What is the difference between those sizes? Is the possession of the 700X20 is higher less than the 700X23 (on dry roads). While climbing is the 700X20 is faster? Which size is recommended?
700X23 Vs. 700X20 Tires
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20mm tires require higher pressures to avoid pinch flats. High pressures will give you a harsher ride. On very smooth roads, 20mm tires might be a little faster than 23mm. 23mm is a better all around choice, unless you are very heavy, then you should use 25mm or more.
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Alot of reviews claim that the 23s are faster than the 20s...I had a pair of EVO CX 23s and went back to the 20s... I've just always had 20s myself and got accustomed to the way they feel and such... If you look at Vittoria tubulars, I believe the Corsa CX is actually a 21.5... right in the middle of the two...
Personal opinion..don't always rely on reviews and what other people say though!
Personal opinion..don't always rely on reviews and what other people say though!
johnny99 said:
That's actually not true. I will search for the research article that shows it, but someone else may beat me to finding it. A 23 is faster than a 20 and it has to do with the sidewall casing and the flexibility of the sidewall. Rolling resistance of a bicycle tire is not determined by the size of the contact patch, but rather by the deformation of the tread and casing. When a round tire contacts a flat road, the tire deforms causing the resistance. This can be helped with higher pressures, as you noted, but that is a totally independent factor from tire width. Between a 20mm tire at 120psi and a 23mm tire also at 120psi, the 23mm tire will deform less and is therefore faster. (You can search for Jobst Brandt's research from IRC tires to verify) but if you have a 23mm tire at 100psi and a 20mm tire at 120psi, it is no longer an equal comparison. Also, it is important to note that with increased pressure, traction is decreased. The thing about bicycle tires is that the rolling resistance is so low that the decrease provided by overinflating the tires is so infinitesimally small that they are quickly outweighed by the trade-off loss of traction (See Sheldon Brown's website for that.)
So all things otherwise equal, say two identical Michelin Pro Race Tires, at 120psi, one 20mm wide, the other 23mm wide, the 23 is actually the faster tire. Now the weirdness happens above 23 mm. Once you get into 25 and 28 mm tires, you now have the tire being substantially wider than the rim which increases the tire sidewall deformation and therefore increases the rolling resistance.
Look at professional tubular tires... there is a very good reason why 99% of them are between 22 and 23 mm wide... least deformation making the lowest rolling resistance = fastest tires.
The real world reality of all this is that you really shouldn't worry about rolling resistance of your tires as the values between like tires is so small as to only make a difference over several thousand miles... you really should pick a tire based on the comfort level of that particular tire as that will make a bigger difference. If you feel more comfortable on a 25mm tire, you may be slightly more efficient on it and that will more than offset the increase in rolling resistance over time.
Maybe you should reread the post... the reason 32 mm tires are not faster is covered.johnny99 said:
My point was that you took one factor out to add another as a reason for your answer to why you thing 20mm tires are faster. A 20mm tire with the same casing and tread at the same psi as a 23 mm tire of the same casing and tread will be slower. Not faster as you stated. But pumping up the 20mm tire to a higher pressure MAY make it faster (which it won't always, that would be dependent on the amount extra you pump it up) and will result in a loss of traction over the same tire at a lower pressure.
It's not interesting to try to answer someone's question by throwing out some factors and adding others in selectively to answer the question. It's much more interesting to answer both those questions instead, which if you read my post, I did. I covered both identical tires at different pressures, otherwise identical tires at the same pressure, but one being a 20mm tire, the other a 23mm tire, and why once you get above 25mm why that no longer holds true. I didn't correct your statement to try to show you up or make you look bad, I corrected your statement to give anyone reading this thread a more complete answer than you gave them. I was not trying to offend you, but your answer was incomplete making it less accurate. By the way, in case it wasn't clear in my other post... all credit to the research quoted in my post, and the reasons stated in my post about rolling resistance goes to Sheldon Brown and Jobst Brandt. I just read what they wrote on the subject and posted a condensed version.
Maybe you should read again what they actually wrote instead of just drop names. Sheldon Brown specifically says that your "equal tire pressure" argument is bogus. See: http://www.sheldonbrown.com/tires.html#widthrussw19 said:
weiwentg said:
Not always true. Best aerodynamics is acheived when the tire and rim cross-sections match exactly.
