Swingarm cracked...

[Dukedesmo]

Riding home yesterday and the bike started feeling a bit 'loose' at the rear end, similar feeling to a puncture.

Stopped to investigate, to find this;



At this point it was difficult to see the cause, indeed it looked as if the spindle had simply slid out due to not being torqued up sufficiently so, rather than call for breakdown and wait the inevitable 2-3 hours and, given I was only 14 miles from home and had some basic tools, I managed to re-seat it with the help of a rock (don't carry a hammer in the toolkit!) and then rode it steadily home.

Today I am investigating and find the right-hand side looks like this;



And the left, like this;



A tricky repair methinks however, I do have a spare swingarm albeit not a braced version.

So I'm now considering my options;

1. Repair the braced swingarm.

2. Fit the standard swingarm.

3. Transfer the bracing from braced arm to standard arm and so continue with a braced arm but without relying on a tricky repair.



Incidentally the braced arm is slightly wider at the front. Wide enough that the circlip grooves in the spindle don't stick out of the end so the clips can't be fitted - having the clips would have prevented the spindle working it's way out even with the cracks.

Always wondered why this was, presumably this swingarm should have a longer spindle? but I've never seen one and I've got 2, both of which are the same length and fit nicely in the non-braced arm but only sit flush with the end of the braced arm.

[900Rebuilder]

Looks like you had a lucky escape there DD - glad to hear that you made it home safely

[Mr Gazza]

Another bullet dodged! That could have been very messy.

Do you know what the braced arm came from? Did you add the brace?
It begs the question; was the brace anything to do with the extra stress at the pivot? Probably not.
Those housings are properly cracked and yet appear beefier than standard. Could it be a well raced 851/888 arm?

I wouldn't bother with the bracing if it were mine, just extra unsprung weight for un-required stiffening on a road bike.
Your spare arm looks as if it may have come from an M900Sie, it has two holes on the rear brace which would mount the extra little mudguard piece of an Sie. But for all I know they probably all have those holes?

Your brake torque arm bracket is on upside down btw.

[Kato]

WOW Lucky you caught that !!!

That is an 851/888 swing arm and they were notorious for cracking, given the distance that the pivot has moved I would also have a good look at the engine casing just to make sure there is no damage ??

That arm is toast the effort/cost to repair if indeed it could be reliably repaired will make it unviable you will need to remove a lot of metal to get weld penetration while trying to keep it from worping and then linebore the spindal holes.

As for the bracing on a road bike it is purely cosmetic and you will not feel any difference on the street, however If you do trackdays and you run high spring rates, slick tyres and you are capable of running with the fast guys it will indeed have the desired effect.

The later arms toward the end of 888 production and susequently fitted to the Monster used a better grade alloy and no bracing, but no reason why you could not move the brace over to your spare arm if you really have to have it on the bike

[900Rebuilder]

Originally Posted by Kato

WOW Lucky you caught that !!!

That is an 851/888 swing arm and they were notorious for cracking, given the distance that the pivot has moved I would also have a good look at the engine casing just to make sure there is no damage ??

That arm is toast the effort/cost to repair if indeed it could be reliably repaired will make it unviable you will need to remove a lot of metal to get weld penetration while trying to keep it from worping and then linebore the spindal holes.

As for the bracing on a road bike it is purely cosmetic and you will not feel any difference on the street, however If you do trackdays and you run high spring rates, slick tyres and you are capable of running with the fast guys it will indeed have the desired effect.

The later arms toward the end of 888 production and susequently fitted to the Monster used a better grade alloy and no bracing, but no reason why you could not move the brace over to your spare arm if you really have to have it on the bike

Good to hear about the 888 arms as that's what I'll be running if I can locate the correct hugger

[utopia]

Blimey.
I shudder to think how that could have ended.
No doubt you've had the same thoughts.

I remember someone else reporting the same sort of failure (on this forum) a few years back but I can't remember who it was.
Of course its difficult to know for sure without knowing the previous history (torque settings etc) but I'm going to stick my neck out and say that it could well be a borderline weak point in the design ... which is worrying for the rest of us (though admittedly there must be many thousands out there which haven't failed).
Certainly it does look visually like there is minimal metal in the area in question.
And I'm pretty sure you will have followed the recommended torque settings (though no telling what a previous owner may have done).
Also I can't decide which I think is more likely .. overtightening being the direct cause of failure or maybe undertightening (or more likely a slackening off in use) allowing the assembly to "rattle" (in a minimal way).

Personally, I think I would be reluctant to trust a repair.
No matter how good the weld penetration is, I would always be worried that it would be less than perfect which wouldn't be good in an already (possibly) borderline area.
You would probably have to recut the spindle bore anyway, which wouldn't be easy.
Of course on the other hand, maybe a repair could beef up the area .. I dunno if there's much room.

On balance I'd be inclined to fit the other arm.
I don't think I'd bother with the brace either .. I suspect you'd agree that its mostly cosmetic, on a road bike at least.

[jerry]

Blimey thats horrible,, my opinion bin the cracked arm fit the spare

[Dukedesmo]

The odd thing about this is that 40 odd years ago, I had the same thing happen on a Ducati 250 Scrambler. Started 'weaving' a little from the rear and when I looked the swingarm spindle had come out of (the same) side.

