giskard

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?

tia

Johno

HGR3inOK

What size are your tires? The AV7 is correct for 20" tires from 1.5" to 2.5". The AV7A may work, but it may not be the best fit. See:
https://www.schwalbe.com/gbl/en/produkte/schlaeuche/

giskard

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?

HGR3inOK

Whoops, wrong language. What size are your tyres?

giskard

The existing tube is 20 x 1.5 so I guess that's what I need. The 1 3/8 tubes I bought were way too big for my wheel even though the wheel is 20" and the AV7's are 20".

I'll post some pics when I can

HGR3inOK

Anyone know the size for the Marathon Racers on a stock Mu P8?

HGR3inOK

I'm pretty sure your tyres/tires are 20"x1.5". If so, you should use Schwalbe AV7 tubes.

vmaniqui

yes. correct. the size is 20x1.5

giskard

Thanks for your responses guys.

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?

Thanks again

HGR3inOK

Just specify that you want the Schwalbe AV7.

jobtraklite

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.