pewe
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| posted on 9/11/09 at 07:00 PM |
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Coilover Springs - question
The F27 came with Spax c/overs on the rear with single bump/rebound adjuster.
Springs are Red, 10.5" long and marked 150 (lbs?).
When the guy set it up for me he jacked up the rear spring adjusters to fully up saying it was better for the ride height at the rear. This of course
reduced the spring travel so that the ride on the road is choppy at the back (much improved by fitting polybushes all round but still quite
unforgiving).
On track it loads up to about 80% and then switches suddenly to oversteer (could have something to do with the fact that the right shock absorber may
have been leaking slightly though).
Any suggestions as to fitting firmer springs please?
If so what's recommended for fast road/ track-days?
Front end seems to be OK on its Gaz c/overs.
Thanks, Pewe 
[Edited on 9/11/09 by pewe]
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MakeEverything
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| posted on 9/11/09 at 07:13 PM |
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If you have oversteer, then either the springs are too pre-loaded or youre running out of travel - As you said.
Wind them down a bit, and see how that goes. Remember to measure the length each time, so that they are equal both sides.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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britishtrident
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| posted on 9/11/09 at 08:32 PM |
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The sudden switch to oversteer suggest the rear end is running out of suspension movement perhaps the springs are becoming coil bound ---- try
reducing the preload.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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pewe
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| posted on 9/11/09 at 09:15 PM |
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Thanks guys.
Will have to see what's to do about the one that's leaking as well.
Cheers, Pewe
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mr henderson
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| posted on 10/11/09 at 09:54 AM |
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The first thing you need to do is to get the ride height correct. You have to decide for yourself what is suitable, based on what sort of roads you
drive over and the need to negotiate obstructions, speed bumps etc, you will also want to get your suspension into its correct working range.
Now, if the springs currently fitted don’t provide this ride height unless the platforms are wound up too high then the springs are wrong. Either the
wrong rate, or the wrong length, but most likely both.
Seeing as you know the current rate, then you’ve got a good start. If you are happy with the rate, and the current ride height, then all you need are
some longer springs.
Remember that the amount the springs are compressed by at any given ride height is a function of the spring rate and the unfitted length, and the
weight of the car at that corner. These things are interdependent., but the ride height is your starting point. Set the car at the correct ride height
then choose the spring rate AND LENGTH that give you the desired stiffness whilst maintaining the ride height with a relatively low setting on the
spring platform. Of course, there is always the possibility that the shock absorber length is incorrect, or that there is some other problem with the
suspension design or execution………………
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dnmalc
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| posted on 11/11/09 at 01:08 PM |
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Mr Henderson is correct if you are running out of stroke then this is caused by the total amount of load being applied. Reducing the pre load will
have no effect on the amount of compression as while preload determins when the suspension starts to compress the spring. The compression to counter
the loading is determined by the spring rate and amount of compression from the uncompressed length.
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pewe
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| posted on 11/11/09 at 04:22 PM |
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Understood but as you may gather I'm a bit out of my depth on this one.
In fact just had an (almost - my ignorance not his) informative conversation with one of the UK's leading coil-over suppliers. He suggested
measuring Bump/compression 4" and Re-bound 2".
Can see that a zip-tie on the damper rod will give me the former but the later?
How is that measured?
Probably switching from 10" 150lbs springs to 12"160lbs - does that sound about right for road/trackday use?
TIA.
Cheers, Pewe
[Edited on 11/11/09 by pewe]
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mr henderson
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| posted on 11/11/09 at 04:41 PM |
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quote:
Probably switching from 10" 150lbs springs to 12"160lbs - does that sound about right for road/trackday use?
TIA.
Cheers, Pewe
First question, are you happy with the ride height as is?
Assuming yes, how much are the current springs compressing, and how far up the threads have the spring seats been wound?
Unable to make spring suggestions without this info, at least
Also, have you tried taking off the spring off the damper and putting the damper back in position, and tried moving the suspension through its full
travel, do you have satisfactory movement above and below the current (presumed satisfactory) ride height ?
[Edited on 11/11/09 by mr henderson]
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pewe
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| posted on 11/11/09 at 05:22 PM |
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Mr. H, checking ride height and susp. movement without springs tonight.
The raceshop which set it up for me originally wound the spring adjusters all the way up to achieve a reasonable front to back height difference as
it had a tendency to feel like it was going into a tank-slapper when higher speeds were reached.
I'll probably take some time off tomorrow after putting a zip tie on the piston rods to see what's going on there.
BTW coil-over expert also suggested maybe one of the front wheels is lifting under heavy cornering giving the over-steer but think I'll wait
until I've sorted out the back before addressing that issue (altho. I appreciate it could be related).
Cheers, Pewe
PS checked the amount of drop/rise on the back axle - it's about 50mm!!
No wonder the fricking things bottoming out. Back to the drawing board I guess. There is a mod which has been mentioned by other live axle F27 owners
so think I'll have to track that down.
In the meantime thanks for your help guys - it never ceases to amaze me how much knowledge there is on here and how willing people are to share
it.
Pewe.
[Edited on 11/11/09 by pewe]
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