# Gear ratios for 4" wheels

Is there any way that I could apply something like a 5:3 gear ratio to a drivetrain using 4" wheels, or would I need to use 3.25" wheels? I wanted to increase the speed of my bot but the 4" wheels are too large, and I can’t guarantee that I’ll get the 3.25" wheels.

I’m pretty sure that the 3.25 inch wheels would make your bot slower. I believe larger wheels means more speed.

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Not necessarily the case as speed ratios (and even torque ratios if done correctly ) can allow 3.25" wheels to run on par or even faster to the usual 4" wheel.

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Wait with torque? I’m interested.

the best thing that you can do is use a calculator and calculate the distance you travel per second.

A regular 200rpm motor cartridge on a 4" wheel will travel at 20043.14 = 209 ft/m = 3.48 ft / s
A regular 200rpm motor on a 3.25" wheel will travel at 2.833 ft / s

A gearing of 5:3 on 200rpm 3.25" wheel will give you 4.724 ft / s
A gearing of 3:5 on 600rpm 3.25" wheel will give you 5.103 ft / s

A gearing of 7:5 on 200rpm on 4" wheels will give you 4.88 ft / s

So no, you don’t necessarily need just a bigger wheel to go faster, you can use gearing to get faster / slower robots. At the end, the difference between certain speeds may not be the final deciding factor, it may come down to whichever gearing is easiest to implement - for example 3:5 vs 5:3 might come down to wanting better clearance, or just a little bit more torque. a 7:5 might be used because your wheels are bigger (mechanums for example).

Keep drive acceleration in mind too though- a 200 rpm drive is sometimes just as effective as a 300 rpm drive cause it accelerates faster

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It’s not an issue of speed yet, my issue is getting the wheel out of the way of the 60 tooth gear shaft. The wheel is too large, so I wanted to know if there way to create a gearbox that’s 5:3, or if I should just go with 3.25 omnis.

Under the same torque output, a smaller radius wheel would have a greater acceleration while a larger radius wheel would have a smaller acceleration but a higher linear velocity.

When you’re selecting drive train gear ratios, another thing to keep in mind is the difference between free speed and adjusted speed.

Depending on how your drive train is setup (chained, geared, belted), your efficiency and speed loss constants would vary

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no - there is no way to 5:3 on 4" wheels ( in fact, the ratio would probably be faster than you’d want it to be ). The next best thing is 3:5 on 3.25 or 7:5 on 4" (7:5 on 4" is actually pretty great).

Ok, will try that out, thanks!

Question: isn’t 5:3 on 4” wheels about 333 rpm (assuming a 200 rpm cartridge) Couldn’t you just go 3:2 200 rpm and use chain? That’s only 33 rpm lower.

I’m using HS Cortex motors because we haven’t ordered v5 motors.

Ah. High speed 3:2 is still 240 rpm though… I’d say that’s an ideal ratio between speed and torque a lot of the time

c o r r e c t. Although - using chain this way scares me way too much - I’ll stick with gears thanks

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We did it with an idler gear.

240 RPM is the same RPM as a Turbo v4 motor.

Might be better to use a turbo instead of gearing to reduce efficiency loss through the geard

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Using larger wheels also means less precision, as the backlash on the motors would be greater.

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