Front Upper Control Arms...

ValidusVentus

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?

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jchammond

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.

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BGTV8

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".

ValidusVentus

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

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Rusty

What are your alignment specs?

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BGTV8

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

MaysEffect

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.

ValidusVentus

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.

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MaysEffect

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.

Rusty

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.

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MaysEffect

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.

Rusty

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

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Hotrodz

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.

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Rusty

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

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MaysEffect

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.