smashndash

Sorry, but do you have a source to prove that a bigger contact patch does not result in more grip?

https://en.wikipedia.org/wiki/Tire_load_sensitivity

There is TONS of evidence to suggest that wider car tires do have more grip. I’m not saying this proves that wider tires on bikes have more grip, but even on very smooth roads, most people can discern a 35mm tire at 40psi vs a 23mm tire at 110psi. A 40psi tire will grip harder.

I think cyclists are the only grip-depedent enthusiasts who still believe there is no correlation between contact patch and grip. Motorcycle and car enthusiasts have known this for a very long time. I believe cyclists were told this so they’d buy ridiculously narrow (19-23mm) tires but people are getting a bit smarter now.

Kapusta

I think he is technically correct thaf the coefficient of friction does not necessarily increase with contact area. And that might be the end of the story if you are on a very smooth surface.

But we are usually not on a very smooth surface, and the ability of lower pressure to allow the tire to conform to the surface essentially increases the coefficient of friction.

I think it can be hard to separate the benefits of low pressure and larger contact patch, sinse they fo together.

And again, contact patch is NOT a function of tire size, but of tire pressure (and weight).

smashndash

Yeah the pressure is what determines the contact patch size, but you can’t run a 23mm tire at 40psi.

But also... hoop stress, aka the “stiffness” of a tire and how much it’s able to conform to a road surface is the same in a low pressure big tire as it is in a high pressure small tire. A huge tire will have a bigger contact patch even if it is actually run *harder* than a narrower tire. There are some very ignorant people out there who think a 35mm tire at 110 psi and a 23mm tire at 110psi have the same “stiffness” but the 35mm tires will be ROCK hard. They’ll probably blow.

So yeah, it is possible to separate the effects of “low pressure” and contact patch size. Just set the pressures in each tire such that they both have the same hoop stress. The bigger tire will have a bigger contact patch (shocking) and, assuming the compound is optimized for that level of load, will suffer less from load sensitivity than the narrower tire.

https://flocycling.com/blogs/blog/fl...or-wider-wheel

wphamilton

I'm not sure what you mean the wiki link to show. It doesn't refer to the size of the contact patch. Neither does the discussion on slip angle, if you follow the links in the wiki page.

The Coulomb model of friction, referenced in that page, is well known and accepted and I don't think I need a link to support it. Look up Amonton's (so-called) Law.

wphamilton

I'd say increases "road grip" rather than coefficient of friction, since the coefficient of friction is considered to be invariable under these conditions.

Otherwise all spot on as far as I know.

Kapusta

I am pretty sure “coefficient of friction” encompasses all aspects of how much force it takes to slide one surface against another at any given moment.

wphamilton

The force normal to the surface? Bouncing? Tread deformation? I don't think so.

Kapusta

Tread deformation does not count?

I am curious how you would meaningfully measure the coefficient of friction of a soft rubber on pavement, then, if the micro-deformations of the soft compound matching the surface irregularities do not count as friction.

Bill in VA

Historically correct. I also think style had an influence as did pro racing. Keep in mind pro teams both motor (car, motorcycle), and bicycle are notoriously secretive about tread compounds. A big dollar, specially developed skinny tire, used in a race, may not necessarily be what was available to the public. However the 'Look' was very recognizable and played well in advertizing.

With cars, wider is often better, but only if the entire width of the tread is in contact with the road. That is why there are carefully calibrated suspensions with adjustable caster, toe, camber, and bounce and rebound. All of those adjustments are not usually present on bikes or motorcycles, which have a rounded tread surface due to 2-wheeled reality. A road race motorcycle tire may have 2 or more compounds depending on how leaned over they are in specific corners.

A softer (inflation) tire gives both a longer and wider contact patch, but I would venture that tread compound is also a critical component, as is treaded and/or no tread designs. Sticky is good, but too sticky gets debris stuck to the tread.

The Continental GP4000SII tires in 28mm seemed incredibly sticky on damp roads to me when I started using them, both in corners and in braking. Far better than any previous tire I had used. I also suspect the softer inflation and lessened vibration contribute to a more stable contact patch.

smashndash

Please watch this video. It explains everything you need to know about why wider tires grip harder.

wphamilton

Correct, the road grip lost due to tread deformation is not a factor in the coefficient of friction.

Surface texture and adhesive forces at the micro scale - there are some myths there also that we could go into but it's beyond the current scope.

Kapusta

No that is NOT correct. I was suggesting that somewhat sarcastically to point out the flaw in your assertion.

