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Discussion Starter · #1 ·
O.k., so I had the second pinch flat this year, after having gone longer than I can remember (many years in any case since my last flat, and never having had a pinch flat in my whole life). So I wonder, why do I all of a sudden get these? To get the obvious out of the way, I weigh 170lbs, and my tires are inflated properly, always. As an aside, don't give me that bull about "properly inflated tires will not pinch flat". A girl in our group flatted on the same damn sharp ledge (road repair in progress...) that got me, with her tire at 110psi. She weighs less than 100lbs...

Here are some things I could imagine: I use different tires (Michelin Pro 3 Race) and tubes (Michelin Aircomp Ultralight) than I had in the past. Could either of these be the problem? I have seen hints that Latex tubes are more resistant to pinch flats. Anybody agree with this? Should I use different tires (see below)? Could the rim be the problem (Zipp 404)?

More generally, I am not sure I even understand why pinch flats occur. I know that those are supposedly caused by the tube being pinched between the rim and some hard/sharp obstacle. Note, however, that a properly installed tube will never touch the edge of the rim. If the tire is squished flat, then the tube will be squeezed between the tire sidewalls, with the edge of the rim on one side of the tire sidewall, and the sharp bump in the road surface on the other side. Given that situation, I could imagine that it's the properties of the tire sidewall that affect the probability of getting pinch flats. Interestingly, I have never seen any actual sidewall damage due to a pinch flat incident. Am I on the right track here?
 

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Your tires aren't inflated enough or you're using defective tubes. If your tubes are exploding you might be getting the tube stuck between the bead and the rim when you mount the tire.
 

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Pirx said:
More generally, I am not sure I even understand why pinch flats occur. I know that those are supposedly caused by the tube being pinched between the rim and some hard/sharp obstacle. Note, however, that a properly installed tube will never touch the edge of the rim. If the tire is squished flat, then the tube will be squeezed between the tire sidewalls, with the edge of the rim on one side of the tire sidewall, and the sharp bump in the road surface on the other side. Given that situation, I could imagine that it's the properties of the tire sidewall that affect the probability of getting pinch flats. Interestingly, I have never seen any actual sidewall damage due to a pinch flat incident. Am I on the right track here?
You're sort of on the right track. The tube only touches the tire, and it gets squished between two parts of the tire sidewall when it gets compressed by the obstacle. The tire is not usually damaged by this, because it takes a lot more force to damage the rubber-and-fiber tire casing.

I don't think the properties of the tire have much influence, though a more flexible sidewall might increase the risk. The factors that determine whether you get a pinch flat are tire size, inflation pressure, weight of bike and rider, speed, and the sharpness of the obstacle.

There's no absolute protection against them. With a sharp enough obstacle and enough speed, you can do it to any tire.
 

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Discussion Starter · #5 ·
Bocephus Jones II said:
Your tires aren't inflated enough or you're using defective tubes. If your tubes are exploding you might be getting the tube stuck between the bead and the rim when you mount the tire.
Sigh... And here I am, trying to explain that none of the above have anything whatsoever to do with this.
 

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Discussion Starter · #6 ·
JCavilia said:
You're sort of on the right track. The tube only touches the tire, and it gets squished between two parts of the tire sidewall when it gets compressed by the obstacle. The tire is not usually damaged by this, because it takes a lot more force to damage the rubber-and-fiber tire casing.

I don't think the properties of the tire have much influence, though a more flexible sidewall might increase the risk. The factors that determine whether you get a pinch flat are tire size, inflation pressure, weight of bike and rider, speed, and the sharpness of the obstacle.

There's no absolute protection against them. With a sharp enough obstacle and enough speed, you can do it to any tire.
Thanks for confirming my thoughts. That ledge we hit last night was really nasty: They had cut the asphalt with a machine, which left a ledge that was razor-sharp. I tried to bunny-hop over it, but that's hard to time at 25mph, so I exactly hit the ledge with my rear wheel. :mad:
 

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Discussion Starter · #7 ·
Hank Stamper said:
Two doesn't make a trend.
Hmm, yes and no. Going something like 20,000 miles with no flat sort of does establish a trend. I used to think that flats are things that only happen to other people.

Hank Stamper said:
You probably just hit two sharp things you never hit before.
That could well be the case. On the other hand, I find it hard to believe that in 20,000 prior miles I had never hit any such sharp things.
 

