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Technical FAQ: Cyclocross tire gluing and pressure

  • By Lennard Zinn
  • Published Nov. 24, 2009
  • Updated Nov. 23, 2010 at 12:06 PM EDT
A hand-held gauge is always more accurate than one on a pump.

A hand-held gauge is always more accurate than one on a pump.

Dear Lennard,
Your article on gluing ‘cross tires was well written, per usual. Your “lift my feet up off the ground” tip has been useful to me going back almost 20 years (and that came after ten years or so of occasionally blistered thumbs).

I just note, though, that the tire pressures you mention in the article relate to the road bike context, not ‘cross. Some (young?) readers may be confused.
-David

Dear David,
Thanks for catching that (and thanks to the other readers who also pointed this out). When I did this column late last Monday night (it’s late on a Monday night once again – when will I ever actually get these done in advance?), I cut and pasted the tubular gluing section from “Zinn and the Art of Road Bike Maintenance” into it. Then I edited it to apply specifically to cyclocross from there, but I did not edit the inflation pressure from my recommendations for road racing tubulars.

So here’s the bottom line: it’s not a great idea to inflate cyclocross tubulars over 75 psi or so, and while doing so may not actually damage the casing of premium tires, it will round the tire out more, thus further decreasing the contact with the bottom of the rim bed, which generally has a smaller radius of curvature.
-Lennard

Dear Lennard,
With the low tire pressures typically used in ‘cross applications, it seems as though a few psi can make a significant difference in traction and performance. Consequently, I’m wondering just how useful the gauges are on my (and most?) pumps. Do you have a particularly accurate, stand-alone tire gauge that you would recommend to check pressures?
-John

Dear John,
You’ve hit on one of my pet peeves that I shake my head at – people using the gauge on their pump to determine tire pressure in a cyclocross tire, particularly for low pressure in a tubular. You need to use a separate gauge if you want to measure even close to accurately.

When you pump a tire, the gauge on the pump is at the base end of the hose, and it is measuring the pressure in the hose. With each pump, the needle swings up as the piston creates higher pressure than in the tire to overcome the seal on the tire valve and create a pressure gradient to force more air into the tire. Then the needle drops after the piston has reached the bottom of the pump barrel. So what do you read? Clearly not the spike in pressure as it blows the valve open. And if you read the pressure after the piston has completed its plunge, you are reading the pressure in the hose, which now has nothing to do with what is in the tire, since the tire’s valve has sealed off once air stopped flowing in. If you just leave the pump to sit, that pressure continues to drop as the air bleeds out of the hose, but the tire’s pressure has not changed.

Get a separate tire gauge. German digital ones, like from SKF and Schwalbe, are nice and give you a clear, accurate reading until you get them wet and fog the little viewing window. They are slower to use than one with a needle, due to turn on/off and resetting. But they seal better than many of those with a needle. I have one with a needle that I love that has a long, brass nipple that fits great on the shafts of valve extenders that you screw the valve core back into. I have other needle types that have a smaller seal trying to just seal on the valve’s top threads, but the flats on removable valve cores on tubulars render these ineffective. I have as many that I hate as that I love, which is a good reason not to order them, as I did. You should try the thing out in the shop, on your tire, then buy it.
-Lennard

FILED UNDER: Bikes and Tech / Cyclocross / Technical FAQ TAGS: / / / / /

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance 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. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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