[Thylacine]

Quite a while ago I was listening to a science show on the radio about motion vectors and wheel velocity, and at what speed an object travels in relation to wheel speed.
Anyway, one of the things I remotely understood (maybe) was that irregardless of the speed of an object, the bottom of the tires' vector is in the opposite direction as the motion of the object, meaning, that the speed of the contact patch is effectively, zero.

What I can glean from this in terms of tire design, is that, as I expected, most tire designs are bogus. They should be designed based on a stationary tire as it relates to steering angle and the angles upon which it could possibly slip.

Are most tire designs based on trends and 'agressive looking' patterns, or is there some physics at work here? Are tire designers just copying motorbike tires and hoping for the best?

Anyway, all you physics heads, put your foil helmets on and lets bust this tire design thing wide open.

 

[NRSracer]

from your "experiment" or what you heard, i am hearing that either this rolling resistance thing is a load of crap, but if you think about that for more than a second, that's impossible. so what i can conclude is that you mean all off the treads that form sort of an arrow are bogus. i can see what you mean if what i understood was right. if that is the case, then yes, i can see how tread placing should be different.


damn, didn't know so much work went into a tire.

 

[Atomic Dog]

Not a physics head, don't have an foil beanie (though I'm thinking I could use one.), and I'm probably talking out my arse, but in the case of the rear tire wouldn't there be a directional force due to pedalling forces? As for the front tire, I dunno.

I'll shut up now and let someone with a clue take over.

 

[Rik]

Thinking about this made my brain hurt for a bit...
but whilst the tyre may stationary be stationary on the contact, there is also the transition to ground contact, and from ground contact.

I'm guessing that the arrow design tyres are designed for that transition in mind, and think about it, the contact point is stationary, so the tyre's got to do its damn best to grip in and stay stationary for that time.

Also, I think there is alot of fluff on the market in regards to tyre design, you're probably right in that some are designed for looks more than anything, but they can hopefully be spotted and avoided.

 

[D_D]

Tread designs are a lot more imortant where the ground in contact with the tyre can move easerly.

Swaping tyres in mud can cause the bike to go from ridable to almost unridable. For most riding it appears to make little difference

The marketing on tyres is pretty daft, belive it or not some people will buy one tyre over another because it has a direction arrow on it. Also look at the width measurments which are never correct.

 

[riderx]

Damn near everything you want to know about a particular tire

 

[buck]

About rolling resistence, there has to be due to friction right. But about tire design Im a bit confused. I took physics last year and Im taking AP Physics this year, Im in high school. But the angular velocity is going the same way as the bike, which Im not sure if that matters or if its right, Im pretty sure Im right though. Just trying to think some physics. We've been doing a lot of electicity lately which is all right but Id like to do more momentum, velocity and such. Good luck figuring out this tire problem.

 

[ChrisRobin]

Originally posted by Thylacine
Are most tire designs based on trends and 'agressive looking' patterns, or is there some physics at work here? Are tire designers just copying motorbike tires and hoping for the best?

Anyway, all you physics heads, put your foil helmets on and lets bust this tire design thing wide open.

I'll let the others take care of the rest of your post....

From what I've seen and from what I understand, if you take one particular tire, it'll have treads that are meant for braking and some treads meant for traction. So no, I don't think they try to make tires look cool...not the expensive ones anyway. I might be wrong here so correct me if I am. I have noticed if you take a Mich 2.5 DH tire, when you install it according to the directional arrow, the tire looks like it's on backwards. I assuming those center 'backwards' looking treads are meant for braking stability while the outer treads are meant for traction.

 

[Rik]

Originally posted by ChrisRobin
I have noticed if you take a Mich 2.5 DH tire, when you install it according to the directional arrow, the tire looks like it's on backwards. I assuming those center 'backwards' looking treads are meant for braking stability while the outer treads are meant for traction.

I have fun and games with things like that all the time. You can tell what's meant for braking, what's meant for accelerating, and what's made for turning. Put a directional front tyre on the rear, pointing backwards, and feel the bike slither as you lock it up on soft surfaces, feel it squirm through corners. Turn that tyre around, and it'll brake like nothing else, but put down the gas and it'll spin a bit.
That's through my experience playing in the mud, your mileage may vary.

 

[MikeD]

Wierwolf

*cough*

 

[Eng-Rider]

If all mountain bikes did is go at the same speed in a straight line then your argument about tread design would be correct. You could use a road slick and get the same performance as a Nokian Gazzalodi. (this neglects rolling resistance)

The pieces the discussion is missing are force and acceleration. Any time you go faster/slower, turn, encounter and obstacle or even shift your weight you are placing a force on the tire by accelerating the bike (trying to or actually changing its velocity vector). Your ability to do this while maintaining zero net velocity at the tires' contact point with the ground (i.e. not skidding) is based on the ability of the contact patch to resist that force.

The ability to resist the force of the bike accelerating is related to contact area and frictional coefficient with the ground. For example, large volume - low pressure tires have larger contact patches and so given the same frictional coefficient as a smaller contact patch will generally resist a greater force without skidding (this is more of a statistical argument based on greater likelihood of having a higher frictional coefficient area under the larger patch).

