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MG Midget and Sprite Technical - Leaf Spring geometry/movements

Hi all.

As the front is now better, it's possible to go round corners substantially faster than it was previously. I've burst the stitching on the side bolster of the driver's seat...

This also means that the inside of the left hand tyre is sometimes catching on the leaf spring. Everything is really tight and poly-bushed. I'm also hitting the bumpstops on both sides on lumpy roads. (btw, at 'normal' speed, none of this is an issue)

Obvious solution is a panhard rod, but I was thinking I'd like to know a bit more about how the suspension behaves first.

Trawling the archives has brought up some points:

1) the front part of the leaf is the bit that does the location

2) the back part of the leaf is the bit that does the springing

Somebody wrote a book once (can't think who ;]) that has a picture in it of a clamp that goes on the spring, there was one thread mentioning this but no real conclusion.

Which bit of the suspension is flexing sideways? Is it the metalastic bush in the front eye, or is it the actual leaves bending in a horizontal arc?

If it is the latter, is a spring clamp on the front part of the spring that reduces sideways flex (and makes the spring a bit harder re bumpstops) a good idea? Presumably this would reduce tramp a bit.

If it isn't a good idea, please say so! I'm aware of the fact that the back needs to be softer than the front. My springs are fairly flat, so maybe this is part of the problem, they're not that old but are the 'self lowering' cheaper type.

Car handles really well at the moment, so maybe just more restraint is the answer.

interested in some views :)

Rob Armstrong

<< so maybe just more restraint is the answer >>

?

!
David Cox

You heathen !!!... Have you no respect for the rules of the road

...hahaha, yeah nither do I

To be honest i cant remember which suspension you have...is it the 1/4 or the later model 1/2 eliptic

But regardless...the pan hard should have came from the factory as orginal....its a great addition, i doulbt id ever own another midget without one...it really locks down the car to the road....i have the 1/2 eliptic
Prop and the Blackhole Midget

My thoughts exactly David!

How does the Panhard cope with genuine road use? I feel it has some limitations with larger suspension movements.

I wasn't actually breaking the speed limit on this particular piece of road ;). Dibble Bridge Bank, if any of you Yorkshire wanderers know it, was the piece of road causing the issue. Can't have been too silly as the passenger was asleep for most of it.

Guess the piggy bank will have to refill fast. I want some better seats as well...
Rob Armstrong

I have a (Peter May) panhard rod on my RWA. No problems at all on "genuine" road use (which is what I do most), though my car is somewhat lowered, which may reduce somewhat total suspention movement.

I do agree with Prop that it's a seriously good and straightforward improvement.
Philip Dodd

Leaf springs don't really give suspension a geometry, IMO... they keep the body raised off the axle, but have so many ways in which to distort under load, the relationship between the back axle and the car is somewhat fluid. The video on Youtube of the Austin-Healey is instructive, esp. when you realise it has an additional locating link (or 2).

The secret to making it better is providing additional constraint that doesn't bind up (or create undue forces on mounting points) with movement such as roll.

I think spring clamps are an example... given that the spring must distort to allow roll, it seems to me that a clamp ensures that this distotion takes place (eg.) at the very leading (un-clamped) edge of the spring and the bush... leading to binding and unwanted forces on the mounting point. (Binding of course leads to more friction and which means the bush tends to grab until the force builds up and is released by sudden movement... not good for feeling your way around the limits...).

Anti-tramp bars seem to work well, provided they have compliant bushes at either end for that reason.

Lowered rear suspension helps, since the CoG of the body comes nearer the roll-centre and so roll is reduced with less sideways movement, and the car feels more controllable.

A Panhard is hard to fault, since they normally use spherical joints which don't bind up; and the constraint on geometry doesn't cause binding of the leaf-springs and bushes.

But a Panhard isn't symmetric and has two pivot points normally about 1/5 of the body width from each side. But with little suspension movement they are a good, cheap (well, everything's relative), easily-fitted solution.

If you're a spirited driver and use sticky tyres, a Panhard is a near necessity.

A Watt link is better still, since geometry is improved; and Mumford more so... but this is a trade-off of complexity/cost and benefit.

