Seven spring rates
Discussion
Having sampled my first trackday with my Locost I have a question about rear spring rates.
I have 275lb on the front and 160lb on the back currently.
It seams to roll too much at the back on the track. Given that the weight distribution with me on board is 46% front 54% rear this does not seem right. If the inclination of the front shocks is factored in a rate of around 250 on the rear looks more correct.
All up weight, full tank, driver etc is 590kg ie the same as a road sport Caterham.
Any suggestions / comparisons?
I have 275lb on the front and 160lb on the back currently.
It seams to roll too much at the back on the track. Given that the weight distribution with me on board is 46% front 54% rear this does not seem right. If the inclination of the front shocks is factored in a rate of around 250 on the rear looks more correct.
All up weight, full tank, driver etc is 590kg ie the same as a road sport Caterham.
Any suggestions / comparisons?
This is very difficult to answer as how your car handles is dependent on so many things and it's very subjective as to what makes a good handling car. Some like a car to grip like it's on rails but when the limit of adhesion is reached they can tend to let go very quickly, others (like me) prefer a much more progressive style of handling whereby the backend will let go earlier but be much more controllable. Some people actually like understeer (weirdo's 
) and some like lots of oversteer.
In an ideal world a neutral balance between the two is what you're really after, that way if you enter a corner too quickly you'll get understeer but you should also be able to set up your car nicely to drift on demand also. This is where the black art of suspension set up comes in. Now I don't know very much about this, few do properly, but it will help to either read a book on race suspension design and setup or go to someone who knows what they're doing.
Contrary to the popular belief of the Max Power brigade, slamming your car and making the suspension rock hard does not tend to improve handling. For example doing this will reduce the travel of the shocks making them more prone to bottoming out, also any bumps will cause the car to skip and clatter around. This is bad. Then again a stiffer ride and less roll on a smooth circuit will dramatically improve things over a wallowy car...
It all comes down to striking a compromise if the car is to be used on road and track. You need decent ride otherwise the car will be a nightmare on the road (trust me, I've been here). But you want it to be stiff enough so it isn't a mess on the track.
To give you an example my car is now set up how I like it. I'll go though the settings and try and explain why we've done certain things as I go along. BTW I was helped massively by Paul Aspden at Plays-Kool Motorsport in Bradford who did a lot of adjustment and playing around with our cars. He is one of those people who knows what he is doing with this sort of thing. We're not worthy
My car was originally a mess on the road, very hard, borderline painful. On smooth circuits it was a revalation, however hit a kerb and you had to be ready to dial in armfulls of opposite lock as it kicked the car all over the place. What I wanted was a car that rode the road well but also worked well on track. As far as I'm concerned understeer is evil and must be eliminated. Luckily being a live axle car with a plate limited slip differential and LOTS of torque, power oversteer on demand is not really an issue
First to go were the cheapo steel Spax shocks to be replaced by AVO alloy Pro-race shocks (not as expensive as they sound). These shockers were not only half the weight of the Spax (unsprung weight is the enemy remember) and their valving is much more suited to a car as light as a Caterfield. Both single adjustable shockers, the Spax damping only seemed to be adjustable on varying degrees of harshness where as the AVO's can be set from soft and very road friendly to very hard indeed. The AVO's have around 30 clicks of adjustment. In the real world the first 10 clicks actually make quite a lot of noticeable difference but once you get to 10 they're so firm (too firm) it's difficult to detect any change.
Spring-wise we took a fairly controversial route. The Spax had some massively compressed 200lb springs on the front, maybe this was to attempt to compensate for the firm shockers? I don't know, but they're way too soft for a Westie with a big heavy Cosworth lump in it. Now popular opinion would have you believe that anything from 275lb to 300lb is what you should be fitting to a Westie with a Cossie/Vx in it. However Paul was fairly insistent that given the choice, 250lb springs would be more suited to what I was after. I'm guessing this is probably because my car is actually quite light compared to later Westies as it's an oldish SE and is pretty much stripped bare... On the back, as the springs are upright on a live axle they work much more efficiently, also there is not a lot of weight at the back of an SE so we used 150lb springs.
It's dead easy to set up the geometry on mine as it's live axle so you only have to deal with the front end. 0.5 of a degree of toe in keeps the wheels going in a straight line when you're moving along. Some Caterham owners I know actually run about the same in toe out saying that this improves turn in on track. Can make it not so pleasant on the road though. I also have about 1.5 degrees of negative camber this seems to work well with the tyres I'm currently running. If I was to run crossplys like Avon ACB10's this setting should ideally be reduced to pretty much no camber at all, maybe 0.5 of a degree tops.
