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.

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hello sir 

 i am a dong mechanicalengineering and doin stair climbing wheel chair as my final year project using this tri wheel mechanism.

i want to know about the calculations of the gears used. i will be very thankful to you for your help. his is my email id please contac me



it seems you have a better education than me. I did not go to university. I cannot help you with calculations. That is what your university professor and all those text books are for.

Is the torque for just one wheel or is it the total torque?

The construction is awesome, All the best OddBot

Can you tell me the length of the chassis?

Also provide a video link . please. Thanks

Look at the MkII version here: http://letsmakerobots.com/node/33857

Do you think you could just ask the question once rather than 3 times each seperated 10 minutes, kinda spammy.

Can you tell me the length of the chassis?

Also provide a video link. Thanks

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.