# X drive problem

I have a question. So I am in the making of an x drive with 4 inch omni wheels. I have made my base 15 holes (long parts) and 6 holes (short parts). I have only made the outside “ring”. what would be the dimensions of the “inner ring” that would line up to my outer ring? BTW, The robot I am trying to make is a smaller version of this:

You don’t necessarily need an inner “ring” for it. You can just have the outter frame and a standoff’ed channel piece that the axle goes through for each wheel. If you feel like you need an inner ring for whatever reason, I recommend just finding the ratio between the lengths of the outer and inner ring of the design in the picture. Looks to me like it’s 20:16, so the inner long pieces of the frame are 80% as long as the outer long pieces of the frame. You can then just apply this ratio to the numbers you provided to get what you need. Good luck!

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you can slice the inner ring in 2 and attach the outer to the new inner with standoffs and spacers. if you still want an inner ring

For mine I made the inner c-channel 6 holes shorter than the outside (3 holes from each end when centered) and spaced it 4 holes inward from the outer c-channel and the holes line up on the diagonal.

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IMO ur literally doing a cad just play around with the spacing. That’s kinda the point.

This is the closest I can to having the X Drive be geometric (My illustration is a little off but it should be relatively geometric with almost no friction if done correctly):

Red - Axle
I would just prefer doing what @Ethan5956F said and just extend standoffs and make the wheels have their own small ‘wheel well’.

I have understood all of your responses and I thank you for the responses. I have gone with the suggestion that @Connor and @Ethan5956F made. However, I am building from home meaning i only have certain materials. What would be the hole count of the smaller angle (if I were to use the standoff method ) with 4 inch omni wheels? these are the only wheels I have
If you can’t answer the question above: what is the “usual” or optimum wheel size that people use for x drives?

For your second question- It depends on what linear speed you want. As you probably already know x drives are 1.41 times as fast as tank drives (with the same size wheels and same rpm) in all orthogonal directions (with a directly proportional loss in torque) but are the same linear speed as tank drives (yet with 2m less power) in the non orthogonal directions of a tank drive. In addition to using this 1/sqrt2 MA to change the linear speed, you can change the wheel size to also increase or decrease the linear speed. Bigger wheels spin faster but with less torque, while smaller wheels spin slower but with more torque. I’ve seen many people go with the 3.25” omni-wheels because they are better made and have mounting holes on them (not very important for x drives unless you’re gearing them or using a lockbar for less slop), but 4.00” omni-wheels do just was well in most aspects and you can still attach lock bars and gears to them (albeit with more work). All in all, it really just depends on what linear speed you want. You can calculate linear speed by taking the rpm x wheel diameter x pi x the MA due to angle of the x drive’s force vectors (like I said earlier, this value for orthogonal directions is 1/sqrt2) and get the final linear speed.

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As for your first question- I’m not completely sure what you mean by smaller angle; both angles should be the same if you have an inner and outer ring; and with the standoff method, it would be the same angle anyways because the standoffs jut out perpendicular to wherever the outer bearing is mounted. As for how many holes between the two, it depends on a couple factors. I’ll try and break it down
Having the first and second bearing further apart means using a longer axle (more likely to bend, which would increase friction) but it could give you more room to work with when putting the wheel and shaft collars in, but the motors might end up being too close to one another which might influence ball collection depending on your design. Overall I’d recommend keeping the distance between the standoff’ed channels as small as you can to prevent all this

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Lol, I know that you really meant resulting robot speed, because wheels spin at whatever angular velocity motors run at, regardless of the wheel size.

I was talking about their linear speed. They have a faster linear speed when the wheels are bigger

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