Dave76

Hi everyone,
I recently changed the stock 700x32 road tyres on my TREK FX to a wider 700x35 gravel tyre but kept the original inner tube. The max PSI on the original tyres is 6.9 PSI where as it's 3.4 on these new ones (Bontrager GR1 Team Issue) and they seem a bit flat when pumped up. Should i have changed the inner tube size aswell as the tyre? What size inner tube should I be buying for this size tire? I'll need one as a spare anyway.
Thanks for your help :-)

Dave76

I did mean bar and not psi, thanks. I was under the assumption that an inner tube that could be inflated to 6.9 bar would be under-inflated at 3.4 but it's the tyres that define the inflation?

dsbrantjr

You will never be able to inflate an inner tube to 6.9 bar or even 3.4 bar unless it is constrained by a tire. The ratings are for the tire not the tube. The strength comes from the tire's fabric plies, the inner tube serves to make an air seal only.

Dave76

OK thanks

Iride01 

You want the tube to be made for that size tire or smaller. If the tube is made for a bigger width or BSD then you'll likely have wrinkles in the tube when inflated in the tire. You might actually feel them when you ride and certainly they'll provide a place for the tube to rub a hole in itself. In severe mis-sizing of tube and tire you might have a kinked area that never fully inflates to support the tire around the kink.

Too much stretching of the tube from being too small just makes them inflate more and get thinner. Less puncture protection I'd think and more likely that act like a balloon meeting a pin when poked with a thorn. Maybe not, but makes for great imagery in ones head <grin>.

Many tubes state a range of sizes that they fit. Try to stay within that. But since your previous tire was smaller by only a few mm and you don't know what range tire it was made for, I wouldn't worry about it. Just put the correct size tube in if the old tube ever fails. Or fails too often if you like patching them.