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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.
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....)
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.
I have been avidly following this discussion and while I don't claim to fully understand the calculations, two points come to mind.
1. 400lb/in seems a lot considering weight of car.
2. Robin, shouldn't the last part of your calculation about trailing arm lever effect be other way round : 12.5/16.5. If you do this, the result is approx 145lb/in, much closer to the 160lb/in figure obtained from " other website".
This is just a thought and I am more than willing to be corrected.
With the driveshaft parallel to the ground there is about 2" travel left at the bumpstop = 2 x 16.5/12.5 = 2.64" at thw wheel - not a lot.
I suppose the spring could be weaker and longer. If we take black-cats suggestion and make it 8" long - hang on I need to think about this - back in a minute...
Oh Bother (or words to that effect) Gonna try again. No pictures this time.
Distance between spring plates at full travel 6.75" (actualy irelavent)
Distance between spring plates when drive shaft is parallel to ground = 5.25"
At this point the distance between the bump stops is 2" giving 2*16.5/12.5 about 2.6" max travel at the wheel before the bump stops. (More than my Mini used to have)
Static force at spring plates = 440*16.5/12.5 = 580 lb (See previous post for how I arrived at 440)
If we use a 7" free length spring.
Compression to acheive 5.25" = 1.75"
Spring rate therefore = 580/1.75 = 332 lb/in
If we use an 8" free length spring
Compression to acheive 5.25" = 2.75"
Spring rate therefore = 580/2.75 = 210 lb/in
I think the problem with using a softer spring is the suspension may run out of travel when carrying a heavier load (Heavy passenger for example)
What does Black-Cat think - Patrick do you know how to contact this guy?
How about all the interested parties chipping in for a prototype set of springs - anyone game for that? Could be a better idea than getting 3 or four sets that are wrong...
Right, you need to work out how much force you want it to take to go from 'normal' to fully compressed, this then defines the spring rate and then you get the free length from that.
As for working out the first number, thats the hard one
don't know if this will help, but here is some more data.
I have fiitedd some Apex ''lowering'' springs that i picked up on ebay.(£35 the pair!)
They are 185mm free length before I fitted them. Now they're on the car (with petrol tank, diff, suspension, seats - but no body work, no petrol or anyone sitting in the car) they have compressed to about 165mm.
The halfshafts are just above horizontal, so i think with driver and passenger, they will be about horizontal.
I have no idea what the spring rating is and can't find anymore info from Apex website (but I have a part number, if you want to persue).
However, I did some very unscientific experiment yesterday. if I stand on the font of the car and bounce up and down, I can't generate about 3" movement up and down (God knows what the neighbours thought I was doing?)
If i do the same on the back, i can only get about 1.5" movement. all this tells me is that the suspension is harder at the back.
Is this good or bad, i have know idea!
As they were designed to take the full wieght of a BMW with 4 passengers and luggage, they are probably a bit hard, but at least they won't bottom out over bumps and speed humps, etc..
I'm going to stick with these until the car is on the road and then see how it all feels.
I think we should all be on the "Rough Science" program on TV....
I to bounced up and down on the front of my car - it didn't move 3" (but then I am only about 70Kg's) The front springs are 300lb/in - I have this confirmed by Marlin. The rate is increased by about 10% because they are at an angle - so they probably behave more like 330lb/in.
185mm (7.25" to me) is very short even for a lowered spring! The standard BMW spring is about 9.5" long - i should think the car they came from must have been on the floor! I would be interested in the part number if you have it.
With the driveshafts level I measured the distance beetween the spring plates as 5.25" (133mm) At 165mm I think yours still has a fair way to go.
I want to go for as for as much movement as possible at the back - after all this is where you sit. Also I wold like to keep the fillings in my teeth...
I am still agonising over having some made - could be an expensive mistake if I get it wrong...
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?
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!)
Bruce wrote:
Any advice on whether this sounds like a good/bad idea?
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?
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!
185mm (7.25" to me) is very short even for a lowered spring! The standard BMW spring is about 9.5" long - i should think the car they came from must have been on the floor! I would be interested in the part number if you have it.
Robin
Hi Robin,
I've checked the part number of my Apex Springs.
They are marked 20-10352 HA 219/03
I also re-checked their current compressed length and it is 170mm. As I said, that is with no bodywork, petrol, driver or passenger.
By the way, the centre of my top bump stop "bolt" is directly in line with the outer edge of the rear box section that takes the rear bumper, when viewed from the back of the car.
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
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?
Hi Robin,
Right then, you are right about the leverage effect, this is usually called mechanical advantage; a ratio. As I mentioned earlier the most important factors to get right are the WR (wheel rate) and frequency. If we can find common ground you can check my calculations; suggest the Marlin front is as good a place as any.
For mine the levers d1 and d2, just as in your diagram, are:
d1 = 210mm
d2 = 380mm
This gives a ratio of 0.55. These may be the same on your E30 car?
Important input from the expert (Dave Cooper) is to check this by measuring the spring length carefully and stroking the wheel a few inches, it should give the same ratio.
And squared I make it 0.31, then with a 300lb spring rate (SR), WR is:
300 x 0.31 = 93
So if you apply 400lbs to the corner it will compress 4.3 inches at the wheel and 4.3 x 0.55 at the spring.
Ok, dont drift off, there is an extra complcation due to the spring being inclined, so for a more accurate calculation the cos of the angle should be added, for mine 0.9:
WR = SR x (d1/d2)^2 x cos (spring inclination) = 80 approx.
The rear spring by the way is close to upright so dont worry about it there.
Now then, as mentioned earlier the other important factor is the suspension frequency, this is where the personal preference comes in. On most road going cars, it is in the range 1.0 to 1.5 cps. On track cars, its more like 2.0+
The Marlin front with 300lbs SR, making the following assumptions:
F/R ratio is 45:55
Curb weight is 800kg (I'll come back to that one)
Driver and fuel 100kg
Then front corner weight is about 450lbs
My weighed unsprung weight is 95lbs
And WR is 80lbs, from above.
So the weight on each of the front spring is about 350lbs
The frequency is (WR/Sprung weight)^0.5 x 188 (in cpm)
(The 188 is a constant due to gravity) Now divide by 60 for cps (hertz).
f = ((80/350)^0.5 x 188)/60 = 1.5
This is quite stiff, compared to the beemer, which by my calculations is nearer 1.0
The weight issue is a big problem, really until I've actually weighed a car, I have little confidence in the excercise; we know that Marlin quote 750kg for a finished car - and that was with a 4 cyl engine. I would guess they are easy to make heavy than light - and if I'm going to err, it had better be on the heavy side.
But, back to the problem, having got the front sown-up; the rear should be pretty similar in terms of WR, but the freq MUST not be identical or the car will behave very strangely when set in motion.
There you are, you did ask !!
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. 
