russtik
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 posted on 21/10/15 at 09:31 AM |
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Springs wanted - to buy or loan
My car is unusual when compared to most 7s in that the suspension design requires considerably stiffer springs than say something like a Westfield. it
is however a very hard ride and I'd like to soften it up to get some more road compliance. My current set up is:
Front - 1.9" ID x 8", 600lb
Rear - 2.25" ID x 6", 550lb
I don't really know what poundage of spring I should be fitting to get the desired softening so I was wondering if anyone had some springs
either going cheap or if you're near (I'm in Brighton) that I could trial fit? I guess I'm looking to start out in the region of
400-500lb?
Cheers, Russ.
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AdamR20
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| posted on 21/10/15 at 09:43 AM |
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Is it a RR SR2? If you have some photos of the suspension setup, I would be able to give you some ballpark figures to try if you like. You can use
shock angles and leverage ratios (of wishbone and any rockers) to work back to a wheel frequency, which then gives you the spring rate required.
I think I have some spare 500s but they are 8" from memory.
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russtik
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| posted on 21/10/15 at 10:05 AM |
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It is an SR2 yeah. It's hard to demonstrate the exact geometry in photos as the wishbone pickup points on the chassis are buried some way inside
the bodywork. Essentially the bones are long, the leading edge of the front lower wishbone is at least 50cm.
Here's some pics from the RRR facebook page.
Just to be clear mine has OUTBOARD front suspension but the wishbones are the same.
[Edited on 21/10/15 by russtik]
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AdamR20
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| posted on 21/10/15 at 10:29 AM |
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Cheers for that! You do have VERY high wheel rates in that case.
I'll do a few quick sums at lunch and jot down what I reckon would be a ballpark spring rate... hopefully others will be able to chip in with
their experience too.
To check - is the car all road, road / track or just track? Any anti roll bars? Ta.
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rodgling
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| posted on 21/10/15 at 10:40 AM |
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I probably have some springs suitable for the rear in the right sort of range, will take a look.
BTW in my non-expert opinion, your front ride height looks a bit low, I believe normally you should aim to have the line between the two pivot points
on the bottom wishbone no further than parallel with the ground as a starting point (or outer pivot point lower than inner point). Although obviously
having a low centre of gravity has benefits too, there's a trade off to be made.
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russtik
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| posted on 21/10/15 at 10:58 AM |
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quote:
Originally posted by AdamR20
To check - is the car all road, road / track or just track? Any anti roll bars? Ta.
Road & track with no ARBs. Thanks for your input.
quote:
Originally posted by rodgling
...your front ride height looks a bit low, I believe normally you should aim to have the line between the two pivot points on the bottom wishbone no
further than parallel with the ground...
The photo I posted is a bit misleading, it makes it look like the car is "sagging" whereas if I were to move the camera a little lower you
would see that the bottom wishbones are pretty much horizontal. The ride height is a touch too low though.
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AdamR20
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| posted on 21/10/15 at 11:33 AM |
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The car does look pretty low, all the SR2s I've seen look a bit 'wrong', but if the suspension is designed to be run at this ride
height it's no problem. There is a little more to it than just lower wishbone angle, but that is a good 'starting point' for most
double wishbone systems to have them level, especially for road-going cars.
Here's a bit of light reading related to wheel frequencies - www.optimumg.com/docs/Springs&Dampers_Tech_Tip_1.pdf. In short you probably
want to be around the 1.7-1.9Hz sort of range from my experience / testing, with slightly higher (10-15%) frequency at the front. This is what I would
call 'soft' compared with most kit cars, but it gives good compliance and confidence in the car.
Looking at the pics and taking some angles / lengths from them, plus guesstimating corner weights (using the ones from my CEC Westfield), you will
probably need to be around 450-500lbs at the front and - weirdly - 200ish at the rear. It would be useful to get some dimensions and angles from your
car (happy to provide details of how / what to measure) to confirm these, but that's what my calculator is saying... The main factor seems to be
how much the front shocks are 'laid down'.
To note, you may need to raise the car a touch to prevent it bottoming if you reduce the wheel rates.
The other alternative would be to try something softer on the rear - 250 to 300 would match the front springs you already have, but give higher wheel
rates overall. The rear is where you feel most of the 'jarring' given where you sit, so this could be a good port of call. I think I have
some 300s or 275s in 7" if they would be of use.
Cheers,
Adam
[Edited on 21/10/15 by AdamR20]
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russtik
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| posted on 5/2/16 at 05:02 PM |
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Just as a bit of an update one this... I've finally gotten round to taking some rough measurements. It's hard getting a tape measure to it
as there's things in the way so these aren't as accurate as I'd like but they should at least be a good enough ball park.
Can anyone help me work what wheel rates/frequencies I currently have and where I need to go to get it more towards the ideal?
I've not had my car on the scales but I've seen a figure published before for a caged SR2 with fluids at 560kg and I suppose I'm
about 80kg. I weighed one front corner unsprung and it was 31kg.
Thanks
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AdamR20
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| posted on 6/2/16 at 02:03 PM |
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Guesstimating 52% rear weight distribution, and 35kg unsprung mass at the rear, with your 600 / 550lb springs the wheel rates come out to 2.12hz front
and 3.25hz rear (!).
Changing the rears to 200 and leaving the 600s up front will get you somewhere near. Alternatively, something like 500s up front and 160s at the rear
will be a little better overall for road driving.
Personally I'd just change the rears for now so you can appreciate what a difference it will make to the car. It's easier and cheaper,
too!
[Edited on 6/2/16 by AdamR20]
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russtik
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| posted on 8/2/16 at 04:12 PM |
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Thanks for your response Adam. I have managed to pick up some second-hand 300lb springs and fitted them on the rear. I've only completed a
couple of short test drives so far but it's made a big improvement in the ride quality as you might imagine. I've yet to assess what the
impact is in cornering ability as that will take longer and I also suspect it may bottom out on some bigger bumps and higher speed. The shock
platforms have yet to go back to their original positions as a C spanner is in the post.
Undecided whether I will do anything at the front, may try a 500lb spring but they're somewhat harder to come by second-hand. More testing...
Cheers.
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