First ride with a passenger.

[jonnythan]

Inflating to max value will ... dramatically reduced traction.

By altering the laws of physics where traction is solely dependent on weight and the coefficient of friction.

But second to all the rest of it. Max pressure isn't at all what you want to ride. Don't thank us, thank your not sore "O Ring".

Uh, no.

The higher the air pressure in the tire, the lower your traction. This is because higher pressure = smaller contact patch.

A tire at 20 psi has more traction than the same tire at 40 psi because the contact patch is larger.

This doesn't mean that lowering the pressure beyond recommended is a good idea - it leads to more and uneven tire wear, more rolling friction, etc.

[sv-wolf]

Inflating to max value will ... dramatically reduced traction.

By altering the laws of physics where traction is solely dependent on weight and the coefficient of friction.

But second to all the rest of it. Max pressure isn't at all what you want to ride. Don't thank us, thank your not sore "O Ring".

Uh, no.

The higher the air pressure in the tire, the lower your traction. This is because higher pressure = smaller contact patch.

A tire at 20 psi has more traction than the same tire at 40 psi because the contact patch is larger.

This doesn't mean that lowering the pressure beyond recommended is a good idea - it leads to more and uneven tire wear, more rolling friction, etc.

Hi jonnythan

If I remember my college physics, storyunfolding is correct. Friction (traction) is not dependent on the size of the contact patch between tyre and road. It is dependent only on the weight of the bike (the Normal Force between the two surfaces) and the specific value of the coefficient of friction between the tyre compound and the road.

The physics is very complex I seem to remember, but in simple terms it goes like this: when you increase the area of contact between two objects, you lower the pressure exerted by one on the other (that is, if you assume the force between them - the weight of the bike in this instance - remains constant). This reduction in pressure exactly compensates for the increasing area on which frictional forces can work. In other words, increasing area and reducing pressure cancel each other out.

So, as counterintuitive as it sounds, the size of the contact patch does not affect the amount of frictional force generated between a tyre and the road. That is true of any two surfaces.

I've never thought about this in terms of tyre traction before, so I'm puzzled. If the size of the contact patch is immaterial, why does tyre pressure make such a difference? (Perhaps it isn't immaterial. Are there other forces operating here that are being overlooked?) Anyone know the answer to this?

[blues2cruise]

Just my ....from the point of view as the passenger.....another reason to brake smoothly is so that your passenger isn't constantly bonking the back of your helmet with the front of their helmet.....

[koji52]

I think i have to side with Johnnythan on this...

Isn't there a total traction number that is determined primarily by the contact patch? Less contact patch, less total traction. Larger contact patch means greater total traction. I could be wrong but i don't know.

In any event, under inflating your tires increases running tire heat and increases the chance of a blowout.

Edit:

Based on what I understand in articles online (seemingly from credible sources, and it's been years since college physics) the kinetic coefficient of friction is affected by the surface area. Static friction is not.

This would have to be true as increasing the contact patch on your tires increases heat. Heat would be created by increased friction.

Maybe?

[jonnythan]

I'm still thinking about this.

Academically, I know that static friction is directly proportionaly only to the coefficient of friction and the normal force. And nothing else.

But intuitively, I have serious trouble believing that a 6-lb stiff rubber sheet attached to the bottom of a 6-lb 2'x2' piece of plywood is going to precisely as easy to slide across the ground as a 12-lb rubber tire with a tiny contact patch.

I know I'm trying to rationalize and generate some intentional cognitive dissonance here.

I'll have to think about it. I think there are other things at play here, though.

[jonnythan]

Based on what I understand in articles online (seemingly from credible sources, and it's been years since college physics) the kinetic coefficient of friction is affected by the surface area. Static friction is not.

This would have to be true as increasing the contact patch on your tires increases heat. Heat would be created by increased friction.

Maybe?

You may be on to something with this. Tires do wear down, especially when ridden hard. So perhaps kinetic friction is a significant part even on the road.

I know auto and cycle racers vary tire pressures to vary grip and harshness. The rear tires on top fuel dragsters are filled to under 10 psi.

Maybe I'm totally wrong though.

[koji52]

yea, i may be totally wrong too...i got a D in physics in college for skipping too many classes.

Oh well, i'll stick with inflating my stock tires at the recommended pressure. I have a solo seat on my sporty so no passenger allowed haha.

[storysunfolding]

The heat on your tires is caused by the flexing of the rubber. It's one of the reasons that swerving is prohibited at track days b/c it does bupkiss compared to simply moving. When you start having kinetic friction your tires are spinning out. In that case a larger contact patch is beneficial but hopefully you're never in that situation.


Race tires are actually specially made for each track. For instance, daytona motorcycle tires are slightly harder on the left so they wear evenly. They use entirely different compounds for each track to maximize grip based on track and weather conditions. However, you'll pretty much find all motorcycle race tires being run at 14-16 psi. They are made of stiffer rubber that creates more friction when flexed allowing them to get up to heat faster.

A fun fact, racing qualifying tires are the hardest compound used on a motorcycle. The added stiffness creates more friction when they flex which quickly brings them up to optimal heat (180 degrees). They're also only good for two laps, one lap to warm them up, one lap to set your time. After that they're toast.

[koji52]

well i yield haha

[Wrider]

I inflate to full pressure at all times for a few reasons.
1. Full pressure means minimal rolling resistance when going straight.
2. Underinflated tires lead to tire squirm when in a corner. Not good.
3. If I do add a passenger I don't have to find an air hose first.
4. I already know how the bike handles as-is, don't need to get reacquainted with her, just with the added weight.