Author Topic: Surly Cross Check Touring Bike?  (Read 37597 times)

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Offline paddleboy17

Re: Surly Cross Check Touring Bike?
« Reply #15 on: November 30, 2010, 05:42:00 pm »
Regarding this 26" versus 700C argument...

All things being equal, by equal I mean same rim profile and same tire, I believe that the 26" would accellerate quicker.    Why do I think this?  The weight should be the same (OK, the 26" could be slightly lighter since there is less rim circumference), but the fact that 26" rim has a smaller diameter is really important.  A smaller diameter means a smaller moment of inertia.  A smaller moment of inertia means less effort to spin it up.  Once in motion, the work to keep a 26" or 700C wheel in motion should be the same.

So why do some of you have a different imperical experience?  Because all things are not equal.  26" rims tend to be beefier and wider than 700C rims.  I suspect that faster tires (lighter carcas) are more readily available in 700C.  Schwalbe seems to agnostic about rim size, a given tire is available in multiple rim sizes.

That is my take based on the Mechanical Engineering classes I took my freshman and sophmore years in college...
Danno

Offline ducnut

Re: Surly Cross Check Touring Bike?
« Reply #16 on: December 02, 2010, 01:26:26 am »
Regarding this 26" versus 700C argument...

All things being equal, by equal I mean same rim profile and same tire, I believe that the 26" would accellerate quicker.    Why do I think this?  The weight should be the same (OK, the 26" could be slightly lighter since there is less rim circumference), but the fact that 26" rim has a smaller diameter is really important.  A smaller diameter means a smaller moment of inertia.  A smaller moment of inertia means less effort to spin it up.  Once in motion, the work to keep a 26" or 700C wheel in motion should be the same.

So why do some of you have a different imperical experience?  Because all things are not equal.  26" rims tend to be beefier and wider than 700C rims.  I suspect that faster tires (lighter carcas) are more readily available in 700C.  Schwalbe seems to agnostic about rim size, a given tire is available in multiple rim sizes.

That is my take based on the Mechanical Engineering classes I took my freshman and sophmore years in college...

Your take on it makes logical sense.

I'd question the rim weights, though. If one compared comparable models of rim, the 700c should be heavier. Obviously, because of it's larger diameter, but also, because the increase in diameter is going to be more flimsy and need more material to maintain rigidity. I'm just speculating on this, though. It'd be a good question for a rim engineer.

The other thing you mention is "work maintaining motion". If bumps or slight undulations are encountered, the 700c should fare better because its inertia is higher, via the extra weight and the distance it is from the hub, and the larger diameter wheel's ability to roll over terrain easier. In this case, I'm just thinking a variety of roads such as dirt, brick, rock, etc.

Offline paddleboy17

Re: Surly Cross Check Touring Bike?
« Reply #17 on: December 02, 2010, 01:29:25 pm »
I m not a rim engineer, but here is what I know about the manufacturing process.

Rim stock starts out a extruded straight pieces that get bent into circles and but welded.  Sometimes pins are used to join the two haves.  I don't think that you have to make to much of an allowance for using rim stock in a 26" versus a 700C for the same application.  I do think that rim designers make assumptions like, this is going to be 700C road rim for racing and the rider will only weigh 160 pounds. 
Danno

Offline whittierider

Re: Surly Cross Check Touring Bike?
« Reply #18 on: December 02, 2010, 02:23:23 pm »

Quote
All things being equal, by equal I mean same rim profile and same tire, I believe that the 26" would accellerate quicker.  Why do I think this?  The weight should be the same (OK, the 26" could be slightly lighter since there is less rim circumference), but the fact that 26" rim has a smaller diameter is really important.  A smaller diameter means a smaller moment of inertia.  A smaller moment of inertia means less effort to spin it up.

The smaller one has to get to more RPM though, losing the benefit.  For a 20mph road speed, the rim and tire have to get to 20mph (or very close to it) around the hub anyway, regardless of wheel size.  The slightly smaller 26" size will give very slightly less wind resistance and slightly more rolling resistance, the latter being more of an issue at the slower touring speeds and heavier loads, and the fact that the wind resistance of the panniers and other luggage will somewhat dwarf that of the wheels.  The difference in speed between the two sizes is totally negligible (again, all other factors being equal, especially between tires).

The myth of wheel weight (related to what paddleboy brought up) and acceleration has been soundly debunked many times though.  For years, it was kind of like the Paul Bunyan tale which got taller every time it was told.  Take a look at  this wheel-science web page.   Even if that first equation is too heavy for you, you can look down at the charts and see that even cutting your wheel weight in half (an absolutely huge difference in wheel weight!) makes only a fraction of a percent difference in the power required to go a given speed.  The difference made by just moving the weight to the middle so it doesn't count so much as "rotating weight" is even much smaller-- .004% and .008% in one case.

