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MG Midget and Sprite Technical - Oops... another side effect of a heavier ARB

I heard a new noise from the front of the midget while driving home from work today. It was pretty evident when on the cobblestones of Main Street, a block from home... so I had a look underneath when I parked up, and found the following (see pic below).

Some of you may recall that I fitted a 3/4" ARB several years ago, and both chassis mounting points failed a bit later. Pics here: http://www.flickr.com/photos/safety_fast/sets/72157623942645649/

So apparently this is another "benefit" of the heavier bar. :-(

When I replace this plate, I plan to place a washer on either side of the drop-link hole on both plates, to reinforce the area.

-:G:-


Gryf Ketcherside

Wow, never seen that before...even on race cars.

Not much paint on it.
Dave O'Neill2

In this pic: http://www.flickr.com/photos/safety_fast/4609235161/in/set-72157623942645649

... the ARB link seems to be pushing the plate more from the front than below; this will put a great deal more stress on the plate/wishpan edge than they were designed for.

The fail has occurred through the force on the plate bending it very slight 'back-and-forth' over many cycles. Examine the break to look at the history - rusty old cracks and clean parts that finally failed.

Are you sure the links are the correct length?
Are you sure the ARB arms are the correct length?
Are the ARB mounts moved forward compared with standard?

Really, you need to correct the geometry as part of the repair, or you will either have to strengthen the rest of the system to prevent further failure.

A
Anthony Cutler

BTW - with the angle of the ARB from the front, you'll be losing much of the benefit of the 3/4" bar preventing roll... a 11/16 or less with the right geometry will be more effective.

A
Anthony Cutler

Anthony,
Can you explain a bit more? I can see that repeated flexing of the mounting bracket may well be the cause of the break. That's clear.
But with flexible rubber ball-joints at either end of the drop link I don't quite see why you think the ARB is straining the bracket? And what is wrong with the ARB angle?
Guy

This is quite a common failure both in MGB and Midgets. More than one in our fleet have experienced it! (Dave - I don't think tracks are as punishing as uneven country roads taken at a good clip, especially when the inside wheel runs off the tarmac :-)) For that reason we strongly recommend to customers that when fitting uprated bars that the area is reinforced first.

Our approach for the Midget is shown on our latest project at http://www.mgparts.co.nz/about-us/our-cars/matt-1961-midget-project/ about 2/3 down the page.
The B is much simpler as the captive plate is inside the chassis rail, leaving a simple flat surface to reinforce.
Paul Walbran

Paul,
I don't think the issue here is the failure of the ARB mounting plate - that is a common fault and Gryf had dealt with reinforcing that some while ago.
The issue now is the fracturing of the drop link anchor plate
Guy

Ahhh, too late not reading properly again :-)
Paul Walbran

Hi Guy

My ARB strut seems to be very nearly vertical. Take a look a the Flicker pic I reference - in it the ARB strut appears nearly horizonal (slight exagg) as it approaches the plate mounted on the wishbone.

Imagine this strut was shorter... then the ARB mounting point would rotate backwards and upwards and push/pull the plate nearer the vertical plane.

Take a look at yours and compare; mine strut is very near vertical.

[By extension, if the strut is entirely horizontal, it would pull the plate forwards/backwards causing max stress with no ARB effect whatsoever.]

A
Anthony Cutler

Anthony, I must be being thick! I don't see it. What I see in that photo is a drop link which is pretty well vertical, although partially hidden behind the end of the ARB. And arallel to the vertical drop link mounting plate that has broken.

Just checked mine and it is much the same. Actually on mine there is a slight marking of the flanged leading edge of the bottom wishbone where the drop link is apparently making occasional contact. Either my ARB "arms" are bent back too far or possibly when I fitted my reinforced ARB mounting plates (like Gryf's)I positioned them a fraction too far back on the chassis rail "horns".
Guy

>>> ...the ARB strut appears nearly horizonal (slight exagg) as it approaches the plate mounted on the wishbone. <<<


Anthony -

I think what you're seeing in that photo is the bottom edge of the wishbone. My drop links are pretty much vertical on both sides. I did a double-take as well on seeing the pic again, but it's easy to spot if you view the enlarged version:

http://www.flickr.com/photos/safety_fast/4609235161/in/set-72157623942645649/lightbox/

Here it is from another angle:

http://www.flickr.com/photos/safety_fast/4609236099/in/set-72157623942645649/lightbox/

-:G:-
Gryf Ketcherside

Right - see it now... much better!

Still find it hard to believe the plate has broken... like many pieces attached to the Midget, it looks easily strong enough (would be slimmer on a modern of 2x the weight!).

If it's under that much stress, I'd consider welding washer in place...

BTW - I had my eyes tested only a couple of weeks back... need +1.5 for reading... which I normally wear.

A

Anthony Cutler

How does the link on the other side look, Gryf?

If it isn't as bad maybe it has been cracking for a while.

I don't think putting washers there will do much (unless you weld them on) to unload the stress on the plate from the stronger ARB.
Bill1

>>> How does the link on the other side look, Gryf? <<<

Hi, Bill. I took a quick look at the other plate, and the upper corner looks like it might be ever so slightly bent. So this morning I ordered two new plates from Moss, along with a new pair of drop links, as the rubber in the old ones is looking mealy.

