Rear Springs

[GreatOldOne]

This seems to have been one of the lost threads due to the move, so I'll put it back.

One of you guys (Martin IIRC) had worked out the spring rates and lengths and was going to order some, and was asking the rest of us wether we eanted to get in on a 'group buy'

I'd like to know an approximate price, as as long as they're not overly expensive, count me in on an order.

[MartinClan]

It was me...

I have had a price of £48 each plus vat and delivery from Faulkner. This is for a barrel spring (fat in the middle) rated at 400LB/IN and a free length of 7". Minimum length of 3" which means, if they are in the right place LOL, the bumpstops will work before the spring binds. I have been waiting for Marlin to confirm the corner weights - but they seem unable to tell me.... (Reason given it varies from car to car - but, as I pointed out I was only after a ball park figure)

I will post my calculation up tonight to see what people think - Im am fairly confident its good - but not 100%

What we need is a proper engineer to check my figures (as opposed to me - an improper engineer....)

[Patrick]

I'm still up for a set of these when you order Are your calculations still the same as the Excel you send me a while back? If not send me what you have now I know someone who may be able to help with checking.

[MartinClan]

Here is my fag packet calculation. Comments anyone?

[Patrick]

Robin, I got this back from Tom on the other forum:

Originally Posted by black-cat

You want the free length to be more than the maximum droop length.

Go for an 8" spring, then you've got to squish it by 2.75" which gives 160lb/in at the wheel.

How much rear suspension travel do you have?

[GreatOldOne]

Other website? Tell me more... (I'm bored at work....)

[Bruce]

Hello
One of the bmw's I broke came with 60mm lowered G-Max springs, they are about 10mm shorter than standard uncompressed but when you stand on them the standard one hardly moves but the G-Max one seems a lot softer.
Is it a dual rate spring (if such a thing exists) ie easy to compress initially but a lot harder further down?
I have no idea what the spring rate is I looked at the G-Max website but there was no info.
My intention was to fit them and see what happens.
Any advice on whether this sounds like a good/bad idea?

[peterux]

Sounds like a good starting point. Another trail version for all of us to compare with!

BR
Peter

[Patrick]

Originally Posted by Bruce

Is it a dual rate spring (if such a thing exists)

Yeah they do! Soaks up soft bumps while cruising along, but compress then the harder bit takes over when push hard in the corners (is my understanding!)

Originally Posted by Bruce

Any advice on whether this sounds like a good/bad idea?

Sounds like a good idea

[Patrick]

I've see on page 20 of performance BMW this month (May 2006) Gaz Shocks have a new coil over set for the E30. The rear ones will fit into a reinforced normal top mount from the look of it. Doesn't look cheap but I plan to have a chat with them! Those shocks combined with a spring with the same rate as the front might be the answer?

EDIT: I've sent a mail to Gaz to see what they have to report about the new shocks and if they can supply custom springs - persumably the same rate as the front would work well?

[Patrick]

This is what came back:

Hello Patrick
the part no. GP8-2015 is the coilover unit shortened for the E30, this must have
the bearing top mount to support the vehicle weight, springs are £16 each and
available in 250-275-300lbs rate (all prices are plus VAT and carriage)
Kind regards
David Lyon
P.S these will allow a -70mm ride height.

I think I might go this way as it allows easier swapping of springs (and cheaper!! ) If the rates are wrong. Its only the higher initial costs of buying the coil overs. I would hope a -70mm over stock right height adjustment aught to be enough!

[morris]

Hi All,

Been back through all this and came to the conclusion that as I'm E36 based I'll follow mperman's suggestion and go for these as a first attempt

http://www.merlinmotorsport.co.uk/p5...duct_info.html

They are close to the
REAR
MR = 0.63
WR = 90lbs (220lb spring, freelength 8")
Freq = 1.24 cps (softer than front)

he quoted being 225lb, 2.25" ID 6" length

Only difference is that the free length is 6" not 8". Does this affect the calculation. I can't see the free length figuring in any of the equations directly but I may have missunderstood.

I've gone for 6" as that's the maximum travel my dampers allow so any extra length is just more hard work compressing them in the first place.

When the half shafts are almost horizontal (just sloping diff to hub slightly) the length with the top adjuster at a low point (40mm of lowering left on the thread) is 5".

