I'm relatively new to off-roading, so maybe this is common...

How much air do you typically lose in a month of normal road riding? (On stock tires and rims)

How much in a day when hard off-roading?

Over a month's time of road riding and light off-roading, my rear went from 25psi to 12psi.

On a ride yesterday, I aired the rear down to 12psi. Five hours of riding later it was at 8.8psi

Aired it back to 12psi (at top of mountain) and an hour later it was down to 9psi. (Bottom of mountain)

Would the changes in altitude be a possible cause? There was about a 900 meter altitude difference in my last ride.

It's the same for the front tire.

If it was only one tire I would think a leak. But both?

 

That's a lot of loss. If the OEM tubes on your bike have been replaced at some point with el cheapos maybe the valve cores are poorly sealed. Try a bubble test on them.

Short of actual holes in the tubes that's about the only place loss can occur.

 

The tubes you're using can make a difference. Apparently Latex Rubber leaks air (through the tube itself via permeation) at about 10 times the rate of Butyl Rubber.

I find that every month or so I have to top up the pressure in my tires. Your pressure loss doesn't sound like permeation, but a slow leak somewhere. Sitting there, the slow leak is probably very slow, but when riding, a combination of movement, heat, changes in altitude may expedite the process.

 

I find monthly air checks are needed. Fortunately, I can top it off with just a bicycle pump.

 

Thanks for the input.

They are the original oem tubes. I've had the bike for about 5 months. I had the dealer put on rim locks a couple months ago. But problem seemed to start before then. I'll take it back to the dealer and see what they think.

With both tubes doing this at such a slow-ish, uniform rate, do you think it could be some kind of defect?

 

You're losing air way faster than I do.

I like to do my own work. A heavy duty tube and the tire of your choice isn't too expensive. You can add ride-on sealant for more peace of mind, too.

 

I'll need new tires in a couple months I think. So maybe this is a non-problem as long as it doesn't blow. I'm planning to get heavy duty tubes as well.

I'm probably too fat for this bike and this is its way of making fun of me.

 

Pressure will change with elevation and temperature:
http://www.tirerack.com/tires/tirete...jsp?techid=167
http://www.tirerack.com/tires/tirete....jsp?techid=73