There aren’t multiple threads on wheel size but I haven’t found any on wheel width. Is there a benefit to using 1” wheels over 1.5 wheels or 2” wheels over the latter 2? Why might you choose one width over another?
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By thickness, you mean width, and not something like tread or rim thickness?
That’s correct, I fixed the title of the thread
If you accept the common wisdom that wheel force is proportional to the contact force, wheel width makes no difference at all. There are a number of teams at the high end who do not accept this wisdom, with some success. Based on this anecdotal evidence, I suspect that selecting a tread material and pattern which grips the carpet (or other surface) well is the most important thing; wheel width/size of contact patch appears to be a secondary consideration.
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In 2014 we used the VexPro VersaWheel’s “W” tread. Most teams report getting one event or less before they wear out (most recently: What type of wheels does your team use?). We put two of them side by side, doubling the width, and they barely wore. We didn’t replace them through the whole competition season (2 regionals and championship).
The sort of, er, general consensus, if you will, from the anecdotal evidence, is that wider wheels generally have a slight increase in traction compared to narrower wheels overall, even in 2009 (when everybody used the same wheel–some teams reported afterwards that doubling their wheels widthwise increased their traction slightly).
I believe some teams have actually tried to research the question, but I can’t recall if their results were ever made public or not.
The dubious physics behind the wider wheels having more traction appear to have something to do with why wide Velcro is harder to get off than narrow Velcro–there’s more roughness in contact to peel away when the wheel rotates (when magnified in the case of the treaded wheels or Lunacy wheels), so the effective coefficient of friction goes up just a little bit. Maybe .01 as a wild guess. However, the wider wheels also, as Joe pointed out, take longer to wear out their treads, keeping them fresher longer. Going up against other robots in their 2nd or 3rd event, if they haven’t replaced their treads, you have an advantage until they figure that out.
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Traction is certainly a big factor to consider. Physics (with its associated assumptions) says that wheel width has no affect on traction. I think that is a bit of an over-simplification as there is a lot of details at the contact patch that can have an effect on the real traction of the wheels.
But traction is not the only consideration when selecting wheels. With swerve, wheels at the narrower end of the spectrum have the advantage that when you are not moving, you can spin the module with minimal scrubbing and associated wear.
For this reason we have settled on the 4" Colson wheels with the 7/8" tread width for the past 2 seasons. We have been involved in numerous pushing matches during that time and have not had any issues with traction of those wheels. Anecdotally, we have been able to push other robots sideways. I am not sure what type of wheels those other bots used, but I would attribute our ability to push them sideways more to the weight of their robot compared to ours than anything else. We are right at the max weight limit.
The wheels hold up reasonably well. We did not replace any wheels in 2018, but by the end of the year, the tread was worn through to the hard, black plastic in numerous places on most of the wheels. We just replaced the wheels on our 2019 bot this past week.
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To state (perhaps) the obvious: 1in wheels are lighter and package into a smaller footprint than 1.5 or 2in wide wheels.
There can be game-specific advantages to different sizes. For example: in the 2016 ‘rough terrain’ feature there were gaps up to 1.75in wide (or something like that), so we used 2in wheels to avoid getting stuck.
Usually wheel width is a tradeoff of weight and size vs traction and wear rate.
Here is a generalized plot for car tires. I suspect the same behavior applies to other polymers, like FRC wheels. Note the idealized dotted line.
Some of the mechanisms are explained well here: Tire Technology, excerpt from The Racing & High-Performance Tire
There is also an argument to be made that larger diameter wheels have more traction than a smaller diameter wheel with the same tread and width. Larger diameter = larger contact patch = lower contact pressure = better µ. I thought a team did testing on this, but I can’t find the information.
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I know some teams double up wheels such as the vex pro Versa wheel (spike tread) because it helps slow down wear. I’ve heard of similar results with wider colsons. This all makes sense to me, although the results are probably not linear. Anything over 2 in is probably overkill.
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This anecdote about 2009 is true, we used wheels stacked four wide and had reasonable traction compared to a one-wide setup.