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Pirx said:
Thanks for confirming my thoughts. That ledge we hit last night was really nasty: They had cut the asphalt with a machine, which left a ledge that was razor-sharp. I tried to bunny-hop over it, but that's hard to time at 25mph, so I exactly hit the ledge with my rear wheel. :mad:
Are you running your tires at max pressure then? You didn't say in your OP.
 

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Discussion Starter · #9 ·
Bocephus Jones II said:
Are you running your tires at max pressure then? You didn't say in your OP.
I said they were properly inflated. I also gave the example of the girl that flatted on the exact same ledge.
 

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Pirx said:
I said they were properly inflated. I also gave the example of the girl that flatted on the exact same ledge.
Well then it sounds like the ledge was your issue. Stop riding over sharp ledges and you should be fine. :rolleyes:
 

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Pinch flats can happen to anybody at any time, regardless of how perfect their equipment is set up.

Properly inflated, quality tires and tubes of the right cross section reduce the odds, but nothing makes you bulletproof against them.

The typical pinchflat from a hard bump comes when the corner compresses the tire to the point that the wall is folded back on itself trapping the tube between. It gets cut the same way you can cut your knee through your jeans without ripping them. BTW- consider a pinch flat a stroke of luck, any more energy and you'd be dealing with a dented rim.

The best way to avoid the problem is to avoid the kind of sharp bump that causes them, like the opposite lip of a deep pothole.

If pinch flats are rare for you, and happen only when you can identify the cause and you accept that compost happens, fine. If they're not rare, consider larger section tires which greatly increase the energy absorption capacity. Just as auto and truck tires are sized according to weight capacity, bike tires should be too. If you need over 100psi in a tire to get it to ride right, and handle road hazards, you're probably on too small a tire.
 

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That's the key...

Pirx said:
Thanks for confirming my thoughts. That ledge we hit last night was really nasty: They had cut the asphalt with a machine, which left a ledge that was razor-sharp. I tried to bunny-hop over it, but that's hard to time at 25mph, so I exactly hit the ledge with my rear wheel. :mad:
That is the worst situation.

Best: correct bunny hop (do not hit edge at all)
Next: Standing with knees bent (minimal weight impact)
Next: Sitting (the bump and edge must pick up the wheel with your body mass having to accelerate)
Worst: Too late a bunny hop (now bump and edge must decelerate your body and re-accelerate it so that the force of the edge into the tire is more than when you are just sitting dead on the seat).

DNM
 

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Discussion Starter · #15 ·
FBinNY said:
BTW- consider a pinch flat a stroke of luck, any more energy and you'd be dealing with a dented rim.
Oh I know, and that was a fairly expensive rim to boot. As a matter of fact, the first time this happened this spring, I had hit a pothole so badly (I was right on the wheel of the guy in front of me, and didn't see it coming) I thought I had totalled the wheel. However, it seemed the wheel was fine, still perfectly true, and no rim damage to be seen (and believe me, I went over that rim with a magnifying glass). But then, about a thousand miles later on that same wheel, I lost a non-drive-side spoke while on a reasonably smooth and level road, broken off right at the nipple. I am almost certain that this was due to damage that was done when I had hit that pothole.
 

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DNM said:
Best: correct bunny hop (do not hit edge at all)
Next: Standing with knees bent (minimal weight impact)
Next: Sitting (the bump and edge must pick up the wheel with your body mass having to accelerate)
Worst: Too late a bunny hop (now bump and edge must decelerate your body and re-accelerate it so that the force of the edge into the tire is more than when you are just sitting dead on the seat).
This is exactly right. Clearly the way you hit the obstacle will have a lot to do with how much damage it does. If your bike has essentially no shock absorption capability (and this is true of most road bikes - not counting the tires/tubes themselves), your body has to absorb the shock.

Get your butt off of the saddle.
 

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DNM said:
That is the worst situation.

Best: correct bunny hop (do not hit edge at all)
Next: Standing with knees bent (minimal weight impact)
Next: Sitting (the bump and edge must pick up the wheel with your body mass having to accelerate)
Worst: Too late a bunny hop (now bump and edge must decelerate your body and re-accelerate it so that the force of the edge into the tire is more than when you are just sitting dead on the seat).

DNM
I disagree: BEST: avoiding the pothole altogether.
WORST: Hitting the pothole at 25+mph and getting into a nasty fall, and having a group of people on your tail crash too, because you didn't warn them and go around it.

Oh, from my understanding, latex tubes aren't less pinch-flat prone than butyl. I'd bet they're more prone, due to being thinner. Yes, your tires will also contribute to it too, different tires will need different pressure to be "optimal"
 
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