Choosing springs and dampers
Discussion
Hi,
Can someone give me a few technical tips on choosing springs and dampers.
1) Rates - Spring wise, I presume this is a case of roughly matching the rears in proportion to the front, given the difference in suspension leverage and weight? ie So both ends deflect the same ammount for a given input? .....and I presume if I ask for a pair of dampers "suitable for a lightweight road car" the range of adjustability should be enough to fine tune the damping force.
2) Mechanical fit - presume you need to know the extended length, the compressed length and the diameter (for spring fitting) of the dampers and the springs can be ordered to a suitable length?
Can someone give me a few technical tips on choosing springs and dampers.
1) Rates - Spring wise, I presume this is a case of roughly matching the rears in proportion to the front, given the difference in suspension leverage and weight? ie So both ends deflect the same ammount for a given input? .....and I presume if I ask for a pair of dampers "suitable for a lightweight road car" the range of adjustability should be enough to fine tune the damping force.
2) Mechanical fit - presume you need to know the extended length, the compressed length and the diameter (for spring fitting) of the dampers and the springs can be ordered to a suitable length?
Its not quite as simple as that unfortunately. If your kit is something "known" like a Westfield etc then its quite easy, but if you've designed / built it yourself then you need to be careful because even similar looking / weight cars could have vastly different spring / damper requirements.
The problem you have is that unless your suspension pickup points, wishbone lengths, damper pickup points, damper angles and car weight are all the same as another known quantity, you can't necessarily compare it as like for like. As an example I changed the top damper pickup point on my book Locost so it was mounted on the side of the chassis rail rather than underneath, and that small repositioning (around 1.5" ) meant the damper sat at about 30 degrees rather than at 40, which in turn neccesitated a 50lb reduction in springrate to keep the same wheel rate. Another example is a friend's race car which although weighing similar to mine, it has 3:1 ratio rocker suspension so runs silly stiff springs even though the compliance when you actually compress the suspension is similar.
As you might have guessed, its the wheel rate you need to look at really, if you can find the wheel rate figures for a known car of similar weight / performance you can then use a bit of trigonometry to work out what spring rate you'll need with your suspension setup, and that will then help dictate what valving the dampers need.
Having said all that, if you're planning on using comparatively basic dampers like Avos, Spax etc (ie ones that don't cost £200+ per corner) then 90% of the time I suspect they'll supply you a standard damper regardless and just rely on the coarse adjustment to give you a narrow window of suitable damping, rather than valving them to suit with usable damping available across the entire adjustment range.
Chris
The problem you have is that unless your suspension pickup points, wishbone lengths, damper pickup points, damper angles and car weight are all the same as another known quantity, you can't necessarily compare it as like for like. As an example I changed the top damper pickup point on my book Locost so it was mounted on the side of the chassis rail rather than underneath, and that small repositioning (around 1.5" ) meant the damper sat at about 30 degrees rather than at 40, which in turn neccesitated a 50lb reduction in springrate to keep the same wheel rate. Another example is a friend's race car which although weighing similar to mine, it has 3:1 ratio rocker suspension so runs silly stiff springs even though the compliance when you actually compress the suspension is similar.
As you might have guessed, its the wheel rate you need to look at really, if you can find the wheel rate figures for a known car of similar weight / performance you can then use a bit of trigonometry to work out what spring rate you'll need with your suspension setup, and that will then help dictate what valving the dampers need.
Having said all that, if you're planning on using comparatively basic dampers like Avos, Spax etc (ie ones that don't cost £200+ per corner) then 90% of the time I suspect they'll supply you a standard damper regardless and just rely on the coarse adjustment to give you a narrow window of suitable damping, rather than valving them to suit with usable damping available across the entire adjustment range.
Chris
Edited by LocoBlade on Wednesday 5th September 20:52
In rough terms:
Determine the weight on each wheel.
From the suspension geometry calculate the leverage between the spring and the wheel i.e. if the spring pushes with 1 unit of force how much force is applied to the wheel. From this you can determine how much force the spring will need to support at normal ride height.
Decide what static deflection you want i.e. crudely how firm you want the suspension. If you don't know yet then take a guess at 4" as a starting point.
From the leverage you calculated earlier, determine how much spring deflection this corresponds to.
Divide the spring force by the spring deflection to determine the spring rate.
If your damper supplier does a thorough job he will need to know the spring rate, and the leverage between the spring and the wheel, to work out what dampers you need. Or he might just pick a number out of the air based on the spring rate.
