Commando Crankshaft Porn

comnoz said:

splatt said:

I also noted that JIM C is starting to use heaver cranks, will that have all of the sheepole tearing their injuns down to get weight added, you have done a full circle it would seem JIM, have you found them nicer to ride or is it just where it ended up and thats where it will stay :?: :?:

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Well here is my story. After blowing up the second lightened stock flywheel in my racebike many years ago I built the crank with the center main and made it as light as I could. I built the rest of the motor the same as it had been before the explosion. After that I found that I could no longer run with Larry Kirby and the the rider from Big D Cycle who had been the guys I had been swapping leads with for some time. I fought that for most of a season trying to figure out why. One day I was talking with Ken.C and he said a light crank isn't always better so I started doing some reasearch. Kirby was racing a shell 750 Yamaha and I asked him what he had for a crank. He said it was stock except for the steel bands around it for added weight. I also talked to a few other people who had a reputation for being fast. So I pulled my motor apart and welded on the two steel bars around the center two porkchops which added several pounds near the edge of the flywheel to get maximum effect. The very next time out I beat Larry. The next crank I built was even heavier and my track times improved even more. The bike actually felt slower with the heavy crank but the watch doesn't lie.
Maybe this is just me and my particular riding style I don't know but it worked for me and after talking to knowledgeable people I found I was not the only one who felt this way.
The only reason I like to add weight to my streetbike crank is the fact that it makes the motor vibrate less. I also have to think the reduced vibration has to help the life of the cases, bearings, ect.

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COOL thankyou, so the seat of the pants dyno can be a compulsive liar :?:

COOL thankyou, so the seat of the pants dyno can be a compulsive liar :?:[/quote]

No doubt about the seat of the pants being a compulsive liar. The thing I see most often is when someone installs a performance cam or puts an exhaust system on their bike. They say WOW it really made a big improvement. Feels a lot faster. Then you put it on the dyno and find the peak horsepower increased by 2 horse and now there is a hole in the middle of the powerband. There is no way a seat of the pants dyno can feel a 2 horse change but what makes the bike seem faster is the hit you feel when it goes from the flat spot to the powerband. If you put it on most roadrace tracks the bike without the hole in the powerband is going to win. Jim

comnoz said:

Well here is my story. After blowing up the second lightened stock flywheel in my racebike many years ago I built the crank with the center main and made it as light as I could. I built the rest of the motor the same as it had been before the explosion. After that I found that I could no longer run with Larry Kirby and the the rider from Big D Cycle who had been the guys I had been swapping leads with for some time. I fought that for most of a season trying to figure out why. One day I was talking with Ken.C and he said a light crank isn't always better so I started doing some reasearch. Kirby was racing a shell 750 Yamaha and I asked him what he had for a crank. He said it was stock except for the steel bands around it for added weight. I also talked to a few other people who had a reputation for being fast. So I pulled my motor apart and welded on the two steel bars around the center two porkchops which added several pounds near the edge of the flywheel to get maximum effect. The very next time out I beat Larry. The next crank I built was even heavier and my track times improved even more. The bike actually felt slower with the heavy crank but the watch doesn't lie.
Maybe this is just me and my particular riding style I don't know but it worked for me and after talking to knowledgeable people I found I was not the only one who felt this way.
The only reason I like to add weight to my streetbike crank is the fact that it makes the motor vibrate less. I also have to think the reduced vibration has to help the life of the cases, bearings, ect.

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Wow! That's a blast from the past. I'd forgotten that conversation, but I remember Larry Kirby and his Yamaha. I've always preferred a heavier crankshaft in the Commando engine, but I've been reluctant to mention it much on the forum for fear of abuse from the light crank true believers. I've tried light cranks, and they do give the sensation of revving faster, and it's hard to argue with anything that drops the overall weight of the bike. Years ago Ron Wood told me an interesting story that started me thinking about the effect of crankshaft weight. He had progressed from modified standard Commando crankshafts in his 750 Norton flat track bikes to a batch of one-piece crankshafts made for him by Moldex. The Moldex cranks were heavier than he wanted, but he used them anyway. He said that his riders liked the lighter crankshafts because they felt faster, but that they actually did better times on the track with the heavier crankshafts. He speculated some about how it might have to do with carrying more speed into the corners, but wasn't really sure if that was the reason. I asked Axtell about it once, and he said that the heavier crankshaft made the engines seem "happier" on the dyno. He's the one who sent me to the crankshaft experts of the time for heavy metal slugs, custom steel flywheels, bigger bolts, radiusing, shot peening, magnafluxing, and balancing, and they all recommended not lightening the crank.

