Frictional Discrepancies

[usbcd36]

It does make a huge difference, though. If the numbers from AndyMark are correct, it means inline > transverse, which makes skid steer a reasonable choice. If the numbers from FIRST are correct, it means the opposite.

[AndrewN]

Doing an incline plane test with four wheels and a frame we found almost no difference between the lateral and transverse static COF as measured by the angle of the incline. We are using the correct surface too.

Just to make it clear:

1. Point the locked wheels down the incline and gradually raise one end of the surface until the frame breaks free slides down. Measure the height at which the frame breaks free (8.5" over a 6ft sheet). This is a measure of the lateral static COF (tan of the angle between the horizontal and the incline). Our result is around 0.12 or an angle of 6.7 degrees.

2. Turn the frame 90 degrees. The wheels are now sideways down the slope. Repeat test. This is the transverse static COF.
Our result 8.5 - 9". Almost the same as the lateral value.

We expected that the heights of the two tests should almost be a factor of two or more different given 0.6 and 1.4 as the printed static COFs.

Can other teams please repeat this test and report the angles they are finding for both lateral and transverse static friction.

[martin417]

As an engineer, I have an issue with the "lateral" and "inline" numbers. On an ideal surface, and the interaction between the wheels and "regolith" this year com as close to ideal as you can get, friction depends ONLY on friction coefficient, and normal force. There is no directionality component. I looked at the wheels, and went to Home Depot and looked at the surface. I can see no reason, theoretical or otherwise, for a difference. As AndrewN's testing shows, in-line and lateral should be identical.

[Bongle]

Originally Posted by martin417

As an engineer, I have an issue with the "lateral" and "inline" numbers. On an ideal surface, and the interaction between the wheels and "regolith" this year com as close to ideal as you can get, friction depends ONLY on friction coefficient, and normal force. There is no directionality component. I looked at the wheels, and went to Home Depot and looked at the surface. I can see no reason, theoretical or otherwise, for a difference. As AndrewN's testing shows, in-line and lateral should be identical.

My team thought this was weird as well. Our theory is that since the wheels will mostly be spinning forward and back more than they'll be sliding sideways (ignoring any holonomic-type drives), then they'll tend to get heavily scuffed in a forward-back direction. So perhaps after a few matches worth of use, you might see the kind of transverse coefficients that the manual describes.

[rfolea]

Originally Posted by AndrewN

Doing an incline plane test with four wheels and a

Can other teams please repeat this test and report the angles they are finding for both lateral and transverse static friction.

We measure COF by drag - we simply drag the load with a scale attached on a flat surface.

We are getting around .1 in either direction, dynamic. Static was sightly more (.12 I think).

There is no noticeable difference between dragging it sideways or not with the wheels locked.

[Matt C]

My only guess is the testing may have been done on the wheels with the mold lines still on them (not worn)?

Could this be trying to dig into the field material, thus raising the effective CoF?

[writchie]

Originally Posted by AndrewN

Doing an incline plane test with four wheels and a frame we found almost no difference between the lateral and transverse static COF as measured by the angle of the incline. We are using the correct surface too.

Just to make it clear:

1. Point the locked wheels down the incline and gradually raise one end of the surface until the frame breaks free slides down. Measure the height at which the frame breaks free (8.5" over a 6ft sheet). This is a measure of the lateral static COF (tan of the angle between the horizontal and the incline). Our result is around 0.12 or an angle of 6.7 degrees.

2. Turn the frame 90 degrees. The wheels are now sideways down the slope. Repeat test. This is the transverse static COF.
Our result 8.5 - 9". Almost the same as the lateral value.

We expected that the heights of the two tests should almost be a factor of two or more different given 0.6 and 1.4 as the printed static COFs.

Can other teams please repeat this test and report the angles they are finding for both lateral and transverse static friction.

Perhaps you have discovered what's behind the "fish" clue

The transverse/inline ratio is a very critical parameter. If it's no where near the 2.3 advertised, then many preliminary design decisions about drive configuration will be dead wrong. We will try to confirm your findings as soon as we can locate the actual surface material.

It is possible that the ratio changes significantly with normal forces closer to 1/4 of the nominal weight of the robot due to the way the materials deform under load. It could also be that the type of backing underneath the regolith is a factor. The wheels are very hard and provide a very small contact area. If the backing is carpet (rather than a very hard material), there could be a small depression that presents differently in transverse and longitudinal directions. Your numbers may reflect light loading, before such effects manifest themselves. Based on your data it does looks like we will need to confirm the Mu values under a range of loads.

