Quality Billet crank

Been chewing over Jims sketch diagrams with the forces arrowed.
Not entirely sure the central flywheel weight doesn't add to the bending moment though.
Having the weight central may strengthen the entire structure however.
Swings and roundabouts, swings and roundabouts.

Mr Axtells shifting the weight outwards certainly accords with the engineering principle
of keeping the weights as close to the bearings as possible - if it would fit !!

1. If you or I can find it again BSA research showed the 52% BF gave the least load spikes so that's likely why Norton picked that for the isolastics while prior higher BF for the survival of solid frames and people. If going over this BF then better to add it near the end supports. Most common vintage solid mount BF falls in the 80's ballpark. 93-94% BF was highest I heard of an isolastic racer using though no data on why or result.

2. It s'not the piston jerk down mass on TDC intake stroke that bows crank as much as the centrifugal sling out force of the off centered mass of the central flywheel.

3. An over rev'd bowed crank ends will clash rotor on stator in an arc from 7>5 o'clock area which d/t the robust bearings in cases act as lever pivot points with the heavy flywheel moving upward as the ends deflect downward > which implies central flywheel mass induces most the bowing of crank and that occurs when pistons at BDC, [as has somewhat less piston jerk inertia crank load than at TDC] ^which agrees with Axtell's hard earned wisdom - demo'd by the common parting of cases even with 'mere' 52% BF.

4. Might be educational to compare the reports of blowing case or cranks in cheese of isolastic racer vs chalk of solid mounts and what BF involved. Ron Fratturelli, [of east coast sales hot spot era] made racer flywheels 4.5 lb lighter and 1/2" smaller OD. Peel got the last of em and helped make me the annoyance I am today and still in the very heart of her next intarnation.

5. So ask yourselves Punks, would ya rather stand back a hand full of seconds and watch your type cranks or Ms Peel's bounce tach needle off zero peg from both sides? Flywheel-crank mass on handling and track time or comfort are one thing * engine surviving it is quite another issue altogether.

I think the difference between a rigid mounted motor and one with isolastics is that the isolastics allow the crankcases and bearings to move in the opposite direction to the vibrations, and allow a lower balance factor to be used which is optimal for street use at normally lower revs, than used in racing. I could use the low balance factor in my rigidly mounted 850 motor, however it would vibrate badly at the high revs used on a race circuit and start destroying itself internally. If you used the high balance factor in an isolastic mounted motor, it would be better at high revs however the isolastics make that consideration redundant. Isolastics on a race bike interfere with the handling so the rigid motor mounting high balance factor is probably better for that. I think I would go insane trying to get isolastics right on a Mk3 Seeley - probably kill myself in the process because of the missing front mount.

When it comes to balance factors and why one would want to vary from the physics determined minimum forces balance factor of between 50% and 55% has at least two distinctly different schools of thought.

The reasoning that I learned for differing balance factors is quite different from the belief held by a lot of people -particularly in the vintage motorcycle scene.

Debating balance factors was not my intention in looking for peoples thoughts on couterweight positioning. Jim

Only reason for higher quality cranks is to press rpm luck higher so its on point to comprehend how much and where best to put the counter balance mass.
Racers that first tried mid 50's BF in rigid mount frames broke frames and injured nerve tissue until BF mass added till frame &or pilot could stand it. You can a crank balancer so can experiment with a weakened crank so the bench rpm would show if lighter of heavier center mass deforms crank most.

comnoz said:

From the rough measurements I did on 750 crank some years ago they were approximately 25%-50%-25%. It is a tough thing to measure real accurately.
I have not tried to measure an 850 crank but I would bet it is similar.

I would bet the Axtell crank was overbalanced for a rigid mount application and I would think adding weight only to the outer cheeks would probably be the right thing to do. Jim

Click to expand...

It was balanced at 62% with stock rods and Powermax pistons, and I originally used it in my Commando PR, and later in a featherbed race bike. The flywheel did have lightening holes to keep the BF at 62%. The intention at the time was to move the weight closer to the bearings, not change the BF. Not sure if it made any real difference, but that was the thinking at Axtell's and the balance shop at the time, ca. 1974.

Worked fine for many years. It now resides in a friend's rarely used 920 featherbed race bike.

Ken

I was talking to someone at the Norton rally about his friend who had been pretty sucessfull flat track racing with a Triumph 650. The trouble with that type of racing is that to win you need to set the engine up to have a lot of power at very high revs. It is possible by reving the engine over 8000 RPMs to get around 80 HP from one of these engines. Triumph engines are often reved past 8000 RPM for this type of racing and although the engine produces power at this speed it will blow up eventually. The main problem is the crankshaft. They tend to break just inboard of the drive side main bearing. If you are lucky when it breaks sometimes nothing else breaks. Often everything gets ruined.

