Fundamental principles of tire friction for off-road biking

Question

This is a two-part question.

I've been watching various videos and getting various advice about mountain bike cornering. My impression is that all of the advice cannot be true, because the advice, taken together, seems self-contradictory. Also, some logic out there seems rather disconnected from the physics.

A few years ago, I read this book on race car driving: "Going Faster! Mastering the Art of Race Driving". It is the best physics-based explanation of car handling I've seen. Based on the laws of physics, each tire has a maximum amount of traction for a given amount of downward force per tire. I want to point out two things that result from this:

1. If you need more traction, you can: (a) get stickier tires, (b) increase contact patch, (c) increase downward force (i.e. the 'normal' vector, perpendicular to the surface).

2. For a given amount of friction, it may be used for some combination of acceleration (forward), braking (backward), or cornering (sideways). By combination, I mean the sum of the force vectors. So, the more friction you use in acceleration or braking, the less you have available for cornering.

Unless I hear a better explanation, I think the above explanation above serves as a useful fundamental grounding of tire traction, whether it be on-road or off-road.

Question, part 1: Is my summary of the physics detailed enough to be useful without overlooking any key aspects? Are there problems in borrowing the logic of automotive tire grip and applying it to mountain biking?

Here is one way to apply this knowledge. As an example, I heard something like this from a video about mountain bike handling (sorry, I lost the link):

"pedaling out of a corner gives more traction at the rear wheel" (paraphrased)

This seems bogus to me. If you pedal, you are asking the rear tire to use grip for acceleration, meaning less is available for cornering. Question, part 2: Is my logic correct?

Answer 1

So let me get this straight before it becomes a mess:

..increased downward force from pedalling..

When you pedal the downward force is not increased - you are not getting any heavier. You can only increase the downward force on a rear wheel by shifting your body back - this will decrease the downward force on front wheel though, which is good for steering actually. Controlling your body position is easier and more precise when not pedalling.

Cars actually act differently while cornering than bikes - bikes lean into the corner. But for the tires and traction that part is the same and you are right.

Maximum cornering angle is proportional to the coefficient of friction of the tires to the gravel/road/whatever (this is only affected by the tire tread and tire compound). The faster you want to go in corner the more you need to lean-in with you center of gravity. So if you manage to throw your body off seat closer to the ground the tires won't have to lean that much - see the picture - giving you a better cornering grip.

So the correct way is: Pedal before the corner to get your speed, then off the seat and control the centre of gravity for maximum cornering grip (you are not able to pedal now), keep weight on both wheels equally for steering, focus on hitting the corner apex (this is actually the most important part), then pedal again off the corner.

With bikes and cornering also the width of the rim plays a role. If running lower pressure to get more grip (bigger contact surface for the treads to bite into ground) with narrower rims you can get a tire blowout (when tubeless) or lose balance as the tire sidewalls have less support. Wider rims provide better support for cornering (but wider are also heavier giving less acceleration).

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TL:DR: your logic is correct, tire tread veeery important, pedaling is BS, use your body weight to your advantage, lower pressure, wider rims.

Answer 2

Adressing your second question where you give the example "pedaling out a corner gives more traction at the rear wheel" and you state that this should reduce traction available for cornering - have you considered that the act of pedaling generally shifts the rider's weight back while also increasing the normal force through putting pressure on the pedals?

This is why suspension designers work so much to reduce 'pedal bob' on full suspension bikes due to compression under pedal load.

The increased downward force from pedalling and the shifting of a rider's weight may provide more rear wheel traction than is lost from acceleration - thereby aiding cornering ability.

Adressing your first question regarding your understanding/aplicability of the physical principles of road cornering to mountain biking - The physics of how the wheel and tire system behaves may not be terribly different between a car and a bike, but you have to consider that overall the bike system is dominated by rider weight (I weigh ~6 times more than my MTB) so the dynamic repositioning of rider weight has a huge effect on traction. Much moreso than a car where the driver is porpotionately smaller part of the system's weight.

So no, there are a few more variables to consider when understanding MTB cornering, and it mostly boils down to rider position.

Answer 3

pedaling out of a corner gives more traction at the rear wheel

The reason racers sometimes pedal out of corners is unrelated to grip. They do this in order to maximize the exit speed, especially in case they didn't corner perfectly (probably tapped the brakes) and lost momentum.