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]

Are you just replacing the factory links with adjustable links, or are you changing the mounting points as well.

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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[SubieDude]

Thanks for the response and questions.

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.

 

[patprimmer]

Looks OK to me then, but I am not a professional suspension guy.




Regards
pat

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[GregLocock]

That looks like a typcal race car setup. Note that your camer control is via the strut, and that brake torques are also resolved via the strut.

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

 

[SubieDude]

Thanks for the help.

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

 

[patprimmer]

I don't know if it matters, but it will also increase the track width

Regards
pat

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[SubieDude]

But only to a minor degree right? I would have to measure to see what an additional degree or two would do to the bottom of the tire.

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

 

[patprimmer]

My only concern would be if it affects guard clearance if they are already very close, or if it puts it outside a class rules maximum measurement. Otherwise it is just nitpicking on my behalf.

Regards
pat

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[GregLocock]

Yes, that will work, but note that your drag link will now come into play, changing your toe in bump curve.

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

 

[GregLocock]

Also it might be worth noting that if you replace all the bushes by spherical joints in the original suspension then it is overconstrained and will be relying on the elasticity of the metal to move. It might be best to leave the drag link mounted using rubber bushes.



Cheers

Greg Locock

 

[SubieDude]

Exactly my thoughts 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.

Thanks for all the help guys. I will post some pics and some alignement results when we get it done.

 

[JakubMech]

Instead of making the 2 lateral links longer to gain more negative camber, why dont you use 2 offset camber bolts to get more negative camber, one in the top position, the other on the bottom position on the strut/hub interface!

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

 

[SubieDude]

Jakub,

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 )

Thanks for the suggestions everyone.

J

 

[shanba]

Adjusting through the lateral links may require significant change to get 2 deg change in camber. Essentially it is the ditance from the top of the strut to the lower link times 2 deg*pi/180. I'd be worried about maintaining the half-shaft in the diff if the track change gets to be excessive.

Can you have multiple mounting points on the subframe for the lateral links?

 

[SubieDude]

Yes, are actually using a one off subframe. We can mount the links as required.

J

 

[shanba]

The reason I asked that was that you could possibly change camber gain and RC height between stages if one stage was on loose stuff and the other on paved stuff. Probably want more camber gain on paved surface and therefore higher inner hardpoints would do this. Higher rear RC would give effectively stiffen the rear end up too, desireable direction of balance change for paved surface. Just a thought. It may be more effort than desired to get alignment right when doing something like this.

 

[SubieDude]

Honestly this sound sounds like a great idea. So if we did this the only adjustment we would need would be for toe changes, which is very minimal. I think we will pursue this idea.

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

 

[GregLocock]

Get your spring testing machine (or a hydraulic ram). Get a copy of one of the arms that failed in buckling. Find out what the load is that buckles it. Design new arm to be 50% stronger.

Until you know the failure loads you won't be able to design a sensible new system.




Cheers

Greg Locock

 

[SubieDude]

Most of the failed links were a result of not enough ground clearance and the link failed from a force perpendicular to its length. I will just start bending links and see what it takes.

J