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MG Midget and Sprite Technical - Coil springs at rear?

Any one converted to coils on the rear axle? What lb/sq inch rate did you use?

Daniel Stapleton (christmas prezzie!) quotes 75 to 80 lb/sq inch in a sidebar for a 1275 leaf spring, but then states that he's still collecting data on leaf spring rates!

And are leaf spring rates the same as coil, do I need to make an allowance for inter leaf friction?

It's for a fast road car.

Thanks
Andy
a borris

Andy,

I can't help with the value, I worked it out years ago for my frogeye but can't remember the value, IIRC higher than you mentioned though. The value you want is the spring rate which is quoted in lb/inch ie the force per unit deflection, lb/ sq inch is pressure or stress.
David Billington

Hi Andy

Yhe front spings are 270lb-in; they operate on a 1:2 lever, being 1/2 way along the wishbones. Therefore, a good starting point would be ~135lb-in for vertically oriented springs...

Use adjustable dampers....

A
Anthony Cutler

Anthony,

You failed to take into account the square law that effects the spring to wheel rate conversion due to the lever arm effect. The lever arm give you an mechanical advantage but also reduces the movement of the spring so a 1:2 lever would give the wheel rate of 1/4 the spring rate.
David Billington

So very much a "suck and see", think I've a range of springs (80 to 150 lb inch) sitting in garage.

Not sure where the springs will be mounted yet, on coil overs (obviously!), but exactly where along the leading arm depends upon the best place to locate the upper mounting.

Also the further along the arm and away from the axle the spring is mounted will alter the effective spring rate.

Need to get under with a tape measure!

Andy
a borris

Andy - not coils but we do have the leaf springs you were after.

Max

Max T


Max, please email me? Would still like to buy, coils on the back is a long term project.

First job is to sort the car ready for the Spridget weekend in Germany and then driving on from there to Italy!

So will fit parts I know work without a long and painful development process! And leafs work.

Andy
a borris


Blimey! Steels got expensive, started pricing up the bits I'll need for this conversion and it's scary.

Bushes: £13 each, I'll need 4
Steel for the links £50
Shocks £55ish X 2
Was going to make it adjustable, adjusters £12 X 2
Welding £50ish
Springs £15 X 2 assuming I guess correctly on spring rate!

Over £300! Think it might wait till next winter.

Andy
a borris

Andy,

What does £50 get you in steel for your links. Sounds rather expensive to me so maybe shop around for another supplier. Also bear in mind what amount of material you need. I know my local supplier and if you need much more than half a length you may as well buy the whole length as that way you get charged length price rather than the higher price per ft.
David Billington

The steel should be nowhere near that price. WOuld have thought £25 would get you a metre or two of tube.
Tarquin


£25, that's what I thought! But that is a length of 50mm x 25mm 2mm thick RHS, which is about 5 meters more then I need, but just one cut, so I only buy only the length I actually need is dearer!

Plan A is to use a trailing and leading link (each side) located from the leaf spring brackets on large bushes to allow for axle twist.

The front spring brackets may (with a little luck!) be able to be used without modification.

I'll either make some rear brackets to use the existing rear hanger bolt holes or just weld a bracket on to the chassis rail.

Hope to fit in-situ adjusters to fine tune the axle position.

I'm not going to trust my welding skills on suspension parts, so will find an engineering firm to do the welding and machining.

Andy
a borris


And the reason for such large tubes, the shocks will be mounted on them!

Andy
a borris

why?

if you mount them directly to the axle the amount of force will be entirely vertical (or such offset as you desire) to the axle and there will be no bending effect.

Mine (yes I am drawing up a similar set up) is going to be based on U section steel with 1.5 to 2mm wall thicknesses.

It could probably even be more simply covered by using drawn steel tubing at 25mmx 2mm thickness

Still drawing and working out as yet, but having looked at lots of coilspring racers' set ups I can't see any need for redundant structure.

The good thing about discussing it though is that we have a chance of good feedback from those pioneers that went before us.

Now then where is there a free axle tube to play with?

Ah yes, pick it up on Sun Tobes...
Bill

Andy,

Sounds like a Watts linkage each side with the axle forming the centre vertical link?. IIRC that was done on the alfa sud or GTV. What are you planning on for the lateral location. If you mount the coilover off the front beam you would need a section capable of taking the bending loads but the rear link could be much lighter as it is basically just taking compressive and tensile loads.
David Billington

David for a couple of years now I have been blowing hot and cold with this idea and I am now in the mood for having a good go. I have modified my axle almost for rear discs and have managed to shorten it to get my centre lock minilites within the square arches.

