Commuting - High test tires with a heavy load?
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05-27-09, 04:44 PM
Would switching to higher pressure tires (110 vs. my current 75 PSI) be a bad plan for me? I ride 17 1/2 miles each way, mostly along a paved rail/trail conversion with lots of bumps. I'm a big guy (over 250 lbs) and I have panniers for the 10-20 lbs of additional stuff i'm generally transporting to and from work.
I'd like to cut my rolling resistance by switching to higher pressure tires, but I'm wondering if it's going to result in more flats and/or more broken spokes. Any advice?
Also, how much difference will switching from 75 PSI to 110 PSI make for my ride? is it noticeable?
Higher pressures do not necessarily correlate to lower rolling resistance.
higher pressures do not necessarily correlate to lower rolling resistance.
05-27-09, 05:27 PM
Something Like a 700c x 25-28 sounds like a good compromise.
I'll post it again.....
Why do wide tires roll better than narrow ones?
The answer to this question lies in tire deflection. Each tire is flattened a little under load. This creates a flat contact area. At the same tire pressure, a wide and a narrow tire have the same contact area. A wide tire is flattened over its width whereas a narrow tire has a slimmer but longer contact area. The flattened area can be considered as a counterweight to tire rotation. Because of the longer flattened area of the narrow tire, the wheel loses more of its “roundness” and produces more deformation during rotation. However, in the wide tire, the radial length of the flattened area is shorter, making the tire “rounder” and so it rolls better. http://www.schwalbetires.com/images/e_img_1164_3.gif
Why do Pros ride narrow tires if wide tires roll better?
Wide tires only roll better at the same inflation pressure, but narrow tires can be inflated to higher pressures than wide tires. However, they then obviously give a less comfortable ride. In addition to this, narrow tires have an advantage over wide ones at higher speeds, as they provide less air resistance (known more commonly as wind resistance) . Above all, a bicycle with narrow tires is much easier to accelerate because the rotating mass of the wheels is lower and the bicycle is much more agile. At constant speeds of around 20 km/h, the ride is better with wider tires. In practice, the energy saving is even greater than in theory as the elasticity of the tires absorbs road shocks, which would otherwise be transferred to the rider and so saves energy.
I wanted to paste in the full page, but these two paragraphs boil it down pretty well. If you want to read the whole page: enjoy.
A larger diameter tire will in most cases have a higher weight load limit.
I find the ride quite comfortable on a steel bike with 700x23 tires (Continental Ultra Gatorskins at present, in my case) inflated to 110 psi. I'm 260 lbs. and usually roll with 15-25 lbs. in my panniers. I have 32-spoke wheels (several sets - Ultegra hubs with WTB rims, Ultegra hubs with Mavic rims, Campy Chorus hubs with Mavic rims), and as long as my spoke tension starts out at the high end of the range I experience very few spoke failures. If my wheel starts out undertensioned (and, for the way I like my wheels to be tensioned, most do), I'll have fatigue failures of spokes eventually. If everything's in good order, I have very few flats, don't break spokes, and I fly with my 23s.
Why do wide tires roll better than narrow ones?
A rather simplistic discussion with a rather limited and unsurprising conclusion - an underinflated narrow tire will not roll as well as a properly inflated wide one.
Missing from the discussion is the impact of the tire carcass on tire deformation, which is certainly nontrivial. For a given tire construction, a narrower tire will have a smaller contact patch because of the structural support lent by the narrower carcass. Likewise, at the higher pressure at which narrow tires are properly run, the contact patch will be significantly smaller. When you take into account factors such as asperity interlocking (the microscopic adhesion of projections on the tire into pores in the road, and vice versa) and the more limited deformation of a narrower tire run at its proper pressure, the conclusions are not as clear cut.
Does a narrower tire ride more roughly than a wider one? For the same bike, and for the same tire construction, it's difficult to argue otherwise. However, if you throw into the mix the variety of frame materials, and particularly if you include carbon fiber with its ability to be tuned, though varying the layup schedules, for the desired vertical compliance without sacrificing lateral rigidity, and again, the conclusions are not as clear. My steel Jamis, for instance, rides better on 23s than my aluminum Cannondale does on 38s. Different bikes, different tire constructions, different pressures - it's a nighmare of experiment design, but good anecdotal evidence that things are not always as they seem.
05-28-09, 07:51 AM
I am enjoying an improved ride with the switch I made but there were more factors that changed than just pressure. My ride is better, meaning it feels a little faster and precise, but did not become harsh.
I went from cheap 27" gumwall tires. They were fine but their center line was so imprecise it was interesting to watch as I rode it wandered ever so slightly. I cranked those tires up to 90 psi and thought I had it good.
I went to 27" Specialized Armadillos and they are cranked up 120 psi. They are slicks and so, no center line. They feel much more precise and even at 120 psi are quite comfortable.
The gumwalls were like $10. online or up to $17 in a shop. The Armadillos were $34. at one shop and $38. at another. I currently have 2 sets for 2 bikes. I highly recommend them. But my "time" has not changed. It is noticeably easier rolling and easier work, but over 13 miles makes no difference in my time. Well, maybe 5 minutes but I'm not sure for the 5 minute better time these days. Could be me, could be the tires, but I highly doubt I can get under the 45 minute mark.
I weigh 213-218 depending on the day ...
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