Ride Height

[chris.richard]

Spent the afternoon changing my front dampers to nice Gaz rose-jointed ones. Bugger of a job single handed - propping spanners up with bits of wood, mole grips etc. Oh for an assistant! :rolleyes:
Two thoughts occupied me between swearing. What sort of shock, if any, will fit onto the lower mounting hole on the Hawk Track control arm? The body of the shocker won't fit into the U-channel.
Secondly, what actually determines ride height? Not chassis or TCA mounting holes. It must be the overall length of the coilover unit, which is pushed to its maximum by the spring. So is it damper maximum length. The length of the spring won't affect that, or does compressing the spring affect things? The softer the spring, the more the unit is compressed by the sprung weight, so lowering the body. I'm going round in circles, someone put me out of my misery! :(

[Martin K]

A number of things affect the ride hight. The uncompressed length of the spring, the spring rate and the spring platform adjustment (the distance between the upper an lower spring seats). If the mounting point of the lower end of the shock can be moved to the lower position, this too would affect the ride height.

Certainly the inner TCA mounting point does not affect ride height - but it does affect the the front suspension geometry - particularly the roll centre -so be careful! The overall length of the shock absorber won't affect the ride height unless it is fully closed - or on the bump stops - at normal ride height/COLOR]].

[COLOR=Red]The fully open and fully closed length of the shock absorber should be chosen so as not to interfere with the suspension travel throughout it's intended design range of travel. If it does limit travel in any way you can end up with a very dangerous front end!

Explanations:

Lets say your car weighs 1000 kilos and the weight is evenly distributed between each wheel. i.e. a 50/50 fron/rear distribution and 50/50 left/right distribution.

Each 'corner' is therefore bearing one quarter of the weight i.e. 250 kilos - about 550 lbs.

At the rear of the car the rate of the spring is the same as the rate at the wheel, because of the Chapman strut. Therefore a 550 lb spring would compress 1 inch, so the ride height would be reduced by one inch from full droop if the weight of the car is put onto the wheels.

If the car was too high, shortening the spring (uncompressed) by one inch but maintaining the rate (550lb) the compression would be the same (1 inch) but the ride height would be reduced by two inches (because the spring is on inch shorter. If you kept the original spring, but adjusted DOWN the lower spring seat, the car would be lowered by the amount you lower the spring seat. i.e. lower the spring seat one inch, the ride height will decrease one inch.

At the front the problem is different for two reasons. 1) because the mounting point of the coilover is not at the same postion as the lower ball mount of the upright, but somewhat inboard, a 'lever arm ratio' is involved resulting in the wheel rate being less than the spring rate and 2) because the damper is inclined away from the vertical, the effective spring rate itself is also reduced.

However, ignoring actual spring rates and dealing with just the springs themselves, spring seat adjustment and the position of the the lower shock absorber mounting, the same rules apply but not in the same, linear way as at the rear.

1) Reducing the rate of the spring will reduce ride height (brecause the spring will compress more)

2) Shortening the free length of the spring will reduce ride height.

3) Moving the lower spring seat down will reduce ride height.

4) Moving the lower shock absorber mounting to the lower position on the wishbone mount (and thus effectively lowering the lower spring pan position)will reduce ride height.

Hope that helps!

Martin

[Sando]

Hi Chris

Here's my simple tuppence worth. Both my old Spax and the new Gaz units (Bushed mounting) will go in to the bottom hole in the arm. It is tight and I must admit I did have to do a little persusion with a big leaver just the once to open it out a little. :eek:
A lot will also depend on how much caster you have put on and the for / aft angle of the shock. The Spax were a pain because the adjuster was on the side and fouled the botton arm too (but they were slightly thinner dampers and crap at damping!) There have also been revisions to the bottom arms by Hawk along the way so it may also depend which type you have. The Spring length and adjusted position along with the bottom mounting all have an effect on ride height, but this has to all be in relating to the open and closed lengths of the unit in question. the Spring length is usually dictated by where and how much ajustment you have on the shock body for the height you want to run at. Getting the shock open and closed length right is crittcal if you are going to have any damping left when you wind the spring down to lower the car. I've gone about as low as I dare with those GAZ coilovers we bought with only about 2" of Bump damping, but then I run mine fairly stiff (but not compared to David May!!!!) at the front.
There is a little sag with the weight of the car on them but then again there's not much weight that end of the car!
I suppose if you unit was quite short or the spring is quite soft then there could be some preload on the spring, but I found that with the stiffer springs this just lifted the front of the car. I ended up with shorter springs to get the height I wanted. They get a little loose on full droop and if I did ever get airbourne they are wired on to the top collars so the stay in place for the crash landing! :D Standard Hawk spring was 325lbs I think and about 9 inch long. Daft thing is I fitted those GAZ shocks without measuring the open and closed length. I just tried them for size and ended up finishing the job off there and then because they were ok. (well perhaps could have done with being 1" shorter overall but I was desperate to get rid of the Spaxies.
Blimey I'm rambling tonight...must be the Whiskey!
enough!
Rob :cool:

[SUSIT]

must be the Whiskey!

