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Author: Subject: Rear suspension quandry - what to do? (UPDATE)
David Jenkins

posted on 20/8/06 at 03:12 PM Reply With Quote
Rear suspension quandry - what to do? (UPDATE)

When I built my car I had 180lb springs on the rear shocks. This gave a nice ride, but I was hitting the stops too often on bumpy roads. Eventually I changed the springs to 200lb, which stopped me hitting the bump stops - but the ride was too harsh and I never got a confident feeling about the rear end, as it always felt like it was fighting the road and skipping around. Today I put the original 180lb springs back on, and once again the ride is far more pleasant and feels 'well placed' on the deck... but I have the risk of running out of travel again.

Now that I have more experience with the car I can analyse things a bit more logically (perhaps!).

With the current setup I have almost no preload on the shocks, so the damper rod is only 1.5" - 2" out of the top of the cylinder when the chassis is sitting at the maximum ride height - not a lot of movement left. With the 200lb springs I didn't use that travel (but bounced around everywhere) but with the 180lb I will probably hit the stops fairly often.

I can see a couple of solutions:

1. Get some shorter shocks, so that I start off with a respectable bit of damper rod sticking out of the cylinder.

2. Move the bottom mount to the back of the axle suspension mount so that I can extend the current shocks, put some pre-load in, and get a decent amount of spare movement.

Option 1 is by far the easiest method, but would cost a lot of cash.

Option 2 would be a real PITA to implement, but would cost me about 1 hour fabrication time for new brackets, then a day or so to get the axle out, modify it and replace it - but I'm not sure about whether I'll have space to move the shock mounts, and also the new bottom mount would mean that the shocks would be at about 10 degrees from vertical (not sure if that's a mechanically sound option). I'm not sure I want to go and hack the back end around, as I recall that it was a real pain to get the axle in there in the first place - and that was without the bodywork and propshaft to contend with!

So - the question is whether to go for option 1 or 2 - or have I missed some other options?

cheers,
David

[Edited on 26/8/06 by David Jenkins]






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andybod

posted on 20/8/06 at 03:17 PM Reply With Quote
i'de go for option one yes more financial outlay but quicker repair and no messing with the axle or shock angles you could sell your old springs and shocks to get some money back
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nitram38

posted on 20/8/06 at 03:27 PM Reply With Quote
Springs affect roll in corners and ride height. If you are happy with both, then just change the shock to adjustable ones and dial in your prefered settings.
It is the damping rate of your shocks that is causing the bump stop problem and not your springs.
Your shocks are set too soft.



[Edited on 20/8/2006 by nitram38]

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planetester

posted on 20/8/06 at 03:29 PM Reply With Quote
are you saying that you dampers are fully extended at normal ride hieght, if not What happens if you put some more preload in & raise the ground clearance a bit, this will give more rod protrusion, this will keep the ride the same.
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Surrey Dave

posted on 20/8/06 at 03:52 PM Reply With Quote
Too Short

I think the book rear shocks are too short, they should actually be longer than the front as they are upright not working at an angle, Westfield rear shocks are longer.

I too fitted lighter springs at 1st 150 lbs , the ride was much more compliant with the road surface , but would hit the stops and smash my piles

So as a quick fix I upped the rear springs to 200lbs which controls the movement but is quite hard / nervous.

I keep the damperes quite low 4 or 5 clicks and run the rear tyres at 18 psi to make it reasonable.

But ideally I would like some longer shocks and softer springs , the axle could follow they road better and it would be more comfortable.


I believe Paulf ? has recently fitted longer rear shocks/springs and finds it much improved.

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TangoMan

posted on 20/8/06 at 04:34 PM Reply With Quote
I have just gone the other way.
I had 180 springs fitted and these were still too stiff. The back end would break away too easily and although it looked good, it was not the fastest way around the circuit. I had the shcoks wound right off.
I have now fitted 150lb springs but with 5 clicks on teh shocks. I have yet to try it as I have been of road awaiting anew steering rack. If the MOT is sorted this week I will do some miles and give feedback. I am lead to believe that the locost races only use 150lb rear springs.





Summer's here!!!!

