Why only two main bearings?

I think it was due to horizontally split crankcases. It’s difficult to have a middle bearing except for maybe the way AMC twins did it as mentioned above. Horizontally split cases make multi cylinder support much easier. The British motorcycle industry was not big into updating tooling as in spending money, so they stuck with the way things were done with the single cylinder machines.
 
EstuaryBoy said:
A lot of (admittedly low power) Brit four cylinder car engines first appeared with only three main bearings, though most grew to five eventually (Rootes, BMC A and B series, Ford 'Kent')...
Cheap and cheerful???
Click to expand...
Some early motors only had two.
 
The Austin Seven had a four cylinder engine with only two mains. This was widely copied/produced under license by the Germans & Japanese amongst others, & was & still is raced. I wonder if at higher revs the two centre cylinders would be higher compression?
 
Hi,
Years ago I spoke to a chap who raced MGBs.
The early Bs had a 3 bearing crank which quickly was replaced by the 5 bearing crank.
Basically a late B series engine has an indestructible bottom end.
I guess the Ill fated MGA Twin Cam only had a 3 bearing crank and although beset with failures it was not the bottom end that was it’s Achilles Heel and they revved extremely highly.
But here’s the interesting part. He told me it was quite common in racing a later MGB to remove two of the main bearings (blanking off the oil ways of course), converting it back to a 3 bearing crank to obtain extra hp by minimising bearing drag.
Take that for what it’s worth
Alan
 
Maybe Norton would have been better off with more counter weight on the outer webs closer to the bearings and less in the middle trying to bend the crank .
 
Shelby-Right said:
Maybe Norton would have been better off with more counter weight on the outer webs closer to the bearings and less in the middle trying to bend the crank .
Click to expand...
It could be that the mass was centralised between the bearings to prevent a rocking couple which may be present with outside weights, amplified by the firing pulses.
Wish I'd paid more attention in the shaft whirl lectures now...

Pity Steve Maney no longer does his lightened steel flywheels.

There are quite a few examples of a small amount of flex being fine, and I'd venture to say the Norton crank (and most other Brit twins, AMC 650s excepted) is one of them. The gearbox layshaft is definitely in the 'not fine' category.
 
I think only the Commando flywheel is cast iron, the rest is cast steel. The 1962 CSR had the oiling problem to the drive side fixed and I don't think the conrods had the sizing holes where they use to break. Probably the only reason Commando cases break, is the stupid crank balance factor they used to stop vibration at low revs. When you rev the motor to 8,000 RPM with that factor, something has to give. I never heard of the 65SS or Atlas breaking crankcases. With a 72% balance factor, the 850 motor is dead smooth at 7000 RPM. My mate uses 79% with his Altas. It probably revs to 8000 RPM.
As far as the Norton gearbox is concerned. I revved my 850 to about 6000 RPM on the startline, dropped the clutch and nothing broke. But I was worried. It is the reason I bought the TTI gearbox. If the box breaks, during a race start, you can get hit from behind.
During races my 850 is usually revving between 5,000 RPM and 7000 RPM. I sometimes see 7,500 RPM
I sometimes think about the steel plug which is screwed into the hole in the bob weight. . It is threaded in tight and stuck with blue Loctite. If it ever comes out, I am dead
 
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Except for the antivibration nonsense the Commando 850 motor is pretty perfect. It is bloody fast, for what it is. I never believed in it, until I really started to use it.
I am not bragging when I say my Seeley 850 cannot be beaten in any race against other bikes of the same era. - It just happens to be a fact. It is not me - it is the bike.
When I was a kid, I regularly raced my 1958 Triton 500 against Kawasaki 900s. I could always stay with them. My Seeley 850 would have made them look stupid, around Winton Raceway.
Two things make it fast - jetting methanol fuel lean - the other is the steering geometry - I get the power down onto the ground where they cannot
 
