1994 VD FC Caster?

[Rick Kirchner]

Last few times I've run the car it seems like the steering is way too heavy - especially in high speed corners. A couple of us were discussing this and the fact that the caster effect causes you to try and lift the front of the car under downforce caught my attention. I haven't changed the front alignment settings - besides camber the specs for various tire brands - for years. Always 1/16 out toe each side, and Dave Freitas once measured caster at 13 degrees. There has been some changes to the car since then so I don't know how accurate that number is - but it's likely close.

Thought I'd try something different - On each side I lengthened the top rear by 1 1/2 turns and shortened the front to fit the mounting points, thus pushing the top of the steer axis forward - while it doesn't seem like a lot it sure changed the front toe!

just got the car back on the platform and was wondering what typical caster settings are for these cars, and what sort of performance differences I might see, if any.

will try to measure what I ended up with and post back in a few hours.

[Steve Demeter]

I have always shot for about 5 degrees, but I have a Citation.

13 Degrees sounds like a ton to me.

One of the main things caster does, besides making the steering heavy is to give the wheel a bit of self centering and tends to keep the car stable in a straight line.

[Rick Iverson]

Dave Freitas once measured caster at 13 degrees. There has been some changes to the car since then so I don't know how accurate that number is - but it's likely close.

Rick, I have data from VanDiemen. I’ll get it to you tomorrow

V/r

Iverson

[RobLav]

13 degrees of caster seems way too much.

[Rick Kirchner]

well, after the changes I made the right is 2.4 and the left is 0.6. Don't know why there's asymmetry, but it looks like I need to do something else.

I found some specs from 1994, looks like it should be 4.5 deg. if it was 13 before then the measurement is incredibly sensitive to the rear leg length. It probably got changed from when Freitas measured it and I just didn't have the means to measure it well since.

Bought a longacre aculevel jig last year, so I'm getting around to figuring out a few things.

[Daryl DeArman]

If you can measure camber accurately, you can measure caster accurately, you just need some math.

Change in camber from 20 degrees steering angle in to 20 degrees steering angle out (40 degrees sweep) * 1.5 = caster

[alangbaker]

It's not the same year, but I have a manual for the RF98, and they have setups for 4 different tires. Three of the four use 4 degrees of caster and the last setup uses 4.5 degrees.

Make of that what you will.

[Nardi]

4.5 is typical spec. I personally run much less than that for the exact reason you mentioned.

[Rick Kirchner]

Well it helps if I could read. When I took the car in to Freitas it was not too long after I bought it and re-built it. it had the setting on it that the previous owner used, and I read the chart wrong - it was 2.5 deg, not 13 (the 13 was actually a camber setting of 1.3 and the decimal points were so small i needed good light and high mag to see them). Since the I've been jigging the parts to an old VD setup sheet from 95, and based on my last set of measurements, it did yield ~4.5

So based on what I've heard here, and what was originally on the car I'm going to shoot for 2.5-ish. Thanks all. It always helps to have APEXSPEED to help get your mind around a topic!

[Rick Iverson]

I have 4.5 degrees. I have two automatic tables, one Excel, the other is Word. But I cannot figure out how to post them

[JHerscher]

Rick, for what it’s worth, I always ran about 2.5-3.0 when I had a 94.

John

[Rick Kirchner]

Thanks All. I ended up with 2.6 on the right and 2.0 on the left after multiple on/offs and re-jigging the left against the right with a rod ruler. I'm still perplexed regarding the offset, but the car has some weird offsets - was hoping last time I had the bodywork off to put all the suspension together with the bare frame on a surface plate and try to figure it out. That was in 2012 and never got around to it.

[lmpdesigner]

If it was 13 degress you would not be able to turn the steering wheel when under a load. That is way more than normal on a non power assist race car.

I would expect 2.5 to 5 degrees as being correct. Maybe 6 degrees if you have elephant arms.

[CGOffroad]

Rick,
Did you purchase a tool for measuring your caster? If so, which tool did you purchase? Any recommendations after doing this? Any photo you can share about how you set up in your shop? I am needing to do check caster on my car and haven't done this before.

[mark elder]

You dont need a special tool to measure caster. A post above said

"If you can measure camber accurately, you can measure caster accurately, you just need some math.

Change in camber from 20 degrees steering angle in to 20 degrees steering angle out (40 degrees sweep) * 1.5 = caster"

You just need to be able to measure camber. Don't bother with 20 degrees, just use full lock each way (its about 18 degrees on my RF91). And dont bother with the maths, just add the two cambers (remember one is negative, if you have -1.5 straight ahead, then you might have -3.5 on full lock and +0.5 the other way; equals caster of 4.0).

Once you are in the ball park you can think that 4.0 measured this way is about 6 is the real world (in this example)

More important is equal.

the 20 degrees thing is nice but unless you have turntables, its a PITA.

[Rick Kirchner]

well luckily with the mold release still on the tires they slide on my scales pretty easily.

I have a longacre aculevel with the three-legged thingy that goes up against the rim. There's only a couple of things that make this a PITS: you have to make sure the unit is level, which means staring straight down on the top of it. then you have to look around the side to see the display.

With this tool you turn the wheels full one way, turn the unit on, put it up against the rim, level, and hit "caster", then "zero". then you turn the wheels the other way, put it up against the rim, level, and read. I managed to screw up the zero a couple of times and had to re-cal the unit. I'd go to make a camber reading and go "that ain't right" and then re-cal.

this unit requires 15 deg of turning, which is pretty much the limit of the VD.

[Daryl DeArman]

this unit requires 15 deg of turning, which is pretty much the limit of the VD.

It's all in the fancy math. Camber change over 40 degrees just results in needing the 1.5 multiplier. Change over a smaller number just needs a bigger multiplier.

For starters just get the left and right even and start with something in the 2.5 - 3 degrees of camber change from lock to lock. Tune/tweak from there. Don't be surprised if you find that unequal caster L/R produces better lap times at tracks where you spend a whole lot of time turning right .

[Rick Kirchner]

its just way too much of a PITA to mess with this as an assembled car. I'm going to make some measurements so that next time I strip it down to the frame I can put it on a surface plate and work the problem - maybe with locating pins instead of bolts. I can tell you that it is extremely sensitive - a half turn on one of the rod ends changes the result ~30%. I'm guessing a lower arm is off that much since I was only playing with the uppers.

[MotorCade]

As previously mentioned, 13 degrees is definitely overkill. 4.5 is factory recommended, between 3-5 is reasonable, and can make enough of a difference in steering weight and stability to suit most preferences. I've been happy with 4.

Cheers!