Why only two main bearings?

lcrken said:
Jim Comstock's center main bearing conversion, back in his road racing days.

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View attachment 109056

View attachment 109057

I've also seen one done by a Canadian racer, but seem to have lost the pictures. That one was also quite a while back in time. I think it might have been written up in a Classic Racer article.

Ken
Click to expand...
Wow!
That crank seems so logical in comparison to the two bearing bolted up original. It looks indestructible, if it is as strong as it looks it seems such an advance.
Norton might have felt their crank was adequate for its intended task but history seems to indicate it was marginal as power was increased.
Such a shame they didn’t ‘bite the bullet’ and retool for something similar way back in the sixties. I’m sure the investment would have paid off if Norton was known for a rock solid bottom end that could take any amount of use and abuse.
Alan
 
With a 3 main bearing crank, horizontally split crankcases are probably desirable. then width might become an issue. When the gearbox is integral, that removes design versatility. Going down that path would have been a big gamble for Norton. The Yamaha XS650 has a 3 bearing crank. Nortons are faster.
 
I suggest the Commando 850 is 'fit for purpose' as long as it is revved above 5.500 RPM. The balance factor is absurd, but it had to be what it is. Most people probably do not need a rorty sports' bike.
 
acotrel said:
I suggest the Commando 850 is 'fit for purpose' as long as it is revved above 5.500 RPM. The balance factor is absurd, but it had to be what it is. Most people probably do not need a rorty sports' bike.
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Hi Acotrel,
I don’t understand your post, could you please explain what you mean?
 
H
acotrel said:
With a 3 main bearing crank, horizontally split crankcases are probably desirable. then width might become an issue. When the gearbox is integral, that removes design versatility. Going down that path would have been a big gamble for Norton. The Yamaha XS650 has a 3 bearing crank. Nortons are faster.
Click to expand...
hi (again),
I note that Tridents did a reasonable job with their centre mains on a vertical split crankcase. Anyway the photo of the modified engine with the centre main looks very prac and doable. Obviously I’m not in a position to say if it makes for a better bottom end but I suspect it would.
I tend to doubt that the addition of a main bearing would account for the perfect difference between the Yamaha and the Norton.
 
Alan L said:
H

hi (again),
I note that Tridents did a reasonable job with their centre mains on a vertical split crankcase. Anyway the photo of the modified engine with the centre main looks very prac and doable. Obviously I’m not in a position to say if it makes for a better bottom end but I suspect it would.
I tend to doubt that the addition of a main bearing would account for the perfect difference between the Yamaha and the Norton.
Click to expand...

As I recall, it worked quite well for Jim. When I met him, we were both racing Commandos with AHRMA. I think most of his racing with the trick center main bearing engine was against modern sport bikes in a Colorado series. He was quite competitive against them. I think he cut the crank down from a Volkswagon diesel crankshaft. My memory is starting to get a little fuzzy about the details after so many years, but more details about his bikes have been posted here on the forum in past years. A little searching might turn up more info.

This is the Commando he started racing with when we were both doing the AHRMA races.

Why only two main bearings?


And this is, I think, the last version of his Norton racer, when he was racing in the modern series. There were several other variations in between.

Why only two main bearings?


And a shot of Jim on the bike.

Why only two main bearings?


All of the above pictures are from Jim, and I'm pretty sure they have been posted here Previously.

Ken
 
Alan L said:
Hi Acotrel,
I don’t understand your post, could you please explain what you mean?
Click to expand...
A 650cc Triumph with a one piece crank, can be revved to 8,000 RPM all day without collapsing. And there is little reason that a 750cc Atlas could not be similarly revved. The hole on the Commando flywheel to lower the balance factor is sinply a bodge job to try and get the duffle-coat brigade to buy Nortons. It actually works - but not if you want to fang the bike. My 850 has a piece if steel threaded into the hole - the balance factor is aboutb 72%. When my motor reaches 7000 RPM, I change up, because I am superstitious. It can probably rev safely to 8,000 RPM. Most road-going Commandos are considered to be revving high when they reach 5.500 RPM . - My motor never goes below that during a race, except at the very beginning.
Anybody who has ever raced an early British twin or single knows what the balance factor has to be. Modern bikes changed people's thinking - the two are not the same - old is different. Just a simple thing such as steering geometry is radically different. A 60s Manx Norton never leans as much as a 60s two stroke, because the two stroke is forced to handle neutral. You cannot squirt an old two stroke when you are in the middle of a corner, but with a Manx, you can. Modrern two-strokes have more torque and are less peaky.
Would you buy a motorcycle which moved backwards and forwards while it is idling - what is important ? The duffle-coat brigade never were motorcyclists.
I like Commandos, but a lot of things about them are rubbish. Triumph did a similasr thing with the 650cc Saint - detuned and made to run smooth - they are a waste of space. What do you think that bathtub was on the Triumph Thunderbird ? - catering for whom ?
You cannot have it both ways - fast and smooth - four cylinders are different. And who wants to ride a two-stroke ?
A Commando is probably a nice ride, if you go slow.
 
