OzVMX Forum
Clubroom => General Discussion => Topic started by: 211kawasaki on October 14, 2013, 03:17:50 pm
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In the process of finishing the 74 KX250 that's been an on going project for 10 years; the bikes (73/74 KX) had a reputation of destroying the front brake backing plate when you least expected. Its a dangerous combo and I wasn't going to fit the 74 backing plate in Magnesium. It was only ever going to be a club day ride so a restored the 74 front end for show and have a 75 front to ride it.
But!
When I was installing the rear wheel yesterday the hub (magnesium)that retains the sprocket is cracked in 3 places such if the bearing wasn't holding it together it would have fallen apart. My question is what about the actual hubs! I remember when I did a YZ250A once that I dropped an original wheel and the hub just shattered in bits all over the floor; can you ever be safe on these things or have I just created an expensive museum piece not to be ridden?
Thoughts?
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Not 100% sure. I have welded Magnesium and had good results. I would call Heat Treatment in Brisbane and ask this question. They for sure could normalize it for you.
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maico hubs in mag. seemed to crack up- of the AW era.
I have just been through the process of renovating '75 magnesium hub ccm wheels, the hubs seem fine and I know of others in regular use that are ok, so can go either way =this doesn't help you though!
I guess careful monitoring is the key....and how corroded or looked after the original hubs were
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Magnesium degenerates over time.p
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The front brake backing plate on Maicos brake the bolt hole that holds the brake stay. That is why most Maico riders change to the billet alloy twin bolt backing plate. Maybe any bikes being ridden in competition after a certain age should not use magnesium hubs or backing plates. I'm sure the road race guys would know.
Kevin
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Just stick YZ A/B wheels in it Dave....you''ll end up with the best brakes in the business....the backing plates on them don't break (mag) and the hubs are alloy....then it will be a true Yamkawsuki.!!! :)
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the backing plates on them don't break (mag) and the hubs are alloy....
Fibber Crocket! You know quite well that the little cable locating boss snaps off on the front backing plate all the time. Use an aluminium one Dave....Magnesium is awful stuff when it gets old..
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what was it, Charles Udall (velocette) called it "electrified dirt" ;D, :P
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They only snap off when Claude the klutz over tightens the cable Firko....how's Mrs Firkin today?...is she going easy on the nurses?
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how's Mrs Firkin today?...is she going easy on the nurses?
She's driving them nuts with her hourly trips to the toilet. Because she can't walk they have to lift her out of bed and into one of those special wheelchairs with no bottom that slip over the dunny and she's making a grand old ceremony over it. She's going in for a skin graft operation on her heel today.
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All the best for her op Mark.
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mark give my love to Edna
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Firko and I have had a very drunken chat about magnesium. Funny stuff. Burns helluva brightly though....
I'm no metallurgist but it seems that mag has a very course crystalline structure that becomes more course with time. That mag is used extensively in the aerospace industry is a bit confusing (to me). But I suspect, like alu, it work hardens.
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I've seen Magnesium components cause (or almost cause) a fair bit of vintage heartbreak over the years.
* I was standing taking photos on the inside of turn one at Nepean back in 1993 when the great Roy East crashed his Metisse coming into the corner. He hit the deck so hard that I thought that he must have been either killed or at least badly hurt. Luckily Roy was a tough old bugger and he escaped the accident wit nothing more than a headache and some missing bark. When we picked the bike up we discovered that the magnesium Rickman front hub has shattered like a broken bottle when Roy grabbed a handful of front brake.
*I was waiting at the pit gate to go out for a race at one of the early vintage meetings at Amaroo Park. While checking out the sidepipe CZ next to me I noticed that the spoke flange on the magnesium front hub was cracked almost the entire 360 degrees...only a 20 mm section holding it all together. I tapped the bloke on the shoulder and told him but he wasn't that interested, he just wanted to race. I had to actually get off my bike and physically draw his attention to the hub. When we later had a closer look, the hub had about 5 other cracks that would have spelled disaster the first bump or jump he hit.
*Broken Hill Racer Robbie Tumes crashed his brains out in practice at the 93 Kembla Nats when the mag backing plate on hos '74 KX250 shattered like a broken plate.
* At the same meeting Jason Cater holeshot the pack on his Maico and when he hit the front brake the magnesium backing plate exploded in an almost identical manner to that on Robs Kawasaki. Jason hit the deck and took out half the field, calling for the race to be stopped and restarted.
How many mag Honda water pump housings, early YZ clutch covers, XL Honda clutch and ignition covers and other parts have been trashed over the years? If those parts had been made from aluminium they'd probably still be in use. I'm that wary of the stuff, it's preventing me from using CZ hubs on any of my project bikes, despite their superior brakes and light weight.
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In the early days of true mag wheels on sports and racing cars they had an expiry date stamped on the rim. I'm not talking about "mag wheels" that are aluminium shiny things but the real thing.
Having said that, my experience with V.W crankcases is although they are magnesium alloy they don't seem to disintegrate and apart from a few stripped threads requiring 'case savers' or helcoils they seem to last.
I assume its a balance between light weight or long life depending on mag to alluminium % in the alloy.
pancho
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I remember checking the rear Marvic mag wheel on my Honda RC30 (VFR750R) Superbike back in the early 90's after a friends tore the spokes out when coming onto the PI straight at 230 K's..... Mine was riddled with cracks in every spoke ........ phew!!!
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Its the KX that Rob Tumes was on that nearly killed him that confirmed to me that its dangerous, was there that day as well; Roy Easts last from memory - confirms the need to flick the Mag for the sake of not getting hurt and with the Freakshow at the moment its comes home that the older we get the harder we fall.
Best wishes for Mum as well; no wonder the project is in the house! wouldnt stand a chance if she was home ;)
211
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Mag by nature is also a brittle material, that’s the trade off for light weight. It also decays over time, how much it decays is relevant to what sort of exposure to moisture it’s had.
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SHITE! Is there an inexpensive way to crack test magnesium parts, particularly the hubs etc? All these horror stories have me thinking now.
I can recall one Mr Motocross round I went to at Amaroo as a kid and seeing Anthony Gunter ( I think ) shatter a rear wheel coming onto the straight off the ski jump on an RM370. Now I know why! He stayed on the bike too, which in hindsight, was bloody good riding!