Parallel 4 Link Spacing

[Simmo]

I'm in the mock-up/planning stages of a mini-tub/4 link build. Trying to package a sway bar, watts link, coil overs and four link brackets into a narrowed 9 inch rear is proving to be a bit of a challange....haven't even thought about the exhaust yet lol.

My question relates to what are the negative effects of having the 4 link brackets relatively narrow (28 inches) as opposed to as wide as possible (rails are 38 inches inside/inside). I can run them wider by incorporating them with coilover mounts - which will be set out at the extremities - but that will require a re-work as far as the swaybar is concerned (upper arms will conflict when set @ high AS #'s). I get how this would effect roll centre for a T4L for a given UCA angle...but I'm a bit lost as to how it works with the bars parallel in plan view. Oh, and ignore the parallel bit it the title, it'll be parallel in profile view only some of the time, kind of an adjustable 4bar/4link hybrid.

If someone has come across a good book that covers this sort of thing I'd appreciate a link....got a few gaps in the skill set at the mo :)

[Norm Peterson]

In your specific case, the narrower you set the wider-spaced pair of links - either the uppers or the lowers - the greater the steer effect for any given amount of compliance gets. Compliance here includes all of the link bushing compliances and at least the link pickup point bracket flexibilities.

There isn't any theoretical difference as far as either roll center height or axle roll steer are concerned for either wide-set or narrow-set plan-view link spacing, as the convergence points of the upper and lower links are both at "infinity" either way. Small fabrication deviations from perfect plan view parallelism won't affect this much.

For both structural and control of axle steer reasons, you're better off going wide with the links and making the rest fit to that. There are other ways to recover any roll stiffness lost to less than optimum bar mount/endlink locations.


But unless you're using at least one birdcage I'm afraid that the four links and the axle are going to make for a pretty big "swaybar" all by themselves.


Norm

[Simmo]

Thanks for the reply Norm...was kinda hoping you'd chime in.

The whole purpose of the rear is to be dual purpose. I just want it to do a GOOD job at both the strip, and the track. I understand there will be compromises here to gain this flexibility but I doubt the design compromises will be as detrimental as the compromised driver!

Re the steer effect - got ya, so say for a given amount of compliance, there will be more amplified effect at the wheel the narrower your links are spaced. I can see that :)

What I envision is something very similar to this minus the 3rd link set-up, I dont think it will be required in my case, and will occupy space I plan to use for a fuel cell.

63034

I'm trying my hardest to retain the bed (El Camino) and stock appearance - minus the subtle mini-tub. Plan is to use lower profile rails to suit the A Body and the mounting the links inboard of the rails to avoid a large kick-up in the frame rails. I was trying to use a CW frame mounted swaybar (makes it sprung weight...not that it'll make a huge difference though!) like this:-


63035


but as you say, I think i'm going to be better off using that space for brackets and coming up with another way to add a swaybar....back to the drawing board there!

Will be using JJ's/Rotojoints to try to minimise bind, and a watts for lateral location. I'm just shopping for the parts at the moment as the hard numbers will will be done by my builder, I just want to get a shipment of gear that he can maximise the potential out of what I get...and it would be good to get a better handle on the physics at play here.

I'd appreciate your thoughts on what I'm tryin to achieve!

[Norm Peterson]

Four parallel trailing arms would all need to be equal length and at least parallel in side view in order for them to not turn the axle into a 3" tubular stabilizer bar.

Ideally, they'd be inclined at an angle consistent with the axis about which the car actually rolls (this is not necessarily the same as the "roll axis" as defined by the front and rear "roll centers").


Norm

[Simmo]

Yeah the brackets will take a while to work out, I figured the joints would be enough with say 3-4" bump travel. I never thought of putting the uppers "tops tipped in" but I guess it's got to help minimise any bind. Thanks