I don't think people today give enough credit to the fellows that designed and assembled these engines. I've owned and worked on nortons since 1965. I have had five Mk-3, one of which back in 1975 I had modified , Bored out and sleeved to a true 940cc, over size valves, heavy duty valve springs, the highest lift cam I could get for the street, 34 mm mikuni carbs ,had the push rods shortened ,had to have the front of the block case relieved to allow for the over size cylinders to fit ,had 12:1 compression ,dyno tested at 80 hp and never did anything to the crank. Put many a jap and hog on the slaughter wagon with that bike and never had any malfunctions let alone with the crank!!
 
hershey said:
I don't think people today give enough credit to the fellows that designed and assembled these engines. I've owned and worked on nortons since 1965. I have had five Mk-3, one of which back in 1975 I had modified , Bored out and sleeved to a true 940cc, over size valves, heavy duty valve springs, the highest lift cam I could get for the street, 34 mm mikuni carbs ,had the push rods shortened ,had to have the front of the block case relieved to allow for the over size cylinders to fit ,had 12:1 compression ,dyno tested at 80 hp and never did anything to the crank. Put many a jap and hog on the slaughter wagon with that bike and never had any malfunctions let alone with the crank!!
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I think there really is something to be said for careful assembly. This is just anecdotal from my experience working on these bikes when they were new or near new but Nortons seemed to be the bikes that came with the most careful assembly. Triumphs were a distant second and BSA wasn't even in the running. Sorry, to all you BSA guys, but these bikes, in my opinion were just lashed together and pushed out the door. To be fair, I think that a great deal of the BSA's problems stemmed from an engine design that was quite happy to be 500cc but never really took to the performance upgrades or the displacement increases. I might be wrong. These engines too might be a whole new animal if assembled with care.
 
ILLF8ED said:
I think it was due to horizontally split crankcases. It’s difficult to have a middle bearing except for maybe the way AMC twins did it as mentioned above. Horizontally split cases make multi cylinder support much easier. The British motorcycle industry was not big into updating tooling as in spending money, so they stuck with the way things were done with the single cylinder machines.
Click to expand...
I think the Britisk motorcycle industry found the shortest way to a good answer Most recent bikes which have three main bearings probably have roller bearing big ends. So oiling is not such a big problem.
 
EstuaryBoy said:
A lot of (admittedly low power) Brit four cylinder car engines first appeared with only three main bearings, though most grew to five eventually (Rootes, BMC A and B series, Ford 'Kent')...
Cheap and cheerful???
Click to expand...
Not only British 4-cyl cars, I´m a Fiat guy and they adapted the 5 main bearing strategy sometime around 1968-69, before that it was 3 main bearings. My 6-cyl Fiat (straight 6) from 1959 have only 4 main bearings, that engine was made until 1968. I´m sure lots of other car manufacturers did the same.
 
When I first looked inside my 850 motor, I could not believe that anything so ugly could be any good. My bike sat for 20 years, unraced. But when I got around to using it, I discovered the motor was like the Featherbed frame. I do not know how Norton got it so right. - 'Racing improves the breed' ?
I have had stacks of Triumph twins - they were never so good.
With Superblend main bearings, you do not need 3 main bearings. The crank must flex - if it does not, it will break. Especially if the balance factor is wrong. Think about what you a doing when you rev a motor with a low balance factor - you must know when it is running with distress. With a high balance factor, the motor just spins-up and runs smooth. And the bike is faster. A Commando is obviously not intended for continual high speed running - it is a commuter bike.
When car guys get involved in designing motorcycles, the result is often a stuff-up. What would they know which was not speculation ?
 
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Excessive flexing of any steel given enough cycles causes it to crack from a stress riser and eventually fail .A Radius can make a huge difference in strength, It happened to me on a suspension arm of a car failed , not good .
 