Ride height is generally determined by clearance to the sump. As mine is dry sumped and has an ali floor on the engine bay, the lowest point on my car is the bell housing that protrudes maybe 10mm through the floor. Clearance is not an issue. On a Westie a ideal height would then be one where the front wishbones lie pretty much horizontal to the road and the back of the car should be about half an inch higher. However we found that set to this, with two up and as the axle is effectively floating, hitting a bump could cause the prop shaft to hit the handbrake mechanism in the transmission tunnel. Compromise time; Paul raised it up by a bit and got it all level.
Mine has been set up pretty close to right. It is possible to set it up absolutely spot on by 'flat flooring' the car. This is really only necessary if you're racing and you want the car set up to be perfect. For trackdays and road use where you're not against a clock or other competitors, it's not really an issue as long as the car isn't miles out. It basically involves someone like Paul, a set of corner weight scales and your ass sat in the drivers seat. He would then set the ride heights with you in the car to balance out all the corner weights and get it right for when you are in it. A very time consuming process. As I say it only really works for racing because as soon as you add a passenger it goes all to pot.
Finally my car is stored in Kirklees which has probably the worst roads known to man, ideal for setting up the damping. Starting with the dampers turned down to minimum we went for a run. The trick is to increase the settings uniformly on all the dampers until it becomes too hard (you'll know) then back it off to the best setting. Then you try adjusting the front an rear individually. I found it really easy to do on the PPE as you're sat on the back axle pretty much and your arse tells you when that setting is ok. I still thought the fronts were a bit bouncy (handy that you can see them) so they were cranked up two more notches. and that was that. IIRC I've got about 3 clicks on the back and 5 on the front.
Our other car, A Vx powered SEW has 4 on the back and 6 on the front IIRC. It is running steel AVO's but that's not the reason for the difference. Steve runs Yoko A539 rubber and I run A032R's. The 32R's have much stiffer sidewalls than the 539's so Steve's is effectively getting more damping from the tyre itself. Other than that his car is set up pretty much the same as mine. I'd like to swap the springs on it as the current ones are too long and the car is riding too high. But as I've said, it's all subjective, Steve likes it as it is so that's how it'll stay.
Saying all that our cars still roll quite a lot, especially in comparison to cars equipped with anti roll bars. ARB's can add yet another level of adjustability to your suspension set up as effectively they're like another spring that operates between the front and rear wheels. These eliminate a lot of roll but can cause a car to understeer dramatically (or less likely the opposite). I like how mine handles so have not investigated equipping them. Their effect on a setup is something you've really got to suck and see, not a cheap thing to do if you don't like them.
For more info on this sort of thing grab a copy of Competition Car Suspension by Alan Staniforth. Some fascinating stuff in there.
Have a good Xmas everyone!
>> Edited by juansolo on Wednesday 25th December 16:14
) and some like lots of oversteer. In an ideal world a neutral balance between the two is what you're really after, that way if you enter a corner too quickly you'll get understeer but you should also be able to set up your car nicely to drift on demand also. This is where the black art of suspension set up comes in. Now I don't know very much about this, few do properly, but it will help to either read a book on race suspension design and setup or go to someone who knows what they're doing.
Contrary to the popular belief of the Max Power brigade, slamming your car and making the suspension rock hard does not tend to improve handling. For example doing this will reduce the travel of the shocks making them more prone to bottoming out, also any bumps will cause the car to skip and clatter around. This is bad. Then again a stiffer ride and less roll on a smooth circuit will dramatically improve things over a wallowy car...
It all comes down to striking a compromise if the car is to be used on road and track. You need decent ride otherwise the car will be a nightmare on the road (trust me, I've been here). But you want it to be stiff enough so it isn't a mess on the track.
To give you an example my car is now set up how I like it. I'll go though the settings and try and explain why we've done certain things as I go along. BTW I was helped massively by Paul Aspden at Plays-Kool Motorsport in Bradford who did a lot of adjustment and playing around with our cars. He is one of those people who knows what he is doing with this sort of thing. We're not worthy
My car was originally a mess on the road, very hard, borderline painful. On smooth circuits it was a revalation, however hit a kerb and you had to be ready to dial in armfulls of opposite lock as it kicked the car all over the place. What I wanted was a car that rode the road well but also worked well on track. As far as I'm concerned understeer is evil and must be eliminated. Luckily being a live axle car with a plate limited slip differential and LOTS of torque, power oversteer on demand is not really an issue
First to go were the cheapo steel Spax shocks to be replaced by AVO alloy Pro-race shocks (not as expensive as they sound). These shockers were not only half the weight of the Spax (unsprung weight is the enemy remember) and their valving is much more suited to a car as light as a Caterfield. Both single adjustable shockers, the Spax damping only seemed to be adjustable on varying degrees of harshness where as the AVO's can be set from soft and very road friendly to very hard indeed. The AVO's have around 30 clicks of adjustment. In the real world the first 10 clicks actually make quite a lot of noticeable difference but once you get to 10 they're so firm (too firm) it's difficult to detect any change.
