Steering Rack Shims

[Briggs1]

Elan Plus 2

I have what looks like a Lotus Replacement Frame that I have been getting ready to drop the body back on to.

Everything is ready to go for this weekends body drop - BUT - I am not sure about the steering rack shims.

I do not see any markings on the frame indicating shim thickness.

This is one of the things I'd like to address while everything is right
there in the open for me to work on.

The Shop manual just says put the shims back in based on the frame
stampings.

Without the full body weight it does not seem like I can determine the shims needed prior to setting.

I guess it is possible that no shims are needed with the replacement frame
(bought second hand so I can't just ask).

What are the collective words of wisdom - bolt it up now without shims and
resolve when everything else is done?

Thanks,

Briggs Pletcher

[john.p.clegg]

My twopenneth
I don't think you can set the "bump-steer" shims etc without the body on and car loaded as normal as the ride height will not be right.....I am open to differences of opinion on this but the bump steer is a deviation from normal ride height geometry up and down.....

John

Flood gates open...

[RotoFlexible]

There is at least one long thread concerning bump steer, with the usual contradictory information. But you can simply install shims as needed to bring the rack mount surface to exactly the right height relative to the wishbone pivot spindles. If you had the marks on the frame, installing shims of the specified thickness would position the rack where God and Colin (is there a difference?) intended it to go, and the bump steer will be correct.

The other approach is to go through the whole bump steer procedure to determine shim thickness and the rest of the suspension settings. You probably will want to review that bump steer thread first and decide if you want to deal with all that hoo-rah. I certainly didn't.

There should be a dimensioned drawing that provides the distance between the lower spindles and the rack mount surface. Brian Buckland's (Baby) Elan book shows how to measure from the top spindles (and provides the magic dimension), but you need the body off in order to do this. That's how I determined which shims to use.

[Rob_LaMoreaux]

As a very good starting point go to the workshop manual and find the page with the front end dimensions.

It should list the lower control arm mounting bolt to rack bottom face dimension.

add sihims until the bottom of your rack is this distance from the center of the lower control arm mounting bolt.

This is what the factory shims would be.

Now whether or not this is the best for bump steer is a totally different discussion.

If you want the best for bump steer you can do it without loading the car since it is best done without shock and spring so you can move the arm through it's travel, but the chassis needs to be level.

Rob

[Briggs1]

Thanks Rob,

That is good input. I'll check the manual for dimensions as soon as I'm done typing

Appreciated !

Briggs

[bill308]

On a related topic, I recently took delivery of a new TTR racing chassis, which I had powder coated gloss black. I don't see any stampongs where the proper shims are defined. Could it be that fabrication is so good that the rack can simply be bolted on?

Bill

[ceejay]

Been following this thread with interest.
Curing bump steer is not done by simply placing
a couple of shims under the steering rack, (Regardles
of what was done at the lotus factory) it requires
many hours of working with measuring gauges and
creating a bump steer graph while the suspension
is operated throughout it's full movement from full
droop to full bump.

The links below will provide excellent description
and data of how to go about curing the problem
of bump steer.

Bump Steer: Lotus Seven Club.
http://7faq.com/owbase/ow.asp?BumpStee

Bump Steer.
http://www.longacreracing.com/articles/art.asp?ARTID=13

Tech Article #9 - BUMP STEER GAUGE INSTRUCTIONS
http://www.irvansmith.com/scart/article ... cles_id=11

Good luck
Col.

[john.p.clegg]

Well I couldn't get into the first one but the second says....


"Set the car at ride height."....and if yours is at ride height with the body off etc. then O.K.....but mine isn't....

John

[garyeanderson]

Well I couldn't get into the first one ....

John

Hi John

Try

http://7faq.com/owbase/ow.asp?BumpSteer

If you read the link, you may see its missing the "r" in BumpSteer

sometimes you have to think about the links and what people were trying to say when the links don't open.

Gary

[john.p.clegg]

Gary

Many thanks

Once again.....resting the chassis on blocks at its normal ride height....

It seems that bumpsteer uses normal ride height as the mean,and equates the deflection either side of this to negate bumpsteer.
If your car is as per factory spec.(springs weight etc.etc) then O.K. but any change of weight (driver,passenger,fuel,spring length,strength etc.) will need the shims re-calculating as per the examples...


John

[gerrym]

Briggs, you can set up the bump steer shims with the assembled chassis, complete with front suspension and steering, without the body.

Basically you will need to remove the damper/spring assembly, rest the front cross member on blocks at approximately the correct ride height and then stroke each front suspension up and down. This can be done with a small trolley jack etc. Lock the steering in the straight ahead position.

As the suspension moves into bump and rebound, the steering knuckle will change angle slightly. This is the definition of bump steer. You will need to measure the change in the angle with a dedicated bump steer gauge (eg checkout demon tweaks website) or use a mirror and laser to measure the change in angle. Refer to the many postings on this site.

Regards

Gerry

[HardTop]

I don't see the spindle to rack mount dimension in the workshop manual, am I missing it?, a page number or a value for the dimension

thanks

[types26/36]

This may be of help, its on the Elan.net site
http://www.lotuselan.net/publish/steeri ... hims.shtml

[ppnelan]

Once again.....resting the chassis on blocks at its normal ride height....

John, I don't think it is critical to have the car at 'normal ride height'.
The aim is to eliminate/minimise bump steer throughout the WHOLE suspension travel.

I did this on my +2 (which used to veer alarmingly across the road, one way under braking and the other under acceleration ) using a bent piece metal as a pointer clamped to the trunnion with mole grips .
I set it to point to some part of the upright (I forget exactly where) then, with the steering fixed, I watched whether the pointer remained pointing at the same part i.e. see if the upright rotates relative to the trunnion as the suspension moves. After adjusting the shim thicknesses several times and rechecking each side, it is now much better .
Note that adding shims to one side will tilt the rack & change things slightly on the other side...

Matthew

[HardTop]

Thanks for the information