I had a flat today,rode with it to the nearest puncture repair shop.The tube had ruptured at 3-4 sites.The repair guy advised me to replace the tube but he didn't have one in 18"( stock size for my bike).He told me it was ok to put in a 17" tube into an 18" tire. I had no option but to let him do it...Should I be concerned?
I would be more concerned about the cross-section of the tube being large enough for your tire than about 17" versus 18" wheel diameter. The 17" tube will fit a little tighter on the rim, but can it expand to fit the inside of your tire without surpassing its specs? You will likely be okay for a while, but I would be inclined to get a high-quality tube of the correct size ... and especially for a front tire.
When I raced BMX we did it all the time to save a little weight...16" tubes in the 20" tires. In all my beating, banging, curb hopping, rock smashing and general holliganisms I never "popped" a tube. Kenny
I would think it would be ok. I've put 21" front tubes in (much wider 18") rear tires with no troubles, though I wouldn't consider the front-tube-in-the-rear a long-term fix, it'll get you home.
