• Ian Watts

Test 2 : 'Stuttering' Motor

After 'playing about' with the Volt / Amp meter it was time to re-visit the motor 'stutter' and the un-modified clutch.


I grabbed my hand-held tacho (nothing special here, 15-oddEuro on Amazon), removed the chain between front / rear sprockets and applied a little reflective tape the motor shaft / gear.

Ahhhh.... All became apparent... The 'stutter' kicked in at between 5700rpm and 5850rpm. The light dawned... with ears alone, I had absolutely no idea the motor was spinning that fast !! Oops !

The maximum motor revs is stated as being 5000rpm and 5900rpm absolute tops ! OK the controller was either deliberately limiting the revs of I'd reached the motor / controller max capability / limit... either way, I had a challenge...


Additionally, using the tacho, it became apparent that the onboard clutch only kicked in at around 3000 motor rpm.


The motor arrived with a 13tooth sprocket and the lowest rear (driven) sprocket I could find with a large enough centre through-hole (to pass over the rear vari-drive / clutch drive shaft) was 69tooth. I should have done some more math... Idiot !


Anyways, couple the 13 / 69 sprocket ratio, the clutch and the, internal-to-rear-wheel, gearbox ratio of 14:1 and... nothing surprising here... high motor revs = low wheel revs ! I should have thought of this way before now. Idiot (again!).

OK. I immediately reverted to a plan I'd been concocting that was to simply lock off the clutch by welding 3 short studs between the outer (driven) clutch bell-housing and the inner (drive) clutch pad plate. 10 minutes later; I'd popped the bell housing (right hand side in the image) and clutch assembly off the end of the gearbox input shaft, welded on 3 short 10mm studs and reassembled everything

The 3 x studs simply locate in the 3 similarly spaced holes in the driven (inner) section of the clutch.


This was just a quick'n'dirty test to see the impact of bypassing the clutch. So, stand up, wheels back on the ground and... OK... 11kph max speed but.. although I'm disappointed at the, not-so-dazzling, top speed, I'm also grinning - I have converted a petrol scoot to an, albeit rather slow, electric scoot !!


After a poodle (I wanted to say 'whiz' but that really wasn't the case) around the garden and patio with my wife saying... "Nice dear... but won't won't the milk have gone off by the time you get it home ??? " I reverted to the garage. I stripped down the clutch assembly and started to re-work the plan... but this time with some math first !

Gearbox opened up (the wheel locates on the splines)

I opened up the gearbox (Just to be sure ! ) and accepting that :

there was very little I could do with the14:1 ratio and

the maximum motor revs = 5000 (forget about the over-rev portion (that can simply be a later date 'Brucie' bonus ! ) and

the rear wheel circumference = 1.45m...



I did the math for a 45kmh top speed (matches the original top speed - although still not dazzling) :


5000rpm x chain/sprocket ratio (CSR) x internal gearbox ratio (GBR) x 1.45 = speed (mtrs / min)

=> 5000 x CSR x GBR x 1.45 x 60 = Speed (mtrs / hr)


convert metres to km (divide by 1000)

=> (5000 / 1000) = 5


convert revs per minute to revs per hour (because I want Km per hour at the end)

=> 5 x CSR x GBR x 1.45 x 60 = Speed (Kmh)


I knew GBR = 14:1, I either need to divide by 14 or (1 / 14 = 0.0714) or multiply by 0.0714

=> 5 x CSR x 0.0714 x 1.45 x 60 = Kmh


... and to isolate the CSR (chain sprocket ratio)

5 x 0.0714 x 1.45 x 60 x CSR = Kmh

=> 31.1 x CSR = Kmh

=> CSR = Kmh / 31.1


so, for a top speed of 45Kmh :

=> CSR = 45 / 31.1

CSR = 1.45


and, for a top speed of 60Kmh - illegal :

=> CSR = 60 / 31.1

=> CSR = 1.93


and for 75kmh - also illegal but fun to work out ;) :

=> CSR = 75 / 31.1

=> CSR = 2.41


That was to say;

for 45Kmh, the front sprocket has to have 1.45 more teeth than the rear sprocket

for 60Kmh, the front sprocket has to have 1.93 more teeth than the rear sprocket

for 75Kmh, the front sprocket has to have 2.41 more teeth than the rear sprocket


So, math over, the motor arrived with a T8F sprocket / shaft fitting, that's to say a double 'D' fitting on a 10mm shaft or... a 10mm shaft with two flats cut into it 180' opposite each other.

