What about time trials then? I think some riders do use 650c, a size closer to 26". And time trials really are all about speed rather than tactics with the long ride days.

I'll guess the reason 700c wheels are used is a combination of frame tradition and tyre availability, and the fact that bigger wheels handle bumps a little better. There might also be a rotating weight/momentum argument in there as well for good measure.

From personal experience I've found that my bike with 700c wheels always feels faster than any of my bikes with 26" wheels -but I know it's far from scientific as usually my 26" wheels are heavier, attached to a heavier frame (e.g. tourer or mtb) with a more upright position, have different gearing and slightly wider tyres at a lower pressure. So I'm not convinced they really are faster (or slower) if all things were equal except for the wheel size.

And what is the scientific basis for this opinion?

Scenario: I'm on my cyclocross bike with 700x28c slick tires. The bike weighs 20lbs with all the doodads I put on it. I weigh 180lbs. My friend is on his GT mountain bike which weighs 26 lbs and is equipped with 26x1'' (25c) slicks. He weighs 215lbs. When starting to roll down hill at the same speed I continually accelerate ahead of him although neither of us is pedaling and there's no aerodynamic advantage. Our tires are inflated to within 5 psi of each other. He has a 41 lb advantage. WHAT IS GOING ON?


As for tire choices, the popular and good touring tires are available in both 26'' and 700c sizes.

Are you talking about a 29" mtn bike frame?

In the grand scheme of things whatever performance differences there are between 26" and 700c wheels are small enough to be of no consequence to a touring rider. Consider that tri bikes for many years ran 650c wheels sacrificing a small amount of rolling resistance in favor of improved aerodynamics. Also consider that recumbents, the absolute fastest way to travel on two wheels, use all sorts of different wheel sizes. Even on the same bike! A Moulton folder with wheels smaller than 26" was used to set an indoor HPV record.

Equivalent 26" wheels are marginally stronger lighter and more aerodynamic than 700Cs even though they have slightly more rolling resistance. They'll be a little less comfortable, but that can be compensated with slightly wider tires or lower pressure which may slow them back down a bit. As a practical matter, it's a wash.

So why should you choose one or the other?

1) Fit. A smaller rider should use smaller wheels because the frame geometry has to be compromised to fit 700C wheels in a smaller frame.
2) Tire availability. If you're doing extended touring in the third world you'll have an easier time finding spare tires in 26" size.

Most of the rest of the brouhaha on this issue is pointless. You're straining at gnats.

Get a set of 26" wheels and then Conti SportContact 1.6" tyres. Fast, cushy, virtually puncture-proof slicks. And they're pretty cheap too!

I use them on my singlespeed commuter. I've taken it on club rides with the fast group and easily kept up with them.

Well, that is not scientific either, though I don't think you meant it to be. The difference in speed is caused by many factors. You and your bike weighing less is actually to your advantage. Because of the smaller amount of mass, you have less inertia. This means it is easier to change, in this case it would be your speed. The smaller wheel will also have less rotational inertia, allowing it to start faster, but there are other factors, like the rolling resistance of the tires. Since the tires are not the same, then the rolling resistance is not the same. Even tires that look the same can have different rolling resistance. The friction of the hubs also needs to be overcome, your bike might be tuned better.

700c wheels have a greater rotational inertia than 26" wheels. 26" wheels are easier to get up to speed whereas 700c wheels want to stay at that speed better.

This is not true. If a 700c tire and a 26" tire of the same width and equal pressure will have the same contact patch. The contact patch is determined by the air pressure and the total weight. When the contact patch x air pressure = weight, the tire stops deforming.

Wide tires have less rolling resistance than thinner tires at the same pressure. Though the contact patch will be equal, the thinner tire has a longer patch. This means more sidewall length needs to deform, this causes a greater rolling resistance. Though wider tires at the same pressure have less rolling resistance, most wide tires are of smaller air pressures, which means that a longer patch is needed to equal air pressure and total weight. This causes greater sidewall deformation causing greater rolling resistance. The sidewall strength is also a factor in rolling resistance. If a tire's sidewall is made to easily deform, there is less rolling resistance.

Rolling resistance is not a function of wheel size, it is a function of tire width, air pressure and sidewall strength.

