Lotus Elan

Modifying rear suspension arms

PostPost by: drtarkir » Tue Jan 10, 2017 3:54 am

I am finally ready to do the suspension on my +2S and I want to modify the rear suspension arms so that I can adjust camber and toe. I have looked at the options available from the various vendors and the mods others have done and I feel that what has been done is not enough. It seems to me as if everyone has cut one arm and added an adjuster. I think that this is only a partial solution as this will twist the unit more than adjust it.

Below are 2 rough drawings. The first is the original arm and the 2nd is what I am considering building. The red lines are where the kinks are and the yellow lines are where I am planning to cut.

By cutting the arm completely and using 2 adjusters I should be able to more accurately adjust the camber and toe. I would appreciate any comments or concerns.

Dennis
Attachments
arm-original.jpg and
arm-mod.jpg and
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PostPost by: worzel » Tue Jan 10, 2017 11:03 am

Hi

Don't cut- try to contact Mike Duff.

I recently sold him a pair of one-off rear frames (for a 2 seater) that utilises the original metalastic bushes but with the adjustment possible on the car and without distorting the tubes in any way. They were made by TTR for me but I never got around to fitting as I recently sold the car.

If you trawl thru past posts he goes under the handle Miked- he posted pretty recently if I recall.


Regards


John
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PostPost by: john.p.clegg » Tue Jan 10, 2017 12:22 pm

What seems blatantly obvious to me is that if you cut as per diagram the bottom adjuster will be in alignment but the top one will be askew...

John :wink:
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PostPost by: vincereynard » Tue Jan 10, 2017 6:25 pm

Why not simply have them adjustable at the chassis mounting end. Then you can adjust both camber and toe in. The requirements are quite small.

screenhunter_21-oct.-24-14.23.jpg and
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PostPost by: drtarkir » Tue Jan 10, 2017 10:19 pm

As John stated I have some concern about the angle between the arms.

Also, I prefer to work on the wheel end to avoid changing the downward angle. And I also want to install utilize jackscrews so that the arm can be adjusted in situ.

Below is another option.
Attachments
arm-mod2.jpg and
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PostPost by: drtarkir » Wed Jan 11, 2017 5:14 am

Ya, YA, YA I agree. That last one was done with no thinking. Below is what that was meant to be.

The distance between tubes A and B is 1.5" to clear the rotor
Tubes B and C are 1" diameter. This allows me to drill them for the threaded inserts D and E. F and G are also threaded inserts
H and I are 1/2 x 20 x 3.4" double threaded jack screws

By working at the wheel end I am avoiding the rotor clearance kinks in the tubes. Also this end is in a flat plane making jigging and alignment easier.

Yes I know that it will add some weight but I am planning on Wilwood brakes and minilite wheels which should balance out.

If you think this may be overkill I have just been through something similar. I also own a Unipower Gt that I got back on the road last year. It also has a funky chassis and the suspension had no camber adjustment capability. When I first got it together the camber was a mess and I had to redo it with adjustable joints to solve the issues. I decided at that time never again.

Dennis
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arm-final2.jpg and
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PostPost by: john.p.clegg » Wed Jan 11, 2017 8:51 am

H and I look like stress points to me...

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PostPost by: toomspj » Wed Jan 11, 2017 2:07 pm

That looks about as far away from Chapman design philosophy as you can get!. You might think TTR solution is not great from a pure science, stress point of view - but it works. And it has been tested many times on the track with pretty sticky tyres. So, I'd vote for the simple, reasonably elegant proven solution.

You could also go the 26R route, which uses threaded rose joints, and just substitute bushes for joints, if you're worried about longevity or transmitting harshness?

Or, if this beast is being used primarily as a road car, how about you just adjust the wishbones to give you desired camber and toe-in. How often are you planning to change the angles?

I have another proprietary solution that is simpler than any of the above, and gives adjustable toe and camber without removing wishbones, and without modifying the steel at all.....

