I thought I'd throw this out to the experts. I have a front end shimmy at about 105mph. It's not a wheel balance issue as there is zero evidence at ~60mph where those usually show up. The car is a fresh build and everything on the front end is new or in very good shape. Some details on the car.

69 Camaro RS/SS 350
Stock subframe (modified for tires and fully seam welded)
Hotckiss springs
Bilstien shocks
DSE upper arms
Speedtech lower arms
Factory quick ratio PS box
Guldstrand mod
P275/40ZR-17 tires
17 x 8 5.25 backspace wheels
2002 LS1 discs on 69 drum hubs
Factory disc spindles
2002 ls6 engine (lighter than stock smallblock)

That's all I can think of..........

The alignment is currently:

Right:
Camber -3/8 degree
Caster 4.625 degrees

Left:
Camber -1/2 degree
Caster 4.25 degree

Toe-in 1/8 total

I'm very confident on the alignment numbers as I've checked/done it three times now looking for something. The car drives great and very straight down the street. It have very nimble response to turns. Overall I like everything about it except for the shimmy.





I aligned it to basically "David's specs" on this page:

ALIGNMENT SPECS
David's Specs

Caster 5 deg positive, or as much positive as you can achieve up to 5 deg, can use .5 deg more positive on the passenger side to compensate for road crown.
Camber -.25 degee for mostly freeway driving, -.5 for street and occasional hard cornering, -1 deg for street, Autocross, and Open Track. you can use even more if tire temps show outside edge of tread is hotter than inside. If the inner edges of tread show excessive wear, use less neg camber. If you have not done the Guldstrand mod, you can add .25 to .5 deg to the above camber settings.
Toe in, 1/16" to 1/8".
Guldstrand recommends the following specs for a "touring car":

Caster: 3 - 4 degrees positive (+) PS = more, manual steer less.
Camber: 1/4 to 1/2 degrees negative (-)
With upper a-arm relocation Camber: 0 degrees
Toe-in: 0 - 1/8 inch
Guldstrand recomends for racing:
(assumes "Guldstrand mod" is done)

Caster 3 to 4 1/8 deg pos
Camber 1.5 to 2 deg neg
Toe in 1/8" out to 1/8" in (I would try 1/8" out first) more neg camber = more toe out.
Herb Adams recomends these alignment specs for the street:

1 degree positive caster ( I would use 3.5 degrees if PS)
1 degree negative camber.
1/16" toe in
Stock factory alignment specs:

Caster +1/2 deg
Camber + 1/4 to 1/2 deg
Toe-in 1/8" to 1/4" (Radial tires need less toe-in, use 1/8" or less for them)
IN ALL POWER STEERING EQUIPPED CASES, USE AS MUCH POSITIVE CASTER AS YOU CAN ACHIEVE UP TO 5.5 DEGREES POSITIVE MAXIMUM. MORE CASTER HELPS TILT THE WHEEL INWARD WHEN CORNERING, HELPS REDUCE BUMPSTEER BY LOWERING THE TIE ROD ENDS. MANUAL STEER CARS WILL REQUIRE MORE EFFORT AT HIGH CASTER SETTINGS, YOU MAY HAVE TO USE LESS.




Anybody have any ideas ?

You might return and have the wheels 'fine' balanced meaning the operator has to hold the button down while spinning to collect the finer/higher imbalance. you're looking for a higher imbalance, higher harmonics.

If you can find a plca with the motorized wheel balancer on the car, sure helps.

Measure the runout of the mounted wheel at the bead, should be less than 0.010".
Might take and give the wheel bearings 1/2 flat/slack on adjustment rather than one flat slack from tight.

Ask the road rally guys where they get their wheels balanced and take yours there. Most Goodyear stores do this.

I have two. First one is wheel balance. I spin balanced the tires on a 240Z back in the day. The owner came back with a shimmy at over 100 MPH.(140 Maybe?) we rebalanced them in calibration mode and found one tire off by .25 oz. Shimmy solved. My other thought is caster shake. This problem is much more common on 4X4 with oversized tires and requires a steering stabilizer shock. I kinda doubt that you have enough tire weight for this to be an issue but it might be worth considering. The easy cure for it would be less caster.

Jeff

Make sure that the tires are checked for both Static (single plane) and Dynamic Balance (multiple planes). Then do what Everett suggested have them "fine" balanced. You are going to need a modern tire balancer machine to do what you need (especially for the fine balance). So you might have to check around for a shop that can do it.

So three votes for just a balance problem. Boy that's sure gonna be an easy fix! I'll come back and let you guys know if that was it.

I'd just try to keep it under 100mph :D

If tire balance is confirmed, check your brake rotor balance.
What subframe to unibody mounts do you have?

If tire balance is confirmed, check your brake rotor balance.
What subframe to unibody mounts do you have?

Solid aluminum sub-frame bushings.