nervous rear fast cornering
Discussion
I have a kitcar.
I have set the suspension, but not 100% statisfied with it.
On fast corners - especialy if the road surface is rough, the rear of the car becomes "nervous".
The car is RWD, the suspension is mini based (two front mini fron subframes.
I run -1deg negative on the font, -1.5 on the rear. The castor on the front is 3.5deg, the toe in on the front is 1.5mm and 1mm on the rear. The tyres are 1.5bars on the front and 1.8 on the rear. I use Yokohama A008 165/70 R10 on 6in alloys.
How should I alter this setup?
I have set the suspension, but not 100% statisfied with it.
On fast corners - especialy if the road surface is rough, the rear of the car becomes "nervous".
The car is RWD, the suspension is mini based (two front mini fron subframes.
I run -1deg negative on the font, -1.5 on the rear. The castor on the front is 3.5deg, the toe in on the front is 1.5mm and 1mm on the rear. The tyres are 1.5bars on the front and 1.8 on the rear. I use Yokohama A008 165/70 R10 on 6in alloys.
How should I alter this setup?
First thing I'd be inclined to check would be that you're not getting bump-steer at the rear.
Presumably if you're using the Mini subframe and suspension at the back, you have tie rods to replace the steering track rods, and if the chassis pickups for these have been incorrectly or inaccurately located, it would create bump steer that will cause exactly the symptoms you describe.
Having said which, doubling up the front suspension geometry of a FWD car to serve at the rear of a mid-engined RWD car is always going to be problematical, as your roll axis will be all wrong. You may have to play around with spring rates, ARB's and wheel widths to regain some semblance of balance - but check the bump steer first!
Presumably if you're using the Mini subframe and suspension at the back, you have tie rods to replace the steering track rods, and if the chassis pickups for these have been incorrectly or inaccurately located, it would create bump steer that will cause exactly the symptoms you describe.
Having said which, doubling up the front suspension geometry of a FWD car to serve at the rear of a mid-engined RWD car is always going to be problematical, as your roll axis will be all wrong. You may have to play around with spring rates, ARB's and wheel widths to regain some semblance of balance - but check the bump steer first!
I have checked the suspension for bump steer (actualy with the original setup there were lots), but I have modifed the suspension with different steering arm, also relocated the pickup point of the tie rod, soo now it is well within controll (maybe 0.3-0.4mms on the whole suspension travell). But as the spring on the rear is very stiff, I think it is even less in reality.
Edited by camelotr on Saturday 4th September 12:12
Can You please explain me what does wron "roll axis" means? Is this the wrong ackerman angle? As I have corrected that also by using different steering arms on the front.
By the way the car is using different lenght halfshafts (normal mini items) and a lsd diff. If it has anything to do with the problem...
By the way the car is using different lenght halfshafts (normal mini items) and a lsd diff. If it has anything to do with the problem...
camelotr said:
Can You please explain me what does wrong "roll axis" means?
It's rather complicated (hence difficult to explain in words alone), but I'll try to cover it in simple terms.In a corner the car will try to roll around an axis between the front and rear geometric roll centres. It won't actually succeed in doing so, not least because the springs and anti-roll bars at either end of the car will resist the roll by differning amounts, but that's another discussion...
Anyway; the roll axis is part of the complicated compromise that makes the car handle correctly.
You will almost always find that on FWD cars, the combination of roll axis and front and rear roll resistances are arranged so that the car will tend to lean diagonally on its outside front wheel in a corner (hence all the photos you see of Minis and hot hatchbacks picking up a rear wheel when cornering hard).
Almost all RWD cars will be aranged so that they are leaning on their rear wheel as they exit a corner (though since they will be transferring weight backward under acceleration anyway, the diagonal weight transfer doens't need to be as pronounced as in a FWD car, which is one of the reasons why RWD can be inherently better balanced). You can see this by Googling images of RWD cars cornering hard, where you will come across plenty of pictures of Porsche 911's and Ford Escort Mk. 1's with a front wheel dangling in thin air.
