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MG Midget and Sprite Technical - Caster angle and self centering

I'm finding the rebuilt Frog tends not to self centre the steering. When jacked up, the steering feels quite light lock to lock. Toe in is set about 1/8". The steering returns from a definite turn, it is more to do with slight turns where you have to hold the wheel steady to go straight. The rear of the car has Moss quarter eliptics which has made it rather high, about 8cm more above the wheel arches than the front.

Assuming the car shoul be level to give the specified 3 degrees of caster then I calculate the car is running with the front to rear axis at 2.5 degrees to the horizontal which, in effect, reduces the built in caster from the front chassis member upturns to about 0.5 degree. Does this make sense and should I expect less self centering of steering?
Bill Bretherton

Bill,

Sounds like you need to make or buy some lowering wedges, some here https://www.ahspares.co.uk/austin-healey/sprite/suspension-rear/lowering-wedge-14-elliptic.aspx and frogeyespares list them as well.
David Billington

Bill
I have used the Moss 10 leaf H/D springs on my Sebring Rep since 04. You need wedges. I use Peter May wedges. Beware of any with a slot: easy to slide in but equally easy to work their way out. Pick a wedge with a locator hole for the spring bolt. I removed the top three leaves to enable fitting with no apparent problems encountered over the years.
Alan
Alan Anstead

Is Bill correct in his analysis, that raising the back influences the effective castor angle? I'm not contradicting his assumption at all as I don't know, but wondered if anyone can back this up from known facts.
GuyW

David, Alan

Thanks, I've considered wedges and may go that route. Alternatively I could attempt to rebuild the original springs which I've kept.

Guy, I did assume the caster would reduce, but as you say, it would be interesting to have other opinions. Three degrees caster isn't much so I assume the car geometry needs to be as close to specification as possible.
Bill Bretherton

Bill,

What about putting some classic mini wheels on the back for a test drive and see if that improve the steering behaviour.
David Billington

David

Not a bad idea if I could get hold of some.
Bill Bretherton

Bill,
I’ve had a slow return, similar to what you mentioned with a rebuilt rack on a ‘B’ - it ran in quite quickly when we put some miles on it.

Not saying that’s your issue though.

R.
richard b

Bill
I 100% agree with your take on the reduction of castor against the angle of the car
The wedges would appear to be the best fix
Have you got your rack shimming correct so that there isn't any binding in the column--? It doesn't take much to bind things up
With the car jacked off the ground you should be able to turn the steering wheel with your little finger, easily and smoothly--from the straight ahead position it should move from the stationary without any feeling any stiffness----smooth
The spec for toe in is 0-1/8"
If you're on crossplies then 1/8" is ok but if you're on radials I'd be pulling it back near to 0
Cheers
willy
William Revit

According to Horler’s “Original Sprite and midget”, Frogs are supposed to sit a bit high at the back. There are photos to illustrate the proper setup. Of course if yours is TOO high, that’s another issue.

-:G:-
Gryf Ketcherside

"Ride height can also affect caster. Spring sag or overloading a vehicle can alter ride height up to several inches, which can increase caster readings by up to a degree or more. Jacking up the front of a vehicle or dropping the nose will have the opposite effect and reduce caster readings. This may contribute to a steering problem, such as instability or poor steering return. That’s why ride height should always be measured prior to checking wheel alignment.

If ride height is below specifications, weak springs should be shimmed or replaced. Upgrade opportunities here would be to recommend variable-rate replacement springs, air springs, overload shocks or air-assist shocks to customers who pull trailers or haul heavier-than-normal loads."

Read the whole article. It's very good.

https://www.import-car.com/using-caster-to-properly-align-vehicles/
anamnesis

Hi Bill, If you are about Worcester way I have some rims with 175/50s on (about 30mm lower ride height than standard 145/80 size), if you want to try them.

Malc.
Malcolm

Bill

Unless the original springs are completely U/S I would have a go at rebuilding them. Personally I was never happy with the replacement springs provided by the current suspects. I fiddled about with them without success until I found a pair of original NOS. Made a massive difference. The car had a slightly 'tail up' stance as suggested by Gryf but had nice predicable handling and was more compliant on the road. The replacement ones were too stiff and was much less comfortable than the original ones.
Depends how much you drive it of course!
Bob Beaumont

Willy, I'll check the steering smoothness again.

Gryf, Anam, thanks for the info.

Malc, thanks for the offer!

Bob, I'm very tempted to rebuild the old springs as a "winter project".
Bill Bretherton

Castor: angle to the road is what detetmines castor action. The maths is that in a spridget each 35mm variation in height at one end will change road castor 1 degree.

Ride height (and weight distribution): our cars have the driver weight biased well to the rear (approx 2:1 rear to front) so when working out effects of ride height or weight distribution take the measurements with someone in the driver's seat. Real cars can't be driven without someone there!

Springs: Having seen springs from numerous sources they all have the same issue - they ride too high. More serious than the actual high ride and the castor effects is the reason they ride high. The leaves are way too thick, resulting in substantially stiffer springs. While this in turn makes for a much harder ride, the real issue is the effect on handling as a stiffer rear end promotes oversteer and early rear breakaway. 1/4 elliptic Spridgets were already lively in that respect!
We measured the rates in the new springs at over 200 lb/in. That compares with 140 for the original springs and in the 80s for the semi elliptic springs. After several hours of measurement and experimentation the final combination was removing every other leaf (taking account of those leaves with clamps rivetted on) and adjusting the leaf lengths to get an even reduction in size from bottom to top. Spacer blocks were then added to the top to make up for the missing leaf thickness in the u-bolts (ride height being determined by the main leaf position).

Result was great. Spring rate a bit softer than original but higher than semi elliptics, ride height correct, better ride than the new springs gave "out of the box" and felt much more stable when pushed hard without losing that nice feel we all love.

Paul Walbran

Paul

Thanks, that's really helpful and confirms my original assumption. I'm very inclined to rebuild the original springs. Interesting about the oversteer effect.
Bill Bretherton

When rebuilding springs, a key point to check is wear caused by the end of each leaf on the next (longer) leaf. If it has worn enough to reduce the thickness there will be a marked weakening of the affected leaf, translating to a softer spring.
Paul Walbran

Bill
If you are inclined to try classic mini wheels as David suggests I have a pair of 12" Weller steels you are welcomed to have free (no tyres fitted).

I have been using 12" wheels on the rear of my 73 car when I compete in autotests, which work very well, fitted with 165/60R12 tyres. A year ago I found a pair of 12 alloys to match my front wheels, so the steel wheels are surplus to requirements.
Ian
Ian Webb 1973 GAN5

Ian

Thanks for the offer but I now intend to rebuild the original springs as I'm convinced the problem is the excess rear height reducing castor.
Bill Bretherton

This thread was discussed between 07/10/2021 and 15/10/2021

MG Midget and Sprite Technical index

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