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Old 08-14-09, 10:29 AM   #1
Riverside_Guy
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Tire Inflation?

I have 700 x 35cs that are marked to hold 50-75 psi. I'm about 235, so at first I inflated to 70... but found after 1-2 rides, they were down to 50-55. Last 2 times I inflated to 63, and the same happens, 50-55.

It seems it gets into that range and tends to stay there, so I'm pretty sure I'm not having the exact same slow leak in both tubes.

Am I right to inflate to 60-65? Is it not unusual for it to deflate to roughly the same pressure? Is it "normal" to lose 10 psi after 2 rides?

Thew bike seems every so slightly more comfortable at 50 and 65. However, I have not noticed any specific issue with the speed I can do, factoring out wind conditions.

My "rides" are 3 per week, about 10-14 miles each.

Should I be topping up before every ride?

Time to get in the saddle now, we got some SUN!!
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Old 08-14-09, 10:34 AM   #2
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i top up every morning and usually cram an extra 10% in. suggested psi ratings owe more to lawyers than engineers after all.
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Old 08-14-09, 11:10 AM   #3
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I use the settings for my tires, but I top up every day I can. I miss a day once in a while, but do not miss two.

Bike tires are notorious for not holding pressure. Check and fill every day if you can.
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Old 08-14-09, 03:57 PM   #4
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Loss of air in a tyre is quite normal. Time to get worried is when you lose all the air overnight and you can't find the puncture.

The tyre pressure on the wall of a tyre is only so you can keep within safe guidelines. The heavier a rider and the higher the pressure required to stop Snakebite punctures. This is where you hit a rock/kerb/pothole and the tyre is deformed so much that it compresses againt the rim- trapping the tube between the two. This gives two neat holes in the tube- just like a snakebite. That does not necessarily mean that you have to go to the Max as your potholes may not be that severe- but it is a good guide to what you should use. Going low on pressure is not always a good idea. Snakebites are one thing but "Extra Drag" caused by a low pressure can be tiring. And going too high will give you a rough ride.

And it is always a good idea to check pressures before a ride. You have found the main reason. You always seem to lose the same amount between rides but if one of the tyres loses an extra 20psi- you will know you have a "Slow" puncture that could turn into a flat tyre at any time. Better to repair it at home than on the road.

Just thought of one reason you could lose pressure after a ride. I use 700x23's at 120psi and I only lose a few lbs pressure weekly. It could be the quality of your tubes. There is a reason why not all tubes are only a few $'s each. The better quality tubes- and more expensive- are better quality.
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Old 08-14-09, 07:25 PM   #5
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I weigh 240 and run Specialized tires and tubes... They are rated at 75-100 pounds.. I blow them up to 95 and then a couple weeks and a couple hundred miles later I check them .. They are usually around 80-85 pounds so I blow them back up to 95 again and go another couple weeks.. If my tires (tubes) were looseing air as fast as the OP's seem to be I would be getting a better quality tube.. He is looseing more in a few miles then I loose in a couple weeks..
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Old 08-15-09, 11:41 AM   #6
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Thanks for the input guys. The pattern seems to be lose 10 pounds in the first 1-2 rides, but not so much after that. Which leads me to ask what ya'll might think I SHOULD be riding at. I get this sneaky feeling they WANT me to ride around 50-55, even though I'm heavy and 50 is the supposed minimum.

Seat of the pants does tell me that having more rubber on the road is not impacting speed much at all.

Way back in my motorcycle days, I recall it was all about real rubber vs. synthetic. Rubber was what to get, part of which was durability, and part was it's ability to hold pressure longer. Is there such a thing as real rubber vs. synthetic for bike tubes?

BTW, I didn't remember correctly, they are 700 x 38cs, although I don't think it's much of a factor from the 35 I previously mentioned.
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Old 08-15-09, 03:53 PM   #7
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The way I see it the tire has very little to do with holding the air pressure... It is the tube that is blown up and holds the pressure.. I am not an expert on tubes but it has appeared to me over the years that some tubes are more porus then others.. I would simply try another vrand of tuve and check the differance... If I was loosing 10 pounds of air the first day I would be very upset
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Old 08-15-09, 06:21 PM   #8
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I'm heavy too, and I find that I have to increase the pressure about every other ride. tubes are porous, not heavily, but they do leak naturally. also, remember to keep the caps on your stems (they actually create the seal). If your caps are old, the internal seal in them might not be solid.
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Old 08-15-09, 07:44 PM   #9
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Wow, what a lot of misinformation. The cap does not form the seal. The tire does the job of holding the pressure, though the tube makes the seal, i.e. it holds the volume.