In the end, I think it comes down to personal preference, particularly on a street bike. If you like the way it feels with a light crank, go for it. I know Steve Maney is a fan of lighter crankshafts, and he certainly knows what he's about. Jim Schmidt has some good arguments about being able to use a lighter crank with his lightweight rod/piston combo, but I don't have any experience to base that on, so I'll probably stick with heavier crankshafts. I think Jim Comstock is right about the reduced vibration from a heavier crankshaft helping the engine live longer.

I've got my asbestos suit handy in case of violent disagreements.

Ken

splatt said:

With the radius pto shaft, are you undercutting the bearing face, and what size radius is being used,anyone ever had the radius rolled, or is that like trying to find rocking horse crap.

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You are cutting the groove into the face of the cheek, not into the shaft. I use a .090" radius tool to cut the groove. That's what the crankshaft guru's recommended back in the '70s. It's also necessary to polish the groove and/or shot peen it to reduce surface stress risers. I use a wooden stick with valve grinding paste for the task. You can see the groove pretty well in this picture.



Ken

comnoz said:

No doubt about the seat of the pants being a compulsive liar. The thing I see most often is when someone installs a performance cam or puts an exhaust system on their bike. They say WOW it really made a big improvement. Feels a lot faster. Then you put it on the dyno and find the peak horsepower increased by 2 horse and now there is a hole in the middle of the powerband. There is no way a seat of the pants dyno can feel a 2 horse change but what makes the bike seem faster is the hit you feel when it goes from the flat spot to the powerband. If you put it on most roadrace tracks the bike without the hole in the powerband is going to win. Jim

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The other "hole" or "flat spot" that induces this perception of increased speed or power is the hole or flat spot left in your wallet after your investment for the parts!

RennieK said:

comnoz said:

No doubt about the seat of the pants being a compulsive liar. The thing I see most often is when someone installs a performance cam or puts an exhaust system on their bike. They say WOW it really made a big improvement. Feels a lot faster. Then you put it on the dyno and find the peak horsepower increased by 2 horse and now there is a hole in the middle of the powerband. There is no way a seat of the pants dyno can feel a 2 horse change but what makes the bike seem faster is the hit you feel when it goes from the flat spot to the powerband. If you put it on most roadrace tracks the bike without the hole in the powerband is going to win. Jim

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The other "hole" or "flat spot" that induces this perception of increased speed or power is the hole or flat spot left in your wallet after your investment for the parts!

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I've been a Norton fan too long to even remember what a lump in the wallet was like.

Ken, your right about the flak when you mention a heavier crank. But then I love a debate. Jim

I would think the weight thing would be relative to the energy produced in the combustion chambers and then the rpm range you want max power plus weight of bike and rider etc etc. At what point is the heavier crank drawing more energy on it's own than is necessary or does that even enter the equation as there is such a wide margin from lightest to heaviest.

RennieK - the symptoms of my broken crank were just a bit of a knock like a big end failure. The crank nose didnt break square but rather half way trough then screwed off. I presume the end float of the crank was not enough to loose drive completely and the stripped motor showed signs that the mains had spun in the cases and had to be lock tighted back in. Incidently the 750 was definitely smoother but not as revvy with the 850 crank. I always preferred the 750s, a much nicer engine to ride - this one just felt like a slow 850 afterwards.
Back to lightweight cranks I've just been given a 7inch diameter 3/4inch thich piece of EN24T - any clue what I wanna do?

RennieK said:

I would think the weight thing would be relative to the energy produced in the combustion chambers and then the rpm range you want max power plus weight of bike and rider etc etc. At what point is the heavier crank drawing more energy on it's own than is necessary or does that even enter the equation as there is such a wide margin from lightest to heaviest.

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Somewhere in my pile of papers I have a formula to figure the correct crankshaft inertia for an engine to obtain maximum conversion of the reciprocating power into rotation. It took into account bore,stroke,compression ratio, rpm, and probably a few things I don't remember. Working out the formula gave you an inertia figure in In.lbs/sec. I never did set up the equipment needed to measure the inertia of a crankshaft. It involves more than just the weight of the shaft. I have a feeling the engineers at Norton probably knew what they were doing when they made the crank weight what it is. I also know that according to the formula increasing the compression ratio or displacement required more flywheel and raising the RPM required less. From what I could gather it looked to me like stock inertia was about right even for a hot motor. Jim

I should probably be more careful in talking about "heavy" and "light" cranks. What we're really talking about is moment of inertia (or rotaional inertia), not crank weight. Our ideal crank construction would start with what we wanted for moment of inertia, and then put as little mass along the axis of the crank as possible, and the mass required to reach the right inertia value as far out from the axis as possible. That would give the lightest overall weight for a given moment of inertia. Maybe something like a titanium plate, pork chopped flywheel with heavy metal inserts at the outer rim, and as much mass as possible moved over to the outside counterweights.

