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Technical Q&A with Lennard Zinn – Large molecules and short frames

The rate of lossDear Lennard,

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By Lennard Zinn

It's a quick fix, but not a long-term fill. Why?

It’s a quick fix, but not a long-term fill. Why?

Photo: QPB

The rate of loss

Dear Lennard,
Since a CO2 molecule is larger than either an oxygen or nitrogen molecule, why does it leak out of a bicycle tire faster?

Dear Glenn,
Upon receiving your question, I put CO2 in a clincher tire with a Michelin butyl inner tube (latex tubes leak air quickly, as you’re probably aware). This particular tire and tube hold air pressure faithfully for weeks on one of my road bikes without needing pumping. And sure enough, within three days after inflating with CO2 to 90psi, the pressure had dropped to 45psi.

Since then, it has not dropped significant pressure in a week. This probably explains why I’ve never noticed this phenomenon before, because, while I often use CO2 inflators, I only use them while mountain biking (I don’t have the patience to pump a fat tire on the trail with a mini pump, and I do with a road tire), and on a mountain bike, I never run pressures over 40psi.

Wanting a bit more detail than my own simple experiment, I consulted my local atmospheric chemist/bike nut Alan Hills.

Answer from Alan Hills

I’ve been told it was true by no less than (our local triathlon legend) Dave Scott. Wading through the web yields some insights on tire pressure loss from tires/tubes inflated with carbon dioxide (CO2) cartridges. Two polymers are used for bike tubes; latex rubber and butyl rubber (isobutylene rubber).

Butyl rubber dominates the market and is used for almost all tubeless tires and bike tubes as its permeability to air is incredibly low — butyl tubes have only 10 percent the leakage rates of natural latex rubber tubes.

Permeation by diffusion predicts gas leakage rates proportional to the inverse of the square root of their molecular weights. Using air as a reference the predicted leakage rates for common gases are: helium 2.7, air 1.0, nitrogen 1.02, oxygen 0.95, argon 0.85, carbon dioxide 0.81.

It turns out however that the leakage rate of CO2 is huge, and the reason is that it is actually soluble in butyl rubber and is thus not constrained to normal permeation loss, it can transfer straight through the bulk rubber resulting in severe tire pressure loss on the order of a single day. CO2 is not likely to be replaced by argon or other gases in refill cartridges, however, because CO2 is much more easily liquefied than other gases and can be contained in a moderate-pressure cartridge in a patch kit. An analogous cartridge holding N2 or argon (non-liquified gas) would be dangerous and would require a thick (and very heavy) steel-walled storage vessel. A reference dealing with CO2 transfer through latex rubber sheds light on the loss process.

Well, there you have it.

A matter of geometry

Dear Lennard,
Can you use S&S couplers to lengthen a frame? My theory is to use the couplers (which are basically lugs) to lengthen my top tube by 1.5-2.5mm and have a travel bike. I haven’t done the math, not my strong point, but I think the down would be longer I think. Any way that’s my theory. S&S said that it wouldn’t work. But I disagree. I think they or all frame builders could make good money at it as a kit or something.

Dear Elvis,
The answer from S&S is right. It won’t work. If you cut the two tubes and lengthen the top tube, the two ends of the down tube cannot line up, whether you intend to lengthen it or not as well. An entire new down tube would need to be installed at a different angle. If you lengthen one side of a triangle, the angles of the opposite side must change.

Draw the front triangle of a bike on a piece of paper, cut it in two pieces, across the down tube and top tube, and slide the two pieces apart along the axis of the top tube, simulating lengthening it. Then look at the down tube. See what I mean? You’d get a down tube with a zigzag in it.

Loss by adaptation?

Dear Lennard,
I am upgrading to a Jamis Xenith SL later this month and will be fitting it up with Dura-Ace 7900 kit. As you know the Jamis Xenith comes with a BB30 FSA crankset. I want to get the Dura-Ace 7900 crankest for superior shifting performance up front.

I know FSA makes an adapter to allow for a BB30 to take a traditional 68mm crankset such as DA7900. Will there be any performance downfall it terms of stiffness/power transfer with the DA 7900 on a BB30 adapter versus a regular 68mm road bike frame- and with this adapter I would assume I would have to use the DA7900 cups (which I want to).

Dear Shane,
It will work fine. It is a press-in threaded BB shell. So all you are adding is some weight. Stiffness should be the same as if that frame had a threaded BB into which you installed a 7900 crank.

Undoing adaptation

Dear Lennard,
I am following up on a question posted on regarding a BB30 bottom bracket. I have recently purchased a Specialized Tarmac Pro SL 2009, which I understand has an oversized or BB30 bottom bracket.

The bike is second hand with only a few miles and comes fitted with a standard SRAM Red crankset which is 172.5mm. As I currently ride with and prefer a 175mm crankset I was going to purchase a new SRAM Red crankset, which is BB30. This is where it gets confusing. I have read that in order to run a standard crankset in a Specialized frame an adaptor is required and that it cannot be converted back to a BB30 compatable crankset after.

Can you please clarify if this is the case or whether I can in fact install a SRAM Red BB30 crankset.

Dear Kim,
I did not have a quick answer for you, so I decided to go to the source.

Answer from Specialized

If this rider has our OSBB bottom bracket shell in his frame, then yes he can run any BB30 crank/ bottom bracket set.

The only question I don’t understand is if his frame has a threaded bottom bracket or our OSBB? It is easy to see the difference. If the customer sees threads in the bottom bracket shell, then his frame is not convertible to BB30.
Mark Schroeder


Dear Lennard,
I just read a tech column on where a reader asks how to keep a chain on a front chainring when using a rear derailleur. The possible solutions you list did not include our chain guide. It does take a bit of modification to make it narrower to fit a road or ‘cross setup (I’m trying to get the factory to supply shorter standoff posts) but it’s a great solution. The Alan team has been using them all season with great feedback.
Rohloff USA

Technical writer Lennard Zinn is a frame builder (, a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.