Lateral Link replacement
Lateral Link replacement
(OP)
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.
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.
RE: Lateral Link replacement
Is the diagram the factory set up, or your proposed set up
Are the green and red lateral links equal length, parallel and at equal height to each other at each end.
If they are different lengths, you will get bump and roll steer.
I expect that most competent rally drivers do not like "passive 4 wheel steer" as they would sooner control all steering input themselves.
Regards
pat
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RE: Lateral Link replacement
We are hoping to replace the factory setup with adjustable units for toe and camber adjustment. The diagram I posted would represent both versions.
The green and red links are parallel to each other and the same length. They mount to the same position on both sides of the upright and rear differential mount. Other than sway bar mounts they are interchangeable.
What I am hoping to do is get rid of fixed length factory versions and replace them with adjustable length all metal units with some sort of spherical rod end.
RE: Lateral Link replacement
Regards
pat
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RE: Lateral Link replacement
You might consider making things more complicated by mounting the shock to the hub via a ball joint, and then adding a camber control arm and a second drag link, if you want to keep everything orthogonal.
Cheers
Greg Locock
RE: Lateral Link replacement
You state that camber control is via the strut. From the factory the strut is mounted with to bolts through the upright and does not provide any camber adjustment. In the past we used an offset bolt to adjust the camber in the top hole. The changed the wheel camber without effecting the angle of the strut, but we are limited in the ammount of adjustment we could get this way. On tarmac stages, more camber may be required. In order to achieve this, would it not be possible to lengthen the lateral links to push the bottom of the upright out from center. This would increase the camber angle of the wheel and strut as a unit. The struts contain a spherical bearing at the top mount, so there would be minimal binding there.
Does this sound correct, or would it just make a mess of things?
J
RE: Lateral Link replacement
Regards
pat
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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Lateral Link replacement
Really we are talking about a max adjustment of +- 2 degrees. With an offset of 1/8" total (both sides cumulative) for toe adjustment.
J
RE: Lateral Link replacement
Regards
pat
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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Lateral Link replacement
That is exactly what I meant about orthogonality - the ability to change only ONE parameter at a time.
I must confess I don't think it is a big deal, in my opinion rear suspensions mostly just follow the car around (grin).
Cheers
Greg Locock
RE: Lateral Link replacement
Cheers
Greg Locock
RE: Lateral Link replacement
Thanks for all the help guys. I will post some pics and some alignement results when we get it done.
RE: Lateral Link replacement
You effectively double your camber range.
Another option, cut of the flanges on the strut and get new ones on their with red-rilled holes for the hub.
Jakub
RE: Lateral Link replacement
That is the common approach that a lot of teams have been using, either slotting the holes in the strut flange, or reworking the upright.
This presents a couple of problems to us. During a race there are only two 20 min service times allowed. During this time we have to fix anything that gets broken and prep the car for the next set of stages. In order to adjust camber using the two bolt method or slotted strut method, the wheels would have to be removed, the strut adjusted, and the wheel remounted. If everything is good with the car, there may be more than enough time to get this done on both wheels. By going to an adjustable link, one mechanic can access the links from under the car and adjust both sides, without interfering with any repair or service work going on at the wheels or brakes.
If there is a reason not to build the stronger adjustable links then we will probably have to rework the strut flanges.
Last year we went through a lot of lateral links, after they were bent. In rally racing the rear end of the car may spend a lot of time traveling sideways to its intended direction. The high torque numbers we generate and the loose surface material often lead to large ammounts of oversteer during cornering. (And the crowds love it :D)
Thanks for the suggestions everyone.
J
RE: Lateral Link replacement
Can you have multiple mounting points on the subframe for the lateral links?
RE: Lateral Link replacement
J
RE: Lateral Link replacement
RE: Lateral Link replacement
Now, on to the next question.. how strong do I need these to be? I found a source for some nice chromoly spherical rod ends. They are intended for off road 4x4 suspensions, so I think they should be tough enough, but what about the pipe in the middle. I would like the links to be strong enough to take the abuse and not fail, but I also would like them to bend before the subframe does. This sounds like a pretty fine line, I know.
Do you guys think a thick wall seamless DOM tube would be sufficient? I could get the tube threaded and just thread the rod ends into it directly.
Should I ask this in the materials list instead?
Thanks for everyone's ideas!
J
RE: Lateral Link replacement
Until you know the failure loads you won't be able to design a sensible new system.
Cheers
Greg Locock
RE: Lateral Link replacement
J
RE: Lateral Link replacement
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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RE: Lateral Link replacement
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
RE: Lateral Link replacement
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
RE: Lateral Link replacement
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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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Lateral Link replacement
RE: Lateral Link replacement
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
RE: Lateral Link replacement
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.