What torque is needed to make a bicycle move.

[alebbing]

Hey I am starting an electric bike project and I need to know how much torque is needed to get a bike going from a stationary position in ft/lbs in first gear that has a gear ratio of .93? What do you guys think?

 

[Travis_King]

I think you have an average weight of people, the weight of the bike, the gear ratio, the size of the driven wheel and the length of the lever arm on whatever means of power you intend to use...

 

[alebbing]

I think you have an average weight of people, the weight of the bike, the gear ratio, the size of the driven wheel and the length of the lever arm on whatever means of power you intend to use...

Ive been using an old huffy that is in pretty good condition. I've been using around 300 lbs as the weight. In first gear the gear ratio is 1:.93. the drive wheel (smallest chain ring) has a 5 in diameter, and I am going to be a direct drive from motor to the drive shaft. The motor I am using is a 48 volt 750 Watt motor, which produces around 25.35 ft/lbs.

I found a few good formulas online that dealt with "burning rubber". Those help me get a better idea of what it takes to beat the COF of rubber vs pavement which is (.75 N)

so it states:

F= MA

F= (136.08 kg)(9.8 N/kg) = 1333.58 N

F(normal) = 1333.58/2 = 666.8 N

F (friction) = (.75)(666.8N)

F= 500.1

Torque = radius * force

26 in wheel means the radius is .33 Meters

T= .33 * 500.1

T = 165.033 Nm which equals 121.72 Ft/Lbs

So if you take that and factor in the .93 gear ratio I believe that to "burn rubber" you need to have a motor that can produce 113.20 Ft/Lbs

Does that make sence? are my numbers flawed what do you guys think?

 

[Travis_King]

Remember it is a BIcycle, so the weight is distributed some. Yea. You will need a ton of torque for a 300lb guy to peel out while sitting on a bicyle...

 

[alebbing]

ya that is a lot or torque, and the 300 lbs is the guy and all the batteries and the bike and the motor that's a total mass. I then did the calculations for just the bike which is about 100 lbs and it came to a little over 40 ft/lbs of torque.

 

[Travis_King]

Go out to the bike and sit on it with the front pressed against a wall. Put your foot on the pedal and see if you can get the tires to spin while sitting down. You've got a 6-7" lever on the flywheel and the strength of your leg. You will find it extremely difficult to spin those tires.

 

[alebbing]

your right it is a hell of a lot or torque which is why I am questioning the formulas that I found.

 

[SteamKing]

You haven't mentioned the speed your electric motor can operate at.

 

[alebbing]

sorry the operating speed for the motor is 2500 rpm

 

[Travis_King]

You may not be accurately accounting for the weight balances, and surely the surface area of the tire as it flattens on the ground adds to the friction, but the formulas are sound. There are others (well, to be more accurate there are versions of these formulas) that account for more of the variables invovled in this problem, but for this type of thing a general idea is all that is needed.

2500rpm is fast for an electric bike...especially if you are driving at a ratio of .93 haha obviously that won't be its operating speed, but consider that you will likely have a "throttle"

 

[tennisthehili]

Hi I am making an electric scooter also.

What top speed for your scooter are you looking for?
Wouldn't a gear ratio of .93 mean a high top speed and very low torque?

Use this online calculator to find your top speed:
http://www.4qd.co.uk/faq/roadspd.html

What is the total diameter of your wheel 26 inches? in your calculations you said 26 = .33 but 26" to m = .66
Why did you divide by two to find F(N)?

 

[alebbing]

Im shooting for a top speed of around 50 or 60 in top gear.

I redesigned the project about a week ago to get a .23 gear ratio or roughly a 4:1. Which means the motor doesn't have to work as hard ,therefore you get more time per charge of the battery.

I don't trust that calculator at all. Their formulas doesn't take into consideration all the different frictions that make a bike or scooter move. I made a excel document that gets me the MPH based on rpm and gear ratios. (if you want email me at andylebbing@gmail.com and we can compare notes)

The reason that you divide the friction by 2 is because the weight is distributed to two wheels. For a car you would divide by 4. I derived that calculation from a formula that I found that honda used for their new electric car.

What size motor are you using? and where are you getting your parts?