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Question about tire size(5 posts)

Question about tire sizeMr Nick
Aug 24, 2003 5:01 PM
I just posted about my first group ride. All of the people in the group ran size 25 tires instead of 23 and only put in about 100psi to increase comfort. I thought it sounded like a pretty good idea because I am always looking for ways to make the ride my supple. Is this a good idea? If so what would be a good brand.
Question about youKerry Irons
Aug 24, 2003 5:39 PM
If you're getting pinch flats at your current pressure and pumping harder makes for an uncomfortable ride or poor traction, then go to the next larger tire size. You don't state your weight, but at 180 lbs./82kg and 105 psi/7 bar in a 23mm, I have never pinch flatted, so I see no need to move to a 25. It must be an unusual group where everyone runs 25s - did you actually see their tire sizes. Did this come up as a topic of conversation or was someone claiming that "everyone" runs 25s. The short answer is that if you can ride around 100 psi w/o pinch flats, then you're on the right size. YMMV.
Did talk about and wasn't about pinch flatsMr Nick
Aug 24, 2003 6:36 PM
We did start talking about it because people were checking out my fairly new bike and it just came up. No one ran the tires because of pinch flats, but because of comfort. Just wanted to know if there was anything bad about the tires or if running 100psi is not a good idea since most tires are rated at 120psi?. I am only 165lbs. by the way.
re: Question about tire sizetube_ee
Aug 24, 2003 7:58 PM
There are some very good reasons for running wider tires. First is comfort. A larger tire holds more air at a given pressure. Since air is your suspesion on a rigid-framed bike, more is better. The wider tire also allows you to run lower pressures without risking snakebites.

Second, rolling resistance. Given two tires differeiing only in width, at equal pressure, the wider tire will have LESS rolling resistance. Not only has this been proven several times in instrumented testing, it also makes physical sense.

Area of tire contact patch = load (lbs) / tire pressure (lbs / in^2). punds cancel, leaving square inches, Note that there are no dimensional terms in this equation. In other words, the tire's dimensions have no effect on the area of tire in contact with the road.

The primary contributor to rolling resistance is casing deformation. Tires are round, but the contact patch is flat. So, the tire casing must deform in order to create the contact area. This deformation results in energy loss, mostly as heat.

Since area = length * width, and width is relatively fixed, the narrower tire's contact patch must be longer to achieve the required area. this results in a longer section of the tire's circumference being deformed, and thus in higher polling resistance.

Here endeth the lesson. :-)

Most cyclists do not know this, and so buy tires that are too narrow. If riders in your club are really riding mostly 25's, they are an enlightened bunch, indeed.

Peace and Grease,
Agree--liked 28s so much I'm using 32s nowretro
Aug 25, 2003 7:55 AM
For general riding, I can't think of a reason to use 23s--I haven't bought anything smaller than 28s, and usually 32s, in about 10 years. Granted I'm a Clydesdale (225 lbs.), but I go just as fast and enjoy the ride a lot more with some cushioning. Now I'm looking at 35mm Paselas....