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Old 07-11-06, 12:30 PM   #1
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Which runs the greater risk of a flat?

Overinflation, or underinflation?

And, in either case, how much leeway until the tire pressure is unsafe?
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Old 07-11-06, 12:41 PM   #2
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Depends on tire width, lateral width of the rim, tire construction, tire fit on the rim, bike geometry, the weight and distribution of said weight of any load you're carrying, stated operating pressure on the tire, weight of the rider, terrain you ride on, terrian you avoid, personal preference, and how many chicks you want to pick up per ride...as well as a bunch of other things!

So the answer is, "It depends!"
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Old 07-11-06, 12:41 PM   #3
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My understanding is that overinflation makes it easier and more likely that objects will puncture the tire.

On a car, underinflation might drive up the heat factor and cause some seperation. If you hold your cycling speed under 50 mph though this shouldn't present a problem.
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Old 07-11-06, 01:34 PM   #4
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An old point of discussion here. Lower pressure is a little more puncture resistant but doesn't roll quite as efficiently on smooth road. It tends to absorb bumps on on rougher road and stay "grounded" better, thereby letting you go a bit faster (you're bouncing less)....and being more comfortable, go somewhat farther, too. At a certain point, lower pressure tires feel "sluggish" and a bit lower still and you have danger of "snake-bite" punctures where your tube gets pinched between the rim and the bump...pssssttt!

Recommended tire pressures are about as dependable as the advertized width....as Paulie says above, there are many variables.

And as that prominent virtuoso of the medical check-up has indicated, much of it is how your head perceives it to be.
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Old 07-11-06, 01:45 PM   #5
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Quote:
Originally Posted by Digital Gee
Overinflation, or underinflation?

And, in either case, how much leeway until the tire pressure is unsafe?
If you hear a loud "BANG!", then you are overinflated.
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Old 07-11-06, 02:07 PM   #6
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I operate mine at 10 lbs under the sidewall recommendation. Many of the rural roads in my area are chip sealed and have a rough surface. Roads in the city are not well maintained due to budget constraints and also are more and more chip seadled. I saw a recommendation to run somewhat underinflated on www.roadbikerider.com some time ago and tried it and liked it.
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Old 07-11-06, 02:32 PM   #7
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I like to run mine about 10lbs under too. Here is some interesting reading..
http://sheldonbrown.com/tires.html#pressure
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Old 07-11-06, 06:28 PM   #8
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Too much underinflation leads to the greater potential of snake bites. However my experience has been that there is not a correlation between over and under (within reason) to flats.

There is correlation that wet conditions lead to more punctures.........and also any roads that Dnvr rides with goatheads!!!
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Old 07-11-06, 07:21 PM   #9
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To much of either is bad.
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Old 07-11-06, 07:41 PM   #10
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"Once upon a time, there was a little girl named Goldilocks. She went for a walk in the forest. Pretty soon, she came upon a house. She knocked and, when no one answered, she walked right in.

At the table in the kitchen, there were three bowls of porridge. Goldilocks was hungry. She tasted the porridge from the first bowl.

"This porridge is too hot!" she exclaimed.

So, she tasted the porridge from the second bowl.

"This porridge is too cold," she said

So, she tasted the last bowl of porridge.

"Ahhh, this porridge is just right," she said happily and she ate it all up.

After she'd eaten the three bears' breakfasts she decided she would go for a bike ride. So, she walked into the garage where she saw three bikes. Goldilocks sat on the first bike.

"These tires are too hard!" she exclaimed.

So she sat on the second bike.

"These tires are too soft!" she whined.

So she tried the last and smallest bike.

"Ahhh, these tires are just right," she sighed. But just as she settled down on the bike to ride, she had two flat tires!"
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Old 07-11-06, 08:09 PM   #11
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I weigh 175 lbs and I inflate the 700x23 tires on my singles and andem to 120 lbs. The track tubulars get 130 lbs (195g Tufos) unless I'm racing, in which case they may get 160 to 180 lbs.

RoadbikeRider has this dude named Uncle Al who claims that using too much pressure is bad. He advocates 90-95 lbs. I gave it a try, and ended up with pinch flats, so I now disregard anythang that dude says.

Two possible problems with too much pressure: It could be bad for the base of the valves, especially if they are the long 160mm variety. And if your tread is almost gone, it could exacerbate weakening of the underlying cord, leading to a blowout at the worst possible time and place. Yes, I speak from experience.

Bottom line: Overinflate! The bike feels much more lively.

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Old 07-11-06, 08:53 PM   #12
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I weigh upper 160's, ride 25's, and inflate the back to 95, front to 90. Pinchflats an extreme rarity...maybe once 2 years ago. What Ihb. above calls lively, I find rough and bouncy. I ride some pretty crappy back roads and welcome a smoother, grippier ride...especially on curves which may be sandy, gravelly, etc.