If the British could not build a decent twin cylinder sports bike, they should have built four cylinder Gileras.
 
acotrel said:
A 650cc Triumph with a one piece crank, can be revved to 8,000 RPM all day without collapsing. And there is little reason that a 750cc Atlas could not be similarly revved. The hole on the Commando flywheel to lower the balance factor is sinply a bodge job to try and get the duffle-coat brigade to buy Nortons. It actually works - but not if you want to fang the bike. My 850 has a piece if steel threaded into the hole - the balance factor is aboutb 72%. When my motor reaches 7000 RPM, I change up, because I am superstitious. It can probably rev safely to 8,000 RPM. Most road-going Commandos are considered to be revving high when they reach 5.500 RPM . - My motor never goes below that during a race, except at the very beginning.
Anybody who has ever raced an early British twin or single knows what the balance factor has to be. Modern bikes changed people's thinking - the two are not the same - old is different. Just a simple thing such as steering geometry is radically different. A 60s Manx Norton never leans as much as a 60s two stroke, because the two stroke is forced to handle neutral. You cannot squirt an old two stroke when you are in the middle of a corner, but with a Manx, you can. Modrern two-strokes have more torque and are less peaky.
Would you buy a motorcycle which moved backwards and forwards while it is idling - what is important ? The duffle-coat brigade never were motorcyclists.
I like Commandos, but a lot of things about them are rubbish. Triumph did a similasr thing with the 650cc Saint - detuned and made to run smooth - they are a waste of space. What do you think that bathtub was on the Triumph Thunderbird ? - catering for whom ?
You cannot have it both ways - fast and smooth - four cylinders are different. And who wants to ride a two-stroke ?
A Commando is probably a nice ride, if you go slow.
Click to expand...
Thank you, interesting.
Yes, our T140V definitely vibrates back and forth when idling and actually lurches backwards when the throttle is blipped if it is on the centre stand on a smooth concrete floor. So, you are saying that this is because it has a higher balance factor which allows it to operate at higher revs? It goes very well but without isolastics it feels very busy over about 70mph but revs to 7000 comfortably in a pinch.
So, to summarise what you have said, the stock Commando has a low balance factor to make it comfortable to lope around at lower revs? Not quite sure what you mean by the ‘duffle coat brigade’?
As for me, I have a very stock Mk2 74. I find its power output a little disappointing ( I’m also very heavy) but it lopes along well and it’s never disgraced around modern stuff if I put my mind to it. I do tend not to rev it much past 5500 as it doesn’t ‘feel’ comfortable above those revs.
As it is running very well I would be loath to pull it apart but I would love to somehow liberate a few more ponies and have it a bit happier towards 6000rpm. Are you suggesting that raising the balance factor would be a good start?
As for the finer points of handling geometry, I really must increase my knowledge. It’s hard to get my head around the fact that ultimately, the angle a bike can be leant over to, isn’t just a function coefficient of friction of the tyres on the road, providing the frame is ‘rigid’ (doesn’t flex) and the shock absorbers keep the tyres in contact with road. Of course I do comprehend steering geometry, wheelbase etc affecting how a bike turns in etc but I don’t understand how lean angle can vary between different bikes going around the same corner at the same speed, providing the rider’s COG is the same? After a lifetime of riding I have so much to learn. Why is a 2 stroke forced to handle neutral?
No matter which way I look at it, a Norton crank (in fact all big british twins) looks archaic to me and a bit of a bodge up. The photos of the crank that began this thread looks (at least to my eyes) a huge improvement with the extra support of the centre bearing. We do know a Norton’s crank is flexing considerably with the problems encountered by the Combat bikes and the need for special bearings. As I said earlier, a trident crank looks a lot more businesslike.
Any tips to help me increase my (limited) knowledge is appreciated.
Alan
 
The older type of two-stroke had insufficient torque and a very distinct power band It was safer to keep it at constant lean in corners while riding carefully, but fast - then wait until the bike was upright before giving it a blast down the straight. If the bike oversteers when you gas it, as long as the power dekivery is smooth, you can gas it hard all the way from beginning to end of any corner. A modern two-stroke has more torque and smoother power delivery. - Not so easy to beat.
The same sort of thing applies to racing cars - the Vanwall used to understeer, so was not very successful. They could not keep the power on in corners.
The problem with a bike which oversteers - you always need to be careful when you are riding faster and under other riders. There is a point as you leave the corner where they can turn into you. I never ride into a corner without braking before it. If the bike does not squat, it does not turn. The bike rotates around it's centre of gravity, so the position of the motor matters - if it is not far enough forward., it can feel wrong. When we are in corners, we are not all like flies stuck to a wall.
If I am at a race meeting, I usually know what is going to be quicker and where. Some bikes look 40 MPH faster down the straights than my Seeley, but lap slower. I never sweat it, when they make me go backwards, I just wait a bit. A very experienced rider will usually win.
Hsd to laugh at one of my mates - he looked around at all the old idiots at the start line, and thought he was in with a chance - found out he did not know how to ride a motorcycle.
 
