# Arduino Forum

## Topics => Robotics => Topic started by: drewdavis on Jun 22, 2013, 06:30 pm

Title: How does wheel size affect speed???
Post by: drewdavis on Jun 22, 2013, 06:30 pm
I have a question about wheel size.  If you have two robots, one with 2inch wheels and one with 3 inch wheels, and they are build exactly the same besides the wheel size which one will be able to achieve a faster possible speed? Also, which one will be able to produce more torque or will they both be the same? My guess it that the one with larger wheels will go faster and the one with smaller wheels will have more torque but I would like to double check.

Thanks!
Title: Re: How does wheel size affect speed???
Post by: robtillaart on Jun 22, 2013, 07:18 pm
first order math:

the speed is # revolutions * circumference of wheel.
circumference = PI * R^2 /4
so the 3 inch wheel will be 9/4 == 2.25 as fast as the 2 inch wheel.

update:
circumference = PI * R / 2
so the 3 inch wheel will be 3/2 == 1.5 as fast as the 2 inch wheel.

second order math:
The 3 inch wheel is a bit heavier so it takes a bit more energy to move. (ask a MechEng for the details)

Title: Re: How does wheel size affect speed???
Post by: drewdavis on Jun 23, 2013, 03:34 pm
Thanks!
Title: Re: How does wheel size affect speed???
Post by: aarondc on Jun 24, 2013, 02:44 am
circumference = 2 x pi x r

Title: Re: How does wheel size affect speed???
Post by: robtillaart on Jun 24, 2013, 07:02 am
Hi Aarondc,
my mistake you are 100% right, I hope it wasn't a school assignment ;)
I'll update my post
Title: Re: How does wheel size affect speed???
Post by: aarondc on Jun 24, 2013, 07:07 am

Hi Aarondc,
my mistake you are 100% right, I hope it wasn't a school assignment ;)
I'll update my post

I hope it was ;)

Title: Re: How does wheel size affect speed???
Post by: robtillaart on Jun 24, 2013, 07:10 am
LOL
Title: Re: How does wheel size affect speed???
Post by: drewdavis on Jun 24, 2013, 01:05 pm
It wasn't for a school assignment (i'm not that lame). I'm building a sumo bot using Arduino and I'm trying to chose the best wheels. Assuming that the same motor was used would the smaller wheels have more torque to push the other robots.

Thanks!
Title: Re: How does wheel size affect speed???
Post by: aarondc on Jun 24, 2013, 01:12 pm

It wasn't for a school assignment (i'm not that lame). I'm building a sumo bot using Arduino and I'm trying to chose the best wheels. Assuming that the same motor was used would the smaller wheels have more torque to push the other robots.

Thanks!

Was just messin' ;-)

My understanding is that torque is measured as force x distance. foot-pound, oz-inch, kg-cm, Nm are examples of units you will often see.

The torque of the motor is a constant at a constant voltage / current, so as the distance increases, the force is reduced.

eg: 20 kg-cm at 1 cm affords 20kg of force. At 2 cm, you're going to max out at 10kg. Assuming constant current.

So the smaller wheels should afford you more force at the circumference of the wheel, but the torque is the same.

Title: Re: How does wheel size affect speed???
Post by: drewdavis on Jun 25, 2013, 11:58 pm
Thanks!
Title: Re: How does wheel size affect speed???
Post by: aarondc on Jun 26, 2013, 01:58 am
To answer your OP: force will be higher but speed lower. Something to keep in mind when designing a battle bot - do you want speed for agility, or a slower bot with a bit more grunt?
Title: Re: How does wheel size affect speed???
Post by: mech_eng on Jul 01, 2013, 05:41 am
A larger tire will require more torque output from the motor, this will cause the motor to slow down for a given voltage which in turn increases power consumption by said motor.

How much the motor slows down will determine wether the bot will move faster or slower for a given wheel size.

EDIT:

In other words you can oversize a tire when trying to get more speed out of a bot and actually slow down the bot

Speed = diametertire*pi*angular_velocitytire

If angular_velocity decreases more than the diameter increases then speed will decrease, the other way around speed increases