suspension....how much pre-load on dampers - if any?
Discussion
Those of you with height adjustable dampers.....how much pre-load (if any) have put on them?
That is to say with the seat wound up so that the spring is just in contact with the top and bottom spring platforms is zero preload (and the spring will compress as soon as the weight of the car is placed on them. From that point winding the seat platform down just lowers the ride height and gives slack spring on suspension droop. Winding the platform up increases the static ride height until the pre-load is the same as the chassis weight on that corner, there after raising the platform has no affect on ride height but does increase the force required to compress the spring any further i.e. pre-load.
That is to say with the seat wound up so that the spring is just in contact with the top and bottom spring platforms is zero preload (and the spring will compress as soon as the weight of the car is placed on them. From that point winding the seat platform down just lowers the ride height and gives slack spring on suspension droop. Winding the platform up increases the static ride height until the pre-load is the same as the chassis weight on that corner, there after raising the platform has no affect on ride height but does increase the force required to compress the spring any further i.e. pre-load.
magpies said:
Those of you with height adjustable dampers.....how much pre-load (if any) have put on them?
That is to say with the seat wound up so that the spring is just in contact with the top and bottom spring platforms is zero preload (and the spring will compress as soon as the weight of the car is placed on them. From that point winding the seat platform down just lowers the ride height and gives slack spring on suspension droop. Winding the platform up increases the static ride height until the pre-load is the same as the chassis weight on that corner, there after raising the platform has no affect on ride height but does increase the force required to compress the spring any further i.e. pre-load.
Good question, but I think you asked a similar one on damper length and were advised "depends....", spring poundage/ride-height etc?That is to say with the seat wound up so that the spring is just in contact with the top and bottom spring platforms is zero preload (and the spring will compress as soon as the weight of the car is placed on them. From that point winding the seat platform down just lowers the ride height and gives slack spring on suspension droop. Winding the platform up increases the static ride height until the pre-load is the same as the chassis weight on that corner, there after raising the platform has no affect on ride height but does increase the force required to compress the spring any further i.e. pre-load.
Your statement: "Winding the platform up increases the static ride height until the pre-load is the same as the chassis weight on that corner, there after raising the platform has no affect on ride height..." is not exactely correct.
Essentially, there is no defined "preload" on coil-overs with adjustable seats. You set the ride-height but this must be tempered with cross-weights? Changing the ride height at any corner will change the cross-weight percentage.
If you raise the ride height at a given corner (one or two turns of the spring seat), the weight on that corner will increase, as will the weight on the diagonally opposite corner. The other two corners will lose weight.
That is but a precis of what is done to get a car/TVR to feel balanced at speed turning left or right. A miss match on cross-weights (ride-height) results in understeer turning right and oversteer turning left (or vice-versa). Not a nice feeling if you are competing - indeed not nice even not competing.
Set your car up for 50/50% cross weighting, not a static preload/shock length - But measure them after you have achieved the balance. This is called Corner weighting and takes time and a very flat floor and expensive weighting pads for all 4-wheels.
Hope that clarifies that pre-load is not an aspect which determines ride-height and a balanced car?
Trev
Edited by K4TRV on Friday 4th October 14:10
Hi Mick,
This looks like a continuation of your earlier thread. The only useful purpose of so-called 'pre-load' is to prevent your springs rattling or becoming dislodged when you get airborne or jack the car up. You are, in effect, adjusting the damper stroke so that it reaches full stroke just before the spring comes loose.
Consider the car sitting correctly on its suspension. Each spring will compress by a sufficient amount to support the load at that corner. The damper should have free travel in both directions at this stage, so is adding nothing other than a reluctance to move. If you jack up one corner (at the chassis) slowly, the suspension and spring will extend, and the force in the spring will reduce accordingly, but there is still no load carried directly by the damper. Eventually one of two things happens: either the damper extends sufficiently so that the spring loses all load and becomes loose, or the damper reaches the end of its stroke, takes the remaining load in the spring and lifting the wheel off the ground.
When setting ride height on these cars, one is normally adjusting the position of the spring seat on the damper body, and hence the extension of the damper when the spring is carrying the weight. If you raise the ride height sufficiently, you will reach the position where the damper extends to its full length, and this is when your 'pre-load' is equal to the static load in the spring. You then have no rebound capacity and your suspension only works in one direction. You can't raise the ride height any further because the damper is already fully extended.
