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MG MGA - Parabolic Springs

I have seen parabolic springs advertised for the MGB but not for the MGA. I understand that these types of springs require the modern style of shock absorber, but other than that I wonder why they have not been developed for the MGA. Or have they, and not proved successful?

Steve
Steve Gyles

Hi Steve,

These were all the rage with the RV8 was release.

I'm interested in these for my twin cam racer.
They were once available for MGA's but then ran out of stock as I understand it as I enquired several years ago.

See Barney's page here:
http://mgaguru.com/mgtech/suspensn/rs202.htm

Which states about 1/4 the weight of steel springs.

also here from 2001:
http://www.mgbmga.com/tech/mga10a.htm

the only difference is MGB springs have a larger mounting eye at the front which is OK to fit to MGA's anyway, so probably why they dropped them for MGA's unless someone knows otherwise

Here is a good article:

http://www.mgbmga.com/tech/mgb14d.htm

What I'm interested in (hoping for 160 BHP) is the tramp properties of these vs standard springs.

Barnies site states the working load for the standard springs is:

http://mgaguru.com/mgtech/suspensn/rs105.htm

Average load rate: 125 Lbs/Ins.
Working load: 450 lb. at 3.6 ins. deflection

So we would need to know what the ratings of the composite springs are.

Moss UK show 100Lbs or 116 Lbs MGB Parabolic springs for £150 here:

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=7978


Thanks
Mark
Mark Hester

My web pages do not say anything about parabolic springs. Composite springs (fiberglass) are not necessarily related to parabolic springs (although they could be parabolic in design). Are we off topic, or was the original question wrong?

Leaf springs are commonly called elliptical or semi-elliptical,as they are shaped like a section from an ellipse. Held at both ends and loaded in the center, when the three points are in line the leaf spring which started out as an elliptical section will be flat and straight, which is the desired end condition, which is why they are shaped like an ellipse to begin with.

Circle, ellipse, parabola, and hyperbola are all conic sections that can be expressed using the same quadratic equation. Parabolic springs are a very special configuration of a leaf spring, but it takes a bit to explain why they are parabolic rather than elliptical.

The typical vintage design leaf spring has a leaf or leaves that are made from flat strip stock, equal thickness and equal width for the full length. When a leaf like this is bent by opposing force at three points, it sees highest stress at the center point and lowest stress at the end points. Low stress metal in a spring means extra weight required for no useful purpose, as the whole spring has to be thick enough in section to withstand the highest stress at the center point with the desired load.

Next step is to stack up multiple leaves that are progressively shorter. This effectively places more material near the middle of the spring and less material near the ends. This reduces stress in the middle and increases stress near the ends. The idea is to have more equal stress over the length of the spring so you don't have excess material at the ends, and therefore a lighter spring.

The "parabolic" spring takes this one step farther and turns the spring of varying thickness into a single leaf. To have equal stress over the full length of the single leaf spring, it has to be thicker in the middle and thinner at the ends. When you do this the thicker section in the middle bends less, and the thinner section near the ends bends more. So to end up with the desired flat condition under load, the initial unloaded curve form needs to be a parabola rather than an ellipse. Therefore the name, parabolic spring.

As fate would have it, these newer technology composite springs with single leaf may be parabolic as a simple exercise of good engineering design, along with being thick in the middle and thinner at the ends.

Now if the original question was targeted at composite springs (that coincidentally may also be parabolic), then lighter weight may be the primary objective, and loss of inter-leaf friction is a collateral bonus. For a leaf spring that is attached to the frame at both ends, and to the live axle in the center, slightly more than half of the spring weight will be un-sprung weight. Changing two springs form 40 pounds to 10 pounds reduces vehicle weight by 30 pounds, and un-sprung weight by almost 20 pounds.

That's all good in my book, but I'm not sure I want to pay for it. I suspect the parabolic or composite springs are not allowed in most racing classes, especially vintage racing. For street use is may be more a matter of feeling rich and wanting to spend some money to improve ride quality and handling a bit. I kind of like my vintage MG the way it is. If you reduce weight and improve acceleration and handling and ride quality, you might spoil the vintage car experience. How much money do you want to spend to drive a Mazda or Honda the looks like an MG? Anyone checked out the cost of an S2000 engine and chassis conversion recently?

On the other hand, if the special composite springs were same cost or cheaper than standard replacement springs, we might have something popular (like aftermarket electronic fuel pump).
Barney Gaylord

Further, composite suspension locating parts can be subject to no-warning catastrophic failure. Friend of mine flipped his modified down the front straight because of such a failure, on a new part, though it was a radius rod rather than a spring. Composite valve train pushrods are known to do the same. It seems to usually be the result of insignificant, invisible, and possibly internal flaws or damage. There are designs used which use composite springs but real metal locators. I believe you can find some on MGB on Curtis Jacobson's fine British race car site.

FRM
FR Millmore

Interesting stuff. I do not have any burning urge nor funds to fit parabolic springs. It was just that I have seen them advertised for quite some time in MG magazines, but always for the MGB. Knowing that there are many design compatibilities between the 2 cars I was interested to know if the springs advertised were one such item, provided of course that the damper units were also changed over.

The follow-on question would have been: any particular advantage for the ordinary street car MGA? Weight would have been one obvious answer; the other was handling.

Steve
Steve Gyles

Weight but especially unsprung weght, and no uncontrolled/variable interleaf friction.

My big truck trailer had parabolic steel springs, and they broke sometimes, but were cheap and light and compact. And the axles were located by big steel radius rods.

FRM
FR Millmore

Hi Barney,

Good reply. Will you be updating your web site with any of this wisdom?

I guess the jury is still out on failure rates.

Cost of these springs was probative but now clearly not. Plus postage would be alot less

I have no issue with improving the ride quality. Telescopic rear shocks achieve this nicely and these have been accepted practice for decades.

Do we know if they sag any faster than steel springs?

Also I'm interested in their tramp properties. Are they comparable?

If this topic had come up before MGLive the other week, I would have looked under several race cars at Silverstone.

If composite springs shown on your site Barney are made from fibreglass, then what are these Parabolic springs likely to be made from ?

http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=7978

Thanks
Mark
Mark Hester

This article makes good reading:

http://www.british-cars.org/mgb-technical-bbs/parabolic-springs-the-report-2010061100551210161.htm

Mr Booker writes:

My own experience is that parbolics bend under load, remain bent and have reduced lateral stability such that tyres rub on the arches on mild cornering. Ground clearance was greatly reduced as a result of the bending. I junked the ones I had after a few weeks.

If you look on the MGOC site, they sell Parabolic springs for £199 and state:

16 - Parabolic Rear Spring Kit PSK06

IMPORTANT: Parabolic springs have been designed and developed to provide increased comfort levels when fitted to factory standard and mild tuned vehicles. Where performance significantly exceeds factory levels i.e performance V8s or Supercharged MGBs, there is the potential to wind up and flatten parabolic rear springs; in such instances we would recommend the use of an anti-tramp location kit. Should you wish to fit parabolics on a non-standard application please contact our sales team for more advice

With precise stress distribution throughout their length, the springs can flex to absorb minor impacts while retaining their load bearing capability. Thus ride comfort is greatly improved, and so is handling because of the in conjunction with telescopic rear shock absorbers.greater tyre to road contact achieved. Parabolic spring kits also include fast road urethane rear suspension bushes and must be fitted

Mark Hester

Mark

Most interesting. Like Barney, I think I will keep to the vintage feel. I am glad I asked the question. Broadens the knowledge.

Steve
Steve Gyles

This thread was discussed between 10/07/2012 and 13/07/2012

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