Gosh it’s crazy that it’s been that long now
I did some research on the topic and like most of others said, the wheel width is not the reason for greater traction. The reason wider tires are used (in racing) is to have greater slip angles to help cornering and softer compounds needing wider tires which I don’t think has much effect in FRC. The other thing I read that makes sense is reduced wear. Since the wheel is skinnier, it would wear much faster since the force on the contact patch isn’t as spread out as it would be if the wheels were wider. It seems as though skinnier wheels help with turning as well since it’s the opposite of the racing car analogy. So another question I have is, would it be fine to have different wheel thicknesses? It seems as though 2" omnis and then a 1" traction in the middle would be weird but it could work. Right? I hope what I’m saying is making sense.
In FRC, anecdotally, wider wheels DO have more traction. It’s noticeable. See: Two posts above yours. However, nobody’s been able to a) quantify how much traction wider wheels give AND b) provide an explanation. There’s an explanation that works (see my last post for the non-illustrated version), possibly more than one, but no quantitative data that’s public.
I don’t dispute your racing reasons, but I recall a pretty heated thread back, er, a few years (like, '09/'10 time frame) where the same arguments came up. Racing was brought up then, too (specifically dragsters) and it was pointed out that not only are we talking softer compounds, but heat dissipation comes up. Not a phenomenon we see in FRC.
Reduced wear does help out a lot.
You’re not making sense at all with “skinnier wheels help with turning […] since it’s the opposite of the racing car analogy”. Please clarify. (If you’re referring to the greater slip angles to help cornering–there’s almost literally no comparison at all. MOST FRC robots don’t spin their wheels to turn the robot.)
As far as your last question… You’re correct, it would work just fine. Wouldn’t use omnis on both ends of the drive unless you want to be spun every time a defender looks at you funny, though. Try one on each side of the robot, it’ll resist being turned just a little bit better–and that’s just a characteristic of omnis, nothing to do with their width.
Uh… what? Generally, wider tires operate effectively at lower slip angles than otherwise-comparable narrower tires. The wider shape of the contact patch, straighter sidewalls located further apart, and flatter shape of the tread and sidewalls tends to limit the compliance of wider tires compared to narrower tires. Usually the smaller slip angles of wider tires are preferable because they improve the response time and feel of a given car’s handling.
My race team used wider tires for the following reasons, demonstrated over thousands of laps and a dozen different drivers:
-More grip, observed as faster lap times
-Longer tire life, observed as more laps per tire set
-Cooler tire temps, observed as less tire chunking and consistent performance lap-to-lap and in varying ambient conditions
But, again, wider tires usually result in more grip because of load sensitivity. Why does tire width matter here? All else being equal, wider tires will experience a smaller change in contact pressure compared to a narrower tire for the same change in load. Smaller pressure change = higher coefficient of friction = better performance.
Is this always true in racecars? No. You can go too wide, to the point where the tires can’t get into a happy operating temperature or other effects ruin your day. But generally speaking, and within practical limits, wider tires provide more grip than narrower tires.
Kind of? This goes more to the wear rate side of things. Wider tires simply have more tread to wear away, so going wider with a softer compounds leads to the same lifespan of the tires. It can also be important to have wider tires with soft compounds if the compound is failing cohesively before losing traction, but that is not a common failure mode.
If your argument is one based in slip angle then this conclusion is completely erroneous. Tank-drive does not really have slip angle in any meaningful way. Slip angle occurs primarily as a result of compliance in an air-filled tire. Most FRC wheels are totally rigid and thus can’t really generate a slip angle. Even if a robot had pneumatic wheels one would not be concerned with lateral grip until the slip angle was 90deg (being pushed sideways) which is a whole other ball of wax.
Skinnier wheels will be easier to scrub sideways during a turn because they have less traction than wider wheels due to load sensitivity.
It is not.
There are a lot of other factors that will affect tire performance (wheel width, alignment settings, damper settings, spring selection, inflation pressure, vehicle weight and weight distribution, compound selection, etc), so proceed with caution when investigating this area.
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