Could you sketch out a basic drawing of what you have suggested there please as I am not sure of what you mean.

I had also considered simply using trailing arms onto a normal axle and supported by coil over shockers and some kind of lateral location.

would be much abliged
Robert (Bob) Midget Turbo

Robert,

From here http://alfasud.alfisti.net/indexe.htm in the "technical data" section. The Alfasud rear axle assembly which effectively has a Watts linkage each side and lateral location by a Panhard rod.




David Billington

David

Exactly right, using the leading link to mount the shock/spring, only because it seems easier to fit a bracket to and gives me more flexibility in shock length.

Also the car I've cribbed this from did, it that way!

I also believe that by having the front links pick up point below the axle and the rear above the axle gives a degree of anti-dive.

Using a panhard rod for lateral control.

F**d Cortina MK4 rear bushes for the links, but Superflex. Big but not too soft. Have to allow for the axle to twist in relation to the links. Could use rose joints at one end of each link, but not for all four.

Really doing this because I'd like to improve the ride.

Andy

a borris

david just to help me understand are the anchor points for the arms solid mounted to the axle? if so then as the axle compressed the suspension upwards would it tend to twist?
Robert (Bob) Midget Turbo

Robert,

The anchor points on the axle would have to be capable of rotation, either a bearing or compliant, otherwise you've effectively going to have a weird shaped cart spring.
David Billington

The Watt link you describe for front/rear mounting looks very like the Sabre (Reliant) system, that was replaced to good effect for the later Scimitar Coupe / GTE with twin trailing arms (per side) and Watt for sideways location.

There are several midgets I know with twin trailing arms; I have many pics if needed.

Unless you mount the spring directly on top / in line with the axle, the weight of the car will make the axle want to rotate. For this reason, many mount the spring foward of the axle so this effect is opposed by the torque from the engine.

A
Anthony Cutler

Hello Andy

We converted our midget to 4 link set up a couple of years ago. We used a 4 link kit for an escort from Rally design which from memory was £60 (with out body boxes). This worked out really well as it comes with all the rose jointed ends and poly bushes pre assembled. Seems a bargain compared to buying al parts individualy. Had to shorten some of the tubes by cutting and sleeving with another piece of tube. It also came with the axle bracket which I cut and welded to the existing spring bracket on the axle. Mounted the dampers On top of the axle as I was nervous of the loads going through angles if the dampers were below. Cut holes in the floor a near th rear bulkhead and welded in a cross brace to attach the dampers too. Adapted the front spring hanger for lower arm and made simple bolt in brackets for upper arms It has done numerious track days and sprints, regularly inspected the welds and all has been well. We are currently fitting escort turrets to relocate the dampers further out board and vertically to hopefully see further improvements. Can email pics if it helps.
James
t fisher

James

Please email photos, andyborris@yahoo.com. Has it improved the ride? I'm hoping to help the car cope with the poorly maintained roads we have around here, car crashes and bangs through the potholes and this upsets the handling!

And also makes me feel as if I'm killing the car

Rallydesign is just 10 mins from me, often buy parts from them, very good prices and nice chaps too.

I had a look at their 4 link kit, realised that the top links are too long and wasn't sure how well it would work with them (the top links) shortened.

But as ever, an once of experience is worth a pound of theory (to paraphrase some one!).

Thanks

Andy
a borris

Also, sorry a bit hung over, what shock length and spring rate did you use?

Andy
a borris

Hello Andy
The ride has improved massively. With 150lb springs and and the dampers on just 2 clicks it rides pot holed roads like a limo. Much to soft for spirited track driving but a change of springs and turn up the damping and its a different car. Best bit is the total lack of axle tramp. We have tried 150 200 300 and 350 lb springs 200 is generally the best but we do some times switch to 300 for tight twisty events. This is with 400lb fronts.
The dampers are GAZ 1.9ID 14" open and 9.5"closed.
Will send some photos, as said before we are currently making some geometry changes.
James
t fisher

James and also Ant, could you also send me any pictures you may have on any set up please to
rjwelchmidget"at"aol.com
Many thanks
Robert (Bob) Midget Turbo

James,

Have you tried fitting an ARB at the rear. Starting off with the usual front ARB on my frogeye, 5/8" IIRC in my case plus some later ARB effectiveness enhancements, I found the understeer became excessive on tight circuits like Wiscombe and Cricket St Thomas at the hairpins particularly. Reducing the rear 1/4 elliptic spring stiffness but increasing the rear roll stiffness with an ARB sorted the issues to my satisfaction quite well. The benefit of the ARB being the increase in roll stiffness with reduced single wheel bump stiffness compared to increasing the spring rates alone to give increased roll stiffness.
David Billington

Could anyone pleasepost these pics 'online' for all to see?? --if the owners dont object!
Mick - The engine is in!

