# Pinch flats, what is it with you folks?



## Cooper1960 (Oct 14, 2010)

I've been on this site for a few months now and have noticed "pinch flats" seem to be a huge problem. The issue seems to cross over to different forums, general, wheels and tires, beginners, components..............

I just don't get it, in all the years I have been actively riding I don't ever remember getting a pinch flat. Road or mountain bike, it just doesn't happen. When I read post on this site and folks talk about getting four, five, six pinch flats a season all I can think of is do something different! Buy better tires or tubes! Check your pressure! Add some air for gods sake! 

So why does it happen so frequently to folks? Do you not check your tire pressure each time you ride? Are you running ultra low pressure trying to get some "cushy" to your ride? Cheap tires and tubes? Heavy weights? (I'm 180/185 so no feather weight myself) Are your roads that much rougher than the roads I ride? (very doubtful, northern Ohio, chuck hole capitol of the world)

Anyway.....to me it seems like such an easy thing to remedy I'm not sure why anyone would deal with it on a regular basis. Just curious as to what the rational is that some of you folks are willing to deal with it, can someone enlighten me?


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## robdamanii (Feb 13, 2006)

It just happens. It depends on the pressure you're running that day, the rims you're running, the tire, tube, where your weight is at the time (which wheel is taking more of it)...all kinds of variables.

Yesterday someone pinch flatted and blew out a tire (quite loudly I might add) in the neutral roll out of the circuit race I raced. It just happens.

Maybe your tires are inflated TOO much, which could be why you never pinch. I always run between 92 and 96psi rear, 90 and 94 front.


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## tihsepa (Nov 27, 2008)

robdamanii said:


> It just happens. It depends on the pressure you're running that day, the rims you're running, the tire, tube, where your weight is at the time (which wheel is taking more of it)...all kinds of variables.
> 
> Yesterday someone pinch flatted and blew out a tire (quite loudly I might add) in the neutral roll out of the circuit race I raced. It just happens.
> 
> Maybe your tires are *properly inflated* , which could be why you never pinch. I always run between 92 and 96psi rear, 90 and 94 front.


I think this is what you meant to say. At least I hope it was.


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## cyclesport45 (Dec 10, 2007)

I try to get in one early season pinch flat so I can practice a "field" tire fix. 
Alternative theory; sh*t happens.


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## AJL (Jul 9, 2009)

I've only had one or two over since 2005. I tend to keep my tires around 100psi front/110psi rear. That's hard on rims, but keeps me from getting pinch flats. I got one last year while sprinting through an intersection and presto, a drainage cover pops up in front of me that is 2" above the tarmac; like cyclesport45 said - sh*t happens.


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## Ibashii (Oct 23, 2002)

In addition to the PSI factor, proper tire installation is the other big item here: some people feel that it is a waste of time to verify that there aren't little bits of tube caught under the bead when changing a flat, especially on the road when people might be waiting for you and/or passing you by.

I do this every damn time, watch my tire pressure and I've have had exactly one pinch flat (1999, downtown PDX, front wheel blowout, nearly killed me)...and that tire had been recently installed at a shop.

Sh*t does happen and no saying I won't have a pincher tomorrow, but safe and happy road biking is also about limiting the holes through which said sh*t can arrive.


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## Reynolds531 (Nov 8, 2002)

Agree. I think that most pinch flats are from pinching the tube between the tire and the rim.


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## martinrjensen (Sep 23, 2007)

I think I disagree on this one. It doesn't "just happen". You have to hit something that you shouldn't in order for it to happen. Bad rocks or pavement with abrupt edges maybe. You will never get a pinch flat running on decent pavement and as bad as the roads are here in Seattle (I actually think that overall the roads here are fine), I can always find pavement to ride on that won't cause a pinch flat. Sometimes it requires slowing down a little or even a bunny hop, but it's always possible. I have never had one, ever. I have had flats though, not a lot but regular flats "are" part of riding. Also saying tires are inflated too much and that's the reason the OP didn't get a pinch flat, well, I'm not certain on how to respond to that one.


robdamanii said:


> It just happens. It depends on the pressure you're running that day, the rims you're running, the tire, tube, where your weight is at the time (which wheel is taking more of it)...all kinds of variables.
> 
> Yesterday someone pinch flatted and blew out a tire (quite loudly I might add) in the neutral roll out of the circuit race I raced. It just happens.
> 
> Maybe your tires are inflated TOO much, which could be why you never pinch. I always run between 92 and 96psi rear, 90 and 94 front.


