nervous rear fast cornering

[camelotr]

"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 mover the CoP rearwards, to see if that improves matters?"

The car is yet a bit nose-down (290mms to the to of the sill on front and 300 on the rear).

As the car is an oldtimer, I dont want to use any fins or other devices. I will go as fa as the suspension setup lets me, and accept the results.

Edited by camelotr on Tuesday 28th September 21:01

[camelotr]

Funny thing is that the car feels much better when the throttle is down. Though if the front end lift is the key to the problem, I would guess accelerating will make it even worse. Or?

[Sam_68]

camelotr said:

Funny thing is that the car feels much better when the throttle is down. Though if the front end lift is the key to the problem, I would guess accelerating will make it even worse. Or?

Yes, you're absolutely right.

As I said earlier, the fact that it is more stable under power tends to point fairly heavily towards something in the rear suspension flexing or distorting under load.

I wasn't suggesting permanent fitment of fins (though if they work, a low downforce rear wing with big endplates would do the same job and not look too untoward), but they might be a simple experiment to see if aerodyanamics are part of the problem.

Edited by Sam_68 on Tuesday 28th September 21:13

[camelotr]

With 2mms on eah side, the car developed. Now I could hit the ton, but still it was scaring. Though I could stabilize it with a bit of left foot brake. This just make my suspect stronger on the rear suspension giving some movement...

But ths may be some kind of structre.related issue. GTM went on to reverse A-arm suspension later.

[Sam_68]

camelotr said:

Though I could stabilize it with a bit of left foot brake. This just make my suspect stronger on the rear suspension giving some movement...

You are giving confusing and contradictory information: you are telling us that it is more stable under power, but now that it is stablilised by braking (which obviously completely reverses the loadings you get when under power).

You have also told us that you have converted the rear to coils, when your photos still appear to show rubber?

I'm afraid I can't offer any further guidance; under the circumstances it would be nothing more than random speculation.

[camelotr]

The rear is running on coils. This is not onvthe photov(above it). What You see is the "hilo" and the arm-balljoint.

The other information may be a bit confusing.

Soo lets take it into 2 different sentences.

After setting the toe-in to 2mms per side, the handling developed. Before this movement, I only managed to reach 75-80mph before getting into a situation where it was becoming dangerous. With the new setup I was able to controll the car up to 100mph.

But still there (at 100mph) the car has got just as instabile as before (at 70-75mph).

After this I have tried slowing down by using left hand brake (rather than using engine brake), and it kept the car hold the straight-line.

Sorry if I was not clear enough.

I think there is still something flexing in the rear suspension. I will try to add an extra 0.5mm on each side just to see.

Unfortunatly I think it is a problem of the suspension design rather than adjusting.

[Lesliehedley]

I'm not an expert but my brother has an early GTM. He mentioned that straight line instability can be an issue with the original GTM setup where the steering rack is mounted onto the fibreglass body rather than the mini subframe. I believe there are kits to remedy this. You should find something if you google the problem. I could be totally off the mark but just thought I'd mention it as a possibility.

[camelotr]

Thanks for Your input.

And You are quite all right. Though my steering rack is fitted on the subframe rather on the bulkhead.

[davepoth]

So, under engine braking the car becomes unstable? does the car go in one direction consistently or does it just become very "loose"?

[camelotr]

It becomes "loose".

I have made a "special suspension device": I have attached a long bar to an old steel wheel . With it i cold test the suspension. Unfortunatly i could move the wheel a bit wit the bar. It looks like the movement is fromdifferent soures: the steering arm flexes a bit, the ball joints move abi and the poly bushes distort a bit.

I think without redesigning the whole rear suspension, it is an issue, which i have to accept. With 2.5mms/side toe-in on the rear, the thing is acceptable now.

[camelotr]

By the way, Leslie You were pointing in the right direction. With the bar, I have also found out that the steering rack could move a bit. After fixing it better, it also improved the handling soo at speed it also added to the instability.