For general, all-around riding, run 25ish tires. Rolling resistance differences are so small as to be "in the noise" for anything in which hundredths of a second don't matter. Cornering grip and traction are also improved with wider tires.
--Shannon
I may be dense, but this seems counterintuitive to me...I would think that since the pressure is distributed over a larger area on the 23mm tire and there is more material to flex, the 23 would experience more deformation.russw19 said:
Riding an old aluminum bike on country roads, 20mms beat me up pretty bad
...but they sure look cool!
Does the difference in total wheel diameter/circumference between 20 and 23 equate to a tooth of gearing, or is this negligible?
It is counterintuitive, but it's true. Here's why:
Tire rolling resistance is primarily a function of casing deformation.
A tire's contact patch area is solely a function of pressure and load. Load (pounds) divided by pressure (pounds / sq. in.) = lbs / (lbs/sq. in.) = lbs * (sq. in. / lbs). Pounds cancel out, leaving a contact patch area in square inches. Notice that tire dimensions do not affect the size of the contact patch.
Tires are round, but the contact patch is a rectangle. The only way for this to work is that the tire deforms in the contact zone, becoming no longer round. Area = length * width. Since the width of the tire is basically fixed, the wider tire must deform over less of its length to equal the same area. Thus, less casing gets flexed, and less energy is lost to heat. Lower rolling resistance.
All of this assumes tires differing only in width, and run at the same pressure. The practical value of all of this is that you can run a wider tire at a lower pressure, and get equal rolling resistance to a narrower tire at a higher pressure. If you play with the numbers, you can figure out where the "break-even" point is between two similar tires. If we assume that rolling resistance is proportional to "length of casing deformed", which is not a bad assumption here, here's how you do it:
Contact patch area = load / pressure. Use 60% rear / 40% front weight distribution to determine load on the tire.
Length of casing deformed = contact patch area / tire width
When the two tires are deformed over the same length, it's safe to assume that rolling resistance is equal.
--Shannon
Tire rolling resistance is primarily a function of casing deformation.
A tire's contact patch area is solely a function of pressure and load. Load (pounds) divided by pressure (pounds / sq. in.) = lbs / (lbs/sq. in.) = lbs * (sq. in. / lbs). Pounds cancel out, leaving a contact patch area in square inches. Notice that tire dimensions do not affect the size of the contact patch.
Tires are round, but the contact patch is a rectangle. The only way for this to work is that the tire deforms in the contact zone, becoming no longer round. Area = length * width. Since the width of the tire is basically fixed, the wider tire must deform over less of its length to equal the same area. Thus, less casing gets flexed, and less energy is lost to heat. Lower rolling resistance.
All of this assumes tires differing only in width, and run at the same pressure. The practical value of all of this is that you can run a wider tire at a lower pressure, and get equal rolling resistance to a narrower tire at a higher pressure. If you play with the numbers, you can figure out where the "break-even" point is between two similar tires. If we assume that rolling resistance is proportional to "length of casing deformed", which is not a bad assumption here, here's how you do it:
Contact patch area = load / pressure. Use 60% rear / 40% front weight distribution to determine load on the tire.
Length of casing deformed = contact patch area / tire width
When the two tires are deformed over the same length, it's safe to assume that rolling resistance is equal.
--Shannon
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205 Posts
And where did Sheldon Brown get his Physics/Engineering Degree?
I'm sure this has already been quantitatively covered somewhere on the net. Just too lazy to look it up.
I'm sure this has already been quantitatively covered somewhere on the net. Just too lazy to look it up.
The rolling resistance of tires differ considerably and shouldn't be discounted. For instance Veloflex Carbon 34.05 watt @ 123.28 psi (8.5bar) versus Vittoria Corsa Evo KS 39.61 watt versus Tufo Hi-Composite Carbon 53.51 watt.
I'm sure someone here can do the math for how much faster you'll go with a given output of 300 watt between the tire with the least and most resistance. It will be noticeable for sure.
(figures relate to 18.75mph and about 176lbs system weight)
I'm sure someone here can do the math for how much faster you'll go with a given output of 300 watt between the tire with the least and most resistance. It will be noticeable for sure.
(figures relate to 18.75mph and about 176lbs system weight)
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