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Inter gear ratios

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I need to exercise my new machine before making changes but already I have ordered a replacement pair if pinions to lower 2nd gear. Have others done this? It's a question of keeping originality or making it more convenient.

Original ratios with kick start are

1.0  1.1  1.33  2.33

And with new gears

1.0  1.1  1.61  2.3

...has anyone else bothered?  And, if so, was it worth it?

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I think it will make a better ratio set, with gaps of 1.4:1 between 1st - 2nd, 2nd - 3rd.  It would be even better if you get a trials 3rd pair, if they can be had.  Try ex-WD suppliers. 

But I haven't actually put a box together with these ratios to try them

Paul

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Thanks Paul

One of the gears arrived yesterday.  The dogs are a bit bruised on a leading corner, so I hope they don't jump out if I do fit it.  Before I do anything, I'll compare it with a spare AMC set I have - in case the AMC individual gears fit after all.

The picture shows my calculation for force on the ground from each gear, with a rev limit of 5500.  The torque curve is imaginary, but the ratios between the heights of the lines will be the same whatever the curve shape.

'Standard' is standard Inter kick start box.

The difference is clear and should make it more tractable at the low speeds it was not intended for.

 

 

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When I work with gearboxes, I start with making diagrams like this. Where blue is original and red is your suggested set.

Inter gearing

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Thanks Mikael - that looks an improvement, although there's now (obviously) a very jump from 2 to 3, which your plot emphasises.  Anyway I think I mightgive it a go!  I've not been inside a doll's head box - I'll practice on my 16H, where the effort won't be wasted because I think a thrust washer is worn which (someone here said) can cause uncertainly going up the box from second to third gear.  That'll tell me how long the job will take, and I shall have gained something on the way.

When the bike spent some time with a restorer a decade or so ago, it went with an Inter box (this one) and came back with a standard box (naughty!).  I suspect the standard one would have been better all along of course!

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One thing that graph shows is that a Norton needed to have a five speed box to make those ratios work properly - all the standard ratios and the second, higher second as well

Paul

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Norton spaced the gears as they did because air resistance is proportional to the square of speed. At high speed most of the engine power is used to overcome drag, thus high revs is needed. At low speed the small drag means that lower revs gives enough power for acceleration. Reason why higher gears are more closely spaced.

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Good point, Mikael. The engine has plenty of torque for practical purposes even on the high second gear at moderate revs. I'll get used to it before doing something drastic. Thanks to you all for the observations.

Tragically, the bike's previous owner, Bernard Harding, who rebuilt it and from whom I obtained it and who was my never-ending source of technical advice for over 40 years died just two weeks ago, and the conversations I enjoyed and relied upon so much have now finally ceased.

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It's great that you had such a long-term relationship with the previous owner of your bike.  Forty years is amazing.

The ratios I suggested give a spread of 20%, 33% and 43%

There's a cluster offered by RGM for the AMC case, which uses Manx sleeve pair, AMC 2 third, AMC 3 second, Daytona first.  That gives ratios 1:1, 1.15, 1.52, 2.05, giving spreads of 15%, 32%, 32%

I would be interested to know what the actual tooth numbers are in a Daytona first gear pair.  Someone published that in Roadholder recently, but I forgot to note it down and don't remember which issue.  Does anyone other than RGM offer them?

Paul

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Thank you, Paul.

I have just been to the shed and pulled out my old box of spare Norton AMC gears. I now find that the gear I bought off Ebay, with its 1930's part number, is identical to a 1960's one I already own! Lesson still not learned: never buy replacement parts before taking the machine apart first!  Is the doll's head cluster identical to the AMC box? We read the later box was developed from the earlier, but that doesn't help the amateur like me.

Now for the spreadsheet calculations of all options. Meanwhile this will remain theoretical for a while.

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Making it a bit more complicated. My previous comments refers to the ideal world (racetrack) where you want best acceleration and top speed. For road use you need a low first for congested traffic. Then you need a fairly tall high gear keeping revs low when effortless cruising around the legal speed limit. You also need a gear where you have more revs and power for overtaking annoying slower lorries and cars. So the ideal set of gears depends on how you use the bike.

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David - you ask if the AMC cluster the same as a Norton one, eg for a Doll's head box.

