Smaller Diameter Wheels - What's the advantage?

[LLogan]

This year, I saw many veteran teams using wheels with smaller diameters than what I'm used to seeing and what my team has used in the past. I've seen lots of 4 inch wheels and some 5 inch wheels, as compared to the 6 inch wheels that my team uses and some of the KOP and Plaction 8 inch wheels that I've seen lots of rookie teams use.

What are the advantages of using these smaller diameter wheels? Of course, you can change the speed/torque ratios through chaining and gearing, so I don't think that's the issue. There is an obvious weight difference too, but I'm not sure if the weight loss is justifiable. The only other thing I can think of is the decreased amounts of rotational moments of inertia (smaller radius, less mass), leading to less strain on the motors when the robot accelerates or decelerates. Though, I am unsure if these are reasons at all for a team to choose smaller diameter wheels.

Are there any reasons as to why smaller wheels are advantageous over larger diameter wheels? If your team uses them, or if you know any good reasons, please share some insight.

Thanks!
-Logan

[thefro526]

One of the biggest advantages of smaller wheels is that they allow your drive base to sit lower, which in turn allows for a lower Center of Gravity. Also a smaller wheel is, smaller - which allows everything to be more compact or allows for more room for other mechanisms.

[AdamHeard]

Smaller wheels have a huge number of advantages.

They force you to make a lower frame, which helps you put a lot of components real low.

They require less torque to travel the same speed, so you need less gear reduction, and there is less load in your gearing; this allows you to make it all lighter, resulting in weight savings.

They are physically smaller, and since volume of a wheel is related to the radius squared, the weight savings are pretty substantial. Our wheels were .23 lbs each this year. This also allows them to be machined quicker and with less material, this is a cost and/or time savings depending on if you make or buy them.

Anyone who says you can't climb with small wheels is just ignorant as well. Both 254 and us had small wheels, and could climb the bump no problem (with a decent battery).

[Pausert]

Originally Posted by AdamHeard

Anyone who says you can't climb with small wheels is just ignorant as well. Both 254 and us had small wheels, and could climb the bump no problem (with a decent battery).

Agreed. We could even climb with 6 inch MECANUM wheels. Climbing ability was not at all the reason for our change to 8 inch.

[MCahoon]

It also allows the wheels at the corners to be 1 inch further toward the end of the robot, making it more stable on sloped surfaces. Effectively with a 4-inch wheeled robot you have a 2 inch longer wheel-base than with 6 inch wheels

[gorrilla]

The only disadvantage I can see would be if two robots had the same gearing(motors+transmissions+chain reductions)
The robot with smaller wheels would have a lower top speed,
But it would accelerate faster.

[AdamHeard]

Originally Posted by gorrilla

The only disadvantage I can see would be if two robots had the same gearing(motors+transmissions+chain reductions)
The robot with smaller wheels would have a lower top speed,
But it would accelerate faster.

how is this a disadvantage?

When we design, we calculate a gear ratio and wheel size based on desired speeds.

[gorrilla]

Originally Posted by AdamHeard

how is this a disadvantage?

When we design, we calculate a gear ratio and wheel size based on desired speeds.

Well,
It really isn't a disadvantage unless your in a drage race

Is the difference in torque even noticeable between a 6' and a 4' wheel?

[Vikesrock]

Originally Posted by gorrilla

Well,
It really isn't a disadvantage unless your in a drage race

Is the difference in torque even noticeable between a 6' and a 4' wheel?

At the same gearing? Heck yes! 150% larger radius means 2/3 the torque. That is a pretty substantial reduction.

The point is that wheel size should be part of your gearing calculations, so there is no reason to compare robots with the same gearing and different wheel sizes. If you want to go faster with smaller wheels, just gear appropriately.

[Chris is me]

A smaller wheel saves weight in the wheel. You also need less of a reduction to get your wheel to an acceptable speed / torque / whatever, which itself saves weight. A lighter wheel allows you to accelerate a little better too. Really, I would rather see this thread posted asking of the advantage of bigger wheels.

[Dad1279]

Smaller wheels are lighter, and extend the wheelbase (can be set further apart) for stability.

[waialua359]

Originally Posted by Chris is me

A smaller wheel saves weight in the wheel. You also need less of a reduction to get your wheel to an acceptable speed / torque / whatever, which itself saves weight. A lighter wheel allows you to accelerate a little better too. Really, I would rather see this thread posted asking of the advantage of bigger wheels.

Compared to our older robots, everything was the same except going from 6 6" to 8 8" wheels. Without getting into gear ratios, we are getting a hell of a lot more torque and pushing power.
As far as CG goes, we adjusted for it, by placing everything a lot lower this year as it was well-suited for the game. We like it so much that I think we are sticking to 8" wheels hopefully for next year.

[Tom Bottiglieri]

Originally Posted by AdamHeard

how is this a disadvantage?

It's a disadvantage when you buy your gearboxes on the internet and don't do any work to figure out what's going to happen when you slap the thing together. Duh.

[JamesCH95]

Originally Posted by Vikesrock

At the same gearing? Heck yes! 2/3 the radius means 2/3 the torque. That is a pretty substantial reduction.

I'm pretty sure you meant 2/3 the radius means3/2 the tractive force and 2/3 the speed, no worries, simple typo.

Smaller wheels mean less gear reduction for the same speed which does mean lower rotating mass (which is important) and it could mean fewer reduction stages which means a higher overall efficiency.

All the CG arguments are moot for two reasons:

1) A large wheel size does not mean you have to have a high CG

2) A "low" CG is not necessarily a good thing. If the robots CG is above or below the bumper zone there will be a moment on the robot if it contacts another robot. However, if the CG is in the height of the bumper zone then when the robot gets hit there is no moment and it can't be flipped, it's a simple bit of physics that is often overlooked that I didn't notice until I saw several "low cg" robots get flipped.

Edit:

@ Chris_is_me

The advantage of larger wheels is a larger contact patch, which generally means more grip. Remember that real coefficients of friction are generally inversely related to contact pressure.

[Vikesrock]

Originally Posted by JamesCH95

I'm pretty sure you meant 2/3 the radius means3/2 the tractive force and 2/3 the speed, no worries, simple typo.

Thanks, fixed it.