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rear control arm question

PostPosted: Fri Feb 16, 2007 4:22 pm
by pinsx3
Are there any engineers out there? I was thinking: why not have two independent adjustable arms for the rear suspension. The Chapman strut eliminates the rotation forces caused by the power being transmitted to the wheels. the suspension arms merely stop lateral fore and aft movement. Would not simply two, totally independent adjustable rods accomplish the same thing as our braced suspension arms? This would make adjus susp easy.

Pete ([email protected])

Re: rear control arm question

PostPosted: Fri Feb 16, 2007 6:36 pm
by MintSprint
The Chapman strut doesn't eliminate rotational forces... the rod can rotate inside the damper quite happily, so without the lower wishbone the rear wheels could 'steer', and the Lotocone is soft enough that the whole strut/upright assembly would rotate in an arc about the centre of the Lotocone if the wishbone wasn't restraining it.

You need the diagonal brace on the lower 'wisbone' to prevent everything folding up like a pack of cards.

As I'm sure you're aware, the conventional approach is to use rod ends and/or a turnbuckle in one of the tubes to allow rear camber and toe adjustment.

I'm slightly confused by your terminology, though, so I may not understand you correctly. What is 'lateral fore and aft movement'... is that, erm... 'longitudinal'?

Re: rear control arm question

PostPosted: Sat Feb 17, 2007 8:38 am
by rgh0
if you have two lower links that met some distance apart at the hub the parrallelgram formed is not rigid and would allow fore / aft movement.

If you had two lower links that met at a common point on the hub the triangle would be rigid and locate the hub preventing fore / aft movement. However the hub would be able to rotate around the single point where the 2 links met and thus not point straight ahead as required. To stop this you need to add another link that connects to the hub some distance from the triangle.

Once you have done this you have in essence the A frame that Chapman used - about as light and simple as possible to create a rigid suspesnion.


Double a-arm rear suspension

PostPosted: Mon Feb 19, 2007 2:10 am
by mct340
Circa 1984 when I visited Spalding to pick up a new Spyder frame they talked me into a double a-arm coil-over set up they had invented for the Elan.
I've been running it for years and it does have advantages. The shock mounts farther inboard to give much greater tire clearance. The hub is smaller and easier to service. The shocks and springs can be changed in a couple of minutes.
I know you'd get punted for this in vintage racing but in autocross you only get moved up to compete with the big boys. They don't stand a chance.
I can hear the teacups rattling over in old Blightey but hey, a Brit came up with this mod.
I'll snap a few pictures of it and post them sometime soon.
Thanks for all your valuable tips here,
Mike (strutless in Canada)

rear control arm question

PostPosted: Tue Feb 20, 2007 11:11 pm
by pinsx3
Don't dismiss my crazy idea so quickly. I know it was risky to even post this, but how could they rotate or "walk" fore and aft? I mean the chapman strut is cast alu and attached to the frame. The lotus arms are not what is stopping rotational forces, but rather the strut. The arm would not be strong enough to stop rotation alone.

As far as fore and aft forces, the tire walking forward into the body, the two independent would remain in control just as before. The rods would not let the chapman cast alu carrier move forward or "walk" up into the wheel well because they make a triangle at the bottom using the carrier as the apex.

As far as steering -- how could that happen? the frame would not allow the rod ends to move, so the chapman strut couldn't steer. Audi URquattros have tie rod ends in the rear to adjust toe. They don't steer under power.

Further, you must agree that braking involves more force than acceleration, yet my audi S4 has simple rod end adjustable rods holding the front suspension with rotational forces located by the shock tower -- just like a chapman strut. See picture. ... ProdID=272

I already have mine modified with rod ends, I just think they would be so easy to build with nice turnbuckle style audi like arms. If there was no chapman strut, those control arms would twist like donuts. Here is a picture of my elan attached.

This is like the cannondale front suspension on their mountain bikes. Everyone said it would pull under braking, but it works fine because the force remains in the same place during braking. Look at this image: ... ty_elo.jpg
At first blush it looks crazy but it works well.


never mind

PostPosted: Tue Feb 20, 2007 11:19 pm
by pinsx3
I just made a model -- you guys are right. One more arm forming another triangle would do it.


At least I'm not afraid to look stupid.


Re: rear control arm question

PostPosted: Wed Feb 21, 2007 1:28 am
by rgh0
No such thing as a stupid question only stupid answers