Differences between 17” and 18” wheels
09-12-2014, 05:55 AM
#61
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I could have sworn he said he'd be happy to discuss it with you just a few posts prior. Huh.
Maybe he just got a little sleepy?
(Remainder of post redacted for compassionate reasons)
Maybe he just got a little sleepy?
(Remainder of post redacted for compassionate reasons)
Last edited by callipygian 911; 09-12-2014 at 06:02 AM. Reason: Unnecessary cruelty
09-12-2014, 05:51 PM
#62
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Josh, I think you found your mental equal in orangecurry right here in this thread and you two should discuss the issue that neither one of you seems to have a clue about.
If you truly want a discussion on suspension, wheels, tires and car behavior, you are welcome to open a new thread when you are sober. I'm sure other Rennlisters will be charitable enough to try and help educate you in return for great contibutins that you've made to this forum so far.
I have to go pick up my A6s for tomorrow.
If you truly want a discussion on suspension, wheels, tires and car behavior, you are welcome to open a new thread when you are sober. I'm sure other Rennlisters will be charitable enough to try and help educate you in return for great contibutins that you've made to this forum so far.
I have to go pick up my A6s for tomorrow.
09-12-2014, 06:33 PM
#63
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You don't seem like a nice person. Your manner smacks of insecurity and bullying. That's really all I need to know.
Funny how those with authentic expertise, the real gurus around here, tend to be kind, gentle, and have nothing to prove. Kind of the opposite of what I'm sensing from you, sir.
Regardless, be safe out there this weekend. May you find whatever it is you need to be less of an insufferable know-it-all...for your sake, and mine.
Funny how those with authentic expertise, the real gurus around here, tend to be kind, gentle, and have nothing to prove. Kind of the opposite of what I'm sensing from you, sir.
Regardless, be safe out there this weekend. May you find whatever it is you need to be less of an insufferable know-it-all...for your sake, and mine.
09-12-2014, 08:00 PM
#65
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BURNNN!!!
Damn. Good one, Mike.
Alright, alright, you won me over with that last one. I take it all back. Adjustable sways are for dumdums, even the RS ones. Let's be friends. Gimme a hug, you old so-and-so.
Damn. Good one, Mike.
Alright, alright, you won me over with that last one. I take it all back. Adjustable sways are for dumdums, even the RS ones. Let's be friends. Gimme a hug, you old so-and-so.
09-12-2014, 10:46 PM
#66
Drifting
Sorry to digress from all the ranting, but has anyone noticed significant changes in 993 fuel economy due to switching from narrower tires on 17" wheels to wider tires on 18" wheels?
More tread width means more grip but also more rolling resistance, right?
More tread width means more grip but also more rolling resistance, right?
09-13-2014, 05:20 AM
#69
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I noted earlier how the 18" wheels I have allow for the use of wider tires, which in turn gives rise to much greater grip in cornering. The subjective experience is a marked one, indeed, but it is also measurable. I can accelerate through corners at much greater velocities before the wider tires begin to chirp and break loose.
This phenomenon is somewhat confounding when one considers the equation for friction is shown by F=(μ)V where F is the frictional force, μ is the coefficient of friction, and V is the normal force.
There is no variable in this equation for area, also known as contact patch in our discussions. Amontons' Second Law reflects this and states that force of friction is independent of the apparent area of contact.
So, if you have twice the area of contact patch then you will likewise observe halved pressure spread across that area. You may pick any size or shape area ranging from a knife's point to a surfboard and the frictional force will be the same. Of course, the tip of a knife might dig into a surface and interlocking could occur, but this extreme example does not enter into a practical discussion of tire contact patch, so let's ignore it.
Since normal force is evenly distributed across a contact patch, it is going to have the same amount of frictional force no matter the tire contact patch size or shape and should therefore require the same amount of lateral force to break loose whether on 17 or 18 inch tires. Assuming diameter, compound, and tire pressure are held to be constant, a wider tire on an 18" wheel and a narrow tire on a 17" wheel will have the exact same contact patch area: it is only the shape of the patch that varies. The area is the same (unless the narrow tire is extremely narrow, then it will begin to affect the available room on the tire for the patch to inhabit. This is another extreme example that should be ignored for the purposes of this discussion).
So why do wider tires generally give rise to greater grip in cornering? According to this equation ... they shouldn't.
