Omni Wheel Information

What are some differences between the 4 inch omni wheels and the new 2.75 inch? What I mean about that include questions like this: Do the difference in size change/affect the speed or torque. In addition to that, what are some pros and cons of both of the 4" and 2.75 inch. Please reply.**

Smaller wheels effectively lower your gear ratio, meaning that your robot travels slower but with more torque, for the same drive shaft speed.
I don’t know about things like roller friction in the omnis because I haven’t used the small omni wheels yet, but I do know that they will be much easier to fit on robots that need lots of space for their intakes!

@Aura- So what youre saying is that the smaller wheels are better for torque? And by small wheels do you mean 2.75 inch or the 4 in? I was thinking that the 4 inch would be better.

Smaller wheels are better for torque, ONLY if you have not changed the gear ratio, meaning 1:1 will be slower but more powerful with smaller wheels. But everyone usually changes the ratio, so i dont believe this is much of a difference.

It makes a difference if it means you don’t want to put gears in for some other reason. Why add an extra level of complexity if you don’t need to? But yes, I agree, generally in games 1:1 is too slow anyway, for either wheel size.
However, choosing a particular wheel size may let you use a different gear ratio to still achieve the same speed and torque, which you would want to do if you wanted to use small gears due to space constraints, or shortage of parts or something.
So, wolfpackpride, you need to consider the combined effects of gear ratio and the wheel size you use.

By small wheels I mean the smaller, 2.75 inch ones :wink:

So what both of you are saying is that, if I want more torque o use the smaller omnis and also if i have size constraints. So is it better to use the bigger ones if I want speed, because I was think a 2:3, but I don’t really know cause I don’t think it have that much torque cause its going to wright around 15 pounds. What do you think?

It really depends on how many motors you have and what you’re trying to do with the robot. Does it need to be able to push things? Or is it for racing from place to place?
I recommend experimentation! A good starting point might be something simple, like 5:3 (60 tooth driving 36 tooth) and 4" wheels. Maybe 1:1 if you don’t have many motors. If you want more speed, try increasing that ratio. If you want less speed/more torque, try reducing it.

The relative speed/torque from the different wheels is purely based on their size. For example, at 1:1 straight off the motor, 2.75" wheels have 4/2.75 = about 1.5 times the torque given by 4" wheels; and 2.75/4 = about 0.7 times the speed.
In other words, to use 2.75" wheels in place of 4" wheels, you would need a gear ratio of about 3/2 in order to get the same speed and torque characteristics.
Experimentation tip - use chain instead of gears if possible, so all you need to do is change the sprocket size, rather than having to move motors every time you want to change gear ratio.

Hope this helps!

Ok, for the picky people out there…
Actually the torque is the same for both wheel sizes, measured as force times distance… The distance (wheel radius) changes, therefore the force changes, the torque is the same.
But it still has the same effect. Smaller wheels mean more force, or greater ability to push things, but a lower speed. Larger wheels mean less force but more speed.

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Other effects of large omni vs small omni:

  • Exothermic hypothesis: small wheels accelerate faster
    – side-effect: less time at near stall condition for motors means less likely to trip the circuit breaker

  • Chrysler hypothesis: move wheels at the corners for better handling, smaller wheels can contact the ground closer to the corner of the sizing box -> bigger footprint -> less likely to tip.

  • footprint: Smaller wheels have smaller footprint, probably sink more deeply into foam on heavy robots, which is probably bad. On the plus side, there is room for more of them. Titan team 1103 had 3 x 4" wheels per side for Roundup. I’ve seen 4 or 5 x 2.75" wheels per side somewhere.

Here are the equations that govern this behavior:

W = wheel rotational speed [Revolutions Per Minute, or RPM]
Wm = Motor rotational speed [RPM]
V = robot linear speed [in/sec]
D = Diameter of the wheel [in]
R = radius of the wheel (D/2) [in]
T = Torque at the wheel [in-lbs]
Tm = Torque at the motor [in-lbs]
F = Force that the wheel has available to push along the ground (pushing force) [lbs]
eff = efficiency of my gear train (it is safe to use .95 per gear stage) [unitless]
GR = Gear ratio. [unitless] A gear ratio greater than 1 is defined as Torque increasing and speed reducing. In other words, if my gear ratio is 2, then my wheel is spinning at 1/2 my motor speed.

Must convert to consistent units first:

W [rad/sec] = W [RPM] * 2*pi [radians] / 1 [Revolution] * 1 [minute] / 60 [sec] --> W [rad/sec] = W [RPM] 2Pi/60
W [rad/sec] = Wm [rad/sec] / GR
V [in/sec] = W [rad/sec] * R [in/sec]
T [in-lbs] = Tm [in-lbs] * GR * eff
F [lbs]= T/R --> F = Tm [in-lbs] * GR * eff / R [in]

So for the same gear ratio, the robot will have more pushing force with a smaller wheel, but will have less robot linear speed with the smaller wheel.

NOTE: I added units for clarity as some people messaged me regarding why Pi was nowhere in y equations. When you deal with angular units (angle, rotation speed, and rotational acceleration) in radians, then Pi takes care of itself. The equation of V = W * R assumes you are using angular units of radians. 2Pi radians is equivalent to 360 degrees or 1 revolution.

Through reduced rotational inertia.

Actually, this is not as obvious as you think. Moving wheels out to the edges of the envelope increases track (width) which somewhat improves stability, but, in the case of Chrysler, has a lot more to do with looking cool than it does to do with handling. In the case of VEX robots with tank steering, the ratio of track to wheelbase DOES affect the forces required for turning. The wider your track and the SHORTER your wheelbase, the easier it is to turn.

Increasing the wheelbase (the distance between the axles) in a traditional skid-steer VEX robot improves the stability and reduces the tendency of tipping fore and aft, but it also makes turning more difficult. Exothermic robot builders started using 2.75" wheels to improve packaging and reduce tipping. For turning, it had the opposite effect – it makes turning harder, and would not be possible without omniwheels. The Exothermic 2009 Elevation robots had six-motor drives to overcome problems with turning, not to make them faster. The shorter the wheelbase, the quicker a vehicle will turn, all else being equal. In a car, making the wheelbase longer reduces pitching*, which improves ride, but it also makes the turning radius wider, which is generally undesirable. Engineering is all about trade-offs. :slight_smile:

  • This is a gross over-simplification, but I’m not really an automotive suspension engineer, and occupant comfort isn’t a design criteria for VEX robots.

you can use this handy speed chart to determine the speed for the wheel you are using
(cant find the links for the HS sprockets)

(cant find the links for the HS sprockets)

Spur Gears
[High Strength Chain


(cant find the links for the HS sprockets)

so. is that information on the chart correct? Also, When someone says for example, 6:30 which gear is motor driven?

Yes. It isn’t measured experimental data, it’s just calculated from the gear ratios and wheel diameters. If you wanted, you could check it yourself.

What that means is that the speeds given are theoretical speeds for when the motors are spinning at their free running speed of 100rpm. They are useful for comparing how geared-up your drive train is relative to drive trains with different wheel sizes, but they aren’t much use for determining how fast it will actually go.

Different people mean different things. If they don’t specify it’s often because they think you can work it out from context - for example drive trains are almost invariably geared up (i.e. for speed) and arms are geared down (for torque).

The fact that there is no consensus is a bit of a pain, but if you always explicitly say “for speed” or “for torque” people will understand you.