LOL re BMWs. Had three of them in the past... now my wife sends me pics like this:



As for the tire debate, here's a starter to point you in the right direction.

People who believe that wider tires produce more grip because of a larger contact patch are wrong. They are wrong in both that they produce more grip and that the contact patch is actually larger. All other things being equal, for a vehicle of a given weght the SIZE of the contact patch will be the same between a narrow and a wide tire. It's the SHAPE of the contact patch that changes, not the actual area. A wide contact patch is primarily about temperature management, not the total amount of friction, and also about cornering stability vs. linear acceleration. For those of you that want to get into the physics of it, here's a link to an interesting article on the subject - http://insideracingtechnology.com/tirebkexerpt2.htm - that deals with slip angles, tread deformation, etc. If you want to skip the physics part and get to the gist of it, the conclusion is that a wide but short contact patch gives better cornering performance while a narrow but long contact patch gives better linear acceleration. This is true with all tires, not just winters.

So - now that we know that the actual size of the contact patch is more or less the same between the narrow vs. wide tires (and as I said earler, wider tires are about heat mangement, deformation and slip angles), when it comes to winter tires all you have to do is look at the tread pattern on the tire (think of how the little sipes that winter tires have are positioned) and figure out what will give you better traction - a wider and short contact patch or a narrow and long one?

For those of you that are old enough to remember Linda Richman's (Mike Myers) Coffe Talk on SNL... I'm getting a little verklempt. Talk amongst yourselves.

 

You haven't actually answered the question. All you've done is said "it's so simple, just look at the tires and the answer is obvious". Well, explain it. If a staggered and wide tire setup is desirable on a C63 in the summer, when you're driving on clear pavement, why is the same not true when you're driving on clear but cold pavement, or packed down snow?

Again man, two tire manufacturers, one of whom only makes winter tires, don't support the old belief that narrow tires are automatically superior in winter conditions.

 

Yes I have. My answer is that a staggered setup is NOT desirable in the summer either! It is necessary for temperature management of the tires when only one axle is driven in higher hp applications when the tires are prone to spin. On any well-balanced track car with a 50/50 weight distribution, the most neutral handling will always be provided by a square setup.

 

Sure, I don't disagree with you there, but that's a hypothetical based on unrealistic assumptions. The C63 isn't a 50/50 balanced car. Evidently Mercedes felt a staggered setup was appropriate, and I'm inclined to trust their judgment on my stock car.

If your claim is not that a square setup is best, why not a square setup running wider tires? I think you know what the real question actually is...

 

No - that's not hypothetical. Every M3, 924, 944 / Turbo and 928 I've ever raced has been outfitted with a square setup.

The reasons why the fronts are usually not as wide as the rears on a street-driven vehicle have nothing to with grip levels. It is about the vehicle's propensity to tramline and also chew up the fronts in a much shorter amount of time as the shearing forces across the contact patch itself during a turn are about 3-5x greater than on the rear (the fronts travel at a much tighter arc - it's why you hear the front tires "skip" on wet concrete).

 

All different cars. Miatas are also square setup. S2000s are not. Every car I have owned, other than the C63, have used square setups so I don't know how you can say "fronts are usually not as wide as the rears on a street-driven vehicle have nothing to do with grip levels".

Here, we are talking about the C63. Not an M3, or Porsche. The engineers at Mercedes evidently thought that a staggered setup was appropriate. That isn't the question or issue here anyways.

Again, why are narrow tires better on cold pavement and packed down snow? That is the question.

 

The engineers determined that you need wider tires at the back to be able to deal with the higher heat. And, those very same engineers also determined that when tire overheating is not an issue - like in winter or in snow - that a SQUARE setup is the prefered one for the C63.

If that is not what you're asking, then I apologize as I honestly don't understand your question... I am NOT trying to pick a fight.

 

Guys we all know staggered looks better haha

 

OK, I'll repeat my question again.

Why is a narrower tire better on cold pavement and packed down snow? That is the question that lead us here, not the staggered vs. square debate.

 

Did you read that after I already posted it?

 

I didn't see you posted it.

 

Those are good informative points from Diabolis from post #76 onward!

 

On a hard surface at *any* temperature, a narrower tire with a longer contact patch (hence the large diameter wheels that you see on 5000 hp dragsters) will give you more longitudinal traction... so if you look at the physics of it, a narrower tire with a longer contact patch gives better grip while accelerating or decelerating than a wider one with a shorter contact patch (keep in mind that the total patch area is the same between the two) because of the way the rubber deforms. This answers the "cold pavement or packed snow" scenario, as well as the "hot drag strip" one. Again - what am I missing? If you are asking why a narrow but longer contact patch gives better longitudinal traction than a wider but shorter one, read the article in the link I posted - it's physics and tribology, which is way beyond the scope of a simple two-sentence explanation.