I think of it as rigidity/suppleness rather than strength. If it bends easily, it will have less rolling resistance. Maybe that means it's less strong too but I'm not sure of that.

I just want everyone to know I'm adding this topic to my Contentious Threads Pantheon Hall of Fame. Whenever I want to wind someone up, and after being inspired by this and previous threads, I shall post the following with random selections given:

About my dream touring bike setup.......

It would be [insert steel/alu/carbon fibre/wet noodles] as it's without a doubt the best frame material. The wheelset size would be [insert 700c/26"] as they are the fastest. For shifting, I would use [insert Shimano STI/Campy Ergo] shifters as [insert friction/bar end/Rapidfire/GripShift or all of the preceding options] shifters are just terrible for touring. For brakes I would use are [insert disc/centre pull/cantilever/v-brake/75lb cinder block tied to a piece of string], preferably anodized [insert blue/yellow/purple] to match my favourite colour.

The saddle would be a Brooks [insert B17/B17N/B66/B67/B52s] broken in by using [insert motor oil/being ridden 5,000 miles/a mallet/repeated floggings by a wet rag on Sunday morning in a leap year -though it should be noted I've only included a very small fraction of possible answers here].

I know there are alternatives, but you cannot tour on [insert a recumbent/an alu frame/two sticks tied together by dental floss] as the ride is too [insert harsh/funny looking] and you'd need custom [insert racks/wheels/dentures]. Lastly, I would always use [insert my noggin/a helmet] which should be made [insert compulsory by law/as full of holes as much as possible for ventilation].

Yes, that is what I meant. I just didn't write it properly. Thank you.

I really want to let this go, but I have to point just a few things, and then I'm done... really! :D

--Lighter bikes change direction more easily when acted upon by an outside force. This is true, but it's a little different when rolling down hill. Now we're talking about potential energy and the object with greater potential energy will accelerate faster unless friction overcomes the difference.

--My wheels are 700c and lighter by 300-400 grams than his, so my wheels have smaller rotational inertia.

--Tire compound - yes, this is a good point! Is it enough to make up for 40lb advantage?

-- Tire size -- we were about equal in that department. His are 25c with a round profile and mine are 28c but run narrow.

--My hubs are road hubs which are more efficient. My wheels are larger and roll over road imperfections better. Blue Ridge Parkway, where we repeated this amazing feat time and time again, is anything but smooth, but the views are spectacular!

No. If you are just rolling downhill, then gravity is the only force causing the bike to move. As we know, the acceleration is independent of mass, a 10 lb ball will fall at the same rate as a 50 lb ball. The heavier bike will have greater potential energy. As the bike moves, the speed of the bike is not greater in the heavier bike because of the mass, but the kinetic energy is greater as potential energy is converted into kinetic energy.
Well, I was giving a general statement, different masses will change the rotational inertia. I don't know the weight of your wheels, so let's say your's are 1500 grams. This means the 26" wheel would be 1800 grams according to you. I=mr^2
Your 700c wheels would have the inertia of: 0.145 kgm^2
The 26" wheels would have the inertia of: 0.141 kgm^2
This is of course using some rounding. As you can see, the 26" wheels, though heavier still have a lower inertia. This should help him get moving faster and you should be able to keep the speed easier.
What advantage? The mass does not matter to the acceleration of gravity. But tire compound is a huge factor.
You say that your hubs are more efficient. I assume you mean that your hubs have less friction. Yes, this will matter.

You say that you accelerate faster down a hill than your friend. This is most likely because your tires have less rolling resistance, your hubs have less friction and the aerodynamics or some combination of these. You say that aerodynamics doesn't matter that they are the same, but I highly doubt that two different people on two different bikes have the same amount of drag.

In conclusion, Weight does not matter on how fast you go down hill with only gravity as the force, before terminal velocity is reached. With all things equal, 26" wheels have a lower moment of inertai, allowing easier change of velocity, 700c wheels will resist a change of velocity more, tire composition plays a major rule in rolling resistance and aerodynamics play a major part in overall speed.

doesn't a larger wheel provide a greater mechanical advantage in braking from the rim?

It does in the motorcycle word.
Full rim brakes have been used on a couple of prototype bikes for some time.
Very expensive though.

