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Technical FAQ: Tire widths, pressures, and more

  • By Lennard Zinn
  • Published Jul. 29, 2014
  • Updated Oct. 31, 2014 at 6:12 PM EST
Wider tires, like the 27mm FMB Paris-Roubaix, can be used at lower pressures to increase ride comfort. Photo: Caley Fretz | VeloNews.com

23mm vs. 25mm tires

Dear Lennard,
I ride with a group of recreational road riders who do, on average, 100 kilometers per week. Recently, most of the riders have switched from 23mm to 25mm tires and like the ride better (whatever that means). As a theoretical question, if you had one 23mm and one 25mm, which would you put on the front and why?
— Doug

Dear Doug,
Well, I would put the bigger tire on the rear. It gives you more cushioning, and that’s where you need it more. The bigger tire size also reduces rolling resistance, and that will make more difference on the tire that is supporting more weight — the rear tire.

The 25mm tire also gives you more traction, assuming you’re running lower pressure in it than in the 23mm one. I suppose arguments about traction could be made for either the front or rear wheel.
― Lennard

Mixing 23mm, 28mm tires

Dear Lennard,
I’m running road tubeless on a pair of Dura-Ace C24 rims. Due to limited Hutchinson availability at my favorite shop (and my own impatience to ride TODAY), I find myself running a 23mm Fusion up front and a Sector 28 in the rear. The rear fusion 23 was beyond repair and I was, and still am, too cheap to pop for two Sector 28s.

I’m aware of the multiple aesthetic violations here and I feel certain that Boulder has municipal codes that bar such a combination. However, I was wondering if you think the handling will suffer. After one ride, the rear end feels completely different — very loose.
— Carson

Dear Carson,
A bigger tire, especially when run at lower pressure (which it ought to be), will have more give to it. This can certainly lead to a loose feeling in the rear. When I was first experimenting years ago with running pressures of around 30psi in cyclocross races on tubulars (32-34mm section), it was such a loose feeling that I couldn’t imagine how Sven Nys and the other top cyclocrossers could do it. But once I became used to riding on squirmy tires, I actually felt MORE secure with pressures as low as 28psi than up around 40psi; I had better grip and I didn’t get bounced around as much. Tire pressure for cyclocross is tire-, course-, and condition-specific as well as dependent on rider weight (I weigh 170 pounds). And if the conditions called for really low pressure, I certainly found I preferred it.

So I think your combination may actually be a good one to ease you into using bigger tires, because I think the loose feeling would be exaggerated were you to have 28mm tires front and rear.
― Lennard

Concerns about wet tires and rims

Dear Lennard,
I got some water in my basement this week. Sadly I was sloppy, and had left my ’cross tubulars — Shimano WH 7850s — sitting on the floor (trying to take advantage of the humidity down there to lengthen glue life). It appears that water seeped into the front wheel. I can force water out through the stem and the weep holes in the rim, but there is still some sloshing around inside. What do you think has been compromised:

1. Is the tire shot, since I have water inside (Tufo Primus 32s).

2. Would you trust the glue job or am I safer just to peel off and re-glue?

3. Any concerns about the rim itself? I was out of town when this happened but my sister pulled them out after about an hour of being wet.
— Rich

Dear Rich,
That is one of the best tubulars you could get water in because it doesn’t have an inner tube. You can’t use a shop vacuum with a thin tip on the hose to suck water out of a tubular with an inner tube in it because it will just suck the tube against the base of the valve stem and prevent anything further from coming out. But you can suck out a Tufo tubular that way. Here’s an article I did on that. (Regarding that link, note that I no longer rinse sealant out of tubulars, as I think getting the water back out is harder on the latex inner tube than the sealant is. I just squeeze out as much sealant as I can, and then I leave the valve closed so that air won’t get in and dry the sealant.)

It’s always a good idea to check a glue job before relying on it. On the other hand, a good glue job should be able to take an hour of being submerged in water without becoming compromised.

I think there is no cause for concern about the rim. Ride it some to let water be thrown out of the weep hole and allow it to dry. But compared to riding for hours in the rain, what happened to your wheel and glue job is pretty minor.
― Lennard

Referencing tire widths

Dear Lennard,
I have seen, on your illustrious site as well as many others, references to tire width where well-meaning people for some reason put the letter ‘c’ in place of the letters ‘mm’ to denote size. One can read at length about the merits of 25c tires and 28c tires but as a person who prefers precision, this makes me want to rip my hair out. I suspect the same may be true for you. I for one am glad I don’t have to explain to customers about which letter denotes which width, as used to be the case. We’ve basically gotten past all that thanks to good ol’ ERTRO, but old (misunderstood) habits die hard I guess. Can you please send out an APB that tire width is measured in millimeters, not in letters, and make sure that at least I don’t see it on VN?
— Carl

Dear Carl,
There’s your APB, in your own words. I think I generally refer to tire width in millimeters for road tires and in inches for MTB tires, but since the “c” doesn’t “make me want to rip my hair out,” I can’t guarantee that I have not used it sometimes. I have managed to live with the letters on the rim size, which have no meaning in millimeters or inches or any other measure (please tell me how to make sense of 700C or 700A or 700D, or 650A, B, or C), so using a “c” in place of “mm” for tire size doesn’t bother me.

I do agree that it’s more useful to give people a meaningful tire-width measurement, but, as you know, many manufacturers use the “c” nomenclature, and in product reviews, we tech writers have a tendency to simply pass on the manufacturers’ specs. I passed your note on to the Velo editorial team, and perhaps as a result you won’t see it again on VeloNews.com or in Velo but alas, I wouldn’t bet on it.
― Lennard

Air quality and latex tubes

Dear Lennard,
I wanted to add a contribution to your recent (and informative) column on latex tubes.

Ambient air quality is another factor that can adversely impact the life of latex tubes (and other bicycle parts made of natural rubber). As a rider/occasional racer in Southern California for nearly 25 years, I have long noticed accelerated cracking/crumbling of rubber goods, including latex tubes. It appears that chronic, high levels of air pollution, especially ozone, can play a significant role in the accelerated breakdown process. In Southern California, these conditions are most acute in the San Gabriel Valley and inland areas to the east. I’ve heard of cyclists storing expensive sew-ups in impermeable bags filled with nitrogen, but can’t personally attest to the effectiveness.
— Stuart

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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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