On powdery snow or slush - which you didn't ask about - the narrower tire also cuts better through the snow / slush in order to make contact with the solid surface underneath, and this is when a narrower tire also works better than a wide one - think a sharp kinfe vs. a dull one, or a pizza cutting wheel vs. a roller... in this case it comes down to simply pressure per unit area.

 

Thanks for the link, I read it in detail and I didn't see where it mentions what I underlined above. I may have missed it, unless you're just extrapolating it from how the tire deforms.

I dont understand how heat management can be completely ignored. Just because the external temperature is colder, doesn't remove heat from the equation.

This is the other part that I am hung up on. If I were to take an identical winter tire in a 225 and 255 section width adding some weight to the 225 so they weigh exactly the same. Then drag each across the ground (both with and against the tread direction) using a newton meter. You're saying they would both require the same amount of force to move?

I'm not seeing it.

You quoted the post I posted it in lol...

 

The contact patch length is virtually unchanged since the OD would barely change.

 

So by your logic, we should all be running narrow tires during the summer, right? Something doesn't add up here. The old timer racers wouldn't run narrow tires during the summer. So by an extension of that thinking, running narrow tires during the winter, when you're dealing with compact, hard surfaces, doesn't make sense either. You may have an initial traction benefit, but I don't think that makes up for the decreased cornering and handling. The same can be accomplished with a higher sidewall (longer contact patch).

I understand the physics side of it, as well as the wider tire myth.

You are correct that I didn't ask about powdery snow or slush because I already stated that there is no denying that in situations where you need to cut through a material, a narrower tire is better.

 

Still missed it, oh exalted one. I'm trying to read posts and work at the same time. It's a losing battle.

 

The tire would need to be quite a bit taller to elongate the contact patch length right?

Where I live, most of the time I'm on cold dry roads. We're also talking a 235 vs a 255 so the contact length should be virtually identical. In the case of cold dry roads there should be no decrease in rotational grip and a slight increase in lateral grip according to that article.

 

I imagine so, but I have heard of people doing it, for the "gainz". To be fair, a lot of us talk about putting wider tires on our cars when the listed change in width is only 1 cm or so.

I am concerned about the same road conditions. We get massive dumps of snow here, but I don't need to drive under those conditions; I have a second car for that and typically use public transit to get to and from work anyways. What I need to deal with is the same cold dry roads.

 

If we want maximum longitudinal acceleration, then absolutely, yes - the largest diameter wheels with the lowest pressures (narrowest and longest tire contact patch) will yield the best off-the-line performance (dragsters are a perfect example, and I do mean the rears, not the fronts). If, however, you're trying to corner at 2Gs, then the narrow tires are not so good, and you need to sacrifice longitudinal traction for lateral - which is where a wide but short contact patch works better. When you need to be able to both accelerate and corner, you end up with a tire design that is a compromise between the two.

Also, I barely touched on this in my previous posts, but the most important thing to remember about tires is that they are designed to work at a particular temperature. This is where heat management comes into play - wider tires with a low sidewall cool better than skinnier tires with a higher sidwewall. If you can't get the tires up to temperature they won't grip. If you get them too hot, you'll cook them and kill them... which is why a tire pyrometer is a very useful tool. The same principle applies to brake pads - track pads don't bite when they're cold, and street pads get cooked at the track. You need bigger rotors and pads - the wider tire in this analogy - to dissipate the heat, not because they will make the car stop any faster than the smaller ones.

 

I get your theoretical arguments. However, I think it is incorrect to conclude that in the real world, a narrow tire is better because of a theoretical increase in longitudinal acceleration for the very reason that you bring up; you have compromised handling.

I assumed, perhaps incorrectly, that we were discussing winter tires designed for cold weather conditions. Again, I assume, perhaps incorrectly, that tires designed to remain flexible and soft at cold temperatures won't have to "get up to temperature" in the same way that a high performance summer tire will, and are designed specifically for cold conditions.

You seem to be avoiding answering the question I've posed several times now.

 

All other things being equal, yes. However, in practice you will run lower pressures when you have more sidewall, which elongates the contact patch length. That's why even F1 cars run 15" wheels. I can't think of any auto racing application where the tires are narrow rubber bands stretched on huge rims... they won't be able to either accelerate or decelerate.

Again, all other things being equal, the decrease in rotational grip on a dry road will be proportional to the increase in lateral grip because the shape of the contact patch is going to change. For most of us driving in winter (and in particular in high-torque cars like the C63), rotational grip is probably a tad more important... when it's -20 outside, I usually don't take any on-ramps at speeds that could get me in trouble.

 

I would put money on holding higher Gs on a dry skidpad with wider winters.

 

Agreed, and I would put money on the decrease in acceleration being minor on dry but cold roads compared to a narrower tire.

 

I don't see how contact patch length would change between a 225/40-18 and a 255/35-18 to make a difference in rotational grip considering the OD difference is less that 2mm.