Setting Bump Steer Height & Shims available...

[pharriso]

I am slowly building up my new chassis & today wanted to check if rack shims were needed since the Chassis did not come with shims or markings on the rack platform. In fact about 18 months ago I called Miles Wilkins to ask him why it was supplied like that & in his own ornery way he told me it did not need shims.

I measured up the Chassis rack platform to top of top spindle shafts as below:

Straight edge across spindles, held down with bungee

Block & transfer punch on rack platform, then use feeler gauges to get exact gap

Using this apparatus this gave me measurements of 6.244" one side & 6.240 the other; comparing to Buckland's 158mm gives me shims .020" one side & .024" the other. OK, all is well....

So I go to various suppliers catalogs/ web sites to see what shims are available:

Sue Miller has .002 & .010" shims
RDent has .035" & .057" shims
SJ Sportscars has .028 & .046" shims
Matty has .020 & .048" shims

So why are Sues shims so much smaller than everyone elses?

Anyone know what the tolerance is for setting the rack height, thought that I read +/- .005" somewhere? but the RDEnt, SJ & Matty shims would be to coarse to allow that.

[snowyelan]

You can use more than 1 shim per side...we stack them all the time in the automotive manufacturing industry. Sue gives you the ability to get it within .001...

[pharriso]

You can use more than 1 shim per side...we stack them all the time in the automotive manufacturing industry. Sue gives you the ability to get it within .001...

Scott I realise that thanks, but that does not answer my 2 questions.

If Matty/RDent & SJ Sportscars only have thick shims, how would you get within +/-.005" if that is indeed the tolerance?

[Certified Lotus]

Phil, I had the same shim question when I built up my new 26R chassis for my S1. RD only had limited size shims and they were the wrong thickness. I called Ray to inquire and he said that's all they carry and what most people need. You have done more research and found others sources. Now I know where to get the additional shims I need (by the way, I was also told I didn't need them if they didn't come with the chassis. My original chassis did not have shims on the steering rack either. But both chassis were off by a few thousands).

I had the same question, how to get within .005 tolerance.

How come your building up a new chassis?

Thanks for the info. Keep us updated on your progress.

[pharriso]

Just called Sue & had a chat, her .002 & .010" shims are for the rack ball joints, not for setting the height of the rack on the chassis. She does not have the rack bump steer shims in stock...

So looks like the only shims available are .020" or thicker...

How come your building up a new chassis?

Thanks for the info. Keep us updated on your progress.

Glen, my chassis has had both turrets overplated at some point (at least 10 years ago...). I'm in no hurry....

[john.p.clegg]

Just purchase some shimsteel ( of whatever thickness you need ) and using sharp scissors cut out your own...

John

[miked]

Ditto. RS components did a nice pack. Sheets about 9 inch by 6.
I used them.
Mike

[pharriso]

Great idea guys, I will buy shim stock .005" & .010" thick & make my own. That will get me within .0025" assuming that I can measure accurately

Sometimes you can't see the forest for the trees!

Many Thanks

[RichardHawkins]

Phil,

At risk of stirring up controversy, I came to the conclusion that I cannot detect any change in bump steer with a shim any thinner than 0.5mm (approximately 0.020"). I suggest this is why you cannot find the shims you need already cut for the purpose.

There are several articles on the web about the subject, and most seem to relate to the Lotus 7. I chose my own method, I attached a laser to the front brake disc, and projected the beam onto my garage door while moving the suspension up and down. After about a month of messing around and plotting results I came to the 0.5mm conclusion.

Along the way I noticed several things. I am using rubber bushes, and how tight the rack is bolted down affects the rack height.

The rack is stiffer than the rack platform, and my attempts to bolts the rack mounts down so that the rubber bushes were compressed and I had metal to metal contact between rack mounts and rack platform, only resulted in deflecting the platform.

My results were shims of 4.5mm left hand side and 5mm right hand side. The numbers etched onto the chassis are 0.034" left hand side a 0.084" right hand side. This is the sort of variation that I had noticed in work carried out by the Lotus 7 owners.

