uncomplicated way of fitting steering rack

[kenny]

Time has come to crack on with the build after a long lay off.
Before I drop rebuilt engine in to hear it rattle and smoke I want to get the car down on it's wheels.
I need to fit the rack........it's going on a new chassis and the in my box of bits were the shims.
On either side of the chassis are numbers which are hard to read.it looks like 109 and 130 something...does this make sense?
The shims were taped in two batches of three........one batch mics up at .165 and one batch at .190.
I don't want to get involved in measuring for bump steer etc. so how critical is this and what would you do with the numbers I've suggested. I'm just tempted to put a couple of shims under each mount and leave it at that.

For grease in the rack is ordinary multi purpose ok?........I'm fed up buying various different compounds for this car now and using a spoonful only from each tin.
The car will not be raced, thrashed or anything aproaching a hard life by the way.

Many thanks for any info,
Kenny

[ppnelan]

how critical is this

It is HIGHLY critical
If the bump steer effect is different on each side, the car will pull across the road as the suspension moves i.e under braking it will pull one way and under acceleration it will pull the other - I speak from experience

Best to start with shims that match the numbers stamped on the chassis - you must decipher them...

I think it is (theoretically?!) possible to measure/calculate it - see 'the Brian Buckland book'.

I sorted mine out (good enough for road use) by assembling the suspension without the springs & dampers, clamping the steering wheel in the straight ahead position (make sure the rack is CENTRED), and checking for relative twist between trunnion & hub over the range of suspension travel. I adjusted the packing under both sides of the rack until there was little or bump steer each side. I used only low-tech equipment (bent sheet metal bracket + cocktail stick clamped to the trunnion with mole grips!). Be aware that any change in shim thickness on on side WILL affect the other side too, as the (tiny) angle of the whole rack changes - think about it...

Good luck!
Matthew

[kenny]

Cheers for that Matt .......although I did ask for uncomplicated and worry free responses

[ppnelan]

OK. The uncomplicated & worry-free response is... just stick a couple of shims under it - BUT you'll regret it when you find out it drives like a Tesco trolley

Matthew

[Elanintheforest]

Matthew...I've thought about it and it still doesn't make sense! Not your explaination, but the whole bloomin' concept.

The steering rack, in standard form, is clamped onto the chassis with rubber bushes...surely this allows a few thou of movement as well? The arms coming out of the rack can move around....the whole chassis is never perfectly square...i.e. tower to tower, or suspension point to suspension point varies by a fair bit. The wishbones are wobbling about on their rubber bushes, again moving by a few thou. And of course, the suspension is moving up and down seperately to all these other movements.

As you can probably tell by now, I'm no engineer But with all these bits moving around, why is it so critical to have the rack so carefully level with the chassis (except when it's moving in it's rubber bushes of course!!).

Steering therory 101 required here I think, or should I just follow what Mr Buckland says and accept that it has to be done, for some reason that us mere mortals cannot fathom!
Mark

[worzel]

Hi all

now I'm really going to spark a debate!

Quite a few years ago I fitted a new spyder frame to my sprint. Unfortunately in transit the shims for the rack went adrift. After reading the bumf on rack height I rang Spyder (then owned by V Moore). I didn't actually speak to him personally but after explaining the problem I was told "not really a problem, try it without any and see what it's like".

Anyway I made up two shims out of stainless sheet approx 75 thou thickness and there's never been a problem. I haven't noticed any difference whatsoever between the car's behaviour (steering and stabilitywise) on the new frame and the old (presumably factory shimmed item). It brakes, accelerates corners perfectly normally.

Now it MIGHT just be that the shims I fitted exactly matched just what the chassis required but the odds against this are I imagine pretty high.

I cannot exactly recall the conversation with Spyder but it went something along the lines of "if you need to, use anything between the following measurements" (sorry I don't recall the figures but I'm guessing that I picked 75 thou because it was the mid point between what Syder suggested.

Regards

John

[kenny]

John, thanks for your reply and what you say makes sense to me.
On my Caterham which I sprinted I was desperate to get the thing more stable and to turn in better. I invested in a full geometry check and was advised that the rack height was indeed causing bump steer and was not right.
The chap did the workings out with laser etc and we fitted shims which were considered correct.............the car was worse.
After many shim changes we eventually left all shims out as a trial and car was sharper on turn in and felt stable so we left it at that.

I then put road wheels back on car,15" as opposed to the 13" sprint wheels........that knackered things up again and car would tramline like a dog.

I posted the question just to gauge a reaction..........with the shims I have with this Lotus, whether I left them out or in would only alter the height 3 or 4 mm which on a road car driven at sensible I don't think there is a problem in any way.

Throughout this rebuild if you followed every single snippet of "how things should be" you'd never finish it, a trap I've fallen into in a big way.
The scare stories and the myths just make you want to close the garage door for good.

I listened to far too many horror stories regarding the twink rebuild when at the end of the day it's only an ordinary engine and not a mystery, even old Miles in his book (great book and very helpful by the way) does play the doom and gloom card on almost every page.

