Tire model improvements

[jlv]

Are you guys sure it isn't "moo"?

Forum suggestion: Add Latex support.

MathML might be possible. I don't know a whole lot about it but if it's simple enough I might be able to just add some tags for it.

How would you go about empirically obtaining a value for μ when the tyre is angled and sliding laterally? Surely the lateral dynamic friction will have some dependency on the velocity of the wheel in the longitudinal direction, as a spinning tyre will have a tendency to cut through the dirt and lower the μ value? Or is there no such effect and it's purely a resultant from lateral and longitudinal frictional forces? I'm now imagining a big box suspended on sliding rails with a wheel attached to a variable speed motor which can be rotated to any angle.

I suspect that longitudinal/lateral thing is distracting from other more important factors. If you think about it, there are two things that should really matter. The speed of the bottom of the tire relative to the ground, and the area of fresh ground it's covering per step. A stationary burnout with the tire moving at 20 f/s should probably be different than a bike moving at 20 f/s with the rear brake locked. I think you'd be able to get both right with those two variables. Other situations would just be variations of those extremes.

[cpt_Slow]

I'm not sure enough maths is done on this forum to make it at all worth it. Alt-0181 saves worrying about mu though. If I'd not seen it written as mu I'd probably write it as 'mew' since that's closer to how I pronounce it...but maybe I'm just strange.

[quote= 'jlv']I suspect that longitudinal/lateral thing is distracting from other more important factors. If you think about it, there are two things that should really matter. The speed of the bottom of the tire relative to the ground, and the area of fresh ground it's covering per step. A stationary burnout with the tire moving at 20 f/s should probably be different than a bike moving at 20 f/s with the rear brake locked. I think you'd be able to get both right with those two variables. Other situations would just be variations of those extremes.[/quote]

That's a fair point. So is the thinking that a locked wheel will build up dirt in such a way that µ increases and a spinning wheel pushes the dirt away, reducing the value of µ? µ becomes a function of the speed of the contact point of the tyre relative to the ground, and the speed of the wheel pivot in the x-z plane?

[jlv]

I'm pretty sure the Greek letter μ originates from the Egyptian glyph for a cow, hence the slight resemblance of the character to a cow's head and the "moo" spelling and pronunciation.

That's a fair point. So is the thinking that a locked wheel will build up dirt in such a way that µ increases and a spinning wheel pushes the dirt away, reducing the value of µ? µ becomes a function of the speed of the contact point of the tyre relative to the ground, and the speed of the wheel pivot in the x-z plane?

Pretty much. I figure that's the area of dirt it would have to break away and displace. Of course, it depends on the soil. If it's sand it'll just keep digging so maybe it wouldn't be a big difference there.

[Phathry25]

You're misunderstanding how the traction settings work. Using the previous example -

friction 1.0 2.0
friction 0.5 3.0

This would give you 1.0 as the static moo. Then for the kinetic moo, it would use 1.0 until it was slipping at 2 f/s. Then it would gradually go down to 0.5 as it reached 3 f/s and stay there as it slid faster. You can make an approximation of any curve you like this way.

I understood it interpolated between points, just did a crappy job of explaining that.

So if I only set my friction to

friction 1.0 2.0

I would always have a friction coefficient of 1.0 even after the tire started slipping? The only change in grip is due to the load being applied to the tire?

I think the problem with the slip speed model is that the friction doesn't scale with the speed. So if the bike is going 100 f/s and the tire is slipping at 2f/s it's not a big deal. But if the bike is going 10 f/s and the tire slips at 2 f/s it's a show stopper. Hence why we can't climb up a hill at slow speeds. It leaves no room for error. If you were to compare to the slip percentage model that's either 2% or 20% slip, two drastically different numbers, but in MX sim it's the same thing.

Or no?

[cpt_Slow]

I'm pretty sure the Greek letter μ originates from the Egyptian glyph for a cow, hence the slight resemblance of the character to a cow's head and the "moo" spelling and pronunciation.

Ahh you have misunderstood, the ancient Egyptians were famed for their appreciation for cats, so the pronunciation and spelling 'mew' is the correct one. The Greeks gave a little nod to this by making the character look like a cat walking on the ceiling.

By the way, I think even 'slight' might be too strong of a word there.

[jlv]

I understood it interpolated between points, just did a crappy job of explaining that.

So if I only set my friction to

friction 1.0 2.0

I would always have a friction coefficient of 1.0 even after the tire started slipping? The only change in grip is due to the load being applied to the tire?

