Maxton magnesium forks

[Dellortoman]

My mate Rick dropped his bike at Mac Park last weekend. It was a a pretty normal low-side fall on the left side in which he slid along on his arse and was unharmed. The bike (a Period 4 Guzzi) had a set of Maxton 38mm forks with fully adjustable cartridge internals. Rick opted for magnesium sliders rather than aluminium to reduce unsprung weight.

While the magnesium sliders are quite a bit lighter than the aluminium equivalent, it would appear that they're quite fragile. This is what the left slider looked like after he picked the bike up (ignore the grey bar in the foreground, it's the paddock stand). I've never seen a low-side drop do that to an aluminium fork slider. It's hard to tell what caused the break. There's no damage elsewhere on the fork leg, axle, disk or rim so it doesn't appear to have hit anything. Maybe it was just a shock from the steering hitting the stop. Who knows? The right leg was still intact.



Mg has the advantage of being less dense (lighter for the same size) than Aluminium. But for the same cross sectional area, Mg is weaker. It appears that the Mg slider above was made from an identical mould as the aluminium version. Whereas to achieve the same strength as the aluminium slider it would have needed a thicker wall.

Ductility is a factor too. Mg is more brittle (or less ductile) than aluminium. A clout that might bend aluminium could shatter magnesium.

I'm not saying Maxton products are poorly made. Indeed, lightness may be more important than strength for racing. As long as it'll do the job when the bike is on its wheels, it's good enough. Race bikes are generally pretty fragile. Just look at how Alex Marquez's bike disintegrated at the Catalonia MotoGP. The forks worked well under normal racing conditions but failed during a reasonably gentle crash. That's fine if you have a generous racing budget or sponsorship, and can afford to replace expensive magnesium parts. But most normal weekend hobby racers don't have unlimited budgets. These Mg sliders were twice the price of aluminium alloy.

The above may be of relevance to anyone considering magnesium fork sliders. Treat them gently. I wouldn't recommend them for road use, or for racing if you're on a shoestring budget.

 

[chrisk]

Bad luck for Rick Cam, glad he is relatively well.

Is not to say the break, under brakes perhaps, may have been what caused the fall in the first instance.

The section between the calliper mounts would be under pretty serious stress under braking, because any possible flex is being restricted by the calliper mounts.

I don’t have a doctorate in physics, nor am I a mechanic or an engineer …. just a hacks suggestion.

 

[Mike_P]

Magnesium is also more prone to stress fractures (fatigue) under repeated cyclic loading ie under repeated heavy braking than aluminium alloy. So like for like with respect to stress/cycles, the fork legs should have been made thicker really

 

[sfcpiet]

A Guzzi V-twin is a heavy lump, perhaps vehicle weight should be more closely considered when choosing such expensive/fragile equipment.

piet

 

[Vince]

Maybe a set of those crash Ogry knobs, rubber doughnuts that fit on the ends of wheel axles, if it was caused by an impact. I would be fitting them to any race bike if it saved damage. Reminds me of missing the chance of riding my cousin's brand new Fireblade, he wrote it off in its first week of ownership. A gentle slide at 60kph that ended at your typical 100mm roadside concrete gutter that completely snapped the whole front end off the bike. Light but fragile.

https://www.hurtlegear.com.au/categories/Frame-Protection/?srsltid=AfmBOopJtTmPxqzfeuaCbAgZYpW0CecgAmlIFd5O4wNr0993ufDnAyqc

 

[Exmoor beast]

It makes you wonder how there are tested- obviously a very low production run. My boss came back from California with 6 sets of the lightest mag wheels I’d ever seen. I fitted a set to a 209 bhp ZZR1100, we dynoed it and I went on a ten mile test ride - all good. Customer rode the bike home and about a quarter of the rim on one side broke off, he wobbled to a stop without falling off. Turns out these wheels were made by an outfit new to wheels & never tested. I know Honda wouldn’t let old mag race stuff go out of there hands- once it had done its job it was destroyed- entire bikes sometimes. Magnesium doesn’t age well either but I imagine these forks were fairly recent.

 

[piranha-bro2]

Looking at the break I'd suspect CKs theory has some weight. Unless there was clear impact, that break looks to be in the zone of highest braking forces. I think Maxton will be quite concerned when they see the photo and hear how it happened. In hindsight I reckon the alu version would be the better choice!

 

[breganzane]

I've got a set of Marzocchi's from my 79 Jota broken in the same place, but the fork legs were bent a good 40deg from whacking sideways into an armco fence. It's probably to do with the position of the leg within the slider at the moment of impact?

But yeah, certainly seems like an option box you might like to leave unticked in the Maxton order form! Most Mg products are indeed considerably beefed up over their Al bretheren. When Laverda made the Mg crankcases for the 73 SFC's they were visibly larger in many places, the top motor mounts were massive.
A good design will get back the part's mechanical properties with a deeper depth of section etc and end up with a part which is as mechanically strong while still being lighter than the Al version. Still has the well known limitations of Mg though - corrosion, poor ductility and high notch sensitivity among them. Definitely implies a short service life and/or extreme testing regimen - worth avoiding in load bearing parts for all but a select few who have the $$$ and resources to use it safely (ie your race transporter has another half a dozen in there!).

Regarding the failure actually causing the fall rather than being a result, I'd guess not. A crack like that would likely propogate for at least a little while before total failure and in this case you'd notice leaking fork oil. Historic races are very short in Oz so the bike would not do much more than 20-30km between inspections so unless the guy was extremely slack (and it doesn't sound like he was the type) I'd guess that a growing crack would have been noticed quite quickly. More likely a casting flaw or stone chip which might have had micro-cracks starting around it (or not) which when subjected to the localised stress of the modest impact just initiated a crack at that point and due to the material's known weaknesses and the perhaps marginal design just failed completely.

An expensive lesson for the guy involved, glad he's ok and hopefully the bike will be back out on track soon (with ally fork sliders).

 

[Dellortoman]

My thoughts exactly Steve. I incorrectly identified it as the left fork in my initial post. The bike fell on its left side but it was actually the right hand slider that broke (calipers are mounted forward of the forks) so it doesn't appear to have taken a direct impact.

The bike was meticulously prepared so any oil leaking from a pre-existing crack in the fork slider would have been noticed. It used to have Maxton magnesium triple clamps as well, until they cracked through the pinch bolt holes. There was very little metal around the bolt holes and they ripped apart the first time the bike was dropped at Baskerville race track a couple of years ago. It now has much beefier Ducati aluminium ally triple clamps. It seems Maxton magnesium parts are best avoided if you want a robust bike rather than a super light one.

The bike will definitely be repaired, but the engine will need to come apart and be washed out first, because it punched a hole in the rocker cover as it slid along the track and it scooped up a heap of dirt and gravel into its innards. The rocker box was pretty much chockers with dirt and bits of blue metal when it was taken off.



I took this photo about 30 seconds before the stack. Bit of a shit photo because it was at long range with my iphone.


I feel as though I have a vested interest in the bike. Not just because Rick is a good mate, but because I had a small role in preparing and tuning the bike. We had it running really well.