-A certain amount of energy goes into deforming the rubber when it hits the road. The square footage of rubber that meets the road per rotation is a function of tire width and does not change with contact patch. In other words, a 195mm tire has 5.4% more area that must make and break contact with the road surface than a 185mm tire.
-In poor conditions, a fat tire means you displace more water and snow.
-A larger/fatter tire may mean more rotating mass, which can be a small factor in city mpg.
-A thinner tire will help shave those last few square inches off your frontal area.
-Fatter tires tend to have lower max psi ratings. Yes, a 225mm and 195mm tire will have similar contact patches at the same PSI, but chances are, the 195mm tire will allow for more pressure (and therefore a smaller contact patch) than the 225mm tire. I'm no tire engineer, but it would seem easier for a tire to preserve its composure under pressure when the distance between sidewalls is minimized.
-A certain amount of energy goes into deforming the rubber when it hits the road. The square footage of rubber that meets the road per rotation is a function of tire width and does not change with contact patch. In other words, a 195mm tire has 5.4% more area that must make and break contact with the road surface than a 185mm tire.
Good points, but I have a question over the first statement.
Generally, a wide tire and a skinny tire have a contact patch of equal area at a given psi, right? If the skinny tire has a square contact patch, the wide tire would have a thin rectangular one.
Wouldn't the area of tire deformity be equal on both? A wider tire has more area than a skinny one, but less of it is in contact with the ground at a particular moment. A wider tire will also cause a smaller area of the sidewall to flex at a time, which intuitively seems more desirable then decreasing tread flexing.
I've only tentatively gone over this in my mind, so sorry if I've made a blatant mistake...
Wouldn't the area of tire deformity be equal on both? A wider tire has more area than a skinny one, but less of it is in contact with the ground at a particular moment.
For any given moment, yes. Per mile however, the fatter tire has a larger square footage making and breaking contact with the road. That square footage has to deform over every imperfection in the road. Visualize a roll of toilet paper unraveling. The fatter the roll, the more square footage it's going to cover per mile of length.
Nicely put. This has become a very informative thread. I think you have answered why square contact patches are better than skinny ones.
I believe this thread has been definitively answered:The lowest rolling resistance can be achieved by the skinniest tire that allows a high enough psi to maintain a square contact patch.
Now that the issue of excessive tire width has been determined, I'd like to look at the opposite end of the issue. When do tires become too skinny?
I've heard that the lowest rolling resistance that is routinely achieved are steel wheels on steel rails (trains). If that is the case, wouldn't the ultimate conventional road tire be one that is under high pressure and sized just large enough to prevent road damage/deflection from localized pressure? I realize tradeoffs are being made between efficiency and practicality, but practicality aside...
Lastly, how does silicon lower a tire's rolling resistance? I'm pretty sure the main additive in LRR tires is silicon, but what benefit does it provide. I'll research on my own, but if anyone knows the answer already...
Just out of curiosity spurred on by this thread and cbergeron's new set of tires, I've been searching the net for the skinniest set of wheels and tires around.
I'd have to run ridiculously high psi to achieve a square patch, or achieve some radical weight loss on the car, or both. It just may be that the stock size is already the optimal one.
Thanks for the Coker idea. I'll take a look. Edit: (Original thread updated)
1.) Could someone verify how you calculate the psi needed for a square contact patch?
I'm assuming it's just:
vehicle weight / (4 * area of square contact patch)
__________________________________________________ _______________
Hypothetically, a CRX HF that weights 1700lbs running a tire psi of 70 (reasonable for a "temporary" tire) would only need tires 2.46" wide (62mm).
My Thunderbird, which weighs ~3569lb, running 55psi would only need a tire 4.03" wide (102mm).
Tire width would need to increase with load (driver, gas, oil, etc.) and traction would be severely reduced, but does this sound reasonable? Those tires would be extremely skinny.
2.) If skinny enough tires don't exist, what is better: a square contact patch or max psi?
A higher psi causes more rubber to deflect than an ideal tire, but it is still an equal amount to the square contact patch of a lower psi. I would say higher psi simply because the amount of rubber being deflected will always equal, but the tire will be stiffer and therefore incur less friction.
I think this is another relevation:In the absence of an ideal tire width, maximum psi offers lower rolling resistance than a square contact patch.
