Lateral Link replacement

[SubieDude]

Hey everyone.

First off hey, first post. I will apologize if this thread is confusing, as I am not really an engineer. I am however a race car builder in the Pacific NorthWest.

We are currently building a Subaru Impreza to compete in Rally Competition nationwide. The rules of the organization allow us a lot of freedom for suspension design. We have had a discussion the last couple of days regarding replacing our factory style lateral links with adjustable units comprised of hollow tubing and spherical rod ends. Obviously material choice here would be important, but are there any other concerns we need to address.

The factory rear suspension is a trailing link with parallel lateral links and a strut.

Is there any reason to think this would not work?

The links are connected with a single bolt at each end passing through the suspension upright and both lateral link ends.

Thanks for the help, and I hope I have not made a fool of myself here.

 

[patprimmer]

If you thread directly into the tube, you will reduce the strength significantly, as you will be creating a notch at the root of the thread.

You should weld a bung into the end of the pipe, then thread the bung. I think you can buy bungs that are already threaded.

Regards
pat

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[Joest]

SubieDude,

I would suggest chassisshop.com for 4130 threaded tub inserts. If us use tubing thick enough to thread directly it will be way to strong to prevent subframe damage. What you really need to do is calculate the critical load for buckling in compression and also evaluate the tensile stress to see how the tub will fail. Analyzing the subframe will be more difficult since it sees more than just tension and compression loads, but also bending. Since it don't look like you have any structural engineers working for you, your best be will be to make some mockup assemblies and test them.

-Joest

 

[GregLocock]

Practical construction details:

Use seamless drawn aluminium tube.

Loctite 600 steel plugs into each end. Do a pull test on one to convince yourself that the aluminium will fail before the bond.

Use left hand thread for one end, right hand thread for the other. Spray paint them so you know which is which. Your mechanics will think you are a god.

Use aircraft grade rod-ends, if you can get them in metric thread sizes.



Cheers

Greg Locock

 

[patprimmer]

Greg is spot on re the testing. You have to have a baseline to make any sense of it.

The way I have seen off road racecars develop. is you start off with everything as strong and light as you can get, then you keep reinforcing the points of fail. You then try to leave an easily replaceable part slightly weaker, and have it so if it fails, you can still limp home.

I guess thats where your at with the arm bending rather than the arm breaking at the end of the thread, which will happen if you thread the pipe directly.

As mentioned elsewhere, you will need to look at half shaft attachments and joints to see how the change in track with suspension travel affects them. We used to assemble the suspension sans springs and move it through full range and look closely and feel, at all points for binding and fit. We would also rotate the wheels while doing this to ensure the CVs were OK at all points.

If the lengthening of the laterals stretches the CVs orr splines to much, you can also pull the strut top in a bit. You might need to modify the towers to allow for the adjustment. If you elongate the holes that mount the strut to the tower, you might need an adjustable brace or ecentric washer etc to prevent slippage at the top strut mounts.

Regards
pat

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[shanba]

If there's room at the top of the strut mounting, you might consider a camber plate like what is used in the fronts of a macpherson strut cars. Check out

http://www.ground-control.com

 

[JakubMech]

Have you considered using shim type adjustemnt on the in-board side instead of thread type adjustment. It would save heaps of time in adjustment and much more accurate.

Also someone does small rubber (washer type) boots for rod-ends, which should be pretty good for rally style racing!

Also if you find that using a LH and a RH rodend gives too much length increase per revolution, use rodends with different thread pitches, and for finer resolution use same thread type (RH) at different pitch's.

Jakub

 

[Tmoose]

"...snip ..... the drag link will retain its softer rubber bushings, but be inforced to take the abuse that a rally could give it. In stock form its a stamped c channel, but we will go ahead and box it in."
Leaving it a channel means it is about 100 X times less torsionally stiff. That flexibility may be a good or even necessary thing as far as the end bushes and suspension compliance are concerned.

GM passenger RWD cars like Chevelles used 4 (u-channel) links to locate the rear axle. HOt rodders eagerly boxed them "for strength" and ended up with strangely stiff or harsh action over bumps and in corners.