You also need to work out what dimensions you need i.e. the damper length at normal ride height, and on the droop stops, and on the bump stops.
Make sure you think about the bump and droop stops too - you need to decide which part of the suspension is going to bottom out, and also how far you are going to let it move in droop. If the permitted droop travel exceeds the static deflection then the spring may become unloaded in droop which is usually bad news.
This only gives you a very rough ballpark and you will need to vary this to get the handling characteristics you require.
Or in other words before you start down this road, buy a few books on suspension and handling and make sure you know what you're getting in to, or get somebody who knows to advise you.
Determine the weight on each wheel.
From the suspension geometry calculate the leverage between the spring and the wheel i.e. if the spring pushes with 1 unit of force how much force is applied to the wheel. From this you can determine how much force the spring will need to support at normal ride height.
Decide what static deflection you want i.e. crudely how firm you want the suspension. If you don't know yet then take a guess at 4" as a starting point.
From the leverage you calculated earlier, determine how much spring deflection this corresponds to.
Divide the spring force by the spring deflection to determine the spring rate.
If your damper supplier does a thorough job he will need to know the spring rate, and the leverage between the spring and the wheel, to work out what dampers you need. Or he might just pick a number out of the air based on the spring rate.
You also need to work out what dimensions you need i.e. the damper length at normal ride height, and on the droop stops, and on the bump stops.
Make sure you think about the bump and droop stops too - you need to decide which part of the suspension is going to bottom out, and also how far you are going to let it move in droop. If the permitted droop travel exceeds the static deflection then the spring may become unloaded in droop which is usually bad news.
This only gives you a very rough ballpark and you will need to vary this to get the handling characteristics you require.
Or in other words before you start down this road, buy a few books on suspension and handling and make sure you know what you're getting in to, or get somebody who knows to advise you.
^^^^ What GreenV8S and Locoblade said.
Only thing I would add is that if you work out the wheel rate (ie. the effective spring rate after you have taken into account suspension leverage, as described by GreenV8S), comparison with the wheel rate of other similar cars can give a pretty accurate point to start from... so, for instance, if you find a Pinto engined live axle Westfield that gives the sort of ride/handling compromise you are happy with and calculate the front and rear wheel rates, you could probably use similar figures for a Pinto engined Dutton (which will have quite similar weight distribution and overall weight)and would find that it is quite close.
As Locoblade said, though, comparing actual spring rates between different makes of cars is pretty meaningless, because the leverage ratios can differ wildly.
As Locoblade also said, unless you are willing to fork out for mega-expensive dampers, you'll get off-the-shelf items anyway. With dampers like Spax and AVO, you'll probably find that even the dampers marketed for 'Sevens' are generally on the firm side for light cars, so you'll be using the bottom end of the adustment range... start off with full soft at the read and only one or two clicks up from full soft at the front, and experiment from there.
What sort of car is it that you have in mind?
Only thing I would add is that if you work out the wheel rate (ie. the effective spring rate after you have taken into account suspension leverage, as described by GreenV8S), comparison with the wheel rate of other similar cars can give a pretty accurate point to start from... so, for instance, if you find a Pinto engined live axle Westfield that gives the sort of ride/handling compromise you are happy with and calculate the front and rear wheel rates, you could probably use similar figures for a Pinto engined Dutton (which will have quite similar weight distribution and overall weight)and would find that it is quite close.
As Locoblade said, though, comparing actual spring rates between different makes of cars is pretty meaningless, because the leverage ratios can differ wildly.
As Locoblade also said, unless you are willing to fork out for mega-expensive dampers, you'll get off-the-shelf items anyway. With dampers like Spax and AVO, you'll probably find that even the dampers marketed for 'Sevens' are generally on the firm side for light cars, so you'll be using the bottom end of the adustment range... start off with full soft at the read and only one or two clicks up from full soft at the front, and experiment from there.
What sort of car is it that you have in mind?
Hi Guys.
I'm aware of suspension leverages, the effect of inclined springs and so, so to use the correct terminology, yes I was thinking in terms of wheel rate not spring rate.
The car is a Sylva Leader. It weighs about 630kg, with a roughly sevenesque back axle with paralell links and quite a strong front weight bias apparently (which seems odd given the very central positioning of the engine). The front springs are 225lbs/in apparently with off the shelf Gaz dampers. I'm told the current setup is rather firm. I'm looking at replacing it for two reasons - firstly there are signs of fluid leaks from one of the rear (donor car) dampers, secondly the previous owner reckons the springs are too long and the car is permanently on full droop!