I made my own Panhard some years back out of an old MGB steering rack parts and spare square tube. I was very impressed with the instant improvement. But once I got used to it (and with 2x the torque of std A-series) I could feel the body/axle squirming when WOT over undulations (quite unpleasant, as if something imnportant at the back has become disconnected).

Staying away from this condition, it did everything I wanted and you could feel the benefit on the road, e.g. you know the bends at the top of the M5? Without the Panhard, the faster you drove, the less steering movement was needed; this can be fun, but demonstrates how much the body is moving on the axle. With the Panhard the steering around these bends becomes much more linear ie. about the same for any speed.

To finish: a pic of BRB at Prescott showing a considerable sideways movement (check how much tread on view compared with stationary). Some weeks later I had a 'off' at Harewood caused by the tyre binding against the body; hence my home-made Panhard shortly after.

A




Anthony Cutler

Good analysis Ant. The photo is highly instructive! Proves how superior square arch cars are as they stop the wheels trying to get out of bed like that hehe. Curious how the back wheel appears to have positive camber too, an illusion created by body and front suspension angle.

My only comment is that it is possible to locate tramp bars so they don't provide any constraint on the axle's vertical movement. However most I've seen fitted don't achieve this because they assume a simple parallelogram, wheras in reality the changing effective length of the spring as it deflects makes it much more complex.

The Healey video is well worth a watch, but I can't remember the link :-(

Paul Walbran

I still get tyre rub on the leaf springs with a panhard fitted. When you look at it parked up you can't believe there can ever be a condition when they could touch but they do. The panhard is still one of the best things you can fit though. By far the best pound per performance benefit I made to my car. Just make sure its parallel to the ground when the the suspension is in it's neutral position.
John Payne

John,

You might consider a spacer between the wheel and the hub...any decent machine shop can make one....im sure others will throw up there hands at me for saying that... But what is worse, a blow out from tire rub or the possiability of a 1 wheel stud breaking 5 years from now

Absolutly correct about the panhard alinment...

It needs to be paraell on all 3 axis....like the corner of a box...vertical, horizontial, and for/aft...as it relates to the car body and the axle
Prop and the Blackhole Midget

I got my pan hard from winners circle...but i had to modify the part that fit onto the axle/spring area as i didnt want to lower the car... And the pan hard kit was made for lowered racing cars...but it was so worth it

Prop
Prop and the Blackhole Midget

will look at building a PH rod then, seems from the archives rubber bushes are a bit flexi. Might raid the old track rod end pile...

presumably the tube is loaded purely along it's length? Might have to beef up the floor a bit.

I already have a 1/4 inch spacer on the back, as John says it's almost unbelievable how much the axle moves. I must have a good inch between tyre and spring.
Rob Armstrong

On my winners circle PH

The body part runs along the front of the gas tank and uses the same bolt holes then it drops down on the left side with an adjustable plate for lowered suspension,1-4 inches of drop... Then a rose hymn jointed rod is connected to that plate and comes back across to the right side of the car and connects to a leaf spring/ axle plate

At least thats how i remember it....then you just adjust the rose hymn rod so that its not activated at stand sill plus a little extra body roll...ive got mine adjusted so it comes on fairly quick...going thur a speeding left turn 90 degree intersection i can feel it coming on...but i think its what you feel comfortable with

I hope that helps in some way... I can post some pics of my setup in a few days if that helps

Prop
Prop and the Blackhole Midget

Hey rob...

.any chance it could be "economy" tires and the side walls are flexing under performance and causing them to rub...id think an inch would be aduquite

"""...I already have a 1/4 inch spacer on the back, as John says it's almost unbelievable how much the axle moves. I must have a good inch between tyre and spring....""
Prop and the Blackhole Midget

>>> I still get tyre rub on the leaf springs with a panhard fitted. <<<

I don't think a Panhard rod will alleviate sidewalls rubbing on springs; arches maybe, but not springs.

-:G:-
Gryf Ketcherside

they are continental ecocontacts on the back, flex is probably due to 70 section tyres.

presumably if the sideways flexing of the springs is reduced (by whichever method), then the max angle between tyre and spring is reduced so there's less likelyhood of it catching?

