How important is caster???

[bcarter]

After a small bang on the right front, I was passing a guy, and he turned in on me, I have found that the caster is all screwed up on the right front, and we can't seem to make it change any...is caster a big deal ??

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G. Brooke Carter
Wheat Buckley Racing
# 10 Challenge Car
2002 Class Champion
Calgary, Alberta

[RX767]

Brooke,
I have been attempting to set-up the new suspension that I have installed. I adjusted the caster with the front trailing arm nuts. I also have the Ground Control camber/caster plates installed on the top of the strut tower. I have not messed with those yet.
What have you tried?

Bill Emery
ITA#23
Glen Region

[bcarter]

The same process, using the adjustment that moves the lower control arm.

I have tried doing the adjustment while the car was on an alignment machine, and was never able to get a lot of caster, but after the bump, it doesn't seem to matter which direction we go in, there is little change in the caster, and on the bumped side I can't get even a whole degree...

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G. Brooke Carter
Wheat Buckley Racing
# 10 Challenge Car
2002 Class Champion
Calgary, Alberta

[lateapex911]

Uh oh...better start measuring....

Generally speaking the lower control arm wants to be pulled to the front by those silly links, to the point of just clearing the crossmenber it nests in thru it's full range of travel.

More adjustibility is available up top if you run the plates that do both caster and camber.

How much is a debated subject, but it will add a lot of self return to the steering, and therefore "feel". Not that any RX-7 has EVER been accused of 'good steering feel'!

An aside: When I first put my car together I took it out for a test drive without setting the alignment, and when I went to turn around, the steering 'auto-completed'! As in it whipped out of my hand and went full lock. Bad. I added some caster and all was better.

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[moto62]

Originally posted by bcarter:
I have tried doing the adjustment while the car was on an alignment machine, and was never able to get a lot of caster, but after the bump, it doesn't seem to matter which direction we go in, there is little change in the caster, and on the bumped side I can't get even a whole degree...

This is quite strange as you can usually get upwards of 3 degs neg caster by merely adjusting the the large nuts on the end of the tension rods. Check the area where, the control arm is connected to the cross member for any binding. At about 1 deg of neg caster, the control arm should be centered in the channel of the cross member.
If it is , then three or four turns on the tension rod nuts should give you ample adjustment in either direction. If it isn't, break out the measuring tape and start measuring.
NOTE: More neg caster increases turn in rate and in a car that half the time is spent chasing the rear, may not be such a good idea. JMO.
Ray

[bcarter]

lateapex911...I really know what you mean, hence the question...HOW IMPORTANT...is caster. Thanks..

So, I know these cars fairly well, and have found that regardless of some of the fine tuning I get into, I can still deal pretty well with the "red fog" of racing, dealing with fast corners, traffic, etc, but, fine tuning makes the whole thing a little less stressful.

At the same time, I am sometimes confused.. Caster confuses me.

I am not sure that in a high speed corner, I have time to think about whether my caster is correct, but I AM sure that if it is "correct" I will have a little less hassle getting my 20 year old car through a corner at high speed, faster than some guy with pistons.

So, My guess is to get the axle/spindle farther forward to get more caster, to get more turn in....?????

So, that means, on the rod from the lower control arm, you want to pull it forward...?

What it comes down to, I am confused by + and - caster..

And again, how important is it... I am not sure I can tell the difference. And I am not sure why I am getting such strange readings on the alignment machine, as the car handles really OK.

Thoughts...?

[This message has been edited by bcarter (edited September 05, 2003).]

[oanglade]

bcarter, Check out Ground Control's "tech talk" on caster. Maybe it will help you.

http://www.ground-control.com/gctalk5.htm

[bcarter]

Still confused.

Moto62 talks about 3 degrees of negative caster, and the Ground Control talks about race cars having positive caster...

So, I gather the bottom of the strut wants to be further ahead than the top...

If I was going to mmasure this, off an alignment machine, want points do I use to measure from..?

[CaptainWho]

I don't know whether it's positive or negative, but you want the top of the strut behind the bottom.

[noresull]

Originally posted by CaptainWho:
I don't know whether it's positive or negative, but you want the top of the strut behind the bottom.

Positive caster is where the top of the strut is forward of the lower balljoint, negative is where the top of the strut is further back then the lower balljoint.

The only vehicles I know of that positive caster helps is some older vans, other than that, most vehicles like equal amount of negative caster. If they are different, then it will cause excessive tire wear, and can cause steering pull if excessive. Watch the top fuel dragsters when they turn their wheels at the end of the run. the front inside wheel dar near is layed down. This is caused by the extreme amounts of negative caster they use to maintain the steering as straight as possible.

Caster is pretty important to a properly steering car and on road race cars, it should be pretty equal side to side. When You are making Your adjustments on the machine, Were You doing a Caster swing after each adjustment? If not, the machine is only calculating the amount of caster adjusted and could be off.

[Quickshoe]

No wonder there is so much confusion...

