Commando Crankshaft Porn

worntorn said:

Jim, just to clarify, the 1/16" runout afer initial heat treating became .004" after stress relief?

Glen

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No, I had recut the center holes and reground everything true and straight. I had left plenty of metal to remove in case it warped a lot in heat treatment -which it did.

The straight shaft became .004 out after stress relief. Jim

A couple of additional questions on the process- have you left material on for truing after nitriding and if so, is there a special stone for cutting the nitrided material away, however little this might be, hopefully a thou or two at most?

Glen

worntorn said:

A couple of additional questions on the process- have you left material on for truing after nitriding and if so, is there a special stone for cutting the nitrided material away, however little this might be, hopefully a thou or two at most?

Glen

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The people who do the nitride would prefer that no grinding is done after nitriding but it is possible. The plasma nitride is supposed to be approximately .020 deep so I suspect a couple thou could be removed without affecting much -particularly toward the ends of the shafts. I would hate to have to remove anything from the rod journals after nitriding. [other than polishing] Jim

comnoz said:

A 53% balance factor places the least load on the crankcase and bearings at any RPM. The counterbalance weight increases at the square of the rpm just like the reciprocating forces.

Increasing or decreasing the balance factor from 53% always increases the vibrating forces regardless of RPM.

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With a standard rod/stroke ratio of 1.6767 :1,the amount of counterweight to balance the small end and piston at TDC would be (1 + stroke/ [2 x rod length]) = 129.82 %
Obviously that would cause an imbalance of 129.82% at around mid-stroke,at maximum piston speed.

With 1/2 that balance factor (64.91% wet),the maximum imbalance forces would be equal at TDC and around mid-stroke.I suggest that any change from there would increase the maximum imbalance force.A lower factor would have more imbalance at TDC.A higher factor would cause more imbalance at around mid-stroke.

When you average in the reduced amount of couterbalance needed at BDC it will drop the percentage into the 50's. Jim

PS
Actually the more I think about this what you are saying makes sense, but I also remember when I was in balancing school we used the Norton engine as an example and the instructor showed how the 53% or thereabouts factor created the lowest average forces.
Now I am going to have to dig out my books and see if I can figure out how that was determined. Jim

The lowest average imbalance force still leaves you with a high maximum imbalance.The maximum imbalance will be the force that breaks things.
Google-search "piston acceleration".At TDC it's proportional to (1 + 0.5/rod-stroke ratio).

comnoz said:

When you average in the reduced amount of couterbalance needed at BDC it will drop the percentage into the 50's. Jim

PS
Actually the more I think about this what you are saying makes sense, but I also remember when I was in balancing school we used the Norton engine as an example and the instructor showed how the 53% or thereabouts factor created the lowest average forces.
Now I am going to have to dig out my books and see if I can figure out how that was determined. Jim

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'lowest average force' - at what revs ?

I suggest it probably refers to about 3000 RPM which is where most Commandos would get the most use. If you want to spin them higher for longer times, I believe the higher BF is necessary to protect the bearings and cases. The fact that the rider might not get a numb bum is a bonus. I know with my bike, it feels great at 7,000 RPM, probably TOO GREAT , and it's likely to pop the top off a rod through over -revving. It is interesting t o ride, it doesn't vibrate at low revs , it feels more like a pulse and at high revs it is extremely smooth. With the methanol fuel and the close gears and relatively low overall gearing it spins up a bit too quickly. and I have to be careful to always catch it. What concerns me is that there is no significant lag under acceleration where I expect there should be one after a gear change. I'm about to receive two larger engine sprockets, so with the 6 speed box, I should find out it's true top speed on our local circuit. I suspect it will go even quicker in top speed, and it is already fast enough to win in period 4 races with the 4 speed CR box if I get a good start and ride my ring off. It is a very strange motor, even a good 650 Triumph doesn't behave like it. I've never believed in long stroke for racing until now.

I didn't quite understand what you meant by moving the forces sideways. If the shaft is spinning in perfect balance at a particular spin rate , surely the forces are equally distributed in all directions at right angles to the axis and become larger or smaller in different directions as the spin rate increases or decreases and as the power output increases or decreases ?

acotrel said:

comnoz said:

When you average in the reduced amount of couterbalance needed at BDC it will drop the percentage into the 50's. Jim

PS
Actually the more I think about this what you are saying makes sense, but I also remember when I was in balancing school we used the Norton engine as an example and the instructor showed how the 53% or thereabouts factor created the lowest average forces.
Now I am going to have to dig out my books and see if I can figure out how that was determined. Jim

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'lowest average force' - at what revs ?

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2 or 20,000 it rpm does not make any difference.

acotrel said:

comnoz said:

Actually the more I think about this what you are saying makes sense, but I also remember when I was in balancing school we used the Norton engine as an example and the instructor showed how the 53% or thereabouts factor created the lowest average forces.
Now I am going to have to dig out my books and see if I can figure out how that was determined. Jim

Click to expand...

'lowest average force' - at what revs ?

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Lowest average force at ALL rpm.
The imbalance caused by the reciprocating piston and small-end increases as the square of rpm,but only acts inline with the cylinder axis and is not constant during the cycle.
The imbalance caused by the counterweight also increases as the square of rpm,but acts radially.Only at TDC and BDC is it acting directly against the imbalance caused by reciprocating parts.If it cancels out 50% of the reciprocating imbalance at TDC,it does that at ALL rpm.
At maximum piston speed (about 75 deg before and after TDC),there is no imbalance force from the reciprocating piston and small-end.At those points,the the imbalance of the counterweight is unopposed and shaking everything in the horizontal (or near horizontal) direction.Before TDC the counterweight is pulling the crank rearward.After TDC,the counterweight is pulling the crank toward the front of the bike.