Please discuss!
Last edited by callipygian 911; 09-13-2014 at 01:29 PM.
09-13-2014, 07:17 AM
#70
Instructor
^Good question.
My take on it is that the coefficient of friction is not constant, and actually decreases as the normal force V increases. So even if for greater Vs you get greater Fs (Frictional forces), the actual ratio between the two gets smaller. The same goes for the pressures (dividing the equation by the area A) so you will get more grip out of a tyre under less pressure (wider) than out of a narrower one.
Again, my take on it and happy to be corrected.
On the other hand, I recently purchased a set of Fuchs for my 993 and actually did not know if moving to 18” or staying with 17”. At the end decided to stay with 17” due to the weight argument that was discussed some posts before. Did a quick calculation and assuming same wheel model and same tyre model (Bridgestone Potenza S02) the weight saving per corner is 1.6 kg at the front and 1.3 kg at the rear.
Great thread
My take on it is that the coefficient of friction is not constant, and actually decreases as the normal force V increases. So even if for greater Vs you get greater Fs (Frictional forces), the actual ratio between the two gets smaller. The same goes for the pressures (dividing the equation by the area A) so you will get more grip out of a tyre under less pressure (wider) than out of a narrower one.
Again, my take on it and happy to be corrected.
On the other hand, I recently purchased a set of Fuchs for my 993 and actually did not know if moving to 18” or staying with 17”. At the end decided to stay with 17” due to the weight argument that was discussed some posts before. Did a quick calculation and assuming same wheel model and same tyre model (Bridgestone Potenza S02) the weight saving per corner is 1.6 kg at the front and 1.3 kg at the rear.
Great thread
Last edited by Nautilus; 09-13-2014 at 07:25 AM. Reason: Typo
09-13-2014, 02:05 PM
#71
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I'm going on a limb here as an untrained physics layman:
You're right about the contact patch shift in shape, and therein is the difference in "grip" or friction. So let's say what you posit about the "pressure" of tire/road remains constant (as in knife edge vs surfboard akin to skinny tire vs wider); though the foot/lbs over the entire contact patch may remain equal between the two tires, the shape of that contact patch, as it deforms under lateral forces, determines which "holds" the road better against this lateral load.
Think of it this way, a wider tire's contact patch is rectangular under stasis, but when cornering it is more a trapezoid ...so let's just extend the analogy to "triangular-ish shaped" for the sake of argument, with a wide area across that one end of the triangle experiencing the most load. That edge of this "triangle" offers more "grip" than were it a skinnier tire, whose contact patch shape is less "triangular," thus offering less contact area where it really counts: the edge under lateral loading. And we all know triangles are stronger than rectangles under lateral force.
All of which to say, the shape of the contact patch under dynamic stress plays a significant role. Perhaps another analogy is two pieces of tubing: thin-walled steel and thick-walled aluminum. Aluminum is about 3x lighter than steel, but also about 1/3 its strength ...which then begs the question, why even bother with aluminum since its strength is equal, right? Wrong. Because the shape of the alum tube's walls (it's thicker cross section) offers greater strength under lateral loading than the shape (thinner cross section) of the steel tube's walls given the same overall weight for both. Another way of looking at this is reduce the amount of alum used, but by retaining the cross-sectional shape one can equal the strength of the steel tubing at less overall weight.
That's enough from this layman. Fire off the criticism
Edward
You're right about the contact patch shift in shape, and therein is the difference in "grip" or friction. So let's say what you posit about the "pressure" of tire/road remains constant (as in knife edge vs surfboard akin to skinny tire vs wider); though the foot/lbs over the entire contact patch may remain equal between the two tires, the shape of that contact patch, as it deforms under lateral forces, determines which "holds" the road better against this lateral load.
Think of it this way, a wider tire's contact patch is rectangular under stasis, but when cornering it is more a trapezoid ...so let's just extend the analogy to "triangular-ish shaped" for the sake of argument, with a wide area across that one end of the triangle experiencing the most load. That edge of this "triangle" offers more "grip" than were it a skinnier tire, whose contact patch shape is less "triangular," thus offering less contact area where it really counts: the edge under lateral loading. And we all know triangles are stronger than rectangles under lateral force.