Assuming the conclusions being made are correct, this thread has been extremely useful to me. I'm surprised...and happy.
- LostCause
Last edited by LostCause; 05-08-2008 at 01:57 AM..
What I have found is, there is quite a difference from the hypothetical calculations and the real world experiment results. All my calculations showed that a 155 series tire would have a square patch at 50 psi on the front; but then my experiment showed that airing up my 185 series tire to 50 psi STILL wasn't enough to achieve a square patch.
"Could someone verify how you calculate the psi needed for a square contact patch?"
I'd go by axle weight rather than total weight/4. Most cars, being FWD, carry about 2/3 of their mass on the front axle unless really loaded. Then once you have a sq in value for the front and rear patches, you need to know how wide the tread is THAT IS CONTACTING THE GROUND.
Good point about the weight distribution. I hadn't though of that. I wonder how much of the tire actually lifts from the sides, but I figured it would be small. I didn't notice any appreciable change when I pumped up my tires, but you are right, contact patch is smaller then tire width and should be noted.
I'm wondering if the psi issue has something to do with the tire being a flexible toroid rather than an impermeable cylinder. The only thing I can come up with is that a tire is flexible, so it is subject to differential stress. Like a rolling pin pushing on dough, some of the force is being redistributed to the sides rather then straight down.
An impermeable cylinder would allow the entire air pressure to support the axle while the toroid must share that pressure with the axle and the further compression of the rest of the air in the tire (plus wheel bulging).
I might be getting a little too deep and lost into all of this, so feel free to set me straight. If anyone has been able to achieve a square contact patch, would they mind posting vehicle weight, tire width, and psi? Now that I think of it...I should probably perform my own test.
- LostCause
Last edited by LostCause; 05-08-2008 at 02:20 AM..
First off, I dont know why we are trying to get a square patch, an article in a tire ad? i measured my mountain bike tire. front was 3 by 1 and back was 4 by 1, at 50 psi, i know those square measurements but i weigh 250 and it come out to 350. The point is though i know that my mountain tires are not fe compared to road tires. The tires should still be skinnier although the patch is way longer than it is square. I then measured my car front 4.75 by 4.24 averaged and figured round (by franks sugestion) give me a weight of 1177 back was 4 by 3.75 both with 50 psi and a weight of 706. Total of 2354 lbs and my car weighed 2300 on the scales. By the way to get an accurate area use carbon paper, jack car up and put under, let down. Use that paper to figure area and make sure that ur presure gauge is accurate. Still I would think that a square contact patch is not a goal, like a bicycle longer and skinnier will be better.
WOW! deep stuff!
All I can say at this point is that my Yota truck will soon need a new set of skins. I intend to go one size skinnier and see what happens. All of this will be documented as a success/fail project. Damn the books!
I think the reason for a square contact patch was well developed earlier in this thread. Using an ideal tire width, a square contact patch minimizes overall wheel deflection (stationary and moving).
The rationale bicycle riders use against excessively high tire pressures is that slipping begins to occur. I don't think the issue has anything to do with patch geometry. If slipping and road surface deformation can be limited, higher pressures should theoretically be ideal.
The circular contact patch issue seems logical, but I'd assume it would look more like a rounded off square than a circle. Without a more advanced mathematical technique or a super-specialized scale, I have no idea what the pressure distribution is like. For that reason, I think a list showing "ideal" and "actual" car widths to achieve a square contact patch should be made. If we get an idea of how far off the ideal is from reality, it'll be easier to say what width tire is ideal for a given car.
To that end, I'm going to measure the contact patch geometry of a few cars I have access to and I'll post the difference between ideal width (taking into account weight distribution) and the actual needed width (if I don't far exceed max PSI). Also, I'll post contact patch width vs tire width.
FrankLee already posted his data:
1993 Ford Tempo - 2520lbs Loaded (Front - 1590lb Rear - 930lb)
Ideal CP: Front - 119mm Rear - 106mm
Actual CP: Front - 127mm Rear - 127mm
Actual/Ideal: Front - 107% Rear - 120%
Actual tire width: 185mm
Actual CP width: 127mm
CPW/TW: 69%
While it's not perfect, it should give a general idea...
- LostCause
Last edited by LostCause; 05-14-2008 at 04:49 AM..
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