Demystifying inner tube sizing

[stickfigure]

I'm confused by the way inner tubes are sized.

Tires I understand. The 140/80-R18 at the rear of my KTM 640 Adv should be approximately 5.5" wide and 4.5" tall, fitting an 18" rim.

Inner tubes come in stranger numbers. I see things like 3.5-4.0-18, 275/300-18, and even 4.5-500-18. The very last number is easy... fits an 18" rim. Ok.

Are the first two numbers a range? Ie, does a 3.5-4.0-18 tube fit a tire with an approximate width between 3.5 and 4.0 inches?

Are the numbers missing decimal points just manufacturers being sloppy in their labeling? Ie, does 275/300-18 mean the same thing as 2.75-3.00-18?

Are we matching the size of the tube to the outside width of the tire or the inside width of the tire? If my tire is 5.5"/4.5", does that mean I need a tube that includes the rough average of 5.0" in its range? Or does the thickness of the tire wall factor into this somehow?

What size tube are you folks using in your KTM 640A rears?

I'm currently using a 3.5-4.0-18 in my rear tire because it's the biggest thing I could find in Nicaragua. It's holding up, although the 2.75-3.something-18 that Scuderia sold me as a spare before I left (looks like they screwed up) lasted less than a mile. Back then a llantera worker in Costa Rica had me buy something marked 4.5-500-18, and now I wish I had bought three or four of them when I had the chance. Sigh.

Another question: What size tube are you folks using in your KLR rears (130/80-17)? My riding buddy has the same problem.

Thanks,
Jeff

[tn-steve]

Conversion Chart

Front Tires			Rear Tires
Metric	Alpha	Inch	Metric	Alpha	Inch
80/90	MH90	2.50/2.75	110/90	MP85	4.00/4.25
90/90	MJ90	2.75/3.00	120/90	MR90	4.50/4.75
100/90	MM90	3.25/3.50	130/80		5.00/5.10
120/80		4.25/4.50	140/80		5.50/6.00
120/90	MR90	4.25/4.50	140/90	MU90	5.50/6.00
130/90	MT90	5.00/5.10	150/80	MV85	6.00/6.25
			150/90	MV85	6.00/6.25

[stickfigure]

Originally Posted by tn-steve

Conversion Chart

#1 This doesn't actually answer any of my questions.

#2 This chart appears to be wrong.

The internet tells me that 140/80-R18 means that the width of the tire is 140mm (about 5.5") and the height of the tire is 80% of that number, which would be 4.4". The chart says "5.50/6.00" which makes no sense. Furthermore this chart doesn't say anything about inner tubes.

Jeff

[Reposado1800]

Originally Posted by stickfigure

I'm confused by the way inner tubes are sized.

Tires I understand. The 140/80-R18 at the rear of my KTM 640 Adv should be approximately 5.5" wide and 4.5" tall, fitting an 18" rim.

Inner tubes come in stranger numbers. I see things like 3.5-4.0-18, 275/300-18, and even 4.5-500-18. The very last number is easy... fits an 18" rim. Ok.

Are the first two numbers a range? Ie, does a 3.5-4.0-18 tube fit a tire with an approximate width between 3.5 and 4.0 inches?

YES

Are the numbers missing decimal points just manufacturers being sloppy in their labeling? Ie, does 275/300-18 mean the same thing as 2.75-3.00-18?

YES

Are we matching the size of the tube to the outside width of the tire or the inside width of the tire? If my tire is 5.5"/4.5", does that mean I need a tube that includes the rough average of 5.0" in its range? Or does the thickness of the tire wall factor into this somehow?

What size tube are you folks using in your KTM 640A rears?

I'm currently using a 3.5-4.0-18 in my rear tire because it's the biggest thing I could find in Nicaragua. It's holding up, although the 2.75-3.something-18 that Scuderia sold me as a spare before I left (looks like they screwed up) lasted less than a mile. Back then a llantera worker in Costa Rica had me buy something marked 4.5-500-18, and now I wish I had bought three or four of them when I had the chance. Sigh.

Another question: What size tube are you folks using in your KLR rears (130/80-17)? My riding buddy has the same problem.

Thanks,
Jeff

That is my 2 cents

[markjenn]

Originally Posted by stickfigure

#1 This doesn't actually answer any of my questions.

#2 This chart appears to be wrong.

The internet tells me that 140/80-R18 means that the width of the tire is 140mm (about 5.5") and the height of the tire is 80% of that number, which would be 4.4". The chart says "5.50/6.00" which makes no sense. Furthermore this chart doesn't say anything about inner tubes.

Jeff

The guy was trying to be helpful and took the trouble to post a useful chart. Lighten up.

The chart says 140 tires are about 5.50-6.00 simply because this is the conversion of 140mm to inches (140mm = 5.51"). The chart says nothing about tire height.

You're making this too hard. Inner tubes don't come in exact sizes to fit a specific tire. They are commonly marked with a range of widths that can be in inches or mm. A simple one inch = 25.4 mm conversion is all you need. Aspect ratio is generally ignored. You do want the diameter to be exact although many people get by mounting a bigger diameter inner tube than spec'd - this is a way you can carry a 21" front tube and make it work, at least for an emergency, in an 18" rear.

- Mark

[dwayne]

#1 the chart does answer at least two of your questions. Your buddy should be running a 5.00/5.10 in his KLR rear, and your KTM is best with a 5.50/600. In fact that is correct.

#2 read what Mattjenn said. Inner tubes are round in cross section, tires are not. Size the tube for the width of your tire not the height. Tubes are really an inexact thing and if your an engineer (or at least one of those who have problems with the idea of tolerances) or an anal perfectionist they will drive you nuts. People have tied knots in there innertubes, or stuffed thier tires with leaves and moss to get out of the bush, have run THOUSANDS of miles and several sets of tires on multiply patched incorrectly sized innertubes. I run a 20" bicycle tire on my 21" front rim when I have my ice racing tires mounted. Pick the closest size in a good quality, appropriate duty tube, add air and ride.

#3 read what Mattjenn said. Crapping on people actually giving you usefull info in not polite. Lighten up, say thankyou, and you will enjoy your stay here much more.

[stickfigure]

Geez, where in my response did I call the guy mean names?

I found plenty of charts like this online, but none are explained well nor do they explicitly describe inner tubes - they're just a way of converting (approximately) between metric, alphanumeric, and square-cut inch size TIRES.

I've already been stranded in Costa Rica once due to an improperly-sized inner tube, so it's clear to me that even the professionals aren't entirely clear about it. You'll pardon me if I want to have a complete understanding.

Sooo... here's what seems to be true based on this thread and further web research:

Inner tube measurements are inches (even when they don't look like inches) and the two numbers are a rough range. The range should roughly correspond to the approximate average diameter of the tire... which in the case of a 140/80-R18 tire should be somewhere around 5" (140/80 has width 5.5", height 4.4").

Online charts that map metric tires to inch tires seem to be inconsistent, at least regarding the 140/80. This doesn't matter unless you want to buy inch-measured tires.

Remaining questions... what happens if you use a tube that is too big? I already know what a tube too small does, it ruptures. I've read online that using a too-wide tube is bad but it the sites offered no explanation why.

I wouldn't recommend putting a tube designed for a 90/90-R21 in a 140/80-R18 rear if only because the width is insufficient. The llantera dude was pretty sure this was the cause of my flat (just having changed due to another flat).

Jeff