L8apex

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An Introduction to Traction.

This is a 100-level introduction into traction and hopefully it can provide some insight and benefit for even experienced drivers on or off the trail. Most of the examples will be simplified but they will be representative. There’s lots of things that go into getting traction – temperature, humidity, surface type, tire compound, and much more. So, let’s begin from the beginning.

What is Friction?

In its simplest definition, friction is the force that resists relative motion. You try to slide a box across rough pavement, you’ll be met with a lot more force than if you slid a box across a waxed linoleum floor. That’s friction at work.

For this discussion we’ll only be considering what is known as dry friction as it’s best for approximating a tire and a surface in good contact.

So how do we calculate friction? Thankfully the friction is quite easy to calculate. We have a dimensionless (no units) property known as the coefficient of friction. The friction force is simply the coefficient of friction multiplied with the force that is perpendicular to the friction and pushing up on the object.

Let’s look at our box example. Our force pushing down is simply the weight of the box, the force pushing up in this case is exactly equal to the weight pushing down. Now, recall Newton’s laws of motion from elementary science – force equals mass times acceleration. Looking at our forces in X we can see that if the push force is ever greater than the friction force the box will slide and accelerate to the right. If we look up a value for our coefficient of friction, μ, you’ll see tons of tables for various surfaces interacting, temperature ranges, wet / dry, but most importantly you’ll see two columns – Static Friction and Kinetic Friction.

So which do we use?
1628197206025.png




Static vs. Kinetic Friction:

Well the answer for the box equation is both. You would use the static coefficient up UNTIL the box is in motion. From there you would use the kinetic friction coefficient. The importance of this is that the static coefficient of friction is ALMOST always greater than the kinetic. Meaning it requires more force to start the box sliding than it does to keep the box sliding – something you’ve probably experienced but never really thought too much about.


1628197221474.png



So what does this mean for wheeling your rig?

No-Slip Condition:

Before we delve into wheeling / friction and which coefficient to use we first have to understand something known as the no-slip condition. The no-slip condition means the velocity of the tire, where it makes contact is actually 0MPH. It can be weird to think about – but let it sink in - Your tire, as it travels down the road, is moving at 0MPH AT the point of contact.

Slipping, however, is simply when that point of contact does have motion relative to the ground.

1628197230580.png


We know the static coefficient is greater than the kinetic coefficient. So, for the same vehicle, in the same conditions, we get MUCH more grip if we’re not slipping. When wheeling this gives you a huge advantage if you’re able to CRAWL obstacles versus doing burnouts.

If you watch experienced wheelers, Kevin from Litebrite is very good at this, he will always attempt to crawl an object first. You’ll see others who like to mash the skinny pedal – one is being smart with their grip, the other is asking for things to go pear shaped.




Crawling


So lets look at the forces acting on our wheel/tire. I’ve cut away the vehicle and just focusing on one wheel.

On flat ground it’s pretty simple. The torque transferred from the axles causes rotation, the weight acting through that wheel creates a friction force – and that friction force is what actually pushes the vehicle forward.

1628197247836.png



On a hill or angle – well things get a little more complicated. Notice now our normal force is much smaller – meaning our corresponding friction force is much smaller. That should be pretty intuitive to anyone who’s watched people wheel. But it highlights that your friction, more importantly, your normal force is not a constant. It will change with loading, angle, vehicle, etc. BUT the important corollary to that is IF it were possible to generate INFINITE Normal force we could have INFINITE friction and climb anything!
1628197265754.png

Circle of Grip

We want grip - so let’s create some more normal force. We could just make our rig heavier – but that comes with a lot of unwanted negative concessions. It also goes against “tread lightly” – which the Bronco and most modern rigs are already pushing that envelope. Side note: the 9000lb Hummer EV will probably be great at crawling because of it’s weight.

Race cars use aerodynamics to push the car into the ground to generate more normal force – unfortunately this is unavailable to wheelers because the speeds are too low for air to be effective. This is where it becomes useful to understand the circle of grip.

The circle of grip is an everchanging “circle” that represents the maximum friction force a wheel can experience in any direction at any point in time. Basically it’s a function of tire characteristics and the normal force the vehicle is experiencing. See below for a few examples. The circle, in actuality, is more of an oval, because tires tend to not have as great of grip side-to-side as they do in the rotational direction, but for this example we’ll keep them circles.


1628197303379.png


The circle of grip explained further.

Using friction effectively - The bump

I know I said earlier it’s best to crawl every obstacle for maximum grip – but sometimes that’s not enough. Sometimes a little momentum is needed to carry us over an obstacle, often referred to as a “bump”. Even then we can maximize a bump to make it over obstacles we would otherwise have been unable.

Watch this example of Loren Healy here:




As he makes it on to the first ledge you can hear him ever so slightly back out of the throttle. Kevin tries to explain it by saying he’s letting the car settle – and he is. But what’s really going on is as the vehicle compresses those shocks the normal force is increasing BEYOND the weight of the car – meaning our circle of grip is also increasing. If Loren were to have WOTed when the suspension was fully extended, he would have simply continued to spin the tires and probably have slid off the obstacle. However, by allowing it to compress he uses his bump efficiently, maximizing his friction and continues to just walk up the ledge.

This post has gotten a little long – but hope you appreciate it and gives you something to think about next time at the track or wheeling your rig.

Stay safe and keep the shiny side up.
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gjproducer

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I'm a millennial - Where is the youtube link, cool intro graphics, and a promise for more videos like this in the future? I promise I will like, subscribe, and slap that bell icon. :p

Edit: Oh wait, now I saw the video link!
 

kabriel

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L8apex,

Wow, you are awesome, thank you so much for taking the time to write this all out! I've been on these forums for over a year but I've never registered. This post was so informative and helpful i just had to sign up to thank you directly. I really appreciate the math/equations and the diagram that go along with your simple explanation. I can picture the whole simulation running in my head now and that is enlightening. I'm looking forward to anything else you write up like this. Thanks!
 

_jeff

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Well written, thank you.
 
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L8apex

L8apex

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L8apex,

Wow, you are awesome, thank you so much for taking the time to write this all out! I've been on these forums for over a year but I've never registered. This post was so informative and helpful i just had to sign up to thank you directly. I really appreciate the math/equations and the diagram that go along with your simple explanation. I can picture the whole simulation running in my head now and that is enlightening. I'm looking forward to anything else you write up like this. Thanks!
A lot of it I think is intuitive for most drivers - but being able to put the science behind it just allows for a deeper understanding and hopefully a benefit.

The circle of grip part I think should be in driver's ed - really helps when you think of track days and balancing a car.

Especially as a guy who spends most of my track days in an old 911, you learn where the weight is and where the grip is, and why you don't want to unweight the rear end of a 911 mid-turn.
 

Natai

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Great write up.
I'm new to off-roading, but one thing I've noticed frequently in videos is you can often tell the pros that really know what they're doing from those that are noobs (or just want to show off).

Pros are slower and more controlled - crawling - while others just immediately want to mash the pedal and bump it.

In most of the Bronco head to head videos I've seen, it appears the Bronco is well-suited to the controlled crawl. I'm assuming at least partly because of its weight (even though that weight is often pointed to as a negative I'm reviews).
 

_jeff

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The circle of grip part I think should be in driver's ed - really helps when you think of track days and balancing a car
Agreed. I also think being able to drive a MT should be included but that’s going away before too long 😢
 

DaveH

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@L8apex I sure appreciate you bringing back memories of college engineering courses. I was certain those modules on friction would come in handy sometime. :)
 
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