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

[E. Roy Wendell IV]

On Jan 11, 2007, at 8:47 PM, Grant Lenahan wrote:

>>  With the Tien's that Taka mentioned, by varying the distance BETWEEN
>> the
>>  adjustable upper and lower perches, you can increase the preload
>> (shorten
>>  the distance btw perches) or decrease the preload (extend the
>> perches), then
>>  ride height is dictated by where along the shock body the top perch
>> is positioned
>
> So I'm really perplexed. What do you adjust the upper spring perch on?
> What is it attached to that it can move on?
> If its not the LOWER part of the strut, with some very strong
> contraption rising up, how does it pre-load? Push on air?
>
> If no, then it pushes against the car. But this is silly, the car is
> already pushing down on e spring with a force = MA where A=G or F=MG,
> the basic law of gravity!
>
> My head is spinning :-)
>
> Grant

I'm with Grant on this one. Unless you are running around with the  
car at full ride height (shocks fully extended) there's no way you  
can preload the spring. That would mean that there is no droop travel  
which just isn't done for a great many good reasons the least of  
which is that it would result a wheel leaving the ground whenever the  
opposite corner compresses. If you move the spring perch, the car  
body moves by the same amount because the compressed length of the  
spring is still dependent upon the spring rate and weight of the car  
which hasn't changed. What has changed is the position of the bottom  
of the spring relative to shock body. Therefore the distance between  
the top of the spring and the shock body has moved by the same amount.

The only shocks I've seen with upper adjustable perches are the  
inverted type where the shock body is at the top and the rod at the  
bottom.

The struts with two adjusters do exist but they're not what you  
think. The second adjuster locates the upright clamp mount relative  
to the shock body. This allows for changes in ride height without  
changing the position of the shock in it's stroke. The upright only  
moves in relation to the shock body. This gets around the problem of  
loosing either bump or rebound (droop) travel that occurs when  
adjusting ride height by way of moving the spring perch. Moving the  
spring perch extends or compresses the shock while moving the upright  
relative to the shock body does not. This is only important if you  
are doing radical changes in ride height or are dealing with a  
suspension that has very limited travel to begin with. It's also  
incidentally why the Bilstein sport struts are shorter than the  
normal ones. They are meant to be used with shorter springs so the  
shaft must be shorter to allow for adequate compression travel before  
the strut bottoms out.

Roy Wendell
erwendell at mac.com
Too many type 44 tq
A pair of MR2s