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Big_Jim59 said:
I think there really is something to be said for careful assembly. This is just anecdotal from my experience working on these bikes when they were new or near new but Nortons seemed to be the bikes that came with the most careful assembly. Triumphs were a distant second and BSA wasn't even in the running. Sorry, to all you BSA guys, but these bikes, in my opinion were just lashed together and pushed out the door. To be fair, I think that a great deal of the BSA's problems stemmed from an engine design that was quite happy to be 500cc but never really took to the performance upgrades or the displacement increases. I might be wrong. These engines too might be a whole new animal if assembled with care.
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Hi Jim,
I must confess I’m a little dubious about what you say as it does sound a little biased.
I guess the true test of the bike as a package of design, material and assembly would be their reliability and service life.
I really cant answer which of the ‘big three‘ pommie bikes provided the best combination of performance and reliability although I suspect the problematic bearing design of the BSA twins might discount them.
Being a Norton forum it would be reasonable for there to be a resounding shout that Norton provided the best reliability and horsepower. I guess the factory’s warranty claim records would tell us a lot about the various bikes reliability and quality control. Even that would be biased by the average demographic of the person who bought the bikes. Is there any evidence to suggest that Triumph owners were wilder or less dedicated to maintaining their bikes than Norton owner or visa versa. again I don’t know.
But all the talk regarding care of assembly is missing one really important point. That is the fact that a really well designed and manufactured engine shouldn’t take extreme skill or care to assemble. It should fit together like a Lego set. I imagine that was companies like Honda’s great advantage. With very accurately machined components they didn’t need highly trained staff to assemble the engines, more like a production line.
just a thought
Alan
 
Alan L said:
Hi Jim,
I must confess I’m a little dubious about what you say as it does sound a little biased.
I guess the true test of the bike as a package of design, material and assembly would be their reliability and service life.
I really cant answer which of the ‘big three‘ pommie bikes provided the best combination of performance and reliability although I suspect the problematic bearing design of the BSA twins might discount them.
Being a Norton forum it would be reasonable for there to be a resounding shout that Norton provided the best reliability and horsepower. I guess the factory’s warranty claim records would tell us a lot about the various bikes reliability and quality control. Even that would be biased by the average demographic of the person who bought the bikes. Is there any evidence to suggest that Triumph owners were wilder or less dedicated to maintaining their bikes than Norton owner or visa versa. again I don’t know.
But all the talk regarding care of assembly is missing one really important point. That is the fact that a really well designed and manufactured engine shouldn’t take extreme skill or care to assemble. It should fit together like a Lego set. I imagine that was companies like Honda’s great advantage. With very accurately machined components they didn’t need highly trained staff to assemble the engines, more like a production line.
just a thought
Alan
Click to expand...
Your points are well taken. My intent was not to cast dispersion on any one brand. There is plenty of that to go around. These were just my observations from a time, long ago. By carful assembly I am including issues such as careful machine work. By the time our favorite machines were being built the machine tooling was not the best. Many components were turned out with "close enough" as the final spec. Add to this the fact that these bikes were being constructed with parts that were themselves not subject to riggorious quality control standards and you have the issue that built the British bike reputation. Least you think me biased I will include all of Harley-Davidson offerings at this time plus a large part of the offerings by Ford, General Motors and Crystler. It was a mess. It took the Japanese to show us the way to zero defect manufacturing that we take for granted today.

As far as the BSAs were concerned, as a group, they probably got the worst of it because they were billed as the hot rod, sport bike of the era. As such were hammered unmercifully. I saw this in Nortons as well but they tended to pay the owners back for such treatment with a hole in the transmission case. I imagine that a BSA can be made a dependable ride with the same type of care we lavish on our Nortons I have just never had the chance.
 
In support of Jim's observation/belief, here is something to consider.
Production racing was very popular in the UK during the 50s/60s.
It is a great way to separate the wheat from the chaff.
The Thruxton 500 , what a test , 500 miles of racing.
Norton punched well above its weight there.
Most probably know that the 650ss won three years in a row.
Even more amazing was the 1962 result where the 2nd place finisher was a little 500 cc 88ss which finished ahead of all the other 650 entries from Triumph, BSA , Matchless and AJS. I believe there was a 700 or 750 RE in that race as well. It had no connection to Syd Lawton either, so it wasn't due to Lawton's " magic touch" as was suggested recently by one poster.

Quite a feat to do that with a 500.

Glen
 

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