The obvious way to solve this problem is by a billet 4140 crankshaft, but this guy figured out another way. What he did was add an aditional bearing outboard of the drive sprocket. He was not using an alternator anyway. Apparenly because of this addition his crankshaft never broke. I guess this might work on a Norton as well.

Nigel

That would tend to show that the shaft under the drive sprocket wasn't strong enough, or wasn't strongly enough anchored,
and was flexing with the torque delivery enough to (eventually) break it.

If you look at 1920s and 1930s motorcycle engines, the output (drive) shaft got upsized on any number of makes and models,
as engine horsepowers grew.
The shaft went from a spindly thumb size to quite substantial shafts in the case of the ohc Nortons and Velos and Ajays.
And the touring versions followed suit too, eventually.

Every Triumph I've ever worked on had a plain roller bearing on the drive side and a ball race on the timing side to stop end float. If the shaft flexes, the fatigue will occur beside the drive side bearing. I think it's the same problem the superblend bearings fixed in Commando engines. I can't see how the extra bearing outboard of the sprocket would help unless the load imposed on the chain was bending the mainshaft. With a superblend bearing all that would happen would be that the crankshaft flex inside the cases would increase slightly.

Here's a good example of the 25% 25% 25% 25% counterweight. Not quite right because of the material between the journals, but close.

Has anyone actually studied crank flexing though to see if having those central weights adds to flexing,
or counteracts it....

A high speed strobe light and a few experimental cranks spinning at revs would show it soon enough.
Sounds cheap and easy if you say it fast enough.

Great pic Jim.
Lotta work gone into that.
tell us more ?

tricatcent said:

The obvious way to solve this problem is by a billet 4140 crankshaft, but this guy figured out another way. What he did was add an aditional bearing outboard of the drive sprocket. He was not using an alternator anyway. Apparenly because of this addition his crankshaft never broke. I guess this might work on a Norton as well.

Nigel

Click to expand...

It worked pretty well for me.

acotrel said:

. I think it's the same problem the superblend bearings fixed in Commando engines..

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And we thought that Superblend bearings fixed a weak bearing problem !!

Did anyone actually say it stopped the crank flexing ?
I understood it simply allowed the crank to flex, AND the bearings not to fail....

The outboard bearing on the gearbox shaft helps prevent the gearbox shaft failing from flexing,
so its difficult to see why the same on the crank output shaft couldn't do the same thing...

comnoz said:

It worked pretty well for me.

Click to expand...

Neat idea, keeps the shafts located the correct distance apart AND supported.

Something along those lines could almost be incorporated inside the primary cover. ?
Although it would have to be located, and locked there, fairly accurately....

The bearing mounts supported by the fabrication between the crank and the mainshaft is a top idea. The only problem I'd have with it would be finding a way to enable adjustment of the distance between centres.

Rohan , What I suggested was that if the tension in the chain tended to cause the end of the crankshaft to bend, the super blend bearing because of its shape would allow the bend to occur in the body of the crankshaft i.e. the stress point moves towards the centre of the crankshaft, and away from the point where the dimension changes immediately inboard of the bearing. With plain bearing on the other end of the crank also has an effect on where the stress point will be. If the whole shaft is able to bow in the middle at high revs, there should be litte fatigue problem unless there is a stress raiser.

'And we thought that Superblend bearings fixed a weak bearing problem !!'

I never believed that, I always thought it was the shape of the rollers which allowed rotation in the other planes as the shaft flexed. Are Superblend bearings stronger than normal bearings with barrel shaped rollers ?

In about 1955 Triumph 650s had the timing size bearing increased in diameter to match the drive side. If you wanted to fit the late fifties big journal shaft to the early cases, you either had to machine the timing side end of the crank, or buy the right OD and ID bearing and fit spacers to the side of the inner and outer . There was however a double row ball race bearing available of the correct size. We never used it because in theory, it would not permit the crankshaft to flex. It just had to fail either in the bearing, or in the shaft. I think that these days there would be a special list bearing to do the job. In the sixties the Hoffman special list bearings were very difficult to get, I had them on the billet crank in my short stroke 500 Triton.

"Superblends" DO NOT have barrel shaped rollers. Measure one & see. They are simply an extra load roller bearing.

I measured one years ago to see if what I had read was correct, well it wasnt barrel shaped , I would guess if there was any change in the shape it would of been that the Superblend had a bigger radius on the ends, but I couldnt guarantee that and I'm probably wrong.