Riverside Guy, tires are tested in the factory at double the rating. I would use 100 psi on the rear tire. You don't need nearly as much on the front, since a typical bike bears 2/3 of the weight on the rear wheel. Experiment to see what pressure is the point of diminishing returns. In other words, if 100 doesn't feel quicker than 90, then stick to 90. Make sure you're not bottoming out on the rim when you hit bad potholes, though. If you are, increase your pressure.
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Old 08-16-09, 03:08 AM   #10
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Main reason for the valve cap is to keep grit out of the valve core, which might stop it seating.
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Old 08-16-09, 06:18 AM   #11
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I never use valve caps. I don't believe they serve a purpose. Perhaps cars need them because of higher speeds.
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Old 08-16-09, 10:33 AM   #12
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Oh I know all about spoked wheels and potholes; after I bent the second front wheel rim on my motorcycle I got cast wheels! Yes I understand conservative ratings.... but 100? That would seem to give me a VERY hard ride. Not such an issue on the Central Park Loop (especially now that the whole of the East Drive is getting new paving) but I fear it would be a major teeth jarring on the Greenway, especially the northern route.

Think I'll do more experimenting once I get a good floor pump in my apartment, I've been using a pump someone left out in the basement bike room which means schlepping my bike up and down in a very small elevator with wood finish (I'm on my coop board so I can't be cavalier about scratching the nice wood finish!).

However, I'm also curious about why you suggest such an inflation... is it more to protect the rim from a heavy rider? For more performance (up to a point, putting less rubber on the road may slow one down)?

So I guess few talk about synth vs. real rubber in biking??? Or is it all moot because everything is synth these days?
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Old 08-16-09, 11:56 AM   #13
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Please forgive my asking if you are not doing this, but are you using the gauge on your pump to check the tire pressure? That works fine when airing tires up, but once the chuck is disconnected from the valve the air in the hose returns to ambient air pressure. If the chuck is then reinstalled on the valve the air from the tube flows back into the hose until the pressure is equalized, giving the vessel containing the air greater volume. Hence, the lower pressure.

However, that being said it is true that all tubes will slowly leak over time. I top my tires off about once a week. More often seems unnecessary.
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Old 08-16-09, 02:48 PM   #14
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I ride very narrow tires at 120 psi, and I weigh only 185. Of course, wider tires need less pressure, but more pressure gives you less rolling resistance, aka better performance. The improvement that higher pressure provides is accentuated for heavier riders or cargo loads. A lightweight rider can't really feel the difference between 70 and 100 psi. Higher pressure also helps protect your wheels, both rims and spokes.

Try it. It's not as crazy as it sounds. As I said, see if you can find the point of diminishing returns. It might even be above 100 psi.

You're on the upper west side? That's where I grew up, on 96th St near Amsterdam. My mother still lives there, and my sister is on W 83 St.
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Old 08-16-09, 11:01 PM   #15
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I run 700 x 32 Panaracer TG tires.I'm not even sure what they are rated at but I've always run my tires around 120-130psi.They lose about 10 psi a week or so.I ride about 200-250 miles a week.Depending on what side of a bike tour you ask,I weigh between 185-200.

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Old 08-17-09, 08:58 AM   #16
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Good story, Booger1.

I have some Panaracer Pasela tires, not the Tourguards. They have got to be the best value on the market today. I think I paid $20 each, and they'll spruce up any bike.

I clumsily put some rips in the sidewall, so I put a boot on the inside. To test it, I inflated it to 150 psi, and it held just fine.

Anyway, they ride great.
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Old 08-17-09, 09:02 AM   #17
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Quote:
Originally Posted by noglider View Post
I ride very narrow tires at 120 psi, and I weigh only 185. Of course, wider tires need less pressure, but more pressure gives you less rolling resistance, aka better performance. The improvement that higher pressure provides is accentuated for heavier riders or cargo loads. A lightweight rider can't really feel the difference between 70 and 100 psi. Higher pressure also helps protect your wheels, both rims and spokes.

Try it. It's not as crazy as it sounds. As I said, see if you can find the point of diminishing returns. It might even be above 100 psi.

You're on the upper west side? That's where I grew up, on 96th St near Amsterdam. My mother still lives there, and my sister is on W 83 St.
104th betwixt RSD & WEA; I'll give your suggestion a shot!
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Old 08-17-09, 09:56 AM   #18
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It's not surprising. The lower the pressure in the tire/tube, the closer it is to atmospheric pressure and the more slowly it'll lose any additional pressure. If you're losing more pressure than you're comfortable with, get thicker tubes. They'll be slower and heavier, but they'll hold pressure better.
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Old 08-17-09, 11:37 AM   #19
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I have Rivendell Ruffy Tuffy tires that Rivendell states are 700 X 27C. I keep them at 90psi as I'm about to head out on them. Most 28C tires state 90psi, some say 100psi. I find 90psi optimal for the mix of pavement and dirt up here.
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Old 08-17-09, 04:20 PM   #20
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I hate pumping up tires. These days there are so many garages around that I just fill up at the station.
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Old 08-18-09, 12:03 PM   #21
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At 235 lb you should have about 90 psi in the back and 70 psi in the front. Use slightly higher pressure if you are interested in speed or slightly lower if you want more comfort.
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Old 08-19-09, 10:41 AM   #22
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Will try that combo once the weather gets more reasonable... although it may make for a harsher ride than I'd like.

BTW, I'm down to 227, which I think is a combo of riding and eating... less.

My guess is "speed" may be limited by my gearing. Can't seem to make it over 25.5 mph even on a long downhill in Central Park.
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