Rear Springs

[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!

[peterux]

Originally Posted by MartinClan

Thans Peter


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.

hope this helps,

Peter

[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

[mperman]

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 !!

Cheers,
Martin

[MartinClan]

Hello Martin

Thanks for the info - I will think about it - tomorrow when this evening's G&T has worn off......

[MartinClan]

Hello Martin

I have made a spread sheet where you can fiddle with all the factors and get the corresponding wheel rates and suspension frequency.

It looks like Marlin make the front suspension for the E30 and E36 models different - unless one of us can't measure...

Rear is different because of differences in design of E30 and E36 rear end.

Am I on the right track?

Cheers

Robin

[Patrick]

I've decided to go the rear coil over route with mine. After speak to David @ Gaz today he things 250lbs it too much saying even the lighter race cars dont run that hard. He's recommended trying 225lbs first and going from there.

[mperman]

Robin,
Looks good to me, please do me a favour and check the front d1/d2 ratio on you car. Measure the spring length, or distance between the eye bolts with the coil-over removed, then place a block of wood under the wheel and measure again. I used a 2" block and divided the answer by 2, in other words if the wheel strokes 2 inches how much does the spring compress - its the same thing. You should get the same value as you calculated. I wonder if you added the king-pin length to your d2 length? I really would expect yours to be similar to mine. In fact most cars are 2/3, squared less than 1/2. At the moment it all looks a bit stiff, I think the car would shake your fillings out!

Cheers,
Martin

[MartinClan]

Martin - OK will do.

Patrick - good luck with the coil overs. The calculation will be different for you because the spring will be in a diffenrt place. Two commencts I would like to make that I hope you will find usefull.

1. Have you checked with Marlin that the shock mounting tower is strong enough to thake the weight of the car continuously?

2. Dont forget the differnce between the un-laden and laden weight of the Sportster. With no fuel and no-one sitting in it it weighs about 800kG's. Two people and a full tank of fuel could add an extra 200kg's (depending on how fat your passengers are!) Most of this extra weight will appear over the rear wheels

Cheers

Robin

[MartinClan]

Ok - it looks like I can't measure or I didn't understand the instructions. I didn't add the king pin length to the d2 length - not sure why you have to do that...

Anyway it looks like the practical method - stroking the suspension - is the most accurate from which I get a ratio of about 0.58 and hence a wheel rate of 90 Lb/In - the same as Martin.

I think I am going to try the practical method at the back as well - hopefully should then be in a position to order some springs.

It was all much simpler when I built my first (Triumph) based Marlin - you just removed leafs from the rear spring until it looked and felt about right

[GreatOldOne]

I'm trying to follow all of this, really... but you lost me several posts ago.

I'm just glad that you all seem to be getting somewhere with it, and I won't have to agonise over the same calculations in the future.

If, once you've got the figures worked out, there's a 'group buy' going on the springs - count me in!

[Bruce]

Hello
Do you need to use a locking nut above the spring seats or will it remain in position with the pressure of the spring?
I wasn't happy with the position of the anti-roll bar when I tried to fit it, the ends were hitting the spring seat, so I thought of cutting the ends off the roll-bar which would have given just enough room maybe.
But instead I cut off the spring seat mounts and rewelded them where they should have been in the first place. So now everything lines up and there is plenty of room for the unmodified roll bar

[mperman]

Originally Posted by MartinClan

Ok - it looks like I can't measure or I didn't understand the instructions. I didn't add the king pin length to the d2 length - not sure why you have to do that...

Anyway it looks like the practical method - stroking the suspension - is the most accurate from which I get a ratio of about 0.58 and hence a wheel rate of 90 Lb/In - the same as Martin.

I think I am going to try the practical method at the back as well - hopefully should then be in a position to order some springs.

It was all much simpler when I built my first (Triumph) based Marlin - you just removed leafs from the rear spring until it looked and felt about right

Robin,
Brilliant news. Although I've spent many hours deliberating over this, it gives me a much better feeling to have someone else confirm the calculations. I didn't want to just send out a copy of my spreadsheet - until we have a bit of confidence in what were doing. The king pin and hub length issue is an odd one, some books say measure to the wheel centre; others leave it out. I'm sure you include it, along with Dave Cooper, he uses the block of wood method to measure the same mechanical advantage on the the car. So now we've both done the same thing both ways and got the same answer; BTW my front is 0.55
So that just leaves the weight issue and choose how hard you want the ride to be, mmm....

Cheers,
martin

[MartinClan]

Originally Posted by Bruce

Hello
Do you need to use a locking nut above the spring seats or will it remain in position with the pressure of the spring?
I wasn't happy with the position of the anti-roll bar when I tried to fit it, the ends were hitting the spring seat, so I thought of cutting the ends off the roll-bar which would have given just enough room maybe.
But instead I cut off the spring seat mounts and rewelded them where they should have been in the first place. So now everything lines up and there is plenty of room for the unmodified roll bar

I think I am going to do the same - how easy was it to cut them off - what tool did you use? they arent in the most accesible of positions...

Cheers

Robin

[MartinClan]

Originally Posted by MartinClan

I think I am going to do the same - how easy was it to cut them off - what tool did you use? they arent in the most accesible of positions...

Cheers

Robin

Forgot to say - I asked Marlin about locknuts - they said they were not required and if I was concerned just to coat the threads with locktight - which kindof defeats the object of them being adjustable in the first place...

[Bruce]

Robin
I just used a 4inch angle grinder to cut them, it left a bit of a stump which took a bit of time and patience to grind flat.
Mask off any vulnerable areas to prevent damage to powdercoat, a few layers of masking tape maybe, I didn't and damaged a little bit.

[peterux]

Hi Robin,
Seems a bit drastic to cut off the top spring mounts?

Mine seemed to be OK and certainly not that far out like yours.

I guess the question is………. Are your spring bump stops in the wrong place? (As you suspect) or Are your trailing arms positioned wrongly in some way? A small displacement at the bushes could give a large movement at the bump stop. Maybe the trailing arms mounting holes are in the wrong position?

If you look in your Bentley Manual section 13 pages 35-36, you will see that BMW do eccentric bushes for the adjustment of toe at the rear wheels. I wonder if you have eccentric bushes fitted? Or perhaps you could fit these as a solution? Are your rear wheels parallel or not?

Do you have proper BMW trailing arm bolts fitted? These have an slightly enlarged shank to locate snugly in the metal centre part of the bushes.

My understanding from reading books on suspension set up is that the most important aspect is the geometry of the rear suspension and wheels, their toe-in or out and their alignment with the front wheels. There are 4 wheel alignment jigs available at some tyre fitting bays and I am going to wait until my car is on the road and the bushes have bedded down after a few hundred miles before having mine measured. (Great article about this in Car Mechanics, April 2006, page 24)


Best regards,

Peter

[Patrick]

I suppose the other question is - what are the ideal toe and camber settings for the Sportster? Do you just mirror the donor car?

[MartinClan]

Unfortunately my spring mounts are definitely in the wrong place. With them in the current position the two bump stops miss one another completely. From discussion with other Sportster builders it seems that there could have been a number of chassis built arround the same time as mine with the same problem...

Still agonising on wether to give them the chop...

I am a bit of a picky b*gger - I like things to be right or at least the best I can do so I think they might have to go.

On a slightly brighter front I have almost sorted the spec for the rear springs now - but it would be good to have some actual measured weights before I commit - anyone????