CorrdoBrit

The problem I'm having, even before replacing the stock CupII 17" rims with 18" Enkei RC5, is that at freeway speeds the tracking would be poor and car would shimmy. I've visually checked the control arm bushings and don't see any obvious damage or degradation. Car has 33K miles. The car had a full alignment performed incl kinematic toe, after replacing the stock suspension with Bilstein HD and M033 springs and RS steering rack brace at ~23K miles.

I know the PO set the car up for more stable handling in the mountains in CO where he was living. In fact the front end was set a little out of spec intentionally. Here are the numbers:

LF ........ RF
-0.5deg -0.4deg camber (OUT OF SPEC)

5.1deg 4.9deg caster

0.03deg 0.02deg toe

0.05deg total toe
0deg steer ahead

LR ....... RR
-1.1deg -1.1deg camber

0.1deg 0.11deg toe

0.2deg total toe
-0.01deg thrust angle

Would bringing the front end back into spec be sufficient to fix the wayward handling issues or do I need to investigate the suspension bushings further?

trophy

Castor on the front should be identical, Same with side to Side toe.

Adjusting the front end should make the world of difference for you.

race911

Loaded or unloaded alignment? And honestly, you'd be amazed how you can do an alignment on a rack, then string it up at the track, coming up with pretty different numbers. (Not to mention eyeball track alignments after having to repair something, then going out and running as fast as you ever did.)

Bill Verburg

LF ........ RF
-0.5deg -0.4deg camber (OUT OF SPEC):That's mid way between stock normal and stock RS, ideally the same both sides but that's ok

5.1deg 4.9deg caster: the lower you go the harder to get to spec, ideally the same L/R but 5° 20'+15'-30' is the spec They are often differ by .1' side to side

0.03deg 0.02deg toe: below spec but a lot of cars run down to zero or even toe out

LR ....... RR
-1.1deg -1.1deg camber: a little below spec -1°10'+/-15' to -1° 20' +/-10'for RS, again lots of us double or triple that

0.1deg 0.11deg toe: light I'd double that at least, less toe means less braking stability and more quicker rear steer effects

You don't mention KT, factory spec w/ the Motorsport gauge is +3° 00' max

This is the Motorsport gauge for indirect caster measurement, it is showing ~5.75 units I'd recommend no more than 3 or 3.5units, the higher the units the faster and twitchier the rear end will be. You can get a very rough idea of the setting by looking at the wheels wrt the wheel well opening, the higher the KT unit setting the more down and forward the wheel will be in the wheel opening, the lower the further back toward the muffler and upp into the well.


shimmy @ highway spoeeds is more often wheel/tire balance or roundness. Irregular tire wear contributes too.

CorrdoBrit

Thanks Bill. Don't have details for the KT setting. All I have is a printout with the info I quoted. At the widest part of the rear tire its 5cm to the skirt and 7cm to the rear bumper.

Since the last time I drove the car I've had the front wheels balanced. One rim had no weights at all so this may have contributed to the effects I was experiencing. I'll report back once I've had a chance to take a test drive.

EMBPilot

they are 20+ years old, you need to change them. they can look fine and still be bad. what you are describing sounds like bad front control arm bushings.

pp000830

If installed on the rear wheel carriers, can one just leave them on the car, maybe slide a little fuel hose over them to protect the sights glass from road debris?
Andy


As an alternative solution I just purchased a electronic tilt gauge, $26 on Amazon, to take some reference settings off the kinematic toe arms as a reference for future alignments.

squidge

+1, just cured the same shimmy in my car by replacing the bushings.

Edward

I'll leave the discussion of specs to the aforementioned gurus. But as for this shimmy you mention, many here have reported having gotten that "50mph" steering wheel shake. For me, I knew it had nothing to do with balance or tires since the shake came in at around 53mph, and then went away at around 59 mph, and then remained even after I had replaced tires. I lived with it for maybe a couple of years until it got annoying enough for me to fix it: "Walrod" bushings for the front control arms and a proper alignment, done.

And yes, my bushings looked perfectly fine to the naked eye. Moreover, I had not done an alignment in many years and the numbers had drifted from where I had spec'd it originally (I checked my records). End result is the car now tracks perfectly smooth and feels more sure-footed than I can recall (remember, since it had been many years since my alignment so it had been degrading gradually). I suspect that once you address the aforementioned alignment for the rear and get the front control arm bushings done, you will be very pleased.

Edward

AX993

Hi Bill,
Somehow I thought that the LOWER the Motorsports KT tool unit number the Looser/ more prone to oversteer the rear end is ???

Bill Verburg

to take take caster out you lengthen the KT arm, this pushes the wheel down and forward(left & down in the pic), take a look at the bubble, where will it float to when that happens?



A lot of the confusion has come from early discussions where people describe maxing the caster or maxing the kt, It was rarely(I never found it anyway) explained what was meant by that, did they lengthen the Kt arm or shorten it.

less caster implies a faster twitchier rear end w/ less rear steer from the KT effects induced by the bushes.

There are 2 different mechanisms that induce rear steer
1) KT designed into the bushes at the KT arm & rear leg of the A-arm
2) geometry induced toe change due to geometry & vertical wheel movement

It's important to keep in mind that there is still kinetic rear steer(aka rear bump steer) even w/ solid bushes. The vertical motion of the wheel still induces some toe change due to the geometry when it's in the front this is also called bump steer.

The same geometry variables apply to front and rear, less caster faster reaction, more caster slower reaction, In front you generally want lots of caster on a track car to make it easier to control, for A/X you might want to dial it down to make the car turn quicker. For street you want as much as you can get.

AX993

Bill,

Let me see if I understand you correctly. On the picture you posted of the Motorsports KT tool, if the bubble is at "1", that wheel is set at a high amount of caster. If the bubble is at "8", then the wheel is set at very little caster ?

If so then the Motorsports KT gauge unit number scale is backwards as far as increasing or decreasing amounts of caster?

MS KT Tool units: 8---------------1
positive caster: Less--------------More

???

AX993

Very Sorry for derailing this thread !!

Bill Verburg

No if the bubble goes to 1 it's min caster, again lengthening the KY arm removes caster it moves the hub down and forward, shortening the KT arm adds caster, the hub hub moves up and back, visually a high caster setup has the tires moved back almost to( and even to) the muffler cans, a low caster setup has the tire move forward in the wheel well.


factory spec for lowered Cup & RSR is +3° 00' max, I take that to mean 3 on the gauge, but could be wrong about that

There is a different aftermarket KT gauge, seen here, it's less precise but is showing low castor

AX993

Thanks for the clarification Bill.