# calculating torque equations help

#### Toby woodman

##### Active Member
I'm wondering how to calculate the torque required for my Homeade v belt drive motorized bike. after some research online it states that I need 100nm to climb a hill effectively. The max torque of the little lifan 152 is 3.1 nm, multiply that by my 12 to 1 gearing ratio, and that gives me about 37nm, that seems awfully low for a 2.5 hp motor that's only geared for 23mph? I know @Sidewinder Jerry has posted some stuff about this in the past, but I can't seem to find it.

Are you sure about that figure of 100nm? That's awfully high.

Anyhow: torque is funny to calculate on combustion engines because it isn't constant. It varies through out the rpm range. To keep it simple I'm assuming a 10 to 1 reduction and a gx160 engine.

At 2k rpm the wheel would be turning with 100nm of torque and at 200rpm. At 3.6k rpm it actually drops to what looks like 9.5nm. Two strokes are even worse. I remember that the phantom 85 had close to 5 ft-lbs at 3.5k rpm and something like 1.8 ft-lbs at 9k rpm.

I may be wrong but I'm pretty sure you just divide by the wheel radius, making sure to keep the units the same. If youre using nm, then divide by the radius in meters.

The size of the wheel is gearing in a way. You get less torque but more speed at the same rpm as wheel size increases. That's why penny farthings had such large wheels: to get higher speeds out of a 1 to 1 ratio.

I'm wondering how to calculate the torque required for my Homeade v belt drive motorized bike. after some research online it states that I need 100nm to climb a hill effectively. The max torque of the little lifan 152 is 3.1 nm, multiply that by my 12 to 1 gearing ratio, and that gives me about 37nm, that seems awfully low for a 2.5 hp motor that's only geared for 23mph? I know @Sidewinder Jerry has posted some stuff about this in the past, but I can't seem to find it.
Think of hp and torque like this you have a 100 lbs weight; you have a long bar, short bar, and a fulcrum. The long bar is going to be able to lift more weight using the 100 lbs weight as a counter weight than the short bar will.

So hp is like the counter weight and torque is like the lever.

So your options are either use a gear ratio which can climb really steep hills with a low max level ground speed. Or build a shifter bike, then you can do what I do, down shift to a gear low enough to climb the hill.

so 27 nm is correct for a 12;1 reduction with an engine max output torque of 3.1nm

this bike is only good for about 25 mph on flat ground, I'm building it as a substitute to an ebike, without the expense and worry from batteries, as I forget to charge them, and don't trust charging chineese batteries when I'm not around. an ebike looks to have a top speed of about 20mph in the UK, with a max torque of 32nm, I know its not an apples to apples comparison, but that gearing and motor should be alright, about 2.5hp at 3600rpm, with a 12.1 gear reatio and a max torque of 37.5