What's the obsession with gear ratios?

I was watching First Canada Live for the last couple days and it seemed that a lot of people were asking about gear ratios. Just wondering why they were so curious.

1 Like

The wrong gear ratio can turn a championship robot into a limping heap of hardware. Gear too slowly, and things will take a long time. Gear too fast, and you may not be able to do them at all.

Added after the next two posts were made: Yes, you also need to know motors and diameter of wheels or equivalent, but you can often see those when you look at the robot; the gear ratio is usually hidden inside.

14 Likes

GeeTwo’s answer is of course correct, but it misses an interesting aspect. FRC only has a limited set of allowed motors. Experienced team members tend to know the motor by heart (or at least have a strong feel for them), so knowing the gear ratio let’s them translate quickly to speed, torque, etc.

If there were 100s of different motors available, knowing the gear ratio would be of limited interest.

2 Likes

I’m not sure what’s up with asking for gear ratios specifically. Gear ratio requires you to also know wheel size. Just asking for free speed and motors tells you everything you need to know.

Anybody sharing robot speed with “adjusted speed” is in for a world of hurt. Sorry JVN clac, but that # is meaningless.

7 Likes

Sort of? Even knowing free speed doesn’t tell you real world robot performance unless you know what motors are used (for the torque curve), and of course all of the other mechanical and electrical efficiency losses.

I prefer to discuss gear ratios than “free speed” because it’s easy to compare. I can see pretty quickly f a team has a different wheel size, or different motors than ours, and ratio accordingly. You’re right that “adjusted speed” is pretty meaningless unless everyone uses the exact same adjustment.

To the original poster, I think people ask because it’s one of those early design decisions that everyone has to make, and it’s useful to ask others to see if they made the same choice and why. There are many CD threads on the subject of selecting gear ratio. Personally I like to use acceleration simulators.

1 Like

That’s why I included that you need motor specs… (type and qty.)
Technically, wheel size and type will also play a role, but from driving robots and messing with gear ratios, free speed and motors alone will quickly give you an idea of how a robot handles with just one number to remember (excluding situations will all omnis). Weight obvious also plays a role, but that’s generally not quite as significant when everybody is 110+/- 15 lbs.

Basically, if I was pit scouting I’d only ask for free speed. The “motor:ground” ratio if you will.

1 Like

Two components are needed to understand the motor/gearbox/wheel selection.
First is a game play approach that provides goals for:
How fast do we need to go, and
How quick do we need to be.
The second is a tool to compare possible choices against the performance goals.

For #1018, the tool has been shared in the papers section of chief:

The total weight (mass) of the robot is also important. Some are around 100 pounds (80 pounds at inspection). Some are up around 150 pounds.

1 Like

Don’t underestimate the importance of wheel size - it plays basically the same role as gearing in determining drivetrain performance. For example, imagine a gearbox designed to drive a robot at a typical 14 feet/second on 3” Colsons (the smallest common drive wheel). Now swap on 8” pneumatic tires (the biggest common drive wheel) - you’ll now be trying to drive 37.3 feet/second and you’ll quickly trip breakers or brown out.

This is obviously an extreme example, but it demonstrates the point.

If you take a look at my first topic reply, I’m looking for free speed which includes wheel diameter to give a “motor:ground” ratio. Obviously, changing wheel diameter changes this ratio.
There are other characteristics of a drivetrain what will lead to two robots, both geared for 15 ft/s but with different diameter wheels to handle differently.