Caliper

Actually, it is NOT true and drag racers are right. Yes, physics 101 says contact area does not affect friction. Basic physics class doesn't get into the little details of tire dynamics...

https://insideracingtechnology.com/tirebkexerpt1.htm

The reason more contact patch = more traction is because rubber is soft and (can be) sticky and conforms to the surface. This lets it use keying and adhesion, or the rubber actually gripping the texture of the pavement. When the rubber conforms to the surface, little bits of the rubbber must actually be torn loose to initiate a slide and thus why a tire leaves a skid mark. This is more pronounced in soft race tires, but keying also happens with grocery getter tires. Race tires will actually pick up loose stones off the track when warm (adhesion) like a lint roller! The extra force generated here is why lower pressures do grip more.

Bike Gremlin

So I explained in the rest of my post. The bold sentence was taken out of context.

"So weight per contact patch inch is important in terms of tyre being scrubbed off (and hence slipping). Cars have advantage here."

wphamilton

Actually correct for "being scrubbed off", but not for "slipping". I will admit that on first reading this, I didn't interpret "scrubbed off" as meaning ablation, and I thought that you intended a more vague reference to friction.

A larger contact patch is required for certain race tires or high performance tires for reasons other than providing greater traction in turns.

Also let me take a moment to reiterate, the question is for a stock, mid-range car straight off the lot, leaving out formula 1 cars, racing tires, aerodynamic downforce, specialized suspension etc.

wphamilton

Yes. However, the traction in the corner comes down to normal force and coefficient of friction, not the size of the patch.


So you're saying that the bike wins in this scenario, when the corolla tires roast?

Phantoj

2013 Honda Accord Sedan: 0.82g on the skidpad (Car and Driver)
Jobst Brandt on Avocet Fasgrips ~ 41° lean = 0.87g

Plus, the bike can carve a better line in the corner.

Conclusion: a good bike and rider can "outcorner" an average car.

Phantoj

Well, as long as I have your personal guarantee, then I guess the argument is settled.

wphamilton

Good example

wphamilton

I'd tend to believe him. He's probably got a high end sports car that he takes to the track on weekends and corners at 4G.

Campag4life

Didn't know English wasn't your first language...you write it well.
But what you wrote about contact patch is wrong. In fact, increasing contact patch is the whole predicate behind wider tires on cars, motorcycles and even bicycles to improve traction and lateral acceleration. Bike makers in fact say that a wider tire with a slightly lower pressure improves cornering by way of a larger contact patch. Contact patch matters for traction and cornering speed.

kcollier5

Well, I'm not an MIT educated engineer and I can't explain all of the physics (although surprised that no one has mentioned the ability of bikes to lean while cornering vs a car? EDIT: Oops, didn't read all of the 2nd page). I'll just tell you my physical experience.

On twisty downhills, I have been passed by cyclists and simply could't keep up unless the straights were longer. But thru the curves, no way without nearing or crashing… And believe me, I was trying to keep up. Perhaps if I was in a grippy sports car, maybe then I could have stayed with them, but that would likely have been due to the acceleration out of the curve more than the cornering ability. Can't tell you why exactly this is true, but like all crazy discussions, it depends on so many factors. But in my experience

Decently skilled cyclist on a twisty descent = faster than a decently skilled driver in an average family car

Bike Gremlin

Explained better than I ever could:

AutoSpeed - Tyres, Grip and All That...

merlinextraligh

It's one person's experience, take it for what it's worth. I do think I have some experience relevant to the point having raced with Professional racers, and having some experiene with sports cars on the track.

merlinextraligh

About 1.2g's on a fairly consistent basis

merlinextraligh

The .82g's on the Accord is on a continuous skid pad. It's quite possible to exceed that number in a particular curve. You might be sliding a bit in the process.

I know I'll see higher than the skid pad numbers on my car on curves that i'm not even pushing as hard as possible.

Vicegrip

This. Knowing how to manage traction and weight is key. Skid pad is mechanical grip only, no weight management, no aero. Carving a curve is NOT the fastest way from a point prior to turn in to a point beyond turn out for a car. A good clean line that uses the entire track or road is important but the car will not always be pointing parallel to the line of travel. I don't coast at all in a corner. Ether on the brakes or the gas.

1.75gs or more in the high speed sweepers. 1.5 in the slower stuff.

Campag4life

Reading that article makes me laugh a bit, sorry. It counterdicts itself.
Some general comments. Take a look at Merlin's rear engine Porsche which has a bit more weight over the rear wheels than front. Do you think Porsche engineers know much about handling and tire grip? Why do you suppose the rear tires on the car are a lot wider than the front? Afterall the front wheels steer the car. The reason is simple. Wider tires DO afford more grip. And yes, I am conversant with the equations at play.

So why does the article contradict itself? Because it stated that tire patch matters relative to heat and softer compounds can be run with a larger contact patch because they won't shred. More? Contact patch 'shape' matters. A wider tire even with the same normal force and psi will have a different L X W contact shape. This matters when it comes to lateral acceleration and a tire begins to slip and scrub, heat builds up which will lose traction on a narrower tire with narrower contact patch and lose traction and why wider tires are preferred for racing versus say taking your Corolla to the grocery store. More? PSI. In the context of bicycles, wider tires can be run with a lower psi, in fact this is promoted for improved ride quality due to lower pinch flat risk. What happens with a lower psi? A larger tire contact patch. Why does this matter? Bike companies pretty much all agree that a slightly wider tire on the road at a lower psi will improve grip for cornering.

