Wheel distance ratio


This could go anywhere but as I'm using a Diablo for this project, ill post here.

Is there a certain ratio in relation to the distance between wheels, left to right and front to back?

The reason I as is my first attempt had a rectangular pattern which I suspect put way to much torque on the wheels when steering, hence the messed up metal gears

I then went for a more square pattern slightly wider than long... with a different body, this did work much better when turning, but also flipped over every time I stopped as all the mass of the batteries were on top of the motors, so I am assuming that the square pattern is right, but I need to make the square bigger so that I can drop the centre of gravity.

If it makes a difference I have 4 PiBorg Motor 12V 300RPM - 37mm (MonsterBorg) motors and the Diablo mounted in a 200x120 box

piborg's picture

Generally speaking a larger width (left to right) between the wheels makes turning easier and a larger length (front to back) makes turning harder. If you keep the ratio the same you should be able to scale the overall size up or down without having too much affect on how the robot steers.

We have found that an approximate 1:1 ratio tends to work quite well so sticking with a square is probably a good idea :)

Turning is also affected by the wheels and the surface you are driving on. If the wheels grip the surface very well it actually makes turning harder as they need to "slip" in a sideways direction to make the turn. For example turning on loose gravel will likely be easier than turning on carpet with grippy rubber wheels.

Lowering the centre of gravity will help stability quite a lot. The only down side is that you may also lower the ground clearance which can be a problem driving outdoors.

@piborg. I came across this thread as I am seeing the problem that turning a carpet is difficult. The diagonally opposite wheels don't turn. It looks like they are 'stuck'. Sounds like what you are talking about that the wheels need to slip. Do you have more details on what is going on and how to make this better. Because it sounds like diddy Borg is the broken by design for these cases. I have diddy Borg v1.

piborg's picture

DiddyBorg v1 can struggle on thick carpets, but should work fine on thinner carpets:

One thing to check is that you have fully charged the batteries, the motors need their full torque for turning on carpet.

With DiddyBorg v2 we changed to more powerful motors. This helps with turning on difficult surfaces.

What you can do is overdrive the motors on DiddyBorg v1 when you need a bit more power. This should be fine for short bursts, but continual use will shorten the lifespan of the motors. For the diddyJoy.py script this can be done by changing the slow button (L2 by default) into a boost button. Look for this line:

slowFactor = 0.5                        # Speed to slow to when the drive slowly button is held...

and change the value to 1.9 instead:

slowFactor = 1.9                        # Speed to slow to when the drive slowly button is held...

Now when the "slow" button is held it should put more power into the motors (+90%) and it should have an easier time turning :)

Subscribe to Comments for "Wheel distance ratio"