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I am finally getting serious about finishing an RM8 part built kit that I bought a few years ago. A lot of things got in the way, I was building in a single car garage, I learnt to fly, I painted my house Blah blah blah....

I am now installed in my (very messy) workshop on Texada Island BC and am putting some time on the car (though I am also working and building a hangar and....)

I bought the car back in 2010, put a Miata motor and gearbox in, put in a pedal box and then really stalled on the rear end. The car came with a Sierra diff, and sandcast uprights but no drive shafts, brakes or shocks. The A arms were there, with a mix of rubber bushings and rose joints, but nothing fitted or lined up. After a lot of time messing around I decided to start from scratch and bought a 7.5" Ford rear end and am building a four link setup. I have that about done now, and am just about to tack on the shock mounts.

One question that I have is whether at ride height the shock should be fully extended, or should there be room for the axle to drop further? I have a pair of 12" shocks with 10" springs (I bought these for the old rear end, 10" would be plenty now).

No the shock should not be fully extended at ride height.

Your suspension design should allow for some droop as well as compression or you will be banging against the shock extension limit in normal driving and it will pick up wheels in chassis roll.

I'm not sure what you should design for but 1-1.5 up and 2.5-3 down seem like reasonable numbers.

Maybe someone will chime in who has more experience in design.

Since your 4 link rods are short and not adjustable your ride height is fixed and working height will be somewhat limited.

Since your rods are parallel, before you finalize your rod placement do an instant center calculation.

I would add that you should make the panhard or watts link adjustable so you can fiddle with rear roll center.

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Honey anyone?

More down than up?

yes and there are other concerns see my edit.

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Honey anyone?

Charles

Here is some good info on your question, I cut a pasted the shock mounting info from Alden's site. This is how I set up my cars.

Hope this helps.

http://aldanamerican.com/suspension-installation-guides/

The more vertical the shock, the firmer the ride; the less vertical, the softer the ride (but less support, especially on cornering). We recommend a mounting angle of 30 degrees from true vertical at ride height. Generally speaking, the weight of the car will collapse the shock 1-1/4 to 1-1/2 inches at ride height. Using this rule of thumb, you will achieve a 30 degree angle at ride height if the shock is mounted at 26 to 27 degrees when it is fully extended (Figure C). When designing your suspension mounting points, extend the shock to 2/3 at ride height; i.e., if stroke is 3 inches, set the car up so that at ride height, the shock as room for 2 inches compression and 1 inch rebound. Do not design your suspension with the shock fully extended and the car at rest–allow it to sag, otherwise damage from excessive rebound force may occur.

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Perry

'If man built it, man can fix it'
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Great, thanks for the replies.

I tacked on the shock mounts with enough travel up and down. The shocks are nearly vertical, and when I put the car down all seems good. I need to put the body back on to check wheel arch clearance before I weld everything finally.

My 4 links are not quite parallel, I read that they work better if they are not?

The panhard is short I know, but I did not want to beef up the rear subframe enough to mount to it, and I could not find a nice clean path for a full width rod in front of the axle without compromising ground clearance.

I'm afraid that I am of the opinion that pretty much anything will work, and am far from knowledgeable about all the technical stuff. These cars were pretty basic in the 50s and 60s, and were still a lot of fun. I think it is too easy to get all hung up with suspension theory.

Are you saying 1-1.5" bump travel and 2.5-3" droop travel?

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-Andrew
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