Technical Q&A with Lennard Zinn: Proper pressure and big molecules

Readers wonder about pumping tires to proper pressure ... and keeping it that way.

Getting the right pressure

31DOILs3frL._SL500_AA300_Dear Lennard,
You recently published on the VeloNews site a chart that showed recommended tire pressure by tire size and rider weight. I neglected to print that and can no longer find it on the site.

Could I ask that you direct me a spot where I can find that chart?

Dear Mitch,
I believe this is the link you’re seeking.

Molecular questions

Dear Lennard,
Typically, I use a standard air pump to fill my clinchers to 100 psi in front and 110 psi in back. After about a week, the air pressure drops to approximately 70 psi. When I have a flat on a ride, I use a CO2 cartridge to fill the tube. I don’t know the psi as I don’t have carry pressure gauge, however, the tires feel harder than 110 psi. For tires that are filled with CO2, the pressure remaining after a week is about 50 PSI. In other words, the CO2 filled tires seem to lose CO2 more quickly than tires filled with air (78% N2, 21% O2, 1% Ar, and 0.039% CO2). Does CO2 escape more quickly from an inner tube than air? One would think that CO2 would escape more slowly as it is 1.5 time denser than air.
Gerald S

Dear Gerald,
I’ve covered this topic in depth before here, starting with this, which led to this, and this, and finally this. I don’t think there’s much more to say about it.

Correction: Pedro’s Syn Lube is still very much alive

Dear Lennard,
Jason from Pedro’s here. I was just browsing your column, and I noticed that a reader asked about using Pedro’s Syn Lube.

I know that you were not the one who responded (that was Bill from Mavic) but he mistakenly said that Pedro’s Syn Lube is no longer available. I believe that he meant to say that ‘Road Rage’ is too light in viscosity and is no longer available (we do suggest Go or Chainj as a replacement) but is it too much to ask for a clarification and let the readers know that we our Syn Lube is still available?
Jason Elhardt
Pedro’s Inc.

Water where it shouldn’t be

Dear Lennard,
Why does water sometimes collect in frames? More to the point, why does water collect in some frames? I have a half-dozen bikes, made by a half-dozen different manufacturers: Half collect water on wet roads, half don’t.

Conventional wisdom suggests that the water gets in through the bottom bracket, seat tube, or head tube. I’m not so sure about the bottom bracket, because (on my bikes) that whole area is self-enclosed. The seat tube seems unlikely, too, because of the tight fit between the seat post and tube (especially at the collar).

In my mind, the head tube makes the most sense. The fact that Lizard Skins makes a headset seal supports that theory, but the Lizard Skins seal doesn’t work well on frames that have internal headsets: The bend in the fork is so close to the head tube that there’s no room for the seal.

Any ideas on how to keep water out of bikes that are prone to collecting water?

Dear Ron,
It definitely comes in around the seatpost. Water that comes in at the headset cannot make its way into the bottom bracket except perhaps on carbon monocoque frames where the entire area behind the head tube is open into the down tube (and top tube) and down into the bottom bracket. Even in that case, it’s a long-shot to make it to the bottom bracket. Water can’t get into the upper head cup, and how it would get up through the lower bearing and continue going up into the head tube high enough to get into the down tube is a mystery, except perhaps when it is hanging on the wall, but then you’d only have the tiniest amount of water left that hadn’t already dropped back out of the headset by the time you hung it up.

On any metal frame or tube-to-tube carbon frame, look at the geometry of the holes in the head tube, seat tube, and bottom bracket shell that go into the frames main tubes. It will be obvious, when the frame is level as it would be when ridden, that water will not be able to move from the head tube into the bottom bracket.

The water comes in through the slot at the back of the seat tube. It gets in past the seatpost when it is thrown up at that area by the rear tire.

You really need to remove the seatpost and leave it upside down overnight to drain whenever you ride in the rain. And then spray Frame Saver down in there after it’s dry, if it’s a steel, aluminum, or magnesium frame (titanium and carbon do not rust, although aluminum inserts in the bottom bracket and at the seat collar can corrode).

A drain hole drilled in the bottom of the BB shell will protect the bottom bracket bearings, but water can still collect above the shell because the hole into the shell from the seat tube is at the high point, and water can sit down along the weld lines around the front and back of the tube. Yes, it is worth it to drill the hole in the bottom of the BB shell, as turning the bike upside down and draining the seat tube won’t get the water back out of the BB shell, so your BB will last longer with the hole there. But a drain hole at the bottom of the bottom bracket shell alone is insufficient to protect the seat tube from rusting.

I cannot stress enough the need to remove the seatpost and drain and dry the bike after getting it wet. In ‘cross season, you ought to be doing it after every washing, as well as after every race in the wet.

We see many steel frames with seat tubes rusted through at the bottom bracket shell, or, if they are hung on a wall by the front wheel, the entire back of the seat tube has become Swiss cheese—you can push it in with your finger because it is totally rusted out from the inside, even though the paint may look fine save for perhaps a telltale bubble here and there. Rust damage is not covered by warranty. And replacing a seat tube on a welded frame is no simple (or inexpensive) proposition.

Not happy with response

Dear Lennard,
I have just read Shimano’s responses to a reader question regarding 105 LH shifter reliability, and it has made me really quite angry.

The mysterious issue which the company representative singularly fails to acknowledge is that, with 2sp/3sp shifters, trying to move to the non-existent 3rd ring when using with a 2-ring setup causes the mechanism to break.

When you purchase the shifters, you get a little leaflet that carefully explains this “feature”. When you get the shifters as part of a complete bike, as I did, you do not get any such warning.

In any case, it’s clearly a cost saving compromise and these levers CAN be broken very easily. My shifter ended up in the big ring position with the chain in the little ring position and without noticing (after putting the back wheel back in following a flat), I tried to shift normally, encountered some resistance, noticed the issue, dropped down to the small ring position and turned the pedals through a couple of revolutions and went on my way. That was enough to break the shifter. Normal pressure, easy “mistake”.

I was then faced with either sending it back for inspection, to be replaced, or not, after a few weeks. Or buy another set so that my means of commuting would be up and running in a couple of days. Would I have got a warrantee replacement, or would this have been “user error?” If the latter, are these shifters really fit for purpose (a UK consumer protection law term)? Chains are known to fall off, etc, and if I can irreparably break these products with moderate force from my left forefinger, how is that at all acceptable?

Now that I know about it, I can avoid (I hope) causing this to happen again, but I can honestly say that I would be keen to try other options for my next bike as a result of being sold an item with this time bomb built into it. It is bad enough that you can’t service Shimano shifters in the first place without being sold ones with fatal flaws in them waiting to be exposed if you happen to use them in a certain way.

I am angry with Shimano because this issue is well known. Very well known. Shimano are simply insulting the intelligence of their customers by feigning ignorance. What I, and I am sure other readers, would like to see is for Shimano to comment on this design issue specifically, not pretend that we customers are imagining something.

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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.