when to try something different.

[260DET]

On those tyres I'm not sure what the 36/30 means, F/R, hot/cold, but anyway that type of tyre typically works best around 40 PSI hot. If they are underinflated then they will feel greasy and there will be a big difference between cold and hot readings.

 

If the car is a daily then the pressures can be dropped off track, another good thing is they should start to squeal when approaching the limits of adhesion, a handy warning

[Zmanco]

Thanks for the additional feedback. Here's what I've concluded so far:

 

1) I need to change to stiffer springs.

 

2) I need more neg camber and maybe more caster too.

 

3) I need to check if the rear ARB is binding and change it if it is.

 

4) I need to cost out buying new springs and LCA/TC rods vs. going with coil overs and camber plates. Seems like for the range of adjustment I'm looking to accomplish both could get me there.

 

5) I should run softer on the Illumina's until I have stiffer springs.

 

6) I need to continue to work at driving smooth. Given I'm used to a relatively underpowered car, this is a continuation for me. I think the challenge now is to stay smooth with the additional power.

 

As for sectioning the struts, I'm thinking I won't need to do that as I don't want to lower the car more than it is now, and I'm not having suspension travel problems, even with the softer tokico springs.

 

Thanks again for the ideas. I've got some part searching/pricing to do...

[Zmanco]

On those tyres I'm not sure what the 36/30 means, F/R, hot/cold, but anyway that type of tyre typically works best around 40 PSI hot. If they are underinflated then they will feel greasy and there will be a big difference between cold and hot readings.

 

If the car is a daily then the pressures can be dropped off track, another good thing is they should start to squeal when approaching the limits of adhesion, a handy warning

Sorry, F/R = front / rear.

 

On the street I run 26/20 which seems to wear pretty evenly. Plus it's a lot smoother on bad roads. There's still more than enough grip to get arrested

 

I'll try going up to 40 in the front next time I'm on the track. And they do squeal just as they loose grip. They're a really forgiving tire and probably the best tire in its price range I've had yet. But they don't have much grip when cold or wet - and even 50 F is cold for them.