Front Upper Control Arms...
 

I currently have the SPL front upper control arms.

With the way they are designed, they change the SAI angle and hence the scrub radius as you adjust the camber. As the SAI increases with the camber increase, and working with the caster, it creates more jacking of the car on the inside tire when the wheel is turned, effectively leaning the car away from corners. I tend to view this negatively. (although I do like the increased steering effort and feedback)

Does anyone have any thoughts on this or comparisons to other FUCA designs for the 370 in regards to this line of thought?

I can't personally answer these question's, but from everything i've heard about the SPL brand parts-is nothing but exceptional.
You currently have the best product on your Z...People whom track their car will chime in & give details about the (Steering Axis Inclination) & any other concern's.
You may need some additional part's to achieve best result's.
:)

The situation you describe puts more weight on the inside tyre which means it contributes more grip towards turning the car rather than the alternative which places more load on the outside tyre.

remember you get lateral weight transfer when rotating the car and the problem is how to "share" the effort across both front tyres.

I actually think that SPL have it "right" .......

As the car rolls onto the outside front tyre with weight transfer, the effect of the FUCA with increased lock puts more weight onto the inside tyre and so it is able to share more of the available grip rather than letting the outside tyre do "all the work".

Thanks for the reply, you may be correct. But my thoughts are along the following line: in order to increase the actual weight over the inside tire the car has to actually push into the ground harder... and to generate a force its got to be pushing against something else. And the amount of weight transfer is only affected by the CG height, track width and weight of the car. All other factors just affect the roll angle attained. I'm thinking that just because the car raises on one side, that doesn't mean its pushing into the ground harder, the only factor changed by the weight transfer would be how hard it has to work to raise one side of the car or another. ...Or would that force possibly even be transmitted back to the outside tires via the roll bars and springs? My head is starting to hurt.

Admittedly though, only in the very tightest of on track corners would this effect possibly be significant I suppose. With its noticeable effect on the steering weight and self centering I was also considering if it would contribute to almost a pendulum like effect with larger quick corrections; which seems to be something I do feel a bit of. Tough to say.

jchammond I would say I've tracked the car quite a bit but I still like to ponder some of these more difficult to measure things (especially while I don't have access to my car or track time) and was just looking for other's thoughts about it. I agree about SPL, have several of their products on my Z :)

What are your alignment specs?

We are talking about "jacking". If you can use suspension setting to "jack" the inside of the car when turning, especially when you are starting the turn and wanting the car to rotate, the act of arranging suspension to "push" down on the road with steering input (eg: LHF with turn to the left), then there will be an equal and opposite reaction of the road surface pushing against the tyre which will improve grip available from that tyre until the roll moment starts the take over.

Take a peek at the smithees website (URL) below where he goes into it in some depth.

Race Car Suspension Set-Up

The SR does not change in this situation. Only the SAI. The SR would only change if you changed the angle of the upright and not the camber and caster.

The only way you'd change the SR is if you change the wheel offset, or somehow modify the upright to change inclination.

Jacking would be increased during turning if you increased caster angle. In this case, don't increase caster.

The design of the SPL FUCAs is such that it adds camber by changing the actual length of the upper A arm.

And I tried to go to the link above but it doesn't seem to work on the gov computers unfortunately, they never seem to like links that end in a foreign abbreviation.

Right. This only changes the SAI. Because the change in geometry also changes the wheel center line angle, there is no change in SR.

If you increase caster along with camber you will have increased rise and fall.

Adding stiffer components won't help much here. It would mostly just make steering more sensitive.

Any adjustable control arm for the Z will change the distance between the ball-joint and the control arm pivots. If you want to change the camber. There is no way around it. You have to live with it. The only way to get around what you are describing, is to move the control arm pivots in or out. Ain't going to happen. With the SPL's. Now you can change the caster too. Slicks need caster. Go to Hooser's web site. They will tell you to run as much + caster that you can. When you start making changes, you will have to live with the compromises.

That's an extremely vague suggestion that should be taken with a grain of salt.

Racing tires needed additional camber, adding positive caster is another way of getting more camber with additional steering lock. The tires dont need caster, in most cases its worse for the tires as it pulls the tire away from the road.

Increased negative scrub and wider tires is necessary if you increase camber and caster.

A little info from Toyo tires.