Again, its all a matter of perception and taste and, thankfully, there's room even in tire pressure choices for most of us to find our own style and taste.
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Old 07-11-06, 08:56 PM   #13
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DG
You know we did this 3000 mile tour cross country on expressways and secondary roads.
I had only three flats (and did not like those) Some had up to twenty flats and four flats/day. That is no fun.
OK, why?
One guy used Specialized Armadillo with Kevlar strips. Had 4 flats in one day. I was close enough to him to know why. -----Low Pressure-----. He used 100 PSI. Had snake bite punctures. You could not tell him anything, he knows everything better. He is German also.
The king of flats used (IMHO) cheap wheels. He was a very tall guy on a lower budget bike and wheels.
The tires rolled in the not deep enough rim and guess what. Twenty flats.
I did not make a study, just throw money at the problem. Used Continentals with Kevlar strips, pretty good wheels and thicker tubes. It worked well enough.
BTW, the problem was the residue of the steel belted radials at the side of expressways.
I was very tempted to lower the pressure because of the lousy road surfaces we were on. My teeth rattled. But I do not like to fix flats. Takes too much time.
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Old 07-11-06, 09:11 PM   #14
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Will above speaks well. I ride Conti Gatorskins and find them to be a very nice blend of flat resistance and nice ride. Tough, but not as heavy as Armadillos. Again, snake-bite punctures have never been much of a problem at my 90's pressure for me. Maybe riding style is also a determiner here.

Next to our local goat heads, and slivers of glass, those little thread like wires from belted tires are often at fault for punctures.
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Old 07-11-06, 09:23 PM   #15
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Quote:
Originally Posted by CrossChain
Will above speaks well. I ride Conti Gatorskins and find them to be a very nice blend of flat resistance and nice ride. Tough, but not as heavy as Armadillos. Again, snake-bite punctures have never been much of a problem at my 90's pressure for me. Maybe riding style is also a determiner here.

Next to our local goat heads, and slivers of glass, those little thread like wires from belted tires are often at fault for punctures.
Yes, riding style it is. We were VERY aggressive bikers. It was not supposed to be a race but guess what?
In the quest of one-upmanship, we disregarded common sense and sometimes safety.
BUT, we did have fun, and flats. The above mentioned snake bites were caused by going over bricks for heavens sake.
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Old 07-11-06, 09:29 PM   #16
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BUT, we did have fun, and flats. The above mentioned snake bites were caused by going over bricks for heavens sake.
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Old 07-11-06, 09:32 PM   #17
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I'm light.... for me, it's crap in the road, mostly glass
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Old 07-11-06, 10:48 PM   #18
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Quote:
Originally Posted by will dehne
BTW, the problem was the residue of the steel belted radials at the side of expressways.
Quote:
Originally Posted by CrossChain
Next to our local goat heads, and slivers of glass, those little thread like wires from belted tires are often at fault for punctures.
My most bizzare flat ever -- and I've had about a million of them -- was one time when I was out on a new set of tires. I got a flat in the rear, and when I inspected the tire, there was a wire from a steel belt going entirely through the tire....in one side, through the tire, in the other side, and out. And, it was right against the end of the rim. Perfectly parallel.

How did it get there?

How did it have enough force to go through both sidewalls???
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Old 07-12-06, 12:48 AM   #19
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The stated pressure on the sidewalls is a very good guide to what you should be using but I never go by that. The pressure I use on the solo MTB is just within the top limit of the tyre at 55psi. top limit is 60. Any lower and I find that drag is too great on the hard pack trails, and snake bites are a problem on the rough downhills. Now the Tanden is different. Same tyre but a minimum of 65psi. Any lower and we have a tyre that is running flat (Due to 400lbs all up weight) and the snakebites mean a wrecked tyre and a big off.

Higher pressures may mean a higher chance of punctures, but low pressures are lethal. Overheating carcase, tyre rolling off the rim and a bike that is 20lbs heavier due to drag. Then again-If you can get my Kendas up to 120psi with my frame pump- you are a lot stronger than me.
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Old 07-12-06, 06:33 AM   #20
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I ride my Conti 4000s at 120psi. I have had 2 flats in 4000 miles and one was due to tire wear. I ride on pretty good roads and if the tires get below 100psi I feel slower.
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Old 07-12-06, 06:53 AM   #21
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Next to our local goat heads, and slivers of glass, those little thread like wires from belted tires are often at fault for punctures.

What are goat heads?
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Old 07-12-06, 06:59 AM   #22
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Goathead is an annual weed in the caltrop family. The prostrate stems radiate from a tap root. The opposite leaves are pinnate. The flattened fruit breaks in five nutlets, with two strong, woody spines. The flowers are small and have 5 yellow petals.
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Old 07-12-06, 07:20 AM   #23
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Goathead is an annual weed in the caltrop family. The prostrate stems radiate from a tap root. The opposite leaves are pinnate. The flattened fruit breaks in five nutlets, with two strong, woody spines. The flowers are small and have 5 yellow petals.

Thanks Rule
I never seen these culperts before, are they only in the SouthWest? I hope I wouldn't want to run into a sworm of them.
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Old 07-12-06, 07:30 AM   #24
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Great photo, they are very abundant in colorado
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Old 07-12-06, 08:26 AM   #25
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Well, underinflation can cause pinch flats. But beyond that extreme, I have never noticed any kind of association of tire inflation and flat production. I figure it takes something over 1000 miles of riding to produce a flat and flat frequency seems to increase with tires that are worn thin. However, that is merely an impression.

In order to really get a fix on it, one would probably have to ride something like 50,000 miles at 90 psi and 50000 miles at 120 psi and look at the number of flats and see if they differed significantly. I don't know anyone who does that sort of thing.
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