Ok, call me nuts! 18" wheels...
12-29-2012, 03:17 PM
#21
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Agreed that a narrower tire is less likely to "float" over the top of snow. However, the contact patch is essentially the same overall size (discounting differences in tire construction) on any given car, regardless or the wheel/tire combo. The reason for this is that a tire is, for want of a better description, a well-constructed balloon. Contact patch size is determined by inflation pressure and vehicle weight - psi/p(lbs) = si(Sq. inches).
What changes, however, is the shape of the contact patch. A wide tire's contact patch is short and wide (which adds stability in cornering), while a narrow tire's cp is long and narrow (which, as you pointed out, is more easily able to cut through snow to reach the pavement underneath).
Also agreed about the benefits of a taller sidewall in these circumstances.
What changes, however, is the shape of the contact patch. A wide tire's contact patch is short and wide (which adds stability in cornering), while a narrow tire's cp is long and narrow (which, as you pointed out, is more easily able to cut through snow to reach the pavement underneath).
Also agreed about the benefits of a taller sidewall in these circumstances.
The first reason is that even given the physics you are quoting, you would have to assume that the tire pressure remains the same when you change tires...which it does not necessarily do.
The second reason is that your sidewalls actually do a lot of the weight bearing...modern tires are not balloons any more.
If you get bored some day you can prove this to yourself by driving your car onto 4 sheets of graph paper, tracing the outline of the tires onto the paper and measuring the contact patch area. When you multiply by your tire pressures you will come out with a number below the actual weight of your car...As you point out, however, the aspect ratio of a contact patch's shape is important...probably more important than any of what I wrote above.
As for the looks of the 17s, I'm not a massive fan of the look, but it definitely is getting a lot of positive attention from a lot of people. I went with 17s for the grip and tire choice, not for looks (who would choose 17s for looks on this car anyway?) and I'm getting more attention. Go figure...
The wheels are: http://www.tirerack.com/wheels/Wheel...=Sport+Package
12-29-2012, 03:26 PM
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12-29-2012, 03:44 PM
#23
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Well, either I don't know how to use Shutterfly or it nerfs photo detail...it's kinda hard to see what the wheels actually look like on the car in the version of the pic I posted above. Those who are interested may find this version of the pic more useful in judging the look...
12-29-2012, 04:22 PM
#24
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This actually isn't quite correct...for two reasons.
The first reason is that even given the physics you are quoting, you would have to assume that the tire pressure remains the same when you change tires...which it does not necessarily do.
The second reason is that your sidewalls actually do a lot of the weight bearing...modern tires are not balloons any more. If you get bored some day you can prove this to yourself by driving your car onto 4 sheets of graph paper, tracing the outline of the tires onto the paper and measuring the contact patch area. When you multiply by your tire pressures you will come out with a number below the actual weight of your car...
As you point out, however, the aspect ratio of a contact patch's shape is important...probably more important than any of what I wrote above.
The first reason is that even given the physics you are quoting, you would have to assume that the tire pressure remains the same when you change tires...which it does not necessarily do.
The second reason is that your sidewalls actually do a lot of the weight bearing...modern tires are not balloons any more.
If you get bored some day you can prove this to yourself by driving your car onto 4 sheets of graph paper, tracing the outline of the tires onto the paper and measuring the contact patch area. When you multiply by your tire pressures you will come out with a number below the actual weight of your car...As you point out, however, the aspect ratio of a contact patch's shape is important...probably more important than any of what I wrote above.
So, if people really want to get into it, the factors that determine contact patch size include, but are not limited to, tire width, sidewall ratio, sidewall construction, wheel width, vehicle weight and tire pressure. I still hold that these last two are the major variables.
Tire pressures for the street are set through manufacturer recommendations, so pressures for your winter and summer set will likely be the same. Regardless, the formula accounts for changing vehicle weight as well as pressure (disregarding the other variables stated). For the mathematically-disinclined, ceteris paribus (all else being equal), if four people climb into your car, the contact patch size will increase. Likewise, if you add air pressure, contact patch size will decrease.
12-29-2012, 07:02 PM
#25
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As for the looks of the 17s, I'm not a massive fan of the look, but it definitely is getting a lot of positive attention from a lot of people. I went with 17s for the grip and tire choice, not for looks (who would choose 17s for looks on this car anyway?) and I'm getting more attention. Go figure...
The wheels are: http://www.tirerack.com/wheels/Wheel...=Sport+Package
The wheels are: http://www.tirerack.com/wheels/Wheel...=Sport+Package
It's interesting that I clicked on the link and TireRack has the notice "This wheel is not available for your 2012 Audi A7 Sedan 3.0 Sport Package"
How much clearance is there between the front caliper and the rim? That seems to be the issue with 17's and some 18's. Thanks for the info.
If anyone is going to buy a set of wheels for snow use - try the bare rim in the car before mounting a tire to check clearance. Some places will not refund rims that have had tires on them.
12-29-2012, 07:13 PM
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c
It's interesting that I clicked on the link and TireRack has the notice "This wheel is not available for your 2012 Audi A7 Sedan 3.0 Sport Package"
How much clearance is there between the front caliper and the rim? That seems to be the issue with 17's and some 18's. Thanks for the info.
If anyone is going to buy a set of wheels for snow use - try the bare rim in the car before mounting a tire to check clearance. Some places will not refund rims that have had tires on them.
It's interesting that I clicked on the link and TireRack has the notice "This wheel is not available for your 2012 Audi A7 Sedan 3.0 Sport Package"
How much clearance is there between the front caliper and the rim? That seems to be the issue with 17's and some 18's. Thanks for the info.
If anyone is going to buy a set of wheels for snow use - try the bare rim in the car before mounting a tire to check clearance. Some places will not refund rims that have had tires on them.
There's about 1/4" clearance between front brake caliper and the wheel.
12-29-2012, 07:44 PM
#27
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12-30-2012, 01:39 PM
#28
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The minus 1 wheels (18") with Pirelli snows (235.55.18) are working out very will.
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