Does anyone know whether the regolith is over carpet?

Good catch.

[SWIM]

Originally Posted by writchie

Perhaps you have discovered what's behind the "fish" clue

The transverse/inline ratio is a very critical parameter. If it's no where near the 2.3 advertised, then many preliminary design decisions about drive configuration will be dead wrong. We will try to confirm your findings as soon as we can locate the actual surface material.

It is possible that the ratio changes significantly with normal forces closer to 1/4 of the nominal weight of the robot due to the way the materials deform under load. It could also be that the type of backing underneath the regolith is a factor. The wheels are very hard and provide a very small contact area. If the backing is carpet (rather than a very hard material), there could be a small depression that presents differently in transverse and longitudinal directions. Your numbers may reflect light loading, before such effects manifest themselves. Based on your data it does looks like we will need to confirm the Mu values under a range of loads.

Does anyone know whether the regolith is over carpet?

Good catch.

From how I interpreted the rules, the entire field is covered in carpet, and the regolith is placed over that.

[Ozeaden]

One thing that my team found out is that if u wear down the wheels, you get better traction. With having a rough tire, it will give more of a stick to the flooring material. We tried it on one of our past robots and it worked really good. Just run the wheels on asphalt and run it down a bit. Its not against the rules at all.

[comphappy]

Originally Posted by Ozeaden

One thing that my team found out is that if u wear down the wheels, you get better traction. With having a rough tire, it will give more of a stick to the flooring material. We tried it on one of our past robots and it worked really good. Just run the wheels on asphalt and run it down a bit. Its not against the rules at all.

Read <R06> and I think you will find you are very wrong, and are now out $100 in usable wheels. This is intentional damage, which is explicitly prohibited by that rule.

[SWIM]

Originally Posted by comphappy

Read <R06> and I think you will find you are very wrong, and are now out $100 in usable wheels. This is intentional damage, which is explicitly prohibited by that rule.

I'm sure they'll get away with it, unless they're worn down to the point where it's obvious they were doing something far beyond just driving the 'bot around. But really, getting grooves in the tread shouldn't change the coefficients of friction much at all, since the surfaces are so hard.

If they do try and cheat to get more traction out of their wheels, that's a pretty rotten thing to do, but I don't think other teams will have to be too concerned with them somehow getting considerably more traction with worn down tread.

[Al Skierkiewicz]

Roughing wheel surface by driving on abrasive surfaces is already under discussion in inspection circles. Read R06 very carefully and watch the Q&A and Team Updates for anything that pertains to wheels. Conjecture in these fori is a personal opinion, the GDC is the official, last and final word.

[IKE]

Originally Posted by Al Skierkiewicz

Roughing wheel surface by driving on abrasive surfaces is already under discussion in inspection circles. Read R06 very carefully and watch the Q&A and Team Updates for anything that pertains to wheels. Conjecture in these fori is a personal opinion, the GDC is the official, last and final word.

If you get word on Policing procedure, that would be great. I know we are planning on buying extra wheels. Having practised drivers I feel will be really impoartant, and doing a full floor will cost around $2K. We have a large open smooth tile floor (like most HS cafeterias), but I am worried about picking up a little grit while pracitising.

[Peter Matteson]

Originally Posted by Al Skierkiewicz

Roughing wheel surface by driving on abrasive surfaces is already under discussion in inspection circles. Read R06 very carefully and watch the Q&A and Team Updates for anything that pertains to wheels. Conjecture in these fori is a personal opinion, the GDC is the official, last and final word.

I'm interested in how this comes out because I believe many teams will practice on carpet, tile or concrete floors because they can't afford a full field. This will leave many teams with interesting wear on their wheels when they get to their first regional.

Al, I don't envy the job of yourself or the other head inspectors this year.

[Al Skierkiewicz]

Originally Posted by Peter Matteson

Al, I don't envy the job of yourself or the other head inspectors this year.

Thanks, tell me again why I am doing this? This is one of many decisions your inspectors go through each year and which the GDC must answer. If you think the GDC just comes up with the game think again. My job is simple compared to the work they go through every year. Remember this game is the end result of months of work, testing and planning, and picking game pieces. And all done in secrecy so we all find out on the same day. Manhatten project comes to mind.