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

["Tom Niemczewski" <dmctom@xxxxxxxxx>]

Mike

Thank you for your input. I will have to stay on my side though ;)
I have spent a lot of time and effort trying to understand how this setup 
works. And I came to a similar conclusion on a few separate occasions. OK, I 
might be wrong... but I believe that the current setup is just wrong. From 
what I know the suspension should work nice and smooth with all the rubber 
bushings replaced with rigid material. The only exception are sway bars, 
torsion bars and reaction bars. They are designed to bend and twist. Every 
other component should work fine. Just imagine what would happen if the 
bushings were rigid in a DeLorean. The rear suspension would simply be 
locked in one position with no movement possible.
The rear hub should not move in arcs! Right now as the suspension moves up 
and down the hub moves in two arcs - front to back and twist left to right. 
My design eliminates both arcs and makes the suspension work 'right'. It 
will move smoothly from top to bottom even with bushings made of metal - for 
testing of course. Before the car is put into service the bushing will be 
replaced with urethane.
I understand why the TAB is supported on just one side and why it has that 
huge bushing... but this is not a proper way to design a suspension. This is 
why the TABs bend and break. There should be no twisting and bending forces 
at the TAB and the bushing should only be there to soften vibration coming 
from the wheels.
We'll see what comes out of my new setup. I'm pretty confident that it will 
be far superior to the original setup. The whole feel and handling should 
greatly improve. I guess the time will tell ;)

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: Saturday, June 28, 2008 2:32 PM
Subject: Re: [DML] Lotus Esprit Rear Suspension - very long and boring ;)


Tom - I don't think your analysis is correct.  The
rear hub is suppposed to travel in an arc perpendicular
to the frame, just like the front hub.  If it is not, there
is something wrong.  You have to consider the
upper and lower links in your analysis, and they are
not mentioned in your text.  They define the
shape of the arc.  The purpose of the trailing
arm is to limit front/rear motion of the hub - not provide
the primary pivots for the suspension.  You also
have to consider how the front trailing arm and upper
shock mounts work.  They are both essentially
point mounts, not axes like the upper and lower
links on the rear or the lower control arm at the
front.  They are designed to allow more degrees
of freedom to account for the bending forces
you are identifying.

What concerns me about people talking about
fixing the free end of the trailing arm bolt is
that you change that joint from a point to an
axis.  When you do that, you get the motion
you describe at the hub - an arc parallel to
the frame.  The end of the trailing arm bolt
is free to allow the trailing arm to twist
slightly as the hub moves up and down.  The
shape of the trailing arm and the way it
is mounted to the hub is the clue as to
how the front mount works.  That big,
wide base pinned to the hub perpendicular
to the arc in which the hub moves plus
the single bolt at the front means that
front joint can't be an axis.  It has
to allow the trailing arm to twist.
The bushing there absorbs the parallel
arc motion, keeping the upper and lower
links (and the shock mount) in the proper
paths of motion.  Otherwise you put all
of the twisting energy back into the
upper and lower links, which are on axes.

If I had about an hour with you, my bare Esprit
Esprit frame and a torque wrench, this would
become a lot more obvious.  If you remove the
shock and disconnect the TAB, the hub moves
in an arc perpendicular to the frame.  Next,
reconnect the TAB and disconnect the upper
and lower links at the hub and see how the
trailing arm and hub are allowed to move - it's
not an arc, it's a cone.

The trailing arm bolts of the Esprit are not
a failure point like they can be on the
DeLorean.  I think the differences in the
way the shims are mounted is the main reason.
If that joint is properly torqued (and stays
that way), the shear stresses on the bolt
are not as big as you might think - they are
distributed into the bushing.  Once that joint
loosens (as in a shim falling out), then the
bushing no longer works and the shear forces
go into the bolt, causing the bending.

--
Mike



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