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## AJL (Jul 9, 2009)

@ 30 mph after taking a 90 degree turn through the intersection with cars on my left and a tall curb on my right - I had no where to go and not enough time to brake. I'm a clyde, so I don't bunny hop very well unless I have more than a second to set myself up - that and I'm not a pro, so bunny hopping @ 30 wasn't appealing anyway. C'est la vie.

Now I know that drainage cover sticks up and I won't hit it again.


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## georgewerr (Mar 4, 2009)

I'm not sure why but cheap tiresa seam to get a lot more flats then good tires. The last new bike I bought I was useing the tires that came on the bike and I got 4 flats in the first weekend of riding. Changed out the tires with the old tires on my old bike and never had anouther flat and its been 2 years


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## Len J (Jan 28, 2004)

Reynolds531 said:


> Agree. I think that most pinch flats are from pinching the tube between the tire and the rim.


I agree......most pinch flats are from lack of care when seating a tire on the rim.

Len


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## bigbill (Feb 15, 2005)

I can't remember my last pinch flat. When I MTB'd quite a bit, I would get snakebite flats from impacting the front tire on rocks, but with suspension forks and better tires, I don't see it that much anymore. 

IME, sudden flats are typically due to leaks around the stem. If a tube has been in a tire for a long period of time, the tube weakens around the stem due to repeated inflation (pump head on/off) and movement against the rim. I use heavy tubes in my commuter (28-38mm tubes in a 25mm tire) and use a piece of old tube around the base of the stem to protect it.


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## Gimme Shoulder (Feb 10, 2004)

I've only been riding 15 years or so, but in that time, I've never pinch flatted or been snake bit. My weight ranges from 180 to 200 lbs. I use mostly 23mm tires at 100 to 115 psi (depending on the tire). Not say'n it couldn't happen tomorrow - just that it hasn't so far.


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## Ibashii (Oct 23, 2002)

georgewerr said:


> *I'm not sure why but cheap tiresa seam to get a lot more flats then good tires.* The last new bike I bought I was useing the tires that came on the bike and I got 4 flats in the first weekend of riding. Changed out the tires with the old tires on my old bike and never had anouther flat and its been 2 years


Ummmm...because cheap tires are made more cheaply??:wink:

Seriously, that's just a question of what the manufacturer puts in the tire to prevent puncturing, which is the first step to a non-pinch flat: naturally, the less the tires cost the less they're going to protect your tube from road debris.

The exception to this rule being, of course, designer race tires, which cost a fortune and flat more often and more easily than Stanley.

I just changed out a back tire after more than 5k kilometers (half in front, half in back, a Hutchinson Intensive): the contact surface tread was long gone, there were zillions of surface holes of varying size and depth, and the whole tread piece was starting to separate from the radial section just above the sidewall...but from the inside of the tire you couldn't even tell it was worn out. That layer of steel or kevlar or kryptonite or whatever they put in there is almost invincible.


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## robdamanii (Feb 13, 2006)

tihsepa said:


> I think this is what you meant to say. At least I hope it was.


Running tires at 120 PSI is a joke, unless you're running tubulars.

You might as well be riding on concrete at that pressure.

Edited to add: Overinflated tires will not pinch flat. Tires that are correctly inflated to provide the best ride quality and most efficiency always have a possibility of pinch flatting if hitting a sharp edged surface (pothole, curb) or running over something (rock, piece of wood, what have you.)


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## Touch0Gray (May 29, 2003)

I generally put the tube in, them pump it up to maybe 60 psi, then let the air back out and check the perimeter on both sides to make real sure the tube is not peeking out under the tire, then pump it back up to 90 psi. The only pinch flats I have had were my own fault for NOT doing that. That is one of the problems with using CO2 inflators on the road.......


