I Didn't Know Spring "preload" Affected Ride Height

[KRISKARRERA 2 1 Cars]

A while back, noticing that my dad's old GTI6 sat lower than my 405 I decided I'd measure the distance between top and bottom coils on both cars. (Yes with springs still on the cars). GTI6 was a couple of centimetres lower. "Maybe that's the extra weight of the 306 causing it to be lower", I thought. But no, because I put my 14 stone on the front of both cars and they dropped by bugger all. So I concluded GTI6 springs must be shorter. I figured being made by Pug for a heavy car they'd make good quality lowering springs. So I got hold of some standard GTI6 springs and noticed they measured about the same as a 405 Mi spring. Hmm, so why does it sit lower on the 306...

Just couldn't figure it out. Then last night I was chatting to Rippthrough and he mentioned "preload".

 

When you mount a spring into a strut you have to compress the spring slightly in order to fit all the top mount gubbins and get the big nut on. I think that's the "preload" right?

 

I've mistakenly always thought that ride height was all down to the spec of the spring and the weight of the vehicle on it. If you have 2 identical springs and mount one into one strut and the other into a strut with longer piston travel and then put both on same car the smaller piston'd strut will give a lower ride height because it's already holding the spring lower even before it's got the car's weight on it.

[Henry Yorke 269 3 Cars]

Yep. that is why proper uprated shocks for lowering springs have a shortened insert so the pre load is correct. Otherwise you put a lowering spring on a standard shock and you instantly lose the preload. Put the spring on a shorter insert and you will find you need to use the spring compressers more. Ever wondered why when putting lowering springs on a car, it is always easier to put them on than take them off with the number of turns on the spring compressers?

 

The presload also keeps the springs seated properly. Back in the early days, spring chopping was common, but often the spring fell off the seat when the car was jacked up (or jumped a humpback bridge!).

 

The interesting bit with springs is how they are made; not just the coil spacing for progressive handling, but the way they are made. Eibach are hot wound, which means the metal is the same density all the way through the coil. Cold wound (cheap) springs will compress on the inner edge and stretch on the outer, so giving unequal compression and spring properties. This is why Eibach are more expensive than PI etc. They are worlds apart for ride quality and handling.

 

Actually was that bit actually interesting or just sad?! :$

[Rippthrough 98]

The interesting bit with springs is how they are made; not just the coil spacing for progressive handling, but the way they are made. Eibach are hot wound, which means the metal is the same density all the way through the coil. Cold wound (cheap) springs will compress on the inner edge and stretch on the outer, so giving unequal compression and spring properties. This is why Eibach are more expensive than PI etc. They are worlds apart for ride quality and handling.

 

Actually was that bit actually interesting or just sad?! :$

 

There's a few do it now though, notabaly Apex and Koni.

The other interesting bit though, is that Eibach tend to use very high quality alloy spring steels, which allow more twist at the same strength level compared to the more common springs steels, or vice versa.

Which means they get away with thinner wire and less coils to get the same rate as a lesser spring, so they can have far more travel. It's the main reason why Eibach springs are so expensive, the material alone is around 3 times the price of the more common grades.

There's a place in Sheffield which make springs for the trade, normally it costs us around £15-20 per spring, depending on length, when I specced some up in similar material to the Eibachs, in order to get a little extra travel on an existing suspension setup, that jumped to £45 per spring, purely due to material cost, the labour is basically the same.

 

So, if you see a cheap Eibach spring set, it's probably a copy!

Edited April 27, 2008 by Rippthrough

[welshpug 1,626]

So with this in mind, I could fit uprated shorter dampers to a car with the original springs, and have a decent upgrade over standard dampers?

[Rippthrough 98]

So with this in mind, I could fit uprated shorter dampers to a car with the original springs, and have a decent upgrade over standard dampers?

 

Not exactly, you'd end up with far more preload so the dampers would be topping out all the time, you wouldn't have as much suspension travel as with the matched springs either, and the spring rate would still be same as standard, so unless you only got a mild upgrade it would be overdamped to boot.

[brianthemagical 1]

If you have 2 identical springs and mount one into one strut and the other into a strut with longer piston travel and then put both on same car the smaller piston'd strut will give a lower ride height because it's already holding the spring lower even before it's got the car's weight on it.

 

That depends on strut specs. If the short strut was only 2" long then yes, but if the difference is something in the region of 2" then no, as as you jack the car up the wheels drop. This is known as sag, at least in the MTB world, and is there to maintain tyre contact if the car 'lifts' of raises slightly in relation to the road. So the spring is the only realistic thing that can have an effect.