Toyo Tires recommends the following general set-up guidelines for the Proxes R888:

Operating Temperature: 160°F to 220°F
Hot Inflation Pressures: 32 to 38 (psi)
Camber: -1° to -3°
Caster: As much positive as possible

My r888 setup:
Hot Inflation Pressure: less than 36 psi tire will get squirrelly 36 psi or better.
Camber: -2.26 front -2.00 rear
Caster: 6.5 and I think 7 to 8 would be better

There was another member here running -3.3 front and -1.8 rear with caster at 8 for the same tire. He was running 18x315 and I am running 18x295.

Yeah, I'm running -2.0 camber and +7 caster front and -1.75 camber rear. :driving:

That is equally vague. 32-38psi covers a huge variety of load ranges of about 300ibs per axle. 2 degrees of camber variation is also a huge margin for both steering geometry and tire contact. Changing the caster by that amount (4 degrees to 6 degrees) would possibly change your camber at full lock by 1-4 degrees.

This also doesn't take into account the wide variety of suspension setups these tire companies are advertising to. Cars without sway bars, cars with solid rear axles, cars with McPherson struts.

None of this is a problem for the Z. So additional caster AND camber is not a necessary change, its the reason nissan knocked the Z34 down to -+5 degrees of caster. If you want more response out of the tires, properly match the SR for the given wheel/tire you are trying to use. Not an easy thing to do when dealing with clearance and fitment issues (brakes or fenders). So yes, this is the ultimate compromise. Adding caster and camber ruins your instantaneous roll center. Something that will severely compromise how the suspension balance SHOULD be setup.

Have you tries 3 degrees with low levels of caster and less rake? and possibly different shock settings.

No I have not. I am dialing things in slowly. I disconnected my rear sway bar and to help rotate the rear a bit more and so far I like. My next move was to add more negative camber, -2.8 to -3.0 since I don't do much street driving with her anymore. What do you consider low levels of caster?

5 degrees. I don't think i have any fair bases of suggestions for your situation though, you are well beyond the level of modifications to start making large dynamic changes again. Unless you like doing that...

My point was simply adding both camber and caster is not necessary for this chassis or the scenario of changing the SAI. It doesn't suffer from suspension limitations like some other cars with inferior suspension setups.

8 degrees of caster is up there with rally cars, and RX cars use low levels of camber for better steering linearity and tracking through loose gravel. Something that is not a issue for cars on tarmac.

It also comes down to how anyone adjusts driving techniques for the setup implemented. Caster levels and rake significantly changes steering feel. If you're making modifications to the suspension to alter this, then changing geometry again will just slow you down and cause confusion. If you're not wearing your tires out incorrectly then there really isn't really a reason to flip the script.

Could you explain or provide a link explaining this. We have gotten slightly off topic on the thread but its a good discussion. I'm going to do some research on my own regardless.
I am currently at +6deg caster and -2.2 camber up front I believe. IIRC it wasn't possible to get more caster with the SPL arms without forcing the other variables off, though I was under the impression -mostly from scuttlebutt- that even more caster would have been favorable and was seeing this as a limitation for the SPL arms. I know I need additional camber all around (from tire evidence) but I still drove it on the street a bunch over the last few years in Cali; with no AC/blower/interior, race seats and a full cage. Now I'm up in Washington its a different story.

I don't want to turn this into another "what's the best suspension settings!?" thread but I think there is an opportunity to learn something here.

I think the discussion is well within the original scope of things. All these problems stem from the concern of rise/fall...weight jacking, and tire traction loss.

Compromised instantaneous roll center is the dilemma behind having too much rise/fall with "incorrect" front wheel geometry. Increased SAI, increased camber and increased caster will be the biggest upsets for this. The notion behind adding as much caster as possible is to get as much camber as possible, but at the cost of reduced pitch control (rise/fall). There are several workarounds for this. One of which is Hotrodz common choice of removing the rear ASB. The other is playing around with rake. The most uncommon is reverse rake, usually only seen on WRC cars and drift cars. The least common setup is reducing the front ASB stiffness.

Like Rusty eluded too, there are trade offs and compromises. Too me...adding caster AND high levels of camber are not ideal trade offs. -2 degrees of static camber is probably the most optimal setting if you are going to the route of increased caster, the issue is when you starting getting close to -3 degrees and around 7+ degrees of caster. Right now you are currently in a safe zone.

The issue is much more evident on low profile tires or tires smaller than the rim width and setups with positive SR values.


PM me if you want links and guides on the roll center issue. There are a lot of references.

Thanks for the detailed reply. I find this stuff fascinating. Will PM.

Agreed, I am not looking to change much as my tire ware has been good. I'm looking for that balance of grip and handling. Which is a never ending chase lol!