Ken

I always wanted to build a press up crank for a Norton. Use full circle flywheels for maximum inertia and heavy metal for counterbalance but reduce the centers for weight reduction. TI would be great for the flywheels but it would sure be a challenge to get it cut and fitted. Not to sure how TI would work for a press fit. Then use some high grade steel for the hollow crankpins and shafts. 9310VAR would be great. Jim

comnoz said:

I always wanted to build a press up crank for a Norton. Use full circle flywheels for maximum inertia and heavy metal for counterbalance but reduce the centers for weight reduction. TI would be great for the flywheels but it would sure be a challenge to get it cut and fitted. Not to sure how TI would work for a press fit. Then use some high grade steel for the hollow crankpins and shafts. 9310VAR would be great. Jim

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You mean like this one?



This is one Kenny Dreer had made by Falicon. IIRC, it used parts from Rotax single press-together cranshafts.

Ken

Yes that looks like the real deal. I had one that I started from a couple KTM 495 cranks like that but I changed direction and went ahead with the plain bearing VW cranks. If I knew then what I know now I might have stayed with the KTM cranks. Jim

lcrken said:

This is one Kenny Dreer had made by Falicon. IIRC, it used parts from Rotax single press-together cranshafts.

Ken

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Jeese, can you pull a rabbit from a hat Ken? How much longer can you keep doing this? Ken wins! he's got the most toys!

Was this last one running ohc's? Where do the rods go, I just see a cyndrical casting above the cam position.

Kool stuff guys, keep it coming : )

Or maybe just the most pictures. Sorry, Rennie, I don't have this crankshaft, just a picture Kenny sent me a while back. It's still a pushrod engine. I'm not sure what's going on in the camshaft area. The new Commando still uses pretty much the same crrankshaft design. This is a picture of the current production crankshaft, from Norton's web site, and you can see that it is very similar to the one Kenny did. The rods in the new one look very much like the stocck rods in my Rotax single. The rods in Kenny's look like Falicon rods.



Fun, isn't it.

Ken

that looks like the crankpins have been welded in........

The idea of a 3 bearing crank means a far stronger bottom end, and the potential for some relatively serious performance being delivered reliably.

The inertia stored in a crank tends to make a bike much easier to ride, and running heavier cranks is something that has been common for many years in speedway and classic MX motors.

What are you going to do today?
I think I'll design and build a new crankshaft for my motorcycle.
What about the cam?
Oh, I made a couple of those yesterday.

An exaggeration? Sure! I'm just saying that it truly impresses me what you guys here have the ability to do.
I am humbled.

That is all

ludwig said:

RennieK said:

I.. At what point is the heavier crank drawing more energy on it's own than is necessary ..

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The crank doesn't draw energy but stores it for later use . I try to make everything as light as possible , but I would never remove one gram of the crankshaft .
I don't race , but if you accelerate uphill and shift up at 6000+ rpm , that heavy crank really kicks you forward ..

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I disagree that a crank doesn't draw energy. I agree it stores it but anything set in motion requires force and a heavier object requires more force to turn and keep in motion.

My theory is if you have 2 identical bikes traveling at say 120 mph on level ground at 7000 rpm's, the one with a lighter crank is using less energy to maintain the speed because it has less weight to turn. Being the engines are otherwise identical the lighter one has unused energy potential to go faster and if nothing else would be using less fuel. There will be a threshold where a crank can be either too heavy or too light for optimum performance. From the examples on this thread so far it would seem there is a wide tolerance band of the ideal weight of the crank.

Once you have your intake, exhaust and combustion chamber maximized it will be capable of producing a fixed amount of energy or power. Given the fixed amount of power the lighter crank will accelerate (and decelerate) faster. It could be the combustion chamber is capable of producing so much power a heavier crank is no burden and you can easily pour on the extra fuel required to turn the extra weight and benefit from the stored power however there is a tipping point where the raw power from combustion is more beneficial because that is the real power on tap.