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Old 05-21-13, 08:01 PM   #1
vol
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Does it do harm to the tire if it's 75~90% full of air?

I am not heavy, and my tires can last weeks, even months without needing pumping, but I wonder if it is harmful to ride it often with not fully-pumped tires?
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Old 05-21-13, 08:40 PM   #2
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The optimum pressure for tires gets rather controversial. Certainly the pressure rating molded right on the tire is a good first guess. I like Schwalbe's guidelines:

http://www.schwalbetires.com/tech_in...ation_pressure

The right pressure seems to depend most on the width of the tire (wider tires need less pressure), then on the weight the tire is carrying (more weight requires more pressure), then maybe the road (soft or rough wants lower pressure, smooth pavement can take higher pressure). Maybe rims a bit too: narrow rims take less pressure for the same tire.

If the pressure gets too low then it means more work pedaling, it can stress the tire sidewalls and wear the tire out more quickly, and when you hit a big bump you can bottom out, hitting the rim, which can cause a flat tire (a pinch flat) or also put a dent in your rim.

I ride 2 inch / 50 mm wide tires and probably pump them up maybe every couple weeks or so. Maybe I pump the rear tire up to 48 psi and in a few weeks it will go down to 42 psi or so. If I am hauling a big load of groceries then I do try to be sure to have them pumped up beforehand but otherwise they could go down a bit more with no problem.

But those are heavy tires and also pretty thick tubes. If you have skinny lightweight tires like racing tires, those you really do want to pump up before every ride. They lose air more quickly, have less air in them anyway, and need to stay at much higher pressure to avoid pinch flats etc.
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Old 05-21-13, 08:43 PM   #3
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I should add - my 2 inch tires have printed on the side a max pressure of 70 psi. I never go above 50 psi. Even 35 psi is OK if I am not carrying a big load. The max pressure is not really the optimum pressure. So "full" is not such a clear term when talking about pumping up tires!
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Old 05-21-13, 08:45 PM   #4
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"Fully pumped" is a topic that gets debated endlessly.

If you get frequent flats characterized by 2 parallel slits, like the inner tube was bitten by a snake, you're allowing your air pressure to go too low.
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Old 05-21-13, 08:53 PM   #5
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Thanks. My tires are 700x35c, min. 50 and max 85 psi. Squeezing the tire with fingers feels still quite hard with just a slight give-in. Never had a flat since buying the bike nearly 3 years ago.
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Old 05-21-13, 09:01 PM   #6
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Gotta say, the squeeze test is really rotten. A bit better is just to look at how much the tire deforms when you get on the bike. I like, too, to see how it feels when I go over a small bump in the road. But really you want a pump with a gauge. Schwalbe recommends 65 psi for 35 mm tires. The squeeze test will tell you if you are over maybe 30 psi but you don't want to get that low.

I hope your pleasant experience with reliability continues long!
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Old 05-21-13, 09:11 PM   #7
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The Topeak Morph G (or is that G Morph) frame pump's gauge is so tiny to be readable it's useless for me. One reason I try to minimize pumping is to reduce the wear and tear of the part near the valve to prevent leaking.

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I hope your pleasant experience with reliability continues long!
Thanks.
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Old 05-21-13, 09:23 PM   #8
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The Topeak Morph G (or is that G Morph) frame pump's gauge is so tiny to be readable it's useless for me. One reason I try to minimize pumping is to reduce the wear and tear of the part near the valve to prevent leaking.
Don't you have a floor pump?
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Old 05-21-13, 09:43 PM   #9
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Don't you have a floor pump?
No, the frame pump is very good, actually better than my floor pump, so I gave the floor pump away.
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Old 05-21-13, 10:06 PM   #10
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The instructions that came with my 23 mm Continental 4000s tires says do not exceed maximum pressure (120 psi), and, do not go more than 20% Lower. That works out to 96 psi. I run them at about 96 psi. I can ride all day at 60 psi with no problems, but I assume that the lower pressure will let the side walls flex too much, and wear early.
I have ridden home with 40 psi on other tires rated at 120 psi with no problems. It also depends on the terrain (road condition), the riders weight, and how good the rider is a not hitting things
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Old 05-22-13, 07:34 AM   #11
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You must have been very lucky or have nice roads. When I was a little lazier about allowing my tire pressure to get too low I used to get pinch flats all the time. Now I check once a week and haven’t had any pinch flats since.
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Old 05-22-13, 08:43 AM   #12
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better than my floor pump
There is a lot of controversy about what is the optimum pressure for this or that tire etc., but much more consensus that keeping tires reasonably close to optimum is up around the number one maintenance routine for bikes.

A good floor pump, like most any good tool, makes the job a lot more pleasant. Worth spending a bit of money to get, after food and rent but before movies. My nicest pump is a Topeak JoeBlow Pro. These days though I mostly use my JoeBlow Mountain pump. The gauge tops out at 70 psi which limits its application but on the other hand reading the gauge between 40 and 50 psi is very easy so it fits my needs nicely.
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Old 05-22-13, 09:50 AM   #13
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My tires are always 100% full of air. I use Frank Berto's guidelines for how much.
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Old 05-22-13, 10:36 AM   #14
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My tires are always 100% full of air. I use Frank Berto's guidelines for how much.
Seconding this! Recent tests by Jan Heine et al., published in the latest issue of Bicycle Quarterly, show that there's a pretty wide range of pressures at which tires roll more or less the same. If your ride feels too harsh, let out a little air. If you start getting pinch flats, put in a little air. I usually pump my 35-622 Paselas at 60 psi in front and 80 psi in the rear (values determined when I weighed about 210 lbs., so I could probably go lower); when they get below 50 and 70 I'll pump them up again. My new bike will use 42 mm Hetres (650B wheels); I'll run those at lower pressure.
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Old 05-22-13, 10:40 AM   #15
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One problem : underinflated tires can creep around the rim, and carry the tube with it ,
stem may look less than straight, or it can just shear off in the tube and tear out, an unrepairable hole.