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JoelP

posted on 20/8/06 at 04:35 PM Reply With Quote
longer or shorter

i think david means that, with the shocks he has currently, to get the correct ride height he has to wind them down until there is only a little travel left. Hence i think he actually wants shorter shocks, with the platform wound higher to give more travel. This would allow him to keep the 180lbs springs he likes, and still not run out of travel. If he finds the car bounces too much over bumps (or too low through dips), then he can worry about adjusting the damping to limit it.
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rusty nuts

posted on 20/8/06 at 06:23 PM Reply With Quote
Time for a DeDion rear end?
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paulf

posted on 20/8/06 at 10:20 PM Reply With Quote
I did go through this same dilemma.
I bought some Avo dampers at Stoneleigh expecting them to fit but they were an inch too long .I had the option of Ebaying them or making them fit and decided to remove the axle and modify it.
I orginally had 12inch dampers and 160lb springs but kept bottoming out, I couldnt adjust the damping as they were converted mini shocks.
I removed the axle and cut off the brackets and modified the top trailing arm mount to fit the new dampers, it was a couple of days work and a PITA to remove and refit the axle with all the body work in place, ended up remaking brake pipes and repainting the axle and chassis where i damaged them but it was worth it in the end.
I have now got 13.5 innch dampers and 180 lb springs and think that the springs are now to strong as it doesnt bottom out anymore but isnt as good a ride and a bit more twitchy on bumpy roads.
I plan to refit the 160lb springs when I get around to it and increase the damping to suit.The dampers I got were valved for the front of a GTM and seem to be better suited as they are valved softer than the standard Avo coilovers I have on the front.
I think this is the way to go as westfields seem to use 14 inch rear dampers obviously for the same reason.
Paul
quote:
Originally posted by Surrey Dave
But ideally I would like some longer shocks and softer springs , the axle could follow they road better and it would be more comfortable.


I believe Paulf ? has recently fitted longer rear shocks/springs and finds it much improved.

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procomp

posted on 21/8/06 at 08:24 AM Reply With Quote
Hi to get the best out of the axle most people run 100-140lb springs max. The other thing to cosider is to controle the unsprung weight of the axle you need dampers that are vaved with the right ratio and valving not what most damper suppliers supply as standard.

cheers matt

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leto

posted on 21/8/06 at 08:25 AM Reply With Quote
Linky to last time (?) this subject was up here.
Unfortunately the last question isn't answered yet.





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I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)

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David Jenkins

posted on 21/8/06 at 08:45 AM Reply With Quote
quote:
Originally posted by procomp
Hi to get the best out of the axle most people run 100-140lb springs max. The other thing to cosider is to controle the unsprung weight of the axle you need dampers that are vaved with the right ratio and valving not what most damper suppliers supply as standard.

cheers matt


So am I right in thinking that my best solution would be to fit lower poundage springs (around 140lb seems favourite) and up the damping rate on the shocks?

I have basic Avo shocks with just the one adjuster, so (a) I know they're not great shocks, and (b) I only have limited adjustment possibilities.

I went out for another play yesterday evening, with the 180lb springs: the shocks have 15 clicks available, but the ride is a little bit too harsh when at 5 clicks. The ride's quite good at 3 clicks, and I only hit the bump stop once (lightly) on quite a severe back lane bump; this was much better than when the 200lb springs were fitted, when it felt like I was skipping all over the road. I guess that I'd have to use a higher click setting if I used lighter springs - I wish I knew more about suspension...

Maybe some fiddling may cure this problem, rather than major surgery!



[Edited on 21/8/06 by David Jenkins]






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Liam

posted on 22/8/06 at 01:23 PM Reply With Quote
I'd carry on fiddling a little for now. Sounds like you've made an improvement already upping the damping, but i'd also try winding on some more preload, raising the back maybe half an inch or so. Unless it's already excessively high at the back, this shouldn't give a noticable impact on handling and might just be enough to give you what you need travel-wise.

Liam

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David Jenkins

posted on 22/8/06 at 01:28 PM Reply With Quote
I'm already as high as I wish to be - I think - 6.5" at the chassis just under the seat.

The idea of using 140lb springs appeals - except I don't want to shell out the money and find that they don't work!

