inner tube sizing

[D36-108A]

Is there nothing as confusing as the sizing of tire inner tubes?

I'm looking to put some street tires on my KTM 440 and ride it on a road course. Supermoto Sportsman, if you will. I'll be d_mned if I can figure out which tube sizes I need.

I've got a standard (1.85" wide?) 21" front wheel and a wide (2.5"?) 18" rear wheel. What are the corresponding tube sizes?

 

[FruDaddy]

Depends more on the tire sizes than the wheel width, and tubes usually cover a modest range of those. So, what size tires do you plan to use?

Before you answer that, check out this link:
http://www.maxxis.com/Services/motorcycle_tire_school.asp#motorcycle_tire_conversion_charts

 

[D36-108A]

I plan to use Maxxis Promaxx tires, a 120 rear. My bike came with a 130 knobby and I've got a Michelin 140 knobby on it right now.

 

[FruDaddy]

According to the link that I provided earlier (from Maxxis) you need a 4.50/4.75 x 18 tube for a 120mm x 18" street tire. The 140 knobby needs a 5.00/5.10x18 tube. This really isn't hard if you look at the conversion chart.

 

[D36-108A]

I don't know where you get the idea that chart has anything to do with tubes. It simply shows the tire sizes in metric, alpha, and inch.

Am I missing some underlying assumption that the tube size should match the tire size?

 

[dirt bike dave]

According to the link that I provided earlier (from Maxxis) you need a 4.50/4.75 x 18 tube for a 120mm x 18" street tire. The 140 knobby needs a 5.00/5.10x18 tube. This really isn't hard if you look at the conversion chart.

FWIW, he's got a 140 Michelin knobby. That is the same size as any other brand's 120.

Seems to me that a tube that works with a 120 knobby should work with a 120 street tire.

 

[D36-108A]

I ended up ordering a 110 tire because that's what the Maxxis site recommended for a 2.5" rim. (The 120 was for a 2.75".) I also ordered their only 18" tube, which made it easy: 100/100-18, 120/100-18 TR6.

 

[FruDaddy]

FWIW, he's got a 140 Michelin knobby. That is the same size as any other brand's 120.

Yep, that's what it looks like on the chart, a metric size 120/100 is the same as the alpha size 140/80 (the only 140 on the list), and they both share the same measurement in inches.

D36-108A, tubes stretch to fill the void, so there is flexibility, so yes, I would choose a tube that is roughly the same nominal size as the tire.