You are making one big mistake. Two items of dissimilar weights will fall at the same acceleration in a vacuum. However, aerodynamic drag is a function of surface area (n^2), while mass is determined by volume(n^3). So let's look at this situation:
First, let's look at the forces of acceleration:

Rider 1:180 lbs + 20 lb bike = 200 lbs total
Rider 2:215 lbs + 26 lb bike = 241 lbs total.

So the ratio of acclerative forces due to gravity 1:1.205 from rider1:rider2. "So what?!?" you say, "Rider 2 is heavier than rider 1, so those forces even out!" But that is if you don't take aerodynamics into account. I'm making an assumption here, but to determine the surface area of the riders, let's just get the cube root of the total weight, and square it, which should give us an appoximation of surface area, and therefore drag(no, this isn't a perfect ratio, but I don't have a wind tunnel to do it properly).

Rider 1: 34.1
Rider 2: 38.7

The ratio of forces due to drag is 1.13. In order for the 200lb rider+bike to out accelerate the 241lb rider+bike, he needs to reduce his overall drag by 13% relative to the larger rider in order to make up the difference. If rider positions and clothing are the same, the only reason why the smaller rider out accelerates the larger is rolling resistance.

Here is a thoughtful review of why 26 wheels are judged best for touring bicycles by Thorn Cycles:

http://www.thorncycles.co.uk/why26inchwheels.html

Schwalbe has technical information comparing rolling resistance of 26 and 700. They show that under the same air pressure (and same tread, etc.), 26" tires have less rolling resistance. However, 700 can take more pressure, so they show less resistance under normal conditions due to higher pressure:

http://www.schwalbetires.com/tech_in...ing_resistance

And another on general discussion consider both 700 and 26:

http://www.precisiontandems.com/artbillwheelsize.htm

Thank you.

Given the differences in purposes -- 700s are generally for road, 26 are generally for off road -- probably the biggest difference is the selection of tires. Otherwise, if you want to go faster, work on the engine.

BTW, the Schwalbe website is excellent. If you want to understand why a wider tire can have less rolling resistance than a skinny tire at the same tire pressure, there are some pretty pictures that demonstrate the point. Although, as Sheldon wrote, the ceteris parabis argument breaks down once you realize that skinny tires almost always run at higher tire pressures.

Yes, this is true. Air resistance is a factor in freefalling objects.

Nope. You are assuming identical bikes and riders. As I stated above, there is a combination of factors that will determine the rate of fall. Friction of the hubs is one. According to you it wouldn't. So, I would go just as fast downhill if I applied my brakes as if I didn't apply the brakes? Of course not.

Your assumptions of aerodynamics is wrong. You are assuming the drag as a factor of weight. It is not. Your assumptions mean that a 50lbs will fall faster than 10lbs no matter the shape. This is not the case, the shape is much more a factor than weight before reaching terminal velocity.

For some reason, you don't want to mention rotational inertia. I can only assume this means that you don't think it is a factor. I don't know why, because it is a big factor.

How fast someone rolls down a hill has many factors, weight would be the least factor, even negligible. The rate of speed and acceleration has to do with rotational inertia, friction, air resistance and rolling resistance.

You guys and your silly 26 inch tires!
Just kidding.
Use what you want, go with what works best for you and your given situation.
As a retro grouch who tours on paved roads with the odd gravel road thrown in I go with the 700C. But I started touring on 27 inch tires in the early 80's so I have already "stepped down" a size in tires!
I will continue to use the 26 inch tires on my MTB. As I run them at 40-50 lbs for a comfy ride off road.

Hardly. I am looking at the forces involved. The poster made a statement, that on a 700c bike, he was able to out-accelerate his partner who even though had 40 lbs more weight, still could not keep up with himself on a hill. I made an attempt to describe the forces involved. As far as total friction from the hubs, friction is a function of RPM as well as total friction. 700c wheels turn at a lower rpm than 26" wheels.

