I noticed something yesterday that may have the potential to be a huge problem. I took our new AndyMark omniwheel and put it on a shaft next to a new IFI traction wheel (both 6") - its not by much, but they aren’t quite the same size. I went over to my other team and looked at the AndyMark trick wheel and IFI wheel (both 8") and found the same thing.
Minor differences are not much of an issue, but when set on the ground, the IFI wheel made contact and the AndyMark wheels did not. In a setup like ours (2 traction in rear, two omni/trick in front), all wheels will be in contact, but the robot will have a forward lean. I have not taken the time yet to see how much this throws the top of the robot forward (nothing too significant, I hope), but for teams that are pushing the envelope, this could lead to problems.
Again, this is just based on both wheels on the same shaft - our chassis is not quite complete (…a bit behind we are…) so I don’t know how well the four wheels sit on the ground (especially the carpet).
I’m wondering if the AndyMark traction wheels, when used with the trick/omni wheels, produce the same result, or if this is a result of two different manufacturers?
Bottom line - I hope that I am getting worked up over nothing. If there is a legitimate issue here, I hope that teams using a similar setup will see this thread and take this into account before going to the inspection sizing box and finding out that they need trim down their front edge a bit.
well the easiest thing you can do to quick fix this problem is to offset the larger wheel by 1/8" if thats what the diffrence is, but like you said if its nothing to major you shouldnt have anything to worry about
Yes, that is an easy solution if you catch it now. Its not so easy once you have a completed robot and have to make that change in the pits on Thursday. A team will have to drill a new hole accurately on both sides of the chassis, possibly recut chain, or even relocate gearboxes. As you can see, a simple fix can quickly become a massive undertaking, depending on the complexity of your robot.
If it were the other way around (IFI wheels are the smaller wheel), an extra layer of something (double tread might be TOO much) would do the trick, but the omniwheels being the smaller one makes it a trickier issue.
In the case of the 6" wheels, you could always see about switching to the AM traction wheels. I am confident that Andy could tell us if there is any noticeable difference between their traction wheels and omni wheels. I have no big ideas about how to solve the 8" problem.
I also noticed a similar thing, however it was with AM traction wheels… they appear to have been 6" before the tread was applied… but they are now bigger than the AM omni wheels.
The leaning robot issue I’m not too concerned about because it is a small amount comparatively. The main issue i can think of is one end (omni wheels) will be dragged along by the traction wheels as they move, IE they don’t travel the same distance in one revolution, if the wheels are linked together as ours are with chain to one transmission then either the smaller wheel will be dragged along, or the bigger wheel will have to spin a bit.
Are they the same size when you put weight on the traction wheels?
I’d almost guarentee that the difference is a discrepancy between the tread being fully compressed, and the wheel sitting on the ridges of the wedgetop, causing the omniwheels to not contact the ground. Either that, or the OD of the wheel without tread is 6" and the tread adds 1/8-1/4" or so.
The “problem” could very well go away once the robot is up to full weight. Like I said, these were just on the shaft. I really don’t think it’s going to be an issue in the long run, though. I hadn’t thought about the wheels dragging, but ours are not tied together, so that shouldn’t be an issue for us. Between the squishiness of the tread/omniwhees and the carpet, I don’t expect to see a lean on the field…just in the sizing box - where it matters.
I don’t have expierence with these specfic wheels but over the summer I learned through communication with Skyway that few if any of their wheels are actually the dimension they are described as, ie an 8" wheel could actually be 7 3/4. This is definatly something to take into account when using varying wheels on a robot.
I ran through a quick and easy CAD model and even your robot is 37.5" x 59.5" with a 30" wheel base, you should be okay. Therefore, HOPEFULLY no one will have a problem here, unless you are relying on perfectly level surfaces.
We would’ve liked to make our Omni wheels and Traction wheels the same exact diameter. The Omni-wheels are right at 6" and 8" while the Traction wheels are about 1/4" larger than their nominal diameter. The Traction wheels are purchased wheels that have been altered to receive a tread and driving hub. We cut a wide groove for the tread, but did not want to cut into the diameter of the wheel any more than we did, to save the strength of the wheel.
This is just one example of why you MUST NOT make engineering decisions based on assumptions. You should check/measure your parts and make appropriate design adjustments.
Our 10" X 3" Skyway pneumatic wheels (used this year and the previous two years - we LOVE 'em) actually measure approximately 10.25" diameter. We are also using 10" omni wheels, and our axle/frame mounting heights are different by the corresponding 1/8". Also since we are driving both from the same transmission, the sprockets are different by 1 tooth to give us similar “wheel surface” speeds.
