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    Newbie tire pressure question

    How much pressure do you keep in the tires? Mine are marked 40 to 60 psi... That's a wide range.

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    Gears? CliftonGK1's Avatar
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    My 28mm Gatorskins are max marked at 115, I think. I run them between 112 and 120 depending on the terrain. Lower for rough and rumbly chipseal, and higher for smooth pavement.

    My 32mm Pasela TGs and max marked at 85 or 90psi and I keep them at 95 all the time. They're cushy enough for any surface at that pressure.
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    Senior Member late's Avatar
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    Just enough.

    The tire should keep it's shape when you sit on it, but no more.
    Old Man Maine

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    Senior Member cyclist2000's Avatar
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    It really depends on what you are doing. I would keep higher for road riding and bit lower for dirt riding. If you find that the 60 psi is too hard then let out a few psi. don't go too low otherwise you may get pinch flats.
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    And I thought 60 was high! I guess I'll fire up the pancake compressor... Doing that by hand might take a while!

    I guess I should have mentioned I'm talking about a mountainbike, not a road bike.

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    Bicycle Quarterly did some rolling resistance tests which showed that, as expected, the resistance drops as you increase the pressure. But the shape of the curve showed that once you get the pressure high enough so that the tire sidewall only drops 15% of its height when loaded then there's very little gain from adding still more pressure and you mainly just lose in comfort. They made a graph of the tire pressure needed for this optimum situation as a function of tire width and the per-tire load (usually the rear is more loaded than the front and should have a higher pressure).

    The article and graph are at:
    http://www.bikequarterly.com/images/TireDrop.pdf

    Their results are based on riding on paved roads. Should be a good starting point and then you can evaluate if you'd prefer the tires to be a little softer for more comfort or harder for improved resistance to pinch flats or slightly less resistance on smooth surfaces.
    Last edited by prathmann; 02-04-11 at 09:27 PM.

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    I am the Snail~! Peter_C's Avatar
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    Quote Originally Posted by prathmann View Post
    Bicycle Quarterly did some rolling resistance tests which showed that, as expected, the resistance drops as you increase the pressure. But the shape of the curve showed that once you get the pressure high enough so that the tire sidewall only drops 15% of its height when loaded then there's very little gain from adding still more pressure and you mainly just lose in comfort. They made a graph of the tire pressure needed for this optimum situation as a function of tire width and the per-tire load (usually the rear is more loaded than the front and should have a higher pressure).

    The article and graph are at:
    http://www.bikequarterly.com/images/TireDrop.pdf

    Their results are based on riding on paved roads. Should be a good starting point and then you can evaluate if you'd prefer the tires to be a little softer for more comfort or harder for improved resistance to pinch flats or slightly less resistance on smooth surfaces.
    Am sorry, but I think you said that a bit wrong?

    The tire's (when you and gear is on it) *height* should drop 15% of the *WIDTH* of the tire. So, if the width of the tire is 40mm, 15% would be 6mm. Then pump your tires to their max pressure, measure the height of the tire unladen (not on it), note that height, than have someone let air out of the tires one at a time, with you and everything you normally carry on the bike - until each tire has dropped 6mm from it's original height.

    This is supposed to give you the best ride with the least increase in rolling resistance.

    Remember, never go below the minimum pressure regardless.

    And, if decreasing rolling resistance is MOST important to you, regardless of comfort - pump them to max pressure, and be done.

    I am into comfort - so I use the theory above.
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    Quote Originally Posted by Peter_C View Post
    Am sorry, but I think you said that a bit wrong?

    The tire's (when you and gear is on it) *height* should drop 15% of the *WIDTH* of the tire. So, if the width of the tire is 40mm, 15% would be 6mm.
    Sure, but the cross section of a bicycle tire is pretty close to a circle so the width of that circle is also equal to its height. The diagram in the BQ article shows the width measurement, but AIRC, Berto talked about the percentage drop in the tire height, i.e. distance from the contact point to the bead. The two are generally close enough that you can use either measurement.

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    I researched this about 2 months ago. These are the best articles I could
    find on the subject. For everyone above 180 lbs it says you should run
    them at max inflation.

