nervous rear fast cornering

[Sam_68]

camelotr said:

Whats Your opinion about fitting a coil spring to the rear - obviously a stiffer setup than on the front.

Yes, absolutely. You need either coils all round, or rubber cones all round, to give consistent characterstics front and rear, and tuning coil springs is a lot easier than tuning rising rate rubber suspension.

camelotr said:

This spring is rated as "fastroad" by the shop .

That'll be the 'fast road' spring kit specially developed for the coil-spring converted 1968 Cox GTM, then, will it?

Forget Mini stuff if you want to do it properly: as I said above, the two cars are quite different in their dynamics. You can't expect to just turn Mini suspension components back to front and expect them to work!

As I said above, you need to calculate appropriate spring rates (in lbs/inch or N/mm deflection) and buy springs with those rates, not just buy a kit for the front of a Mini and bit it to the back of your GTM!

[camelotr]

Can You suggest me literature to learn from?

[camelotr]

Or if only You have the time to write down the basics here, it would be tremendous for the whole pistonheads society.

[Sam_68]

camelotr said:

Can You suggest me literature to learn from?

First thing you need to read and understand for your present purposes is Allan Staniforth's book 'Competition Car Suspension', particularly the chapter on weight transfer (actually written by David Gould, not Allan Staniforth) and the bits about suspension/spring frequencies.

Milliken & Milliken's 'Race Car Vehicle Dynamics' contains the same information and is much more thorough and comprehensive, but also a lot more impenetrable to the beginner.

If you turn the Gould chapter in Staniforth's book into an Excel spreadsheet to calculate the weight transfer for you, you can then play around with numbers... I'd suggest you might like to model the Lotus Elise's dimensions, spring rates, ARB's and roll centre heights as a starting point for comparison, as it's not a million miles away from the GTM in terms of basic configuration and weight distribution.

Edited by Sam_68 on Saturday 4th September 18:20

[camelotr]

I have ordered the book now. Waiting to see.

[camelotr]

Thanks for the advice on the book!
It is realy good! I have analyzed my suspension, but it realy lookes ok.

???

I think my problem should be a flexing suspension pickup point. I will check them on the weekend.

Edited by camelotr on Thursday 16th September 23:37

[Sam_68]

camelotr said:

I have analyzed my suspension, but it realy lookes ok.

???

No disrespect, but if you've analysed a mid-engined set-up with constant rate coil springs at the front and rubber with a steep rising rate at the back, and it looks OK, then your analysis is flawed.

[camelotr]

The car is running coils on the rear now. And the rates looks more or less ok.

My suspect now is the rear steering trackrod pickup point. I think it may flex a bit if the car is pressed. I will make a support and have a look if it makes any difference.

Edited by camelotr on Friday 17th September 05:36

[camelotr]

I have strengtened the suspicious suspension point on the rear, and voila, the car is now stabile as a rock!

Thanks for Your help! The book is realy good.

[camelotr]

Please help me thurder if only possible.

Now the car is driveable under ormal conditions. But at high speed (70-80mph+) it becomes a bit uneven. Plus it still has a bit of oversteer especialy if driven at speed.

Requested toe-in values?

[davepoth]

I'd try raising the front tyre pressure a little - 1.5bar (22psi) seems a bit low.

[Sam_68]

camelotr said:

...at high speed (70-80mph+) it becomes a bit uneven. Plus it still has a bit of oversteer especialy if driven at speed.

It's very difficult to know what you mean by 'uneven'. Do you mean lacking in straight-line directional stability?

If so, are you sure that it is geometry/suspension related? Aerodynamics would be my first suspect on the GTM...

Crosswind instability at high speed is a common problem with mid- or rear-engined cars, as the aerodynamic centre of pressure is ahead of the centre of gravity: you're basically trying to throw a dart backwards.

Check chassis rake and fit a front spoiler, if you haven't got one already, to kill as much front-end lift as you can, but don't expect to ever completely resolve the problem.

