Coupe Alignment

[redmt]

Does anybody have some solid numbers for caster settings? I'm getting numbers from 3 to 9 degrees from different sources. The FF5 coupe is set around 4 right now and it's twitchy as hell on the rough roads. I want to put the final alignment on it but need some reliable numbers to go with.I think I'm pretty good with .5 on camber and 1/8 toe in.
The rear is .5 camber and 1/8 toe

[db replicas]

Twitchy is usually more to do with bump steer than caster. Caster is more to do with self centering when coming out of corners.

Sorry if you already knew this and were looking for finer refinement.

[redmt]

My issue is that it's wanting to dart and dive for every little bump and dip in the road. Here's the road. 2nd try - YouTube

[db replicas]

Two things to consider:-
Deep dish front wheels will try to follow whatever the road is doing.
Bump steer will do all sorts of horrible things.

Definitely investigate the above two things first.
If you can get your hands on some wheels and tyres that would have been used originally on the front hubs would be an easy starting point, but I think you have bump steer problems.
There is plenty of info on the net about how to check for bump steer. Fixing it can sometimes be simple but other times it can be much more involved.
It you do get into it and work it out you will be very pleased with the results. It will make the car a pleasure to drive and inspire confidence.

[Bobcat]

Couple of things ... are you running radials or bias ply tires ? Also , what tire pressure ? The bias ply will tend to dart more than the radials .
You gave a camber of .5 degrees ... negative or positive ?
As db said , you may also have a bump steer problem , but are you sure there isn`t excessive play in the steering linkage , tierod ends etc ?

[redmt]

I should have clarified -.5. I'm going to align it to +7 on the caster and see what differences it makes if any. The components are either new or as new. I'll see how it handles when I get the adjustments made and go from there. Thanks for all the pointers.

[Gofastoldguy]

Are you running power steering? If so, 6-8 Positive caster should be about right. It helped me in the roadster. Lost the "twitchiness".

[redmt]

Standard steering. I'm chasing every tip on this Coupe. It was poorly built , badly engineered and pathetically assembled by the original builder. After digging a little deeper, I find the bump steer kit was assembled to the wrong side of the spindles. I'm going to guess that it will make a huge difference in the handling!

[Dominik]

If you look from the front at your (front) suspension, draw a line from upper control arm inner mounting point to lower control arm inner mounting point. Do the same for the outside points.

Your tie rod pivot points should sit on those lines - or give the same length as if sitting there. Essentially imagine in the letter "A", the "-" being your tie rod (seen from the front).

If the pivot point of the tie rod is positioned further out, then it must also end further out at the steering arm by the same amount.

And your steering should sit that the tie rod is level.

Looking from the top, you want the steering arms pointing a bit forward. That should give you acceptable Ackermann steering angle. But don't worry about that too much. Hard to change anyway.

[redmt]

The "A"is more of an "H" and do you mean that the tie rod should sit level or parallel? For the simple measurements I've heard parallel to the lower A arm.

[Dominik]

It's kinda hard to explain in one letter ;-)

If your lower control arm is level (to be precise, the line drawn from control arm mount to lower balljoint center), so should the tie rod be.

But parallel to the lower control arm covers either!

upper right hand corner shows nice picture:
AutoSpeed - Another Human Powered Vehicle! Part 6 - A Unique Front Suspension

[db replicas]

There are a few problems with that picture.


What it doesn't show is that the (Centres that is) steering tie rods position in relation to the upper and lower arms should be a percentage length.

Meaning.
If the tie rod is mounted at the same height as the lower arm it should follow that same line, and if it is on the same level as the upper arm then it should follow the line of that.
Anywhere in between should be treated as a percentage.
If the tie rod is mounted centrally (height wise) at the chassis end then it should be fixed centrally (Again height wise) at the hub end.
In the image it shows the tie rod mounted about 35-40% higher at the chassis end but at the hub end it is only looking like around 25%. This car will bump steer.

The length of the tie rod is also critical (As mentioned above). As also said above it is difficult to explain in short forum posts but....

The "H" and "A" discussed are as close as you can get, the actual is somewhere in between. I would say "A" with the apex points separated, but not to the point of becoming a "H".
The length of the tie rod should be determined (Using what I posted above) by the centre lines from the upper and lower ball joints and the upper and lower chassis mounting points of the arms.
That will give you the exact length, but.... (It just goes on and on!!!!)
If you want to incorporate Ackerman to stop your car from squealing like Frank Cannon's Lincoln on parking lots then the whole tie rod needs to be set further outboard (rack in front of axle) or inboard (Rack behind axle).
This is set my the position of the tie rod on the upright.
This point is found by drawing a line from the centre of the rear axle to pass through the centre line of the upper and lower ball joints on the upright.

There is more to it, but that is to do with slip angle and overall car set up and how you want the car to feel.
Another constraint is often the brake rotor, which can get in the way when you have the rack mounted forward of the axle.

I think I got it covered, but chances are if this is not something you are familiar with then it might as will be in Swahili.

[Dominik]

That should work!

Let's not get into Ackermann... ;-)

Just briefly: I consider Ackermann completely overrated. The steering angles we get during racing will hardly turn the wheels into Ackermann behaviour. Only in very slow corners. But it's the fast corners where you win races!

[krausewich]

Hi Redmt,
Before getting too into the physics of it all, is it possible to unbolt a shock/spring assy. and get some static reads? If so, get it up on floor stands, place a jack under the tire and measure your camber/toe. Jack it up 3" or so to replicate road travel and take your measures again. Your camber shouldn't change more than a degree or two(worst), but your toe definitely shouldn't change more than 1/16 (1/8 combined for both sides). Zero change would be ideal. You may consider using washers to tweak rack position to see if you can reduce the numbers if so.

Good luck,
Mike.

[redmt]

Thanks for all the replies. The steering geometry explanations are pretty straight forward. The biggest problem with this coupe is that it was cobbled together by a supposed professional builder that used whatever parts he could get his hands on and then installed the majority of them incorrectly. I'm working the way through it. It will come together.

[xb-60]

....and also, if your steering offset is excessive, that will make the steering twitchy. The offset I'm talking about is....if you draw a line through the two ball joints (on the same side of the car!), the steering offset is the distance from the centre of the tyre contact patch to the point where that line through the ball joints hits the ground. Ideally the offset should be a little inboard of that point. All the other previously mentioned factors are important as well (including Ackermann!)
Good luck.
Cheers,
Glen

[Dominik]

Also don't forget to check the rear...