POSITIVE caster is when the top is behind the bottom. (Ball joints or Strut).

Positive is good.

As to how to measure: my caster gauge requires 1. the steering to be turned 20 degrees. 2. zero the gauge 3. turn the steering back 40 degrees and read it.

Without using a gauge you could use a string, plumb bob, accurate tape/ruler and a calculator with trig functions. This will be hard for me to explain but here it goes.

Take the string and plumb bob and go through the center of the top ball joint or strut. Mark the floor at the appropriate spot, now do the same with the lower ball joint. The distance between these two marks on the floor, when viewed from the side of the car (not a connect-the-dot, your negative camber will make the top mark more inboard than the bottom) is your "adjacent". The distance between the upper and lower ball joints is the "hypoteneuse(sp?)". Now make certain your calculator is in the degree mode. Enter your "adjacent" and divide by "hypoteneuse". Take that answer and find its' arcosine or "cos-1". That answer will be your degrees of caster.

Or you can simply make the bottom rod as short as possible. Move the top of the strut back as far as possible. Drive it. If it behaves differently (self corrects too easily/too much steering effort) move the strut more towards upright.

Happy tuning.

[bcarter]

Quickshoe....your answer makes the most sense. I am not so bothered with which is + and -, as I know it has to be the lower end forward, just that we had some real confusion onm the machine, and got silly readings. I am going to ddo the last suggestion first, set the darn struts equally far forward, and hope for the best in the last race weekend coming up...

Thanks for the interesting comments, seems not too many of us are sure about caster

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G. Brooke Carter
Wheat Buckley Racing
# 10 Challenge Car
2002 Class Champion
Calgary, Alberta

[steve s]

remember don't move the bottom of the wheel too far forward as you will increase your wheel base by whatever you pulled it forward..also the control arm starts to bind.good luck tuning.

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steve saney
it-7 /it-a #34

[lateapex911]

I would begin your setup by measuring the rear axle. Make sure it is straight to the body. Now, when you have established that the axle is square, adjust the front links, pulling the wheel forward, and measuring the wheel base side to side to maintain equality. Now, pull of the wheel and check the relationship between the control arm and the mounting frame. Should have the same clearance side to side. If not, something is bent. At this point, any alignment machine should give you logical and similar readings side to side. Remember that you want to have each side set the same for ride height, camber, and upper location of the strut in the caster setting.
I don't think it will affect the caster setting, but unhooking the sway bar will help get the corner weight perfect.

As for your original quetion, I don't think that the cster is critical to the lap time over it's normal range of adjustment. I'll check the good book on it. More (further forward at the tire contact patch) means stronger self centering, a personal taste thing.

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[bcarter]

First off, the car has been bent a few times, wall contact, and straightened on a frame machine, but how accurately I don't know.

Second, the ride height is a little different from side to side due to spring height adjustments made on the scales.

I also have a little more neg camber on the right, due to the previous bangs (????), and partly preference, as we race mostly left turns

I like the idea of using the wheelbase to measure the caster, and that's what I'll do.

I also agree that it probably isn't that critical anyway.



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G. Brooke Carter
Wheat Buckley Racing
# 10 Challenge Car
2002 Class Champion
Calgary, Alberta

[moto62]

Originally posted by bcarter:
Still confused.

Moto62 talks about 3 degrees of negative caster,

Oops! Sorry for the confusion. Anywhere in my post that I mentioned neg caster should be positive caster.
Ray

[bcarter]

Well,

Seems caster is not a precise science, and it doesn't have to be !

I used the simple method suggested, measuring wheelbase, ( thanks lateapex911 ) and,....lo and behold, the car handled just fine.

I ended up replacing one of the caster adjustment rods as it was stripped, but if there was any difference in the handling, I didn't notice.

Season is done as of today, and now I can go to sleep.

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G. Brooke Carter
Wheat Buckley Racing
# 10 Challenge Car
2002 Class Champion
Calgary, Alberta

[Festus E. Simkins]

My understanding of caster is that it helps with straight line stability and self centering of the steering. It also helps to keep the front wheels upright in a turn. I can get about 5.5 degrees of Postive caster in my 79 RX-7. You have to be careful of some interference problems if you get too much caster. If I could get 7 degrees without any interference I would.

Drive well.

[jake7140]

I agree that caster can be important to the feel and behaviour of the car. It affects straigh-line stability, turn-in speed and camber. Looka at the old Volvos, they can have lots of pos caster which makes the big bricks turn in. Get what you can and feel comfortable with.

When I slotted my towers, I didn't just go straight down, I went at an angle toward the firewall. (no camber plates in spec)

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Steve
[email protected]
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</A><A HREF="http://www.geocities.com/elrss" TARGET=_blank>Elkhart Lake Racing_&_Sipping Society
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[Scott Nutter]

Originally posted by bcarter:
Season is done as of today, and now I can go to sleep.

So sorry And none of us believe for a minute that you can go to sleep

And now to start planning for the next!