Paul
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PostPost by: Orsom Weels » Wed Jan 11, 2017 3:46 pm

Forgive me, but you do seem to be over complicating things a bit or I'm not understanding what you're trying to achieve? Or both

In your first post you state
?The red lines are where the kinks are and the yellow lines are where I am planning to cut.?
What kinks? There shouldn't be any kinks where you have drawn red lines, they are straight bits of tube.

Then later you say
?I prefer to work on the wheel end to avoid changing the downward angle.?
What do you mean by that?

Vince's (vincereynard) suggestion above will achieve toe & camber adjustment with the A frame on the car & won't introduce any unwanted stress points, nor will it interfere with brake disc clearance in any way or change any other suspension geometry. If I remember correctly, it's also pretty much what John (worzel) had for sale a while back, made by Tony Thomson. What more do you want/need?

Tim
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PostPost by: Peter +2 » Wed Jan 11, 2017 5:30 pm

OK Paul, I'll bite, what is your secret?

Knowing your a racer I would be keen to hear.

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PostPost by: patrics » Wed Jan 11, 2017 9:05 pm

Hi,
From my point of view I would build the car and then get it measured - who says it needs adjusting might be good enough for the road.
On my race car I just measured it and then using a 4 jaw chuck machined nylon bushes to give the right off set

I have since gone for spherical joints but in reality I don't think it has actually changed much regarding toe-in but now I can easily change camber

I would not modify the arms on the wheel side as it looks to weak - modifying at the chassis will be a far stronger option.

Regards
Steve
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PostPost by: vincereynard » Thu Jan 12, 2017 4:27 pm

drtarkir wrote:Ya, YA, YA I agree. That last one was done with no thinking. Below is what that was meant to be.

The distance between tubes A and B is 1.5" to clear the rotor
Tubes B and C are 1" diameter. This allows me to drill them for the threaded inserts D and E. F and G are also threaded inserts
H and I are 1/2 x 20 x 3.4" double threaded jack screws

By working at the wheel end I am avoiding the rotor clearance kinks in the tubes. Also this end is in a flat plane making jigging and alignment easier.

Yes I know that it will add some weight but I am planning on Wilwood brakes and minilite wheels which should balance out.

If you think this may be overkill I have just been through something similar. I also own a Unipower Gt that I got back on the road last year. It also has a funky chassis and the suspension had no camber adjustment capability. When I first got it together the camber was a mess and I had to redo it with adjustable joints to solve the issues. I decided at that time never again.

Dennis


No offense Dennis but that is a horrible design. The whole bending moment will be on H and I. It would probably distort somewhere about tube C where it is put under torsional twist by E.
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PostPost by: vstibbard » Sat Jan 14, 2017 12:53 am

If you want to be able to adjust to TTR solution without removing the wishbones, i.e. in situ, all you need to do in use a barrel nut arrangement.

I've done this on my 26R rear wishbones for that exact reason as we were trying different tyre types and got sick of removing refitting wishbones. It also allow us to use smaller high quality rose joint removing weight...

V
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PostPost by: rgh0 » Sat Jan 14, 2017 1:11 am

Structurally the Elan rear suspension A frame arm is a real compromise to begin with as it does not have a true diagonal brace to fully carry the fore / aft thrust loads from the wheel to the chassis. The un-braced ends of the A frame are in bending as a result of this compromise design and this is where they typically break. If you add adjustable links to replace the fixed un-braced ends then the adjusters are in bending and you need to ensure these are rigid and strong enough to withstand that bending force.

cheers
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PostPost by: stevebroad » Sat Jan 14, 2017 4:02 pm

Looked at TTR wishbones, didn't like the concept - not good engineering practice IMO.

Decided to replace rear chassis mounting bush with a rod end (which is turnable between the chassis bracket so no removal required other than the bolt and a couple of spacers. The small amount of adjustment I am anticipating will be easily accommodated by the existing bush. If I am proved wrong then the bush will be replaced with a rod end.

Only did the one end as camber is adjustable via the top wishbones on my setup, but if you do both then camber would be adjustable with no additional stresses to the bone.

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