Dougs951S

I've been chasing a front end issue for a while, and finally took the car down to get laser aligned, where I promptly realized that my understanding of the caster eccentric was backwards and therefore, it was way off. (For reference, Maximum caster is when the tabs are at 3 O clock, pointed directly toward the center of the car, and minimum caster is when the tabs are pointed away from the car) They did not have the tool to align the rear and I did not bring it, so I will be returning to have them realign it next week. Here are the specs when I left the shop.

Front Camber: -4 degrees both sides
front caster: 3.4 degrees on driver side, 3.0 degrees on passenger
Front toe: .1 degree toed in on both sides

Rear Camber: -1.3 degrees both sides
Rear toe: .38 degrees toed in on driver side, .78 degrees toed in on passenger side
Rear thrust angle: .28 degrees to the right

The front camber is so negative because I am running late offset control arms and tie rod with early hubs/spindles/brakes. -4 degrees was as low as we could get it.

The car drives well and almost perfectly straight, I feel like it just needs ~.2-.3 more caster on the driver side to make it straight as an arrow. If I let go of the wheel on a straight highway going 70, it will take a few hundred yards of forward motion to drift far enough left that it will exit the lane. I'd also like more caster overall because the on center stability isnt as tight as I'd like. My concern is this, most cars are set up with cross caster in order to compensate for road crown. On most cars though, the caster is biased toward the passenger side. Why does my car require the caster being biased on the driver side in order to drive straight? Is this a cause for concern, or should I just get it realigned and drive it? I'm planning on converting to early offset steel arms soon to pull back the extremely negative front camber, but my question is solely concerning the caster split.

Cheers gents

StoogeMoe

I don't think your problem is the castor. Your toe-in for the rear is way off and no wonder why it pulls to the left. You need to get that straightened out first (pun intended)! Having that much camber in the front will also make it more sensitive to straight line stability. If this isn't for the track, then you need to get your camber to more reasonable levels.

Dougs951S

I had thought it might be the rear as well, but with the thrust angle being slightly toward the right I figured it would pull right. When I bring it back to get it aligned, should I have them set it with a bit more caster on the passenger side as is typical, or should I keep the driver side bias I have now? Do you think it will pull to the right a lot after If I keep the current cross caster? I'd planning on adding 1* to both sides as well.

The Camber is being addressed, I have a new set of early offset tie rod, early caster clips, 1 early sway bar mount, and steel control arm bushings in my inventory, and I ordered new steel control arms and ball joints last night. Its all ready to go, I just need to get a hold of 1 extra early sway bar mount so I can transfer my 24mm hollow bar. My car should be early offset by sometime next week, and then I'll take it in for a final alignment.

StoogeMoe

I assume you have looked at the FSM for the castor setting. They specify 2 degrees 30 minutes +30 minutes / -15 minutes, with a maximum difference of 30 minutes left to right. You're at the max on the passenger side and a bit over on the drivers. You are within spec for the difference between left and right.

The reason why I would think it would pull left is because your right rear toe is much larger then the left. You have 0.78 degrees and its supposed to be 0, plus/minus 5 minutes. So your rear wheels are pointing to the left effectively. Then again, maybe since its the rear, it works the opposite. In any case, you need to get those to zero. Toe-in probably affects handling more than the other two settings combined.

Dougs951S

Just wanted to recap this thread, I am now running early offset steel arms on my 86 951. It feels great to have all new bushings and ball joints, really tight. Here are my new alignment specs.

Front driver side: -1.2 camber, +6 caster, -.02 toe out

Front passenger side: -1.2 camber, +7 caster, -.02 toe out

Rear driver side: -1.1 camber, .01 toe in

Rear passenger side: -1.2 camber, .01 toe in

0 thrust angle

This leads me to ask a question, on the front end on the driver side, 6 degrees caster was maxed out. On the passenger side, the 7 degrees we were able to get was the MINIMUM caster we could dial in, and I could have easily dialed in 8+ degrees. The car drives straight with the 1 degree split, but do the early cars naturally have much more caster than the late? I was never able to get more than ~3.5 caster with late offset arms. Is it ok to have that much caster, and is there a way to change the amount of caster I'm able to get by moving the rear brackets, such as by loosening the front control arm bolt before trying to adjust the caster at the rear? I'm just wondering why the amounts I was able to get were so different on both sides with all new parts. I really would have preferred less caster on both sides but it just wasn't possible. Running early 951 brakes, early 951 hubs and spindles, early control arms, early N/A strut bodies.

Van

I have never heard of it being "typical" to have more caster on the passenger side... I've always wanted (and had) it even on both sides.


On the early cars (with the steel control arms), the *only* caster adjustment is by loosening the bolts that hold the caster mounts to the body (at the rear of the control arm) and slide the mount side-to-side. As the mount moves inboard, the caster angle will decrease.

The late cars, with the aluminum control arms, have the eccentric caster mount for this adjustment (though there is still a little adjustment you can make with the mounts).

Also, to make sure you understand the mechanics of caster, this is the angle of the strut compared to vertical when you're looking at the side of the car. The point is to have the contact patch of the tire trailing behind the pivot axis for the steering. This trailing behind is was make the wheel want to "self straighten" while you're turning - like a shopping cart's "caster wheel".

When the wheels are pointed straight ahead (assuming your toe-in settings are correct) caster won't pull the car left or right... Imagine you're ice skating... If both skates have the blades going parallel, you're going to go in a straight line - even if you move one foot slightly ahead of the other. When the blades are straight ahead, you go straight ahead.

If your caster settings are different, what you'll feel is a different amount of steering force required in the steering wheel to turn one direction or the other when the cornering g-forces build up. As you hold the steering wheel on a steady arc, your holding force is countered by the caster - and because of weight transfer, the outside wheel will be doing more work. This is one of those "for every action there's an equal and opposite reaction" things.

kev951

thrust angle needs to be as close to 0 as possible. .5 degree caster split is normal. even up to 1 whole degree on some cars. fix your toe and why the hell are you running -4 degrees camber up front?

Dougs951S

See above post, I'm not running anywhere close to those alignment specs anymore. My question still stands concerning the evidently much larger caster angle built into the geometry of the early control arm cars vs the late cars.