Gearboxes, Chicken & the egg?

[johnnyboy-wonder57]

Hi,
Now that the subject has come up & now that with magnification I can see  the position of the gearbox in the OHC Norton Model 40, being upright with a long gear-change pedal & the Model 7 Dominator twin being laid- down with a shorter shift, something I hadn't bothered to take much notice of before, I realise I am going to have to expand my knowledge of the World of motorcycle gearboxes!
And again becoming increasingly aware that gearboxes were & are designed with various finite strength limits , selecting overall ratios to maximise optimum engine performance, may be trickier than I first thought!
 Are there any sure-fire modifications needed beforehand, to ensure avoiding impending destruction thereof of said  gearbox unit,  when an automobile engine such as the Imp 875cc unit is matched to a  standard Norton gearbox with respect to rpm/speed, economy & durability 

I have already kindly been informed  of upgrades like Andover -Norton strengthened cases, require a right-foot shift box, being a traditionalists & as yet are unsure about 4  of 5 speed options.

I realise that various Norvins  & the fragile Combat  engines put reasonably high outputs through the Norton/AMC  gearbox, but were the gearboxes further modified to cope with this first, rather rather than being standard Commando fitments coping with a claimed output of 58bhp?

Anybody out there done some fiddling with gearboxes, upgrades  & ratio calculations?


Cheers


JBW

[R]

Unless you are planning on racing this thing, and using full throttle all the time, a standard imp engine is not going to trouble the gearbox internals all that much ??

Spinning the gearbox at closer to engine revs, and doing all the gearing down to the back wheel will also reduce the torque loadings in the gearbox. Later Commando gears are stronger than early AMC gearsets, and the torque spread of a 4 cylinder should make a 5 speed somewhat redundant... ?

[johnnyboy-wonder57]

R,
No its, (the bike that is), for the road only, a moderately enhanced Imp engine, 875/930cc with some top-end modification and bottom-end work plus other  mods, higher lift cam 0.300", increased valve overlap,  28.66.66.28, a modern ignition set-up, Weber carburation, oil-cooler, higher capacity oil-pump, carefully designed exhaust,  should get into the mid 60s bhp heading towards  70 bhp, a little bit more that a standard Commando output!

I'm just trying to avoid any mistakes or short-cuts I will regret later on, build it once, build it right!


Cheers

JBW

[esometisse]

You won't need 5 speeds with the torque characteristics of an Imp engine - and the gears in a 4-speeder are stronger!
The most likely trouble a Norton gearbox will develop when coupled to a high torque engine is a bent mainshaft or cracked case as experienced by grasstrack and motocross outfits using for example the 950cc Weslake twin (about 80hp).
This can be avoided by using a sturdy outrigger bearing between clutch and sprocket.

Cheers
Andy

[johnnyboy-wonder57]

Andy,

Any pictures of the modification?


Cheers



John

[esometisse]

I once built a 920cc-Commando-engined Wasp outfit for a friend who rides it on the road.
I used an outrigger plate as offered by Steve Maney but took no pictures of the modification, unfortunately.
The plate goes where the inner primary normally sits so has to be used with a dry belt drive system.
There is a pic of Steve Maney's solution on his website http://www.stevemaney.com/products.html but it's no big deal to manufacture something similar yourself.

Cheers
Andy

[johnnyboy-wonder57]

Andy,
Thanks, I will check the site out.



Cheers

JBW

[johnnyboy-wonder57]

Andy,
I have found the technical reference to the outrigger bearing being used , it seems to have been developed by Peter Williams, on his Norton Commando based monocoque- chasis machine in the Formula 750TT in the early 1970s.  To correct main-shaft flexure, an outrigger bearing was placed  as a third bearing in the Inner half of the primary chain-case, directly in line with the pull of the chain, stopping the mainshafts previous habit of stripping teeth.
It also, according to the write-up made the transmission more efficient only losing 8bhp from 76 @ the crankshaft to 68bhp @the rear wheel!

Source Classic Motor Cycles Vic Willoughby  ISBN 0 600 34961 6 (1982/3)


Cheers


JBW

[esometisse]

I'm not surprised that Peter Williams was involved in this. He was/is an excellent engineer as well as a very talented rider.

Cheers
Andy