Crank balancing

[LaceyDucati]

Hi Kev

Your tale of crank failure at the Giro sounds familiar, was the crank subsequently repaired in the UK? Only asking as it looks familiar to a job I did back in that sort of time. If so I may be able to shed some light on the components and why it was done like that.

I run my 450 racers at 75%, seem fine and not had any complaints or problems.

Nigel

[machten]

As a man that must see an awful lot of these things, you must have a very good memory, Nigel!

As you may recall, I was one point removed from the work (and back in Aus by then) and I was very pleased when I found out who the crank had gone to.

I should also say that it has served me very well since for a lot of riding in the UK and Europe and in situ, it still feels as good as new.

Regards,

Kev

[LaceyDucati]

Kev

Yes I'm known for having a good memory, as long as it's not names then I'm pretty useless!

The rod was sleeved and narrowed to use a I think a Yamaha 34/42/20 bearing (may have been 35/42/20 INA) for two reasons. Firstly the rod was too worn on the side to use a 22mm wide bearing and secondly so as to leave as much original strength in the rod as possible. Being 1mm smaller than the original 43mm it meant less had to be removed to put a sensible thickness sleeve in. Sleeving rods is not something I like, but as there was nothing original length and width available at the time it was the best compromise. I wouldn't normally do it but the later "B" rod is substantially thicker section than the earlier rod and my guess was it would be as strong as the earlier rod and fine for road use. Using Honda rods is fine, but you do need to machine the wheels away to allow the rod to pass through the wheels. You then of course need to re balance the crank as it will be wildly out. Like many of these things, okay if you are doing the whole build, otherwise best keep it as stock as possible. I would also say that a repaired original is preferable to a dubious pattern alternative.

The polishing certainly wasn't me, very Italian, remember if you can't make it right make it bright!

Glad it's lasted well so far. Out of warrenty now

Best Wishes Nigel

[machten]

Hi Nigel,

Many thanks for that detailed description, as I never actually knew in detail what had been done down there. A very elegant solution indeed and the passing of time and many km's of spirited alpine riding validates its effectiveness.

Glad it's lasted well so far. Out of warrenty now

You mean I didn't buy the 10 year 50,000 km extended warranty???? Damn!

Kev

[Eldert]

Hi Kev

the crankshaft drawing is in Massimo Clarke´s book Ducati Scrambler, Desmo e Mark 3 page 78

it says percentuale equilibrium con pistone 0615.47.406 = 70 %

Eldert

[machten]

Hi Eldert,

Brilliant! You are a font of knowledge! Many thanks.

Kev

[machten]

Thanks to all of you who contributed to this. It is not something I have known much about before, but now have had a good opportunity to learn a bit about and to have the benefit of your own personal experiences has been invaluable to me. One of my degrees is in Applied Mathematics, which is useful to help me understand the theoretical physics, but no amount of theory can substitute for real experience.

Thanks guys.

Kev

[graeme]

Balance can be a "feel" thing with different frames, bars, foot pegs etc changing what a rider feels.
I raced a 450 with 70% balance and it was smooth over 4k
I balance my road 450 at 70% and it was terrible and bloke things, tried 66%, 60%, and finally 55% which is good.
A standard 450 engine in a road bike, same engine in an RT both bikes feel and vibrate totally different.

Graeme

[Jordan]

I heard that BF is sometimes etched on the flywheels with an electric pencil.

[LaceyDucati]

I've seen percentages etched on narrowcase 250 cranks a few times, I think 72 or 71%. I would presume this was a balance factor as I can't see what else a % symbol would be there for. Most other cranks have + or - symbols and numbers on them, which I guess was the deviation in crank width from a nominal figure when new. Perhaps used to shim cranks at the factory during assembly.

I would agree with Graeme that the balance factor is not hard and fast, as some of the effects you feel are resonances dependant on other factors. If you have different bars or footrest mounts then the vibration experienced may be different. For instance 72% for a Saxon framed 350 racer was fine but the same factor used on a standard 350 narrowcase racer broke the engine mounting bolts! Of course a single will vibrate at some point, so depending how the bike is used will depend on what factor will give the best results. Hence why race and road bikes may need differnt factors.

There is also another issue (more with widecase bikes with very different size wheels) where something called a rocking couple can be in existence even with the crank appearing to be statically balanced correctly. My experience of Bevel twins is that Ducati paid scant regard to balancing in the 70's and apparently according to the balancers I use, things have changed little at the Ducati Factory over the years since!

Nigel