Offline paddleboy17

Re: Surly Cross Check Touring Bike?
« Reply #19 on: December 02, 2010, 04:14:01 pm »
whittierider, I don't think we are talking about the same thing.

Your reference talks about keeping a bike at speed over time.  I could believe that aerodynamics is the biggest factor, but I suspect is data is skewed for road racing.  At lower speeds, I would guess other things are important, but that is a guess.

I was talking about getting a bike up to speed, and that has to be moment of inertia.  This is the whole flywheel effect. 

So I think it comes down to what you define as zippy or tanky.
Danno

Offline Pat Lamb

Re: Surly Cross Check Touring Bike?
« Reply #20 on: December 03, 2010, 04:05:57 pm »
I was talking about getting a bike up to speed, and that has to be moment of inertia.  This is the whole flywheel effect. 

So I think it comes down to what you define as zippy or tanky.

We are talking about touring, right?  Where you have 20-50 pounds of luggage on the bike?  If that's the context, I don't see how a few ounces are going to be noticed when you start riding.  Zippy doing a crit start out of a gas station driveway isn't going to matter much compared to a 3-6 mile, 6% climb.

(And for the engineers/mathematicians, how does the rotating weight [~r^2] compare to the rotational energy and angular velocity [~w^2] for a smaller wheel?  Without real numbers, my gut instinct is it'll be a wash!)

Offline whittierider

Re: Surly Cross Check Touring Bike?
« Reply #21 on: December 04, 2010, 02:06:18 pm »

Quote
I was talking about getting a bike up to speed, and that has to be moment of inertia.  This is the whole flywheel effect.

Sure, and the tables on the linked wheel-science page are for crit racing which is full of accelerations, unlike a time trial or touring.

Quote
We are talking about touring, right?  Where you have 20-50 pounds of luggage on the bike?  If that's the context, I don't see how a few ounces are going to be noticed when you start riding.

So it's even more irrelevant than when the guys on another forum are willing to spend a couple grand to lighten up their wheels by the weight of a few swallows from their water bottle (nevermind the extra several pounds around their middle) and they totally ignore aerodynamics or even the difference in rolling resistance from one tire to another.

Quote
(And for the engineers/mathematicians, how does the rotating weight [~r^2] compare to the rotational energy and angular velocity [~w^2] for a smaller wheel?  Without real numbers, my gut instinct is it'll be a wash!)
You still have to accelerate the weight of the wheel to your speed, plus the rim and tire (most of the wheel's weight) at that speed around the axle, meaning the outside rotating wheel weight has twice the effect on acceleration that something like your handlebars do, regardless of wheel size.  A 2-ounce difference in rim & tire weight affects acceleration as much as a 4-ounce difference elsewhere.  If your total weight is four thousand ounces (rider + bike + touring load), 4oz won't matter, and neither will the different wheel size.

Offline ducnut

Re: Surly Cross Check Touring Bike?
« Reply #22 on: December 04, 2010, 06:31:49 pm »
So it's even more irrelevant than when the guys on another forum are willing to spend a couple grand to lighten up their wheels by the weight of a few swallows from their water bottle (nevermind the extra several pounds around their middle) and they totally ignore aerodynamics or even the difference in rolling resistance from one tire to another.

^^^ You sure hit the nail on the head there.

I used to compete in the weekly TT, near here. I was blown away by the amount of money people were willing spend on aero parts when they needed to spend time on themselves. The course record holder rides a fairly un-aero TT bike that's all aluminum construction and doesn't have any of the aero aids of a modern carbon frame. The Kona Tri, bike split, record holder wasn't able to beat the guy and he has all the latest and greatest. Proof that training means more than parts.

BTW, I have a set of those fancy wheels. I didn't buy them so much for the aero as I did for the ride. The carbon construction really damps the vibration from the road surface. I ride a lot of chip and seal, so the improvement was worth it. The locals thought I was nuts for riding them as everyday wheels. I asked "What's the point of spending the money on them if you're only going to keep 'em in a wheel bag?".

Rollin' down a dirt road......

Offline paddleboy17

Re: Surly Cross Check Touring Bike?
« Reply #23 on: December 06, 2010, 01:29:17 pm »
I will propose an experiment, and let us see what the outcome is.

Take a test weight, and attach a 12" long twine to it.  Spin your body holding onto the twine.  The test weight will go to end of the twine, and pull the twine tight.

Repeat using a 12' long twine.  Yes, 12" of twine weighs less than 12' of twine, but not very much.  Let me know if you worked harder with a 12' twine.  If 12' is unmanagable then try 6'.  Getting the the test weight flying at the end should be more work.  Keeping the test weight flying once it is flying should be little additional effort.
Danno