Opinions-- 920 Commando in a Featherbed

mdsaxon

Joined
Sep 6, 2015
Messages
4
I know a few of you guys are running 850's without shaking your teeth out but wanted to build something tricked out. Would it be over the top?
 
Nothing is over the top if your willing to give it a go, be a intresting project.

Ashley
 
mdsaxon said:
I know a few of you guys are running 850's without shaking your teeth out but wanted to build something tricked out. Would it be over the top?
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It would probably be okay with lightweight pistons and longer rods from jsmotorsports, but your probably looking at some stronger/thicker maney cases or maybe the andover/norville ones, with a worked over crank or new one piece aftermarket like nourish or similar, balanced for the solid mount, then some 920 aftermarket cylinders as others here with more experience have said that they had trouble getting long life out of bored over factory iron barrels.
 
I've never run a 920 in a featherbed as a street bike, but I did run one last year at Bonneville. It was rideable, but did have a lot of vibration at higher rpm, and I think it would have been pretty unpleasant on the street. It shook enough to break the nitrous injectors off where they threaded into the intake manifolds, and that was on it's first run.

A Norton racer friend of mine told me years ago that he'd raced a 920 featherbed back in England before he emigrated, and that it was a real beast for vibration.

On the other hand, neither of these had particularly light weight pistons, and I'm sure lighter reciprocating weight would help in the vibes department.

Ken
 
Why not buil it with an ofset crank to reduce the vibrations? That is what the new Commando is running and many have done this exact mod. I don't know if longer rods and lightweight pistons would go with such a crank, but it would be something to try for sure, after all it's not my money :wink:

Jean
 
Ken, were the bikes that you refer to as bad vibrators dynamically balanced?

My hypothesis (NOT based on actual Norton experience) is that increasing capacity to 920 should be irrelevant, as the main factors involved in determining vibration are revs and weight of moving components.

I therefore believe that a badly balanced 750 with heavy pistons will vibrate way more than a dynamically balanced 920 with lightweight pistons and the correct balance factor for the application (iso / rigid / etc)

However, my only practical experience that supports this is with Triumphs. Can any 920 Norton owners comment?
 
Fast Eddie said:
Ken, were the bikes that you refer to as bad vibrators dynamically balanced?

My hypothesis (NOT based on actual Norton experience) is that increasing capacity to 920 should be irrelevant, as the main factors involved in determining vibration are revs and weight of moving components.

I therefore believe that a badly balanced 750 with heavy pistons will vibrate way more than a dynamically balanced 920 with lightweight pistons and the correct balance factor for the application (iso / rigid / etc)

However, my only practical experience that supports this is with Triumphs. Can any 920 Norton owners comment?
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The crankshaft in the 920 in my featherbed was balanced dynamically by Bob Milliken, originally the balancing expert at Ditronic Balancing Company, a division of Hank the Crank in North Hollywood, and later operating as Milliken Balancing in Sun Valley, CA. Bob was pretty much the "go to" balancer for Socal racers back in the day. He retired some years back, but I'm still using a couple of the crankshafts he balanced.

I don't know the details of the bike my friend raced in England. Knowing him, I'm quite sure the rotating assembly was balanced, but it's quite possible it was all static, not dynamic. That was pretty common practice back in the early '70s.

I would certainly agree that the principle reason a 920 shakes more than a 650 or 750 in a rigid mount like a featherbed is the increased reciprocating weight of the larger pistons (as well as the additional weight in the counterweights to keep the balance factor correct). If you worked at it, you could build a 750 with heavier pistons than a 920 (and rebalanced to suit), and it would probably shake more, but I don't see why anyone would intentionally do so. Assuming you use the same piston and rod design criteria for 750 and 920 with the same stroke, the 920 piston is going to weigh more, so the 920 is going to shake more, no matter how carefully it is balanced. That doesn't mean that you can't build a 920 with less reciprocating weight than a stock 750 Atlas, and if you did, you would expect it to shake a bit less. But even a stock 750 Atlas is a bit of a bone-shaker.

In my case, I was running a good dose of nitrous oxide in the 920, so piston strength and durability were more important than light weight. If I were building a 920 for the street in a rigid mount frame, I'd certainly try to keep the reciprocating bits as light as possible.

As long as we're discussing personal hypotheses, here's one of mine. I've always liked running heavier crankshafts, because I think the additional rotational inertia reduces the acceleration/deceleration of the crank assembly during a rotation cycle, thereby reducing the forces. The light crankshaft fans have other reasons they prefer them over heavier ones, and I'm not trying to re-start that argument here, just throwing out a thought.

