When I bought this GasGas donor bike I thought I'd ride it with its existing 2-stroke engine for a bit.
But having ridden the OSET for some time now, I have really become spoiled with its torque delivery characteristics and ease of use. No clutch, no gears, no noise.
Needless to say, I took one ride on the 2-stroke and that was the last time. That engine has now been ripped out and might be turned into a wakeskate winch (pic below).
For the record, here are the specs for the original 2-stroke bike (at least as much as I could find):
Wet weight: 75kg
Max power: 15kw (20hp) @6000rpm
Max torque: 25Nm - 45Nm (varying specs on the net, let's split the difference and make it 35Nm)
Gearbox 1st gear: 2.9:1 (first gear is crazy low, good for rock crawling)
Gearbox 2nd gear: 2.5:1 (most guys use 2nd gear for power actions)
Rear/front sprockets: 42:11 (3.8:1)
Rear wheel dia: 0.67m
The trials guys tend to use the clutch a fair amount, essentially storing energy in the flywheel for actions requiring short bursts of more power.
On my OSET I found that you can achieve the same result without a clutch, because all the power is available on-demand and it doesn't drop off when the flywheel has expended its energy.
However, the technique is different. Same end-result though.
I am fairly impressed by how much torque the OSET 20 puts down. I normally run it at the 50% power setting, which means it ramps up to full power over a period of about 1 second.
I almost never run it wide open (or should I rather say full-scale, seeing as the throttle is not actually opening an airway
).
Full power is frankly rather intimidating. I have in fact mounted a cut-off switch on the rear brake because it's too easy to go full whiskey throttle standing up. And this is on a 1200W motor, 48V SLA 9AH battery pack.
Generally I potter around at about 10km/h, with occasional bursts up to 30 km/h. High power actions (wheelies/splats) are few and far between, probably 10% of riding time.
The 2-stroke, in 2nd gear, with the standard sprockets, at max engine torque, should deliver 35Nm x 2.5 x 3.8 = 332.5Nm at the rear wheel.
With the standard rear wheel, that equates to 332.5Nm / 0.67m = 496N of forward thrust (provided we have lotsa grip).
If me and the bike weigh 165kg together, that thrust provides 496N / 165kg = 3m/s^2 of accelleration, which means a 0-100 time of approximately 9.2s.
Going electric, I would prefer similar or better torque performance. Top speed is somewhat academic, but let's say 40km/h is sufficient.
The rear wheel stays the same, so let's start with rear wheel torque, i.e. 333Nm.
I can use a gearbox or I can use a direct drive from the electric motor. Direct drive is easier to build and has less frictional losses, but sprocket size can be an issue.
Let's say I use #35 chain, I can go 10T front and 100T on the rear with a sprocket size of 0.308m
(https://www.rollerchain4less.com/Sprocket-Diameters--Sprocket-Diameters-By-ANSI-Chain-Size_ep_88-1.html)
#35 chain has got a max allowable load of 2kN and up, which is at 5 times my requirement.
(https://www.did-coltd.com/english/products/pdf/sr_35.pdf)
On a 0.67m wheel I can run a 0.308m sprocket, giving me a 10:1 ratio. I think this is fine, given that my OSET 20 has a large rear sprocket and I've not had any issues fouling rocks or terrain with it.
I can also drop down to #25 chain if I need even higher ratio.
(https://www.did-coltd.com/english/products/pdf/s_102-103.pdf)
With a 10:1 ratio, I can get away with a motor that provides 333/10 = 33Nm shaft torque.
For 40km/h (666m/min), and a wheel size of 0.67m dia (circumference = PI x 0.67 = 2.1m), that gives 666m/min / 2.1m = 317rpm.
So. Motor.
Minimum: 33Nm, 317rpm.