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Who is that old snoozer who rides at Goodwood Revival in the Barry Sheene, and cannot even run across the track, but ends-up up front, against the MotoGP riders ?
 
acotrel said:
Who is that old snoozer who rides at Goodwood Revival in the Barry Sheene, and cannot even run across the track, but ends-up up front, against the MotoGP riders ?
Click to expand...
...and the link to a two bearing crankshaft is?
 
If you rebalance the crank in a Commando engine, revving it is not a problem. I just choose to avoid revving my 850 over 7000 RPM. In a race, it never drops below 5,500 RPM.
I agree with the comment that the crank looks horrendous. That was the reason my bike sat unraced for 20 years. I now believe that crank is of very sound design. It works well.
Witn Commandos, the design concept was a problem - what it is and what it could be are two different things. Atlas crank might be better.
 
Alan L said:
Thank you, interesting.
Yes, our T140V definitely vibrates back and forth when idling and actually lurches backwards when the throttle is blipped if it is on the centre stand on a smooth concrete floor. So, you are saying that this is because it has a higher balance factor which allows it to operate at higher revs? It goes very well but without isolastics it feels very busy over about 70mph but revs to 7000 comfortably in a pinch.
So, to summarise what you have said, the stock Commando has a low balance factor to make it comfortable to lope around at lower revs? Not quite sure what you mean by the ‘duffle coat brigade’?
As for me, I have a very stock Mk2 74. I find its power output a little disappointing ( I’m also very heavy) but it lopes along well and it’s never disgraced around modern stuff if I put my mind to it. I do tend not to rev it much past 5500 as it doesn’t ‘feel’ comfortable above those revs.
As it is running very well I would be loath to pull it apart but I would love to somehow liberate a few more ponies and have it a bit happier towards 6000rpm. Are you suggesting that raising the balance factor would be a good start?
As for the finer points of handling geometry, I really must increase my knowledge. It’s hard to get my head around the fact that ultimately, the angle a bike can be leant over to, isn’t just a function coefficient of friction of the tyres on the road, providing the frame is ‘rigid’ (doesn’t flex) and the shock absorbers keep the tyres in contact with road. Of course I do comprehend steering geometry, wheelbase etc affecting how a bike turns in etc but I don’t understand how lean angle can vary between different bikes going around the same corner at the same speed, providing the rider’s COG is the same? After a lifetime of riding I have so much to learn. Why is a 2 stroke forced to handle neutral?
No matter which way I look at it, a Norton crank (in fact all big british twins) looks archaic to me and a bit of a bodge up. The photos of the crank that began this thread looks (at least to my eyes) a huge improvement with the extra support of the centre bearing. We do know a Norton’s crank is flexing considerably with the problems encountered by the Combat bikes and the need for special bearings. As I said earlier, a trident crank looks a lot more businesslike.
Any tips to help me increase my (limited) knowledge is appreciated.
Alan
Click to expand...
 
You are correct about coefficient of friction of the tyres is important, but if the bike leans less, it becomes less important. It is almost imposiible to make a 1962 Manx Norton lean as much as a TD1C Yamaha. A Manx oversteers when you gas it while on a lean. You do not usually use a TD1C Yamaha that way. With a two stroke, good tyres are more important. The TD1C Yamahas spelled the end for the Manx. They were lighter and faster down the straights. A good guy on a Manx should beat a TD1C Yamaha in corners.
 
If I race, my main opposition ride 1100cc methanol-fuelled CB750 Hondas which are built and sold by Rex Wolfenden. I think on his website he claims to be getting 180 BHP out of their motors. And I do not doubt that. When you race a bike wirh that much power, you become forced to ride in a certain way - extremely fast down the straights but up on the ripple strips at maximum lean in the corners usually in a procession. My bike handles differently - it is slow down the straights and faster in the corners - so it IS competitive. Race bikes and race circuits conform to patterns, which determine cause and effect. Those patterns are made up of variables - just changing one variable does not do much. To corner better, you need softer suspension, more trail on the steeering and lower tyre pressures. For faster down the straights, you need the opposite. A bike which oversteers and is extremely fast down straights is probably dangerous.
I know of a bike like that - it is a 250cc Parilla. It has crashed everyone who ever rode it, and the crashes came from nowhere.
A Manx Norton is a slow motorcycle. It was designed to race on the IOM. On a normal race circuit. it is slow down the straights, but it takes a lot of power to beat it with a bike which does not handle as well as a Manx. Most guys in Australian historic racing ride converted road bikes.
A Commando 850 motor is fast enough. the rest is gearing and handling. I never worry about horsepower.
 
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