Tuesday, October 2, 2012

Heat induced experiment

With the outside temperature hovering around 100F, the second floor of our otherwise always cool townhouse uninhabitable, my bronchi congested, voice gone and an inflamed middle ear, I went to the garage to look at my bikes. The machines have been sitting idle there for the past three days, and I would take riding in 100 degree heat over being sick any time. Instead, read a few biking magazines and realized that you cannot open any of them without headlines like "the 650B revolution!" or "Death of the twenty-six inch wheels"...etc. screaming at you. I won't go into a debate on what wheel size might be best for mountain bikes, but I will share results of my primitive experiment here (it is really hot out there).

After I installed the 700x35cc slicks on my 29er, I noticed how much smaller the wheels looked. So I decided to measure the size of wheels plus tires on all our bikes. Now, I should be clear here: this is not a well controlled scientific experiment. I measured different wheels (different brands) mounted with different tires. No attempt was made to compare wheel weights or their ability to roll over terrain obstacles. My tires are at various stages of wear, although none worn to the casing, and they were inflated to various pressures.
So here are the four players:

AMC 26 tubeless/ Nobby Nic 26x2.25
Specialized 29/ Vittoria Randonneur
Hyper 700x35cc
Neuvation M28/Conti
GP4000S 700x23cc
Vuelta XRP 29 / Kenda SB8

All wheels were propped against a cardboard box in as much upright position as possible and a tape measure was used to mark the wheel diameter. I estimate the measurement accuracy to be about +/- 5mm. 
And here is the result:

Vuelta 29er wheel with a large volume Kenda Small Block Eight ..... 73cm=28.4"
Specialized 29er wheel with a commuter / touring Vittoria 35c tire.... 69cm=27.2"
American Classic 26 tubeless with Schwalbe Nobby Nic ................... 67cm=26.4"
Road rim with Continental 4000S 23c tire ...........................................66.8cm=26.3"

Conclusions: twenty nine inch wheel and mountain tire is not quite 29, presumably because the low tread profile on the SB8. Put a touring or cyclocross tire on a 29 or 700cc rim and you almost get 27.5 (650B). A big knobby tire on a 26-er rim will be more than "just 26". And a large diameter road rim with a skinny tire is pretty much a 26". Could somebody make a fat bike style tire that would fit onto a 26" rim but be a quarter inch taller? It would sure be heavier but I would not have to buy the revolutionary third wheel size and worry about fitting it into my 26" bike farme, or (horror!) buy a 650B new bike! Or how about an on-the-fly expandable tire for tuning your ride to the surface under your wheels while you ride? It is 9PM and 90F in the bedroom and I'm running a fever.

Edited Tuesday October 9, 2012: Here is a comment from a guy who knows a thing or two about bikes: John Nobile, the 2008 Great Divide Race winner and record time finisher, and an overall genius engineer:
"The added overall diameter of a tire (of a given diameter rim size) should be in direct proportion to the tire width, since the (inner) cross section is typically designed to be nearly round (although the rim width affects the roundness a bit), because the pressure tries to force perfect circularity.  Outer diameter is of course also effected by tread thickness, as you mentioned.  Car tires can get away with non-circular cross sections only because they are heavily reinforced along the circumference, usually with steel."
So I guess there is no future for bike tires that would be taller than wider, plus such a tire would probably ride like sh!t.
A table listing all possible rim / tire sizes is also here: http://www.bikecalc.com/wheel_size_math

1 comment:

  1. Back when fat bikes first started to become popular, there was a lot of forum chatter about how Surly should make 29" rims for its Pugsley. Eventually I measured my wheel and the 26" Large Marge rim with a 3.8" Endomorph tire was, surprise, almost exactly 29." I concluded that most people pretty much have no idea what they want.