James, thanks for the photos, very helpful.

Can I have your permission to post them?

Does seem to be quite a bit of interest!

Thanks also for the spring data.

Andy
a borris

I suggested 135lb-in rear as a guess for std front springs at 271 lb-in.

James suggests 200lb-in at rear for use in his 400lb-in front spings - approx the same ratio.

James' Midget ('shared' with Tamsin) is a highly developed car, that started with Tam's dad fitting FL front susp, Turbo-A, then a Kseries for MGCC SC events (in an arms-race, possibly with BRB). James has taken on engine and suspension development making a very rapid car (with either at the helm).

Tamsin is organising a Midget-specific track-day early in 09 so we can check out the Kseries engines and suspensions... Ollie Neaves has suggested he should also contribute his Kseries B racer. I'm interested in trying BRB (relatively standard, with FL at the front and FL dampers/RTL at the rear) with the 4-link, 5-link and independent conversions.

Anyone else interested in taking part?

A
Anthony Cutler

Ant, yes very possibly, it might be useful to have a FISC-spec car as a yardstick for more sophisticated mods; do we know where and when?
David Smith

Ant - sounds perfect for me to run the motor in, can you put me on circulation list?

My 4-link rear was somewhat more basic. It used a 1/4 elliptic rear axle with springs replaced with trailing arms similar to standard upper arms. The bottom link had a mounting point for the Jaguar XJS coil spring/damper unit which went up and mounted on the (reinforced) check strap mount. Latterly we cut'n'shut an Alfasud (? memory dims) panhard rod. It also had an anti-roll bar mounted on the body and clamped to the lower link, with adjustable sleeves to change the settings.

I wish I'd known then what I know now about how cars were handling.....I just did it because it copied other suspension setups, not because it made me any quicker.

I also wish I'd gone testing not drinking....

Last time I saw this setup it was on a trailer heading to David S's place - did you ever sell it David or do you still have the parts for reference?
Max T

http://www.spritespot.com/gallery/view_photo.php?set_albumName=toomanyspridgets&id=redb

Quite a few more pics on this page if you want to have a look. No memories of the spring rates but the whole lot came off a Lotus 7 replica.
rob multi-sheds thomas

....or.....



....which uses the standard late axle.


rob multi-sheds thomas

Max said - "Last time I saw this setup it was on a trailer heading to David S's place - did you ever sell it David or do you still have the parts for reference?"
Good lord I know I'm a hoarder but that must be well over 10 years ago, gone to the tip long ago. Although I admitt I still have your old ally & f/glass detachable rear end....
David Smith

The trackday was at Bedford in February booking via Mgs on Track - I'm not organising one from scratch. Just druming up interest to get more midgets out together doing this track day.
TVF


t fisher

Hello Andy
Fine publish the photos if you like. On the ARB question, the traits you describe sounds similar to us. Rather than using arb I am relocatng the dampers further outboard and more verticle to try to improve the roll stiffness by making better use of the springs and dampers.
James
t fisher

>>>>>>>>The trackday was at Bedford in February booking via Mgs on Track

Blast means I can't go as the car's not MOT-ed and I hope to have sold the car with a tax before then ;-)
Max T

... I'm otherwise engaged for that weekend (work)...

A
Anthony Cutler

Can those people that have advised spring poundage also put those poundages into context by supplying the weight of the car or car and driver.

Thanks
Daniel Thirteen-Twelve

Added James pictures of his rear end (the cars!) very interesting and nicely done.

To save bandwidth I've put them into my Flickr account.http://www.flickr.com/photos/11305116@N05/

Andy
a borris

Hello Daniel
You don't expect us to give all our secrets away! As Ant pointed out it has a K series, so engine is lighter and glass fibre front with no radiator cowl so the front of the car at least should be lighter than a standard midget.
James
t fisher

This thread was discussed between 29/12/2008 and 04/01/2009

MG Midget and Sprite Technical index

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