I find this difficult to justify. At the time our cars were current Lotus were world champion constructors, so they probably knew more about the subject than I do. Did they intentionally design in an amount of bump steer? I am only an average driver, so will probably never notice. My car is a restoration project, and my intention is to drive the car with both shim thicknesses to see which I prefer.

Richard Hawkins

[lotocone]

Phil, ...

My results were shims of 4.5mm left hand side and 5mm right hand side. The numbers etched onto the chassis are 0.034" left hand side a 0.084" right hand side. This is the sort of variation that I had noticed in work carried out by the Lotus 7 owners.

I find this difficult to justify. At the time our cars were current Lotus were world champion constructors, so they probably knew more about the subject than I do. Did they intentionally design in an amount of bump steer? I am only an average driver, so will probably never notice. My car is a restoration project, and my intention is to drive the car with both shim thicknesses to see which I prefer.

Richard Hawkins

It's interesting to see what you got Richard. I ended up with shims of about 0.2 in. which is very close to 5 mm.
I did help improve driving the car.

Bob

[alan.barker]

You say bolting up the rack deformed the support lugs on the Chassis is interesting.
When i fit the body to a Chassis i like to see gaps between the Body and the Rack support lugs. That way i can shim the gaps and the Rack lugs are not deformed by the Body weight.
Alan

[Foxie]

Phil,

There are several articles on the web about the subject, and most seem to relate to the Lotus 7. I chose my own method, I attached a laser to the front brake disc, and projected the beam onto my garage door while moving the suspension up and down. After about a month of messing around and plotting results I came to the 0.5mm conclusion.

I find this difficult to justify. At the time our cars were current Lotus were world champion constructors, so they probably knew more about the subject than I do. Did they intentionally design in an amount of bump steer? I am only an average driver, so will probably never notice. My car is a restoration project, and my intention is to drive the car with both shim thicknesses to see which I prefer.

Richard Hawkins

From my own experience I have to agree with Richard. The surest way of getting the shims right is to set them up with a laser, test drive, and adjust again as required. It didn't take me a month, but it is a time consuming process, every time you adjust one side, you have to go back to the other side and make a further small correction.

There is no "standard" shim set-up, every chassis is different due to manufacturing variations. Before fitting the fully adjustable front and rear (Spyder) wishbones, I measured the original (fixed) settings, they were horrendous !

Car is now a joy to drive as regards steering stability.

[bill308]

Hi Phil,

I used a home made, laser pointer based system, to measure toe change on my 26R build. I can resolve the effects of toe changes of 0.001 inch thick shims. Toe effects are small, but measureable.

The other thing to pay attention to is the range over which you are looking at the toe change. I am using TTR fixed length lower A-arms, with adjustable, 26R uppers. The chassis, with bare bones front suspension, was mounted on a fixture/dolly, that held the body horizontal. No springs or dampers were fitted. An aluminum angle was fixed to the uprights, on which a mirror was mounted. A laser pointer, about 10ft away, was aimed at the mirror and the reflected beam was plotted on paper near the laser source. So, sensitivity to toe angel measurement was doubled. The toe change was plotted from droop to neutral (lower A-arm horizontal) to bump. When everything is good, there is little toe change. Small shim changes make a difference. How close you need to be to not notice any handling issues is unknown to me.

Bill

[john.p.clegg]

Bill

Can I take you to task on this

"The toe change was plotted from droop to neutral (lower A-arm horizontal) to bump."

If all was standard , springs , weights, etc,etc,etc at " neutral " the lower arms may be horizontal , but in practice it may be best to take the measurement from " Static Ride Height " to bump/droop .?

John

[661]

Bill

Can I take you to task on this

"The toe change was plotted from droop to neutral (lower A-arm horizontal) to bump."

If all was standard , springs , weights, etc,etc,etc at " neutral " the lower arms may be horizontal , but in practice it may be best to take the measurement from " Static Ride Height " to bump/droop .?

John

I see where you're coming from but for practical purposes when the body is off and one is playing with a rolling chassis without springs in, the static ride hide is a horizontal lower arm.
This is what I'm doing. I must say I'm using a dual gauge bump steer gauge.
I ordered stainless steel shims from ebay. They come as a pack of varying thicknesses.