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Changing Suspension Mounting Points


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I've been reading all of the very good (and some very long) threads on suspension mods and was curious if there was any firm concensus on optimizing the suspension mounting points on a dedicated race car?

 

Many of you are familiar with my car ( http://sth2.com/Z-car/ ), if not, it is a purpose built F Prepared 280Z autocross car and about the only area where the car is not fully optimized is in the suspension geometry. That is my project for this winter.

 

I started by using some suspension analysis software and took my best shot at measuring the current setup. I think I did a pretty accurate job, using level setup stands, laser levels and virtually every measurment tool in my garage. The results were well below ideal. My front roll center is about 1" underground and jumps around laterally about 4 feet either direction when I induce about 2 degrees of roll. The camber change is going a good way into positive with that much roll. The rear is not as bad with the stock control arms, the RC is about 3.3" above ground and only moves a about 7.5" from side to side in roll...

 

So my main questions are:

1) Is there concensus on where the pickup points should be on a very much lowered car? Front and Rear...

2) I've read the threads on TC rods, but didn't get a solid feel for the benefit of moving those points. I played with those points in my suspension software but didn't see things change much. What am I missing there?

3) Is there a downside to increasing the length of the control arms? Provided I have clearance, I don't see a negative. Is there an area I need to pay attention to to avoid binding?

4) Caster. I've never had much, mainly because I have never made any modifications to make it more adjustable. Currently I only have about 1.25 degrees. I willing to take the plunge and try this stuff, i hear it can work wonders on turn in! Any recommendations on how much I want?

 

That's "all" I can think of right now, but I am sure I will think of more as we go! Thanks for any help.

 

Tom

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IMHO...

 

1) Is there concensus on where the pickup points should be on a very much lowered car? Front and Rear...

 

No concensus. Do like Jon M and make your inner pickup points adjustable vertically. Then you can adjust them to your liking and driving style and also adjust bumpsteer.

 

2) I've read the threads on TC rods, but didn't get a solid feel for the benefit of moving those points. I played with those points in my suspension software but didn't see things change much. What am I missing there?

 

Personally I don't see all the fuss about raising or lowering the TC rod mounting point. Again, you can add some adjustability and see what works for you. I am pretty emphatic (based on my own experiments) about not shortening the length of the TC or the distance between the TC rod pivot and the LCA.

 

3) Is there a downside to increasing the length of the control arms? Provided I have clearance, I don't see a negative. Is there an area I need to pay attention to to avoid binding?

 

Lengthening the LCAs is always a good thing in a strut suspension.

 

4) Caster. I've never had much, mainly because I have never made any modifications to make it more adjustable. Currently I only have about 1.25 degrees. I willing to take the plunge and try this stuff, i hear it can work wonders on turn in! Any recommendations on how much I want?

 

Positive caster is your friend. Just make sure you don't induce bind or friction. Monoballs and rods ends help a lot in this area. Moving the upper strut mounting point is a way of gaining caster without increasing friction.

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4) Caster. I've never had much, mainly because I have never made any modifications to make it more adjustable. Currently I only have about 1.25 degrees. I willing to take the plunge and try this stuff, i hear it can work wonders on turn in! Any recommendations on how much I want?

 

Factory is 3º, so I would at least get it back to that if not more. I run about 7º.

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Here is my take:

 

1. No consensus. The general theme is you can have a lower roll center (even underground) but the lower you go the more stiffly sprung the car needs to be to control roll. Once the control arm points up from the center to the hub, introducing a side load compresses the suspension. There seems to be a lot of guys in OR who are having really good luck with what would have previously been thought to be ridiculously heavy springs. I definitely want to try some of the heavier spring rates for myself.

 

2. Raising the TC rod mount promotes anti-dive. I don't have a way to quantify how much of a difference it is going to make, but my car had a lot of nose dive under braking before, so I think it will help. The other reason for doing what I did was to get the TC pivot inline with the control arm pivot. This required the new bracket, and slotting the bracket was in a sense a byproduct of the whole thing.

 

3. As John says, can't go too long on the LCAs.

 

4. I agree with the caster comment. This depends to some extent on what tires you are using, some like more than others. I haven't yet hit the point at which I feel that I have enough, and I was running about 5 degrees. You'll see some people have run as much as 7 or 8 degrees. There is a point at which it doesn't help anymore, but I'd have to say caster is the #1 thing that helped my front end to hook up. I am having a hard time believing that you haven't messed with it at all, and even harder time with the fact that you're running less than stock...