All this excited my curiosity (I'm easily excited these days!) so I measured the compressed length of my springs with the car sitting on its wheels in the garage. It appears that the peculiarities of suspension geometry is such that on my V8S the front springs are carrying ~900lb each and the rears ~700lb each. I expect the numbers are much the same for the other S variants. If you wanted to estimate a maximum 'pre-load' as you call it, I would guess no more than half these values to give your suspension some working room.
Regarding corner weights, I agree entirely with K4TRV. It need not be expensive if you have a flat floor to work from - a couple of cheapo bathroom scales and assorted planks of wood will do the job. The flat floor IS important though - if you have stiff springs, you can see that a 1/4 inch out of level corner to corner may upset your measurements by 20-30lb.
Regards,
Graham
This looks like a continuation of your earlier thread. The only useful purpose of so-called 'pre-load' is to prevent your springs rattling or becoming dislodged when you get airborne or jack the car up. You are, in effect, adjusting the damper stroke so that it reaches full stroke just before the spring comes loose.
Consider the car sitting correctly on its suspension. Each spring will compress by a sufficient amount to support the load at that corner. The damper should have free travel in both directions at this stage, so is adding nothing other than a reluctance to move. If you jack up one corner (at the chassis) slowly, the suspension and spring will extend, and the force in the spring will reduce accordingly, but there is still no load carried directly by the damper. Eventually one of two things happens: either the damper extends sufficiently so that the spring loses all load and becomes loose, or the damper reaches the end of its stroke, takes the remaining load in the spring and lifting the wheel off the ground.
When setting ride height on these cars, one is normally adjusting the position of the spring seat on the damper body, and hence the extension of the damper when the spring is carrying the weight. If you raise the ride height sufficiently, you will reach the position where the damper extends to its full length, and this is when your 'pre-load' is equal to the static load in the spring. You then have no rebound capacity and your suspension only works in one direction. You can't raise the ride height any further because the damper is already fully extended.
All this excited my curiosity (I'm easily excited these days!) so I measured the compressed length of my springs with the car sitting on its wheels in the garage. It appears that the peculiarities of suspension geometry is such that on my V8S the front springs are carrying ~900lb each and the rears ~700lb each. I expect the numbers are much the same for the other S variants. If you wanted to estimate a maximum 'pre-load' as you call it, I would guess no more than half these values to give your suspension some working room.
Regarding corner weights, I agree entirely with K4TRV. It need not be expensive if you have a flat floor to work from - a couple of cheapo bathroom scales and assorted planks of wood will do the job. The flat floor IS important though - if you have stiff springs, you can see that a 1/4 inch out of level corner to corner may upset your measurements by 20-30lb.
Regards,
Graham
I entirely agree with Trev and Graham. Talking of pre-loading the damper is really a misapprehension. It is the spring which is 'preloaded' by the static body weight. Technically setting the spring position by ride height isnt an accurate way of preloading the springs as there can be marginal differences in the spring rates and geometry geometry, and indeed in the outriggers relative to the suspension, and even tyre pressures, but it is really the most practical method. At least it is as a start reference point.
Mention has been made of flat ground. It needs to be level and flat and to be honest you are going to find that really difficult. Is that garage floor really flat to a couple of mm over the whole area of the car?
See if your local garage will let you check it on their ramp.
Get the ride heights right according the suspension wiki THEN see if the suspension arms are at the same height either side of the car. If the chassis is level but the arms are not then you have a geometry discrepancy and you need to check what it is.
It might also be worth checking the static sprint length on both sides of the car, if they are significantly different then start trying to find out why.
One obvious but easily forgotten thing is that you need to 'bounce' the car before checking heights. If you have fitted new springs/dampers, or merely taken the old ones off and replaced them, it is very unlikely they will have settled down at static load height. 'Bounce' the car at each corner then check heights. If you adjust any unit then bounce it again before checking again.
Also do not forget that if you change the height at one corner it will marginally alter the weight distribution and very probably at least 2 of the other ride heights will change! It isnt a 5 min job.
We have a lot more adjustment options than in the vast majority of road cars, and it will take time to get those adjustments right.
Mention has been made of flat ground. It needs to be level and flat and to be honest you are going to find that really difficult. Is that garage floor really flat to a couple of mm over the whole area of the car?
See if your local garage will let you check it on their ramp.
Get the ride heights right according the suspension wiki THEN see if the suspension arms are at the same height either side of the car. If the chassis is level but the arms are not then you have a geometry discrepancy and you need to check what it is.
