Caster Issues

[Ron R.]

So I finally got around to taking my car to a good alignment shop for a corner ballance and alignment. The local wizard informs me that the caster is off 2 degrees side to side, and of course, isn't adjustable.

We talked a little bit about how to compensate, but everything is just a compromise. It may not sound like much, but 2 degrees is a lot.

Anybody else have this problem? What did you do about it? Should I talk to SPF?

 

[Seymour Snerd]

...2 degrees side to side, and of course, isn't adjustable.

I don't know if I have it, but thanks for the heads up.

My first question would be "what is it supposed to be?" After all, even if you could figure out a way to correct it, what would you set it to?

Regarding that, take a look at Dave's thread: http://www.gt40s.com/forum/gt40-tec...s/25964-castor-heavy-steering.html#post234739

Phill suggests that the originals adjusted it with shims, and I think the SPF frame could be modified to do so by shortening the tubes through which the upper (or lower) A-armspivots pass, and then shimming as needed. Failing that, or perhaps better yet, the inner pivot ball joint spacers could be shortened as well.

 

[Ron R.]

Alan,

I don't know...the specs aren't published anywhere that I'm aware of. But, mine are 4 degrees on the left, and 6 degrees on the right. This would cause the car to naturally pull to the right, all things being equal.

I can't imagine its suppose to be that way.

 

[Seymour Snerd]

Alan,

I don't know...the specs aren't published anywhere that I'm aware of. But, mine are 4 degrees on the left, and 6 degrees on the right. This would cause the car to naturally pull to the right, all things being equal.

I can't imagine its suppose to be that way.

I suspect we have to go with what works for other similar cars and tires. I kept editing my post so you may not have seen this reference:

http://www.gt40s.com/forum/gt40-tec...s/25964-castor-heavy-steering.html#post234739

And per my edits I think the hat-shaped spacers within the a-arm inner ball joints could be machined down to create some adjustment room using shims. Obviously not a job for a normal alignment shop but a full-service race shop should be able to help.

Also see http://www.gt40s.com/forum/gt40-tec...-heavy-steering-racing-speeds.html#post194505 and the books referenced therein. Warning: it's a long thread and meanders a bit but there's some really good discussion in there.

 

[Ron R.]

Alan,

The possibility of offset ball joints, shims etc were some of the things that we discussed. I just wondered if my car was the only one that had this issue.

I'll probably end up giving Dennis Olthoff a call.

 

[Ron R.]

Ok,

Per "a well known" shop, they don't mess with the caster, and just set the car up without addressing the issue.

 

[Cliff Beer]

Huh?

Caster should be checked in the process of a proper alignment by any race shop. For a GT40 that includes both front and rear caster, though the front is obviously much more important.

That said, caster is generally less sensitive (by far) than camber or toe. An 1/8 degree on toe can be a big deal, a 1/2 degree on camber can be similarly significant. However, if you're +/- 1 degree on caster that's probably OK. Your net delta left v. right is 2 degrees and that's not ideal, but I'd be surprised if you felt that too much in just general driving.

My CAV has adjustable fore/aft slots for the inboard mounts of the upper wishbones. Basic, but it works well for setting caster. Perhaps that's a possible adjustment or modification for you car? Alternatively, you can adjust caster by cutting and inserting an adjustable link in one of the upper wishbone legs. This is less ideal as adjusting in this way monkeys with camber also.

Caster can be sensitive to relative ride height so make sure the caster readings are done with a car that's perfectly leveled left-to-right.

At my last race shop alignment I believe the caster was set to 4.5 degrees left/right, making the steering fairly light in turns but not a lot of self-centering to be honest.

Just my $.02.

 

[Ron R.]

Not a high priority right now, I'll decide after I get off the track the first time.

Long term, it will probably bug me enough that I'll end up doing something about it.

BTW, had it corner weighted, wasn't off by too much straight from the factory...100lbs, but now all four courners are within 4 lbs with me in the car, and a half tank of gas.

....only took 5 hours of shop time to get there.

 

[Phil]

Gents:
I have done one or two sets of upper wishbones for original chassis with an offset because the arm could not be moved rearward enough to get sufficient caster, usually about .450 offset to the rear with the ball joint.
There should be some adjustment at the mounts, spacers or whatever to at least even out the caster side to side.
If you do add positive caster you will go more positive camber because of the taper of the mounts at the chassis, so that would need to be addressed.
I myself prefer 4-5 deg positive caster and some say it makes steering heavy, but with a light vehicle it is negligible and provides a lot of stability at speed, just my preference.
Cheers
Phil

 

[Ron Earp]

We just got done finishing setting up three of our cars for an SCCA points championship in Savannah in less than two weeks. Clearly everyone has their hot buttons for setup, but caster isn't at the top of my list.

We do adjust caster and with the Mustangs we spent considerable setup time going between a lot (7-8 degrees) and a little (2-3 degrees) as we modified the streering geometry over the course of the year. The effects are well covered here, but the left right balance with caster isn't going to cause you concern within a degree or two. As mentioned, I doubt you'll notice it at all. Far more important are getting the spring rates correct, corner weighting, camber, toe, height/roll centers, wedge (if any), and damper settings all spot on for the best handling experience.

Yay setup. As you know, setup is by far the most time consuming part of messing with a race or track car. I sort of thought moving to the Mustang from the Lola might ease that a bit as the rear axle takes away a lot of adjustability. Sort of - but then you have to work like the devil to get the axle to behave and be adjustable.

 

[Ron R.]

Gents:
I have done one or two sets of upper wishbones for original chassis with an offset because the arm could not be moved rearward enough to get sufficient caster, usually about .450 offset to the rear with the ball joint.
There should be some adjustment at the mounts, spacers or whatever to at least even out the caster side to side.
If you do add positive caster you will go more positive camber because of the taper of the mounts at the chassis, so that would need to be addressed.
I myself prefer 4-5 deg positive caster and some say it makes steering heavy, but with a light vehicle it is negligible and provides a lot of stability at speed, just my preference.
Cheers
Phil

Phil,

Thanks, I will definitely keep you in mind!

 

[Ron R.]

For you suspension gurus, the shop I had do my work is very good, and comes highly recommended. One thing they mentioned was that they had to preload the rear springs a fair amound to get at the correct ride hight. Didn't sound like it was an issue up front.

Does that match your experience with the car?