Discussion
Having got my sticky fingers on the new turbo beastie on saturday and clocked up 600-odd miles so far, I think its too low. Definitely too low for practical road use, but in handling terms it feels quite unresolved in the back end on bumpy roads - hard and crashy on small bumps/potholes etc but it feels quite soft and bouncy on bigger bumps, and a big bump under acceleration can see you light up the rears at 70mph (or a couple close together will start it pogoing quite disconcertingly. So I'm wondering if being set too low is having a negative effect on bump/droop control or somesuchlike, it may be a damping issue but before looking at that I plan to sort out the ride heights and see what that does.
Its got Cusco adjustable platform suspension so shouldn't be hard to adjust, any idea what the optimum ride height is for suspension geometry?
The optimum height for geometry isn't any particular height. Just make sure the lower wishbones are horizontal and that will be the ideal height. It sounds to me though like you need to play with the damping - is the damping adjustable? The ride height will affect natural roll-resistance and of course suspension travel but shouldn't affect damping as long as it's off the bump-stops.
Edited by MX-5 Lazza on Tuesday 23 March 10:02
I was under the impression there was a height (or range of heights) which gave the best results in terms of camber change, wishbone geometry etc?
The lower wishbones on the rear slope up to the wheels so its obviously too low there. No adjustable damping unfortunately but the suspension looks to be right at the end of its adjustment to sit that low - so it may be reducing bump or droop travel by a significant amount, I'll get a better idea when I have time for a closer look.
The lower wishbones on the rear slope up to the wheels so its obviously too low there. No adjustable damping unfortunately but the suspension looks to be right at the end of its adjustment to sit that low - so it may be reducing bump or droop travel by a significant amount, I'll get a better idea when I have time for a closer look.
GravelBen said:
I was under the impression there was a height (or range of heights) which gave the best results in terms of camber change, wishbone geometry etc?
There probably is but getting the wishbones set in the right positions is more important. If the rear angle up to the wheels then they are too low. Many try to have the rear slightly higher than the front but the ideal is to be the same front & rear - it's how the racers set up their cars. They also use floor to sill measurements rather than hub to wheel-arch as it's more accurate (and doesn't depend on the condition of the wings).the problem is down to teh design of the rear suspension - there's just hardly any travel. so when you lower it, you don't have much room before you're bouncing off the bump stops and throwing the back all over the place. sounds to me like what's happening.
put the rear up so you have 30 front, 31 rear. also, make sure the springs are the right way round front to rear. the fronts should be thicker.
put the rear up so you have 30 front, 31 rear. also, make sure the springs are the right way round front to rear. the fronts should be thicker.
Edited by skinny on Wednesday 24th March 07:40
Thanks - will crank the ride height up to a more sensible level on Saturday and see if that improves things - its far worse on bumpy roads than the tired original suspension on my old '89 Mk1.
Its really not ideal as it is, a big bump while overtaking had me lighting up 3rd gear (briefly) at 70mph. Fine on smooth roads though - its possible the previous owner deliberately set it up quite soft in the rear to improve traction, in general it hooks up very well unless provoked.
Its really not ideal as it is, a big bump while overtaking had me lighting up 3rd gear (briefly) at 70mph. Fine on smooth roads though - its possible the previous owner deliberately set it up quite soft in the rear to improve traction, in general it hooks up very well unless provoked.
Edited by GravelBen on Wednesday 24th March 10:22
Its a new one for me.
Nope, just jack it up, loosen the bottom lock ring and wind it down out of the way then adjust the top ring (spring seat) up/down the required amount then relock the lock-ring. It'll probably be quite an effort to adjust the height upwards but it should be do-able. I needed to use a cloth around the c-spanner to do mine to make it more comfortable. Before locking put a smear of copper-grease between the 2 rings and only pinch them up lightly - the lock-ring is just there to make sure the spring seat doesn't turn, it doesn't need to be very tight.
Remember though that if you need to raise it 10mm that doesn't equate to 10mm on the spring seat - I don't know what the ratio is but raising 10mm will be something like 6mm on the spring. Also, make sure you do both sides before trusting a measurement and then measure again after a trip around the block (preferable a bumpy-road block) to make sure the spring is settled on the seat properly.
Remember though that if you need to raise it 10mm that doesn't equate to 10mm on the spring seat - I don't know what the ratio is but raising 10mm will be something like 6mm on the spring. Also, make sure you do both sides before trusting a measurement and then measure again after a trip around the block (preferable a bumpy-road block) to make sure the spring is settled on the seat properly.
Stick the jack under the corner of the car your doing to release the pre-loaded pressure off the spring 
I'd also look into corner-weighting for a more accurate setup - while the majority of people seem to think coilover are so you can 'slam' your car - actually there purpose is to correctly setup the spring preload and sag on each corner to even out the weight distribution across the 4 tyres.