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## Cooper1960 (Oct 14, 2010)

Ok, I never meant to imply that I have never got a flat, heck I probably get one or two a season but it's almost always from something I picked up off the road. And not installing a tire and tube properly can happen but that still isn't what I meant, nor hitting an unavoidable object. I am talking about the basic snake bite pinch flat that people just accept as routine. 

Depending on what tires I am running at any given time and what the days ride will be I'm probably around 10%, maybe 15% below max pressure, and many times closer to max. I think my tires are "properly inflated", and I certainly don't think running properly inflated tires is a "joke" My bike rides just fine, I don't cry about any discomfort (unless it's cold and windy!), my glasses aren't bouncing off my nose, the bike doesn't pogo off the road, good traction, good cornering, gee, just like I would expect a properly set up bike to perform.

IMO there are other ways to make your bike ride more comfortably besides running such low tire pressure that you routinely pinch flat. Wider tires, better seat or shorts, proper fit, better bars or seat post, maybe better wheels. I just wouldn't put up with constant flats, and frankly it's a situation that can turn dangerous very quickly. 

Anyway..... you can disagree with me if you want, it's such an easy thing to avoid I just don't understand why it's such a common issue that it's mentioned so often in these threads.


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## robdamanii (Feb 13, 2006)

Cooper1960 said:


> Ok, I never meant to imply that I have never got a flat, heck I probably get one or two a season but it's almost always from something I picked up off the road. And not installing a tire and tube properly can happen but that still isn't what I meant, nor hitting an unavoidable object. I am talking about the basic snake bite pinch flat that people just accept as routine.
> 
> Depending on what tires I am running at any given time and what the days ride will be I'm probably around 10%, maybe 15% below max pressure, and many times closer to max. * I think my tires are "properly inflated"*, and I certainly don't think running properly inflated tires is a "joke" My bike rides just fine, I don't cry about any discomfort (unless it's cold and windy!), my glasses aren't bouncing off my nose, the bike doesn't pogo off the road, good traction, good cornering, gee, just like I would expect a properly set up bike to perform.
> 
> ...


And this is the point I was making:

Some people use the sidewall pressure as gospel. Frankly, I believe it to be improperly high. Sure, you'll rarely if ever get a pinch flat, but the bike will ride like a rock. Hence, running printed pressure is "a joke."

And I never said I put up with constant flats. Don't put words in my mouth. In 3 years I've had 1 pinch flat, and that was from riding an utterly awful gravel road. My initial comment was no matter what happens, even with "properly inflated" tires, there's the potential to pinch when hitting a pothole or object wrong, especially if you plant your butt on the saddle and just let your weight pound the tire into said object. It pays to be attentive, use your body as a shock absorber, and avoid situations that would lead to a pinch in the first place. 

My other point was that flats are a routine part of riding. Puncture or pinch/snakebite, they DO happen, often times in spite of any carefully laid plans.

And finally, tire pressure and selection is probably one of the most predominant factors influencing comfort on any bike, especially for the price (free.)


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## Kontact (Apr 1, 2011)

When you compare car tire pressures for the same tire, you'll notice that the pressures are higher on heavier cars, or higher on the heavier part of the car (front end on most). This principle probably applies to rider weights, but no one talks about it.

A 150lbs rider might compress 100psi tires about the same amount as a 200lbs rider compresses a (for example) 110psi filled tire. Given the same level of compression, the two riders will get similar ride, pinch resistance and traction qualities out of these two different pressures.

Unfortunately, no one has really shown how to correct psi for rider weight - and it probably isn't simply linear. Aside from incorrect installation, I'd wager that underinflated tires by larger riders is a factor.


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## FTR (Sep 20, 2006)

Another vote for WTF.
2 years no flats.
Pinch flats on my road bike = never.
Every other flat has been due to glass or other debris.


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## wim (Feb 28, 2005)

This thread pings back and forth between two very different types of flats. One is the classic "pinch flat" in which the tube is pinched between a collapsed section of sidewall. It's caused an impact at low tire pressure.