In summary, even though F = coef. of friction X N is an immutable tenant of physics taught in every introductory engineering class, the reality is changing contact patch does change coef. of friction because of heat and shape and the irregularity of road surfaces which changes grip between a small patch of contact with the pavement compared to a larger one. This is why every superbike race where speeds reach 180 mph on the straight has a much wider tire in back compared to the front. Power to the rear wheel causes tire spin which creates heat which would shred a narrow tire subjected to the same cornering force of the front tire. Contact patch and tire width matter for traction, just not because of a simple equation but rather due to other factors and why tire widths are so carefully choosen in every application balancing rolling resistance, ride compliancy, handling, weight, wear....a very long list of reasons.

meanwhile

Ok: you obviously don't know anything about cars - the 911 design was notorious for decades for breaking away during cornering. This was inherent to the design features you are ranting about, eg

The Complete Story of Porsche 911
The rear-bias was always a problem to 911's handling. Any tail-heavy cars have a tendency to oversteer. If such oversteer is not adequately suppressed, lost of control may occur.

..Modern 911s are the products of decades of engineering to fix that design problem while still reaping the other benefits of their design.

They're often not; it varies with model. The ones that go that way are the huge power output turbos, which would otherwise wheelspin in acceleration.

No, you're not. Trust me: I have a physics degree. The equations for tyre behaviour are horribly complicated and no one who could say that bs about the 911 has them down. (In fact, the real physics are so complicated that in practice no one uses them - instead people work with a sort of complex non-physically based set of approximations called Pajeka's Equations.)


..This is taught for INELASTIC objects. Ie. NOT FREAKING TYRES.

However, wider tyres - well ones with a wider contact patch - do grip better. And not just slightly better - the difference can be very large. This is because
...This forces doesn't behave like inelastic friction: at a certain intensity it just fails. So a bigger contact patch pays off by allowing total cornering force to be increased. An MTB or BMX with good slicks can corner in a way that is impossible for a road bike.

(But even the above is an over-simplification because camber force also plays a large roll in bike cornering: Camber thrust - Wikipedia, the free encyclopedia)

meanwhile

Because you say so. Completely. Utterly. Wrong.

..Unless you're using solid steel "tyres", which would indeed obey the laws of friction you were taught for inelastic objects.

Seriously: is this hard? Does anyone have any problems understanding that tyres are soft and springy instead of hard and smooth for a reason? Columb's Laws Of Friction are for INELASTIC OBJECTS; tyres are ELASTIC. In fact, this is the point of having them, rather than using solid steel wheels.

meanwhile

If you are saying that more weight per area of patch is good, then no - that's exactly wrong.

meanwhile

..Literally For Dummies. The laws here are for INELASTIC objects only, which the dummy of an author forget to state! They do not, not, never in this universe, apply to tyres. (You can prove this by experimenting with a grippy object like a pencil eraser - it will grip more when you use the sides with larger area.)

You should be able to see why patch size matters if you look at this diagram from the wikipedia article:



..This is completely unlike the coulomb friction model Dr Dummy was explaining. Which is why we use rubber for tyres and not steel.

This may seem confusing, but it shouldn't be once you understand that what you were told were "the" laws of friction are just a model for friction in one class of objects - inelastic ones.

Caliper

Sadly, so do most Physics 101 profs. Many things in those type of courses that are "true, but...". Sure, you've gotta fit it all into one semester, but a quick disclaimer to state the limitations of what you are teaching would do the subject far better service.

meanwhile

Oh great - depress me even more!

The wikipedia friction article is technically correct, but the language is so obscure that 99% of people won't understand it.

Caliper

Moving back to our topic though... Has anyone out there actually reached the limits of road bike handling? I mean, a tire sliding on clean level pavement simply because you are cornering too hard? Not sliding due to water, sand, gravel, bumps, etc? I know I've never hit that point personally.

In an entirely unscientific test, I have taken several bikes and leaned them over in the driveway while pushing as hard as I can against the seat and stem. Even the mountain bike with knobby tires could be leaned past 45 degrees before a tire slipped. Sure, it's a static test with forces that are too low and I'm sure the knobbies would give a healthy slip angle in a dynamic situation, but it's interesting to see the rubber still holding at angles I've never leaned the bike to. Perhaps someone is versed in tire dynamics for two wheel vehicles and could expand on those a bit?

wphamilton

Tires or steel - you were mistaken when you said that it only applies to inelastic substances.

meanwhile

Again your argument is based on "Because I say so!" - and utter nonsense. From wikipedia:

The Coulomb approximation mathematically follows from the assumptions that surfaces are in atomically close contact only over a small fraction of their overall area, that this contact area is proportional to the normal force (until saturation, which takes place when all area is in atomic contact), and that the frictional force is proportional to the applied normal force, independently of the contact area

So steel plates, but not rubber tyres! (Because tyres are flexible and under pressure, so they are under "close contact" across the entire patch - which is why tyre pressure matters and lowering it increases grip.) Once again, look at the diagram. Tyre grip is not the simple product of Coulomb friction but of the tyre making contact with the surface and then acting like a series of tiny springs - ie tyre slip. Tyre slip wouldn't exist without coulomb friction, but it is not the same.

If this is still too complicated for you then consider skidding - this is impossible under coulomb friction! Skidding isn't just movement because adhesion limits have been passed, it is the REDUCTION in grip because said limit has been passed. In coulomb friction, this never happens - if you move because force is too great, then braking/cornering force is not reduced. So ABS, turning into a slide, feathering the brakes, etc, would be pointless.

In fact what happens during a skid is that you lose the spring force of the tyre but still have columb friction - which is much lower.