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More Caster With Power Steering?

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With a few new power steering kits out there, would dialing in more caster in the front be helpful? I just installed power steering and will put on short steering knuckles soon, so will need an alignment. I was going to use these specs from the alignment sticky:

Street

For 225 to 245 width radial DOT tires try these alignment settings:

 

Front

 

Camber: 1.25 to 1.5 neg

Caster: 4 to 5 positive (even on both sides)

Toe: 1/16" out or 0

Ride height: 6" to 6 1/2" measured at the front of the rocker outboard of the pinch weld seam.

 

Rear

 

Camber: 1.25 to 1.5 neg

Toe: 1/16" to 1/8" in

Ride height: 6 1/2" to 7" measured at the front of the rocker outboard of the pinch weld seam.

 

For tire widths over 245 try these alignment settings:

 

Front

 

Camber: 1.25 to 1.5 neg

Caster: 4 to 5 positive (even on both sides)

Toe: 1/16" out or 0

Ride height: 6 1/2" to 7" measured at the front of the rocker outboard of the pinch weld seam.

 

Rear

 

Camber: 1.25 to 1.5 neg

Toe: 1/16" to 1/8" in

Ride height: 7" to 7 1/2" measured at the front of the rocker outboard of the pinch weld seam.

 

Would more caster be a bad thing? And what about 0 or 1/16 toe in?  I have slight toe out, between that and bumpsteer, my car wanders on the street.

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Toe Out is an old Autocrosser trick and yes it will lead to wandering on the street. Some tires will " Tramline" more than others. Toe Out can be wicked bad in the Rain. Fact is, Toe out is seldom needed in Autocross. It's a crutch to get the front end to turn in, when other solutions are available. I've Road Raced, Autocrossed and Hill-Climbed for many, many years. Mostly with Big HP RWD cars. Have NEVER needed to use Toe Out to get the front end to bite on Turn In or during any other phase of cornering.. And I like a nose that turns in very sharply.  

Most people run too much front spring and anti-roll bar in Autocross. And not enough rear brake. A good Autocross car should be wickedly unstable at Road Racing and Hillclimbs. Totally different setups.

Only exception is FWD cars... they may need Toe Out in Autocross. But those are horrible things any way!! LOL. 

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I also have been looking at a lot of new RWD cars with IRS suspensions in traffic. Porsche, BMW, some American cars.... It appears that they have more negative camber on the rear than on the front. Is this a better way to setup suspension? Something like back in the 50s, 60s, some cars had positive camber, and over the years they figured out negative is better for handling, so now everyone goes for negative?

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Posted (edited)

I think there are a lot of variables.

 

The factory ALWAYS designs cars to understeer at the limit.  That means that they always want the front suspension to reach its traction limit before the rear.

 

Many newer sports cars have an SLA front suspension and a strut rear suspension, or they have struts at all four corners with tons of front caster.  The SLA will almost always have more camber gain than a strut set-up, so the designers may be adding rear camber so that the rear will be optimized relative to the front at the lateral limit.  Likewise, the newer front strut designs utilize a ton of caster compared to older designs.  The added caster adds camber gain to the front wheels as steering angle is applied.  So again, the rear needs added camber to match the front when both ends are near their limit of traction.

 

The S30 is different than the newer designs (at least in stock form).  The rear struts have more camber gain than the front (the fronts have about .5 degrees per inch of travel, and the rear have about 0.75 degrees per in of travel).  You can see this on a lowered Z with stock suspension:  The rear gains a lot of negative camber, but the front gains very little.  Also, the stock front caster is really low compared to newer designs (<3 degrees), so there is no camber gain as the wheel is turned.

 

If you add significant caster to the front of an S30 (>6 degrees), then you get the benefits of camber gain as the wheel is turned.  You can then run less static camber, and you can run almost equal camber front and rear.

 

I run 6.5 degrees caster and -1.8 degrees camber up front.  I run -1.5 degrees camber in the rear.

Edited by 74_5.0L_Z

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Posted (edited)

For a street driven car, I would say dial in as much caster as possible without having clearance or binding problems.  Unfortunately, adding caster is difficult to do correctly.  If you add caster by lengthening the T/C rod then you have to have spherical bearings at the LCA to cross-member connection to avoid binding.  You have to push the lower ball joint forward 1.25 "  to get 6.4 degrees of caster.  Because the LCA attachment to the cross-member is fixed,  the LCA becomes angled forward pretty significantly as seen from above (See attached image).

 

I used to set the car up with the control arms angled forward, but that causes other issues.  The angled LCA causes loss of static camber.  With camber plates you can compensate, but that is a band-aid.  Another issue to deal with when you angle the arm forward is clearance between the tire and frame and tire and forward section of fender.  Again, things that can be dealt with but have to be considered.

 

In my current set-up, I moved the top of the strut back 3/4", lengthened the T/C rod to move the lower ball joint forward ~.5 inches, and made a cross-member that relocates the LCA attach point forward so that the LCA is perpendicular to the chassis center line at my desired caster.

TopView.png

Edited by 74_5.0L_Z

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This conversation has got me really interested in adding in some front caster angle. In the case where you moved the upper mount backwards, how did you accomplish that, and did your changes have any notable effect on the wheel's position in the wheel well? I came across this tidbit on ZHome about just moving the crossmember and lengthening the TC rod, but I'm worried that will move the wheel too far forward.

 

Of course in any situation of moving the crossmember, engine mounts and steering changes will have to be made to compensate (unless I move the rack back an equal amount like I've seen some threads mention...)

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How much caster do you think it's safe to go with an all adjustable suspension (TTT) for a street car? I can't remember what I have now, but I think it's a little more than stock. I'll have to see if I still have my photo with the settings of everything.

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Unfortunately, it is really hard to get more than about 5 degrees on an S30 without major modifications.  I'm at about 6.5 degrees and I have moved my cross-member forward and the top of the struts backward.   

 

Yes you do get benefit from the effort.  Just know that there is a lot involved in getting it right.  This is what 6.5 degrees looks like in side view.

Front_Suspension_2.JPG

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