I recall from somewhere like analyticcycling, weight trumps aero when the grade exceeds 6-7%. But that only applies while you're climbing, so if you ride 179km on the flat and a 1km climb, a heavy aero wheel trumps a light, less aero wheel as the gains on the flat outweigh the extra weight on a climb. If it's 50% uphill @ 6-7% and 50% downhill, a lightweight wheel will be better as you are spending more time climbing than descending. I personally would use my 808's for a climbing course as they are lighter and more aero than my training wheels, but I race on flat courses so I always use a cover in the rear.

Good question, you can get into lots of interesting, complicated detail on this question, for instance, on some bikes, in certain wind conditions, a zipp 808 might be a little faster. Or if you are going uphill only and it is steep enough a lighter wheel might be faster, or if the course has nothing but uphills and technical downhills, the lighter wheel might be faster. You can do a bunch of math and try to figure out when those situations will occur in a race.

BUT, pretty much that never happens. Even crazy crazy courses like Savageman, a disc is pretty much just *as fast* as a lighter wheel, not slower, so you can just always use a disc and simplify your life.

This tech article explains the basic idea of "When does weight trump aero" very well. Of course it depends on HOW aero, HOW much weight, and how powerful you are:

http://www.cervelo.com/.../weight-vs-aero.html

The key thing to remember is weight matters less than most people intuitively imagine, aero matters more, even at low speeds, than people intuitively imagine.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

When you are M40-44 and are in the same race as me. Aero helmets are also slower in those races, as are wetsuits.

No easy answer.
Weight is one consideration - but a disc will only be a few hundred grams heavier than a 404 or 101 wheel. That will make more difference to a 4'11", 90 lb lady than to 6'5" Matty Reed as it will affect their power-to-weight ratios differently.

Then there is the moment of inertia consideration - it takes more energy to spin up a disc than to spin up a 101. But you don't get all that energy back. So if the course is full of rollers, sharp turns or anything that causes you to change your speed often - the disc may not be the ideal choice. Though I doubt the difference is all that big.

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Remember - It's important to be comfortable in your own skin... because it turns out society frowns on wearing other people's

Another important variable besides the grade of the course is wind speed and direction, I use both an 808 and disc on the back to race but on very gusty days I'll chose the 808 since it handles much better in those conditions.

My .02 YMMV

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WyoJoe

If you think you can do a thing or think you can't do a thing, you're right - Henry Ford

Does it REALLY handle better in gusty conditions, or have you not tried the disc in gusty conditions?

Do you change the front wheel too in gusty conditions?

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

Most commonly if its a very gusty wind and the gusts make you nervous so you get out of the aerobars. Other than that it would be very rare. Basically a hilly race where the decents are so twisty that you brake so much and never get to use the aero advantage.

But the first situation happens a lot, you see all sorts of people on discs and deep wheels not on the aerobars. Its not all the discs fault, experience and bike fit are bigger issues by far, but if you commonly dont ride in the aerobars during a race then I wouldn't add a disc the the equation. And yeah I know all about the pressure on the disc and stabilizing the bike, but that assumes a constant wind. When it gusts you are going to feel the wind hit the disc and if you are nervous you wont like the feeling.

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Styrrell

My experience with discs and gusty winds is that it is a non issue... compared to what can happen to the front wheel if the wind catches it just right (or wrong). The pressure on the disc will tend to push you over but it doesn't steer your bars.

The people who aren't riding on the aerobars probably shouldn't have gotten deep rims in first place. There isn't much benefit to them unless you have high yaw.

I run a Jet 9/Jet Disc 95% of the time. If it were really gusty I might switch to my Jet 6 in the front. For me the front wheel makes the biggest difference.

Isn't it because of the wind that they don't allow discs at Kona?

The pressure on the disc will tend to push you over but it doesn't steer your bars.

And thats enough to keep lots of people off aerobars in races. Coming from a cycling background it never occurred to me that people would try a few tris on an o.ld beater bike, then plunk down $10K on the best tri bike in the shop with almost 0 experience riding aerobars. But it happens all the time.

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Styrrell

Wind speed and direction matter, but in the non-subjective sense (i.e let's exclude "handling" for a bit) the wind speed and direction only matter insofar as they dictate your yaw angle. So let's look at a drag-vs.-yaw chart:

We'll have to suppose for the sake of argument that the data is valid--I just pulled this off an image search so I'm not sure where it came from, but if I had to guess I'd say it probably came from HED :).

