[A4] suspension theory thoughts, questions, etc - grooved shocks

[Grant Lenahan]

Jim,

See below.

On Jan 10, 2007, at 4:07 PM, thejimrose wrote:

> wow guys thanks for all the responses. i've read and reread several 
> times to pick out next steps from here.. still crunching on it all.
>
> so a few clarifying points - yes i understand that my grooved struts 
> have no real preload feature [unlike the lovely shock that resides on 
> the back of my triumph... why aren't car shocks made so nicely? =]. 
> and i also understand /agree that changing the lower spring perch 
> height in relationship to the upper perch [which is fixed, mounted the 
> car, and only as far away as the maximum shock piston stroke allows it 
> to be.

Yes, btu more importantly, except under full extension, it is, or 
should be, in the middle of its travel, not the top, when at rest.
>
> i guess what i was wondering is if + how, in theory and in practise, 
> changing the maximum overall spring distance effetcs

Define what you mean by spring distance.

> ride, performance, etc. it seems to be a bit of a hack, or an 
> ancilliary effect, that if you put the spring under more compression 
> [hence my thought that it was simply preloaded] it will have a raising 
> effect on the car, since it's pushing harder.

I'm not getting it.  If you lower the perch of a stock spring, it 
lowers the car, but does not affect travel nor does it affect spring 
rate.

If you put in a stiffer spring, it may or may not be shorter.  Most 
are, btu need not be.

If you put in a shorter spring, yes, it needs to be stiffer to suspend 
the car without bottoming (since bottoming is a shorter travel in that 
case).

NONE of this has anything to do with pre-loading.

>  
> what i forgot about is sag [in motorcycle speak] since i spent to much 
> time looking at it unloaded. the car can not sit higher than the shock 
> will physically allow. however since the spring is under more 
> compression with the perches set higher, the car will sag less when it 
> is under it's own weight. thus raising the car. i'm probably using all 
> the wrong terms, im as far from an engineer as you can get, but i 
> *think* i am getting how it all works.

Well, maybe its all terminology.  The springs you have will compress a 
very specific amount with the weight of your car on them.  They will 
compress (sag) no more nor less regardless of where you set the lower 
perch.  No difference.

But if the bottom of the spring is lower (lower perch), so will the top 
of the spring, by the same amount.

Since the car is resting on the top of the spring, the car is lowered 
by exactly that amount as well.  Its really that simple. If you stand 
on the bottom rung of a ladder, your head may come to the top. If you 
stand on th top rung of a ladder, your head's above it.  Really, that 
easy.

> in looking at 'real' "coilover" designs [i too have always thought it 
> funny that they're called coilovers when pretty much every suspension 
> i've come across is a shock inside a spring] they do all use the same 
> principal - an adjustable lower perch. with the exception of the 
> stasis motorsport coilovers, these look like they have an adjustable 
> upper perch as well. and they outta at 4grand.

I have never seen an adjustable upper perch. What you may have seen is 
a camber plate, whcih allows you to adjust CAMBER, but not ride height. 
  Its a slotted plate you attach the strut top to.

>  
> so it seems that it comes down to proper spring spec, legnth etc. i 
> need to do some research + checking. i will also take taka's advice 
> and look into having the shocks valved to match the springs. the car 
> isn't too bad, but i do get the feeling that it could be a little more 
> controlled.

Yes, spring length and stiffness are your key. Taka's also absolutely 
right that the shocks need to be matched to the springs.  They are 
nothing but oscillation dampers after all.  Too much makes the ride too 
harsh ( and can cause bouncing over bumps). Too little and you have no 
real wheel control.  Shocks are not another device to add stiffness to 
a car - although most people use them this way.
>  
> ill reprot back when i have some more info. thanks again all for the 
> help and input!

Hoep you sort it out.

Grant
>  
> jim
>
>