The other flat is the one in which the tube escapes from under the tire bead, which some people also call a "pinch flat." This one's caused by improper installation. Not sure which one of those flats the OP meant in his title.

/w


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## looigi (Nov 24, 2010)

Lots of new potholes this spring in New England. I was cruising up to an intersection looking for potential cross traffic when I hit a pothole going ~20 mph and flatted front and rear tires a week ago. Two nice side-by-side holes in each tube (AKA snakebites). Rode by today and they had patched the hole. In many cases, it's just 1.5" to 2" veneer or resurfacing that comes off, but the resulting sharp edge can get you. I'm currently running 95 in front and 105 in the rear with 23mm wide C2 rims at 160 lbs. I used to run 110 front and rear with 19mm wide rims got snakebites then too.


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## Touch0Gray (May 29, 2003)

That's too bad, we don't have any potholes here in WI


uh.........yeah........I got some land in FL for sale too.....high, dry land.....


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## Cooper1960 (Oct 14, 2010)

robdamanii said:


> And this is the point I was making:
> 
> Some people use the sidewall pressure as gospel. Frankly, I believe it to be improperly high. Sure, you'll rarely if ever get a pinch flat, but the bike will ride like a rock. Hence, running printed pressure is "a joke."
> 
> ...


I didn't say YOU specifically "put up with constant flats", that statement reflects a common issue I see written about across these boards, sorry if that wasn't clear. And I have read plenty of write ups about tire pressure and not once have I saw something written by a reputable tester that referred to max pressure as a "joke" I won't for a second say that max pressure is the best, as I said earlier I don't often run max pressure. And lower pressure is a way to increase comfort to a point, and in the right circumstances increase performance, again, no argument from me. And hitting a bone jarring pot hole, sure it will happen. But....if I got a pinch flat and I was running 75psi I would never run that low of pressure again, seems to just make sense.


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## JoelS (Aug 25, 2008)

Reynolds531 said:


> Agree. I think that most pinch flats are from pinching the tube between the tire and the rim.


Yup.

Though I suspect that the term "pinch flat" doesn't apply to an improperly installed tire, but instead to a tire that hit the rim, thereby pinching the tube and causing a flat.

I've never had one of those. And don't recall ever seeing someone get one.


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## Touch0Gray (May 29, 2003)

JoelS said:


> Yup.
> 
> Though I suspect that the term "pinch flat" doesn't apply to an improperly installed tire, but instead to a tire that hit the rim, thereby pinching the tube and causing a flat.
> 
> I've never had one of those. And don't recall ever seeing someone get one.


oh I have....my daughter was riding and decided to try to hop a cub with the tire pressure down around 25 or 30 pounds.....and to top it off she didn't pull up on the bars, she underestimated the pitch of the curb...she cut two slots in the tire and had 2 matching slits in the tube. To make it even more interesting, it was a slime filled tube.....WHAT A MESS!!!!!!!


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## danl1 (Jul 23, 2005)

I'm in NEO, so know what you're talking about. It's always a toss up whether the highway department is going to fill them before the DNR decides they need to put in a ramp and stock them with smallmouth. 

At 185lbs, I run 100 rear, 90ish front on 23's (lower on the 25's that I normally ride) and have had one pinch in, ever. 

That was from running over a fist-sized rock that was hidden by the person in front of me. Riding with groups / others makes the road hazards harder to miss. Solo, it's hard to find something that you can't either go 'round or bunny hop. Pinch flats 'Just riding along' are an irrational fear, the result of riding or maintenance carelessness.


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## nyvram (Apr 11, 2002)

nobody puts 120psi in their tires??? i have over 1000 miles since january on my gp4000s tires and i always put 120 in both. routinely ride 50+ mile routes on the weekends as well.

what am i doing that is wrong here? is my ride less comfortable? i dont know..i do know i went the entire year last year without a single flat. only one this year so far after i bunny hopped and realized later in the day i had gotten distracted while airing up my tires and completely forgot to air up the rear one before the ride.

i simply never thought about this before..if my ride is harder because of the air pressure i guess i'm used to it. i do have nice carrbon bikes so maybe that helps. seriously..i did jack & back last year..170ish miles in 2 days..and i felt great getting off the bike when done. i dont disagree with using less pressure..if i had a good argument to


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## heathb (Nov 1, 2008)

I use 25mm Gatorskins for my training tires and have never pinched flat, I don't think I could even if I wanted.. I run them at 90 to 110psi. 