Perhaps new springs may also be needed. The healey video is great, really helps with trying to visualise what's going on.
Rob Armstrong

I do not like my panhard.
But the rubbing in the arches is not acceptable without it.
I do not like the side ways action on the rear it creates.
Most noticed on speed bumps taken at some speed.
On flat roads and in corners it is okay, but we have a lot of speed bumps here in Holland.

If you go through the trouble of making one then why not go the smallest bit extra and go for a watts link.
Almost as simple and non of the bad points
Onno K

<<< we have a lot of speed bumps here in Holland >>>

probably the highest things over there...

I'll get me coat...
David Cox

Onno..has a great point...

if your going to hand fab a PH, might as well go all in and do the watts link..or even the mumford

In for a penny, in for a pound

Prop
Prop and the Blackhole Midget

Watt linkage looks much better from an engineering view, and not a whole lot more work to build. Think I'd go for a chassis mounted pivot behind the diff, similar to the frontline rtl mount, some bits of tube and rod ends under the spring hangers. More research is needed into the lengths needed and roll centres and things, shouldn't be too challenging!

Any tips for Watt linkage design?
Rob Armstrong

Yes - given that the main force on the rod is along its length, don't make it too big in diameter; something like 5/16 is enough... (assumes you will use rose-joints or equivalent).

For a Watt link, remember the pivot point is the roll-centre; so this needs to be low - no higher than the axle/diff-housing centre.

The plate to mount the pivot gets in the way of the diff fill-up hole; hence many implementations mount the pivot on the back of the diff in the middle, usually a little lower than the fill-up hole to allow access.

Check articulation of the 3 rods for whole of suspension vertical movement, so you can be certain that they don't bind.

The forces on the mounting plate will be mainly side-side; so only light bracing needed front-rear.

I had a Watt on the Salisbury axle on all 3 of my Scimitars (GT (Coupe), GTE 5a, GTE 6) and all worked well with no maintenance/repair required (bushed joints...). I recently re-furbed my RTL after 10 years and needed to replace 3 rubber boots, 2 spherical rod-ends and one spherical bearing (one of the pivots); not bad but good to keeep in mind that rose-joints aren't 'for life', even booted.

A
Anthony Cutler

thanks Ant. I might use Track rod ends (If we have enough 'spares' around) - d'you reckon they'd be strong enough?

Any views on chassis mount pivot vs axle mount pivot? To me it seems easier to do an RTL style bracket and axle mount the ends of the link.
Rob Armstrong

Rob,

Here are some pics of a Watts link I made for a mates Austin Sprite http://www.metal-arts.co.uk/temp/Watts/ . The full linkage picture is actually of the one for my frogeye but the rest are from the Austin Sprite, the difference being the one on the Austin Sprite needed a kink to clear the fuel pipe. The links used TR6 inner wishbone bushes (1/2" ID inner sleeve in 1" ID tube) at the outer end and Spitfire bushes (3/8" ID inner sleeve in 3/4" tube IIRC) at the inner end and a 3/4" ID Oilite bush for the pivot. Not sure how it has faired as my mate is slowly rebuilding the car but he has said before that the link is the only thing on the car still in good condition.
David Billington

those are plenty helpful David - thanks a lot! I have to say that doesn't look much more difficult than an Panhard to make!

Quite like those radius arms as well... NO! one job at a time!

Cheers :)
Rob Armstrong

Rob, as a quick easy method of radius arms you could use early midget mounts at the front and simply weld tabs to the axle for the other end and buy the pre made rose jointed arms off ebay.

But then, why not just do a full 6-link and forget about having to fix or replace anything in the back end for a long long time
PeterJMoore

Track-rod ends will cope fine.

A
Anthony Cutler

BTW - meant to add:

DB's frame neatly overcomes one of the points I raised; also a single upper trailing-arm per side moves you 80% of the way to a 5 (or 6) link but without the need for coil-overs... so only 20% of the effort needed (all %ages approximate!).

The only downside is that leaf-spring has a little more unsprung weight and a lot more inherent friction and consequent hysteresis.