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

    http://www.michelinbicycletire.com/m...rpressure.view
    http://www.michelinbicycletire.com/m...essuremtb.view

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    My tires list a max pressure of 60 psi (IIRC). Do inner tubes have a seperate max pressure?

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    Senior Member Wogster's Avatar
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    Quote Originally Posted by LACamper View Post
    How much pressure do you keep in the tires? Mine are marked 40 to 60 psi... That's a wide range.
    It depends on a number of factors, generally though the articles are right, but the pressure tables don't go high enough. Fill the tires to the maximum pressure, measure from the edge of the rim to the ground, multiply this by .85, get on the bicycle and measure again, if the number is more then you calculated, you can drop a little pressure until it matches up, if it's below that amount, then you need a higher PSI or a wider tire at the same PSI. The width of the tire and the height of the tire are almost the same, most pressure gauges are 5 PSI, you never will get it exact, but it will be close enough. You just want to make sure you stay above the minimum.

    Unfortunately the bicycle tire industry tends to make the assumption that all riders are under 140lbs, so you see things like a tire in wider widths having lower pressure ratings so that you can't use a wider tire to handle more weight. I also find it interesting that a frame will be designed to hold up a 500lbs rider then have chainstays that only allow a 23mm wide tire, a tire that would need to contain over 170PSI to hold the load.

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    Quote Originally Posted by prathmann View Post
    Sure, but the cross section of a bicycle tire is pretty close to a circle so the width of that circle is also equal to its height. The diagram in the BQ article shows the width measurement, but AIRC, Berto talked about the percentage drop in the tire height, i.e. distance from the contact point to the bead. The two are generally close enough that you can use either measurement.
    If you will look at the original post, an article was quoted. My point is the article was mis-quoted, so I corrected the mis-quote. Your point (when used in light of the quoted article) is not revealant. Your point on it's own merits may be, but that's for a different post.
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    Question:
    Tires show max pressure actually of 65 pounds. I need more pressure than that to be at only 15% depression. How much can you overinflate tires by safely?

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    That means you need a bigger/wider tire for your weight. On the BROL Forum this was discussed a great deal, and that going to a wider or narrower tire, you can get to a point where you have the correct cross-section at max PSI, or at minimum PSI.

    In this case, you would gain speed by going to either a higher pressure tire in the same width, or a better ride by going to a wider tire. No matter what, the current tire is *not* the best choice for you. I went to a max PSI of 65 on a marathon 26 X 2.00 on my Suede DX to get the best cross-section at the max PSI.
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    I have 26x 1.95 Schwinn's right now, (which are practically brand new, btw). Suggestions?

    And until I get to my local bike shop to replace these, how much can I safely overinflate these for concrete use?
    Last edited by LACamper; 02-06-11 at 01:47 PM.

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    I am the Snail~! Peter_C's Avatar
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    No one can tell you how much you can *SAFELY* (or should) overinflate them.

    *I* have overinflated about 10 PSI is all...
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    Gears? CliftonGK1's Avatar
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    Quote Originally Posted by LACamper View Post
    My tires list a max pressure of 60 psi (IIRC). Do inner tubes have a seperate max pressure?
    Inner tubes aren't rated for pressure, they're rated to a specific size; like 19-25mm or 28-35mm). The maximum size rating is supposed to assure that you aren't stretching the tube too thin and compromising it, although you can save weight by running a smaller tube in a larger tire (a racer's trick.)
    The casing of the tire keeps the tube from inflating beyond the maximum size, so as long as there aren't any punctures through the tire casing, the tube can't expand beyond that size.

    That said, I've run a 19-25mm tube in a 32mm tire for 100 miles without a problem. When changing a flat in the rain and the dark after already riding 150 miles, using a smaller tube makes it a little bit easier to make a quick change.
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    I've read that tires are supposed to be tested at up to twice their rated maximum pressure, but I wouldn't recommend going that high. I have inflated tires on our tandem up to 30 psi over the stated maximum (120 vs. 90 psi) without any problems.