What units are dampers rated in? IIRC, it's the force required to produce a certain acceleration of the piston through the fluid so presumably it's be N/msec^2 or something?
This may be irrelevant, as at my level and budget, I suspect I shall work out the spring requirments than get off the shelf dampers, possibly Gaz units to match the fronts! Any back to springs.....
Once I've worked out the suspension leverage (more on that in a second!) the next step is to work out what spring rate I'd need to generate a given static deflection (say for example, you want the wheel to move 4" upwards when the car is placed, loaded on a level road)?
So, going back to the leverage, what would that be? If both wheels hit the same bump simulataneously, there is no roll on the axle, so for springs acting vertically onto the top of the axle, the change in height is effectively the same as that of the wheel. As the paralell links make the axle (and hence the pickup points) move in an arc, you'll have to calculate the change in rate due to the spring deflection, but this will be the only ratio, if the axle just moves vertically without any roll. If onme end moves more than the other, it starts to roll at which point a second ratio comes into affect - the ratio of lateral distances between the roll centre and the contact patch, versus the roll centre and the spring mounting point?
.....am I on the right track? I actually did this sort of thing (for double wishbones!) in some detail at uni, but I've realised I've forgotten ALL OF IT! Doh!
Trying to remember what the other location is on the axle - it's not a regular panhard rod I don't think and presumably that will affect it to? I seem to remember it has two links coming down to the back axle in an A shape or something?
I'm aware of suspension leverages, the effect of inclined springs and so, so to use the correct terminology, yes I was thinking in terms of wheel rate not spring rate.
The car is a Sylva Leader. It weighs about 630kg, with a roughly sevenesque back axle with paralell links and quite a strong front weight bias apparently (which seems odd given the very central positioning of the engine). The front springs are 225lbs/in apparently with off the shelf Gaz dampers. I'm told the current setup is rather firm. I'm looking at replacing it for two reasons - firstly there are signs of fluid leaks from one of the rear (donor car) dampers, secondly the previous owner reckons the springs are too long and the car is permanently on full droop!
What units are dampers rated in? IIRC, it's the force required to produce a certain acceleration of the piston through the fluid so presumably it's be N/msec^2 or something?
This may be irrelevant, as at my level and budget, I suspect I shall work out the spring requirments than get off the shelf dampers, possibly Gaz units to match the fronts! Any back to springs.....
Once I've worked out the suspension leverage (more on that in a second!) the next step is to work out what spring rate I'd need to generate a given static deflection (say for example, you want the wheel to move 4" upwards when the car is placed, loaded on a level road)?
So, going back to the leverage, what would that be? If both wheels hit the same bump simulataneously, there is no roll on the axle, so for springs acting vertically onto the top of the axle, the change in height is effectively the same as that of the wheel. As the paralell links make the axle (and hence the pickup points) move in an arc, you'll have to calculate the change in rate due to the spring deflection, but this will be the only ratio, if the axle just moves vertically without any roll. If onme end moves more than the other, it starts to roll at which point a second ratio comes into affect - the ratio of lateral distances between the roll centre and the contact patch, versus the roll centre and the spring mounting point?
.....am I on the right track? I actually did this sort of thing (for double wishbones!) in some detail at uni, but I've realised I've forgotten ALL OF IT! Doh!
Trying to remember what the other location is on the axle - it's not a regular panhard rod I don't think and presumably that will affect it to? I seem to remember it has two links coming down to the back axle in an A shape or something?
Ok, so just in bump....
For a damper which mounts vertically onto the top of the axle casing - something like this,
there would be no leverage initially - the damper is vertical and so is the force coming up fromt he wheel. However, because the axle is constrained by trailing arms, it moves on an arc, which means the damper will get tilted slightly as the wheel starts to move up, causing the effective rate to drop.
But, as far as I can see, when static there is no leverage to take into account.
For a damper which mounts vertically onto the top of the axle casing - something like this,
there would be no leverage initially - the damper is vertical and so is the force coming up fromt he wheel. However, because the axle is constrained by trailing arms, it moves on an arc, which means the damper will get tilted slightly as the wheel starts to move up, causing the effective rate to drop.
But, as far as I can see, when static there is no leverage to take into account.