For intentionally running low pressure , like studded tires on the Ice,
a Good Idea is use sew-up Tire Glue and
glue one bead to the rim, the other one remains as is, to remove the tube for puncture repairs.

Last edited by fietsbob; 05-22-13 at 10:44 AM.
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Old 05-22-13, 10:42 AM   #16
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Isn't the inner tube prone to tear/damage/leak near the valve area if it's manipulated too frequently when putting on and removing the pump nozzle?
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Old 05-22-13, 10:46 AM   #17
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Isn't the inner tube prone to tear/damage/leak...
if you often do it badly , you can screw it up.
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Old 05-22-13, 11:09 AM   #18
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My tires are always 100% full of air. I use Frank Berto's guidelines for how much.
I found the diagram and it seems I only need 30 psi.

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if you often do it badly , you can screw it up.
I try to be careful but it always take some effort to remove the pump from the valve. This is why I pump only when necessary.
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Old 05-22-13, 11:43 AM   #19
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Isn't the inner tube prone to tear/damage/leak near the valve area if it's manipulated too frequently when putting on and removing the pump nozzle?
I think the biggest danger is from frame pumps or other small pumps that don't have a hose. The pump attaches directly to the valve so the valve is easily stressed back and forth with each stroke of the pump. Those Morph pumps are nice because they avoid this danger. Yeah, there must be some stress too just attaching the hose connector to the valve. Usually though there is some sort of clamping lever so that connector decouples easily enough... you shouldn't have to fight with that too much!
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Old 05-22-13, 04:34 PM   #20
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I think the biggest danger is from frame pumps or other small pumps that don't have a hose. The pump attaches directly to the valve so the valve is easily stressed back and forth with each stroke of the pump.
This. All my mini pumps have hoses;in addition to not having to worry about trashing the valve stem,they're just so much easier to use.

Just curious,why are you running your tires that low? From your initial post it sounds like you just don't want the effort. 30psi in a 2+" knobby ridden in the dirt is fine,30psi in a 35mm tire that calls for 50-85 ridden on the street sounds really low.
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Old 05-22-13, 05:11 PM   #21
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Isn't the inner tube prone to tear/damage/leak near the valve area if it's manipulated too frequently when putting on and removing the pump nozzle?
I'd say no.

The chances for damage occur when the tube has lost pressure and can shift position when you are trying to attach the nozzle. If you are not a complete clutz topping off presents few problems.
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Old 05-22-13, 05:15 PM   #22
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I think the biggest danger is from frame pumps or other small pumps that don't have a hose. The pump attaches directly to the valve so the valve is easily stressed back and forth with each stroke of the pump. Those Morph pumps are nice because they avoid this danger. Yeah, there must be some stress too just attaching the hose connector to the valve. Usually though there is some sort of clamping lever so that connector decouples easily enough... you shouldn't have to fight with that too much!
I don't recall having had any issues with that.

BUT I pretty much only use the frame pump only after getting a flat and then pump to pressure before putting the tire back on the bike. That lets the tire move enough to minimize stress.

If pumping a tire already on the bike I think what you said is something that once mentioned is obvious. And just the kind of thing that can bite someone over and over and not get figured out until someone like you brings it up!
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Old 05-22-13, 06:15 PM   #23
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I found the diagram and it seems I only need 30 psi.
I think the Berto rule of a 15% drop makes reasonable sense. The charts I see on his paper

http://www.bccclub.org/documents/Tireinflation.pdf

don't really look right, though. I did a bunch of theoretical analysis and came up with a formula that the pressure should be proportional to the -3/2 power of the width of the tire. If you fit that to the Schwalbe pressure recommendations at

http://www.schwalbetires.com/tech_in...ation_pressure

it is a pretty good fit. I would recommend the Schwalbe table and just scale it up or down depending on your weight, whatever cargo you're carrying, etc.

So I think 65 psi is a lot closer to the mark than 30 psi, unless you and your bike together weigh like 90 pounds or something.

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Old 05-22-13, 07:36 PM   #24
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Just curious,why are you running your tires that low? From your initial post it sounds like you just don't want the effort. 30psi in a 2+" knobby ridden in the dirt is fine,30psi in a 35mm tire that calls for 50-85 ridden on the street sounds really low.
I'm not running it at 30 psi. My tires are about 80-85% full, still feel quite hard. 30 psi is what I figured from the above mentioned diagram what the mininum I seem to need, but I'm sure at 30 psi my tires would be unridable. I guess that diagram should be taken with plenty grains of salt. I usually just pump by feel (by hand). The main reason I didn't pump frequently is concern about the inner tube damage near the valve.
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Old 05-22-13, 09:11 PM   #25
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I tried to fit Berto's chart to my -3/2 power rule. I eye-balled the chart for 190 pounds - the fit isn't too bad. But looking carefully at the chart, it's easy to see a fundamental problem. Really the lines should all converge at 0 psi for 0 load. They seem to converge at 0 pressure more around a load of 30 pounds or so. So the chart can't be quite right.

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