 

Cary McAllister (tube80z) should have some good insight for you. He knows his stuff and I know he'll be looking at this thread as it's right up his alley.

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Thanks for the feedback guys! I can't believe I never realized how much info was on this forum... I've had a login here for years, I must have posted more than once before.

 

Allright... so I think adding caster is a given. I will just have to sneak up on that to find the right amount. I am currently running bias ply slicks but I may try switching to the Hoosier radial slicks. That might play into how much I want. Looking at the difference in camber gain I may want to start low with the bias tires and add more if I want to try the radials.

 

Jon, on the rollcenter and stiff springs comment, I am currently on 400 lb springs all around. If I pull the RC up above ground, is that going to drive a change to softer springs?

 

I've got this nagging fear that if I "improve" the suspension too much I won't be able to drive it. I've had pretty good luck with the less than ideal setup, for a long time. I'm not sure I can adapt at this point! :)

 

Tom

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Thanks for the feedback guys! I can't believe I never realized how much info was on this forum... I've had a login here for years, I must have posted more than once before.

When I first came here it was because the IZCC list didn't have very good answers to my racing suspension questions. I was very anti-V8 at the time too. The more I stuck around the more I realized this place has a metric buttload of information about EVERYTHING Z related. Not just engine swap info, that's for sure, but since coming I've decided to do an engine swap too... :D

Jon, on the rollcenter and stiff springs comment, I am currently on 400 lb springs all around. If I pull the RC up above ground, is that going to drive a change to softer springs?

I doubt it. I think you're going to be fine with those rates, should work out really nicely.

I've got this nagging fear that if I "improve" the suspension too much I won't be able to drive it. I've had pretty good luck with the less than ideal setup, for a long time. I'm not sure I can adapt at this point! :)

What I'd be worried about is when the front end hooks up so much better the back end may get really loose. If and when that happens I'd look at adding some toe in out back to counter it. I was running 3/16" total in back and that had it pretty balanced with the front in my old configuration.

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I've been reading all of the very good (and some very long) threads on suspension mods and was curious if there was any firm concensus on optimizing the suspension mounting points on a dedicated race car?

 

I think the answer to that is probably no. The safe route is to try and keep things in the sweet spot and not to get carried away with running anti-geometry. So that means keeping things near level side to side and front to rear when in a dynamic condition.

 

I started by using some suspension analysis software and took my best shot at measuring the current setup. I think I did a pretty accurate job, using level setup stands, laser levels and virtually every measurment tool in my garage. The results were well below ideal. My front roll center is about 1" underground and jumps around laterally about 4 feet either direction when I induce about 2 degrees of roll. The camber change is going a good way into positive with that much roll. The rear is not as bad with the stock control arms, the RC is about 3.3" above ground and only moves a about 7.5" from side to side in roll…

 

A word of caution. The geometric RC most programs calculate is only a rough approximation. Most programs don't take into account tire or component deflection at all. And most programs roll the car around the centerline, which isn't correct. Depending on how the RC migrates you could have different results if you corner first and brake, versus braking and then cornering when you look at the car as a whole. I never really thought about this until I listened to a long explanation from Claude Rouelle. It seemed to be a sore point for him and he went as far to create a software package called optimum K that does take this into account. The price is about $5K for this program, which is well out of my league.

 

In the end Claude explained the geometric RC is really a way to visualize how the elastic and inelastic forces are distributed. That's a fancy way of saying what proportion do the springs and shocks deal with and what proportion goes through the linkages. Why this is important is that control the speed of the transfer seen at the tire contact patch. Generally underground RCs are slower to transfer weight compared to above ground RCs. So to speed up reaction you can raise the RC. For an autoxer you may actually want to slow things down a bit and I've found the underground front RC works for me.

 

Your current setup is very similar to where my car was. I ended up at this point through experimentation and it seems to work well. General consensus (not necessarily here) is that you should start with the spring rate equal to the corner rate for slicks and test harder/softer from there to see what works best. So you're setup is a little on the soft side, which is probably okay if your running where it's really hot and worried about tire temps. If you're running where it is cold you should think about going stiffer to work the tire harder and build more heat.

 

So my main questions are:

1) Is there concensus on where the pickup points should be on a very much lowered car? Front and Rear…

 

As mentioned above staying in the sweet spot would be the easy thing to do - basically close to level in a dynamic condition.

 

2) I've read the threads on TC rods, but didn't get a solid feel for the benefit of moving those points. I played with those points in my suspension software but didn't see things change much. What am I missing there?