I believe you are making arbitrary distinctions (and ultimately meaningless ones in regards to the topic at hand) and thereby over-complicating and confusing what is actually a very simple concept. You seem to be focused solely in the properties of the materials themselves. While that obviously has a big influence on the real world coefficient of friction, it is not the only factor.

Frictional force is simply the amount of force that resists movement of two surfaces or objects against each other. The relevant coefficient of static friction (which is what we are concerned with here) is simply the frictional force divided by the force pressing the two surfaces together (normal force) at the point that the two surfaces begin to slide. And it is (at least in theory) a measurable number.

The coefficient of static friction with regards to a tire on a road (be it smooth pavement, wet pavement, rough pavement, dirt, gravel, ice) is simply the lateral force on the tire/road interface divided by the downward force on the tire/road interface at the point at which it begins to slide. That is basically it. If lowering the pressure (or changing the tread pattern, or adding studs) increases that ratio, then you have increased the coefficient of static friction. This is also what I believe one would refer to as "grip". Of course, the reality is that this coefficient is constantly changing, as the surface conditions change, as different parts of the tread engage, and a host of other factors.

Don't over-complicate this. "Road Grip" IS essentially friction.

wphamilton

I sense that there is a fundamental misunderstanding about what the coefficient of friction really is, so I'm not going into it more than very briefly.

It is an empirical observation only. It is not based on physical laws, and not derived from physical laws. One cannot provide a common sense analysis of friction.

The CoF does not require any analysis, hand-waving style or otherwise. It is a number from a chart of observations, period. However, these charts *eliminate* the extraneous factors that can apply to road grip. Basically, the coefficient applies specifically to two material surfaces. Road grip applies to an object and a surface, and will consequently involve properties of the object which are not properties of the material or surface.

Tread with respect to slip is in fact an excellent example of the difference. If you're interested (rather than just convinced already) I recommend a study of it.

​​​​​

Kapusta

You are correct, it is based on empirical observation..... such as a tire on a road. That is my whole point.

And when you talk about "eliminating" extraneous factors, what that really means is simply ignoring them, so whatever tests you ran are not a valid re-creation of the environment in which you are wondering about. If you want to know the CoF of tires, you test them ON THE ROAD where all those "extraneous factors" can be accounted for.

79pmooney

Have you looked at race photos of 1970s criteriums? Easy to measure the lean angles of the bikes. That tells you the Gs pulled. All of that is provided by the very skinny tires pumped to 110-1290 psi we all raced then. Yes, those courses were swept. But we were also getting very high friction from very skinny, hard tires (with VERY supple cord).

And bike tire friction vs either car to motorcycle - I suspect the big difference is the power. Race cars and motorcycles can apply power to the rear wheel in a slide, bring it to a line further out and that power "pushing" the car or bike into the corner. We roadies rarely get to pull that off with our massive 1/4 hp. (Besides, we'd hit the pedal.) I haven't studied friction (got the quick version in engineering school) but I suspect that large area helps a lot for sliding friction, a state we roadies try very hard to avoid.

Now, poor road conditions and especially things like gravel, sand, wet leaves ... start slides and now the wider tire could well be a large advantage (as is really grippy tread)..

Ben

Banzai

I love tire threads.

I am absolutely faster, and the rides feel easier, on my CAAD9 with 25s, than on my Ritchey with 32s.

There are lots of variables involved, and the Ritchey is a more comfortable ride, but that is how it is.

wphamilton

It means that it's a constant. Coefficient of Friction is a constant for two materials. In your road tests you will have other variables for road grip: suspension, speed, gravel, lubricants and a slew of other factors. We don't just turn a constant into a variable.

wolfchild

Rolling resistance of tires must be one of the most boring uninteresting subjects to debate and argue about. All the charts, numbers, studies and data are completely irrelevant to me.

Marcus_Ti

Heh...well the OP article that 'busts' 'another myth' is completely devoid of numbers or studies or data.

himespau 

I think his claim was that wider tires make the rider faster because the tires themselves are not significantly slower (until they get large enough that weight matters - and he's more than willing to sell you tires with ultra thin sidewalls to cut down that weight) and the fact that wider tires offer more cushoing makes you more comfortable so that you are less tired (and therefore have more energy to put into speed) later into the ride. So he claims the wide tires themselves aren't faster, but they do allow you to be faster. At least I think I read that in one of his articles on the subject.