A
Anthony Cutler

here's some interesting (if you like that kind of thing) screenshots from a migrane inducing video, left hander and right hander viewed from on top of the wheel.

For starters, here's what it looks like with me in it, at rest. Tyre looks OK, and there's a little bit of wheel between the centre cap hole and the wheel arch.

For reference, tyres are 175/70 on 13*5 minators. Camera is suckered to the rear deck on an extension (and tied on as well!) This is the driver's side. Tyre pressure is 27 cold.






Rob Armstrong

Cue a left hander.

90 degree left hander in second gear, accelerate through, not terribly fast, and absolutely nowhere near the limit.

Look at that tyre deflection! That's surprised me.

Wheel's also gone under the car a lot, partly due to body roll probably.


Rob Armstrong

and a right hander

Hello tyre!

this is what can be felt as Ant describes on the M5 sliproad.

Flex is enough to get rid of the squashed piece of tyre at the bottom.

I can upload the videos if anyone wants to test how good paracetamol is vs neurofen.

Interesting stuff. (I think, at least)




Rob Armstrong

Rob,
always interesting, did you miss the videos of the track cars showing rear suspension and axle movement

tyres in particular are very complex (and by some under valued) components and not even the manufacturing engineers have a full grasp on them
Nigel Atkins

Hi Rob

Nice pics; would like to see the vid.

BTW - top of M5... I'm referring to the (relatively) sharp series of bends at the top of the M5; I always treat them as a bit of a suspension test, whatever car I'm in (only kidding, officer...); a bit like the bump-steer test when turning off the M5 southbound at M5 J11a.

A
Anthony Cutler

will stick video up. I couldn't get anywhere good to stick the camera under the car, I'll keep trying though!

Saw the track car vids, that's what gave me this idea!
Rob Armstrong

Rob,

BTW on the Watts linkages I did what isn't shown in the pics is a IIRC 1/2" spacer that goes between the tank flange and boot floor on the right hand side so the mounting frame on that side clamps the tank flange.
David Billington

Cheers David :) that's a much simpler arrangement than the one I was thinking of...

Video 1

http://www.youtube.com/watch?v=gQsNgDSMiS4&feature=plcp

Video 2
http://www.youtube.com/watch?v=CLobOcY9WM8&feature=context-cha

Noise is pretty bad, and it looks a helluva lot faster than it actually is! Managed to link some google maps to it sort of so really interested people can work out what's happening.
Rob Armstrong

Peter Moore - you mention a 6 link - any particular reason for preferring this layout?

I've been very seriously considering a 3 link, with the lower link being a Lotus style A Frame to offer sideways location as well (and a low roll centre). Seems to me to be about the easiest, simplest, and lightest option if going to the effort of ditching the leaves. I've also seen some pictures of a Dave Headley MGB suspension (well regarded US race builder: http://www.fast-mg.com/) of this design. If anyone has any thoughts on this idea then I'd love to hear them?
Andrew F

this sort of thing?

http://www.mgexperience.net/phorum/read.php?41,1966779,page=1

I thought of that, but I'm not sure I could get it to mount strongly enough. My idea had it the other way round than that (one point on body) to try and reduce cracking the axle in a lotus fashion, but this puts the loads into the bodyshell at a point where it's not strong. I looked at the single link and thought it wouldn't be strong enough...

I suspect that *fast & on the limit* road use may well be more hard on components than track use? Reason behind that thinking is the bumpy surfaces and more 'sudden' response of suspension needed, as long with having to cope with more travel.

Rob Armstrong

Andrew - its not only proven time and time again to be the most effective, but the strongest and requires the least maintenance. On top of that, axle location is very positive.

WDT Corry on here is able to offer a complete rear end conversion. The sheer grip available is outstanding and the way the car rides over rough ground and uneven surfaces is simply mind blowing when compared to a leaf sprung midget.

There are MANY other options, and no doubt hundreds of members which will happily argue that their £250 back end conversion is better, cheaper, more effective, but I can almost guarantee you those cars have not or will ever see the abuse that Wills car does and as far as rear end development goes for the Midget/Sprite there is simply nothing out there to beat the 6-link setup.