I think the problem with the slip speed model is that the friction doesn't scale with the speed. So if the bike is going 100 f/s and the tire is slipping at 2f/s it's not a big deal. But if the bike is going 10 f/s and the tire slips at 2 f/s it's a show stopper. Hence why we can't climb up a hill at slow speeds. It leaves no room for error. If you were to compare to the slip percentage model that's either 2% or 20% slip, two drastically different numbers, but in MX sim it's the same thing.

Or no?

Even worse, when the bike isn't moving but the tire is spinning the slip percentage is infinite! Infinite is an even more drastically different number!

Jokes aside, I had assumed slip percentage was to model tire flex, not friction. If you imagine applying a lateral force to a stationary wheel, it will flex the sidewall a bit but not result in any sustained motion. But if you do that while the tire is rolling, it'll creep sideways as it flexes the relaxed parts of the tire as they touch the ground and the flexed parts leave the ground. As the tire rolls faster you'll get more creep. It's hard for me to see a rationale for anything else keying off of rolling speed. Why and in what way should the friction change?

I'm pretty sure the Greek letter μ originates from the Egyptian glyph for a cow, hence the slight resemblance of the character to a cow's head and the "moo" spelling and pronunciation.

Ahh you have misunderstood, the ancient Egyptians were famed for their appreciation for cats, so the pronunciation and spelling 'mew' is the correct one. The Greeks gave a little nod to this by making the character look like a cat walking on the ceiling.

By the way, I think even 'slight' might be too strong of a word there.

Well, you have to account for the way language evolves over the years. While the Greek letter "moo" may have looked like this:


The Phoenician version could have had more pronouced horns:


And obviously, an authentic Egyptian logogram uses an accurate depiction of a cow:

[yzmxer608]

It's hard for me to see a rationale for anything else keying off of rolling speed. Why and in what way should the friction change?

I assume you mean slip speed? I agree though, the friction stays the same if you get that 2 ft/s wheel slip if you were going 100 or 10. The difference just seems drastic because of how much less momentum you're carrying at 10 ft/s to keep you going with that loss of forward drive.

Unless our mxs dirt is non-newtonian .

[cpt_Slow]

An accurate depiction of a cow

How can I argue with hard evidence like that?
From this day forward I shall refer to µ as 'moo', just as the extra-terrestrial overlords....I mean erm, Ancient Egyptians willed it to be.

[jlv]

It's hard for me to see a rationale for anything else keying off of rolling speed. Why and in what way should the friction change?

I assume you mean slip speed? I agree though, the friction stays the same if you get that 2 ft/s wheel slip if you were going 100 or 10. The difference just seems drastic because of how much less momentum you're carrying at 10 ft/s to keep you going with that loss of forward drive.

Unless our mxs dirt is non-newtonian :lol:.

I meant slip percentage which is apparently slip speed / vehicle speed. I said "rolling speed" since the tire creep I had just described varies with the rolling rate of the wheel.

How can I argue with hard evidence like that?
From this day forward I shall refer to µ as 'moo', just as the extra-terrestrial overlords....I mean erm, Ancient Egyptians willed it to be.

This whole "moo" thing might have been my lamest joke ever...

[cpt_Slow]

This whole "moo" thing might have been my lamest joke ever...

Right....."joke"

[Phathry25]

Why does rolling resistance go up with speed? Shirley in sand it should go down, no?

[m121c]

Wouldn't roll resist stay relatively the same for the most part as the dirt doesn't change just your speed? The higher rate of speed will make it seem as if it is less resistance, and if you want to counter that and make it a proportional roll resit at 10 as 100 you would need to increase the roll resist proportionally with the speed?

[yzmxer608]

Have you tried only changing the low and high pressure resistances and keeping the speed the same?

[jlv]

Why does rolling resistance go up with speed? Shirley in sand it should go down, no?

You're flexing the tire and displacing that much more dirt per second. The fact that your tires heat up more at higher speeds is more evidence of it. In sand I'd expect it to go up, but not scale linearly with speed. It'd be pretty easy to do some coast down tests and come up with some decent numbers based on the rate of deceleration. Sadly, my local officially illegal but the cops tell you to ride there sand pit has been shut down for a long time now.

What Shirley does in the sand is her own business.

[MxRicer211]

I have no idea what the hell you guys are talking about. I wait I have solved it, you just change the arm around the other way and it should click right back in. Wait never mind, that's how to click an arm back in.