The challenge here is that for the (theoretical) power involved (3000W / 3Kw) this type of fitting doesn't allow for larger than 17tooth sprockets (not unless custom built - or someone has a source I haven't found ;) ) To be fair, above 17tooth would probably placing a HUGE amount of additional stress (even with the 14:1 rear gearbox ratio) on the motor shaft and, as the number of teeth are increased, so too does the sprocket diameter and, therefore, so too does the side or cross stress and the torsional stress. Long and short? 17 teeth from sprocket is (probably) the limit - having said that, if I can find a 19tooth item I'd willingly try it !! ;)


So, taking the 17tooth front sprocket and the CSR for the three 'required' top speeds above :

to calculate the drive ration of these two sprockets :


Front Sprocket Teeth (FST) / Rear Sprocket Teeth (RST) = Chain / Sprocket Ratio (CSR)

(FST / RST ) = CSR

=> RST = FST / CSR


Front sprocket teeth (FST) = 17teeth

...feeding the numbers into the final equation :

for 45Kmh and a CSR of 1.45 :

=> RST = FST / CSR

=> RST = 17 / 1.45

=> RST = 11.7

obviously, a sprocket can only have a WHOLE NUMBER of teeth.

A 12tooth sprocket will provide a top speed of slightly under 45kph, whereas an 11tooth sprocket would provide a slightly greater than 45Kmh top speed.


for 60Kmh and a CSR of 1.93 :

=> RST = FST / CSR

=> RST = 17 / 1.93

=> RST = 8.8

A 9tooth sprocket will provide a top speed of slightly under 60kph, whereas an 8tooth sprocket would provide a slightly greater than 60kph top speed.


for 75Kmh and a CSR of 2.41 :

=> RST = FST / CSR

=> RST = 17 / 2.41

=> RST = 7.1

An 8tooth sprocket will provide a top speed of slightly under 75kph, whereas an 7tooth sprocket would provide a slightly greater than 75kph top speed.


NOTES

It should be noted here that a 9tooth sprocket is the smallest I was able to find BUT I'd be very surprised if the chain ran smoothly over it on a standard shaft. Obviously, sprocket tooth stress increases with a lower number of teeth on that sprocket - there are simply fewer teeth for the chain to engage with and transfer power to.

It should also be noted that whilst some even number sprockets exist within these ranges, there are more odd numbered ones that there are even numbered ones. I believe this is because sprockets with odd numbers of teeth wear more uniformly than sprockets with even numbers of teeth. If a sprocket has an even number of teeth, the same tooth will be engaged by the same chain rollers upon each rotation. This, in turn, leads to uneven wear on both the chain and sprocket and a shortened service life for both.


I thought about the above for a good few hours whilst 'out and about' and couldn't see any other, relatively simple and low cost approach to my speed dilemma...

As I was originally 'aiming' to replicate the original / standard scoot, I decided to settle for slightly improved 45Kmh performance and opted for an 11tooth sprocket.


Working out the math on this one from the original formula :

5 x CSR x 0.0714 x 1.45 x 60 = Kmh

CSR = 17/11 = 1.55


=> 5 x 1.55 x 0.0714 x 1.45 x 60 = Kmh

= 48Kmh

...and... the max motor rpm was stated (and seen on the tacho as the 'limiter' cut in) as being 5900

=> 5.9 x 1.55 x 0.0714 x 1.45 x 60 = Kmh

= 57Kmh


So, with a 17tooth sprocket up front (on the motor) and an 11tooth sprocket on the rear (gearbox input shaft) I should expect a top speed somewhere in the region of 48 to 57Kmh.


That was OK. I was 'happy' again... the question though was how to get an 11tooth T8F sprocket onto an overly long 38mm shaft !!


In the end, the solution was pretty simple, if a little drastic. But, hey, what the heck, I'd already gone way past the point of no return...

All I had to do was adapt the gearbox input shaft !.... and what's so hard about that ?


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© 2017 Ian Watts