Gearing questions for G15/N15

Elfix

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Im puttering with my ‘68 G15 a preparing it for some road work this summer- hopefully a trip over the Top Of The World Highway to Dawson and back. I’ve just pulled the primary to make sure all is happy in there and to get to the transmission sprocket.

There is some disagreement on the “correct” gearing for these, and depending on where you get your info the engine sprocket was a 21, 22, or 23, and the transmission sprocket was a 17 or a 19. Mine is presently set up with a 21 on the engine and a 17 at the transmission.

Most of the riding I expect to do this summer will be road based with a fair bit of gravel so I plan to maintain the 21 at the engine and switch to either a 19 or 20 at the transmission. I’m hoping to get an easy lope at 65-70 and still be able to navigate some rougher stuff in 1st and 2nd. The present gearing was a bit too low for long highway pulls, and the vibration comfort zone pretty much stopped a little over 60.

What have you other owners tried? What worked for how you were using the bike? Unfortunately experimenting with sprocket changes is a tiny bit more involved than with my other machines.

Thanks!
 
What have you other owners tried? What worked for how you were using the bike? Unfortunately experimenting with sprocket changes is a tiny bit more involved than with my other machines.

Thanks!
Somebody will say this is irrelevant, and it could be because my experience is with a P11:

On my P11 I have tried several combinations of gearing while maintaining the stock rear 42 tooth hub. If the rear sprocket is a 42, a 21T engine and 20T gearbox is the only way there will be anything close to an easy lope at 70MPH. It is a good combination, but not perfect on the open HWY. Still good enough if that is your plan.

A 23T engine and 19T gearbox is a very usable combo, but still wound up some at 70MPH cruising. I've geared my P11 up to 23T engine and 21T gearbox. It would be OK off road, but I'd probably never get out of 1st gear. Around town I seldom shift into 2nd. 65-70 MPH in 4th on the HWY is very tolerable. Climbing mountain passes I have not done since gearing it up, but I have a feeling I'd seldom use 4th gear.

If geared too tall, and I am very close for my setup, you'll find out how weak a Norton actually is short shifting to keep the revs down. For example, I can ride the bike at 50mph on flat ground in 4th, but it is gutless for a few seconds longer than I like going WOT. However, it's probably over carbureted, and over cammed for the street anyway. 930 Amal's would probably work much better with a stock cam shaft.

Ludwig knows what works for exactly what you are planning to do. Maybe he will say something and clear things up.

I agree about changing gearing. It is a tedious exercise on pre and post classic Commandos.
 
I’ve made the same trip to Dawson (twice!) on my 1972 Yamaha RT2, a 360cc two stroke single. On the first trip I had geared it up with an added tooth to the countershaft sprocket (either a 15-16 or 16-17, I’d have to look in the parts bin) and while the taller gearing was brilliant on the flats it suuuucked on the hills. The RT just couldn’t pull 5th with the tall gear and the gap between 4th and 5th was so large that your speed dropped considerably- so 65 on the flats and 45 on long hills. It didn’t help that the RT would heat soak itself under those conditions and loose power as well.

Two strokes.

Back to bone stock it would maintain pace on the hills without quibble, at least until it got too hot. The heat issue was later solved by Harry Klemm with a reshaped combustion chamber. If you own a 1970’s two stroke and intend to use it he’s a guy you want to talk to.


Anyway, the takeaway lesson is that the factory (in this case at the very least) tends to be pretty good at getting the gearing right for a machine’s power characteristics. I’ve mucked about with gearing a lot at the track and have found that a top down approach works best- first you match your top speed to the track then fine tune to individual track features as required. The bike I ride now is set to just reach peak revs at the end of our longest straight, but a previous machine had to be set to overrev there in order to get a good drive out of a few important corners.

As far as the G15 goes my few hundred miles of use so far has all been with the very short 21/17 gearing, so a change to 21/19 (standard for an Atlas) will be a large jump up. I found the bike to be happy enough at an indicated 60/65 with the lower gearing, so a swap to the 19 on the gearbox will probably get me in the ball park without overdoing it. I’ll likely begin with that.
 
OK then, sounds like you already have it figured out.

I just got back from a HWY test of my latest gearing change. Sweet. I could eat up a lot of miles with the 23T engine 21T gearbox gearing. I don't have a tach, but I was loping at 70MPH. My mirrors were steady and vibration low at the grips. Much better ride for my use. It'll still work in the mountains as well until the elevation makes the motor get flabby. Then I won't be using 4th without a lot of open road in front of me.
 
My ‘68 N15CS has 17T gearbox sprocket and will only do 70 miles on a full tank before it’s bone dry, you’ll get better mileage with a 19T sprocket.
 
My ‘68 N15CS has 17T gearbox sprocket and will only do 70 miles on a full tank before it’s bone dry, you’ll get better mileage with a 19T sprocket.
Hard to believe your bike is this thirsty. Assuming you have the 2.25 imp. gal. petrol tank, the consumption equates to 9.08 litres /100 km.
You'd have to drive a car with a 2.5L petrol engine to reach a comparable consumption (when engine is warm). Are you sure your bike combusts this amount of petrol, no leaks?

- Knut
 
Mileage becomes very dependent on one’s right wrist…. I can easily hit reserve on my RT2 inside of 70 miles if I’m having fun. Ridden with care that extends to over a hundred.
 
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