Broken crank and high compression

acotrel · May 14, 2017

The bump on the head is a worry. A friend of mine once said 'every time we fall off, we could be killed'. I never think like that, but as another friend said 'it all depends on luck'. I don't know whether I believe in that, either. Perhaps we need to minimise the risks to a tolerable level ?

Dances with Shrapnel · May 16, 2017

lcrken said:
Nitriding adds compressive strength of the material, thereby requiring higher stress to start cracks. The effect is the same for other surface treatments, like shot peening, that leave added residual compressive strength near the surface of the material.

As Dances said, plenty of info on all this sort of thing on the web nowdays. I kind of miss having to go to the library to dig through technical books for info, but not much. The wealth of engineering and science knowledge available on-line to anyone willing to look is incredible.

Ken, I think the precise term is compressive stress near the surface. Though the Nitrided surface material may have a greater compressive strength, it is the residual compressive stress that increases the durability - similar to rolling the fillets or shotpeening; at least that is my understanding of it.

I stumbled across this summary in the link below on Prince Rupert Drop and thought this might be a natural analog to Tuftriding or Nitriding ability to increase the durability of a component through increased surface compressive stress.

http://www.iflscience.com/physics/r...400-year-old-mystery-of-prince-ruperts-drops/

acotrel · May 17, 2017

Hard chrome lasts longer as a wear surface.

lcrken · May 17, 2017

Dances with Shrapnel said:
Ken, I think the precise term is compressive stress near the surface. Though the Nitrided surface material may have a greater compressive strength, it is the residual compressive stress that increases the durability - similar to rolling the fillets or shotpeening; at least that is my understanding of it.

I stumbled across this summary in the link below on Prince Rupert Drop and thought this might be a natural analog to Tuftriding or Nitriding ability to increase the durability of a component through increased surface compressive stress.

http://www.iflscience.com/physics/r...400-year-old-mystery-of-prince-ruperts-drops/

You might be correct about the terminology. I've seen both terms used to explain why nitriding makes crankshafts less likely to crack. I saw it as the residual compressive stress increasing the local strength of the material, but that may not be the correct engineering description. I started out as a physicist, and only switched to engineering later in life, so I don't always get the engineering terminology right. in any case, there doesn't seem to be any real question about the benefits of nitriding crankshafts.

Never ran into Prince Rupert Drops before. Amazing stuff.

Ken

jseng1 · May 18, 2017

Dances with shrapnel gave me an idea. I have several punches I made with ball bearings brazed to the ends of cupped shafts. Fitting a 3/16" ball punch to a hand held pneumatic hammer and peening the drive side journal radii and PTO radius while spinning the crank in a lathe should work out fine. I've seen the diesel cranks with their shot peened radii and you can see the little dents from the peening balls. The air hammer and ball end should burnish the radii and give the desired hardened surface. Easy to do and you don't have to ship it out. You do have to cut the radii in a stock crank but I've done that. The crank in the photo just needs the burnishing - I may do that next tear down.

acotrel · May 18, 2017

Whenever I've had a crank ground, the machiners have always removed the radius in the corners of the journals. I've always believed the radius can be put in by shaping the corners of grinding wheel - is this correct ?

lcrken · May 18, 2017

Yes. But normal automotive crankshaft grinding shops don't like to do it, because it's not their normal grinding form, so they have to dress the wheel for a one-off Norton job, and then dress it back to their normal form, using up some of an expensive grinding wheel in the process. Specialists that are used to doing Norton crankshafts are a better bet.

Ken

Beaneater · May 19, 2017

lcrken said:
I'm not sure a close ratio box makes much difference. I broke a fair number of stock crankshafts racing, and I only used Quaife 5-speeds, which have a close spread between gears. I think it's more a case of fatigue from the crank flexing that finally catches up with the race engines. I've not had one break in a street bike yet, but I've seen some that have. I do think that high rpm is the big culprit here. Race Commandos spend a lot of time at 7,000 + rpm, because that's where they make power, and more power means more crank flex. In any case, with the aftermarket cranks available now, that shouldn't be the problem it's been in the past.

Ken

Is it easier to downshift coming into corners with that Quaife 5-speed? I do that on my modern bikes but I have not tried it with my commando because it seems as though the gears are much farther apart then the ones on my modern bikes. Thanks

Dances with Shrapnel · May 19, 2017

Though you may bump engine speed with too large a gear spread upon downshifting; typically with a Norton twin and its relatively large inertial mass crankshaft, the wheel hop takes over and/or you break the gearbox which protects the engine from over reving.

Beaneater · May 19, 2017

I see. Thanks for your response. Generally I pull in the clutch and rev the bike up to about what the bike would be revving in the next gear down at that particular speed. I match the revs in order to provide less load on the gearbox. I just don't want to match revs up to redline if you know what I mean. Sorry for my lack of terminology of this sort of thing. When I downshift my 6-speed triumph for braking I just match the revs and switch down a gear and it's a nice smooth transition because the gears are so close together. I'm curious to know if that quaife 5-speed would provide a nice smooth transition to the next gear down because the gears are closer together than the stock gears (I think).