It might also be worth checking the static sprint length on both sides of the car, if they are significantly different then start trying to find out why.
One obvious but easily forgotten thing is that you need to 'bounce' the car before checking heights. If you have fitted new springs/dampers, or merely taken the old ones off and replaced them, it is very unlikely they will have settled down at static load height. 'Bounce' the car at each corner then check heights. If you adjust any unit then bounce it again before checking again.
Also do not forget that if you change the height at one corner it will marginally alter the weight distribution and very probably at least 2 of the other ride heights will change! It isnt a 5 min job.
We have a lot more adjustment options than in the vast majority of road cars, and it will take time to get those adjustments right.
Hi Mick, I used to 'engineer' a rally car, which had adjustable suspension all round. As you can imagine a bit of geometric misalignment is pretty well inevitable on a rally car!
We never weighed the corners, it wasn't the important issue really. The issue was to get the spring damper units on each corner working complementary to each other, that meant having paired units at pretty well the same amount of spring preload as well as the same damper setting. Measuring spring static length was more important, it also gave us an idea when something was bent or about to break!
Mind you, there is a complication on the S! Are you running 2 up or solo. The rally car was always 2 up.
Try getting the car set up perfectly with two of you, then remove the 15 stone (? kg!) passenger and drive it again. There should be a noticeable difference as the passenger weight is a significant proportion of the gross weight! I can very definitely feel the difference, especially on undulating roads.
Don't get too carried away, at the end of the day you will just have to try it and 'play' around until it feels right for you. Use the wiki figures as a datum starting point then fine tune from there.
We never weighed the corners, it wasn't the important issue really. The issue was to get the spring damper units on each corner working complementary to each other, that meant having paired units at pretty well the same amount of spring preload as well as the same damper setting. Measuring spring static length was more important, it also gave us an idea when something was bent or about to break!
Mind you, there is a complication on the S! Are you running 2 up or solo. The rally car was always 2 up.
Try getting the car set up perfectly with two of you, then remove the 15 stone (? kg!) passenger and drive it again. There should be a noticeable difference as the passenger weight is a significant proportion of the gross weight! I can very definitely feel the difference, especially on undulating roads.
Don't get too carried away, at the end of the day you will just have to try it and 'play' around until it feels right for you. Use the wiki figures as a datum starting point then fine tune from there.
greymrj said:
Hi Mick, I used to 'engineer' a rally car, which had adjustable suspension all round. As you can imagine a bit of geometric misalignment is pretty well inevitable on a rally car!
We never weighed the corners, it wasn't the important issue really. The issue was to get the spring damper units on each corner working complementary to each other, that meant having paired units at pretty well the same amount of spring preload as well as the same damper setting. Measuring spring static length was more important, it also gave us an idea when something was bent or about to break!
Mind you, there is a complication on the S! Are you running 2 up or solo. The rally car was always 2 up.
Try getting the car set up perfectly with two of you, then remove the 15 stone (? kg!) passenger and drive it again. There should be a noticeable difference as the passenger weight is a significant proportion of the gross weight! I can very definitely feel the difference, especially on undulating roads.
Don't get too carried away, at the end of the day you will just have to try it and 'play' around until it feels right for you. Use the wiki figures as a datum starting point then fine tune from there.
Strange you should mention rally car suspension......as I intend to rally the S (tarmac only)We never weighed the corners, it wasn't the important issue really. The issue was to get the spring damper units on each corner working complementary to each other, that meant having paired units at pretty well the same amount of spring preload as well as the same damper setting. Measuring spring static length was more important, it also gave us an idea when something was bent or about to break!
Mind you, there is a complication on the S! Are you running 2 up or solo. The rally car was always 2 up.
Try getting the car set up perfectly with two of you, then remove the 15 stone (? kg!) passenger and drive it again. There should be a noticeable difference as the passenger weight is a significant proportion of the gross weight! I can very definitely feel the difference, especially on undulating roads.
Don't get too carried away, at the end of the day you will just have to try it and 'play' around until it feels right for you. Use the wiki figures as a datum starting point then fine tune from there.
up to now making sure ride height is about the same as in the wiki. but also the platforms are the same positions side to side. I am using a dummy weights in the seats for both driver and navvy. and trying to make sure the ride height position still allows for a little adjustment before going int preload. I have some 'helper'springs I can fit in case the springs go loose on full droop.
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