David






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Liam

posted on 22/8/06 at 02:07 PM Reply With Quote
Hmmmm if you're sitting up that high but only have 1.5 - 2" of damper rod protruding (is that including or in addition to the bump stop rubber?) then those shocks are definately too long. You need more suspension travel if you want to go nice and soft. Restricting bump movement to the small amount you have available by just cranking up the dampers is not an ideal solution imo. Springs and dampers should be well balanced to work well together and going even lower to 140 lb springs will mean you have to be even more overdamped to avoid the bump stops.

Definatley go for one of your two first options.

Liam

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procomp

posted on 22/8/06 at 02:22 PM Reply With Quote
HI you have definatly got the wrong lenght dampers on so the thing to do is get it looked at and get the ride hight that is most suitable for the car sorted then get new dampers to suit whith the right valving and ratio and run 140lb springs with the right length to suit the dampers. If you can work out the right ride height and damper lenght your self you will be sorted if you can't then get it looked at by some one who dose or you will be back to square one .

cheers matt

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Surrey Dave

posted on 22/8/06 at 02:29 PM Reply With Quote
My car is setup with 1/3 travel at ride hieght leaving 2/3 for compression.

If your dampers are only 3" total travel that corresponds to 1" versus 2".

Longer dampers will give you longer available travel, say 6" ,that would give you 2" versus 4" travel.

As I understand it 140lb springs would take 140lbs to compress 1" , 240lb to compress 2" etc, therefore with more available travel you have available in the dampers you could run lighter springs without hitting the bump stops.

And not have to over damp to compensate.

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David Jenkins

posted on 22/8/06 at 02:49 PM Reply With Quote
Thanks folks!

It does look like I need to pick one of the options - new shocks, or bracket moving.

Buggerit...


cheers,
David






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Surrey Dave

posted on 22/8/06 at 04:44 PM Reply With Quote
One last thought , you could use your existing dampers but mount them at angle inwards at the top, (John Isons locost was like this) , then you get more vertical axle travel from the same dampers. probably lower effective spring rate as well ,just like the front.
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David Jenkins

posted on 22/8/06 at 06:15 PM Reply With Quote
Unfortunately there's boot-box in the way!

That would involve even more hacking around, although it is a good idea.

Need to give it some serious thought...

David






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David Jenkins

posted on 26/8/06 at 12:14 PM Reply With Quote
UPDATE

OK - I took off a damper this morning and measured its length - 14" extended (ooer!), which is the dimensions recommended in The Book. Unfortunately this is a dumb recommendation, as the distance between centres when the car's jacked up and the axle's on the chassis is just over 12"!

So it looks like I need 12" shocks, with maybe a rubber bump stop to prevent the shock from going fully extended on hump-back bridges. What length have other builders used?

And what length springs should I get to go with those?

cheers, and thanks for all the help so far,
David






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MikeRJ

posted on 26/8/06 at 12:39 PM Reply With Quote
Do you have a standard English live axle with "book" brackets?
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David Jenkins

posted on 26/8/06 at 02:41 PM Reply With Quote
Yep - the only difference is that I corrected the book's mistakes (hole spacing on the axle's brackets)

Otherwise all is standard.






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JoelP

posted on 26/8/06 at 03:49 PM Reply With Quote
hi david.

i have a pair of 11.5" shocks from gts, standard ones. Not sure on springage and the bushes are a little sagged from having a car on them for a year, but basically unused besides that. Also some 12" ally protech ones, adjustable damping and rosejointed ends. Any use?

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paulf

posted on 26/8/06 at 07:25 PM Reply With Quote
Or remake the bottom mountings to use the trailing arm bolt and an extra bracket welded on the axle to locate the outer end so as not in single shear.This will give an extra 1,5 inches of travel and correct amount of bump clearance at 5.5 inch ride height.
Paul.
quote:
Originally posted by David Jenkins
OK - I took off a damper this morning and measured its length - 14" extended (ooer!), which is the dimensions recommended in The Book. Unfortunately this is a dumb recommendation, as the distance between centres when the car's jacked up and the axle's on the chassis is just over 12"!

So it looks like I need 12" shocks, with maybe a rubber bump stop to prevent the shock from going fully extended on hump-back bridges. What length have other builders used?

And what length springs should I get to go with those?

cheers, and thanks for all the help so far,
David

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