Dances with Shrapnel · May 19, 2017

I am with you on the transition. I run a TTI 5-speed with a 750 Nourish in a Seeley and from top to fourth often times there's no need to even blip the throttle as the change is not that significant for slowing down but makes a world of a difference for being postured to get a better drive coming out of the corner. With the standard four speeds, there was considerably more braking and blipping the throttle in order to avoid upsetting the bike; the whole cornering process was slower. I have an 89mm stroke Commando race engine in another Seeley and it ended up with a TTI six-speed and it's almost stupid as to how tolerant it is to downshifting because of the close ratios and borderline PITA because it really does not need that many gears.

Your observations and questions were spot on and I have found, as you have suggested, that the downshifting is less dramatic with the more gears (closer the ratio). As for impact on crankshafts, I don't think it makes one bit of difference unless one continues to abuse and abuse during downshifts.

Fullauto · May 19, 2017

Marc Sullivan said:
I see. Thanks for your response. Generally I pull in the clutch and rev the bike up to about what the bike would be revving in the next gear down at that particular speed. I match the revs in order to provide less load on the gearbox. I just don't want to match revs up to redline if you know what I mean. Sorry for my lack of terminology of this sort of thing. When I downshift my 6-speed triumph for braking I just match the revs and switch down a gear and it's a nice smooth transition because the gears are so close together. I'm curious to know if that quaife 5-speed would provide a nice smooth transition to the next gear down because the gears are closer together than the stock gears (I think).

Are you talking road or race here? If you are talking about road riding, quite simply, you aren't using the strengths of the Commando motor. You should be aiming to use the mid range power, which is much more useful. Riding a Commando like a modern bike is a self defeating exercise in frustration. Keep it smooth.

Use the torque, Luke.

acotrel · May 19, 2017

When I first raced the Seeley, I tried to use the standard gearbox. It was impossible to do anything smoothly, so I bought a 4 speed close ratio gear set. It was perfect everywhere except off the start-line in races. I now have a 6 speed TTI box which has first gear halfway between std commando and manx ratios. I have not yet tried it in anger. You will find your acceleration up through the gears is much faster with close ratios and the down-changes are much smoother. With the heavy Commando crank it is always better to keep it spinning high than try to accelerate it up a long way from low revs. I never believed in my Commando motor, however now I love it. It is a real weapon when used in a certain way.

acotrel · May 19, 2017

I don't know why anyone would fir a racing gearbox into a road-going Commando unless they intended to ride in a highly illegal manner. Both of my cars are Mazda 6s fitted with 6 speed manual gearboxes. So you can actually DRIVE them. I've only ever really used one of those gearboxes on one occasion. That was on the road between Queenstown and Strahan in Tasmania, when it was almost as good as being on a motorcycle. There is very little technology which can be transferred between modern bikes and early Commandos. They are completely different concepts. It all comes down to choice. Perhaps what some guys need is a good RETRO bike such as the Ducati Paul Smart replica ?

Dances with Shrapnel · May 19, 2017

acotrel said:
I don't know why ............

Well if you have the money and inclination, it is nice to have an extra gear on a stock Commando but more tangible is the much greater durability of a race gearbox (TTI) as well as the sweet sure shifting mechanism.

Dances with Shrapnel · May 19, 2017

Removed duplicate post

acotrel · May 20, 2017

If I had a stock Commando I'd love to have a close ratio 5 or 6 speed box in it. That is why I don't ride on public roads. I like having my licence as well as staying alive. If you've got the gearbox, you have to use it - motorcycles are about having FUN. In any case, where I live most of the roads are straight for miles.

Dances with Shrapnel · May 21, 2017

An interesting approach there jseng1. I know Comnoz has some experience with diesel cranks and seems knowledgeable about rolling the fillets. You might do some online research on the matter. Maybe you could set your lathe up with a roller held by the tool block - similar to a knurling tool. I know the knurling tools handle quite a bit of thrust; I remember using them and lubricating them with white lead and cutting oil. You might be challenged with getting a roller into the undercut fillet you are showing.

In this case, when "burnishing the radii" as you say, you are not really hardening but rather leaving the surface with residual compressive stresses. You could even roll a crank with a stock fillet radii.

jseng1 said:
Dances with shrapnel gave me an idea. I have several punches I made with ball bearings brazed to the ends of cupped shafts. Fitting a 3/16" ball punch to a hand held pneumatic hammer and peening the drive side journal radii and PTO radius while spinning the crank in a lathe should work out fine. I've seen the diesel cranks with their shot peened radii and you can see the little dents from the peening balls. The air hammer and ball end should burnish the radii and give the desired hardened surface. Easy to do and you don't have to ship it out. You do have to cut the radii in a stock crank but I've done that. The crank in the photo just needs the burnishing - I may do that next tear down.

jseng1 · May 22, 2017

There's plenty of "deep rolling" info online but the drawback is that it can reduce the diameter of the shaft and you don't want that. Ken Canaga once showed a Ron Wood crank that used stock nitrided cheeks. Maybe you could dig into your photo bucket Ken and show us again. If you could cut the PTO radius and get it Nitrided then it should be good with no crack worries.

acotrel · May 22, 2017

I suggest you are being ambitious if you think you can make a standard crank bullet-proof. My problem with billet cranks is that I've seen to many sulphur-prints from bar stock. There is usually a string of inclusions right up the centre where the main shafts would be, if you made a billet crank from it. You could start with a piece of billet, but the machining required would be horrible.

Broken crank and high compression

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