Spring-wise we took a fairly controversial route. The Spax had some massively compressed 200lb springs on the front, maybe this was to attempt to compensate for the firm shockers? I don't know, but they're way too soft for a Westie with a big heavy Cosworth lump in it. Now popular opinion would have you believe that anything from 275lb to 300lb is what you should be fitting to a Westie with a Cossie/Vx in it. However Paul was fairly insistent that given the choice, 250lb springs would be more suited to what I was after. I'm guessing this is probably because my car is actually quite light compared to later Westies as it's an oldish SE and is pretty much stripped bare... On the back, as the springs are upright on a live axle they work much more efficiently, also there is not a lot of weight at the back of an SE so we used 150lb springs.
It's dead easy to set up the geometry on mine as it's live axle so you only have to deal with the front end. 0.5 of a degree of toe in keeps the wheels going in a straight line when you're moving along. Some Caterham owners I know actually run about the same in toe out saying that this improves turn in on track. Can make it not so pleasant on the road though. I also have about 1.5 degrees of negative camber this seems to work well with the tyres I'm currently running. If I was to run crossplys like Avon ACB10's this setting should ideally be reduced to pretty much no camber at all, maybe 0.5 of a degree tops.
Ride height is generally determined by clearance to the sump. As mine is dry sumped and has an ali floor on the engine bay, the lowest point on my car is the bell housing that protrudes maybe 10mm through the floor. Clearance is not an issue. On a Westie a ideal height would then be one where the front wishbones lie pretty much horizontal to the road and the back of the car should be about half an inch higher. However we found that set to this, with two up and as the axle is effectively floating, hitting a bump could cause the prop shaft to hit the handbrake mechanism in the transmission tunnel. Compromise time; Paul raised it up by a bit and got it all level.
Mine has been set up pretty close to right. It is possible to set it up absolutely spot on by 'flat flooring' the car. This is really only necessary if you're racing and you want the car set up to be perfect. For trackdays and road use where you're not against a clock or other competitors, it's not really an issue as long as the car isn't miles out. It basically involves someone like Paul, a set of corner weight scales and your ass sat in the drivers seat. He would then set the ride heights with you in the car to balance out all the corner weights and get it right for when you are in it. A very time consuming process. As I say it only really works for racing because as soon as you add a passenger it goes all to pot.
Finally my car is stored in Kirklees which has probably the worst roads known to man, ideal for setting up the damping. Starting with the dampers turned down to minimum we went for a run. The trick is to increase the settings uniformly on all the dampers until it becomes too hard (you'll know) then back it off to the best setting. Then you try adjusting the front an rear individually. I found it really easy to do on the PPE as you're sat on the back axle pretty much and your arse tells you when that setting is ok. I still thought the fronts were a bit bouncy (handy that you can see them) so they were cranked up two more notches. and that was that. IIRC I've got about 3 clicks on the back and 5 on the front.
Our other car, A Vx powered SEW has 4 on the back and 6 on the front IIRC. It is running steel AVO's but that's not the reason for the difference. Steve runs Yoko A539 rubber and I run A032R's. The 32R's have much stiffer sidewalls than the 539's so Steve's is effectively getting more damping from the tyre itself. Other than that his car is set up pretty much the same as mine. I'd like to swap the springs on it as the current ones are too long and the car is riding too high. But as I've said, it's all subjective, Steve likes it as it is so that's how it'll stay.
Saying all that our cars still roll quite a lot, especially in comparison to cars equipped with anti roll bars. ARB's can add yet another level of adjustability to your suspension set up as effectively they're like another spring that operates between the front and rear wheels. These eliminate a lot of roll but can cause a car to understeer dramatically (or less likely the opposite). I like how mine handles so have not investigated equipping them. Their effect on a setup is something you've really got to suck and see, not a cheap thing to do if you don't like them.
For more info on this sort of thing grab a copy of Competition Car Suspension by Alan Staniforth. Some fascinating stuff in there.
Have a good Xmas everyone!
>> Edited by juansolo on Wednesday 25th December 16:14
Wow thanks for all that. Generally the handling is fine, Hereforshire roads give it a fair work out.
The speed of roll is the issue, I think, in sudden changes of direction. It makes it a little difficult to manage the weight transfer through an S bend.
It looks like anti roll bars could be the way forwards we shall see.
