How good of a cyclist do you have to be for a disk wheel to be be the optimal choice for a rear wheel in a 1/2 Iron or full Iron event. In other words, does a rear wheel disk benefit someone who is able to sustain a 19 mph or 20 mph pace?
I’ll tell you after my next race. I just bought one.
All the gear no idea.
A disc wheel will benefit everybody by the same %. Actual time saved will be more for slower riders.
This is a good read:
Thanks
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BUt is the marginal aero advantage of disk versus a deep rim aero wheel worth the added weight if you are not a great cyclist?
A disc wheel will benefit everybody by the same %. Actual time saved will be more for slower riders.
No it won’t. For slower riders, gravity and resistance are a bigger percentage of the total forces they have to overcome, compared to aerodynamic drag. Furthermore aerodynamic drag is not linear compared to speed (it’s a square or a cube law, I forget which). The percentage gain should actually increase at higher speeds, though you’re correct that for slower riders a smaller percentage can still be a larger amount of time.
If you look at some of the newer discs the weight is not that much different. I personally ride a disc on everything and every course.
“No it won’t. For slower riders, gravity and resistance are a bigger percentage of the total forces they have to overcome, compared to aerodynamic drag. Furthermore aerodynamic drag is not linear compared to speed (it’s a square or a cube law, I forget which). The percentage gain should actually increase at higher speeds, though you’re correct that for slower riders a smaller percentage can still be a larger amount of time.”
I (and a degree in Aerospace Engineering) second that. As for the square or cube law thing it really depends on the situation. Truth is it isn’t well defined/known.
If you can ride 17mph then a disk will benefit you.
$400 for a Renn or more for a 404 $880, H3 $640, 808, stinger, Deep etc
Why spend the money twice?
jaretj
stanglor, so based on your educatio, how do you perceive the value of a disk for a person riding 18-21 mph
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“As for the square or cube law thing it really depends on the situation. Truth is it isn’t well defined/known.”
For a cyclist moving at reasonable speeds through air, under what conditions would the drag be linear with speed? F_d=1/2 C_dA rho v^2 has worked pretty well for all cases I’m aware of. What am I missing?
"BUt is the marginal aero advantage of disk versus a deep rim aero wheel worth the added weight "
Absolutely. Read the url that I posted erlier in this thread. It talks about weight vs aero.
“a degree in Aerospace Engineering…Truth is it isn’t well defined/known.”
What exactly is the truth. I’ve read many conflicting things from various “experts”. Some say the % gain is always the same while others even claim that aero wheels don’t even begin to make a difference until above 25 mph. It can get very confusing for the lay person.
The 5-7-5 is $450 now… Wish I had got mine before the price bump.
Drag is in the range of proportional to velocity squared, especially concerning cyclists. Some data would argue that certain instances might be closer to velocity cubed, but as mentioned earlier a squared value is probably the best for cyclists. Definitely not linear. Gigs, if you are averaging 18-21 mph I would say DISK DISK DISK! Then again I’m a 190 lb aero guy so I’m biased in favor of aerodynamics over weight savings.
For myself I would use a disk on pretty much any course except for an aggressive uphill TT, this includes hilly long courses like IMLP or hilly short ones like the 6.6 mile TT that started with a 2.25 mile climb at the Cascade Classic last Friday. Now I just need to save enough money so that I can buy one instead of borrowing one…
If you already have an aero rear wheel it’s definitely a case of diminishing returns for dollars spent in order to purchase a disk. Then again you can’t take it with you, so if you have the cash lying around why not go for it.
Regardless, the best money spent is time on the bike. I know that whenever I pass someone with wheels that cost more than my complete bike I feel like a total stud (of course this is getting harder to do now that I have an income and will soon be able to buy some sweet wheels)
“Drag is in the range of proportional to velocity squared, especially concerning cyclists. Some data would argue that certain instances might be closer to velocity cubed, …”
First you say drag is proportional to velocity squared or velocity to the first power. Now you say drag is proportional to velocity squared or velocity cubed. First of all which is it, and second, could you share the data showing a third order relation between velocity and drag?
Oh for God’s sake: If you don’t already have a “racing” rear wheel, get the Renn Disk. If you already have a deep wheel (Hed Jet or the like), you most likely won’t experience any real time savings.
Now, if you just want a disk for the cool factor, and don’t underestimate that, then get the disk anyway. There’s always ebay if you decide to sell it.
I’m an engineer also, but I’ll dispense with the math. If you need to pick up somewhere like 1 min or less over 25 miles (you can extrapolate for longer distances) to hit the podium or make yourself feel better, then the disk will be worth it. If you need 1/2 hour, well, I’d spend a little more time training.
“First you say drag is proportional to velocity squared or velocity to the first power. Now you say drag is proportional to velocity squared or velocity cubed. First of all which is it, and second, could you share the data showing a third order relation between velocity and drag?”
At what point did I say it was linearly proportional to velocity? Either you misread it or I made a typo. If it was my fault, my bad, but that is definitely not what I intended to say or believe.
My point (not clearly made as I tend to ramble aimlessly) is that drag produced by a body (in this case bike + cyclist) moving through a fluid (in this case air, and yes it’s a gas but for aerodynamics purposes we/I use the term fluid) can’t be easily and generically summarized by one formula. It’s faaaaaaaar too complex for that. There are different flow regimes different wake and vortex shedding patterns etc., and those complexities can be seen in something as simple as a cylinder in a wind tunnel. Try factoring in the complexities of a moving person in an ever-changing environment and you are HOSED. That being said, as I mentioned earlier a good **approximation **for a cyclist is
drag is proportional to velocity squared.
Sorry, but I don’t have any data that looks like a velocity cubed relationship. Actually I don’t have any data at all. What I can say is that I have seen this data (though not for cyclists) and fluid dynamics is pretty complicated
I second everything TriPA just posted
Sorry for the confusion, back to work.
I have a question about the Wheel Covers -
I thought that one of the reasons that a disc is more aero than a standard wheel, is that the solid disc does not widen from the rim to the hub - it can be a solid, straight surface, thus not creating much of a cross section in the wind, like standard spokes. On a regular wheel, the spokes widen from the rim to the hub, creating drag. Wouldn’t this still be the case for wheel covers?