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MG Midget and Sprite Technical - Heritage shell issues

Looking back through the BBS archives there is mention back in 2000 of the front left suspension mount issues.

My Heritage shell is early 1990's and only just being built up. Now freshly painted, I have just discovered that the front left kingpin doesn't align with the shock absorber arm (out by a mere 8mm) and apparently this problem is known on other shells.

If the components are forced together then undue stress + strain will be put onto them. Fulcrum pins will wear excessively in the wishbones, and I have seen shocker arms pulled out as a result of such pressures.
I have tried fitting other suspension parts for comparison and it is definitely the chassis/bodyshell which is out somewhere by a very small amount. Measurements against the opposite side of the car are not consistent or reliable due to the nature of these shells.
If the shock absorber mount plate is rotated then the caster angle will be altered. If the wishbone mounts on the chassis are altered (by cutting and rewelding) both caster and camber angles can be altered, depending on whether the mounts are moved in a vertical or horizontal plane.

The question is, what is actually out of alignment on the Heritage shell and by how much, and how can it be best rectified at this stage?

Once the current lockdown is over I hope to visit other owners who are doing rebuilds in order to compare measurements.






I L Cooke

Just like mine was.

I’d also check the RH side for positive instead of negative camber which mine suffered from - a mid 90’s shell. I used offset bushes to sort it.

As for sorting out the top trunnion to damper arm I’m not sure. On mine I just levered them apart and never looked back! It must be possible to slightly alter the wishbone pivot points but not going to be easy. Or maybe alter the way the top trunnion attaches to the damper? Thinner bush/machine a bit off the damper? Or even open up the holes in the damper casting very slightly to give a little movement of the damper? I suppose a combination of small changes could give you the clearance but it’s up to you to decide if any of those options would work and or be safe!!

My heritage shell car also suffered from dead steering, it just didn’t centre like the other midgets I’ve driven. Probably a lack of caster.

John Payne

I would bet that original shells varied by just as much, and that in the factory they just shoved and pulled until they could get the bolts started and then just tightened it all up.

That's not to say it wouldn't be better to engineer a correcting solution, given that you are not working to time limits on a production line.
GuyW

I was wondering if you could grind the welds off the wishbone mounting brackets, then try a bit of jacking and levering to bring it into line, before welding them back up again.
Dave O'Neill 2

I wonder if the angle set in the chassis rails is the same both sides? I think it is supposed to be 3 degrees, and immediately aft of the rear wishbone bracket.

It would take only a very small variation in the upsweep angle, just a few radians, to give you the 8mm error that you have at the top of the kingpin.
GuyW

The pedestal that the damper is mounted on is attached to the rail, so I can't see that being the issue.
Dave O'Neill 2

Ahh! True, Dave.
Didn't think of that!
GuyW

Hi

Any useful dimensions in the 'Horizontal Alignment Chart' for sale by Sprite Parts in Austrailia: https://www.spriteparts.com.au/shop/brochures.html (note it is for earlier quarter elliptic rear suspension cars - MkI&II Sprites, MkI Midgets - but the front portion might be the same for a later heritage shell)?

Anyone found a chart showing vertical chassis dimensions?

Cheers
Mike
M Wood

An 8mm error at the trunnion corresponds to a 2.5 degree vertical error of the kingpin which could easily occur at the wishbone brackets. Using trigonometry the horzontal line through the wishbone brackets is 9mm too low at the front (midpoint between front pair of brackets) or 9mm too high at the back (with reference to your picture). That assumes the shock absorber is aligned correctly. Ideally the brackets need re-welding imv. My shell (not Heritage) needed this. Someone familiar with chassis welding could do this easily. You have to re-paint of course but only a small area - could be done in black, will hardly show.
Bill Bretherton

I think you need to get something that will accurately measure angles. I expect they are relatively cheap nowadays.
If you can set the shell up so that the chassis rails just infront of the bulkhead is level both across the car and inline you should be able to measure the angle of the front of the chassis. This will tell you if the upturn is as spec and even between sides. If OK then measure the angle across each spring pan in line with the chassis rails. Also measure the inclination of each kingpin and the angle across the tops of the shock towers.
Hopefully, somewhere during this you should be able to identify where the problem lies.
In previous posts I have indicated that I had a similar problem and chose to solve it by a slight rotation of the shock mounting, but it may not have been the root cause. I have yet to drive the car!
Rob
MG Moneypit

Having suggested the chassis rail upsweep might be wrong, I now agree with Dave's point as this would also alter the angle of the damper mount and damper rather than just the lower wishbone and therefore the king pin castor angle.

Some years ago I replaced a complete near side chassis rail that had rusted out, making the replacement top-hat section rail with the upsweep angle. Measuring the angles on the chassis is difficult to do accurately in situ. But it was relatively easy to make a template shaped to fit closely against the underside of the chassis rail, following both planes either side of the change in angle. I made one out of ply, that fitted the offside rail closely and then just made a nearside rail to match. It wasn't therefore necesary to have an actual measurement of the angle though I think it worked out as 3 degrees. The same methd could be used to check one side against the other by cutting a card template.



GuyW

As regards Bill's calculations on the position of the wishbone brackets.

This may sound silly, but the brackets for 1500 chassis are different to the 1275 and earler cars. They appear to be the same, but they are either mounted at a differnt height on the chassis rail, or the bracket itself is deeper to raise the ground clearance. If a single wrong bracket had somehow been picked and fitted then this would cause the trunion alignment problem.
GuyW

Some useful info. here as ever, for which many thanks.
I heard recently that the 1500 chassis may have been used on the heritage shells and this may account for the differences. Perhaps 1500 parts were fitted one side and earlier components to the other?
I have levelled the car with a spirit level. Chassis rails themselves seem parallel.
I will continue measuring and comparing with others cars when I get the opportunity.





I L Cooke

The wishbone mounts for 1500s are noticeably different from earlier ones and yours are earlier, Ian. I’m not so familiar with the shock absorber pedestals to tell the difference between them, though.
Emma





EmmaJacobs

This thread was discussed between 01/05/2020 and 06/05/2020

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