The amount and direction you want of this diagonal weight transfer will depend (amongst other things) on the weight distribution of the car; hence one of the basic decisions the designer should be using when he designs the suspension geomentry is how where to put the roll centres, so that he correctly manages the steepness and direction of the roll axis inclination.
Simply using the subframe and suspension from a Mini at both ends (is it a GTM we're talking about, out of interest?) will obviously mean that no thought has been given to this at all, and the geometric roll centre will basically be at the same height both ends (hence a flat roll axis). All the diagonal weight transfer will have to be managed by differences in the front and rear roll resistances (ie. front:rear spring/ARB rates), which it will be very difficult to optimise for diagonal weight transefer without f
king something else up (suspension pitch control, ride quality, or whatever).Eta: And I certainly wouldn't be running an LSD in a Mini-based mid engined kit. You simply don't need one (Lotus manages quite happily without and LSD in the Elise) and it certainly may not be helping.
Edited by Sam_68 on Saturday 4th September 09:34
Thanks for the info.
I am very sorry, my english is... well getting better, but still very poor
.
Soo roll axis. I have managed to lift the rear roll centers up about 40mms (by raising the subframe), in order to make the weight distribution better in corners. Soo it is something which has been done. Plus the rear spring is the original mini item, quite stiff. On the fron I use a coil spring, which is much lighter than the original rubber cone, as the front subframe was designed to work with the engine's weight.
I use adjustable dampers, which are now set to the softest. I have not tried adjusting them yet, but they are still quite hard, and the weight of the car is about 550kgs - I guess.
I am very sorry, my english is... well getting better, but still very poor
.Soo roll axis. I have managed to lift the rear roll centers up about 40mms (by raising the subframe), in order to make the weight distribution better in corners. Soo it is something which has been done. Plus the rear spring is the original mini item, quite stiff. On the fron I use a coil spring, which is much lighter than the original rubber cone, as the front subframe was designed to work with the engine's weight.
I use adjustable dampers, which are now set to the softest. I have not tried adjusting them yet, but they are still quite hard, and the weight of the car is about 550kgs - I guess.
Edited by camelotr on Saturday 4th September 12:15
The car is a 1969 Cox GTM.
And You are quite all right: the LSD looks like nearly useless. Though the engine is about 100bhps, the car is soo light that the wheels do not spin even under the hardest acceleration. Just if I jump off the clutch, but why should I doo soo?
Though the LSD may became helpfull if the road gets wet or dirty...
And You are quite all right: the LSD looks like nearly useless. Though the engine is about 100bhps, the car is soo light that the wheels do not spin even under the hardest acceleration. Just if I jump off the clutch, but why should I doo soo?
Though the LSD may became helpfull if the road gets wet or dirty...
Photos would help, as I may be completely misinterpreting what you're telling us, but...
If so, then you'll have raised the centre of gravity at the rear, which on a car that is already tail heavy and top heavy at the back, is very bad news.
Obviously steel coil springs are (usually) constant rate, whereas the rubber cone springs used by Minis are not merely stiff, but have a dramatically rapid rising rate (a quick google suggests that the rate of the standard front cones rises from 118 lb/inch at static ride height to 1050 lb/inch at full bump).
This will mean that you have a fairly constant roll stiffness from your front springs, but a dramatically rising roll stiffness at the rear, as you enter a bend. Since the general rule of thumb is that increasing rear roll stiffness increases oversteer, you can imagine where this would take you...
camelotr said:
I have managed to lift the rear roll centers up about 40mms (by raising the subframe), in order to make the weight distribution better in corners.
Does this mean you've lifted the engine/transmission as well?!If so, then you'll have raised the centre of gravity at the rear, which on a car that is already tail heavy and top heavy at the back, is very bad news.
camelotr said:
...the rear spring is the original mini item, quite stiff. On the front I use a coil spring, which is much lighter than the original rubber cone...