    Tubes can be used well outside of their nominal size range but in some cases it may make the installation more difficult and could result in pinching the tube between the rim and tire if one isn't careful. But I've put 700c tubes in 26", 20", and even 12" wheels without any issues

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    Senior Member Wogster's Avatar
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    Quote Originally Posted by LACamper View Post
    Question:
    Tires show max pressure actually of 65 pounds. I need more pressure than that to be at only 15% depression. How much can you overinflate tires by safely?
    Tire pressure limits are not engineering limits, they are lawyer limits, say an engineer designs a new tire and it's good for 100PSI, the legal department wants a much lower limit, something like 50PSI, because if a tire fails and the tire company gets sued, then legal has to get off their fat duffs and actually earn some of that big retainer they are on. Marketing on the other hand wants a high limit so they can prove how durable the product is, so the number moulded into the side wall has a lot more to do with who wins the battle between legal and marketing, we know it will be something less then the engineering limit, we just don't know by how much. could be 5 psi it could be 50....

    One thing to remember the 15% only applies to road use, if your tires are intended for off road use (knobbies) then they tend to be much wider and have a much larger depression. If you are riding mostly on the road, then you need a different tire, I ran Ritchey Tom Slicks on my mountain bike for quite a while, those tires are 1.4" wide (35mm equivalent) and are marked 80PSI, I've run them at 90 on the rear with no ill effects.

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    Senior Member late's Avatar
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    Hard tires go up and down a lot and when you are going up or down you are not going forward.

    Hard is slower.
    Old Man Maine

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    lot of different info in this thread. IMHO making things alot more difficult then they need to be. Tires of the same size can have different max Psi ratings. You do not necessarily have to get a bigger or wider tire. I run my road tires at the max on the side wall of 115psi and my mtb to its max side wall suggestion of 80 Psi.

    If you want cushy and you dont care about rolling resitance let some air out till they feel comfortable, just dont go so low that the bike feels unstable or where you have to worry about pinch flats. On my mtb if i am riding a trail and it is rough I get my butt out of the seat and my weight on my feet, thats the way to ride them anyway.

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    I am the Snail~! Peter_C's Avatar
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    Quote Originally Posted by dgrenthum View Post
    lot of different info in this thread. IMHO making things alot more difficult then they need to be. Tires of the same size can have different max Psi ratings. You do not necessarily have to get a bigger or wider tire. I run my road tires at the max on the side wall of 115psi and my mtb to its max side wall suggestion of 80 Psi.

    If you want cushy and you dont care about rolling resitance let some air out till they feel comfortable, just dont go so low that the bike feels unstable or where you have to worry about pinch flats. On my mtb if i am riding a trail and it is rough I get my butt out of the seat and my weight on my feet, thats the way to ride them anyway.
    Instead of skimming, or speed reading - take the time to read the entire post perhaps?

    My second paragraph started with this:

    In this case, you would gain speed by going to either a higher pressure tire in the same width, or a better ride by going to a wider tire.
    Depending on what the OP wants to do, there are choices. Yes, one of them is to get the exact same tire in the same size, that has a higher maximum pressure, or a narrower tire with a higher max PSI (most speed), or a wider tire with the same or higher max PSI (more comfort, and possibly same or more speed)
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    So, can ya'll suggest some tires? I'm riding a mountain bike. Mainly on concrete but occasionally on dirt trails. Not much in the way of hills here. I'd rather stay with the same size tires (26 x1.95 IIRC) unless there's a good reason to change. I don't want to put a lot of money into a $250 bike obviously. Eventually if I start riding enough I'll spend the money on an upgrade. I don't know diddly about tire brands... educate me please.

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    I am the Snail~! Peter_C's Avatar
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    I love my Marathons - not Marathon plus, just plain old "Marathon HS 420".

    WEBSITE

    I had 26 X 2.00 tires, and did pavement and towpath (crushed limestone), and have a bit over 700 fl@t-free miles on them. You can get them in 1.5, 1.75, 2.00 - and they can carry a heavy load too~!

    There are many, many different tires to pick from, these are the only ones I have personal knowledge of.
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