crossbreak said:
Can you plz post the now-load rpm of the motor? I need it to calc gear ratios. Plz also post input-, output-sprocket and chainwheel number of teeth
I'm totally sold on the Astroflight family of motors. The CPM is being designed around the 3220, but that means it will accommodate the shorter 3215 and 3210 without a problem. For now let's say I'll be using the 5 turn 3220 (or 10 turn 3210 withthe same kv), with a kv of 135. At 50 volts peak (using Castle Creations HV-160 as controller), that's ~5200rpm.
Reduction will go as follows:
Stage 1: 20:80 via belt (1:4).
Stage 2: 16:34 via bicycle chain (1:2.125). 34 is a constraint here, I want the smallest commonly available chain ring that fits 110mm BCD. This gives me 1:8.5 reduction at the input to the NuVinci left side drive sprag clutch.
The 16t driver on the second stage is going to be a freewheel in the very short term, as a failsafe in case the sprag clutch seizes while in use. I can get more reduction by shrinking that freewheel to 15, 14 or 13 teeth (13t = 1:10.46 reduction at the NuVinci input).
The NuVinci output to the rear wheel is a chainring mounted to a 130bcd hole pattern. I want this as small as possible - 38t. If I use a 50t chainring at the rear, I'm getting a further reduction of 1:1.3, leading to a total motor drive reduction of between 1:11.2 for the 16t fw and 1:13.8 for 13t fw. This will put my performance very close to the 1:12.2 I'm currently running on the 20" wheel folding bike (as a design goal: Get the thing moving with performance characteristics I'm used to, then perhaps make changes from there. The folding bike has great torque and a top end speed of 33mph).
Once this thing is built and proven, I can easily swap that 16t freewheel out for 17t or 18, for higher top-end speed...
crossbreak said:
The primary reduction will be a belt, and will be tensioned by slotting the axle and mount bolt holes.
Chain-wheel screws are best use for this method IMO. They have very small thread pitch and can induce high pressure, they will not loosen due vibration.
Chainring bolts are huge, requiring large slots. I'll take your advice and plan on using bolts with extra fine pitch - but I'd prefer to keep the bolts in the 5-6mm range.