Not just on this question, but any question you ask of a Lotus Guru on a simple matter you often feel you shouldn't have asked.
I've virtually finished the car now and the hardest job on it was threading the bloody fuel line through with the body.............and that's the only question if asked I'd scaremonger.

Regards,

Kenny

[john.p.clegg]

All
I've had a play with this and would second setting the rack height as per the manual,mine was out and the "bump steer" was noticeable and after adjusting--yes,alter one side and you have to re-do the other side--the wheels did not have a mind of their own,it's not rocket science, just commom sense when you get down to it....

John

[ppnelan]

The steering rack, in standard form, is clamped onto the chassis with rubber bushes...surely this allows a few thou of movement as well

Yes, but very small indeed - feel how stiff they are...

The arms coming out of the rack can move around....

Yes, but the idea is to position the ball joint on the INNER end of the track rods in the 'right place' to avoid it pulling the steering arm on the hub in or out when the suspension moves up & down. This is only done for the steering in the STRAIGHT AHEAD position. If things are distorted enough I suppose it would be possible to get different effects (steering feeling) when turning left compared to turning right, but would you notice on a road car? It is the straight ahead position that is most important for stability under braking & acceleration (& these days speed bumps!).

the whole chassis is never perfectly square...i.e. tower to tower, or suspension point to suspension point varies by a fair bit.

Yes, this is what shimming allows for.

The wishbones are wobbling about on their rubber bushes, again moving by a few thou.

Yes, but it's stability in the straight ahead position that shimming is for, and the effects under braking & acceleration in a straight line should be near enough the same both sides, unless one side has new bushes and the other old ones...

And of course, the suspension is moving up and down seperately to all these other movements.

Yes, this is why it is important to match the steering rack position to the suspension geometry!!

But with all these bits moving around, why is it so critical to have the rack so carefully level with the chassis (except when it's moving in it's rubber bushes of course!!)..

You'd have to try a car that was poorly set up to feel exactly why. My +2 was like this initially and although you do get used to steering slightly one way under braking and slightly the other way under acceleration or over a crest , it is wrong and isn't safe If you want the precise turn-in that Lotus intended, you need to shim the rack correctly.

From my experience, I know it is quite sensitive to shim thickness. Mine has a 25 year old chassis which is probably no longer very straight judging by the amount of shims and difference in thickness between sides, so it made a big improvement. Perhaps on a NEW chassis (e.g. Worzel's Spyder) it is not so critical as manufacturing tolerances are better.
It may also be acceptable (or even desirable?) to have a certain amount of bump steer, as long as it is the same on both sides. You'd have to speak to a 'racer' about that...

So, if it's a NEW chassis, just use the thickness stamped on the rack mounts. If it's an old one or you can't read the stamped numbers, it is worth checking while it is all in pieces.

Best of luck!
Matthew

[worzel]

Hi Matthew

I suspect you're probably right about spyder building more accurate frames and thus (possibly) making exact rack height less critical. Maybe,(and I didn't check this at the time) my chassis was dimensioned not to need shims in the first place.

Re other owners' comments about making heavy work of what should be commonsense- and this is not to make light of the problems experienced by other owners and is only my personal experience but- and I've said this before on the forum- the problems I've had with this car almost 100% were caused by the previous owner's lack of attention/abuse.

I've had this car since 1978 and during the first two years it gave me nothing but trouble but I gradually got to grips with it and determined that I wouldn't shortcut. By and large it's worked out as planned- after the initial teething problems I ended up using it as a sole everyday commuter car for around 11 years without any real problems. The biggest gripe I had was the marginal cooling but a switch to a full width radiator and a modern fan fixed that.

Generally problems have been caused by "others". I'm not trained as a mechanic but I think I must have done most jobs on these engines over the years and have never really had much go wrong once a fault has been properly fixed.

Some might like to think these cars are exotic but (fortunately) they're screwed together using pretty simple components so if looked after definitely should not keep giving major headaches.

Hope my comments are not seen as too contentious.

Regards

John

[vortz]

hello all and kenny
speaking from experiance, don`t underestimate the effects of bump steer. you will be amazed at the differance of moving the rack one millimetre up or down has on the direction the wheels point.
go to your nearest decent library and look at COMPETITION CAR SUSPENSION. by ALAN STANIFORTH pages 192 - 195.
well worth a read.

regards ian

[Elanintheforest]

The light is kind-of coming on for me...thanks for taking the trouble for the explaination Matthew. I have replaced several chassis over the years, and have always put the shims in that were supplied or as indicated by the size scratched onto the chassis...so I don't have the experience of this dreaded bump steer.

I recently bought a new-old-stiock chassis for a 2 seater that a chum was going to use for his car...the shims were lost and the galvanised chassis was powder coated, so no sizes can be seen either. It's off to the tool shop for a vernier caliper and a long straight edge to carry out Mr Buckland's method...looks to be a very simple job when the chassis is off
Mark