I usually run 120psi in my race tires 20-23mm and I have pinched flatted with 20mm, but it's only happened maybe two or three times in the last 10 years and it was usually running on some very rough roads.


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## sometimerider (Sep 21, 2007)

Kontact said:


> Unfortunately, no one has really shown how to correct psi for rider weight


I don't know about that.


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## sometimerider (Sep 21, 2007)

Len J said:


> I agree......most pinch flats are from lack of care when seating a tire on the rim.


Many, including myself, do not consider problems resulting from that to be "pinch flats".

Yes, the tube is pinched - but that generally doesn't cause damage to it. What causes the damage in that case is inflating the tube when it isn't properly contained. And that damage is a blowout flat, not a pinch flat.

(Yes, I know that many call blowouts caused by the tube escaping from the tire "pinch flats", but I consider such to be poor usage.)


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## ssing20 (Aug 1, 2007)

sometimerider said:


> I don't know about that.


There's also an article on RoadBikeRider that you can download for $4.99:
http://www.roadbikerider.com/e-articles/all-about-tire-inflation-earticle


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## Lelandjt (Sep 11, 2008)

I never heard of someone running less than 110psi before joining this forum so maybe (probably) there's a connection. I've since dropped from 120 to 110 and agree that it improves cornering grip and comfort but I'm now more cautious (get off the seat, keep my arms and legs loose) when hitting stuff. I bet if I ran sub 100psi I'd get a flat every time I ride my West Maui loop.


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## Kontact (Apr 1, 2011)

sometimerider said:


> I don't know about that.


Well, that's worth repeating:









According to that, you have to be below 142 lbs. to ride a 23c tire at 100psi. For a Michelin, anyway. Good stuff.


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## sometimerider (Sep 21, 2007)

Kontact said:


> According to that, you have to be below 142 lbs. to ride a 23c tire at 100psi. For a Michelin, anyway. Good stuff.


That's why I ride 25mm tires: 99psi, no pinch flats (172lbs.)


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## Sisophous (Jun 7, 2010)

The term Pinch Flat is from hitting an object or dropping into a pothole. It is contacting an obstruction or the force of the tire dropping and pinching the tube causes the puncture.

I got a pinch flat going into my driveway, hitting the curb which protruded 3 inches just before it was about to get paved.

I had only one Pinch Flat with larger size tires. The thin, narrow tires, like my 700x23 have caused me 4 flats since last June. When I had 700x25 tires I only got a couple of flats over many years.

If riding on smooth surface and you hear a very loud pop, sort of like a gun shot, and see you have a flat, it is not a Pinch Flat.

If you get a flat, it may not be a pinch flat, but a pinch flat is a flat with a pinch of the tube.


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## nyvram (Apr 11, 2002)

i'm 197 (hopefully less by my next race) and i always go max..i might experiment with 110..but definitely not changing anything for my next race.


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## AlexCad5 (Jan 2, 2005)

AJL while sprinting through an intersection and presto said:


> I hate those pop up drainage covers! I know what you mean though, there are a lot of things to watch when on the road, least of all the cars bearing down on you and you can not see two inch bumps in the road. For people who think they have never had a pinch flat, I'd argue, if they had *never gotten one*, how would they know how to identify one when if they got it? Ignorance is bliss.
> Since going to 25s I've rarely gotten one. They used to be more common when 21c was all the rage.


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## Ibashii (Oct 23, 2002)

AlexCad5 said:


> I hate those pop up drainage covers! I know what you mean though, there are a lot of things to watch when on the road, least of all the cars bearing down on you and you can not see two inch bumps in the road. For people who think they have never had a pinch flat, I'd argue, if they had *never gotten one*, how would they know how to identify one when if they got it? Ignorance is bliss.
> Since going to 25s I've rarely gotten one. They used to be more common when 21c was all the rage.


