westf27
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| posted on 24/7/06 at 11:20 AM |
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what poundage and length springs
Built and sva'ed Formula 27 with pinto and live axle.This is no lightweight in fact at sva the rear was well over 300kg's i think 340 with
the tester.Shocks have a 3" travel and fully extended can just force a12" spring on.So the question is for road use what poundage and
length do i need on the rear.Read a lot of threads but still not sure what to buy.Have tried 175 but still bangs on humps
[Edited on 24/7/06 by westf27]
555
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NS Dev
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| posted on 24/7/06 at 11:59 AM |
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spring length.....as long as you can get on with the abutments wound right down.
spring poundage, certainly not more than 175 on the rear, my vauxhall 16v engined car has 140lb rears.
In fact my opel manta coupe only has 160 lb rears!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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andyharding
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| posted on 24/7/06 at 12:22 PM |
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I'm running 200lb springs on the rear. Around 550-600Kgs with me in it and a full tank.
Are you a Mac user or a retard?
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John Bonnett
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| posted on 24/7/06 at 12:47 PM |
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If your shock absorbers with 175 lb springs are vertical to assume no leverage effect nearly 2 inches of your travel will be taken by the weight of
the car and as you have found, there is insufficient travel left to accommodate bumps. I would have thought a 220 lb spring would be better as this
will still allow nearly an inch and a half free travel. The length of the spring only affects ride height. You cannot increase spring rate by winding
up the spring seats.
I hope this is of help.
John
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Liam
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| posted on 24/7/06 at 04:25 PM |
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quote:
Originally posted by John Bonnett
If your shock absorbers with 175 lb springs are vertical to assume no leverage effect nearly 2 inches of your travel will be taken by the weight of
the car and as you have found, there is insufficient travel left to accommodate bumps.
John
That's what preload is for. 200 or over sounds very hard for a locost rear end - you'll be hopping and skipping all over the place and
have a rather snappy tail if you go too hard (it's often said most locosts are oversprung). For optimum roadholding and ride quality you want
to go as soft as you can get away with. If the car's weight is taking up too much of your travel you need to add preload rather than harder
springs. An oft quoted rule of thumb is to wind on preload so that you have 2/3 of your suspension travel for bump and 1/3 for droop.
Liam
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leto
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| posted on 24/7/06 at 04:30 PM |
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You can find information on how to calculate the spring rate on other forums. From what I understand the subject isn't discussed here because
some people has strong opinions on it (link)  Once you have the spring rate you
can calculate the length from the preload needed at ride hight. You might still have a problems as you need enough suspension travel for the springs
to do there work. If the travel is too short you will have to rebuild, higher spring rate will only swap one problem for another.
“I'm gonna ride around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)
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John Bonnett
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| posted on 24/7/06 at 05:32 PM |
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I think you must have some rebound left in the shock absorber so going for a high preload may make the shock absorber operate at the end of its
travel. I think this has highlighted the problem of a short travel shock absorber coupled with a heavy rear end. It is true that stiffening the rear
springs will promote oversteer.
I just calculated that if the weight of the rear of the car is 660 lbs each spring takes half that. Assuming no leverage effect from an angled shock
absorber a 220 lb spring would deflect 1.5 inches. This is still higher than the 1/3 2/3 rule.
How about this then, using a 180 lb spring which should be good for handling and ride we need a spring length that will deflect sufficiently to
compress the shock absorber by one inch leaving two inches for travel. If the open length of the shock absorber is 12 inches a 13 inch spring will
compress 1.8 inches leaving 0.8 inches for rebound.
I'm not sure of the maths but I am sure you are right it is a question of spring length and not poundage.
John
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Peteff
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| posted on 24/7/06 at 06:09 PM |
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Wouldn't damper travel affect that more than spring length? You could have a 4" spring at the top of the damper if the travel is only
3", admitted it would have bloody thick coils but it would work in theory.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Liam
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| posted on 24/7/06 at 06:23 PM |
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As I said above, there's no need to choose your spring so that it compresses just the right amount under the car's weight to put you at a
decent ride height. That is what preload is for. You can control how much the spring compresses under the cars weight with the preload, from the
whole amount you'd expect to none at all (no droop travel). If you choose springs that sit you at the correct ride height with no preload you
will almost certainly be hugely oversprung.
LIam
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TangoMan
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| posted on 24/7/06 at 10:19 PM |
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Rebound and dropp are functions of damper length and will not be affected by the spring unless you get into coil binding.
The spring length needs to be sufficient to allow the preload to set your ride height. You already have the calculation for the spring deflection.
I have just ordered 180lb 12inch springs for the rear of mine Formula27 as it is too hard at the rear and oversteers too much for me. Good fun but not
the fastest way around a track!!
I am not sure of the current poundage but looking at the springs I am guessing over 200lb.
180lb seems to be a hard but acceptable setting as the Locost racers are using less than this although they will probably weigh a little less.
If your 175's bang on humps then I guess you may have another problem. Are you certain that you have not run out of travel on the dampers. If
you have four linked the axle with panhard rod, are they rubber mounted or all rose jointed.
If you try softer and find it works, can you let me know as it might save me the job of changing springs twice!!
Summer's here!!!!
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NS Dev
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| posted on 25/7/06 at 12:14 PM |
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quote:
Originally posted by John Bonnett
If your shock absorbers with 175 lb springs are vertical to assume no leverage effect nearly 2 inches of your travel will be taken by the weight of
the car and as you have found, there is insufficient travel left to accommodate bumps. I would have thought a 220 lb spring would be better as this
will still allow nearly an inch and a half free travel. The length of the spring only affects ride height. You cannot increase spring rate by winding
up the spring seats.
I hope this is of help.
John
heh heh if only it were that simple!
trust me (and Liam!) over 200lb springs on the rear of a 600kg locost will make it handle like ****.
Preload will increase the initial rate whilst maintaining the same dynamic rate with a linear rate spring.
If you need 200lb springs for the back of a 600kg locost then how come my opel manta road rally car handled so well on 160lb rear springs when it
weighed 980kg???????????????
I know I always rattle on about this on here, but hopefully Syd Bridge might mention it again as well. SOOOOOOO many 7's are built with woefully
overrated springs, using the "max power" stiff and low approach, which totally wrecks the handling of these cars.
Softest springs you can get away with, decent quality dampers properly set up, and small wheels with decent tallish profile tyres all work towards
giving best grip and cornering in these cars.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Syd Bridge
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| posted on 26/7/06 at 09:42 AM |
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quote:
Softest springs you can get away with, decent quality dampers properly set up, and small wheels with decent tallish profile tyres all work
towards giving best grip and cornering in these cars.
Yes Nat, you're spot on.
But, some will never understand.
Wait till autumn again, and the rash of 'offs' and written off cars.
Cheers,
Syd.
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leto
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| posted on 26/7/06 at 03:44 PM |
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NS Dev & Syd
How much suspension travel do you need for a setup like that?
Cheers!
“I'm gonna ride around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)
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NS Dev
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| posted on 26/7/06 at 10:49 PM |
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between 3 and 4 inches.
Would be nice to have a lot more but around 3.5 inches will work fine.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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leto
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| posted on 27/7/06 at 06:29 AM |
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Thanks  Is that total travel or compression only?
“I'm gonna ride around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)
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