• Ian Watts

Gearing - Attempt 2 !

Having experienced the results of my NOT having done some math (I guess, at the outset, I had just 'hoped' all would be well because it was the simplest solution - not to mention that the conversion itself seemed pretty daunting and I was impatient to start. D'Oh !! ) and having just recently worked out the chain / sprocket ratio I was looking for, I ordered up and received three T8F / double 'D' sprockets; 9tooth, 11tooth and 17tooth.

The 17tooth is for the motor sprocket

The 9tooth is for the rear sprocket along with the 11tooth sprocket should the 9tooth be, as expected, simply too small for the chain / shaft diameter... it IS sold for this chain but maybe not this shaft diameter and I'm loathe to reduce the original diameter of the shaft on the gearbox side of the sprocket. Each sprocket was around was 7Euro, so why not give the 9tooth one a go anyways ?!


So, in steps (always easier for my tiny brain ! ) :

I'm going to need to :

  • shorten the existing input shaft,

  • reduce its diameter, creating a 10mm shoulder for the T8F sprocket,

  • reduce the shaft diameter still further (to 8mm) so that the T8F sprocket fits over it and up to the 10mm shoulder,

  • cut two flats on the 10mm portion of the shaft to suit the T8F sprocket, ensuring they are 6mm deep - the depth of the sprocket,

  • cut an 8mm / M8 thread onto the newly created 8mm portion of the shaft,

  • slide the sprocket into place, followed by a lock washer and then the M8 nut (the lock washer not only helps prevent the nut from backing off but also takes up the 'slack' between the 10mm-to-8mm shoulder and the thread on the 8mm shaft due to the shoulders on the thread cutting die).

When put like that, it all sounded pretty simple but how to achieve concentric faces on the shaft whilst the shaft is still fixed to the gearbox ? I really needed to remove the shaft and lathe it up... but... that's definitely NOT within the realms of your average DIY'er I thought and most certainly outwith my budget to have it professionally done (not to mention that it's outwith the original scope of the project)...


What I came up with was slightly simpler: reassemble and back-drive the gearbox with a drill. So, attaching a suitable socket to the drill and driving the gearbox from the wheel end gave me a rotational advantage of 14:1 (the gearbox ratio in reverse) and the ability to rotate the input side of the shaft just like it was on a lathe ! Well, that was the idea... So, sticking to my step-by-step approach, I :

1: reassembled the gearbox and removed the existing clutch / variable-drive unit for the gearbox input shaft :






2: measured the required offset; the distance between the shaft bearing face and the end of the 'to-be modified-and-threaded' shaft, taking into account the depth / thickness of the sprocket, lock washer and nut (+ a wee bit).




3: attached the drill to the output side of the gearbox and had my 'helpful assistant' on the drill trigger ! And now, cut the shaft to suit. No going back now !!



4: carefully re-measured the offset of the 10mm shaft shoulder and, with the help of a spinning gearbox input shaft (drill and helpful assistant), a grinder (for the initial 'get-me-close' work) and a file and some digital callipers (for the finishing / precision work), reduced the shaft diameter to 10mm (for the sprocket).


5: carefully measured a 6.5mm offset on the newly created 10mm shaft and reduced the shaft end to 8mm, thereby creating a second shoulder for the washer and nut...


6: locked the shaft and filed two opposing flats on the 10mm portion to accept the T8F sprocket.

7: threaded the 8mm portion of the shaft getting as close to the 100mm/8mm shoulder as was possible.I was very careful here to get as close as possible but not to have the die come up hard against the shoulder and thereby damage the newly cut thread. The split / spring washer is here to 'take up' that 'slack'.



8: offer up the sprocket and make any adjustments. This was slow work, you can't add a bit back onto the shaft after taking too much off... slow but sure, testing and measuring all the time.


9: fit the split / spring washer and the nut !!

10: Use a rule for the check the final lateral alignment between the motor pinion and the gearbox sprocket. There is around 15mm of lateral movement in the 4 x motor mountings so final alignment was simple enough.




11: Cut and split-link the chain to suit and then tension in the usual 'alternator-style' way at the motor. In the case of a scoot and particularly in the method I'd selected to mount the motor, there is no 'play' needed in the chain to take up suspension travel. The motor, chain and rear wheel all move on the same swingarm / frame... No idler, sprung tension sprocket needed - one of the clever design features of a scoot's transmission.


All in, from start to finish (once I had the math pegged), this took the best part of an afternoon ! Nice 'n' steady was the name of the game here, like I said, I was very aware I couldn't easily remedy removal of too much of the shaft...

As it happened, all worked out just fine and, remember, I'm an electrician (of sorts) to trade, not a machinist or mechanic ! If I can do it, then anyone can !

I should have done this from the start but feared messing about with the mechanical stuff - just go nice and slow, take your time, better to get a good result and take your time than to rush a mistake... especially as a 'mistake' on the shaft work is not easily recovered from.

Chain connected and SwingArm cover re-fitted

That's me for the initial conversion. All-in-all around 37 hours work. The next one would be MUCH quicker (barring any mistakes) IF I was ever to do another scoot - what I'd REALLY like to do is a Kawasaki VN 650 or similar - now that would be fun !


Anyways, I'm off to have some fun and (gently) road-test the Scoot-ee !!


There will undoubtedly be some alterations and additions but they're for another day as is whether this could ever be documented with a view to creating a 'kit' of parts. I'm also keen to make a small video of Scoot-ee on the road after some basic road-tests. Maybe it could indicate the performance, how it sounds and behaves and. . .

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© 2017 Ian Watts