The Scientific Approach

[Browncoat]

I've seen a lot of discussion on these forums about which direction the treads should face on tires such as the Losi Rock Claw. The pattern on these tires is directional, forming a V shape, similar to that of a tractor tire. Some people swear by running them the "right" way, others prefer to run them "backwards". Still others say it doesn't matter, the tires perform the same no matter which way they're ran. There are even a handful of people who run the front set one direction, and the rears in the opposite. In other words, it has become a matter of personal preference, because no real answer was readily available. Until now....

First, let's discuss the purpose of the tires. You can sum this up in one word: traction. This is why we don't use racing slicks to climb rocks. Scientifically, traction = friction. More specifically, rolling friction. This is the resistance that occurs when a round object such as a ball or tire rolls on a surface. When calculating rolling friction, one must also account for the tire's deformity, wheel size, and forward speed. I'm not going to get into a bunch of fancy math and physics.

F = CrrNf

The above is the formula for calculating rolling resistance. Again, I don't want people to fall asleep reading this stuff, because it's important. But there are a few things about force, friction, and traction that I think should be noted.

NORMAL FORCE. This is the force pushing up on the tires. Caused by the weight of the vehicle and tires, it is in fact equal in size (but opposite in direction) to the weight pressing down on the ground. The greater the weight of the vehicle and tires, the larger the value of the normal force, and as a result, the larger the friction force. However, since the weight of the tires is only a small percent of the total weight of the vehicle, tires that weigh more only cause a small increase in traction. Nevertheless, a very light vehicle with light tires will have little traction.

How this translates to crawling: Adding weight to your tires does not increase traction. The weight of the truck would have to exceed the weight of the rocks in order to move them.

COEFFICIENT OF FRICTION. This is an important one. It is a number that is determined by the nature of the two surfaces in contact. The 'rougher' the two surfaces are relative to each other, the bigger this number will be...and the larger the friction. For example, if a box weighs 10 pounds, and it needs to be pushed across a normal hardwood floor, the math works out to the coefficient of friction equaling 0.07. This means the box requires 7 pounds of force to move across the floor. When on an incline, one must factor in the angle of the incline and account for the resistance of gravity. The steeper the incline, the more force required to move the box forward.

How this translates to crawling: We are looking to increase our coefficient as high as possible. Since crawlers encounter numerous angles, the best way to do this is with clean tires that provide exceptional bite.


Now...on to the purpose of this post. Forwards or backwards? We are talking about two main features of the tread here: Lugs and voids. The lugs are the raised portions, voids the lower. I will try to illustrate using the keyboard, and hopefully everyone will understand. Let's say the direction we are going is this way:

<-----

First, we will test the tires in the forward position. So the treads run like this:

<-----
<<<<<

This works best if you have a tire handy, so you can roll it along slowly in front of you. As the tire moves and makes contact with the surface, it first touches the point of the "V". As it moves forward, it continues to increase contact with the surfaces of the tread lug, which in turn, helps to propel the tire forward. Now let's try it the other way:

<-----
>>>>>

As we slowly roll the tire, initially, things look good. Now, instead of just coming into contact with just the point of the "V", we are coming in contact with both tips. But as we continue to roll the tire, the lug begins to fade away, giving us less traction.

In summary, as we all know, the closer to the top of an obstacle you get, the harder it is to crawl over it, because more traction is needed. You want traction when you need it the most. To answer the age old debate, if you subscribe to physics, tires should be ran in the "forward" position for the best traction.

 

[ElChupacabra]

That was a lot of reading and Im still not sure you proved the tire works better in one direction or the other. Why show the formulas and then not implement them as emperical evidence? Can you plug some numbers in and expound on your "theory"???

