Rear Springs

GreatOldOne · #1 29th March 2006, 11:48

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 · #2 29th March 2006, 12:28

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 · #3 29th March 2006, 13:25

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 · #4 30th March 2006, 12:34

Here is my fag packet calculation. Comments anyone?

Patrick · #5 30th March 2006, 13:25

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

Quote:

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 · #6 30th March 2006, 14:24

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

Patrick · #7 30th March 2006, 15:07

Quote:

Originally Posted by GreatOldOne

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

www.talkaudio.co.uk

I spend faaaaar too much time on there

Alfie · #8 30th March 2006, 16:35

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.

Alfie

Alfie · #9 30th March 2006, 17:07

I've just re-read everything and realised I've misunderstood. So I'll just shut up then!

MartinClan · #10 30th March 2006, 18:57

Travel at the bumpstops is about 3 1/2"

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...

MartinClan · #11 30th March 2006, 19:38

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...

Patrick · #12 30th March 2006, 21:21

I PM him on www.talkaudio.co.uk, dont have an email address.

I'll pass this stuff on to him see what he thinks

Patrick · #13 30th March 2006, 21:28

this is what i got back

Quote:

Originally Posted by black-cat

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

peterux · #14 2nd April 2006, 08:39

Robin,

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.

not sure if any of this helps?

Peter

MartinClan · #15 2nd April 2006, 16:07

Thans Peter

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...

Cheers

Robin

Patrick · #16 2nd April 2006, 17:27

Can the place where your planning to order the springs not help a little with recommendations?

Bruce · #17 8th April 2006, 16:01

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 · #18 8th April 2006, 21:28

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

BR
Peter

Patrick · #19 8th April 2006, 21:58

Quote:

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!)

Quote:

Originally Posted by Bruce

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

Sounds like a good idea

Patrick · #20 10th April 2006, 09:07

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?