On that nothing was actually broken but the pinch bolts wouldn't pinch enough to be secure so I stuck a length of threaded rod/washers/nuts through it to stop it sliding out but I was less finnicky back then!

Inspecting everything, I can't find any further damage - the engine cases are fine, as appear to be the bearings but the spindle had pulled back out of the left side of the swingarm, meaning the wobble was due to that side not being attached to anything (single-sided swingarm eat your heart out) and, under throttle the chain would flex the rear end some.

The other arm actually has a little more 'meat' around the boltholes, presumably because it is a later Monster version and they had solved (or at least improved) the problem. So that, and the fact that any repair would be complex and likely doomed to failure means I'll be going with that.

As for the bracing, as mentioned I doubt it really does much, I just bought that arm because it was available when I was looking and it was a nice bit of bling but I have had a few issues caused by the bracing - it makes a very narrow passage for the chain and there is a tendency for the chain to 'bounce' between brace and tyre edge (especially when I tried a 180 tyre) so, not too fussed about it and it does add weight which is not good.

So, onwards and upwards and (hopefully) back together before the weekend to get a bit more riding done during the decent weather.

I've already got 1 bike off the road (916 has leaky fork seals) so I'd like to keep my options open.

Oddly, yesterday whilst riding and before I was aware of this, I was contemplating that if I could only keep 1 bike, it would be the Monster because since I've had it it has been used far more than the other 2 but, at the same time it's been off the road more for various, major repairs.

It's saying something when a 25yr old 916 and a 44yr old Guzzi have been more reliable...

[Dukedesmo]

Originally Posted by Mr Gazza

Your brake torque arm bracket is on upside down btw.

It only fits that way due to the counterbores, bear in mind it's not a Ducati part rather an Aella aftermarket so maybe a slightly different design?

https://shop.aella.jp/prdct_list.html?category_code=1

[900Rebuilder]

Originally Posted by Dukedesmo

It only fits that way due to the counterbores, bear in mind it's not a Ducati part rather an Aella aftermarket so maybe a slightly different design?

https://shop.aella.jp/prdct_list.html?category_code=1

Didnt see the rear brake listed under Monster, but this gave me a chuckle for its chinglish;

https://shop.aella.jp/item/IS00000N10718.html

[Mr Gazza]

Originally Posted by Dukedesmo

It only fits that way due to the counterbores, bear in mind it's not a Ducati part rather an Aella aftermarket so maybe a slightly different design?

https://shop.aella.jp/prdct_list.html?category_code=1

Oh! I don't think it would work at all that way round with standard Monster footrest hangers, as the shoulder bolts would disappear in the counterbores and you'd have to turn it over anyway. Besides being too thick for the bolts to reach.
It looks as heavy, if not heavier than a standard steel one and renders the brake virtually rigid with that geometry.
Bitza made one from a bit of 1/4" Dural I sent him, along with a pattern.

The rose joint will go either side, so that's not important.

[Kato]

Originally Posted by 900Rebuilder

Good to hear about the 888 arms as that's what I'll be running if I can locate the correct hugger

As long as you keep an eye on it should be OK, the problem part is the cast bit welded into the end of the tube, Ducati changed the grade of alloy, the casting process and amended the design somewhere around 1990/1 and the later arms don't crack, the production non - braced arms also cracked but it was always claimed that the pinch bolts were over torqued and not that there was a problem in manufacture

For what amounted to quite a short production cycle, there are actually quite a lot of those braced arms about, reason being they were changed almost every race because of the cracking so teams went through 100's of them

Also had a similar problem with the SS swingarm but they useually cracked on the shock mount

[Dukedesmo]

Getting on with the repair though the spare arm needed a some minor repairs - the inside where the sprocket carrier fits had the obligatory circular grooves (usually because someone fitted the conical spacer the wrong way, causing it to cut into the ally) and a couple of gouges on either side that looked like passenger footpegs or luggage had cut into it?

Anyway, I took it to a friend who's a wizard with ally welding and he patched it all up nicely for me, also reckons he can repair the old arm so I've left it with him, nothing to lose.

I can see where they've added strength to the later model, the bottom half where the bolt goes through is both thicker and longer, so long that it almost touches the front sprocket;



Don't think a 16 tooth sprocket would fit!

As for the thickness, it requires a 5mm longer bolt so 5mm thicker, plus the fact the counterbores have some meat in front of them adds a decent amount of strength, I would think (hope).

All (mostly) together now and, unlike before I can fit the snap-rings on the spindle so it can't pull out even if the clamping fails.

But whilst doing the job I've decided the tyre's not worth re-fitting so I've got a new one coming and I lost one of the rubber bungs when the spindle pulled through so some on the way along with a few spare snap-rings and normal service will be resumed.

Probably won't get the new tyre until Monday so won't be out on it over the weekend but good to have it all back together again.

One plus of the unbraced arm is that, no longer do I need to cut the chain if I want to remove it which helps a little for maintenance purposes.

Out of curiosity, I checked on bike-parts-ducati.com and new swingarms are available at £1,583.89.

[Dukedesmo]

All up and running now, will fit the new tyre when it arrives but, I've just been out for a ride and all is well again in Monsterland.

[jerry]

good result , fast work