02GF74
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 posted on 10/11/08 at 08:22 AM |
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another 2 spring questions
concerns coil-over shocks with height adjusment collar.
1. Am I correct in thinking that when the springs are fitted to the shock, they must be long enough to push the shock to its fully extended length?
If they are too short, once fitted to the car and it flies over a bump, the situation can arise that the exle pulls the shock fully open and the
spring is just flopping about so when the car lands back on the ground, the part of the extended shock will close very quickly before the axle starts
to rest on the spring? I assume this is not a good situation.
2. Fitting a spring that is too long for the shock means that there is preload. This preload may be so big the ride height cannot be adjusted since
the preload exceeds the weigh the car puts on the spring?
(hope I've explained this correctly)
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MikeCapon
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| posted on 10/11/08 at 08:35 AM |
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1 You're right in saying that the free length of the spring should be at least equal to the distance between the two faces between which the
spring sits. Normally the spring seat and the top collet.
The rapid closing of the shock after a wheel has left the ground is not a big problem. What is a bit more of a worry is the top collet dropping out,
for example.
On some rally cars we use a second very soft spring which sits on top of the main road spring. When the shock opens right up the second (helper)
spring keeps everything in order. Some people call these helper springs, "anti-rattle" springs.
2 There always needs to be a certain amount of preload on a spring. The actual amount depends of a variety of factors. Typically it will be a
minimum of 15 and up to 50mm for a "normal" 7 type car.
You are right in saying that too much preload will result in excessive ride height.
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procomp
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| posted on 10/11/08 at 10:54 AM |
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Hi i will just chip in on top of Mikes above post and point out that in the application of a live axle in this type of car the spring poundage being
used should be soft enough for there to be more than enough preload to not have this situation occur.
IE if say using 200 + Lb springs this situation may occur but if using a more sensible poundage of say 120Lb - 160Lb it should not occure with the
preload required to hold the car up at ride height. Unless using a very short spring for some particular reason.
Cheers Matt
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02GF74
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| posted on 10/11/08 at 11:00 AM |
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Well the reason for asking is that I need to specify the spring length for a pair of Spax height adjustable coil overs.
The data I am lacking is where on the body the screw thread.
The shock length (without spring fitted):
extended: 372 mm
compresseed: 260 mm
A too long spring will mean the preload is too high so the body weight may not be enough to compress the springs furtyer i.e. no adjustment in ride
height.
Too short then I am worried that I amy run out of thread, I think 
It is formula 27 rear shocks, live axle. Poundage I am going for is 160.
[Edited on 10/11/08 by 02GF74]
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mr henderson
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| posted on 10/11/08 at 11:32 AM |
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You might find the info you need here
http://www.spax.co.uk/selector_html.php
Bear in mind that if the shocks are mounted at an amgle to the up/down movement of the axle, you will need to allow for that.
Once you have the maximum open length (spring seat to srping seat) and the poundage (supplied by procomp above) all you will need to calculate is the
open length of the spring required to provide the ability to adjust to get the correct ride height
Mike Capon had a good spreadsheet for doing this, I will see if I can find a link
Edited to add, the link is in a worx post halfway down this thread. I don't know how to do a direct link
http://www.locostbuilders.co.uk/viewthread.php?tid=86161
John
[Edited on 10/11/08 by mr henderson]
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MikeCapon
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| posted on 10/11/08 at 01:20 PM |
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Hi John,
I've improved the spreadsheet you mentioned. This is the latest version
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