I used weight as a proxy for volume, and from that, extrapolating surface area, which is a good indicator of total aerodynamic drag, all else being equal. That is why I take the cube root of weight and square it. That will give us an idea of surface area, which is a good indicator of aerodynamic drag. Force of gravity is dependant on mass, which is density * volume. Total drag is a function of surface area, which has an exponential coeffecient of x^(2/3) that of volume. In any event, I am giving the lighter rider the benefit of the doubt in assuming that both riders are of equivalent physical condition, the only difference being size. A more out-of-shape rider is more dense than an in shape rider, meaning more mass compared to surface area.
No, it's just not that important. The force of aerodyamic drag is far, far more important than rotational inertia.

Except for the fact that weight *does* play a factor, or more accurately, the ratio of weight/aerodynamic drag plays a factor. A larger person has less surface area as a function of volume than a smaller person. All else being equal, total surface area is the greatest predictor of drag there is. It is not that a larger person gets a better assist from gravity, it is that they get less of an effect from drag. The greater the difference in size, the greater an effect aerodynamic drag has on the lighter rider relative to the heavier one. (If you don't think volume/surface area matters, explain why a 10cm hailstone has a terminal velocity of 136mph, yet a 1cm hailstone only has a term velocity of 46)

Nope, you are taking identical riders and bikes. You state the example, but your math is for identical riders and bikes. If not, then you would have to put these variables in the equations. You have not done that. You are taking weight as surface area, that is kind of ok, but again, you are taking identical riders and bikes. Just because the total surface area of the riders are roughly calculated, you are not taking into account how the bikes and riders are aerodynamically. I agree that the drag will most likely be more important than the inertia, unless the drag is the same. Just because two things have the same weight and surface area does not mean they are the same aerodynamically.

The real problem I had with your original statement is:
This is totally false, unless you are talking about identical bikes and riders that have the same aerodynamics. Otherwise all the other factors come into play on how fast a bike will roll down a hill, not just rolling resistance.

The original poster wanted to know why the other rider was slower, there are many reasons why this could be. You tried to make it a simple argument about tire rolling resistance by neglecting other factors.

Could just as well be the hub bearings.... How about softer vs. harder rubber tires? There is a lot more that is going on that a simple number crunchers game I think... I could be wrong though.

As for the OP's initial dilemma, I recommend getting the bike that fits, with the added observation that Surly's run big, so test ride one if possible.

On 700c vs 26", in the U.S. I think you will be more likely to find a tire reasonable for touring on pavement in the 700c size than in the 26" size. When I last needed to buy a tire on tour I was in Indiana, and the shop I found (only one in town) had a very limited selection of tires. I was looking only at 700c sizes, but I doubt I would have found a slick 26" tire in that shop.

So if it's a toss up between the 54 and 56 LHT fit wise, then I'd go for the 700c tires if you plan to tour in the U.S.

I know this thread has been dormant for awhile, but I've read through it and find it interesting. I won't pretend to understand all the physics and engineering complexities, but to me there seems to be a simple reason why 700c wheels would be faster than 26" wheels which doesn't seem to have been mentioned (or maybe it has and I just didn't understand):

All other things being equal (which I know is almost never the case, but for argument sake) the larger the diameter of a wheel the farther it will roll per revolution. Since 700c wheels have a greater diameter a bike with 700c wheels would go a little faster (again, all other factors assumed to be equal) than 26" wheels. Does this make sense at all, or am I completely off base?

I know wheel diameter is only one of many factors that determine how fast a bike will be (as this discussion illustrates), but to me it would seem to be one of the most important factors. Granted this speed advantage for 700s would be partially offset by other factors like greater weight, etc, but purely in terms of maintaining speed 700s would seem to have an advantage, though as others have mentioned 26s might accelerate better due to the lower center of gravity and lighter weight.

And just to be clear... I'm not an advocate for one being inherently better because I think that depends on rider size and intended usage. To me it seems to make sense that 26s would be better for smaller riders, while 700s make more sense for larger riders. I'm 5'9" and currently ride a 700c bike that seems to work fine, though in all honesty 26s would probably be more well suited for my size (I just on the cusp of 700s being a little big for me).

I run panaracer pasela tourguards, 26x-1.25 on my lht 54.
I can run with the guys on the 700 skinnys on the roads, but they can't
run with me on the hills or gravel. :-)

I've got touring bikes in 700c, 26" and 20" - given the ability to pick appropriate tires and gearing I'm not significantly slower or faster on any of my bikes on a paved road.