Rob,
your taking this honary Scot a bit on the serious side

[chris.richard]

So winding the lower spring pan up will raise the car as it preloads and shortens the springs? Hmm, counter intuitive. Must be the beer!

2) Shortening the free length of the spring will reduce ride height.

3) Moving the lower spring seat down will reduce ride height.
These seem to contradict each other, or do you mean shortening the uncompressed length will lower ride height?

I'm always wary of modifying parts that were designed by people that know what they're doing more than me! But it's OK to splay the TCA sides a bit?

[Martin K]

Uncompressed length of the spring=free length of the spring

[Martin K]

"So winding the lower spring pan up will raise the car as it preloads and shortens the springs?"

Yes, it will raise the car

No, it won't shorten the springs, umless you have no (zero) re-bound travel in the shocks at ride height.

[Steve Strain]

On this Ride Height theme, has anyone measured the ground clearance of their Transformer/Hawk front compared to rear?
Having had the car apart for a while doing the Alfa conversion I was thinking about suspension set up etc prior to getting a wheel alignment.
I will shortly have the use of a set of electronic corner weights to fine tune the platform heights and thought I would do a bit of preliminary tinkering and decide on an ideal ride height.

With the spring platforms set at approx half way of their travel I found that the front of the car was approx 15mm higher than the rear.
Okay I guessed the Alfa lump is heavier than the beta and so thought about adjusting it back to level.

To level the car front to rear and side to side I lowered the front platforms as far as I could whilst retaining tension on the spring (3-4 threads left). At the rear I did the reverse by raising the platforms significantly. As I am using 250mm length springs at the rear I now find the respective platforms in the upper third of their adjustment range.
The result now is +/- a 150mm ride height at all 4 corners.

Does everyone else have the same?
It seems to me that the front cannot be lowered any more than this and I may need to get longer rear springs.
The upside is that I have lots of rear suspension travel but may have compromised the front shock travel slightly.

Any pearls of wisdom out there?

Thanks

[John]

Chris,
can I come round and watch while you try to shorten a compressed spring?
As I recall Arthur went to some length previously to recall his experiences of ride height experiments and their effect on the car. Might be worth a quick search.

[chris.richard]

Steve, did you have ballast in the car to simulate driver+/- passenger? Their weight compresses the spring and lowers the ride height. I think their weight is split 50:50 between front & rear.
Have you put in stronger rear springs since fitting the Alfa? It's heavier therefor compresses the springs more at static.
Winding up a spring pan and compressing the spring increases the spring rate too. A 100lb spring with 1" of static compression becomes a 200lb spring, so there is an effect on wheel frequencies by winding up the ride height by this method. This bit is wrong - see below :o
Hope this helps! ;)

[Martin K]

Winding up a spring pan and compressing the spring increases the spring rate too. A 100lb spring with 1" of static compression becomes a 200lb spring, so there is an effect on wheel frequencies by winding up the ride height by this method.
Hope this helps! ;)

Not strictly true: A 100 lb/in spring with 1" of preload still has the same frequency, but it has some 'potential' or stored energy in it.

[chris.richard]

So it still acts as 100lb spring dynamically, but it won't compress it's static length further unless >100lbs is loaded on it? Is that right? :o

[catswhiskers]

This is using all the power of my brain....DOH

[Martin K]

So it still acts as 100lb spring dynamically, but it won't compress it's static length further unless >100lbs is loaded on it? Is that right? :o

That is exactly right.

I thought that maybe having preload on a spring might require an increase in rebound damping, but it seems from what I've read that it doesn't.

[Chris J]

This is using all the power of my brain....DOH

It's not just me then Mick? We have to keep the faith though, it's important.

[rutthenut]

I don't think that raising the spring platform will necessarily compress the length of the spring - it may effectively 'move the shock body down' along the damper rod. That would change ride height without producing any preload.

To get preload, doesn't the damper have to be at maximum extension, which isn't good?

[chris.richard]

Oh no, I'm getting confused again. Yes, I think you're right John