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MG Midget and Sprite Technical - Front Chassis Legs

Hi everyone just joined the site after browsing for a while.

Anyway, been restoring a 1500 midget for a couple of years and having got sick of getting crap in my eyes and ears from working under the car I've built myself a rotisserie - which is brilliant - works really well.

Now my question. Now that I've got full access to the car I've noticed that the front chassis legs are slightly "upturned" from just behind the wishbone mounts. There are no horizontal bends, twists or kinks, but they turn up by about an inch over a two foot length and both legs upturn by the same amount.

Does anyone know if this is normal, or just natural sag after 31 years or may it indicate accident damage at sometime.

Thanks

John
J Marklew

normal, it's a cunning way of setting the castor
Will Munns

John

This is a perenial question. Makes me think we should have a FAQ for these... anyway...

... the chassis legs are 'turned up' during manufacture, possible to set the castor angle? Looks like something that was decided late on in the car's design.

I'll login and post a pic of this bending being done...

A
Anthony

and the proof... taken during a visit to BMH to see the shelss being made (a great day out and highly recommeded... the guys on the 'line' are a great bunch to watch and also v friendly with q&a)

A


Anthony Cutler

Thanks guys - one less thing to worry about.

John
J Marklew

I dont think this was an afterthought, myself

Just a naturally easy way to set the caster angle

I have now seen it on very early Frog/bug/crabeyes and the later rubber bumper cars

I well remember my panic the first time I saw it "in the metal"

And hours drilling the garage floor to take Rawlbolts so I could batter it flatter

And the relief when I looked under the next Midget I saw to find mine was like his'n!

And I still have two Rawlplugs in my garage floor

just in case




You can see from the jig angle how the "bend" is maintained during the rest of the build


Bill

Does anyone know of drawings giving the exact dimensions of the kick-up?

I've recently acquired a very nice shell which sadly shows signs of someone having stood very hard on the toes of the front chassis legs.

What I haven't yet been able to do is to bring myself to try and check how far they managed to 'succeed' ...

Tom

PS What does castor angle do exactly, apart from making the wheel spin back nicely?

PPS Great pic Anthony - good reminder of an excellent day.

Tom Coulthard

Tom Caster angle as you rightly point out does help with self centering but it helps to induce "camber" when the car is cornering. Only both wheels will be cambered the same way in a corner and not opposite as is the case when the wheels have camber on them, can help enormously on tight circuits.

I am not convinced by the easy Castor speculation as this would be very easy to do when deciding the position of the top shocker mount. To me that would be how the caster would have been designed into the shell. There must be a very good reason for this bending that has not been identified by us mortals.
Bob (robert) Midget Turbo

Bob,

As I see it as the design uses the kingpin and upper and lower trunnion arrangement the caster angle options are much reduced compared to if upper and lower ball joints were used in the design. While you could move the damper position forward/aft it would produce significant loading on the pivots during articulation with the current design.
David Billington

Tom - courtesy the current project's upturned new bodyshell in the garage, I can report that the kick-up is exactly 1" measured from a straight edge aligned along the rear of the chassis rail.
Paul Walbran

saves me trying to extrapolate from the photo via photoshop and a protractor

and feeble attempts to drive a cad program or two...
Bill

Castor provides the self-centering effect of the steering. Like the angle of the front forks on a bike. Without that angle it makes it very hard to steer straight.
And I am not convinced that it would have been easier to use offset top and bottom ball joints. Using a lever arm damper, the unit itself needs to be canted over at an angle as well so that the axis of the damper spindle is at right angles to the castor angle. So the damper mounting plate is also set at an angle.

Oh, have I just repeated what David was describing?
Guy Weller

yep quaity replies, that all makes sense. I am now of the easy caster angle school :-))
Bob (robert) Midget Turbo

the A pan inner mounts would have to be squiffy too! whereas the factory solution means everything is still aligned evenly with what its bolted to.
David Smith

Dave that is exactly the reason for me changing my mind and becoming a disciple of the easy castor group.
:-)
Bob (robert) Midget Turbo

Just a thought but are there any other um... eccentricities to look out for on the bodyshell.

I don't what to straighten anything that should be bent or bend anything that should be straight - if you get my meaning :¬)

John
J Marklew

I cant think of anything else John

But I expect you will find lots of "out-of-true" bits all over the car

Which seems to be common and not necessarily a problem

Looking at the assembly jigs at the Heritage factory there is lots of feasible leeway where the panels get clamped to allow welding

And in some places there are gaps which get filled by weld to close them

TIG rules


Bill

and afterwards

look

no holes!


Bill

One thing I have noticed is that the inner reinforcing panel to the B-post adopts a slight, but very convincing bend inwards and then straight as a result of ageing cills. I am pretty sure however that it is supposed to be flat, and the bend is a result of a sagging rear end.

Guy
Guy Weller

I've been looking at the front suspension for a while, and cannot find where to apply my castor oil. I bought some a while ago, and would be annoyed to have to drink it!

lol
Neil
Neil Williams

The castor oil is applied to the king-pin; more castor oil = more kingpin castor. That's at least what I heard.

My car seems to fit together fine, i.e. doors, wings, sill gaps are all sensible; no rust in the lower/rear wing join for example; doors close fine on all edges. Hence at some point when I need to replace the sills, I'm going to repair using patches rather than cut the whole sill out so that the current alignment (whatever it might be!) is preserved. (As Bill might say, I'm happy to stick with its current 'out of true' state... cos all the 'out of true' items are at least aligned with each other)

A
anthony

Bill,

Small point but I really doubt there would use TIG in that situation, too time consuming and it's a skilled process. Looks like MIG to me.
David Billington

Probabaly right David

I never bothered to ask

Strike TIG

enter MIG

I must confess I'd rather have seen metal to metal than metal to air but it seems to work*

*tempted to write "seAms to work", but old Ollie my English master would spin in his crypt if I did
Bill

RE:

"...the chassis legs are 'turned up' during manufacture, possible to set the castor angle?"

and:

"I'll login and post a pic of this bending being done."

I may be stating the obvious by saying that the curve is pressed into the chassis legs when they're initially formed, and not after they've been welded into the shell. The jigs shown in the pictures don't actually do the bending - they merely support the shell under the chassis that's already bent, and the jigs as shown are simply conformed to the shape of the chassis.

-:G:-
Gryf Ketcherside

Gryf

No doubt you are right... but it would be more fun to imagine the jig giving the chassis a heave after fabrication and a couple of seam welds going 'pop'.

I leave all my fabrication to an old mate... I'm better at... (well I couldn't think of anything at this point) ... enjoying a couple of pints.

A
Anthony

>>> No doubt you are right... but it would be more fun to imagine the jig giving the chassis a heave after fabrication and a couple of seam welds going 'pop'. <<<

Oh, agreed! ;-) Sort of like my chassis legs did after I'd run a heavier ARB for a while...

Cheers,

-:G:-
Gryf Ketcherside

This thread was discussed between 28/07/2008 and 30/07/2008

MG Midget and Sprite Technical index

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