During my experience with FIRST, I have observed teams using several different wheels for their drive trains. One thing I have noticed is that teams with mecanum wheels generally choose large diameter wheels (8" or above) while teams with non-mecanum wheels choose generally smaller wheels (4"-6"). I am wondering what the reasoning behind this is. Obviously if you can get the same angular velocity on a larger diameter wheel, then that results in a greater linear velocity of the robot but besides that, I don’t understand the advantages of choosing larger wheels, specifically the reason for not choosing the 6" versions offered by Andymarkand Vex. Does the size increase improve the mecanum’s performance in some way? Do smaller mecanum wheels have a greater tendency to break?
Having used both 6’’ mecanums and 8’’ AndyMark mecanums extensively, I’ve had far better results with the former, both in terms of performance and quality of the product.
I don’t know why anyone would specifically go for larger mecanum wheels; this past year 4464 used 8’’ AndyMark mecanums because they were donated, but we honestly would have done better to just buy a set of 6’’ given the problems we’ve had with them.
I look forward to trying out 4’’ mecanums (probably in an octanum setting) when they (likely) become available.
Could you please give more detail about what you mean by performance and quality in this context?
I don’t know why anyone would specifically go for larger mecanum wheels; this past year 4464 used 8’’ AndyMark mecanums because they were donated, but we honestly would have done better to just buy a set of 6’’ given the problems we’ve had with them.
What kind of problems did you have with the 8" wheels that you did not have with the 6" wheels?
We always used 6" mecanums. Even in 2012 while crossing the bump. They are by far and away lighter and massively smaller profile than the larger variants. The large mecanum wheels also don’t look near as cool
In my experience, 6’’ wheels run smoother (less vibration) and track straighter while strafing (in the absence of a gyro).
We also had several defective 8’’ mecanum plates, and one of the backup wheels we purchased was so deformed that not a single roller could turn since they were all in contact with their neighbors. They’re also heavier, and the rollers jam far more often.
I’ve had no quality problems with the 6’’ models, ever, nor any major issues with their performance since 2008 (when the plate was flimsy and required constant maintenance to keep it from bending in on the rollers).
I’d just like to add my two cents on this.
We used 8" AndyMark mecanum wheels in 2010. We wrecked two sets over 7 events, after the constant bashing from climbing over the bump. I have no comparison for the 6 inch, as we didn’t try those, but draw your own conclusions about the 8" wheels’ durability.
Can anyone attest the the 6" VexPro mecanum’s durability as opposed to the AndyMark ones? We’d like to try those this year.
In 2011 we used the standard AM 6" mecanum wheels. In 2012, we used the HD wheels, due to the bump and related forces (even though after calculations, the standard wheel specs would not have been exceeded). After 2 regionals and an offseason, and extensive testing (our first year using mecanums) in 2011, we did not, and still to this day, have not replaced any of the rollers on the wheels. On our 2012 robot, after our 2nd regional we had to replace about 10 rollers, some from each of the 4 wheels. This was probably not due to any difference between the standard and HD wheels, but likely due to slight skidding on the ramps and testing on linoleum floors that wore down the rubber rollers over time.
I have not used any VexPro mecanum wheels, but I would trust the construction of the plastic plates, because of the rigidity other plastic VexPro products (ballshifter housing, versa-wheels). I do not know about the rollers on those wheels, but if they are similar in tread to a Versa-wheel, I would be careful and monitor the tread wear on your mecanum rollers. Damaged and missing tread will cause your robot to “hobble” as it bounces every time the wheel reaches the missing spot. Remember, a mecanum wheel is not a circle, it is a N-gon. With a missing vertex, irregularities occur. This will make actions such as strafing behave a little weird.
Could you please describe in a bit more detail what you mean by “wrecked”?
e.g. the plates got bent, or the roller axles got bent, or the rollers got chewed up, etc
It is possible to design a mecanum wheel whose profile is a true circle. For anyone interested you can find more info here, here, here, here, and here.
We used the 8-inch AndyMark ones and we were impressed. The wheels allowed us to not only move nicely and freely, but also allowed us to push other robots and sideload them! Our drivetrain was quite effective. On the other hand, I saw many teams with ginormous mecanums. I, myself, cannot understand why they did that. It only seems like more weight and it takes a lot of room! 
Why didn’t you use 6" mecanums then?
This is true. And the VexPro wheels are a good example. The problem arises when one (or more) rollers is removed (physically or due to wear), suddenly you have a round wheel with a flat spot. Like a train wheel.
http://www.tsb.gc.ca/eng/rapports-reports/rail/1996/r96t0095/r96t0095.jpg
(someone had a bad day)
Well, we tried the 8 inch wheels and they went past our expectations. So, we just stayed with that! 
Also, how does that happen to a train wheel? Does it go, Thump…Thump…Thump…?
Why is it red hot?
Having used both, I would recommend you at least consider the 6’’ models, if only for the significant weight decrease.
I will talk to our team captain about that!
The funny thing is that every Wednesday (robotics day), I enter the classroom and blast the mentor and the captain with outrageous ideas:D. That’s just me, the curious one
edited for truth: That’s just you, the team Chief Delphi prowler.
Yeah. I agree. However, it should be the curious Chief Delphi prowler! I have been going through every post, but have been posting a little less these days ;), especially because finals are coming 
Yeah, and those lines of green faces have disappeared!
^^Let that be an exception^^
Rarely do train wheels get that bad, but there have been several times I’ve ridden on commuter trains (Metro North and MBTA) that have minor flat spots. At 80mph, it sounds like tick-tick-tick-tick-tick-tick-tick-tick-tick and feels like someone keeps tapping your chair. Generally they are only noticeable on track that has continuously welded rail instead of the outdated 39ft sections (which itself makes the old clickity-clack sounds).
Lol, I wonder how bad it is in India! Most of the transit is by trains, and with the meagre funds and little income, I am pretty sure they do not do much maintenance, especially in the regular cabins, without even an A/C ^~^ (Crabby face. CD is missing that :()
I thought most trains can easily handle those speeds!
^ (2 posts up [artdutra04]) you are a VEX engineer, surely at some point you guys compared the leading competition’s (AndyMark) products to your own, to create a competitive product. Obvious bias aside, what do you personally think is better?
(and don’t worry, your opinion is safe here)