That situation would be about the best you could hope for.The counterweight opposes 1/2 the reciprocating inertia at TDC (there is still serious imbalance).The counterweight,being unopposed at 75 degrees before and after TDC,gives the same amount of serious imbalance at those points (mainly in the horizontal direction,forward and rearward).At BDC the counterweight would balance most of the reciprocating inertia.If the wet balance factor was 70.18%,it would totally balance the reciprocating inertia at BDC with a standard rod length and stroke.I'm suggesting that 64.9% would be a good compromise.That would be about 70% dry.

If vertical imbalance caused the frame to vibrate more noticably than horizontal imbalance does,then you would increase the balance factor for comfort sake.Increasing the balance factor would still cause higher forces in the horizontal direction,but could prevent or lower frame vibration at a certain frequency.If the natural frequency of the frame happened to match your cruising rpm,you would appreciate this change.

That's about as well as I can explain it.

From my notes taken 30 years ago. As explained by Joe Mondello. Jim

The balance factor of about 65% creates the lowest peak imbalance force if the engine were mounted in a infinitely rigid frame.

The balance factor of around 58% creates the lowest average imbalance forces in the infinitely rigid frame.

The Commando engine with isloastic mounts creates the least amount of stress on its bearings and cases at 53% as that is where the engine motion creates a circle on its mounts. This takes into account the different mass of the engine in the vertical plane in comparison to the horizontal plane.

comnoz said:

The balance factor of around 58% creates the lowest average imbalance forces in the infinitely rigid frame.

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There is the crux of the matter: "lowest average"

Taking it a bit further, reducing the "average" forces is another way of saying reducing the magnitude in bending moments in the crankshaft and that is what allows crankshafts to live longer.

With good engineering judgment one can say there is no practical infinitely ridged frame in this context. Every frame of every motorcycle ever built moves and vibrates. Infinitely rigid would be mounting an engine to a massive block of concrete with thick stiff steel engine plates to truly constrain any engine movement. Even then there's movement within the engine due to microstrains of components such as crankcase webbing.

So, for all intents and purposes, 53% is it.

Thanks Jim.

So the people who built these things forty years ago got that part right. Actually, I think they got quite a bit right, maybe more than we give credit for.
At the Vancouver Vincent club dinner yesterday I chatted with a fellow who owns 2 Commandos, a red Fastback that he built up from junk and is now a showbike, plus a 74 850 that he has owned since new and was his only transport for fifteen years. It now has 126,000 miles and the bottom end has never been apart, though it is now very tired. The top end had one redo and is at. .020 over.
At the same dinner there was another Commando owner with over 100,000 miles on his bike, that one had the bottom end renewed at 80,000, not sure how much top end work was done.

Glen

Really cool stuff guys, Wish I was as smart. I guess that's why I like the photos too. :wink:

worntorn said:

So the people who built these things forty years ago got that part right. Actually, I think they got quite a bit right, maybe more than we give credit for.
At the Vancouver Vincent club dinner yesterday I chatted with a fellow who owns 2 Commandos, a red Fastback that he built up from junk and is now a showbike, plus a 74 850 that he has owned since new and was his only transport for fifteen years. It now has 126,000 miles and the bottom end has never been apart, though it is now very tired. The top end had one redo and is at. .020 over.
At the same dinner there was another Commando owner with over 100,000 miles on his bike, that one had the bottom end renewed at 80,000, not sure how much top end work was done.

Glen

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I have always thought that most of the engineering was pretty good for the time period. Sometimes the execution left a bit to be desired however. Jim

Well after 16 hours in the oven at 250 C. the crank actually got slightly straighter. Runout was down from .004 TIR to .003 TIR.

But I still don't have a good feeling that the crank is stable. Partially because the heat treatment outfit that processed it started on it on Monday morning and called me first thing Tuesday morning to tell me it was done. When I asked how it could be hardened, double tempered and stress relieved so quickly the guy said "with new equipment it doesn't take as long as it used to". [OK]

So I went back to the old fashion method, the crank is packed in a steel container with 150 lbs of glass bead and it is in the stress relief oven I use for cylinder barrels. It is set at 1000 F. where I will leave it for 10 hours. After 10 hours I will start the slow ramp back to ambient. If it stays straight through that I figure it should stay straight through nitriding. Jim

Is the .003 " TIR the maximum runout measured on either mainshaft or the sum of max runout of the two mainshafts? There seems to be a couple of common versions of crank TIR measurement, tho I expect only one is correct.

Glen

At this point I'd sure give this fella a ping for the low down on what to do next or not too to get below .002" on either and both ends and keep it or even improve with use. Your call of course ...
http://www.marinecrankshaftinc.com/

My feeling is that the 650ss and the Atlas were both very good bikes , however the need to compete with the Honda CB 750 for smoothness changed things and possibly not for the better in some ways. I don't think consumers would have ever accepted a commando with a rigidly mounted motor, if they could buy the Honda. It depends on the size of the niche market ? Most people don't use high performance bikes to their full potential and will tolerate discomfort while riding around town. The commando was a good compromise. I would own a standard Commando, however never a CB750.

worntorn said:

Is the .003 " TIR the maximum runout measured on either mainshaft or the sum of max runout of the two mainshafts? There seems to be a couple of common versions of crank TIR measurement, tho I expect only one is correct.

Glen

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The reading is with the crank supported on the main bearing journals and the indicator near the end of the PTO shaft. So it obviously would not be the bend from one end of the shaft to the other but just an indication of warpage. Jim