All of which to say, the shape of the contact patch under dynamic stress plays a significant role. Perhaps another analogy is two pieces of tubing: thin-walled steel and thick-walled aluminum. Aluminum is about 3x lighter than steel, but also about 1/3 its strength ...which then begs the question, why even bother with aluminum since its strength is equal, right? Wrong. Because the shape of the alum tube's walls (it's thicker cross section) offers greater strength under lateral loading than the shape (thinner cross section) of the steel tube's walls given the same overall weight for both. Another way of looking at this is reduce the amount of alum used, but by retaining the cross-sectional shape one can equal the strength of the steel tubing at less overall weight.
That's enough from this layman. Fire off the criticism
Edward
09-13-2014, 03:19 PM
#72
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I'm going on a limb here as an untrained physics layman:
You're right about the contact patch shift in shape, and therein is the difference in "grip" or friction. So let's say what you posit about the "pressure" of tire/road remains constant (as in knife edge vs surfboard akin to skinny tire vs wider); though the foot/lbs over the entire contact patch may remain equal between the two tires, the shape of that contact patch, as it deforms under lateral forces, determines which "holds" the road better against this lateral load.
Think of it this way, a wider tire's contact patch is rectangular under stasis, but when cornering it is more a trapezoid ...so let's just extend the analogy to "triangular-ish shaped" for the sake of argument, with a wide area across that one end of the triangle experiencing the most load. That edge of this "triangle" offers more "grip" than were it a skinnier tire, whose contact patch shape is less "triangular," thus offering less contact area where it really counts: the edge under lateral loading. And we all know triangles are stronger than rectangles under lateral force.
All of which to say, the shape of the contact patch under dynamic stress plays a significant role. Perhaps another analogy is two pieces of tubing: thin-walled steel and thick-walled aluminum. Aluminum is about 3x lighter than steel, but also about 1/3 its strength ...which then begs the question, why even bother with aluminum since its strength is equal, right? Wrong. Because the shape of the alum tube's walls (it's thicker cross section) offers greater strength under lateral loading than the shape (thinner cross section) of the steel tube's walls given the same overall weight for both. Another way of looking at this is reduce the amount of alum used, but by retaining the cross-sectional shape one can equal the strength of the steel tubing at less overall weight.
That's enough from this layman. Fire off the criticism
Edward
You're right about the contact patch shift in shape, and therein is the difference in "grip" or friction. So let's say what you posit about the "pressure" of tire/road remains constant (as in knife edge vs surfboard akin to skinny tire vs wider); though the foot/lbs over the entire contact patch may remain equal between the two tires, the shape of that contact patch, as it deforms under lateral forces, determines which "holds" the road better against this lateral load.
Think of it this way, a wider tire's contact patch is rectangular under stasis, but when cornering it is more a trapezoid ...so let's just extend the analogy to "triangular-ish shaped" for the sake of argument, with a wide area across that one end of the triangle experiencing the most load. That edge of this "triangle" offers more "grip" than were it a skinnier tire, whose contact patch shape is less "triangular," thus offering less contact area where it really counts: the edge under lateral loading. And we all know triangles are stronger than rectangles under lateral force.
All of which to say, the shape of the contact patch under dynamic stress plays a significant role. Perhaps another analogy is two pieces of tubing: thin-walled steel and thick-walled aluminum. Aluminum is about 3x lighter than steel, but also about 1/3 its strength ...which then begs the question, why even bother with aluminum since its strength is equal, right? Wrong. Because the shape of the alum tube's walls (it's thicker cross section) offers greater strength under lateral loading than the shape (thinner cross section) of the steel tube's walls given the same overall weight for both. Another way of looking at this is reduce the amount of alum used, but by retaining the cross-sectional shape one can equal the strength of the steel tubing at less overall weight.
That's enough from this layman. Fire off the criticism
Edward
09-13-2014, 03:54 PM
#73
Three Wheelin'
we mix up causation and correlation and think that an increase in width causes an increase in coefficient of friction. Thanks to science, we know that can't be true.
A scientist may strive to increase the coefficient of friction of a given tire (with a different rubber compound) and also may happen to make it wider in the process, (maybe to reduce the pressure on a given area of soft rubber) but that's just a common coincidence.
09-13-2014, 04:34 PM
#74
Interesting meander on this thread, physics chat is above my pay grade but here's some more:
http://www.physicsforums.com/showthread.php?t=330790
http://www.physicsforums.com/showthread.php?t=330790
