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A Question for the Suspension Geometry Gurus


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We are working on developing some control arms and tension struts for race cars (also street cars don't worry, but this question only pertains to the race crowd). Now, is there a reason that the control arm should not be mounted in the middle of the sub-frame instead of behind it? Note that the heim joint and control arm will clear and move freely and that if the control arm was straight across instead of an "S" shape it would bring the cost down a bit. We understand that the triangular shape of the geometry would get slightly narrower, we are just unsure of the pros/cons of this. Pictures for reference.

E4E87B18-0E8D-4369-AFBA-903EB7C703BC.jpg

IMG_6842.jpg

Edited by Ireland Engineering

'67 1600 B Sedan Race Car #5

'72 2002 B Sedan Lite Race Car #13

'83 E30 323i Bauer M30B35 Swapped

 

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probably has to do with the CASTER angle with the original geometry but since you can shorten/lenghten the track rod i guess its non relevant. i own a hunter K111 alignment set up and played with it a lot. i know what i am talking about.

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17 minutes ago, PatAllen said:

probably has to do with the CASTER angle with the original geometry but since you can shorten/lenghten the track rod i guess its non relevant. i own a hunter K111 alignment set up and played with it a lot. i know what i am talking about.

Thanks for the input! Yes, I agree. The original arm has 2 slight bends in an "s" pattern and mount rearward of the subframe. If you had an arm that mounted in the middle of the 2 "shears" of the subframe towards the rear of it (using a spacer to take up slack), you could use a perfectly straight-across control arm and end up virtually in the same ball joint position as stock (no caster adjustment). But as you stated, with the adjustable tension strut that could always be adjusted.

'67 1600 B Sedan Race Car #5

'72 2002 B Sedan Lite Race Car #13

'83 E30 323i Bauer M30B35 Swapped

 

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I guess the real question we want to ask is if the geometry relationship between the OEM control arm and OEM tension strut is that way for a reason? Would we be compromising or weakening anything by, in a way, bringing them a little closer together?

 

PS. This race set-up we are developing we are trying to use a re-buildable ball joint, and you will have various lengths to chose from that will change and correct bump steer (no need for spacers).

 

PPS. We're going all heim joint suspension pretty much because as most racer's know the OEM '02 suspension has a lot of bind and restriction in it especially due to the fact that the car is extremely lowered and cambered.

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'67 1600 B Sedan Race Car #5

'72 2002 B Sedan Lite Race Car #13

'83 E30 323i Bauer M30B35 Swapped

 

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The 2002 doesn't really suffer from bump steer, stock or properly modified.

It does have roll center issues in front.

 

I've never run across binding problems with it either-  obviously, if you do something dumb

like put an aluminum spacer in the tension rod, yes, but even hard urethane doesn't bind it enough

to make spring changes difficult.  Stock, it all moves pretty freely, even lowered.

 

Bringing the rear point forward will increase forces on both the tension rod and the 

lower arm significantly.  I wondered, too, why BMW chose to leave the lower arm in single

shear, and I suspect it came down to reducing rearward deflection at the ball joint with rubber

bushings, as in, with a rubber bushing there, there would be too much movement of the front

wheels under braking.  

 

t

 

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12 minutes ago, wheelieking said:

It's hard to tell from the pic, but could the control arm end up bottoming out in the middle against the edge of the subframe when it's at full compression? 

Not really, at least not in any more danger than the original configuration would be. Plus on a race car the suspension does not really travel a whole lot.

'67 1600 B Sedan Race Car #5

'72 2002 B Sedan Lite Race Car #13

'83 E30 323i Bauer M30B35 Swapped

 

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On 2/2/2020 at 10:40 AM, TobyB said:

Bringing the rear point forward will increase forces on both the tension rod and the 

lower arm significantly.

^ This is bad but also I don't like how it would change the line through the pivot points (red lines). It would affect caster gain in a way that I find uncontrolled - much dependent on ride height. It would be ok if you could figure a nice way to move the tension rod mounting inward, towards the blue line, but that would require heavy modification to the subframe itself. In this case I think it's better to leave the arm where it is.

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Modul Motorsport (site is offline) had quite good pics of the homologated A Arms / Triangular suspensions on their site.

But they all ran rack&pinion steering.
I'd look into the homologation forms and try to copy a homologated front axle which would also increase the chances of selling it outside the us.

 

 

 

 

2020-02-04 11_12_08-homologation_form_number_1663_group_2.pdf.jpg

Edited by uai
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my english is not so good, so translated with google: i think when driving through a curve the front rubber bearing is loaded forward, the wheel swivels in toe-in and stabilizes the car. For this function the wishbone must be mounted in the rear.

Greetings, Karl-Heinz

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Interesting ideas.

 

Racing community: Perhaps it’s worth checking whether your racing organization rules allow for non-factory control arms, spherical bearings, or changing of suspension pick-up points. (Example: current VARA BS / BSL rules do not allow the changes mentioned herein, though they may be legal in GT classes). -KB

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We are playing with several setups.  Some with factory arm and some with a tubular arm.   I have seen a couple configurations like Russ posted.  Nice and simple.  I just questioned hard bolting the thrust rod to the control arm at the ball joint.  As the arm moves through it's arc it seems like there would be a fair amount of twisting force.  I'm leaning toward using a rod end on both ends of the thrust rod.

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51 minutes ago, jireland2002 said:

I'm leaning toward using a rod end on both ends of the thrust rod.

That is not a good idea. Too many effective degrees of freedom. When you brake your contol arm will turn and every geometry is f***ed up

Edited by uai
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