Let's Make Robots!

Tri-wheel stair climber

It seems to me that for a robot to be really useful in the home or workplace then it must be able to handle stairs. With this in mind I want to design a stair climbing robot chassis. This robot is my first experiment with Tri-wheels.

The chassis is just 4x Wild Thumper 75:1 motors driven by a Wild Thumper controller in a clear acrylic base. As this is only an experimental platform for the wheels I am controlling it with a TV remote and an IR receiver.

This first set of Tri-wheels is made using a new 80mm diameter wheel that has been produced by DAGU. The gears are steel to handle the high torque required when climbing.

The clear triangular frame is made from 3mm thick milled polycarbonate. I used CNC milled polycarbonate instead of laser cut acrylic because it is tougher. The acrylic is fine for the chassis but too brittle for the wheels. All shafts are ball raced to reduce friction.

You can see here that the chassis consist of 4 ribs, a top plate and a bottom plate. They are all held together by 56 small steel angle brackets and 112 small M2x5 screws that give the chassis good strength and rigidity. In hind sight I could have used less but this is just a prototype.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Can you please tell me the length of the chassis? Also provide a video link of this experiment. Thank you.

Here is a simple diagram to help explain the torque required.

When the robot is on flat ground, the motor is only driving the wheels (black) via the gears (red). In this case the torque required is just enough to overcome the robots momentum and friction.

If the robot is going up an incline of 30° then the aditional torque required will be:

sin 30° x 30Kg (robot weight) x 7.5cm (radius of the wheel in cm) = 0.5 x 30Kg x 7.5cm = 112.5 Kg.cm


When the wheels get stuck and cannot turn then the entire Tri-wheel (blue) must rotate. The motor must now be able to lift the entire weight of the robot so now the additional torque required is:

30Kg (weight of the robot) x 19.5cm (radius of the entire tri-wheel) = 585 Kg.cm.
Admittedly you can divide the required torque by at least 2 depending on the number of wheels you are using.

Considering the heavy duty truck wiper motors I bought for a big robot base only had a 80Kg.cm rating I would suggest that your wiper motors will stall or break when you try to climb steps or even just a steep incline.

If your wiper motors use plastic gears then they will be no good to you. You will need heavy duty metal gears.You will need to increase the gear ratio by at least 5:1 and then your next problem will be the strength of your 6cm diameter gears. They will need to be steel and at least 3mm thick with big teeth.


why u didnt multibly this in the coeff of friction "

sin 30° x 30Kg (robot weight) x 7.5cm (radius of the wheel in cm) = 0.5 x 30Kg x 7.5cm = 112.5 Kg.cm" and the other equation?

and the diagram . i searched for specification of wiper motors i found that the power is 50 W . Now if i divided the torque required per motor i need about 30 N.m . now  power= Torque * Angular Velociy ==> ang. Velocity= 50/30= 5/3 rad/sec . that means that the wheel will rotate by 16 rpm . sorry but i don't have alot of choices in motors .. if it is no way to use the wiper motor what kind of motor i should use ?in this robot he carried about 70 kg http://letsmakerobots.com/node/23719  using two wiper motors!

The coefficient of friction has nothing to do with the torque required. The torque calculations assume the wheels do not skid.

You cannot calculate by motor power consumption. All motor gearbox combinations have at least 20% inefficiency due to electrical resistance and mechanical friction.

The robot using 2 wiper motors carried a person across flat ground. Your tri-wheels must lift the entire weight of the robot up stairs.

I did not say you cannot use wiper motors. I said they will need to be very strong wiper motors and that you will need to increase the output gear ratio by at least 5:1.

Please understand, I am only trying to explain to that based on the information you have given me, you robot will travel on flat ground but will not be strong enough to climb stairs. Unless your gear train is very strong it will break.

To put it simply: Your robot is 30 x heavier than mine but you motors are only twice as powerful.

OK , thanks alot .

OK , thanks alot .

iam buliding firefighting robot with tristar wheel .. iam worry about it is weght and the motor , it weights approx. 30 kg i will use to wiper motor . the gear diamters is 6 cm the wheel diamter 15 cm . these motors will make it to climp or no? thanks in advance :)

With tri-star wheels you need to allow for when the small wheels lockup at the base of a step and the whole triangle rotates. So if the radius from the center of the triangle to the outer edge of the small wheels is 150mm for example and your robot weighs 30Kg then your geared motor needs a torque of 450Kg/cm.

You will need to drastically reduce the weight or get much more powerful motors.

.iam not sure how to calculate the torque. is it T= coeff of friction *ground reaction*  Distance from motor to ground ?

photo attached for my dimension in mm  .thxx again