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Old 02-05-2008, 04:42 PM   #31 (permalink)
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Thanks, Matt. Good post. I can't remember my college phsyics, and my text books were ruined by a leaky roof ; How does area affect friction *after* the tires are skidding?

I've been following this thread closely, and I've got to throw my two cents in because I've held a strong opinion about tire width for a long time, anyway. IMO, wide, low-profile tires are a fad, with no benefits except under the extremes of racing. I first noted them showing up on high-end sports cars and G machines (muscle cars with beefed up suspensions and tires designed for extreme cornering. ie, trying to pull one G on the skid pad). Then on higher end vehicles. And now they have filtered down to OE on nearly every model except the low end economy autos. Whether for perceived traction increases, a pricey look, a sporty look, or just plain better looks, people like them and they're everywhere now.

As fas as safety goes, my experience is that, as Matt pointed out, increasing the patch area distributes the weight over a larger area, reducing the force per area unit (psi, kg/cm2, <your units here>). While a large sticky patch may be desirable for racing and AndrewJ's high-speed turns under, as diesel_john called it, "ideal conditions", my goal is to get more force between my tire and the pavement for those less-than-ideal conditions. Rain, snow, and mud to be specific. THAT'S when I need traction, not on dry pavement, and a narrow tire gives it to me. Hydroplaning avoidance has already been mentioned, but I also get much better traction in snow with narrow tires.

I have to agree with diesel_john on both of his posts: Some of our practices are not safe. But for normal-driving safety, I believe skinny tires are the way to go. (and the narrow tires' reduced price and increased in FE of are icing on the cake )

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Old 02-05-2008, 05:05 PM   #32 (permalink)
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My personal opinion on this is that, yes, there is a point where certain aspects of ecomodding can become more dangerous then it is worth it, however, if common sense is used while doing the modifications, proper automobile safety can be retained.

On the note of higher tire pressure. Tires are rated for a particular pressure, car manufacturers recommend a pressure that is usually lower then this. The reason for that is to make sure that when you drive and heat the tires up, that the pressure does not exceed the tire's maximum...IE some drivers are less aware of what actually happens to a car when they are driving it so most people would not figure out that they need to inflate at a lower pressure then is on the tire, so the manufacturer of the car tells them. What I do is I inflate the tires pound by pound, at least initially. I will put them say, 4 pounds higher then the car company specifies and then drive and test the pressure after they have warmed up, if it is still below the maximum for the tire, I increase a few pound and repeat. I do this until the pressure just a few pounds, maybe 2 or 3 to give it a little room for extra heat, and I let the tires cool down, then I note what the cold pressure is so I know what to fill it to the next time I have to fill a flat. That is perfectly safe to do as it is within the tested boundaries of the tire's capability.

Doing a side mirror delete can be hazardous in some cases. Many cars have many blind spots as it is with their mirrors factory installed, so those cars would probably not be wise candidates for a side mirror delete modification. However, if done properly and not rushed a side mirror delete can be done such that the amount of gained blind spot is minimized to a very safe level.
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Old 02-05-2008, 07:35 PM   #33 (permalink)
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Quote:
Originally Posted by NoCO2 View Post
On the note of higher tire pressure. Tires are rated for a particular pressure, car manufacturers recommend a pressure that is usually lower then this. The reason for that is to make sure that when you drive and heat the tires up, that the pressure does not exceed the tire's maximum....
I doubt it. It has more to do with ride quality and over/understeering characteristics than minute temp changes.
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Old 02-05-2008, 09:30 PM   #34 (permalink)
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as far as contact patch goes, wider is better in the sense that more traction becomes available be each square each has to work LESS to hold the car in place. no matter what type tire you are using, a wider version of that same compound/tread design will loose traction later. and proper tread design negates most worries about hydroplaning anymore. Snow and mud have 2 completely different needs. snow needs more weight on each square inch to pentrate the surface layer to find traction, mud needs large blocks with wide open grooves "clean" out packed in mud which causes one big, slick, spinning mudball.

the real issue becomes balancing ALL of these needs into a vehicle that can take daily driving abuse in climates from the equator to the poles, all at reasonable pricing.
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Old 02-05-2008, 10:43 PM   #35 (permalink)
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Well if you are concerned about reduced contact patch; take a look at SCCA auto Xsers. We Pump our tires UP to do essentially a minute or so long Emergency maneuver.
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Old 02-07-2008, 02:22 AM   #36 (permalink)
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Quote:
How does area affect friction *after* the tires are skidding?
The equation is the same but the coefficient is different (lower). That is why it is important not to enter a skid, since when that happens you immediately lower the available grip, hence ABS. Contact patch has the same (ir)relevance either skidding or rolling (except when the surface under you can move).

Quote:
as far as contact patch goes, wider is better in the sense that more traction becomes available be each square each has to work LESS to hold the car in place. no matter what type tire you are using, a wider version of that same compound/tread design will loose traction later.
Each square each has to work LESS to hold the car in place but has less ability to do so because it has less reaction force. Its like replacing 8 horses to pull a cart with 16 Shetland ponies so they can do less work each. If they can only pull half as much then they put out the same pulling rate. A given compound on a given surface has a coefficient of static friction, the available friction (force) increases linearly according to that coefficient and the reaction force between the surfaces. Area is not factored into the equation.

Quote:
the real issue becomes balancing ALL of these needs into a vehicle that can take daily driving abuse in climates from the equator to the poles, all at reasonable pricing.
You only need to balance the needs that you actually have not the needs that some other human on a different part of the world may have. I highly doubt anyone regularly makes trips from the equator to the arctic. Each person will have their own needs depending on their driving environment. I live in the city/suburbs of Sydney and never have to drive in mud or snow so it would be a waste to buy all terrain tyres. That's why there are speciality tires which you can cater to your needs. I think the real issue is finding the MOST EFFICIENT tyre that can still meets your needs, which will be less for people in snowy/unpaved regions but the principle still holds. An ecomodder might even decide it is worth an afternoons work and higher initial cost to swap a winter and summer set of tyres so they aren't meeting needs which aren't there.

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