Spring Selection for rear trailing arms

[jimmypet]

I race a BMW E30, I have for over 20 years.
Over the last winter I had Chris at Angry Sheep fulfill a long long time wish of recreating a DTM rear subframe and trailing arms.
What we didn't do was switch to a true coil over like the original DTM cars had.
He did a modification to the rear trailing arm that was done on some of the fast EP Z3s and recessed the rear spring down into the trailing arm, and added an adjustable screw jack to the bottom to make weight adjusting super easy. He did a really nice job.

QUESTION: The recessed adjuster (at its middle of its range of adjustment) added 60mm of length required to the static spring length to get back to where I was before this modification.

My original spring was an H&R 120(h) 60(id) 160(rate) + a 20mm thickness of the old seat and adjuster
My head is saying I now need a 200(h) 60(id) 160(rate)

Am I missing any suspension guru spring physics knowledge that says when I grow the spring length I need to change the spring rate?
Or is it as easy as just adding the length to get back to where I was previously?

Also to further complicate it H&R doesn't make a 200mm x 160nm, they make 200 x 180nm -or- 200 x 150nm
I was thinking to try the softer 150 because the delta was less than going up to the 180nm.

Thanks for your suspension knowledge.
Cheers
Jimmy P

 

[GT6]

Why recess unless you are lowering or going for more suspension travel?
Is it to correct the spring perch angle?

Only issue with longer spring (that is otherwise the same as current) is increased weight and more likely to buckle.

60mm I.D. Race Springs | H&R Special Springs, LP.

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[jimmypet]

I'll be very honest I dont know what the initial reason was to recess the spring.
It was something that was done on some of the faster EP Z3s (which are the exact same rear suspension) and so when we did the plan for modifications to the trailing arms he told me about it (I had already heard about it from Chris Dryden) and I said yeah if its better lets do it.
I'll be very honest in that I dont know "why" they were doing it, so my reasoning was simply well while we are in here...

The recessed pocket does now have a nice, very easy to adjust articulating string perch in it that helps keep the spring properly oriented through the swing of the trailing arm.
Before at the ride height the car is at the spring was a tiny thing that always seemed like it was a few MMs to coil bind (although I have never seen it in action) so the longer spring seemed like maybe it would be better and the jacking adjustment is WAY easier so there is a win there.
I'll be able to do it with a 17mm wrench laying on my side on the ground if I ever wanted to.

Thank you for the links to the alternate springs, I'll look into those options.

 

[Al Seim]

Hi Jimmy -

If I understand correctly, the question is "does the spring rate change is a spacer is added to change the length?". No, a good race spring should have the same rate Or at least very close) at any compression.

 

[jimmypet]

Thanks Al,
Thats what I am gathering from responses I have gotten, that by going to a longer spring, I should just stay with the rate I was using (or close as possible) if that was working for me, just get one that fits the length of the new application.

 

[jimmypet]

More options with Hyperco.

Shop Metric Coilover Springs | Metric Springs for Sale | Hyperco

Buy metric coilover springs online. Available in various free lengths and rates, our metric springs are high-performance aftermarket suspension springs.

www.hypercoils.com

Thanks for that link Anthony, Hyperco had one that was closer than the H&R offerings so I'm trying a set of those first (and they were in the Summit Pro Shop system so with the discount it was much better than the H&Rs).
Thanks again.

 

[Gordon Jones]

I think the reason for the recessed trailing arm is so that you can use a normal spring without bottoming out. The stock springs are what is known locally as football springs due to their shape.

 

[jimmypet]

I think the reason for the recessed trailing arm is so that you can use a normal spring without bottoming out. The stock springs are what is known locally as football springs due to their shape.

Yeah I believe getting away from the short 120mm tall spring was part of it. We got away from the stock type balloon spring a long time ago, but with the ride height the car was at the 120mm spring had not alot of compression before coil bind. I think this may be part of the reason.