The speed of roll is the issue, I think, in sudden changes of direction. It makes it a little difficult to manage the weight transfer through an S bend.
It looks like anti roll bars could be the way forwards we shall see.
At the risk of confusing you further, I was taught two rules when I did suspension design:
1) 3g bump
This is the minimum load the suspension should take before metal-to-metal (imagine the bump stop has been removed/destroyed). With some measurement of suspension points, you can calculate how much compression goes into the spring at full bump, then match this to the 3g load. Remember to allow an extra 100kg for a passenger and some luggage, but you can deduct the unsprung weight.
2)Front & Rear frequencies
You'll need your old physics book for this. There is a formula to give the harmonic frequency of the suspension (ignore the damper) that depends on sprung weight and spring stiffness. You will need to make allowances for angulation of the springs to fix it as a vertical action (but you can ignore how this changes as the suspension articulates - it is primarily realted to small inputs). You need to calculate it separately for a front and rear wheel (we'll assume left right difference is marginal) for both unladen (driver only) and fully laden (passenger and luggage).
At all times, you should ensure first of all that the natural frequency at the rear is 10% higher than at the front. If the rear frequency falls below the front, it will create some strange rocking motions that are unsettling to you and the car. Also, if either frequency goes above 80Hz, you can expect it to feel a bit lively.
This is really a comfort thing, but might prove interesting to calculate. If you stick with your existing front springs, however, do the +10% check for your rears.
Now I suspect that you will find both methods will give you very soft rates, compared to what you have fitted already, but at least you've done the test. This may prove interesting, as the Rocket uses very soft springs and no ARB. Controlling turn-in and handling comes from the dampers, which can easily be wound up for track and down for road. As a result, I feel ride on the road is better than a Se7en, but a quick wind-up of the dampers makes it tight as a gnat's chuff on-track. Of course, it helps having suspension designed by someone who REALLY knows how to do it, but it does show how soft springs can aid both ride and grip.
As I said at the start, this may confuse you even more, but at least it's all theroetical and doesn't require you to buy anything first. It may prove revealing or completely useless...
1) 3g bump
This is the minimum load the suspension should take before metal-to-metal (imagine the bump stop has been removed/destroyed). With some measurement of suspension points, you can calculate how much compression goes into the spring at full bump, then match this to the 3g load. Remember to allow an extra 100kg for a passenger and some luggage, but you can deduct the unsprung weight.
2)Front & Rear frequencies
You'll need your old physics book for this. There is a formula to give the harmonic frequency of the suspension (ignore the damper) that depends on sprung weight and spring stiffness. You will need to make allowances for angulation of the springs to fix it as a vertical action (but you can ignore how this changes as the suspension articulates - it is primarily realted to small inputs). You need to calculate it separately for a front and rear wheel (we'll assume left right difference is marginal) for both unladen (driver only) and fully laden (passenger and luggage).
At all times, you should ensure first of all that the natural frequency at the rear is 10% higher than at the front. If the rear frequency falls below the front, it will create some strange rocking motions that are unsettling to you and the car. Also, if either frequency goes above 80Hz, you can expect it to feel a bit lively.
This is really a comfort thing, but might prove interesting to calculate. If you stick with your existing front springs, however, do the +10% check for your rears.
Now I suspect that you will find both methods will give you very soft rates, compared to what you have fitted already, but at least you've done the test. This may prove interesting, as the Rocket uses very soft springs and no ARB. Controlling turn-in and handling comes from the dampers, which can easily be wound up for track and down for road. As a result, I feel ride on the road is better than a Se7en, but a quick wind-up of the dampers makes it tight as a gnat's chuff on-track. Of course, it helps having suspension designed by someone who REALLY knows how to do it, but it does show how soft springs can aid both ride and grip.
As I said at the start, this may confuse you even more, but at least it's all theroetical and doesn't require you to buy anything first. It may prove revealing or completely useless...
I have yet to do the maths, but it sounds like we may be in the right ball park after all. Winding up the damping certainly changed things for the better cornering wise, even if the ride was poor on our dreadful roads.
What brought all this on was fitting the Motobuild suspension kit to my dad's Elise recently. This contrary to what anyone else does fits 300lb springs all round. With the result of better ride and far less weight transfer.
If the weather improves we will take both of them to the first bat date at llandow to find some real answers.
Cheers for the suggestions .
Happy new year.
Stu
What brought all this on was fitting the Motobuild suspension kit to my dad's Elise recently. This contrary to what anyone else does fits 300lb springs all round. With the result of better ride and far less weight transfer.
If the weather improves we will take both of them to the first bat date at llandow to find some real answers.
Cheers for the suggestions .
Happy new year.
Stu
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