Koyote

Yeah, I've read that at his site. Note that his hypothesis is not really testable, and it supports his decision to sell wider tires.

aclinjury

You have been clear in here with your explanations of CoF.
But, this whole business could be explained even more clearer if we distinguish between the concept of grip (mechanical friction), traction, and handling.
Starting at the most fundamental level is the "grip" or mechanical friction of the tire's rubber property.
Then, different tire sizes and tread pattern (using the same rubber) will give rise to different traction to the tire (and note, wider isn't always better, other wise MotoGP front tires would be as wide as their rear tires).
Then, after adding in suspension elements, you finally come to the "handling" character of the vehicle.

A lot of folks talk about mechanical grip when they are actually talking about traction. Confusions in discussions then arise because now we're talking about different (but closely related) concepts.

RiceAWay

I am a high school dropout that became a self taught electronics engineer and had to lay off PhDs that couldn't keep up with me. In one case in particular I had six engineers and two with PhD's and I still did half of the hardware design, the firmware programming and wrote the entire real time operating kernel. In another case a couple of PhD's in an EXTREMELY important project said that it would require 2 IBM supercomputers to do the project. I looked at the PhD Chemist next to me and then said, "I can do it with a microprocessor" and did, on time and under budget. The Chemist led the project and was responsible for the chemistry on the project. He did a fabulous job down to the point where I had to vibrate the samples to mix the solutions. That's not the easiest thing in the world to do with stepping motors and with the original fairly slow, microprocessors.

A degree doesn't take the place of accurate prediction and there is a reason that you use larger wheels. Why do you think that 29ers became so popular that they have washed the 26" wheel out of the market? It wasn't so much due to tire diameter but the increased diameter putting more weight further in front of the bike reducing wheel lift on hard climbs so that you don't lose control. I didn't care a great deal for his analysis. Especially concerning rally car tires. They have reasons for that - the greater volume of air makes the impact on objects less severe. Unlimited road racing cars have larger wheels to allow the driver and car weight to be below the center of lateral rotation of the car. this then makes the cornering speed of the car dependent upon the traction of the compound of the tires which then increases the speed of wear.

Racing motorcycles use smaller diameter wheels to reduce the power to accelerate the wheels and wider tires to increase the traction surface. Most of the play racers riding rice rockets have no idea how to corner.

Road bikes load tires so little that cornering angle is limited. Racing motorcycles it was normal to drag your knee through every corner. Try that on a bike. The reason that larger tires (not wheels) work is because increased volume reduces the bouncing up and down. 28 mm works well for most of us heavy riders and pros are generally using 26 mm sew-ups. Sew-ups do not corner or work any better than a good clincher rim. They allow (or did before disk brakes.) a team mechanic to rapidly change a wheel for a team member and then inside of the team car he could pull off the flat sew-up and pull on a new, pre-glued one, inflate it with a power inflator to their exact pressure and lean out the car window and insert it into the rooftop wheel rack. There are tubeless compatible tires and tubeless tires. The difference is compatible needs sealant to seal. The actual tubeless tires. have a rather heavy liner of soft rubber that allows it to seal against a rim. Tubeless compatibles are being used in used by most TT riders. Far less a chance of a flat and the sealant is not rotating weight.

Science can only explain reality when they actually know it. Or as Nobel Laureate Richard Feynman said, "In talking bout the impact of ideas in one field on ideas in another field, one is always apt to make a fool of oneself." To underscore this - the idea that CO2 could cause climate change was disproven in 1905 when people learned about spectrometry. After the idea of it was theorized in 1886 by Arrhenius. Do you suppose anything new has been learned since then?
'

70sSanO

29ers became so popular because they can just roll over stuff.

John

Kapusta

It would appear that The Transportation Research Board of the National Academies of Science, Engineering and Medicine did not get your memo:
Coefficient of Friction Between Tires and Road Surfaces

And here they pretty well spell it out See page 19: "The resistive force, characterized using the non-dimensional friction coefficient, μ , is the ratio of the tangential friction force (F) between the tire tread rubber and the horizontal traveled surface to the perpendicular force or vertical load (FW)"

A little bit of Googling Coefficient of Friction Tires and Pavement (or lubrication) yielded me a list of researchers and engineers who do not share your view that CoF only applies to the materials themselves, but are in fact looking at how CoF changes under different conditions (including wet vs dry pavement and speed).

I don’t think you are correct about what "constant" the coefficient is representing. The only thing "constant" about the coefficient is that (in theory) as long as experimental conditions remain the same, the ratio of forces remains constant at different loads (the force pressing the materials together). But in fact even that is sometimes not truly constant.