Looking at the chart, the lines with the lowest drag are mostly discs and deep wheels. BUT...at low yaw (i.e <5 degrees) and at very high yaw (>15 degrees) disc wheels are more or less comparable to other deep wheels--hardly better, hardly worse. So the question becomes: what is my yaw angle? This question is obviously simplistic. On any given day with steady wind conditions over ground, changing direction on the road will give you a different yaw angle. So on average, what wind angle would a rider see? Depends on wind speed & direction, and rider speed & direction. It's an iterative problem.

The answer is that in the yaw angle regimes where discs aren't the best they're very close to the best, and in the yaw angle regimes where they are the best, they're the best by a pretty long shot. As Jackmott said, the weaknesses of the disc are weight and moment of inertia--both very minor considerations when compared with aerodynamics. The result is somewhat of a self-supporting statement: discs are only slower in circumstances that a triathlete would never experience, because the same conditions that make a disc slower make triathletes hate the conditions. You'd need something like a race with a constant 7% grade uphill for the entirety of the course for a disc to not be faster than a lighter, shallower wheel, or a race with inumerable 180 degree turns for a disc to not be faster.

Subjectively though, many people feel like discs are harder to handle. The truth is the disc makes the bike easier to handle, while the front wheel de-stabilizes the bike. The larger the ratio of rear-wheel area to front-wheel area, the more stable the bike will be in nearly all situations. Pretty much only in the case where the wind over ground is blowing from like 4-5 o'clock...i.e. where the resulting apparent wind on the rider is a pure crosswind of enormous strength would a disc destabilize the bike.

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Something to consider is that most disc wheels distribute their weight more evenly over the entire wheel than spoked wheels. That causes the rotational inertia to be lower per gram of wheel weight vs spoked wheels. Completely anecdotal but I've always thought disc wheels accelerate and climb quite well.

Isn't it because of the wind that they don't allow discs at Kona?

It's never made sense to me. I used to live in Hawaii, and the winds they get there are not nearly as crazy as what we sometimes have here. And since it is Worlds, you'd think most of the riders would be pretty good at it...

All I heard there was you prefer standard box to strapons.

It takes more energy to spin up the heavier wheel, but they have more momentum once they're spinning. As I understand it there is no extra loss of power there unless you hit your brakes.

Price for being imprecise I guess - I'm talking about true cross winds with 10+ mile gusts, I tend to use disc primarily except under these conditions. I rarely even then get off the aero bars but know that I tend to stick closer to my intended line in those conditions when using the 808. Being a cautious lad I use a 404 up front all the time.

I'm here to learn so thanks for enlightening me...

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WyoJoe

If you think you can do a thing or think you can't do a thing, you're right - Henry Ford

Those conditions are when you get the most out of a disc, you might try it and see if it is really worse.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

JackMott is the ST expert on this topic so you can trust what he posts. Go Aero.

Despite the "common knowledge" on this subject (everyone remembers their HS physics, I guess) in reality the differences in performance due to varying wheel rotational inertias is in reality EXCEEDINGLY small...even in an all out sprint.

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http://bikeblather.blogspot.com/

I thought it was equally about tradition....therefore they want the bikes to look more like bikes (using somewhat normal looking wheels).

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24 Hour World TT Champs-American record holder
Fat Bike Worlds - Race Director
Insta: chris.s.apex

When you've spent too much money on a rear wheel and coaching, another component, or buying time to spend in the saddle would have made you faster.

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com

I researched this last year before buying aerojacket for my Reynolds Assault. Read a lot of Jackmotts historical posts on this topic. From what I learned is that a disc is almost always faster, however rarely is it ever slower than a non-disc. Thus going with the disc is a win-win.

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The curious task of economics is to demonstrate to men how little they really know about what they imagine they can design.

Some interesting opinions here - not quite the route I thought the thread would take, but it has enabled me to think for myself as opposed to just relying on the collective brain of slowtwitch!

This is a good read...



This is funny...


This is untrue... for me, there is very little correlation between how much money I spend on Triathlon and how much time I spend training. A different topic entirely but to a small degree the inverse is true, I buy a new toy / gadget and I want to spend time on the bike, but again thats off topic.


Fair to say everything I continue to read on the subject, puts me deeper in the disc camp...