Ken
 
lcrken said:
Fast Eddie said:
Ken, were the bikes that you refer to as bad vibrators dynamically balanced?

My hypothesis (NOT based on actual Norton experience) is that increasing capacity to 920 should be irrelevant, as the main factors involved in determining vibration are revs and weight of moving components.

I therefore believe that a badly balanced 750 with heavy pistons will vibrate way more than a dynamically balanced 920 with lightweight pistons and the correct balance factor for the application (iso / rigid / etc)

However, my only practical experience that supports this is with Triumphs. Can any 920 Norton owners comment?
Click to expand...

The crankshaft in the 920 in my featherbed was balanced dynamically by Bob Milliken, originally the balancing expert at Ditronic Balancing Company, a division of Hank the Crank in North Hollywood, and later operating as Milliken Balancing in Sun Valley, CA. Bob was pretty much the "go to" balancer for Socal racers back in the day. He retired some years back, but I'm still using a couple of the crankshafts he balanced.

I don't know the details of the bike my friend raced in England. Knowing him, I'm quite sure the rotating assembly was balanced, but it's quite possible it was all static, not dynamic. That was pretty common practice back in the early '70s.

I would certainly agree that the principle reason a 920 shakes more than a 650 or 750 in a rigid mount like a featherbed is the increased reciprocating weight of the larger pistons (as well as the additional weight in the counterweights to keep the balance factor correct). If you worked at it, you could build a 750 with heavier pistons than a 920 (and rebalanced to suit), and it would probably shake more, but I don't see why anyone would intentionally do so. Assuming you use the same piston and rod design criteria for 750 and 920 with the same stroke, the 920 piston is going to weigh more, so the 920 is going to shake more, no matter how carefully it is balanced. That doesn't mean that you can't build a 920 with less reciprocating weight than a stock 750 Atlas, and if you did, you would expect it to shake a bit less. But even a stock 750 Atlas is a bit of a bone-shaker.

In my case, I was running a good dose of nitrous oxide in the 920, so piston strength and durability were more important than light weight. If I were building a 920 for the street in a rigid mount frame, I'd certainly try to keep the reciprocating bits as light as possible.

As long as we're discussing personal hypotheses, here's one of mine. I've always liked running heavier crankshafts, because I think the additional rotational inertia reduces the acceleration/deceleration of the crank assembly during a rotation cycle, thereby reducing the forces. The light crankshaft fans have other reasons they prefer them over heavier ones, and I'm not trying to re-start that argument here, just throwing out a thought.

Ken
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Hi Ken, thanks for that, great info as always.

I was (of course) thinking of lightweight pistons etc... JS pistons 920 pistons are lighter than stock 750, which is kinda impressive!

And we know that many early 920 kits used ridiculously heavy car pistons and heavy oversized wrist pins, so something like that, especially if not very well dynamically balanced, would be a bit horrid I think!

By contrast, I would imagine a light piston / long rod 920 could be made really quite smooth.

The closest comparison I have is that I put a Norton crank in an 840cc Triton once, rigid mounted into a lightweight Manx rep frame, and it was smoother than a stock Triumph due to the dynamic balancing.

Having said all of this, I must say that personally, I have become quite a fan of the Commando frame, mine handles far better than I had been led to believe. So I reckon that one of the best frame options for a hot Commando... is the Commando frame!

And BTW, I'm with you on the heavy crank topic!
 
Fast Eddie said:
Having said all of this, I must say that personally, I have become quite a fan of the Commando frame, mine handles far better than I had been led to believe. So I reckon that one of the best frame options for a hot Commando... is the Commando frame!
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Me too. I raced a PR for a couple decades, and they handle just fine, thank you. I also raced my 750 Commando-engine featherbed for almost two decades, and enjoyed it too. For me, I suspect the best featherbed combo for the street would be a short stroke 650 (75 mm stroke, 74 mm bore) with lightweight pistons and a 5-speed (and a e-start). Should be no issue with vibration there! I've got the bits to build one, but too many other projects. Maybe in my next life.

Ken
 
Surely if you don't use a staggered crank, you choose where you want the vibration by setting the static balance factor. If you fill the hole in a standard 850 crank with a threaded steel plug, you should be close to right for a rigidly mounted motor. It will rock while idling and vibrate under 3000 RPM, however should be super smooth up to 6500 RPM. Depends a bit on how you are going to use the bike and the gearing you choose.
 
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