Re: [DML] Lotus Esprit Rear Suspension - very long and boring ;)

["Tom Niemczewski" <dmctom@xxxxxxxxx>]

Mike,

Look closely at the lower shock mount on both the front and rear. Both, the 
lower control arm and the rear trailing arm travel in an arc. The difference 
here is that the front arm moves perpendicular to the frame while the rear 
moves in parallel.
In both cases the upper shock mount is part of the frame and is fixed. We 
can omit that small busing that's there in this example.
Lets look at the front first. The bolt that supports the shock at the lower 
mounting point is parallel with the axis on which the control arm rotates. 
In this design the bolt holding the shock rotates inside the bushing as the 
suspension travels from top to bottom - the bolt is fixed to the control arm 
while the shock is fixed to the frame by the upper mount. This joint works 
as it should. The arc created by the movement of the control arm simply 
causes the shock mount to rotate. Great!
Now, lets look at the rear shock mount. The bolt that supports the shock 
runs along the line of the vehicle and it is turned 90 degrees to the axis 
on which the trailing arm rotates. Well, it's not exactly 90 degrees, but 
close to it. So, as the rear suspension travels from the lowest to the 
highest point of it's travel the shock bushing cannot rotate on that bolt. 
Try to imagine what happens with that bolt... with the trailing arm at the 
lowest point that supporting bolt is actually pointing down. When the 
control arm moves to the highest point of travel the bolt will point up. 
This causes bending of the rear shock and very quickly crushes and wears out 
the lower shock bushing. Now, if this bolt was rotated 90 degrees to be 
parallel with the TAB the shock would simply rotate on the bolt without 
damage.
Yes, I agree that this setup works. But just because the car doesn't fall 
apart and can be driven doesn't mean that this is in any way a good design. 
Actually, in my opinion this design is terrible! What would happen if they 
would weld the hubs directly to the frame? Would that work... well, yes. 
Would it be a good design? No way. By saying that is doesn't work I didn't 
mean that it doesn't work at all... because it does as we all know. It was 
an exaggeration to make my point.

I sure hope that all this makes sense ;) It's sometimes hard to put all that 
stuff into words. If what I'm saying does not make any sense, please go out 
to the car, look at the rear shock lower mount and imagine what happens to 
it as the trailing arm moves up and down drawing an arc at where the shock 
is mounted.

When it comes to the trailing arm mount to the frame and the famous TABs I 
have to disagree. The trailing arm should not twist on the mount as it moves 
up and down. Twisting in this joint will cause instability and a feeling of 
"floating" where it might be difficult to keep the car in a straight line on 
uneven pavement. Same thing happens when it comes to the fore/aft movement. 
Besides, the current design puts an awful amount of stress on those TABs. 
The bolt should be supported from both ends with moving joint in the middle. 
The engineers at Lotus should now that what they designed is not the way it 
should be done! It is a critical joint with immense forces at work... To me 
it seems that whoever designed and approved this joint has never taken any 
classes in joint design, forces at play and most of all lacked common 
engineering sense. I know, I'm harsh... but this is what I think when I look 
at this joint. Especially in such an important joint as this one and with 
the rear wheels being driven on top of that. Of course the joint cannot be 
rigid or something will have to give eventually. That is why the bushing is 
needed on the TAB. I am changing this unfortunate Lotus design to something 
that will be a lot stronger, safer and lower maintenance.
BTW, it there any other car on the road beside the DeLorean that you have to 
check the TABs? Have you ever seen a TAB from other cars that were severely 
bent out of shape just with regular use? I have not. On my DeLorean the OEM 
TABs were both bent so badly that I had a really tough time getting them 
out. I replaced them with a much stronger bolts (it was not a Toby TAB, but 
a similar bolt) and guess what.... both bolts were bent again after less 
than 10.000 miles. That is a bad and dangerous design!

BTW, do those TABs hold up ok in case of the Esprit?

You will see what I have come up with as soon as it materializes. For now 
it's just a pile of metal parts that will become the frame and suspension ;)

Take care,

Tom Niemczewski
vin 6149 (in Poland!)
Google earth: 52°25'17.30"N 21° 1'58.00"E
dmctom@xxxxxxxxx
www.deloreana.com


----- Original Message ----- 
From: <mike.griese@xxxxxxxxxxxxxxxx>
To: <dmcnews@xxxxxxxxxxxxxxx>
Sent: Thursday, June 26, 2008 11:16 PM
Subject: Re: [DML] Lotus Esprit Rear Suspension


>I don't see where you are getting that it's 90 degrees out of
> actual movement.  The rear suspension travels roughly in a
> vertical arc.  The shock and spring is arranged to control the
> travel of the hub carrier as it moves vertically.  The arrangement
> is no different than the front control arm, with the exception
> that the lower shock mount trails the hub instead of being
> contained by the control arm.  Obviously what Lotus did
> does work.
>
> When you consider the trailing arm mount to the frame, you
> have to devise a joint that travels along 3 axes.  This is why
> the mount is done as it is.  As the rear wheel is deflected up,
> the trailing arm twists, so the front mount has to pivot.
> When the rear wheel hits a bump, there is some rearward
> motion, so the mount has to accomodate fore/aft motions.
>
> --
> Mike

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