Frictional coefficient is based on mechanical interference between two bodies (the rougher the bodies, the more pieces of them interfere and resist relative motion). Soft compound tires (super sticky) conform better to the contours of the ground (more interference) and so have higher coefficients of friction. Aggressive treads can get you both of those things by having larger contact area (think 3D) and better interference with rough terrain.

To quickly discuss rolling resistance. As far as I can tell there are really two main things that cause it. Deformation of the tire and knobs (bouncing or squirming) takes energy and the only place the energy is coming from is your cranks. So, a hard tire deforms less than a soft tire and a knobby tire deforms the knobs (requiring energy) while the smooth tire does not.

Second cause would seem to be that rolling a tire is like separating a joint in peel. The very thing that keeps your bike from sliding every time you accelerate also resists the rolling motion of the tire. A good but kind of extreme example is velcro. If you press velcro together and try to pull it apart in shear (laterally) it is very hard to separate (just like sliding a tire). If, however, you try to pull it apart in peel (think opening a fruit roll-up) it is much easier to separate but still requires some effort. Every time your tire rolls around it is separating its joint with the ground in peel. The stronger the joint, the more rolling resistance. Think smooth pavement vs. sticky mud.

I may have missed something important but at least this should help a little bit.

 

[spacemanspiff06]

sounds like you got it. tread design is all about dealing with acceleration; be it positive, negative, or lateral, all the while not squrming not wearing to fast, and rolling fast.

 

[A.P]

The michelin sidewall rotation arrow on their downhill tires are wrong. They are correct for the XC versions of those tire, but they never bothered to change it for the downhill verison.

 

[Kornphlake]

If all mountain bikes did is go at the same speed in a straight line then your argument about tread design would be correct. You could use a road slick and get the same performance as a Nokian Gazzalodi. (this neglects rolling resistance)

The pieces the discussion is missing are force and acceleration. Any time you go faster/slower, turn, encounter and obstacle or even shift your weight you are placing a force on the tire by accelerating the bike (trying to or actually changing its velocity vector). Your ability to do this while maintaining zero net velocity at the tires' contact point with the ground (i.e. not skidding) is based on the ability of the contact patch to resist that force.

The ability to resist the force of the bike accelerating is related to contact area and frictional coefficient with the ground. For example, large volume - low pressure tires have larger contact patches and so given the same frictional coefficient as a smaller contact patch will generally resist a greater force without skidding (this is more of a statistical argument based on greater likelihood of having a higher frictional coefficient area under the larger patch).

Frictional coefficient is based on mechanical interference between two bodies (the rougher the bodies, the more pieces of them interfere and resist relative motion). Soft compound tires (super sticky) conform better to the contours of the ground (more interference) and so have higher coefficients of friction. Aggressive treads can get you both of those things by having larger contact area (think 3D) and better interference with rough terrain.

To quickly discuss rolling resistance. As far as I can tell there are really two main things that cause it. Deformation of the tire and knobs (bouncing or squirming) takes energy and the only place the energy is coming from is your cranks. So, a hard tire deforms less than a soft tire and a knobby tire deforms the knobs (requiring energy) while the smooth tire does not.

Second cause would seem to be that rolling a tire is like separating a joint in peel. The very thing that keeps your bike from sliding every time you accelerate also resists the rolling motion of the tire. A good but kind of extreme example is velcro. If you press velcro together and try to pull it apart in shear (laterally) it is very hard to separate (just like sliding a tire). If, however, you try to pull it apart in peel (think opening a fruit roll-up) it is much easier to separate but still requires some effort. Every time your tire rolls around it is separating its joint with the ground in peel. The stronger the joint, the more rolling resistance. Think smooth pavement vs. sticky mud.

I may have missed something important but at least this should help a little bit.

I think you're confusing friciton with mechanical interaction of two slidding objects. One creates heat the other does not, press your hands together and move them back and forth and you'll feel heat, now cross your fingers together and try to move your hands back and forth, what you'll feel is your fingers stretching and knuckles pulling on each other but you will not feel heat.

Frictional coefficents are not affected by surface area, the only way to increase friction is to increase the force pushing two surfaces together. I don't make the rules I just obey them, you can argue that it's wrong all you want, I agree it's counter intuitive but can be proven in a laboratory when mechanical interferance is removed from the experiment.

I'd say traction is the appropriate term for explaining why dragsters use wide tires, why downhill bikes use fat tires with knobs and why you see a narrow belt on something like a go cart and a wide belt on something like a supercharger. Friction holds a nail in a wall when your trying to pull the nail straight out, traction holds the nail in the wall when it's driven in at an angle and a picture is hung from it.

In my mind a tire's knobs should pierce the dirt while compacting the dirt around the knob so that it mechanically grabs the dirt, the compound the tire is made of should have a high coefficent of friction so that the tire will not slip on rocks or asphalt. Actual knob geometry is beyond my understanding of physics.

I'd say that any of the popular treads are adequate for the average ridder, most "observed" differances are a placebo effect.

 

[ChrisRobin]

Wow...blast from the past...

I was wondering why I could recognize the title of this thread in my emails...its from 2 years ago!

 

[mcoomer]

Blast or not, this thread made my head hurt. I think I need to lay down.