It's very hard to explain just how good the setup is, but i can almost guarantee when it comes to rear end traction, predictability and overall feel when driving there wont be another Spridget in the UK comes close.

You can see just how steady the car is during suspension movement in the following videos

http://youtu.be/YkS_fmX06FE?hd=1&t=20s

http://youtu.be/Hpbag6YA5L4?hd=1

PeterJMoore

that's very impressive, both the driving and the car!

I think I'll go for watt to start with, then perhaps radius arms if needed...
Rob Armstrong

Robb your going with what ???

At 1st you said PH rod, So whats ....your 2nd choice

Prop
Prop and the Blackhole Midget

watt is my first choice now Prop ;)
Rob Armstrong

Though I am impressed with the driving and setup Will has I would never go for it.

To me it is as a K engine conversion a step to far.
It just ask for to mutch mods in the shell in my opinion.

Though the midget is a product produced in big series it is a design now over 50 years old.
I like the old design and I want to respect it and make all the mods reverseable.

So a leafsprung setup with a watts and anti tramp bars is the most sophisticated suspention available to me.

Onno K

I'm not a fan of the A-frame, since it greatly increases the load on the mounting points compared with a Panhard/Watt/Mumford; this load increases with the distance between the body mount and axle mount positions - it's like using a larger lever with the same force at the far end.

If you make this lever increasingly short to reduce the multiplication of the forces (ie. "bring the axle forward to the body"), you end up with something like a Panhard.

Panhard and trailing arm are at 90 degrees to each other and each deal predominantly with forces solely in the direction of the length of the link, with no multiplication effect.

I'm not saying an A-frame can't be made to work well - just that it needs to be a great deal stronger throughout due to the higher forces that result.

A
Anthony Cutler

Thanks for all the comments!

Rob: Yes, exactly like that! The advantage I see over an A Frame with the pivot on the body is that it can use the stock leaf mounting points. The disadvantage (depending on what you read/who you listen to) is that the roll centre moves with the axle rather than being a fixed point on the body.

Peter: I can't view those videos at work but will have a look when I get home. I'm located in Australia, so much as your arrangement is probably great, I don't think shipping my car over to have it converted would be very economical! I also like to do things myself where I can. Do you have any pictures or diagrams of the setup you can share, or is that secret information (I understand if so given that he is selling the setup)? By 6 link, am I right to presume a 4 link plus body mounted watts? Any clues on where the roll centre is located?

Onno: I know what you mean and have been pondering that myself - it's actually one of the positives in my opinion of the 3 link A frame arrangement - I believe it could be done with very little modification of the shell, and it retains fairly similar to stock geometry. It is something I consider - it's probably the main reason I stuck with the A Series too. But I may still end up sticking with leafs and some kind of sideways location for this reason.

Anthony: While I hear you on the leverage compared to a Panhard, is it not the same forces, in the same directions, as on a stock leaf spring setup with no sideways location? No question the mount on the axle needs to be beefy and well braced (have a look at the link Rob provided), but it seems to me the forces on the leaf spring mounts should be the same as stock? Admittedly you lose the benefit of the additional rear mount on a semi-elliptic car, but it surely is the same forces as a quarter elliptic car would see?

Is there any differences between the front leaf sping mounts of semi-elliptic and quarter elliptic cars? I've never closely looked at a quarter elliptic rear end.

What are peoples thoughts on ideal roll centre for the rear of a Midget? Has anyone a model or diagram of the front and/or rear suspension offering base measurements that could be completed with ones own cars measurements to calculate roll centres etc?
Andrew F

1/4 elliptic rear end is substantially reinforced compared with semi elliptic. Apart from the very first few frog-eyes which came out without the reinforcement and showed why it was needed.
Paul Walbran

Hi Andrew

The roll centre for 1/2 elliptical springs is a moveable feast; it depends on the instantaneous configuraton of the leaves themselves, which changes due to defelction and distortion. Static, it's on the c/l of the car and vertically between lower and upper spring mounts; often taken to be in line with the diff itself, for simplicity - treating the front part of the springs as radius arms.

