RoadBikeReview.com's Forum Archives - Components


Archive Home >> Components(1 2 3 4 5 6 7 8 9 10 )


700x20 or 700x23?(11 posts)

700x20 or 700x23?nakoosa
Sep 2, 2003 11:22 AM
just wondering what the benefits/drawbacks of either tire size (700x20,23) are. I usually ride 20-30 miles, fast with group or solo training; only race is Mt. Diablo Challenge, once a year.
I'm considering either the Vittoria techno pro, or Vredstein Ricorso.
Thanks for any feedback.
Chris <><<
Good question but here's even another takelotterypick
Sep 2, 2003 11:51 AM
Is there any value in going 20, 21 or 22 in the front and like 23 in the back.

Continental is marketing the Attack Force combo. I'm jsut wondering if it has any measureable, meaningful merit (MMM's ha ha ha)

I'm riding 23's and it's much more comfortable than 20's and I haven't had any flats (statement of death). So if you had to make a single choice I'd say 23's.
re: 700x20 or 700x23?PaulCL
Sep 2, 2003 12:09 PM
Let's see...
23's are more comfortable than 20's
20's will get more pinch flats than 23's
20's have less rolling resistance than 23's, but not much

My conclusion: Use the 20's for races the 23's for everything else. I used to use 20's all the time and got pinch flats all the time. I used them becuase my then LBS sold them to me...I guess he wanted to get rid of his 20's so he sold them to newbie 180lb guy. Jerk. I flatted all the time. Paul

PS. I'm not familiar with the tires you mentioned. I use Vittoria Corsa CX TT clinchers and I love them. I buy them overseas or on sale (currently $33 at Excell). They last about 1500-2000 miles on the rear, more on the front.
re: 700x20 or 700x23?filtersweep
Sep 2, 2003 1:15 PM
I don't know that I'd want to use 20s "for races" if I wasn't accustomed to whatever unique handling issue they may have. I don't know if 20s delve into this realm, but 18s can exhibit "chatter" (finally learned the word for it) where they are at such high pressure that the don't conform to surface irregularities, so they momentarily hop over them. Personally, I don't like the feel of the rear wheel going airborn while cornering.
Wrong on the rolling resistance...tube_ee
Sep 3, 2003 11:17 PM
PaulCL said:

Let's see...
   23's are more comfortable than 20's
   20's will get more pinch flats than 23's
   20's have less rolling resistance than 23's, but not much

I say:

All good up until the last one. Assuming equal pressure and identical construction, the wider tire will have LESS rolling resistance. This was shown empirically by Jobst Brandt in the early 80s, when he was consulting for Avocet. It can also be shown theoretically by thinking about the ways in which rolling resistance is created.

It is counter-intuitive, but it's true. I agree that the differences are small. So the correct answer to the poster's question is probably 25s, or even 28s, if they'll fit in the frame.

--Shannon "trying to be an engineer" Menkveld
...at the plate and batting .667....PaulCLPaulCL
Sep 4, 2003 7:22 AM
It is counter-intuitive, but if Josh Brandt says its true then it must be. But don't 20's have higher pressure ratings??? Is R-R less if your 20's are at 130psi and the 23's at 100psi??? Just throwing more questions out to muddy the water......heehee....Paul
More math...tube_ee
Sep 4, 2003 9:54 AM
Tire pressure ratings are determined by mounting the tire in a specific rim, and adding pressure until it blows off the rim or explodes. The failure pressure is divided by a government-mandated constant, and the max safe pressure goes on the label. It depends more on tire construction than width, and is useless as far as determining the best pressure for a given rider on a given tire.

Rolling resistance decreases with additional tire pressure. So does contact patch area. Lets take a look at the math. Assuming a 180 pound rider on a 20 lb bike with 60/40 weight distribution.

For the 700 x 20 at 130 psi:
Contact Patch = load / pressure
Front: (200 lb * 0.4)/130 lb/in^2 = .615 in^2
Rear: (200 lb * 0.6)/130 lb / in^2 = .923 in^2

Casing deformation is proportional to the length of the contact patch,
so for the 20 mm tire, we get

Length = Area / width
Front: .615 in^2 / .787 in = .781 in
Rear: .923 in^2 / .787 in = 1.17 in

For the 700 x 23 at 100 psi:
Front: (200 lb * 0.4)/100 lb/in^2 = .8 in^2
Rear: (200 lb * 0.6)/100 lb / in^2 = 1.2 in^2

Length = Area / width
Front: .8 in^2 / .905 in = .883 in
Rear: 1.2 in^2 / .905 in = 1.33 in

So in the example you gave, the 20 at 130 psi wins over the 23 at 100. Of course, most people run their 23s at 120 - 130. Whether the difference in comfort and rider fatigue due to lower pressure is more or less significant than the difference in rolling resistance is something I don't know. I suspect that rider issues would tend to dominate equipment issues, but that is nearly always true anyway.

So what have we learned?

First, pressure differences dominate in calculations of rolling resistance. Pressure is not any more significant mathematically, it's just that the available range of pressures is much wider than the available range of widths.

Second, that all things being equal, to minimize rolling resistance, increase both width and pressure.

Third, that all things are never equal.

Peace and Grease,
--Shannon "Math is fun" Menkveld
re: 700x20 or 700x23?deHonc
Sep 2, 2003 3:51 PM
Most experienced bike racers will use a 20 (front and back) for time trials or point to point races - they will have some benefit in friction and wind resistance. Use a 23 in crits where you are cornering much more - the side wall on a 23 is stronger and better designed to handle this type of situation.

Dan
re: all replies to my 700x20 or 700x23 ?nakoosa
Sep 2, 2003 7:12 PM
thanks so much for all the replies. good info.
i'm actually considering doint the 23 rear, 20 front (i actually had a guy at a lbs suggest the opposite -- huh?).
I'm only 5'3", 120 lbs, so I don't even pinch flat on my mtb., in Tahoe boulders, @ 20 psi. However, I'm considering doing the 23 route for comfort and cornering stability over slightly faster and quicker. I've heard the weight and drag difference is pretty negligeable.
thank guys!
Yes, just go all 23s (or 22s in Veloflex Paves). nmSpunout
Sep 3, 2003 3:35 AM
Why you might want to go bigger in front...peter1
Sep 5, 2003 10:22 PM
...think about what you use on your mtb. I have always gone wider in the front, the feeling being that a larger front tire will prevent washout. i.e., the rear wants to follow the front, so if you can get through a corner with the front tire, you're home free with the rear, hence going larger in front and running a smaller rear to minimize weight and traction (more weight on a narrower tire to cut through slop)

The same principles might apply on the road. Having said all that, I use 23s front and back, it being the most commonly available size. And if you don't mix the sizes you can rotate front and back...