I need to replace the inner tube on the front wheel of my Mu P8 and was wondering how I determine which size tube I need given that the bike is fitted with Schwalbe Marathon racer tyres. My lbs has sold me a Schwalbe AV7A tube - it's labelled 20*1 3/8 Schrader which looks okay to me except for the 1 3/8 which I guess is the tube diameter when inflated?
Can anyone tell me if I have the correct tube for my bike?
Thanks for your reply.
Referring to the chart in your linkie, I can see that the AV6 40-406 and AV7 40-406 should fit - my existing Kenda inner tube is a 20x1.5
The only difference is between those two tubes that the AV6 is lighter than the AV7 (I guess the AV7 is a more robust tube?) so I'm guessing I need the AV7?
The ETRTO number on my wheel is 406 x 17 - how does that relate to the tube size?
After following the link provided by HGR3inOK a few days ago, I did conclude that the AV7s were the correct ones, my tyres are 40-406 (20" x 1.5) so that's what I need. BTW, I still don't know what the 40-406 means - I'm guessing the rim diameter is 406mm, but the 40? Wassat?
Just out of interest, here are a few pics (not great quality) of the original Kenda tube and the AV7A's that Evan's Cycles (incorrectly) sold me - you can see that the 20" x 1 3/8 AV7A is way too big, even though it's supposedly a 20" 1 3/8 tube which to me would mean that it should be the same diameter but a wee bit smaller than the 20" x 1.5 that's correct for my bike.
I'm confused by all this tyre/tube sizing as it doesn't seem to make sense to me, even after reading Sheldon Brown's pages about tyres and tubes.
So just to confirm, I need these, but do I need to specify the exact size (i.e. 20" x 1.5) or is that size covered by the tube that is 1.50/1.75/2.00?
The maiden voyage on my MuP8 was to the train station to pick up tickets for a train departing at 6:00AM the next morning to go on tour. On the way back from the station, I had a pinch flat and through stupidity, blew out the tube. The LBS was closed for the day; so my only choice was a generic 20" tube from K-mart. That was over 700 miles ago; and it's still going strong.