suspension theory thoughts, questions, etc - grooved shocks

[Grant Lenahan]

Jim and all:

It actually does lower the car - whcih has clear benefits (if a few 
issues as well). I'll try to treat the two issues separately:

1.) What are the benefits of lowering, if any?
2.) Does this really lower - or just pre-load the spring?

First, #2 - since if it proves the system bogus, #1 is irrelevant.

It really lowers the car.  The problem you are experiencing is that you 
need to compress the spring from the longest point the shock piston 
will expand, to whatever setting you have the lower spring perch at.  
Since the piston only goes out *so far*, and is not adjustable, you 
have to work harder to compress the spring at higher settings than at 
lower.

However, when the car weight is on the spring, it will compress a 
consistent, given amount based on the weight of the car and the spring 
rate in lab/in. So if any particular corner of your car weighs 1000 
lbs, and the spring rate is 333.33 lbs/in, it will compress 3 inches.  
The car will sit higher or lower depending on where the LOWER end sits. 
That's the knurled adjustment you see.  The problem you have is that 
the UNLOADED spring, and the never-varying shock piston length, menas 
that the seat-to-piston stop distance changes.  Since the spring is 
unloaded, it expands to fit the space alloted. And since it might 
really be 333.33 lbs/in - or more - its a PITA to compress.  Ergo La 
Spring Compressor, to mix several languages, badly.

Now, does lowering help?  Assuming you can get suspension geometry and 
alignment in spec, yes, quite a bit.  It lowers the center of mass and 
the roll center of the vehicle, causing less weight transfer. When I've 
done the same, its made a big difference -- but IFF (if and only if) 
you get the F/R rates right; the alignment corrected, etc.  Note also 
that by lowering the spring perch, the spring rate and suspension 
travel is unaffected. Pretty cool.  Curbs, alas remain their previous 
height.

Grant

On Jan 8, 2007, at 5:29 PM, thejimrose wrote:

> hey all - i've got a question and some thoughts and btdt i'd like to 
> throw
> out for collective rumination, enjoyment, infotainment and maybe
> edification. i installed a suspension kit on my a4 from neuspeed [got 
> it
> used, cheap] that includes bilstein sport shocks and neuspeed sport 
> springs
> [a bit lower and stiffer]. the shock bodies are grooved and include a 
> big C
> clip which the lower spring perch sits on, to allow the ride height to 
> be
> 'adjusted'. it's nothing new, but i'm suspicious of its merit.
>
> what i don't understand is how raising / lowering the lower spring 
> perch on
> the shock body will adjust ride height? all it seems to do is preload
> [compress] the spring, since all it does is shorten the distance from 
> upper
> to lower perch. it doesn't change suspension geometry at all. i do
> understand that, as a byproduct of this preload, that the car
> could/would/should sit a little higher since the spring is pushing 
> harder
> against the body, which is more likely to give than the road.. =)
>
> the question comes because, in installing them it was almost 
> impossible to
> get the springs compressed enough to squeeze into the 'highest' ride 
> height
> setting - aka the uppermost lower spring perch setting aka the shortest
> distance between the perches aka the most preloaded spring. set this 
> way
> there is virtually no room between several coils - they touch, so there
> isn't much room to suspend if the spring coils are touching at that 
> point
> it's a sleeve not a spring =]. the NS springs have far more windings 
> than
> the stock sport springs do.
>
> as a review, i'm not thrilled with the setup so far. it does handle 
> like a
> gokart [for good and bad], its a load of fun but very bouncy and the 
> car
> wants to launch itself when the suspension unloads over bumps. there 
> is NO
> dive or squat or sway. it's a crapload of fun in the turns but is 
> unsettled
> easily. when i installed it all i planned to set the perches at full 
> high,
> but i ended up installing them full low, partially because i figured it
> better to have the least preloaded spring, which i thought would ride 
> better
> vs a higher riding, but more preloaded spring. i was also forced to 
> install
> them full low because the crap compressor i have wouldn't allow me to
> physically compress the springs enough to squeeze them between the 
> perches
> [at the highest setting]. ideally i would want slightly higher ride 
> height,
> but i'm afraid of preloading the spring more and losing what little 
> ride
> compliance there is. [i have a feeling that i just asked the 
> suspension holy
> grail question and the only answer is $$$$$$$$$ =]
>
> so i guess my questions are; is my understanding of the adjustability 
> and
> function accurate? i'd like to validate my suspicions before i make 
> more
> changes. what i'm thinking is to install my stock sport springs on the
> bilsteins but i'm worried it would go the other way [overdamped - soft 
> but
> harsh]. ive heard this setup works well on non-grooved shocks.
>
> did any of that make sense? =)
> thanks all!
> jim
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