Ah... I think we may have a culprit!Obviously steel coil springs are (usually) constant rate, whereas the rubber cone springs used by Minis are not merely stiff, but have a dramatically rapid rising rate (a quick google suggests that the rate of the standard front cones rises from 118 lb/inch at static ride height to 1050 lb/inch at full bump).
This will mean that you have a fairly constant roll stiffness from your front springs, but a dramatically rising roll stiffness at the rear, as you enter a bend. Since the general rule of thumb is that increasing rear roll stiffness increases oversteer, you can imagine where this would take you...
Edited by Sam_68 on Saturday 4th September 12:39
Thanks again.
I think I am getting on.
The engine was raised with the suspension, soo You are quite all right about that the weight centre was lifted. To compensate the engine, I have moved everything to the front (fuel tank, battery, put in a spare wheel, fitted a remote servo etc). The car must not be balanced (still rear-heavy), but not that much I hope.
According to the springs. I have a set of stiffer springs, which I might try on the rear...
I think I am getting on.
The engine was raised with the suspension, soo You are quite all right about that the weight centre was lifted. To compensate the engine, I have moved everything to the front (fuel tank, battery, put in a spare wheel, fitted a remote servo etc). The car must not be balanced (still rear-heavy), but not that much I hope.
According to the springs. I have a set of stiffer springs, which I might try on the rear...
Edited by camelotr on Saturday 4th September 12:51
camelotr said:
Soo the engine was raised with the suspension, soo You are quite all right about that the weight centre was lifted. To compensate the engine, I have moved everything front (fuel tank, battery, put in a spare wheel, witted a remote servo etc). The car must not be balanced (still rear-heavy), but not that much I hope.
You are confusing the the longitudinal location of the centre of gravity (which, as you suggest, dictates the split of weight between front and rear) and the height of centre of gravity, which has a big influence on lateral weight transfer when cornering.Imagine a double decker bus with an elephant on the top deck. If the elephant sits somewhere near the middle, you could well have a 50/50 front:rear weight distribution, but the bus would still fall over if you went round a corner at much more than a fast walking pace.
camelotr said:
According to the springs. I have a set of stiffer springs, which I might try on the rear...
For heaven's sake don't just mess around at random: chances are you'll wind up dead or in a wheelchair.Contact the GTM Owner's club; the Cox GTM and its sucessors are fairly well known cars and I'm sure there will be plenty of information on optimum spring rates and set up.
At the very least, read up on the subject to the point where you're confident about calculating the spring and wheel rates and frequencies and roll stiffnesses you need to give some semblance of ride & handling balance as a starting point.
Forgive me for saying so, but anyone who doesn't understand the ramifications of fitting stiff rising-rate rubber suspension at the rear of a mid engined car at the same time as fitting constant rate coil springs at the front shouldn't really be trying to modify their suspension without expert advice on hand.
camelotr said:
ARB - I dont have it.
You may find you need one, at least on the front. If nothing else, an adjustable ARB is a useful tool (used in moderation) to fine tune handling once you've got the basic balance somewhere close.You were honest but I dont find it offending. I wont tell that I am an expert (very-very far from one), but I am not stupid also. I know my responsibility. I am only risking my own life (no passengers, car is tested on a private road and a closed, rather big parking yard) while I test it, and I do it with great care.
Thanks God, the situation is not that bad. The car handles quite good, with only a slight, miserable sloop while cornering fast. It is only a feeling. I have never lost the track or any such thing.
Otherwise the cars holds firm up to 120kmh (topspeed in the longest straight), no oevrsteering, no understeering, no vibrations, no tyre sqeeleing etc. This piece of tin operates just like a car. It is much better than my everyday VW, but still not perfect.