I think you usually know because there are often two matching little slices in the tube where it got pinched. Often just far enough apart to be difficult to patch...

As far as the nomenclature goes, regardless of what is 'correct,' enough people call flats resulting from leaving a bit of tube pinched between the rim and the tire bead "pinch flats" that this common usage cannot be dismissed outright. Besides, it's not totally out to lunch: there is pinching involved. :wink:

As Confucius said (sort of): when enough people call a thing something, it becomes what they call it.


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## Ibashii (Oct 23, 2002)

Kontact said:


> Well, that's worth repeating:
> 
> 
> 
> ...


Sure, the chart looks dumb out of context and without the accompanying explanation...especially the part where it says the chart is a "reasonable guide of where to start when experimenting to find the proper tire pressure for you" and not the definitive word on tire pressure.

What are you, a journalist or something??:wink:


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## GetReal (Jul 26, 2010)

Cooper1960 said:


> I just don't get it, in all the years I have been actively riding I don't ever remember getting a pinch flat. Road or mountain bike, it just doesn't happen. When I read post on this site and folks talk about getting four, five, six pinch flats a season all I can think of is do something different! Buy better tires or tubes! Check your pressure! Add some air for gods sake! ?


I'm with you! I'm a bigger rider and it just doesn't happen if you know what you are doing.

Sad thing is that too many rides are just lacking in comon sense and mechanical ability. I've been on rides with some riders that swear they know it all. But to watch them repair a flat on the side of the road/ Holy ***********!!!!!! I was once on a ride with a self proclaimed "super" long distance rider. He flatted then started the repair. After watching him whine and ***** about not being able to get it right, I just said Jesus Christ give me the freaking wheel. I repaired it then we hit the road again. The guy had the tubed pinched all over the freaking place.

I've seen quite a few internet members try to repair a flat. They talk big on the forums but in reality, they don't know jack! Pinch flats just don't happen if you know your head from your arse! Most riders don't!


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## Kontact (Apr 1, 2011)

Ibashii said:


> Sure, the chart looks dumb out of context and without the accompanying explanation...especially the part where it says the chart is a "reasonable guide of where to start when experimenting to find the proper tire pressure for you" and not the definitive word on tire pressure.
> 
> What are you, a journalist or something??:wink:


I don't think the chart looks dumb. 

It matches the point I made earlier - talking about tire pressure without reference to rider weight makes no sense. The two balance each other - and it appears to be a linear relationship. The exact tire pressure a person uses is up to them, but I think this makes it pretty clear that a 180lbs guy shouldn't expect to run 97psi in 23c tires.


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## Ibashii (Oct 23, 2002)

Kontact said:


> I don't think the chart looks dumb.
> 
> It matches the point I made earlier - talking about tire pressure without reference to rider weight makes no sense. The two balance each other - and it appears to be a linear relationship. The exact tire pressure a person uses is up to them, but I think this makes it pretty clear that a 180lbs guy shouldn't expect to run 97psi in 23c tires.


Sorry, I thought I was reading sarcasm in your first post. My bad.

Tricky sometimes, this whole Internet thing.


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## sand101 (Mar 29, 2009)

AJL said:


> I've only had one or two over since 2005. I tend to keep my tires around 100psi front/110psi rear. *That's hard on rims*, but keeps me from getting pinch flats.


Ummm, no.


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## JCavilia (Sep 12, 2005)

*reading comprehension*



GetReal said:


> I'm with you! I'm a bigger rider and it just doesn't happen if you know what you are doing.
> 
> The guy had the tubed pinched all over the freaking place.
> 
> I've seen quite a few internet members try to repair a flat. They talk big on the forums but in reality, they don't know jack! Pinch flats just don't happen if you know your head from your arse! Most riders don't!


Those aren't "pinch flats." That's not what's being discussed here. read the thread


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## AJL (Jul 9, 2009)

sand101 said:


> Ummm, no.


Uhm, YES! Do you know one freaking thing about mechanical engineering :idea: 
I'll try and find some internet quotes for you later... or maybe Kerry Irons or someone else who knows what they are talking about will pop in.