The AMC cluster is derived from the Norton one, but you can not assume that a gear will interchange from one box to the other.  Besides having different chain lines, the mainshafts are obviously not interchangeable, because the Norton boxes use a thrust washer between the 3rd gear pinion and the sleeve gear to control the end thrust due to lifting the clutch.  The AMC box has a nut on the cover end to lock the shaft to the outer ball bearing. 

That also causes differences in the sleeve gear - you have to have a ground face for the thrust washer to press against.  Also the Norton box has roller bearings at this end of the sleeve gear.  This was one of the 'improvements' over the Sturmey Archer box, which functioned well without them.  The AMC lost this, and relied on two plain bushes without ill effect. 

Also, from memory, the dogs between sleeve gear and 3rd gear pinion are 3-dog on AMC, and 4-dog on Norton.  You have to look carefully at each pinion to work out if they can interchange

Another difference is the first gear mainshaft pinion.  Because of the different method of controlling end-float on the AMC, its first gear mainshaft pinion has a shoulder on its right hand side, to act as a spacer between the toothed segment and the ball bearing.  The International first gear pair is 14:27, and the AMC first gear is 14:28.  I expect that these two 14t gear pinions are a different size and tooth form, but have not checked.  It's important to make sure that you use a compatible pair of pinions.

Traditionally, Norton used pinions pairs which always added up to 42 (apart from that Daytona first, which was fine-toothed to get the highest possible first gear with a kick starter).  With the AMC 2, they started mucking about with the tooth totals and gear forms.  It makes it much harder to be certain which pinions will mate satisfactorily.

One AMC upgrade which can be made to transfer is the cam-plate.  If you machine out the main casting, it will go into a horizontal box, but must be mated to a long-travel cam-plate plunger.  For the Doll's Head and Vertical box, the cam-plate spindle with integral pinion are larger, and have larger splines.  So if you want the superb indexing of the AMC on an early Norton box,  you would have to make up a special spindle with the smaller splines.

So you see, it gets complicated when you try mixing and matching

Paul

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Thanks Paul. All best to leave things alone I suspect. It does however look like the dolls head second gears are unchanged. But I see from 30s parts lists that even those have more than one part number each!

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I'll leave this thread with the attached.  It shows comparisons between the thrust on the road (torque at back wheel) in each gear.

The standard Inter box appears, not surprisingly, to be aimed at getting as much thrust down onto the road as possible between about 60 and 90mph, at the expense of 40 to 60.  Whereas on today's roads, 40 to 60 count for more.

 

Maybe a job for a wet autumn weekend.

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.. will, I expect, have similar ratios to the 16H - at least will if as I assume it is standard. I really like the close 3rd and 4th which allow easy switching between them on 50-70mph roads, but there's a yawning gap down to 2nd. I'd really like something between what I have and what I see from your Inter curves.

Although when I say I would like it I'm not sure if I'd like it enough to pay for it......

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Just looking at Bacon's Norton Singles. The post-1960 AMC box  has almost exactly the same ratios as the standard 1935-on Norton box. The early AMC box had a bigger gap from 3 to 4, but they must have decided they had made a mistake and the later one, like the 16H box from 1935  had the bigger gap from 2 to 3. Maybe you could lower third by using 3rd gear cogs from an early AMC box. You're on your own from here on! Paul Standeven's mail above might help!

It's all too complicated. Anyway I hope to ride mine this weekend now the working week is over!

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I have just spoken to the man at Russell Motors, who deals with classic spares.  He says that the 21:21 third gear is in fact not WD, but they are all gone because the Inter owners bought them all up.  To do my mod.  Damn, other people had the same idea as me, and have used them.

Paul

I've just changed the First and Second gear ratios in the laydown box of my 1957 350cc Inter.

The original ratios were (as you first said) 1  1.10  1.33  2.33.

They are now 1  1.10  1.61  2.70. I changed the first and second gear pinions to those of the standard road box. I've been out on it this afternoon and can say that it makes it much easier to ride on the road.

Doug

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A club friend told me to learn to love it! But he does not live in quite as urban a location as I do. I'll leave it for a while. My 16H gearbox needs a new thrust washer somewhere or so I believe, so I'll learn about the doll's head there first. As far as I can tell it's only a couple of hours work...but I've thought that before!

 

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