 

[Wrecker]

Good text book info here, but text books don't apply directly to real world applications unfortunately. Also, this only works for uniform surfaces, which rocks are not. Therefore for each rock that you're climbing over one way might actually perform better than the other way even though the book says other wise, therefore averaging out either direction. This also does not account for tire ware, after one side gets worn to a certain point flip them around and the tire performs better on the less worn edge of the lugs. This also does not account for lateral traction while turning and the list goes on. Equations just aren't going to give an accurate answer no matter how complicated you get.

From what I've tested, all tires are different and results vary from driver to driver. I've tried them both ways on the same lines and and found no difference on some tires and a difference on other tires. Just try your tires on a couple lines with them one way, then flip them around and try them the other way and decide which you like the best.

Thanks for the effort in trying to contribute good tech though, threads like this are always appreciated."thumbsup"

 

[Browncoat]

Equations just aren't going to give an accurate answer no matter how complicated you get.

Agreed. As you elaborated on in your post, the number of variables probably nears infinity during any given crawling session. It would be next to impossible to accurately calculate everything...which is why I didn't even attempt to show any math.

I'm a big subscriber in real-life experience. I work with closely with engineering every day, and what's shown on a computer screen is often a far cry from reality.
"thumbsup"

Thanks for the effort in trying to contribute good tech though, threads like this are always appreciated."thumbsup"

Thanks! I should've noted that this wasn't intended to be a definitive answer on tires. I was hoping to add something to the discussion using a true method of calculation, and possibly spark some debate.

 

[TURTLE]

Yeah great Tech I'm very impressed. It definatly gives some insite to the long debate."thumbsup" Although it is impossible to test all situations and possiblities I really enjoyed the read."thumbsup"

 

[Wrecker]

I'm a big subscriber in real-life experience. I work with closely with engineering every day, and what's shown on a computer screen is often a far cry from reality."thumbsup"

Yeah, I'm a Mechanical Engineer.;-)

 

[TunaMaxx]

I like the scientific approach, but I have a couple of comments.

1) The 'V' shape of the tire is to help clear debris (especially mud) from the voids. With the 'V' pointing down at the front of the tire (aka normal or forward), mud, debris, rocks, water, etc is pushed away from the center of the tire. With the 'V' pointing up at the front of the tire (aka backwards) all that junk is forced under the the center of the tire.

2) You seem to neglect the contact patch. If tires were infinitely round, then your argument regarding the 'point' and 'tips' of the tread might hold more weight. But since our soft, conformable by design tires aren't infinitely round (especially on the bottom!) now we are talking about a varying degree of flat surface where the direction isn't so important.

Don't get me wrong, I agree with your conclusion about 'normal' and 'backwards' tires. But I think my preference is based on a history of what was 'right' in my area.

 

[Crawlchevy]

Check out real rigs running mickey thompson baja claws, they are a directional tire, alot of people run them the opposite way, and others dont.

 

[Mnster]

Don't forget about what happens to a carcass when the tire deforms under stresses.

There are so many variables its nearly impossible to say which way will work better on paper.

I've experimented several times with bad results. On masher 2ks the tread seems to push in on itself causing the tire to pop and spring when run backwards. I always run my tires in the proper direction. I feel the only reason people like their tires backwards is once the tire is worn and lugs rounded. Then they flip the tire over to the sharp lugs and are like wow what a difference.:roll:

 

[cuban b]

The argument is good, but completely flawed in the real world. There is a lot more involved on a real obstacle. The main thing left out, in my opinion, is the ability of the lugs to grab edges on the rock.

You can't calculate that ability, and how each tire reacts to this factor depends greatly on rock type.

You're probably right on slickrock, but on fractured, irregular rock, who knows?

Oh yeah, I run them forward "thumbsup".

 

[constantmotion]

<-----
<<<<<

This works best if you have a tire handy, so you can roll it along slowly in front of you. As the tire moves and makes contact with the surface, it first touches the point of the "V". As it moves forward, it continues to increase contact with the surfaces of the tread lug, which in turn, helps to propel the tire forward. Now let's try it the other way:

<-----
>>>>>

As we slowly roll the tire, initially, things look good. Now, instead of just coming into contact with just the point of the "V", we are coming in contact with both tips. But as we continue to roll the tire, the lug begins to fade away, giving us less traction.