 

Does your program show you side-view geometries? These are also called anti-dive/anti-squat/anti-lift. Changing the vertical height of the rear TC mount should change these as well as your caster gain. The problem I've found is that a lot anti-dive can cause major understeer in slow corners. I removed most of mine and opted to use droop limiters to get some anti-lift in the rear. As it stands you can't change squat/lift in the back very easily because we don't have a true macstrut.

 

3) Is there a downside to increasing the length of the control arms? Provided I have clearance, I don't see a negative. Is there an area I need to pay attention to to avoid binding?

 

Yes and no. Generally longer arms will slow the RC movement down and you should get a much more stable feel from that end of the car. So it's a big win for that. Where you run into problems is that side-to-side scrub you have as the suspension moves is lessened. In the case of short autox runs it may not get as much heat into the tires. I'll say more a little further below. For me I'd go longer as this has helped in previous cars.

 

4) Caster. I've never had much, mainly because I have never made any modifications to make it more adjustable. Currently I only have about 1.25 degrees. I willing to take the plunge and try this stuff, i hear it can work wonders on turn in! Any recommendations on how much I want?

 

You need a lot more caster. I'd do big jumps to see what your tires think as they are the limiting factor. I'd try 4, 8, 12 degress. You'll get to a point where the turn-in is good but the front falls away (turns good them major push). Back off a couple of degrees from this and try again. Keep in mind that steering effort goes up big time as you increase caster. What you're looking for is whether or not it works and not so much steering effort. Once you find what works you can change what's called pin lead (the position of the spindle on the strut). If you move it forward (looking sideways at the car) you'll decrease the trail seen at the ground. This reduces the steering effort with lots of caster. BTW, simple rule of thumb is caster should be 1/2 of the SIA, which in the case of the Z is the about 12 degrees stock. So when you add in camber you can see how this is easily 7 to 8 degrees.

 

Jon, on the rollcenter and stiff springs comment, I am currently on 400 lb springs all around. If I pull the RC up above ground, is that going to drive a change to softer springs?

 

Yes and no. The key is a concept called a magic number. There are a bunch of them but in this case we're interested in the percent of lateral load transfer handled by the front of the car. If we keep this number the same for a known setup we'll be able to change RC hieght, roll bars, springs, track width, etc. and still keep a setup that works very much the same.

 

What I've found is that I can run really stiff springs in the cold the car works fine. What it won't tolerate is much roll bar. So one idea would be to go with stiffer springs, raise the RC, and finish it off with a small roll bar. The idea being the above ground RC will help to generate more heat into the tires along with the stiffer springs. This might be your Kansas setup. Or you could use your program to see if you can get your RC's up by raising the car. You may be surprised at how you can balance the car with a couple of turns of the spring perch in the front or the back. You lower the end that isn't sticking or raise the end you want to unstick. You can use your program to set limits on what is acceptable, which is a good use for these programs.

 

I've got this nagging fear that if I "improve" the suspension too much I won't be able to drive it. I've had pretty good luck with the less than ideal setup, for a long time. I'm not sure I can adapt at this point!

 

I think this is a little unfounded. What I found is as I worked more and more on this that car didn't get massively faster but it got a lot easier to drive. I could get those demon runs more consistently without feeling like it was always going to bite me. I also noticed the car got a lot easier on the tires and they lasted longer.

 

If you're looking to pickup real performance you need to look at reducing friction. Once you're at the level that your car is prepared to that can make a large difference. And if you have access to a data logger you can start to see a lot more of this if you log suspension displacement, i.e. are the shocks working correctly, how well does the car corner, does it get better or worse over a run, etc.

 

I hope this doesn't come of like I'm some kind of know it all. I really started getting into this about 5 years ago. I've taken a number of classes and put those principles into work. At first I simply measure tire temps and air pressures. I kept good records and learned how to make the tires work better. What I can tell you from personal experience is that this is a never ending game. The more you learn the less you know …

 

Cary

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I think I should point out that if you're working within the confines of the stock front crossmember, you really can't get the LCA's too far above parralel with the ground. I've seen how low you run your car, Tom, and if you drill a hole in the crossmember as high as you possibly can without the control arm touching the top, I think you'll still be really close to horizontal if not a little below it when you're on the brakes hard. That's my guess anyway from messing around with my own suspension. You can go farther than that with the RC by adding bumpsteer spacers, and that's probably where you're going to end up going if you want the RC higher than you can get by redrilling or slotting the control arm holes.

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After looking at the Frisselle car and the amount of caster it must have from looking at the top of the strut I have 2 questions. Is there room to slide coilovers that far back and use a weld on fabricated camber plate?