I suppose another way of looking at it is if I go for the 8" at the lowest ride setting then there is no chance of them popping off where as with the 6", if I adjust to the lowest they could just about slip out of position, not that I plan spending much time shooting through the air dukes of hazzard style.

In this case the best option is
http://www.merlinmotorsport.co.uk/p5...duct_info.html

[mperman]

Ok, I've been a bit quiet on here lately. Back in November I posted a few things on this topic - since then I have been busy sorting out a few things. I've also done a lot of work on this, and with Dave Coopers help at Track Developments, have come to some sort of idea how to proceed.
I'm afraid Martin Clans calculations look a bit different to mine - sorry Martin.
I think were all agreed that collapsing a coil Mark's way is not ideal. BMW springs are also too stiff because the cars are much heavier.
The place to start is the wheel rate (WR) and a desired frequency for the suspension. Using the front spring rate of 300lbs and the front geometry, for mine, this is about 100lbs WR and freq approx 1.4 cps (dependent on the weight). Note that a BMW M3 coupe is 100lbs and 1.1 cps.
What we really need is a good estimate of the weight, that's why I have tried a few a times to get this before. My estimate is 900kg including driver and half tank of fuel!
Then using the rear geometry and wanting the back to be a little softer, say 1.3 cps, the rear springs will be about 250lbs.
But there are differences, mine is an E36 and heavier. Do you guys really have 7” between the rear spring seats at max – luxury mine are more like 5”
Hope this helps.
Cheers,
Martin

Keep up the good work.

[MartinClan]

Hello Mperman

Interesting calculation. How did you arrive at the 250 - do you have some sort of formulae?

Marlin quote the unladen weight of the Sportster at 700 KG's - how did you arrive at 900?

Is the spring rate of 250 for a spring that sits over the axle - or does it take into account the leverage effect of the swinging arm? (16.5/12.5)squared. If 250 is the effective spring rate over the axle the actual spring required to achieve this (on the E30 anyway) would be

250 x (16.5/12.5) x (16.5/12.5) = 435

pretty close to my original 400 estimate

And - yes there is about 7" clerance between the bumpstops at full travel - but I guess the suspension design is different on the E30?

Cheers

Robin

[Stuart]

I took the Sportster to Track developments near Castle Combe. I still have standard springs in the back and Marlin supplied on front coilovers. He recommended the following after doing corner weights and a lot of measurements of the vehicle.

quote:Hi Again Stuart

I have managed to get some more info. At last I have found the formula I needed. Before I was transposing from Metric units and getting conflicting results.

Your choice is :- for comfort go with 250 Front and 260 rear. This would still be stiffer than a family saloon.
For a more sporting set up try 280 Front and 300 rear.
end:

I am yet to source some full length rear springs at 260lbs if anyone has supplier would be apreciated.

Stuart

[MartinClan]

I had mine made. Can't find the details now but there is a thread somewhere on this forum with the name of the company!

Robin

[peterux]

Originally Posted by Stuart

I took the Sportster to Track developments near Castle Combe. I still have standard springs in the back and Marlin supplied on front coilovers. He recommended the following after doing corner weights and a lot of measurements of the vehicle.

quote:Hi Again Stuart

I have managed to get some more info. At last I have found the formula I needed. Before I was transposing from Metric units and getting conflicting results.

Your choice is :- for comfort go with 250 Front and 260 rear. This would still be stiffer than a family saloon.
For a more sporting set up try 280 Front and 300 rear.
end:
I am yet to source some full length rear springs at 260lbs if anyone has supplier would be apreciated.

Stuart

Hi Stuart,
did TD make any other recomendations or changes to your car?

Do you know what the final Camber, Caster, toe settings etc are set to?

Did it make much difference to your cars handling?

I've got Marlin supplied springs at the front and Apex lowering springs at the back, but no idea what spring rates they are?

thanks

Peter

[Patrick]

you can't set anything on the back unless you get ecentric bushes which are quite pricey.

[Stuart]

Correct Patrick you need ecentric bushes which I dont have but he did check the rears and that is what they were as standard.

[Stuart]

Peter I have 2 washers front and back so equally spaced.
I have poly bushes on the front.
Your springs of 291 lb/in sound almost spot on but you must make sure you have different rate springs at the front otherwise you could get harmonics through the car.