Why ball bearing mains?

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Steve, back in the 1920s and early 1930s, when iron pistons gave way to alloy pistons, folks switching to alloy from iron got a high BF, if they didn't play with the BF. They sure didn't like it. Bikes could snap steering heads off - but that just may have been the rough roads, and not related !!

This wasn't a Commando though, so YMMV...
 
110 % ??

You ever see those wacker-packers in action, that stamp down the dirt in your driveway before they tile it.
Not sure I'd like to tame one of those with a saddle, but each to his own.....
 
acotrel said:
How much do you believe the flywheel moves off centre and returns on every rev of a standard commando motor ? What is the amplitude and frequency of the vibration ? I suggest that the flywheel moves until the forces balance, and the vibration amplitude is well within the bearing tolerance at the revs to which the crank is balanced. If the crank is out of balance at any part of the usable rev range, the extra force is destructive , but if the out of balance occurs only at low revs , and not higher up the rev range, the forces involved are less. If you constrain the crank by using close tolerance roller bearings, I suggest you risk concentrating the stresses at the stress raisers on the crank, or cracking the crankcases , or destroying the bearings themselves. Fatiguing the bolts holding the shaft together is always a risk.
If you want a commando to go fast, buy a Weslake motor for it.
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Amplitude is a function of the crank angle phase, crankshaft stiffness, crankshaft mass, balance factor, distribution of bob weights and rpm. Frequency is a function of rpm. The forces in balance because the main bearings are in place and everything is connected.

I doubt bearings have any bearing (no pun intended) on crankshaft failures. In the realm of realistic Norton crank cases, I doubt any particular crankcase will have much of any direct bearing on crankshaft failure other than the crankcase can handle the power required to fail the crankshaft. Fatigue on the bolts holding the crank together is a bit of a red herring. Never came across an instance where bolts factored in to the equation in any way, shape or form.
 
Rohan said:
PPS My veteran early Triumph (single) had a twin row ball bearing on the drive side, OF THE SELF ALIGNING TYPE.
This requirement has been known for 100+ years then...
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Yes, I recall seeing something like that before. May have been a Ducati single of maybe a HD. Not much flexure in those applications though.
 
Dances with Shrapnel said:
Rohan said:
PPS My veteran early Triumph (single) had a twin row ball bearing on the drive side, OF THE SELF ALIGNING TYPE.
This requirement has been known for 100+ years then...
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Yes, I recall seeing something like that before. May have been a Ducati single of maybe a HD. Not much flexure in those applications though.
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You jest ?
The Triumph had quite heavy flywheels, and a very heavy iron piston.
As one roadtest put it, "a quarter of a hundredweight of rotating/repicrocating mass acting through those bearings" - and giving strong torque pulses down to countable revs was quite a load on those bearings. Cranks in early Trumpies are often not in good shape (part rivetted together) as testament to this. ...
 
No, I was serious. The main contrast is a single with relatively minimum span between the main bearings and a twin with a substantial bob weight some +3 inches away from the main bearing supports
 
V-twin guys go to a great deal of trouble to ensure both flywheels are evenly counter-weighted, so they seem to think that even with such narrow distance between bearings its worth the effort. I was a little skeptical, and they were adamant...
 
acotrel said:
I know you guys probably worship Peter Williams and believe in the commando design - I don't ! - Commandos are rubbish. They should have all been fitted with billet cranks, and I wouldn't have been nervous about racing mine.
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Alan, I met Peter once, he is just a nice guy, a talented rider and a smart engineer, hero worship would embarass him...but...he didn't design the Commando anyway....he was just given the engineering and riding tasks of winning races on one when completely outclassed by the competions baseline....and he delvered what he could...which is to be admired...greatly....what he designed was just about everything outside of the engine and gearbox, and they modified those components in the same way any third party race team would have....

But he would have had billet cranks if the budget allowed and a lot more like centre bearings or lightweight valve trains or whatever.....he had to work within extremely tight budgets and very unrealistic expectations, so he applied lateral thinking and concentrated on what he could do....same as many of us would have, except few of us have his engineering background.....and family heritage...

I suspect you are being overcautious about racing yours as it is.....get out on it when you can....
 
lcrken said:
Lots of good ideas there, Thomas. It looks like we all have some experience with bottom end failures, but don't all draw the same conclusions from them. The first set of cases that I had break in a race bike were the early '71 cases that Farrell likes, and the crank had the original roller drive side and ball timing side races. The case cracked on the drive side at the back just above the top cradle bolt, where the early cases were very thin. I had the cracks welded, along with some added reinforcing ribs, and used that set of cases with no more problems for several years of road racing. The original crankshaft eventually cracked around the drive side mainshaft, after a few seasons of racing. Over the next couple decades I had several other cases break on the drive side half, on a line from the mainshaft bearing bore back to the case joint behind the cylinder, just above the top cradle bolt. The cases that broke included another early 750 case that I had reinforced with ribs behind the cylinder and above and below the upper cradle mount, a stock 850 case in a short stroke 750 engine, a heavily reinforced MKIII case in a 920 engine, and a heavily reinforced 850 case in another 920 engine. I also managed to break several race-prepped crankshafts, and there may have been a couple more broken cases. The breakage all happened between 1972 and 1993, and I'm only describing the ones I recall in detail. The conclusions I drew from my experience were that the engines with reinforced cases and one-piece crankshafts lasted longer than the others, and I never saw any connection between whether the timing side bearing was ball or a roller. The 920 engine I'm running now has heavily reinforced 850 cases and a one-piece Nourish crankshaft, and really seems to be bulletproof (so far). I also think that the combination of original Commando cases and crankshafts, of any model, in a road race bike that makes high horsepower and is ridden aggressively, will eventually result in broken cases or crankshaft, or both. It's just a matter of time. With modern aftermarket replacement cases, crankshafts, rods, etc., I think it is possible to build much more reliable Commando race engines. It's still possible to break them, but it takes a lot more abuse to do so.

Just my humble opinion, and it could be wrong. I'm not trying to convince anyone, or prove anyone right or wrong, just passing on some experience.

About the only thing I'm really sure of is that, all other things being equal, the more horsepower the engine makes and the harder the pilot rides, the more often things will break.

Ken
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Ken,
You're really on to something with the one piece crank. I spoke with Nourish about a 90 degree crank for a Norton, the main reason he was hesitant to make one was he wasn't sure about the balance. I'm not a fan of the built up crank, but have wanted for years to make up a steel flywheel, set the cheeks at 90 degrees and add as much mallory metal to the cheeks as possible. The next thing (on an 850) that I would like to do is use the short stroke 750 rods and a Suzuki piston, to have a motor with a better rod:stroke ratio. It seems it would make a much better motor at RPM. For a 750, you'd have to make pistons with a higher wrist pin hole. I've been busy for a few years since I wrote the above article quoting Mike Farrell, who passed a way last year. These are all things I was talking to him about doing, but...well...lets just say it didn't happen. He wasn't really motivated to do much engine work. Obviously a rigid crank and a rigid crank case are the best solution and a Nourish crank is a step in that direction. Dreer did a lot of work inn that direction with his stronger cases and Falicon cranks. The 1990 something article was more about how do we keep our "stock" cranks functioning. They're mostly ok for the street IF your bolts are a drive fit in the holes, but truly rubbish for racing,
Stevan Thomas
 
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