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[Dennis C]

I guess you are correct Les, when I rebuilt my 59 Super Meteor around 6 years ago now it still had the original bearings and I saw no reason to change them.

[DUFFEL]

Hi Les ,how strange you came out with this as I studied my engine when it was apart then compared it with with the more common bikes and wondered myself , two great minds must think alike. I did mail our Host the same question below is his reply
Reg Thomas designed the twin engine and although I do not have any documentation as to why it was deigned this way, I am sure there was good reason. In the absence of the main bearings giving any problems and being long lasting, then I would never question this decision as Mr Thomas was far more qualified than I could ever dream to be to design the engine like this .
Cant be any fairer answer that that

[Les H]

That must be it DUFFEL...the big ball race is rated sufficiently for the drive side but a two part roller bearing is required to make engine dismantling and assembly easy...so in line with the engines heavy-weight build, just stick in a roller with the same dimensions rather than use a smaller one with sufficient load capacity......But HEY....once you've made that buying choice of bearings....why not swap them over and utilise their strengths in the right place.....we shall never know I guess...I'm still puzzled.....Thanks for all your replies....



Les

[Martin]

The original interview with designers R.A Wilson-Jones and E.A. Pardoe
way back in 1949 or so by 'Motor Cycle' simply skips over why the main
bearings are arse-about. I quote, "Both bearings are the same diameter
since there is virtually the same load on each. The single-row bearing
on the driving side locates the end of the crankshaft,and the roller on
the timing side allows sliding to occurf on the rollers should there be
any differential expansion between the case and tohe shaft" Who knows,
maybe they got a good deal on rollers back then, I'm only quoting them..

[Presto]

This is a bit late in the day, but here goes…



The same thought has often occurred to me Les about this unusual arrangement. Strangely it is hard to find any comments on this set-up in various articles of technical analysis of the twins in the magazines of the time, other than the fact that they simply report that that is how it is. The designer was not I think Reg Thomas – he did the 250 unit engine – but Mr R. A. Wilson-Jones. In an interview by Bob Currie on the twin engine Mr W-J commented to this effect:


‘Location of the shaft is by the drive-side ball bearing, inserted into the housing externally; the timing-side roller bearing has no lip on the outer race, so that the rollers can slide with differential expansion between shaft and crankcase.’ (The Motor Cycle, August, 1960)



This is all a bit beyond me but, presumably this feature could not be implemented were the bearings reversed. But in all fairness that is hardly an full explanation.

[Les H]

Many thanks Gordon and Presto...the penny has dropped....Ureka!



The domed drive side of the crankcases is much stronger than the flat and fairly flimsy timing side...so if you want to anchor the crankshaft to a bearing then that bearing has to have lots of support... Now only a ball race bearing can retain the crank so it follows that the drive side is a ball bearing. As said a roller bearing provides the easy dismantling and assembly so this then goes on the timing side. As the designer wanted an immensely strong bottom end then he just spent a little bit more and specified the same size for it, making the bottom end one of the toughest British designs going other manufacturers would probably have used a smaller one to save money...Cheers, feel happier now...Les

[Presto]

Excellent! That makes sense to me now. Another merit star for Royal Enfield! (And one for Les too!)

[Dennis C]

It looks like between us (mostly Les) we have managed to figure out what one possibly two designers did on their own all those years ago, and not a single harsh word in sight, thanks to all involved, what a refreshing change.

[Les H]

Thanks Dennis and Presto and just to add some more meat to the bone (is that the right phrase?) To explain the inherent weakness of the timing side case you can list the jobs it has to do: its drilled to support the cam bushes and cam tensioner and is drilled with oil ways and breather holes and to create the required space it is also tall and flat like a wall so not inherently strong and certainly not that stiff in the sideways plane compared to the drive side case which is pretty solid...However I still wasn't happy about the choice of the 2:1 load capacity oversize of the timing side roller bearing, cost is always an issue with any design so there must be more to it. So with a bit more thought I think I have the answer to that too now...
,br>

Within any given flat area and regardless of load capacity a physically bigger bearing can be supported more rigidly, so by choosing an equally large diameter bearing you have increased the stiffness of its fixing within the more flexible timing side crankcase. By choosing a smaller roller bearing that matches the drive side load capacity you have saved money but lost some support strength on the bearing and you would need an increase in wall thickness of the timing side cover to regain the strength...Summary: The large roller bearing is overspecified in terms of its load capacity but enables the timing side case to hold it more rigidly than a smaller sized bearing.



Well right or wrong, I'm happy with that and no longer have any temptation to swap the bearings over when I do the bottom end rebuild.



Cheers chaps.

Les

[vince]

Hi, I read somewhere that the bearings were arranged that way because the driveside clamps up against the bearing by vertue of the large sprocket nut, the timing side sprocket nut would be inadequate for this purpose. This is also the arrangement on redditch bullets! When I worked on the Met police ISDT triumphs in the 70s one tuning trick was to fit ball bearings on both sides. Also Harley racing specs replaced all roller bearings with ball. Anyway whatever the reason I think the enfield bottom half has stood the test of time. Vince