Higher tire pressures lead to higher forces on the rim sidewall, which will reduce the fatigue life, particularly for aluminium rims. Think about what PSI really means. Think about what it means to have a more rigid tire as the pressure increases (hint, less tire deformation == a higher load being transfered directly to the rim).


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## stevecaz (Feb 25, 2005)

Well whatever kind of flat, all flats just don't happen. I'm on about 9 years and counting. the last flat I had was my fault knowingly running over the very rare glass in the road thinking I'd be OK and wasn't. The only other road flat before that was around 1989 and I can't remember what happened there. My experience doesn't apply to everyone, but I do believe that flats are only the result of bad luck and really should happen often (measured in years). It's sort of like car tires where you may experience a flat only a few times over your whole life. Geography can play a part such as those who have thorns fall onto roads from plants. 

So I have to agree that flats are from riders who don't know what they are doing mounting tires, or who don't know how to finesse their bike over the road, or who observe different visually and don't pick up on road hazards readily. NE roads suck really bad for smoothness and yet I have no problems. 

As for tire pressure and comfort, if you have a frame and components that provide comfort you can ride rock hard tires very nicely.


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## DM.Aelis (Jun 19, 2007)

AJL said:


> Uhm, YES! Do you know one freaking thing about mechanical engineering :idea:
> I'll try and find some internet quotes for you later... or maybe Kerry Irons or someone else who knows what they are talking about will pop in.
> 
> Higher tire pressures lead to higher forces on the rim sidewall, which will reduce the fatigue life, particularly for aluminium rims. Think about what PSI really means. Think about what it means to have a more rigid tire as the pressure increases (hint, less tire deformation == a higher load being transfered directly to the rim).


To add an anecdote, I dented a Cadence VSR rim after failing to completely bunny hop a set of really rough train tracks. The rear tire didn't flat (I weigh 145 lbs, run pressure usually 100-105 psi in the rear), but the rim got ruined from the impact!


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## JCavilia (Sep 12, 2005)

stevecaz said:


> As for tire pressure and comfort, if you have a frame and components that provide comfort you can ride rock hard tires very nicely.


IMHO and IME, that's hooey.


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## sand101 (Mar 29, 2009)

AJL said:


> Uhm, YES! Do you know one freaking thing about mechanical engineering :idea:


Just a Master's degree in engineering. And do mechanical testing on materials for a living. 



> I'll try and find some internet quotes for you later... or maybe Kerry Irons or someone else who knows what they are talking about will pop in.


I typically don't put a whole lot of stock in what some guy on the internet said.



> Higher tire pressures lead to higher forces on the rim sidewall, which will reduce the fatigue life, particularly for aluminium rims. Think about what PSI really means. Think about what it means to have a more rigid tire as the pressure increases (hint, less tire deformation == a higher load being transfered directly to the rim).


You are talking about 20-30psi delta here in this discussion. These rims are designed with a substantial margin of safety. Given that we're talking something designed to carry humans, I would imagine a MOS of 3 or more is what is used. That is fairly typical for commercial systems as well as the space systems I work on. Recall that these rims are designed to hold up to very high transient loads during normal riding. The extra static loading for 110psi vs. 90 just isn't that much. 6061 is elastic to 40ksi or so. That very small increase in pressure should get the stresses in the hoop nowhere near the yield point of 6061. If it does the design of the rim was flawed.

I personally have never seen a rim fail in fatigue.


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## JCavilia (Sep 12, 2005)

sand101 said:


> I personally have never seen a rim fail in fatigue.


I think you've got this right. The poster you're arguing with may be confused by the fact that rim sidewalls do sometimes fail, pushed apart by the pressure of the tire. But this only happens after the walls have been thinned by considerable brake-track wear. The additional load from running higher pressure is irrelevant, as you say.


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## TomH (Oct 6, 2008)

All my road flats have been complete blowouts from cuts/punctures.. Ive had one mountain pinch (waaay too low pressure, hit the rim 10 time before flatting). 

Ive always kinda wondered how people pinch so often too.


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