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MG Midget and Sprite Technical - Rear drums scoring backplates.

Another problem with the Frog. Yesterday I re-adjusted the rear brakes and noticed after taking off the drums that the drums have scored lines in the backplates. The rim of the drums seem to be flying much too close so that when cornering they actually make contact. The RH side is worst wil about 200 degrees of scoring, the LH side being about 100 degrees.

The drums were originally off a 1500 and the backplates off a 1275. I have a new set of (1275) drums and measured the internal depth and it was 0.25 mm less than the 1500 drums. I thought all drums for disc brakes midgets were identical?

The backplates have been powder coated and I was wondering if the process of firing the powder coating has somehow allowed some stresses to bowl the backplates slightly?

Any ideas?

As a temporary cure I have put washers over the wheel studs between the half-shaft and drum. About 1.5mm clearance now and there is no sign (ie scraping noise) of the problem.

Rob
Rob aka MG Moneypit

I think 0.25 mm would be called manufacturing tolerance. With everything assembled and bolted up you can stick a big screwdriver 'tween drum and backplate and lever the backplate away quite a bit, it won't return to the same position thus providing more clearance. Do this at 90 degree intervals around the edge.
David Smith

Rob. David is most probably correct that the .25mm (about .009") is part of the manufacturing tolerance involved in the production of the drums. And, his trick of flexing the back plate to cause it to sit very slightly further away from the rear of the drum is not a bad idea if the axle is perfectly straight with the center line of the axle housing. But, before I tried levering the back plates, I would put a dial indicator (clock gauge) on the axles (without drums installed) and see if the cause might be that the axle(s) are slightly bent. If they are, it is worth knowing about. There is also a slight potential for the drums to be slightly warped. If the axle is straight, mounting the drums on the axle, inside out, and using the dial indicator to see what the straightness of the drums is would be worth checking so that, if there is a problem, you can know what it is. Then, you can make any decision about how to best correct it.

Les
Les Bengtson

afterthought - the drum is wider than the shoe contact patch, when they wear that's the lip you can feel, it's about an eighth or three-sixteenths wide. So you could also take a heavy file to the drum, or just run them against the grindstone, depending on what tools you have available.
Have you got aftermarket halfshafts? I'm wondering if the flange could be thinner than standard.
David Smith

Hi David, everything came off a 1500 except for the Frog axle casing.
At the moment I have spaced the drum away using washers between hub and drum and it seems ok but hardly a final solution.
I will try the brute force method first then grind the drums if that fails. I will also see about axle alignment as Les mentioned.
The axle casing I got from a friend who had it in his garage for 30+ years as a spare all nicely painted with black smoothright. It's been off the road more than on so I expect it is ok alignment wise but will check.

Rob
Rob aka MG Moneypit

Rob, I had the same problem with mine. Rear wheel bearings were slightly slack, but the real cause was the thickness (or thin-ness) of the new halfshaft heads, which are 1mm thinner than the originals, one of which snapped, thus pushing the drums 1mm closer to the back plates.
Dominic Excell

Resurrecting this thread for more advice.

I finally got around to trying to solve this problem. I tried as suggested to bend the backplates back using a big screwdriver, which promptly snapped. It was my favourite too.

So, I took a tyre lever to it and put some real effort into the task. Removed the drums, and the washers I had used to space the drum away from the back plate, then put back the drums.

Took it out for a test drive and the rubbing was as bad as ever. Of course it is a lot worse when cornering but the drivers side rubs slightly all the time even when going straight ahead.

So, for now, I am going back to using spacer washers. Any ideas?

Rob
Rob aka MG Moneypit

Get a millimeter machined off the back of the drum ?
Andy Phillips (frankenfrog)

As we converted the 1275 to wires I also have a set of good disc wheel shafts and hubs available for offer if anyone is interested.

dominic clancy

Rob, are you on wires? We have a simliar problem on one of ours and my thinking at the moment is that there's a bit of movement in the press fit between hub and halfshaft. Certainly when the wheel is given a good top to bottom shake the drum moves relative to the backplate, and changing the wheel bearing made no difference (and it was a good qhality bearing)
Paul Walbran

Paul, I have steel wheels. Not sure what my problem is but I have taken about 1mm off one of the drums which took quite a while with an angle grinder. I still need thin washers between hub and axle to ensure no contact when cornering hard.

The only thing I can think of is that the back plates were blasted and powder coated. I just wonder if the heat treatment for powder coating has released some stress in the metal causing it to go slightly bowl shaped?

BTW been to North Island twice for holidays. Where abouts are you located?

Rob
Rob aka MG Moneypit

Rob, Aucklander born & bred, a westie at that (well known for being petrol heads) ... but we get all over the country.
Paul Walbran

I like Auckland, stayed across the bay in Auckland Takapuna Oaks hotel for a few days and did the usual touristy things. A memorable trip around Waiheke Island being memorable because the tour bus left us stranded at Onetangi on the beach. Took hours to get back to the ferry terminal!!

We will be in NZ again Feb 2016 for about a month touring in a camper van probably doing more free camping this time.

Wish I could afford to take the Frog over. Now that would be a holiday to remember.

Rob

Rob aka MG Moneypit

Stranded at Onetangi! - that must have been tough, right next to the pub no doubt, no wonder it took so long to get back to the ferry. When I was young and fit I used to run from the ferry to Onetangi and back, about 17 miles return as I recall so you must have had a bit of walking to do. Look us up when you get here :-). Better still, ditch that camper van and take an MG - al our workshop courtesy cars are MGF's, OK a bit bigger than a frog but still has the badge and the wind in your hair. And sure to make it rain on you :-)
Paul Walbran

Not enough room in an MGF for two large 20 Kgm suitcases, and the wife!!. Got any with a tow bar and a small trailer?

Will look you up when next in Auckland.

Rob

Rob aka MG Moneypit

That's the cunning plan Rob, no room for retail therapy!!
Only the B with a towbar and a big enough trailer to fit the Midget. Not often that the tow car also competes :-)
(And in case you are wondering, no we don't travel with all the wheels loose, just doing a bit of a reshuffle when this pic was taken)
But yes, look us up.
Paul Walbran

Oops, forgot the photo ...


Paul Walbran

This thread was discussed between 01/06/2014 and 25/06/2014

MG Midget and Sprite Technical index

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