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MG Midget and Sprite Technical - chassis out by 7mm

For some reason which a lot of measurement and eyeballing does not reveal, my left rear (passenger) side wheel is 7mm forward (closer to the front) than it should be.
I can see no discernable floor or structural damage to acount for this. Could it have come out of the factory this way??
Springs are new and are within 1mm of spec distance from front rubber bush to locating bolt at the axle.
All brackets and bushes are new.
Any clues Gentlemen???
Rod
R W Bowers

How/what are you measuring Rod?

7 mm over a wheelbase of about 2 m is only 0.4%. Manufacturing tolerance ;-)

Malcolm
Malcolm Le Chevalier

Rod, there is a fair bit of variation in the manufacturing tolerance on these cars. A lot of it one just doesn't notice and then one day you will spot something that looks odd, get the tape measure out and start to worry about it. Don't! Its normal.

I guess that what you are measuring is the distance between the rear tyre and the pointy end of the cill. And yes, that does vary from car to car and from side to side. 7mm is well within the normal range.
Guy W

I had that problem after fitting new springs.If the U bolts are not done up tight enough you may get axle movement on acceleration / deceleration, which will "relocate" the axle line. Slacken the bolts off, realign, and tighten up. Problem solved.
Geoff Mears

Did that Geoff, didn't fix it. I'm measuring from the front wheel center point to the rear wheel centre point. It may be only .4% but it annoys me greatly and it does cause 3 degrees of misalignment (according to the boys who just did the wheel alignment)
I'm working on a fix involving another method of locating the rear axle to the springs,I'll keep you posted.
Cheers
Rod
R W Bowers

are you sure it's not the front wheel that is misaligned?
David Smith

I presume you are comparing left with right sides. But, surely the front wheel centre point is not fixed but varies with the angle of the wheel (while the other side moves in the opposite direction). Unless you are confident that the wheels are perfectly straight ahead on both sides I don't see how you can measure this distance accurately (and a small error on one side of, say, 3mm will be matched by a similar error on the other thus doubling up the perceived error to 6-7mm).
Chris H (1970 Midget 1275)

Yeah, thats a thought Chris. Isn't it also fairly typical for the back axle to sit off-centre? So the rear wheels are consequently offset side to side? This could add to the error.

Would the angle of the front wheels change it...? As theoretically the wheels would pivot about the centre and Rod is measuring from the wheel centre. That said, they don't really, the pivot about the king pin and the wheel centre is offset from this thus it would change.

Just thoughts...

Malcolm.
Malcolm Le Chevalier

I'm not criticizing just wondering what you use to measure the distance and how accurate this measurement can be with wheel centres recessed from bodywork

as others have put each little discrepancy can add up, difference between each side, front wheel(s) out, rear wheels) out, variation of measurements
Nigel Atkins

7mm over the width of a backaxle is way less than 1/2 deg, not 3 deg out of "square".
G Williams (Graeme)

For a time it was not uncommon for designs to have a different wheelbase on the left to the right. This didn't apply intentionally to Spridgets, but it did to some other cars and sometimes the difference was several inches. I think the theory was that it would reduce pitching on supple suspensions. Certainly Citroens employed the idea and I think some Renaults did too. But then, that's the French for you!
Guy W

if this is a wire wheel car, they flex so much in/out while driving that I bet the 7mm fore/aft you are worried about is like nothing

the front ends are usually a fair bit out of balance with each other as well (camber)

This is such a small car, with such a rudimentary suspension (and small suspension travel, and stiff springing to try and minimize the unkind geometry (like very bad bump steer) that it gets in to during jounce and rebound), that just setting the toe is about all that any of them ever got (and that was primarily to prevent premature tire wear).

If you want to fix the discrepancy that you found, either re-set the rear axle on the springs, and re-tighten the U bolts (probably would have to slot the central spring bracket hole first). Or, an alternative fix would be to shift the rear spring front mounts fore/aft (slot the mounting holes). However, doing either of these things (slotting mounting holes) will open you up to future shifting under load.


Norm
Norm Kerr

"I'm measuring from the front wheel center point to the rear wheel centre point."

Surely, as others have said, this is where the error is creeping in. Unless the front wheels are exactly in the dead ahead postion, there will be a difference between the lh and rh sides.

The wheel centre doesn't pivot about the centre of the king pin. As can be seen from the picture below, the wheel sits several inches out. So any degree of off centre from the straight ahead position, will translate to a difference of wheel base, when compared to the straight ahead position.

You could even have a bent lever arm at the front, and that would cause it.

Lawrence Slater

..... and what's dead-ahead? A "straight ahead" centred steering doesn't necessarily coincide with the wheels and some cars have the track set "off line" with the steering wheel. Setting the steering straight by centering the steering wheel, which would be the natural thing to do, would cause the problem Lawrence describes.
G Williams (Graeme)

It would also be affected if the tracking wasn't spot on for both wheels as well.

Trev
T Mason

Thanks lads all comments gratefully received.
Three seperate measurements have confirmed 7mm, and as you have said this could be affected by the front wheels being not straight ahead. I can assure you the wheels were exactly lined up because I had a straitedge clamped to the buggers. As I posted, the boys with the laser wheel alignment jig confirmed this.
Norm you were close to the money with a possible solution, slot the hole and move the locating bolt back 7mm. But I think that I'll remove the bolt completely and fit track rods (tramp rods?), any thoughts?
Cheers
Rod
PS every component in the front and rear suspension is new except the diff.
R W Bowers

Has it ever had a front end crash?
Lawrence Slater

Dont worry, my shell was out by nearly 3 inches across the diagonals and yet looked fine from the outside.
PeterJMoore

I can't believe any two cars probably came out of the factory with identical dimensions in all planes.
As Guy said, some cars were designed this way. The Renault 4 was apparently with a 1.8 inch shorter wheelbase on the left than the right as they placed the torsion bar suspension units behind one another across the centre of the car.
Clever stuff!
One of the rear wheels on my midget looks definitely closer to the front of the rear arch than the other. I thought they were all a bit like that!
Matt1275Bucks

You're probably right Matt
And yes Lawrence it has had a front ender, but I don't think this is the problem, the front wheels are OK and all measurements here are correct. As I said I'll figure out some way to locate the rear axle e.g track rods. Anything which reduces the inherent "twitchiness" of these cars
is for me.
Cheers lads
R W Bowers

This thread was discussed between 30/01/2013 and 01/02/2013

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