The original car had a terrible suspension. The steering rack was some 23mms higher than supposed to be, giving enormous bump steer. Just like the rear end. The rear suspension had 8mms toe-in (!), the rear steering tierod was held by a 4mms thick piece of velded steel strap, and the thread was cut right INTO that 4mms thick normal steel material. And the damper pickup wasd again a piece of art. I will post the photos if I find them.
Soo I think the car is a way better then it was, just I would like to make it even better.
According to the springs, this setup was advised by a mini specialist, and is used quite frequently (rubber springs on the rear and coils on the front). Originaly my Cox had modified rubber cones on the front also, but they were still very hard, and with thelight front end, it was realy hazardous, as the car had an unpredictable tendency to understeer. I will try to work out some kind of solution to fitt an ARB on the front, though minis do not benefit too much from one.
Thanks God, the situation is not that bad. The car handles quite good, with only a slight, miserable sloop while cornering fast. It is only a feeling. I have never lost the track or any such thing.
Otherwise the cars holds firm up to 120kmh (topspeed in the longest straight), no oevrsteering, no understeering, no vibrations, no tyre sqeeleing etc. This piece of tin operates just like a car. It is much better than my everyday VW, but still not perfect.
The original car had a terrible suspension. The steering rack was some 23mms higher than supposed to be, giving enormous bump steer. Just like the rear end. The rear suspension had 8mms toe-in (!), the rear steering tierod was held by a 4mms thick piece of velded steel strap, and the thread was cut right INTO that 4mms thick normal steel material. And the damper pickup wasd again a piece of art. I will post the photos if I find them.
Soo I think the car is a way better then it was, just I would like to make it even better.
According to the springs, this setup was advised by a mini specialist, and is used quite frequently (rubber springs on the rear and coils on the front). Originaly my Cox had modified rubber cones on the front also, but they were still very hard, and with thelight front end, it was realy hazardous, as the car had an unpredictable tendency to understeer. I will try to work out some kind of solution to fitt an ARB on the front, though minis do not benefit too much from one.
Sam_68 said:
Contact the GTM Owner's club; the Cox GTM and its sucessors are fairly well known cars and I'm sure there will be plenty of information on optimum spring rates and set up.
I am in contact with them. Realy nice folk. I have disscussed the suspension setup pre setting it on their forum, and had my suspensin set around their suggessions. 
camelotr said:
But if we dont have bumpsteer, the rolling centers tend to be in good relation (front lower than rear), the weight distribution is not that bad, I have a nearly spot on ackerman, where should I try any modification?
As stated above: spring rates.And I can't believe that anyone is stupid enough to recommend linear rate steel springs on the front and rubber giving a rate that rises by a factor of 10 between static ride height and full bump at the rear, but if they do, ignore them: it's a dangerously incorrect thing to suggest.
At risk of pointing out the obvious, a Mini is quite different to a GTM. A very stiff front end (relative to the rear) will tend to induce understeer, which is safe. A very stiff back end (relative to the front) will tend to induce snap oversteer, which is not!
Ok then.
Whats Your opinion about fitting a coil spring to the rear - obviously a stiffer setup than on the front.
This spring is rated as "fastroad" by the shop
. In reality it offers the same or a bit more firm ride than the rubber cone.
i think the mini suspension is more forgiving in sense of spring rates as the suspension travel is very short, and whatever You do, the ride is - best say - firm.
People often replace only the front rubber cones as it is an "economical" way to improve the ride quality. I have driven such cars and they behave jsut like a normal mini, only with a bit more body roll.
Whats Your opinion about fitting a coil spring to the rear - obviously a stiffer setup than on the front.
This spring is rated as "fastroad" by the shop
. In reality it offers the same or a bit more firm ride than the rubber cone. i think the mini suspension is more forgiving in sense of spring rates as the suspension travel is very short, and whatever You do, the ride is - best say - firm.
People often replace only the front rubber cones as it is an "economical" way to improve the ride quality. I have driven such cars and they behave jsut like a normal mini, only with a bit more body roll.
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