The movement in an arc caused by the trailing arms is pretty negligible unless you have short arms and a large amount of travel, if it wasnt you'd get a lot of rear wheel steering effect from the axle so Id guess the fore/aft movement is going to be a few mm at most.
As its a live axle you're sorting out Id just look at similar cars with vertical dampers and find out what they're using, the big variations / uncertainties only really apply to an IRS setup. FWIW Id imagine starting off with springs around the 175lb mark wouldnt be too far off, my Locost with a live axle runs about 130lb rear springs but its a BEC so a lot lighter than yours.
If buying new dampers in the sub £100 price range, have a look at Protech. Ive not used them personally but a lot of people seem to rate them favourably for build quality and damping characteristics against Gaz / Avo etc.
As its a live axle you're sorting out Id just look at similar cars with vertical dampers and find out what they're using, the big variations / uncertainties only really apply to an IRS setup. FWIW Id imagine starting off with springs around the 175lb mark wouldnt be too far off, my Locost with a live axle runs about 130lb rear springs but its a BEC so a lot lighter than yours.
If buying new dampers in the sub £100 price range, have a look at Protech. Ive not used them personally but a lot of people seem to rate them favourably for build quality and damping characteristics against Gaz / Avo etc.
Edited by LocoBlade on Thursday 6th September 20:01
Ok, just found out, what the car is suppose to have.... cut down Chevette springs (nice!)
Although it may be tricky to get hold of a pair of Chevette rear springs, if you know the original rate and how many coils were removed I think you should be able to work out the intended rate.
[small]Here comes the science bit[/bit]
According to my old text book spring rate is basically equal to,
(98.98 x 10^8d^4)/(nD^3)
Where d is the wire diameter, D is the coil diamter and n is the number of coils.
So, presumably if I can find the original donor car spring rate and multiply this by the ratio of the number of donor coils over the number of kit car coils, I'll have the rate intended by the factory. Then match these with some off the shelf dampers recommended for something like a Westfield (the Leader is fractionally ehavier, but has a greater front weight bias)
Although it may be tricky to get hold of a pair of Chevette rear springs, if you know the original rate and how many coils were removed I think you should be able to work out the intended rate.
[small]Here comes the science bit[/bit]
According to my old text book spring rate is basically equal to,
(98.98 x 10^8d^4)/(nD^3)
Where d is the wire diameter, D is the coil diamter and n is the number of coils.
So, presumably if I can find the original donor car spring rate and multiply this by the ratio of the number of donor coils over the number of kit car coils, I'll have the rate intended by the factory. Then match these with some off the shelf dampers recommended for something like a Westfield (the Leader is fractionally ehavier, but has a greater front weight bias)
Although Jeremy Phillips (AKA Sylva) is a well respected chassis designer / engineer and has undoubtedly put a lot of thought into his later designs, Im not convinced a lot of science or optimisation went into the original "chosen" springrate or damping of your particular car considering what you've just discovered about the spring/damper's origins 
Therefore Im not sure what you'll get from the calculations so if I were you I'd not spend too much time on the maths, just buy a pair of off-the-shelf dampers and a pair of springs in the 150-200lb range and see how it feels. If its too hard / soft then just get another pair of springs, they're dirt cheap so its not a big issue.
Therefore Im not sure what you'll get from the calculations so if I were you I'd not spend too much time on the maths, just buy a pair of off-the-shelf dampers and a pair of springs in the 150-200lb range and see how it feels. If its too hard / soft then just get another pair of springs, they're dirt cheap so its not a big issue.
Edited by LocoBlade on Friday 7th September 22:29
Maybe, but even the Leaders are said to handle pretty nicely - I've spoken to a chap who reckoned his Striker, although quicker is less fun to drive than the Leader he had before. So I think whatever was on there is quite a good starting point, but I agree, there's no substitute for getting out and driving it.
What sort of rate would you expect on the front springs? Apparently it has 225 lb/in psrings on there currently (it's a normal double wishbone set up, fairly steeply inclined - no rocking arms or anything) These weren't the original set up.
I suppose the same goes for the front really, be prepared to replace those too and see which combination works best.
What sort of rate would you expect on the front springs? Apparently it has 225 lb/in psrings on there currently (it's a normal double wishbone set up, fairly steeply inclined - no rocking arms or anything) These weren't the original set up.
I suppose the same goes for the front really, be prepared to replace those too and see which combination works best.
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