I just don't see how this theory helps out anything. Forward, you're going to get less traction at first then traction gets better as the tread pattern progresses. Backwards, you're going to get more traction at first then traction lessens as the tread pattern progress. This is almost like saying 2 + 4 = 6 and 4 + 2 = 6.

 

[ROLANDROCKSHOP]

great info,but as was said the real world and what looks good on paper
are two different things. too many variables,foam density,the way the foam has been cut,rock surface,tire lug pattern...driver..blah blah
ive got just about every tire thats out there and i keep going back to
rock claws frnt or backwards they just work,badlands,rock lizards.
just orderd some flat irons well see how they do...

 

[Browncoat]

1) The 'V' shape of the tire is to help clear debris (especially mud) from the voids. With the 'V' pointing down at the front of the tire (aka normal or forward), mud, debris, rocks, water, etc is pushed away from the center of the tire. With the 'V' pointing up at the front of the tire (aka backwards) all that junk is forced under the the center of the tire.

2) You seem to neglect the contact patch. If tires were infinitely round, then your argument regarding the 'point' and 'tips' of the tread might hold more weight. But since our soft, conformable by design tires aren't infinitely round (especially on the bottom!) now we are talking about a varying degree of flat surface where the direction isn't so important.

1) If we were discussing a basic tractor style tread, under different circumstances, that would be the case. However, the "V" pushing away debris is not the application for crawling as there is no mud or debris (or very little). For crawling, the "V" is for traction only.

2) The contact patch is one of the x factors that are nearly impossible to determine. However, having more tire against the surface does not equal more traction. Common sense tells us that there is an optimal contact patch. Too little, and the tires would just spin. Too much, and it becomes difficult to move forward. Also consider that no matter how large the contact patch is, at the exact same moment one part of the tire contacts the surface, another part of the tire is leaving it.

I just don't see how this theory helps out anything. Forward, you're going to get less traction at first then traction gets better as the tread pattern progresses. Backwards, you're going to get more traction at first then traction lessens as the tread pattern progress. This is almost like saying 2 + 4 = 6 and 4 + 2 = 6.

It is important because more traction is needed to propel forward, up and over and obstacle. If you've ever been jogging on a treadmill that gradually increases the incline, it takes more effort as the angle increases.

 

[Dumptruck]

The greater the weight of the vehicle and tires, the larger the value of the normal force, and as a result, the larger the friction force. However, since the weight of the tires is only a small percent of the total weight of the vehicle, tires that weigh more only cause a small increase in traction.

How this translates to crawling: Adding weight to your tires does not increase traction.

Im no physics buff, but those statements contradict eachother.

If the total weight on the tires determines friction, why wouldnt weight in the tires add traction. I know when i push down on my crawler, it has more traction, and i lift up and it has less. I understand what your doing, and its cool, just basic physics i dont think are enough to say for sure that a tire works better or worse or the same in any direction. It would have just way too many variables. Surface shape, tire compound (effecting the way the tread pattern reacts to friction), foam, temperature, wheel speed. The list would go on forever if you really dug out every variable.

Buy you have a good start, maybe there will someday be a Unified Crawler Theory, where you could put it the variables of your truck and it would tell you exactly how it would do.:lol:

 

[cuban b]

I think what he is saying there is that more weight doesn't increase traction with reference to the truck, even though it does increase traction with reference to the tire.

Basically, you have added weight holding you on an incline, and an equal amount pulling you off the incline.

 

[Browncoat]

I think what he is saying there is that more weight doesn't increase traction with reference to the truck, even though it does increase traction with reference to the tire.

Basically, you have added weight holding you on an incline, and an equal amount pulling you off the incline.

Yep! "thumbsup"