O.K. O’ wise ST bike gurus…my question is simple a straightforward…the answer may not be.
“What is it that makes a disc wheel faster than a non-disc alternative?”
-is it all aerodynamics?
-is it the solid disc to tire connection?
-is it some combination of both?
-am I missing something?
The reason for asking is I have recently started riding a disc and have not had the chance to compare my times in similar enough circumstances so I’m not sure I’m any faster with the disc. Am I missing something or am I expecting too much “obvious” improvements???
Since i don’t have a degree in aerodynamics let’s make it simple.
Start with a regular box rim. Make the rim dished and the wheel gets faster. Make the dish bigger and it gets faster even more. So keep making the dish larger and larger and the wheel continues to get faster. Make the dish bigger until it connects to the hub. You know have a disc - the fastest wheel available.
That’s my version of “keep it simple stupid” aerodynamic theory.
Didn’t have anything in mind exactly but since I haven’t been able to quantify an improvement I was wondering if there is either a) such small incremental improvements as to be not empirically noticeable b) some other benefit such as to not be “speed related” but more tied to power expenditure…I do not own any sort of power meter…
the increase in speed from a disc is all about and increase in surface area which in turn causes an increase in the aerodynamics. That is why zipp is now putting dimples in their discs so they will have more surface area compared to other smooth discs on the market. But the question is, are you willing to pay the $1000 plus price tag for the increase in speed.
I think you should not expect more than a couple of watts increase which does not add up a hole lot until you are doing long distances, nocking a few minutes off your PR.
The rider. The rider makes anything faster, or not.
A disk on a bike ridden by a slow rider will still be slow. Just slightly less so. And they make that same cool “whoosh-whoosh” noise when they are going past you backwards too.
<< Am I missing something or am I expecting too much “obvious” improvements???
Depends ™ on what other wheel you are comparing it to. If you already had something like a 404 or Alps or H3 or whatever, the difference will be very small. Compared to a standard non-aero wheel, you should see a fairly noticable difference (at least I do w/ 404’s vs Rolfs). Unless of course you are running Tufo tubbies, in which case you will be much much slower.
"are you willing to pay the $1000 plus price tag for the increase in speed. "
A disc is a disc is a disc. Kraig Willet tested a $60. CH Aero disc cover against a Zipp disc in a wind tunnel. The results are available on his pay site for a few dollars. The results would shock a lot of people.
not to answer this too seriouly, but . . . the basic principle as I understand it is that most wheel-aerodynamics revolve around the spokes. the spokes travel very fast (especially at the rim) and create a great deal of turbulance. Hence deep dish rims. The dish smooths the airflow around the fastest moving spokes. If I remember correctly, the advantage of ‘hiding’ the spokes in the rim is more advantageous then the airfow moving directly over the rim (such as in the zipp airflow cross sections that explain the advantages of the “bulge”). Same principle applies with tri-spokes. Less spokes, and then stretched to cut the air more smoothly.
A disk, therfore, removes the spokes completely. The less air resistance, the less wattage is needed to overdome the wheels aerodynamic drag. I think that Cobb wrote, way back in the day, that a disk wheel takes aproximatly 4-6 watts to turn, compared to a box rim at around 30).
Of course, what you do with those watts is what determines the benifit you get. If you keep pushing the pedals at the same frequency and intensity you should be faster. If you use that energy to rest and run faster, then your benifit will be expresed in other ways. I, personally, have a disk and I use it to slack off and still post moderatly respectable bike splits.
Just another way to look at aerodynamics- an object has at least one leading edge and one trailing edge. It takes energy for the leading surface to push air out of the way and the faster you go the bigger the shock wave. Behind the trailing edge you create a vacuum. For a simple shape you have a force pushing on the front (air, shock wave) and a force pulling back (vacuum), both working in the same direction. The disc removes the extra leading and trailing edge that the regular wheel rim gives.
Put your bike on a stand and crank the pedals by hand to feel the force required to keep the rear wheel rotating at high speed. I can feel the difference between a spoked wheel and my Spinergy Rev X, for instance. That’s the eggbeater effect.
Actually, it’s a combination of lower resistance to spinning (no spokes), overall lower drag for the wheel in an airstream, and an aerodynamic property called the “splitter effect”. If you take a blunt object (e.g. you and your bike) in flowing air and then attach a long thin “wall” to the trailing edge (the disc wheel) of the object, the drag will be greatly reduced due to reductions air seperation (vortex formation) at the trailing edge of the object. The fact that the disc is spinning also adds a “suction” component (air is drawn towards the disc) that helps to amplify this effect. I found this nugget a little while back on Google groups in a thread between Dan E., Andy C., and Gerard.
Pay close attention to the post in which Gerard expounds on rider/bike interaction, the “splitter effect”, and discs vs. non-discs on bikes without seatposts.
.