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MG Midget and Sprite Technical - Sprite Rear Roll Centre

Hi All,
Got a question re Rear Roll Centre Height on standard QUARTER ELIPTIC sprung cars.

What is the rear RC height?
and
How have you determined that?

Cant find any definitive info on this, only passing comments in texts that suggest several different answers!

Apreciate your help.

Paul N-M



P A P Neil-MacLachlan

Paul

I have not got an answer, but intrigued - you building/modifying a Frogeye or MkII Šprite/MkI midget or Debring Dprite replica to compete with?

Greetings from Ayrshire!

Cheers
Mike
M Wood

Did you spill salt water on your keyboard Mike?
😎

I'm intrigued too but I can't help with Q. Sorry.

Welcome to the best bbs there is Paul. Tell us more.
Btw we like photos....
Haha!
Greybeard

Paul,

Look for a thread: Front spring rate, Deborah Evans explains everything about your question and more.

Flip

https://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&access=&mode=archiveth&subject=97&subjectar=97&thread=2011041712172231985
Flip Brühl


If you can't get to it then drop the 's' in 'https' -

http://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&access=&mode=archiveth&subject=97&subjectar=97&thread=2011041712172231985

Or the thread is in the Archives here, 'Midget and Sprite Technical Archive' that you can look up, the thread title is 'Front spring rate'.
Nigel Atkins

That thread applies more to a 1/2 elliptic spridgets where the effective RC is near the centre height of the axle, for a 1/4 elliptic it's lower. I've made Watt's linkages for my frogeye and placed the pivot nearer the bottom of the axle, one I made for a mates later Austin Sprite was higher up.

1st image is the linkage for my frogeye, for scale the width of the chassis mount is between the inner faces of the boot floor box sections just behind the axle hump.

The 2nd image is side on on the Austin Sprite. IIRC the centre rocker is about 5" centre to centre on the lateral links and they're 1" OD tube. The image is looking down a bit so makes the centre pivot look lower compared the the filler plug on the back of the axle.

You need to try and keep the RC from the axle location close to that given by the springs or they will fight each other effectively stiffening the suspension at the rear.






David Billington

The off side (left) of the body bracket, 1st image, bolts to the fuel tank flange and then has a 1/2" IIRC spacer between flange and the boot floor, the near side (right) bolts directly to the floor.
David Billington

Thanks for your replies.
Interesting links, however its the roll centre for quarter eliptic springs I am looking for.
Does anyone have this info?

Paul
P A P Neil-MacLachlan

Willy might know if/when he sees this thread.
Nigel Atkins

It's a bit controversial Nigel, I was trying to stay away, but you've dobbed me in so--

There are two trains of thought for this,

The most common thought is that with any parallel arm rear suspension like the Bugeye the rollcentre is at the height where the lateral control mounts to the axle housing--In the case of a bugeye the lateral movement is controlled by the rigidity of the springs so the roll centre will be at the height where the mounting bolt goes through the spring eye where it attaches to the housing

Second theory is that with a four arm rear that swings from the body, the roll centre is half distance from the front pivot point to the ground which on a bugeye would be the mounting point of the front of the 1/4 eliptic spring to the ground
It would be interesting to compare these two measurements--I'm thinking they'd be almost the same height

IF you were to fit a Watts link or Panhard bar that has more lateral rigidity than the 1/4 eliptics then the roll centre will be determined by whatever height the fitted control link is-

It's important not to make a new rollcentre point any more than a couple of inches away from the natural rollcentre height otherwise the suspension will bind when cornering hard

Is that what you're after Paul

willy




William Revit

Thanks Willy, Thats exactly what I was wanting to know. Your point re a panhard rod was my concern but that makes things clearer going forward.

And thanks Nigel for prompting him!!!!!


Paul N-M
P A P Neil-MacLachlan

Just for once it was an innocent remark - but I'll take the praise as Willy can't get his own back and ever drop me in it - as I'm usually in it anyway. :)
Nigel Atkins

Paul
I'm more inclined to agree with option one the point where the spring mounts to the housing ,but it would be interesting to compare the other value as well--just out of interest
If you're fitting a Panhard rod , best to have the body mount on the driver's side and the axle housing mount on the passengers side and make the bar as long as you can possibly make it to fit in the car
I'd think a good height to start off would be about the level of the bottom of the banjo bulge , that seems to be a popular height amongst clubman type sports cars like Lotus 7s etc

willy
William Revit

Grey

Yes, I did spill saltwater in my keyboard! I do not even know how I got that special character on this tablet.

Not really, but I am finding the size the comment box and its text initially appears on my IPad a challenge, combined with my impatience not to proof read my messages properly or increase the comment box size. I quite often miss words, so maybe I am mildly dyslexic (or just impatient, typing in a rush and sloppy not to check what I type) - my wife says do the FACT test just in case. Eye prescription fine.

Cheers
Mike
M Wood

Thanks again Willy. As it happens I am going to locate as you suggest.
Mike in Ayrshire. No nothing very exiting just mucking arround.
Thanks everyone for your help.

Paul N-M
P A P Neil-MacLachlan

Paul

Good to hear the BBS has been useful.

Thanks for your reply to my nosiness - some of us are interested in the not very exciting and certainly also like mucking about too! Do you have a Frogeye or MkII Sprite, or some interesting plans on how to use an axle for one?

Like others on this forum, love to hear of other folk's projects and ideas. As well as as keen to hear of folk in Scotland, NI and Ireland - not many regular posters on the BBS unfortunately from these nearby places. I am a MkII Sprite owner which I am very slowly getting back on to the road and hopefully one day hillclimbed, I am keen to hear of other early cars and projects with 1/4 elliptic axles. Also collecting bits for a Locost 7 project, which might have an A Series engine going into it (and my 948cc Sprite engine could have ended up in a Lotus VI replica if I had been quick enough to get the chassis for one when I saw it; allowing a 1293cc build for the Sprite).

Cheers
Mike
M Wood

This thread was discussed between 17/05/2020 and 24/05/2020

MG Midget and Sprite Technical index

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