[jonnie205 1]

a length of damper piston rod will have no affect on rideheight. Once the cars weight is on the spring the damper will be having no affect on spring. The shortened damper is just used so the damper is working within its range with a lowered spring and not bottoming out internally or comin dislocted on droop

Edited April 30, 2008 by jonnie205

[Rippthrough 98]

But the spring saddle height will, which is what may be different between the two cars.

[jonnie205 1]

yes spring seat height will make a difference

[richsmells 2]

Hmm... This has got me thinking. I have SBC "roadspeed" springs with standard (newish) genuine pug dampers. The ride quality is horrific in my opinion. Really crashy, noisy and often bumpsteers.

 

I assume the roadspeed springs are "hot wound" as Henry explained.

 

I'm considering getting some decent matched dampers. Will this improve the ride? The car is only used on the road.

 

Which dampers would you suggest?

[welshpug 1,626]

Roadspeed springs were meant to be used with Roadspeed Dampers, which were re-valved Bilsteins (B6 IIRC) to SBC spec, not cheap, but a fantastic combination in a fast road 205.

 

(the car I drove was quieter but firmer than mine on Ventura Springs and B4 Bilsteins, but in no way bouncy or crashy)

[weejimmy 1]

ok then

 

while we are on the subject dose anyone know why on some springs the coils alredy touch when the weight of the car is on it. ie at a standstill?

why there isint a gap between every coil?

[brianthemagical 1]

ok then

 

while we are on the subject dose anyone know why on some springs the coils alredy touch when the weight of the car is on it. ie at a standstill?

why there isint a gap between every coil?

 

They're shonky or progressively wound for one reason or another, maybe just a low spring rate to keep the spring seated during droop.

Edited May 1, 2008 by brianthemagical

[Rippthrough 98]

Sounds like you've got too much compression damping for a spring that hard.

[KRISKARRERA 2 1 Cars]

The Venturas that were on my 405 were bloody awful. Half the sodding coils would be touching with just the weight of the car on them and the car would "float" as you went over hills. So then I stick standard springs on and it handles great! Although it sits a bit higher than I'd like now.

[weejimmy 1]

i wouldent think the seting of the shocks would be relevent at a stand still.

its not my car i mean ive just seen it on loads of cars. its a bit strange

[Rippthrough 98]

i wouldent think the seting of the shocks would be relevent at a stand still.

its not my car i mean ive just seen it on loads of cars. its a bit strange

 

 

I meant the bumpy ride

[KRISKARRERA 2 1 Cars]

If I spring is held more compressed by a particular strut than it is / was by another strut it WILL affect the ride height.

 

If I hold a spring down a bit and have my 70 stone ex sit on it, it'll sit lower than if she was just to sit on it without me pulling it closer together.

[brianthemagical 1]

Only if you still have the extra preload acting on the spring with the car weight. The preload is removed as soon as enough load is exerted on the spring to displace the spring ends off the origonal fixed points of the unloaded spring seats.

The only force applying the preload is the reaction force or the end stop of the damper, therefore as soon as a compressive force is applied to the springs the preload force is diminished, therefore it has no affect on the ride hight.

Edited May 2, 2008 by brianthemagical

[Rippthrough 98]

Only if you still have the extra preload acting on the spring with the car weight. The preload is removed as soon as enough load is exerted on the spring to displace the spring ends off the origonal fixed points of the unloaded spring seats.

The only force applying the preload is the reaction force or the end stop of the damper, therefore as soon as a compressive force is applied to the springs the preload force is diminished, therefore it has no affect on the ride hight.

 

Only if you are basing that on the damper rod being a different length, with no differences to the body/lower spring seat, the same length damper rod but with more preload will result in a higher ride height.

[jonnie205 1]

exactly, preload is only when the cas is jacked up or strut on bench, its the end stop in the damper that is giving the preload, as soon as the cars weight is on the strut there is no preload given by damper

[KRISKARRERA 2 1 Cars]

Yeah I spose I see what you mean, hmmm.

[Rippthrough 98]

But they're not shortened piston dampers, they're two different casings...

If they were shortened dampers, the springs would be wrong anyway

[brianthemagical 1]

Rippthrough, i'm slightly confused. Are you saying that on the same damper/strut and spring, the preload will affect ride height? If so any chance you could explain as i can't work it out. Thanks.

[Rippthrough 98]

Rippthrough, i'm slightly confused. Are you saying that on the same damper/strut and spring, the preload will affect ride height? If so any chance you could explain as i can't work it out. Thanks.

 

The two struts on the 2 cars are different, which means the lower spring pan has a good chance of being in a different place, which means the spring from the 306 on the 405 might have differing preload even with similar piston travel, hence the differing ride height is due to the differing preload on the spring from the lower spring pan.

 

Personally I reckon it's from the weight distribution but me and kris have done that one