[camelotr]

"Uneven" means that it lacks straight line stability, plus the bumps throw it out of the line. But just above a certain speed. Morover it is stabile as long as You accelerate, just becomes uneven when You lift off from the throttle.

I have a suspect, but may be wrong.

I have used a normal steering ball joint to fix the rear steering arm. But the suspension link that is fixing the steering arm is pointing in a great angle. Is it possible that this is throwing in some amount of movement, thus lowering my toe-in? I am thinking of converting it to rose joint...

Bad idea?

Edited by camelotr on Wednesday 22 September 14:26

[Sam_68]

Dunno. Very hard to diagnose without being able to see the car, but the fact that it's stable as long as you are accelerating may point toward some problem with compliance/flex in the suspension bushes or mountings that is pulled into toe-in so long as it is being loaded by the engine, but is 'relaxing' into toe-out when you come off the throttle.

The 'only above a certain speed' bit is confusing me, though... if it was some sort of compliance/flex problem, I'd expect it to be more noticeable at lower speeds, where the variations in thrust at the tyre contact patch are more pronounced.

Do you have any diagrams/photos of the overall suspension arrangements that you could post?

[camelotr]

Rear suspension. Sorry for the bad pic.

[camelotr]

Soo...

I have converted to rear steering rod to rose joint, plus gave it 1.5mms each side. It nearly eliminated the oversteer. Soo I have raised the front steering arm a bit (3mms) to give some slight bump-understeer. It was just an idea, but it worked suprisingly well! Now the car is very precictable in the corners.

Unfortunatly I still lack straight line stability...

Still open for ideas...

Edited by camelotr on Tuesday 28th September 19:30

[Sam_68]

camelotr said:

...I have raised the front steering arm a bit (3mms) to give some slight bump-understeer. It was just an idea, but it worked suprisingly well! Now the car is very predictable in the corners.

Unfortunately I still lack straight line stability...

Still open for ideas...

I'd be inclined to leave the clever stuff like roll-understeer to the professionals (who these days would be clever enough to achieve it by means of bush compliance - hence relating it to tyre loads - rather than geometrically).

You don't want any bump steer at all, if you can avoid it, otherwise when one wheel hits a bump when travelling in a straight line, it will change toe and thus try to point the car in another direction (...straight line instability, in other words).

If you need to introduce roll understeer at the front, you can do it by slightly increasing the front roll resistance (ie. increase front spring or ARB rates), though you may wish to try increasing the front toe-in a touch if you genuinely think that the 'bump-understeer' was helping in corners.

As I've said previously, don't expect too much by way of straight line stability from a short wheelbase, mid-engined car: you're throwing the dart the wrong way in terms of centre of gravity and aerodynamic centre of pressure.

[camelotr]

"As I've said previously, don't expect too much by way of straight line stability from a short wheelbase, mid-engined car: you're throwing the dart the wrong way in terms of centre of gravity and aerodynamic centre of pressure."

I've got Your point, but others running GTMs report the car to be stabile at speeds over the ton. Soo I must think that it must be possible.

I will keep on playing with my settings.

The bump steer is still minimal now. Less than 1mm on the whole suspension travell. Soo it is not that bad. But still resulted in a small gain.

I will focus on my shacks now. The front end feels a bit soft to me. How will I see if I adjust the shocks to hard?

Edited by camelotr on Tuesday 28th September 20:02

[Sam_68]

camelotr said:

...others running GTMs report the car to be stabile at speeds over the ton. Soo I must think that it must be possible.

Have you driven such cars for comparison? People have different standards about what constitutes 'stable'...

It's a long time (perhaps 20 years!) since I drove your model of GTM (the factory demonstrator), but I don't recall it being the sort of car that would have a relaxed and stable cruise at three-figure speeds!

In terms of aero, you could try checking the rake (which should be slightly nose down, certainly not nose up) or could experiment with fins at the back to move the CoP rearwards, to see if that improves matters?

Edited by Sam_68 on Tuesday 28th September 20:53

[camelotr]

You must be right with that. I have never driven a GTM. not even this one before the restoration. It was a complete non-mover.