The forces on the A-frame will be higher, since there is no 'give' (free-play) in the system, as there is with leaf-spings; the impulse loading will be much higher.

A
Anthony Cutler

Anthony: Thanks, I was mostly wondering about the front roll centre, but I guess I'll measure it out myself - I figure there is going to be a lot of variables between cars in this area (longer wishbones/shorter Frontline top arms/offset bushes/different spring lengths). I just thought someone may have a diagram that I could use as a baseline and edit with my own measurements. I know the roll centre of a leaf sprung rear is approximate at best. After thinking about it a bit more I had come to a similar conclusion about the loading on the leaf spring mounting points.

Paul: Thanks for the info.

Based on the above and some further thought I will definitely be further bracing the standard leaf mounting points if I decide to use them for an A Frame style suspension.

Sorry if I sidetracked your thread Rob!

I guess my main concern is that any sideways restraint that improves the roll centre (low mounted panhard or watts) will then be fighting the natural geometry of the leaf setup. And if I'm going to be fabbing up mounts and arms for a Watts or Panhard, and probably some traction rods or similar later when I fit the supercharger, then it's not much more work and presumably a better result to just do away with the leafs altogether?
Andrew F

Leaf spring geometry is a whole lot more complex than you might think - get books!
Might want to look up Curtis Jacobsen's British Race Car site for loads of loverly autoerotica. Real cars that really work! If you can't find it go look on MGExperience>Motorsports, link will show up soon. One trick used is to retain the leaf springs, but divorce them from all location duty, as some race MGBs do. Then you can use composite leaf springs too!

FDRM
FR Millmore

True, but I'm pretty sure that the only reason they do that is that they are required by the rules to retain leaves? Not proclaiming myself an expert though. Yes I've spent a lot of time looking at Britishv8 and BritishRaceCar, lots of good ideas to investigate and think about.
Andrew F

Hi Rob
I think you should forget about panhard rod or watts linkages and sort out the tyre rubbing issue. What size tyre and wheel are you running and what pressure. I used to run 185/60 on 6" rims and no rod, and remove the clamps that are on the spring right where the tyre bulges. The only time I had any tyre rubbing was if the tyres dropped more than 2 psi which they did due to old alloys. I ran with this for many years and did Castle Combe practice days with no problems. It tyre movement. Push your car side ways unless your U bolts are loose the movement is in the tyre wall.
N Handford

No sidetracking, it's all about rear suspension location so all good!

It's all in the tyre wall, the U bolts are done up monstrous tight. Polybushed everything (apart from metalastic eye)

Tyres are 175/70 R13 on the KN 5 inch minnilite style. Back ones are run at 28psi, any harder and it's oversteery, and softer and it's wavery at the back. Totally even wear across the tread.

How heavy was your car? Mine's got me, passenger, spare wheel, massive toolkit, spare water, full tank of fuel etc, most of that is hung very close to the rear.

It's already running a 4mm spacer on the back. Any more and I'll hit the arches.

The images show a lot more than just tyre flex (on both mine and Ant's car) I think the rubbing is a combination of flex in the sidewall and in the spring, if I can stop the flex in the spring then I can reduce the chance of rubbing.

This is only under quite severe provocation, normal 'making progress' there's no rubbing.

Just missed out on a scimitar watt linkage on ebay, thought that might be a good starting point but maybe not.
Rob Armstrong

Mine was standard and a boot full of tools, water, fuel and spare on the road and empty on the track. I run the car low by mixing the springs and adding one thicker flat one, see diagram, which all mean when loaded the springs are pretty flat. The more curve the more they flex sideways.
But I still think your 70 series tyres are the problem. My 185/60 never rubbed on the outside because that tyre (inside of bend) wouldn't have the side pressure of the one on the outside. I still run the 6" rims but I now have 165/65 which allows me to play with the pressures a bit more with no worrys.


N Handford

Ant, you have RTL and drivers side anti tramp?

What is the axle location with RTL only ?
I suspect with my panard , the axle was twisting ad locking under compression and WOT
d cusworth

This thread was discussed between 01/10/2012 and 21/10/2012

MG Midget and Sprite Technical index

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