 

I have never measured caster and was wondering how far back a permanent solution like welding on camber plates would or could be? The final setting would then be dialed in by adjustable T/C rods. I just would not want to move the top of the strut too far back (if that is possible in the confines of the strut tower). I think the camber "biscuits" that johnc sells does this caster adjustment to some extent but my towers are already cut for GC plates.

 

FrisselleCaster.jpg

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OK, let me rephrase this. The reason I want to explore moving the caster some up top as well as below is it seems to require more than an inch increase to get 7 degrees or more of caster. You are only allowed a 1 inch increase or decrease in the wheelbase in the Prepared category.

Quoting blueovalz :

 

"I moved the entire crossmember forward to increase my caster to roughly 7 degrees. The shaft that comes out of the steering column to the coupler is splined and will slide several inches with adaquate engagement. To use camber plates on top to account for all of the required movement would push the towers inside dimensions beyond their limits. Not knowing the static ride length of your struts, I am guessing that a minimum of about 1.25" movement will be required to achieve the 7 degree (4 degree change) caster."

 

If Tom wants to explore 7 degrees or more I don't think just lengthening the T/C rod will keep him legal.

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OK, let me rephrase this. The reason I want to explore moving the caster some up top as well as below is it seems to require more than an inch increase to get 7 degrees or more of caster. You are only allowed a 1 inch increase or decrease in the wheelbase in the Prepared category.

 

If Tom wants to explore 7 degrees or more I don't think just lengthening the T/C rod will keep him legal.

 

I hope your happy, but you made me download the 2007 rules to take a look, and I should really be doing my day job :-)

 

I noticed composite doors are now legal. For the above I read 17.8 number 2 to say I can move the tower where I need it.

 

As for the wheelbase it says "The wheelbase of the vehicle shall not be changed or relocated in a fore/aft direction by more than + or - 1". To me this says I can move the front wheels forward and the rear wheels forward by an inch. I guess it could also be the wheel base could get longer or shorter by an inch too.

 

If I had a prepared car I'd try more caster to see if it worked. If it does then I'd move the top pickup point to where I needed it to make this work and use an adjustable bottom as you mention. If more caster than I could easily get didn't really do much for my combo then I'd call it a day.

 

Cary

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Sorry for dropping out for a few days guys... and Cary, I'm extra sorry I caused you to actually read the rules! :P Now that you have read them, I agree with your interpretations... The question of WHERE the wheelbase should be relative to the chassis is a whole other can of worms, since the drawings someone else posted in another thread dimension the wheelbase from the bumpers. So long as you have a wheelbase of 91.7" or less and the wheels look like they are where they are supposed to be, I can almost guarantee you will never have protest problems...

 

Back to my car and what I am planning based on our discussions...

 

Playing with the geometry software, and assuming a fair amount of error in what it is telling me I am planning to raise the inner pivot by about 1.25". My current control arms are pretty close to level, but even with the 1" bump spacers the inner pivot is about 1" below the balljoint (the balljoint was one of the harder dimensions to guess since it is hidden by the rubber boot)(pic below). If I can get the inner pivot up above the ball joint by a 1/4" that brings my roll center to about 2.5" above ground and keeps it within about 15" of the centerline in a roll state. Which is about a 25" improvement over where I am now! If I move the inner pivots inboard, I reduce that 15" by about 2" for every 1" I move inboard, but going inboard forces more extensive modifications of the crossmember and probably screws up my bumpsteer in a way that is not easily corrected. It would also mean making longer control arms... So I am thinking the inboard move is probably not worth the trouble.

 

Next on the change list is moving the upper strut mount back to gain some caster. I am also planning to go with adjustable TC rods to fine tune the caster. My wheel base is kind of short right now (about 1/2" under spec at 90.2"). So moving the upper mount back 1/2" and the balljoint forward 1/2" I should be sitting at around 5 degrees of caster which is a good start.

 

I've included some screen shots of the current and proposed geometry (if I got the files small enough) showing the how the compare sitting straight and with 2degrees of roll and a moderate amount of steering input. I find interesting that the roll center changes sides as you go from below to above ground level.

 

Tom

RC-straight_thumb.jpg

front-arm_thumb.jpg

RC-turning_thumb.jpg

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Off topic a bit...

 

If you're going to tear into things you might want to move your anti-roll bar up and run it through the frame rails (if you have enough clearance to the oil pump and balancer) and attach the end links directly to the strut tube. That was one of the front end mods I really wanted to do to my car but never got a chance. My thinking was the ARB would be more responsive, lighter, and I could probably reduce the ARB rate a bit.

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