eddymcclements
|
| posted on 18/9/05 at 05:20 PM |
|
|
Handling is a bit..err...challenging
I've put about 350 miles on my car since it was SVA'd and taxed about 6 weeks ago. I'm finding that as soon as I turn into a bend I
have to be ready to wind off a bit of steering lock because the back seems to want to come round all the time. Nice on the odd occasion where
there's space for hooning, but a pain in the @rse when you just want go for a brisk but relaxing drive. I was chasing some big sports bikes up
the A6 out of Kendal last weekend, and as we neared Shap and the road opened out and got quicker the car felt so flighty I started getting worried.
Mind you, I looked down at the speedo and thought "How fast...?!???!" so perhaps it's to be expected.
Should I be running a bit of toe-in at the rear? The rear wheels are just about parallel at the moment.
Cheers,
Eddy
|
|
|
|
|
andy d (rizla)
|
| posted on 18/9/05 at 05:25 PM |
|
|
what tyres are you running and what are the pressures
|
|
|
britishtrident
|
| posted on 18/9/05 at 05:28 PM |
|
|
Yes toe-in at the rear is essential but not too much. But I suspect the problem is more to do with roll couple distribution --- the front needs to
be stiffer in roll or the rear made softer -- or both.
Front anti-roll bars are gaining favour with MK owners -- fitting one might be the way to go. If that alone isn't enough think about using
softer rate springs at the back.
[Edited on 18/9/05 by britishtrident]
|
|
|
David Jenkins
|
| posted on 18/9/05 at 05:37 PM |
|
|
Eddy,
I'm with Andy on this one - the basic car has very good handling, so there's something basically adrift if the back end doesn't feel
secure in normal driving.
It's a light car, so pressures should be lower than you think - I'm running 18psi all round, and even that may be too high (but my
handling's good, so I'm reluctant to mess around).
Also - a long shot - are your back shocks set on too high a setting? If too high the tyres might be going light on bumpy bends.
Saying all that, if you're getting a move on then the back end may well swing around! Mine does... 
rgds,
David
|
|
|
zilspeed
|
| posted on 18/9/05 at 10:26 PM |
|
|
Ding !
The three things which I have changed on my GTM to remove extreme tail happiness.
Added toe in at the rear where it was previously toe out. It just whipped round with little or no warning.
Fitted proper performance tyres suited to the car - Yokohama A-032Rs in my case.
Adjusted tyre pressures. I am now at 20psi all round - that's on a 700ish kg car with a mid engine. Higher pressures led to tramlining
previously.
The toe in adjustment made the most difference to oversteer. The tyres fixed the understeer. My problem is now roll stiffness which wasn't
previously evident because of the above issues.
Still - the roll happens at much higher cornering speeds than were previously acheivable 
|
|
|
greggors84
|
| posted on 18/9/05 at 11:10 PM |
|
|
quote:
Originally posted by britishtrident
But I suspect the problem is more to do with roll couple distribution --- the front needs to be stiffer in roll or the rear made softer -- or both.
[Edited on 18/9/05 by britishtrident]
As I understand it you want the back end to be stiffer so that there is less weight transfer at the back when cornering?
Am I wrong?
Chris
The Magnificent 7!
|
|
|
pbura
|
| posted on 19/9/05 at 02:40 AM |
|
|
quote:
Originally posted by greggors84
As I understand it you want the back end to be stiffer so that there is less weight transfer at the back when cornering?
Am I wrong?
Actually, weight transfers no matter how stiff the spring is; a stiff spring will just compress less.
A neutral car will have a 50/50 front and rear weight distribution and will also have 50% of its roll stiffness at each end of the car. If one end is
made stiffer than the other, it will slide first. So, what is usually done is to make the front a bit stiffer to that the car wll have a tendency to
understeer.
To Eddy in particular: There's a great post about finding minimum tire pressures and doing what you can to trim oversteer/understeer with tire
pressures here (Richard Hudgins, about 2/3 down the page):
http://www.clubcobra.com/t20032-15-2.html
He describes skidpad testing, but I think the principles can be applied to road testing.
The only thing I disagree with him about is using the springs to adjust the proportion of front vs. rear roll stiffness. Allan Staniforth and I 
think that spring stiffness ought to be set for optimal tire adhesion and that roll stiffness and distribution should be adjusted with antiroll bars
(front bar only in the case of a solid axle car).
I think Eddy's particularly bad oversteer will be helped by the advice in previous posts. Probably all Locosts would benefit by following the
lead of, er, a well-known 7 producer and have bigger tires on the rear and a modest antiroll bar on the front.
Pete
|
|
|
Syd Bridge
|
| posted on 19/9/05 at 08:51 AM |
|
|
Get your springs and shocks right, CoM low, and any anti-roll bar becomes unnecessary weight.
Next step is take the springs a step lower, and the arb's can adjust the car to your style. Just don't overdo it.
|
|
|
britishtrident
|
| posted on 19/9/05 at 09:54 AM |
|
|
quote:
Originally posted by greggors84
quote:
Originally posted by britishtrident
But I suspect the problem is more to do with roll couple distribution --- the front needs to be stiffer in roll or the rear made softer -- or both.
[Edited on 18/9/05 by britishtrident]
As I understand it you want the back end to be stiffer so that there is less weight transfer at the back when cornering?
Am I wrong?
wrong other way round -- to reduce oversteer make the roll stiffness more at the front and less at the rear.
It is really to do with how the load on on the wheels on the inside of the corner share load.
Making the front stiffer and the rear softer makes the front inside tyre work less and he rear inside tyre work more --- giving the rear a bigger
share of the cornering grip.
|
|
|
omega 24 v6
|
| posted on 19/9/05 at 12:27 PM |
|
|
yep I agree stiffen front and soften rear to reduce oversteer. Get the corner weights all set up first though and take it from there.
|
|
|
NS Dev
|
| posted on 4/10/05 at 02:11 PM |
|
|
exactly right, soften the rear, first the springs, then the dampers to match gradually testing them one click at a time. At a guess, if you are
running AVO shocks, they want to be on aroung 4 to 6 clicks, i.e. a very light setting.
|
|
|
caber
|
| posted on 4/10/05 at 05:17 PM |
|
|
All the original Lotus 7s have an anti roll bar acting on the upper wishbones curved round the top front chassis member. Colin C must have put it
there for a reason!
Caber
|
|
|
eddymcclements
|
| posted on 10/1/06 at 10:50 PM |
|
|
Well, I've followed some of the advice here and these are the results:-
Softened damping all round - improved grip; the tyres don't feel like they're skating over the road surface quite so much and are gripping
better
Lowered tyre pressures - they were at around 25psi (which is what the tyre fitters had put in them) and they're currently at 19psi. Better
again, even more grip.
Next step is to RAISE the rear slightly - I'm running out of damper travel and hitting the bumpstops, sending the spring rate through the roof.
These are 8.5" closed / 12.5" open AVOs which are the right spec for the IRS rear ST Locost. I think I have the rear too low, and once my
lardy carcass is in the pilot's seat I'm already losing some rear ride height.
Need to do something about the oil pickup / sump - the enhanced cornering speeds are making the oil light come on...!
Cheers all,
Eddy
|
|
|
