steering rack shims

[MACCA.GLM]

Hi this might be a difficult question to answer.

Does anyone know the approx thickness of the shims that fit under the steering rack.My sprint has a stressed skin chassis from spider in the late 80s and I am getting a bit of bump steer.There is no damage to chassis and is in excellent condition.

As chassis are jigged I would have thought a general size of shim would have been fitted.

Regards

G

[alanr]

There are no approx thickness shims.
If you want to check or work out what is needed for your car you will need to do a full Bump/Droop measurement check with the car fully assembled and with ballasted weight.
I have my steering rack off the car at present and on one side of the rack there are 6 shims that i guess have been there from when the car was built ( original chassis), according to my measurement they are:-15thou (1), 30thou (2), 45thou(1), 75thou( 2).
On the other side of the rack I have 4shims of 25thou each shim.

Make of this info what you will..... I am currently though being very careful not to muddle them up!


Alan.

[jono]

I think there is a measurement from the upper wishbone pin to the rack mounting face that's supposed to get you about there, is there not?

[MACCA.GLM]

Hi Alan

That is a big difference from one side to the other 100 Thou to 160 thou. There is a figure from center of bottom wishbone spindle to top of rack mount of 2.190 inches , basically. 2 . 3/16ths of an inch, plus minus 5 thou but getting any kind of measurement on these two items is difficult to say the least with the body on. Getting a reading from top wishbone spindle even worse though ive no idea what that is. So it sounds to me all chassis are different which seems strange to me unless some kind of crash damage as come into play.

Regards

G

[alanr]

Yes, on my car that is a big difference. But the car handles well and the only current reason for me removing the steering rack was to change the non-pinion end bush in the rack due to a small amount of play in it at the wheel when jacked up.
My shims will go back as I removed them.
The problem is these cars are are now geting really old and who knows what has really gone on with past owners and their driving.

Alan

[661]

Sorry, I wouldn't recommend a rough guess at the shim heights, it needs to be done properly.

[lotocone]

I agree with the last post by 661. It has been a long time, but we set up dial indicators as shown in the photo and the suspension was bolted on the chassis with no spring and shock so the suspension could be moved up and down. A plate went as shown with parallel lines scribed into the plate to measure the bounce and jounce and various thickness shims were installed to minimize the change in the angle of the plate. We ended up with shims that were roughly 0.200 in but that was a Lotus chassis. They varied a little from side to side. Hope this helps.

[alan.barker]

Hi Macca,
I suggest you speak to Spyder as it's a Spyder Chassis/Subframe. They profess to make a more accurate Chassis.
Alan

[alanr]

Normally you would not do a Bump Droop check until after you have first done a Castor/Camber/KPI check and you cannot accurately do a Castor/Camber /KPI check without the car body on and the car fully ballasted.

Bump/Droop is the last thing you check/adjust in conjuction and alongside with doing the above.

Alan.

[Andy8421]

Normally you would not do a Bump Droop check until after you have first done a Castor/Camber/KPI check and you cannot accurately do a Castor/Camber /KPI check without the car body on and the car fully ballasted.

Bump/Droop is the last thing you check/adjust in conjuction and alongside with doing the above.

Alan.

Alan,
Arguably it is much easier to set the shim height with the body off and the springs / shocks removed. There is no need to ballast anything, just support the chassis at the Lotus recommended height by placing the chassis on blocks.

Much easier to get to all the bits with no body in the way.

[alan.barker]

Hi Alanr,
I thought the only thing you can adjust on a Lotus is the Shims for Bump Steer.
If the others are wrong then something is bent
Alan

[alanr]

Hi Alan,
Yes there isn't much you can adjust on a Lotus, unless you put adjustable wishbones on but until you have done a basic Castor/ Camber/ KPi/Toe check you don't know what you are dealing with i.e. if the chassis is bent/ twisted or a turret is misaligned.
You need to check the basics first before you can move on to checking Bump.
Then you check/adjust Bump, and will probably have to do another Castor/Camber/KPI/Toe check again afterwards.
Checking Suspension/Steering geometry is a circle...one thing can and often does alter another.

Alan.

[snowyelan]

Hi AlanR,

If the opportunity presents itself please measure the bump toe you have with the current shims! From you description it seems possible (likely?) that you have an original lotus setup for the rack height and I would like to compare the bump toe to the model I created a while back.

viewtopic.php?f=42&t=45889

[alanr]

Scott hi,
Doing a full steering geometry check including Bump/Droop takes up a lot of time. It isn't something you can do in a few minutes or even hours!
Slightly different I know but all the same principle, I have spent quite literally days in the past on racing cars going around and around trying to get suspension/steering setups perfected, spot on, or a setup that suited the driver.
Back to my +2...
Although i have my Steering rack in pieces at the moment, i am not doing anything to alter Bump so i am just going to refit the shims as they were before. It handled great before so it should be all ok when back all together. Only if it doesn't handle will i be doing a Bump check however, if i do I will get back to you.


Alan.

[MACCA.GLM]

Hi

I am going to try to find a way to measure the height with the body still on. I have a Five foot grade a straight edge which I think I can get in to take some measurements and see if the 2.19 inches is somewhere near.I think the rack will have to come off to facilitate this so might take some time.

Regards

G