# Is there a way to improve the 3.25 inch omnis' grip on the platform?

that’s a very surprising result, I wonder why additional weight would cause a decrease in traction?

I’m no physics expert, but some googling tells me that the formula for the tractive force applied by a wheel on a surface is:

F = μt W

where μt is the coefficient of friction between the wheel and the surface (which in the scenario of using only 3.25" omni wheels, should remain constant), and W is the weight (or vertical force) applied on the wheel.

So physics tells us that as weight increases, the tractive force should also increase.

however, there might be other factors at play here such as the pivoting center of mass causing the robot to want to tilt backwards, taking weight off of the front wheels and onto the rear wheels, which would probably reduce the traction of the wheels on the platform.

this is true when all 6 wheels are contacting the platform, but if the issue is with climbing the platform, the middle wheels would be lifted momentarily into the air and not contribute anything to the traction of the drive. If you can make to the point where all the wheels of the robot are contacting the platform though, this should help.

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This is not the case for the platform. When on an incline, the traction is a function of the angle the robot is at. So, really, it’d be `F = μ • Fn` where `Fn` is the normal force on the robot from the platform. In this case, the normal force is equal to `cos(Θ) • W` where `W` is the weight of the robot and Θ is the angle of the robot relative to the ground. Basically, the steeper the incline, the less traction

This simplification assumes the robot is in continuous contact with the platform and friction coefficent is constant.

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Like you said, I assume the reason this happens is because the bot is trying to go uphill rather than a flat surface so it also needs to overcome its own weight with the wheels’ friction rather than having the weight be purely increasing effective friction.

We did have a 6 wheel drive with all wheels being powered, but even when the bot had all 6 wheels fully contacting the platform, it still wasn’t enough to consistently drive up or even stay on the platform. Poor weight distribution also could’ve played a role as we had 3 out of the 4 goals held at/behind the rear wheels which could’ve caused the front wheels to have less traction.

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Yeah depending on where that weight is it could actually hurt more than it helps

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This is an added effect of the incline. So not only does it decrease the robot’s traction, but the robot also has to increase its gravitational potential energy some amount. Thinking of it in terms of energy is an elegant way of visualizing what’s going on, otherwise, you need to do fairly complex free body diagrams to account for all the other variables. The work needed to do this is significantly more than just translating across a level surface.

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If you think about it, the formulas for normal force, reactive friction force in the drivetrain, friction converted to traction force, or force required to move the robot forward uphill - they all look like this: F = W * some_coefficient, where the coefficient depends on various things, like drivetrain properties, slope incline, etc…

The important point is that, if system behaves linear, and you write an equation to compare, let say, force required to push robot uphill against the force of max traction you can get, given the robot weight, then Ws on the left and right sides will cancel out.

This means that it shouldn’t matter how much weight you add to the robot: increase in the force required to drive it uphill will be matched by the increased wheel traction.

So, if you have a counterintuitive case of wheels slipping after you add weight to the robot, it means that something is non-linear.

For example, adding more weight to the back of the robot will actually decrease the normal force on the front wheels and you will end up with front motors being underused and the power of the back wheel motors may not be enough to move the robot.

Do you have front and back wheels of the drivetrain linked to redistribute the power for such use cases?

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of course 20 characters

All sorts of stuff accumulates on the omni rollers’ surface and their little axles which makes them less grippy and harder to turn.

You’ve got to make sure they are clean - wash them in the warm soapy water and air dry well.

Also, 3.25" rubber is not as soft as for 4" and hardens over time - some people boil their omnis after a few years to get it softer again.

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yup
clean the wheels

you could do some tests before/after to measure the difference
would make for a good notebook entry

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How about doing locked omnis? Like Blank’s bot in turning point!

I think those would count as traction wheels

there is literally zero point to this as locking the rollers does not change the friction coefficient at all. Basically I’m looking for legal ways to increase this coefficient.
EDIT: grammar mistake.

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you cannot legally change the coefficient of friction of the 3.25" omni rollers.

is your center of mass roughly centered on your robot? if not, try climbing with the center of mass oriented towards the platform, sort of like how a biker would lean forward when biking up a hill.

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Using light pine tar on the wheels would increase their traction and reduce slippage and would be virtually impossible to detect, much like any naturally sticky substance that might be present on the tables, tools, etc

Applying pine tar to your robot’s wheels would be a clear violation of R6 and possibly also R4.

Applying pine tar to your robot’s wheels while knowing it’s illegal and hoping no one will notice would arguably also violate the REC Foundation Code of Conduct.

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If pine tar has antistatic properties it would be vrc legal under R8c:

I am not suggesting to boil your wheel rollers in a bowl of antistatic tar, but there is something irresistible appealing in that idea.

Seriously, as @Xenon27 said you, probably, have center of mass issue. If your robot is back heavy, did you try to drive up the ramp backwards to see if that helps?

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Using it to remove the crud/dust/hair from the wheels would be illegal? Isn’t it just a form of cleaning the rollers that typically are less prone to slipping when they don’t have residue from the field on them?

Ok actually I can see why my statement was misleading;

*because it is able to remove the crud stuck to the wheels that comes between the rollers and the surface.

*because in light amounts the stick won’t provide any significant value that the rollers don’t otherwise have when they are clean. For example, if the wheels contact a sticky surface on the table, it is hard to detect because of how little it impacts the traction. This is why I said:

Light pine tar pulls general smudge off the wheels that interferes with the traction between the wheel and the surface, while having minimal impact on the wheel. Used lightly it can be effective in this regard, but to use it to the extent that it is inherently better than a clean, fresh rubbered omni with no pine tar would be quite obvious due to the marks it would leave on the tiles

Assuming it did have anti static properties, which I’m sure it does not, to use it to an extent that it would actually be useful would very likely leave marks on the field which would be obviously illegal.

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Ok, I had read that and thought, “That doesn’t sound like @JD6…”

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If you’re just using the pine tar to clean the wheels (i.e., return them to the same coefficient of friction they had when they were new), and no pine tar remains on the wheels after you’ve done that, then I think that’d be OK.

If you’re applying pine tar to the wheels as a sticky material that will stay on the wheel and thus increase its coefficient of friction, that would violate R6.

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That’s definitely the idea, but it isn’t unreasonable to expect a small degree remain on the wheel, much like if the wheel was to rub against a table that had glue on it. The robots don’t exist in a perfectly isolated environment

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