I'm hoping that new plates plus washers may be the best combo. The existing setup uses pins from the drop link that have no real shoulder on the front side; just a taper. So ARB forces are concentrated right there at the hole, and my theory (ahem... cough...) is that simply spreading the load with washers will be enough.

Naturally, the weather is delightful at the moment, near 60F. I know as soon as I snug up the last nut on this repair the temps will plummet and snow will arrive. Typical.

Cheers,

-:G:-
Gryf Ketcherside

>>So this morning I ordered two new plates from Moss, along with a new pair of drop links, as the rubber in the old ones is looking mealy.<,

don't do it !

if the drop links are original then keep then, if they are more recent in say the last few years then the new ones could be worse - I know this for a fact

rubbish rubber has been plauging me for last couple of years before that it was petrol hose not a good item to break down unexpectanlty

current I have track rod ends where the rubber boots have completely split and steering rack gaiters that have small crease splits all were put on new last August

Nigel Atkins

Thanks for the bad news, Nigel! ;-)

I'll have a look at the new links and play it by ear. As for the plate on the other side, I took it off a little while ago. See pic below. There's a slight bend to it, not the concentrated bend at the hole that caused the other one to fatigue.

This has me wondering if I'm overworking my A-arms as well. I sure don't want to crack one of those.

-:G:-


Gryf Ketcherside

GRYF,
it's even worse news for me I have to keep replacing parts and redoing work that should have lasted for years:(

now I don't know and have wondered about it on my own uprated ARB (sway bar), I noted in the photo of yours on another thread that you have a jubillee clip acting as an end stop and there appears to be little sideways movement of the ARB at that point

now the standard ARB have end stops but the bigger bars seem to come without them so is it better to fit end stops or not

on mine I can see there's a lot of sideways movement
Nigel Atkins

I use the original arb stops on my 11/16 bar.

Comment on beefed up arb components stressing the wishpan are pertinent; this is fixed as a by-product of using the FL conversion.

A
Anthony Cutler

>>I use the original arb stops on my 11/16 bar.<<

that could explain a lot the 'specialist expert' outfit didn't reuse mine
Nigel Atkins

Gryf,
I wouldn't use large washers either side of the mounting bracket when refitting the drop link pin. The pin is tapered, and without a shoulder, so that when tightened onto the bracket it pulls up tight in the hole with no possibility of any sideways movement. The bolt hole in the mounting bracket has a corresponding chamfer to assist.

Putting a load spreading washer in there would mean that it was relying on clamping action alone. Even the slightest sideways movement would quickly develop into a constant hammer action between the pin and the plate and the hole would rapidly enlarge.

I think that the bend in the mounting is probably simply caused by the leverage of the pin length acting on the mounting bracket. I wouldn't conclude from this that it needs strengthening though. The thicker ARB will increase the loading on it but if it has done its duty for 40 + years that's not bad. If the replacement is half as good then you shouldn't have much to worry about. As Nigel says, the rubber in a replacement drop link is far more likely to fail long before the mounting plate does again!

There is also the danger of beefing up one item and the stress is transferred somewhere else and something else - possibly something more safety-critical - then fails.
Guy

I have a beefed up ARB on my Bug Eye and had no problems for years.

Then after I fitted semi slicks and a beefed up motor and drove hard competition laps I saw both sides break away.

I have welded reinforcing plates and have had no more problems. The original plate is just too light for the greater loads

Bruce
Bruce Roberts

I've had an 11/16 on mine since about '91 (Moss Handling kit complete with Nylatron rear bushes) and havent had any issues in 20+years and about 30K miles when its been on and off the road at different times. There aren't any stops on the bar nor were there instructions to move them over at the time.

It had a new chassis leg "end" on one side at about the same time and has had new wishbones at least once. So far though no issues with any of the normal ARB mounting points.

Dean Smith ('73 RWA)

If you think the standard arb drop link mounting plate(s), is likely to be too weak for your uprated arb, then instead of replacing them with standard plates, replace with home made thicker ones.

Should be pretty easy to make those up from the pattern of the originals.

I don't think thicker ones would transmit any more stress to the wishbones, than standard ones. After all, the standard ones aren't supposed to flex in the first place as far as I can tell.
Lawrence Slater

>>> ...then instead of replacing them with standard plates, replace with home made thicker ones. <<<

Thanks for that, Lawrence. I think I'll feel safer going with the direct replacements I ordered. I've already beefed up the ARB mountings on the chassis, thus eliminating the first weak link. And if these plates are the second weak link, I'd rather have them take the bullet instead of the wishbones.

As much as I'd like to replace or rebuild the front suspension anyway, SWMBO (bless her) would give me the stink eye if I suggested that kind of expense, especially since we're still paying off a large home-improvement loan and my Cooper S.

Cheers,

-:G:-
Gryf Ketcherside

Just to follow up, I received the new plates and droplinks yesterday, and got everything bolted up after supper. I didn't use washers as mentioned above. The taper fit that Guy spoke about seemed much more pronounced on the new parts, so I just reassembled things by the book.

And typically, when I turned out the garage lights and headed for the house, it had begun to snow. So the car's sitting today, but will be back out at our first opportunity.

-:G:-
Gryf Ketcherside

This thread was discussed between 02/02/2012 and 08/02/2012

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