I always thought that, on dragsters, centrifugal force acting on the spinning tire was the largest reason that the tire has a narrower contact patch while spinning than while stationary. The same would be true of passenger car tires except to a much lesser degree due to a much slower spin speed.
according to wikipedia "fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading".
Now what is cyclic loading? In the case at hand, one could probably argue that it happens at every rotation of the wheel at the contact patch, and probably even every time there is a tire warm up cycle. Constant higher pressure does not look like cyclic loading to me, but I might be way off.
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The issue isn't the pressure. The issue is the stress in the tire - and it changes as an individual section of the tire revolves through the contact patch. That's the cycle.
Quote:
Originally Posted by tasdrouille
re fatigue failure:
From my understanding of it, hysteresis plays the biggest role in tire fatigue. This seems to be demonstrated by the fact that tires which have been run at low pressure for extended periods of time are prone to cord failure and tread separation. So reducing sidewall hysteresis by means of increased pressure would appear like a good way to reduce the occurrence of fatigue failure.
That would br true up to a point. But we are discussing way beyond that point.
Normally, the problem with hysteresis is that the heat generated changes the properties of the rubber - and that causes the failure in the form of a separation between plies. You will hardly ever see actual fabric fatigue in a tire, but if you do, it will be caused by the cord going into compression. It's sort of like pushing on a rope - the rope doesn't work very well that way.
Last edited by CapriRacer; 05-30-2008 at 11:49 AM..
I always thought that, on dragsters, centrifugal force acting on the spinning tire was the largest reason that the tire has a narrower contact patch while spinning than while stationary. The same would be true of passenger car tires except to a much lesser degree due to a much slower spin speed.
Take a real close look at the part of the tire that is actually in contact with the ground at - oh, say - the 60 foot mark. The centrifugal forces are indeed causing the tire to grow, but the black marks being left are even smaller than the width in contact with the surface.
I always thought that, on dragsters, centrifugal force acting on the spinning tire was the largest reason that the tire has a narrower contact patch while spinning than while stationary. The same would be true of passenger car tires except to a much lesser degree due to a much slower spin speed.
You are correct about the dragster tires. The latest versions deflect much less, but if you took a picture of the tire near the finish line you would see it is almost oval. The tire narrows at the top of it's rotation (furthest from the road) and then is flattened back down due to the tremendous downforce created by the rear wing.
Passenger car tires have much stiffer sidewalls and cords than racing slicks, so actual narrowing of the tires by cetrifugal force at normal speeds is negligilbe.. Only by overinflating a tire can you reduce it's contact patch.
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You are correct about the dragster tires. The latest versions deflect much less, but if you took a picture of the tire near the finish line you would see it is almost oval. The tire narrows at the top of it's rotation (furthest from the road) and then is flattened back down due to the tremendous downforce created by the rear wing.
Passenger car tires have much stiffer sidewalls and cords than racing slicks, so actual narrowing of the tires by cetrifugal force at normal speeds is negligilbe.. Only by overinflating a tire can you reduce it's contact patch.
I think you will find that drag slicks are bias ply tires. It's the belts, not the sidewalls that prevent the growth due to centrifugal forces - and if the belt isn't enough, circumferential cap plies are added.
Normally, the problem with hysteresis is that the heat generated changes the properties of the rubber - and that causes the failure in the form of a separation between plies.
Like this? The cords appears to be good (or at least unbroken), but all the rubber was el-gone-o.
Happened to me, probably under-inflated. It was a company vehicle, I didn't bother checking the pressure when I took it for the week. Now I do!
Heard a very faint "tick" on the freeway. Slowed down a little, then gradually sped up to see if the rhythme of the tick correllated to speed. It did, and at close to 90mph there was this little crap-your-pants explosion. Handled it fine, despite needing to cross 3 lanes of traffic to get to the shoulder. The biggest pain was using the crappy stamped-steel lug wrench/jack handle by cell-phone light to change the thing at 5 am. I now drive this car full time, keep a real lug wench in the trunk, carry a flashlight, and check tire pressure on a weekly basis!
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Miles displaced by bike in 2008 - approx 68
Miles added to vehicle by driving to rides with buddies - way more than above To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
What I have understood so far is that the optimum tire pressure for safety and ride quality is the recommended pressure. How unsafe hyperinflation is is still a major question.
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If I have this right, the concern is the car following the grooves in the pavement. We tire engineers call that "groove wander" - although the term "tramlining" is also used (I prefer the former - a bit more descriptive, I think.)
What is happening is that the grooves in the tire are lining up with the grooves in the pavement and when edge meets edge, the tire tends to be follow the edge in the pavement. This phenomenon is going to be affected by the number of grooves in the tire (and the number of grooves in the pavement) and the amount of footprint pressure for each groove.
Obviously over inflating the tires is going to put more pressure on the center grooves - and those grooves will dominate the way the tire behaves. Less pressure will tend to engage the rest of the grooves and decrease the effect.
2 problems: While there are standards about pavement grooving - spacing, width - there are so many non-standard pavement grooving, it is difficult for a tire manufacturer to space the grooves in a tire to account for all of them.
Grooves in a tire are necessary to allow a pathway for the water when the tire rolls through the water - which is what the grooves in the pavement are also doing. In designing a tire, well defined grooves work better that grooves that are interrupted by the tread pattern. Plus the edges of a tread element are areas were the wear will be more rapid that in the center of the element. "Heel and toe" wear is a good examples of this at work. Well defined grooves tend to be more resistant to irregular wear, especially diagonal wear.
In your case, I would try decreasing the pressure and see if that doesn't help.
I was experiencing the groove wander when the tires were set to 42 PSI, so I don't think it will make a difference. I predicted this behavior before I bought the tire when I looked at the tread :
The Continental tread reminded me a lot of my old motorcycle tires (circa 1980's), which also had the "clean/flat" tread grooves and suffered from massive groove wander. On the new Continentals, I think the tread grooves are also very wide. If I had known this in my motorcycle daze, I would have gotten a tire with the diagonal grooves you describe :
I agree that the tire behavior is dependent on the groove of the road. There are some "groove zones" on the freeway that do not effect me. It always depends. It's like a "harmonic frequency" thing. In the back of my mind I wonder if these tires were designed for "European Highways" that may not have the same kind of "groovy" freeway design patterns that we do.
Part of being a Hypermiler is "driving the road ahead". This means following the 3+ second rule and looking for escape routes from possible accident scenarios. Despite the bad traffic my LA driving conditions are very mild. I don't have to deal with snow, mud, or torrential rain the way the rest of the country does.
Overall I am very happy with the tires. The GPS correction means I always get a 2% bonus whenever I fill-in my gaslog. That's happy stuff for me, .
CarloSW2
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I have noticed that in order for me to keep my tires @ 50psi I have to stay on top of them more often. At 44 psi I was checking my tires once a month with no problems. Now that I am up to 50psi I am checking them weekly and noticing a drop in pressure each time. The drop varies, 2-3psi in the rear, as much as 8psi in the fronts. I am going to blame the valve stems since they are the cheasy rubber ones you get at the tire store, but otherwise the wheels have no punctures ... and the FE gains I got are indisputable.
But yeah, I may rename my Vibe the Radio Flyer, because it does ride rougher, but coming from low riders, it is very tolerable.
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How unsafe hyperinflation is is still a major question.
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