what is a mecanum drivetrain

A standard mecanum drivetrain consists of four mecanum wheels oriented in an "X" shape, where the rollers are angled toward the center when viewed from above . This configuration allows the robot to combine the force vectors generated by the offset rollers to achieve movement in any direction . Because the wheels must be powered independently, there is no redundancy in the system .

Performance and Characteristics

Movement: Due to friction, perfect movement in all directions is not possible; the drivetrain drives slightly faster forwards and backwards than in other directions, and combining translation with rotation slows movement .
Advantages: It provides excellent maneuverability and agility through strafing, has good acceleration, can achieve high top speeds, and is highly versatile .
Disadvantages: It suffers from low traction because the rollers have a lower coefficient of friction than traction wheels, cannot traverse terrain, and is susceptible to being pushed around on defense .

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide

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One of the most important features of a mecanum wheel is the mechanism that allows the roller to spin---either bushings or ball bearings. Mecanums which use ball bearings strafe better than those which use bushings, as the rollers can spin with less friction.

Note: In the past, some teams have invested in 6 inch diameter mecanum wheels. These are generally much more expensive and generally do not provide significant performance benefits. We highly recommend sticking with wheels that are between 3 and 4 inches in diameter.

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Mecanum TeleOp

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Note: Some common COTS mecanum drivetrain kits are the goBILDA Strafer Chassis Kit (https://www.gobilda.com/strafer-chassis-kit-v5/) and the REV Mecanum Drivetrain Kit (https://www.revrobotics.com/rev-45-2470/).

A standard mecanum drive configuration possesses 4 mecanum wheels oriented in an "X" shape. This means that the rollers are angled towards the center when looking at it from above. This configuration allows one to add up the force vectors generated by the offset rollers and derive movement in any direction. It is important to note that because of friction, perfect movement isn't possible in every direction, so a mecanum drivetrain will be able to drive slightly faster forwards/backwards than any other directions. Combining translation and rotation will also result in slower movement.

In the image above, vectors 1, 2, 3, and 4 are the force vectors created by the mecanum wheels when the chassis is instructed to drive towards the top of the image. All motors are driving forward. The blue and red lines are their X and Y components, respectively. Here are a few examples of how the wheels must be driven to achieve different movements:

Attention: It is strongly advised to not hardcode these movements in; there is a much better way described below that allows for true holonomic movement and is much more elegant.

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide > Not Recommended

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There is almost no reason to use these mecanum wheels---they perform very poorly and are not much cheaper than those in the Recommended section. We cannot recommend the use of these on an FTC robot, if at all possible replace them with mecanums from the Recommended section.

TETRIX Mecanum Wheels (https://www.pitsco.com/TETRIX-MAX-Mecanum-Wheels) ($149.00): These mecanums are designed with a bushing based, hard plastic roller which in turn creates poor strafing performance. The integrated hub is a set screw based design with a round bore, causing unreliability as well as taking up extra space.
Andymark Standard Duty Mecanum Wheels (https://andymark.com/products/4-in-sd-mecanum-wheel) ($77.00): These wheels barely strafe and are super fragile.

(Figure: 8103 Null Robotics, Rover Ruckus, using Nexus mecanum bearing)

(Figure: 731 Wannabee Strange, Rover Ruckus, using AndyMark HD mecanum wheels)

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide > Recommended

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These wheels are the gold standard of mecanum wheels, if you are buying new mecanum wheels there is very little reason to buy any mecanum wheels not from this section.

goBILDA Mecanum Wheels v2 (https://www.gobilda.com/96mm-mecanum-wheel-set-70a-durometer-bearing-supported-rollers/) ($127.49 with team discount): The goBILDA v2 Mecanum Wheels are the gold standard for teams wanting a quality design in all aspects. They feature a 96mm diameter and a thickness of 38mm, with ball bearing supported, 70A durometer rollers that provide excellent strafing performance. With the use of recessed 16x16mm and 32x32mm holes, these wheels have the largest amount of mounting options and can fit all shaft styles commonly used in FTC®.
REV Robotics Mecanum Wheels (https://www.revrobotics.com/rev-45-1655/) ($127.50 with team discount): The REV Robotics Mecanum Wheels come in at only 75mm in diameter, providing a smaller mecanum wheel option to FTC teams; however, they are not the thinnest option at 40.8mm thick. They feature ball bearing supported rollers that provide exceptional strafing capabilities and traction. The hole pattern featured on these wheels is compatible with Andymark Nubs as well as the REV Robotics Universal Hex Adapter v2 (which is included); however, in order to use other shaft options the use of an adapter or physical modification will likely be required.

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Holonomic Drivetrains > Mecanum Drive > Advantages

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Fantastic maneuverability and agility due to strafing, can avoid defense very well
Good acceleration, can have high top speed
Very versatile drivetrain for nearly any game

Suffers in traction, as mecanum rollers have a lower coefficient of friction than traction wheels; cannot traverse terrain
Able to be pushed around on defense
Wheels must be powered independently, so there is no redundancy

CAD Examples of Mecanum Drive (Click to expand) goBILDA COTS Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/df9bcea72fcdd7e4aee4134b)

REV COTS Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/b8d4a6cfe4f26c5170f40a6c)

goBILDA Custom Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/cc2df2fbce0e9ca393696b54)

REV Custom Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/40c3266dab2e444011cce79c)

(Figure: Configuration for mecanum wheels, courtesy goBILDA)

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide > Viable

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These mecanums have acceptable performance but there is very little reason to buy them at this point, as they have been superseded by better ones. If you already own them, they are a viable option, but consider looking at some of the mecanums from the Recommended section.

Nexus Ball Bearing Mecanum Wheels (https://www.robotshop.com/en/100mm-mecanum-wheel-set.html) ($134.00): These wheels are 100 mm in diameter and 59 mm wide. They strafe excellently because of the use of ball bearings. However, they are more expensive, take up more space, have less traction, and are harder to mount to than the recommended wheels.
goBILDA Mecanum Wheels v1 (https://www.gobilda.com/96mm-mecanum-wheel-set-70a-durometer-bearing-supported-rollers/) (discontinued): These wheels are very similar to the Nexus Ball Bearing Mecanum Wheels, but with a different color scheme and better mounting options.
Andymark Heavy Duty Mecanum Wheels (https://andymark.com/products/4-in-hd-mecanum-wheels) ($225): These are easily the most expensive mecanums on the list. These wheels are 4" in diameter and 1.65" wide. These are bushing based mecanums, so while they strafe decently they still perform worse than bearing based ones. They have a good amount of traction, more than the Nexus bearing or bushing wheels.
Nexus Bushing Mecanum Wheels (https://www.robotshop.com/en/100mm-aluminum-mecanum-wheel-set.html) ($80.00): This is the Nexus Ball Bearing Mecanum Wheel with bushings instead of ball bearings. There is not much to say about them except that they strafe decently but worse than the ball bearing based equivalent.

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