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MG MGB Technical - Wheel base tolerance

Does anyone know what the tolerance is for wheelbase on a Mk1 roadster (1963)
Getting my car prepped for new springs and new rack and measured wheelbase - 5mm difference between left and right.
I had been concerned about harsh suspension and jittery steering and decided to get it sorted. The car has obviously been bungled at some point and repaired, but the cross member is possibly a bit crooked, or else the sagging springs are not sagging evenly.
The original bodywork repair manual doesn’t give a tolerance on wheelbase or track
John
John Minchin

Where are you measuring from John
The usual place to wear and throw wheelbase out is the rubber dampener pads on the rear springs, but it could be anywhere really, A good thorough measure up is the only way to track it down

willy
William Revit

Willy
Wheel hub centre to centre. Hence any issue with springs (especially the rear) could cause a difference. With new springs in, I will remeasure. The body repair guide shows hub centre to centre measurement of 91" but without any indication of tolerance.
In an archive post, a couple of people mentioned a maximum allowable difference of 1.5mm, but I dont know where that is specified
J
John Minchin

Measuring from the hub centres is a bit touch and go, really you need to measure from the kingpin to the rear axle housing
Measuring from the hub , you have to have the wheels dead straight and if for example the the rear is out a bit and the steering wheel has been straightened up to drive straight on the road, then it's all up the creek . So kingpin to rear housing or if you can find someone with a good 4 wheel wheel aligner who will take the time to set it up properly then that will tell you straight away which corner the issue is at
Are you replacing the rear springs, if so you will be able to check the dampening rubbers while you're at it
willy
William Revit

Bear in mind that a major consideration is the inaccuracy of the chassis to start with.
I know my V8GT has never been pranged but I couldn't get reference measurements to coincide with my measurements.

I put a CCS 4 link on and meticulously set it up. When I had a 4 wheel alignment done it was interesting to see that some of the discrepancies were generated from chassis mounting points; i.e., the rear axle front eye mounts and the cross member itself.
Allan Reeling

While investigating why the tyres always seem to be closer to the rear arch on the left compared to the right I did similar drops from the hubs rather than the chassis rails and the diagonals were within a couple of mm. The tyre to arch difference can be 10mm or more, even though on my roadster the left-hand arch bulges out slightly whereas the right-hand arch is virtually flat, but that left-hand tyre was still closer.

I know some have cut the spring saddles off the axle and rewelded, but for me the discrepancy is in the panels and how the jigs attached them to the chassis rails, so changing the axle will have made the car crab. It's worse on my car with a wire-wheel conversion done in 1989 even with a correct wire-wheel axle as the wheels have 10mm less inset than the originals which means my rear track is 20mm more than originally, and 20mm more than Rostyles or V8 alloys.
paulh4

As far as wheelbase measurements go, the usual suspects are rear spring saddle rubbers and front inner A arm bushes wearing
For the average roadgoing std MGB ,if the rubbers are ok they drive fine
The most common fault is loose rear U bolts that have allowed the axle to move a bit and wreck the rear saddle rubbers, This usually happens on the LH side here where the wheel hits bumps on the side of the road and can be self diagnosed in a way by the car wanting to pull left in the steering all the time, specially under power--John's 5mm would be plenty enough to make it want to steer off line

willy
William Revit

Interestingly, I recalled that the Renault R16, which was a common car on our roads in the 1970s had a very significant, and visibly recognisable difference in its wheelbase as a result of the transverse torsion bar springing arrangement at the rear.
I've just looked it up , and the figures are
Left: 2,720 mm (107.1")
Right: 2,650 (104.3")

However the difference would be that, unlike John's MGB, the Renault wheels would still be parallel to the car and to each other.
I wonder if the "jittery steering" John describes is not related to the front cross-member being "a bit crooked"?

In my 1965 MGB the rear axle always has seemed off-set to the left side of the car. I have read over the years that this offset at the rear is normal for the MGB.
I am not an engineer or a mechanic. But if the rear axle is twisted in relation to the car, rather than offset, enough to account for a measured 5mm difference, rightly or wrongly, I would picture in my head a tendency for the car to "crab" to one or other side from an element of rear wheel steering.

T Aczel

Mine's got the offset to the left, too (a '72 Roadster). As this is a known phenomena on a mass produced car on various years, why don't all MGBs have the same syndrome? Or do they? As Paul has pointed out, this is exacerbated if one has a non standard wheel arrangement at rear. Mine's got 195/70 section tyres on 6" wide Revolutions. Maybe it doesn't get picked with standard wheels and 165 section tyres.
Peter Allen

I've only ever read of one claim that the narrower gap was on the right, and whilst not everyone has made the comment I've certainly read of probably hundreds over the years where the left does have a narrower gap.

I had a rear spring break on the V8 and the axle moved back an inch or so on that side. Wondering what the bang was and the thump in my backside, it was only when I noticed that the steering wheel was turned when travelling straight that stopped to look round the car. It still travelled in a straight line with the steering released even though the wheel was turned. I had to travel well over 100 miles like that to get home, with no apparent trauma to tyres.
paulh4

When I was setting up my TVR race car we found it impossible to get a proper set up on an accurate modern laser alignment rack. We concluded that cars of this era from Britain (maybe the TVR more than most) were basically a collection of parts flying in loose formation and accuracy past a certain point was an unobtainable ideal.

The TVR had probably been mounted to the chassis at a slight angle by the factory but I have also seen production MGs that had no evidence of having taken any significant hits be significantly different side to side on basic things like wheelbase.
Bill Spohn

With TVR it probably depended on at the factory which very tall guy was paired up with which very short guy and what areas each worked on.

Most product development was done by the customers reporting back to the dealers, who then ignored it. :)
Nigel Atkins

With TVR, it would have been more a cottage industry. I'd guess it was down to how symmetrical were there chassis jigs for the space frame. What I don't get with MG is that they made half a million of the things. Why aren't they all good, or all bad? Where the nearside wing issue was concerned was it down to the monocoque, or the inner/outer wing pressing?

On the other hand, and on the subject of mass production (or the Brit take on it!), my brother managed a line at Austin Rover in the late 1990s in Longbridge. He said it was interesting the amount of hand fettling that had to be done on the Rover 400 before each one went out.
Peter Allen

As far as the MGB goes the nearside wing issue IS pretty-well universal from the number of comments I've rear over the years. The best I have been able to get mine is central - bearing in mind the nearside wing bulges out more than the offside anyway - by slackening the U-bolts with the car on its wheels then giving the body a good hard shove towards the nearside and retightening the U-bolts. As to the cause, as far as I'm aware that has never been established.
paulh4

I made the error once of fitting tyres that were too wide. Even though there was clearance both sides, under even under moderate cornering forces, the rear tyres would rub on the inner lip of the wheel arches; mainly on the left where clearances are less, but both sides.
I hadn't appreciated till then how much sideways deflection there is of a rigid rear axle supported only on a pair of semi-elliptic springs.
This tendency is further accentuated by the rubber (or poly) pads above and below the axle where it is located, and by the rubber or poly bushes at each end of the springs.
So like Paul, but on a hoist, we loosened off the U-bolts, and forced the axle to the right, (and of course re-secured the U-bolts). When I checked again a couple of hundred miles later, the axle was back where it was before we had moved it!
Ultimately I did the obvious, and purchased more appropriate sized tyres.
The car was pretty awful with the big tyres anyway.
T Aczel

This thread was discussed between 01/01/2021 and 10/01/2021

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