I'd also look into corner-weighting for a more accurate setup - while the majority of people seem to think coilover are so you can 'slam' your car - actually there purpose is to correctly setup the spring preload and sag on each corner to even out the weight distribution across the 4 tyres.
5 seconds googling said:
Corner Weighting
As the name suggests, this involves very accurately measuring the weight on all four wheels at the same time. What's the point you may ask? Well, the amount of weight or pressure on each tyre will affect the amount of grip that tyre has. Ideally, all cars would have 25% of their total weight on each corner, as this would give completely neutral handling during cornering, acceleration and braking. Unfortunately, this is rarely true apart from exotic mid engine'd cars where the designer has not had to compromise weight distribution due to other design constraints.
If a car has more weight on one side than the other then the heavier side will have more grip when braking and accelerating. At best, this imbalance will make the car feel nervous and in the worst cases, will cause loss of traction or wheel lock up.
If your car has adjustable spring platform shock absorbers, the platforms can be adjusted up or down to optimise the distribution of weight around the car. This allows us to take into account the manufacturing tolerances in the suspension and chassis and also to compensate for driver weight. Corner weighting can also be useful during the build of a car as once the major items whose position is fixed have been fitted, other items such as batteries, fire extinguishers etc, can be placed to optimise the weight distribution as much as possible.
With many cars it is not sufficient just to use ride height as a method of setting up adjustable platform shocks. Adjustment using ride height only as a reference does not take into account variables such as slight chassis twist or differing spring lengths and especially with many kit cars or rally cars using the body as a reference point is not advisable. This is due to inconsistent body mouldings or inaccurately repaired panel damage.
Needn't be expensive either, its qutie a straightforward process. your best bet is to find somewhere near you that prepares Rally cars, they are guaranteed to have some scales. As the name suggests, this involves very accurately measuring the weight on all four wheels at the same time. What's the point you may ask? Well, the amount of weight or pressure on each tyre will affect the amount of grip that tyre has. Ideally, all cars would have 25% of their total weight on each corner, as this would give completely neutral handling during cornering, acceleration and braking. Unfortunately, this is rarely true apart from exotic mid engine'd cars where the designer has not had to compromise weight distribution due to other design constraints.
If a car has more weight on one side than the other then the heavier side will have more grip when braking and accelerating. At best, this imbalance will make the car feel nervous and in the worst cases, will cause loss of traction or wheel lock up.
If your car has adjustable spring platform shock absorbers, the platforms can be adjusted up or down to optimise the distribution of weight around the car. This allows us to take into account the manufacturing tolerances in the suspension and chassis and also to compensate for driver weight. Corner weighting can also be useful during the build of a car as once the major items whose position is fixed have been fitted, other items such as batteries, fire extinguishers etc, can be placed to optimise the weight distribution as much as possible.
With many cars it is not sufficient just to use ride height as a method of setting up adjustable platform shocks. Adjustment using ride height only as a reference does not take into account variables such as slight chassis twist or differing spring lengths and especially with many kit cars or rally cars using the body as a reference point is not advisable. This is due to inconsistent body mouldings or inaccurately repaired panel damage.
Interesting comments there about corner weighting - its something I thought I'd probably get done once I have the ride heights where I want them as it will be getting track time somewhere down the, er, track. But if an outfit like WIM don't think its necessary then maybe I won't bother at this stage.
when i changed my spring rates on my coilovers i noticed that the measurement between spring platform and end of body differed by something like 10mm on the fronts.
i decided that since they're basic avo's and the only locking device is a plastic grub screw which is fubar'd on every one, that lots of track time had rattled one spring seat down the thread
since setting up, having alignment done etc. at wim the car pulls on cruise and pulls under braking.
i'm starting to think that the car had been corner weighted, and maybe for good reason if there was some chassis flex?!
i'm wondering if having slightly crushed frame rails suggests some twisting has taken place, might be more noticeable if i ever get some of the P5 frame rails
for what it costs (?) might be worth looking into? i've heard from people that used to race metro's that corner weighting really made the car handle nicely
i decided that since they're basic avo's and the only locking device is a plastic grub screw which is fubar'd on every one, that lots of track time had rattled one spring seat down the thread
since setting up, having alignment done etc. at wim the car pulls on cruise and pulls under braking.
i'm starting to think that the car had been corner weighted, and maybe for good reason if there was some chassis flex?!
i'm wondering if having slightly crushed frame rails suggests some twisting has taken place, might be more noticeable if i ever get some of the P5 frame rails
for what it costs (?) might be worth looking into? i've heard from people that used to race metro's that corner weighting really made the car handle nicely
Got it sorted today - lifted the back end 20mm so its 300mm F/310mm R. So much friendlier over bumps now, made a huge difference! Little bit more body roll and feels slightly less planted in the rear but I think thats mostly just a change in camber with the ride height, so an alignment should fix that if I don't want to be a tail-happy loon. 
Edited by GravelBen on Sunday 28th March 08:18
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