Is the following statement true (aerodynamics)

[Shrike]

On any given route that is a full circuit, i.e. same start and finish point, an aero bike will always be faster than a lightweight bike.

Or are there exceptions to this? Without using extreme weight differences.

Been playing around with the aeroweenie calculator and digging around online: http://www.aeroweenie.com/calc.html using R5 and S5 as examples (as Cervelo say there is a 0.013 m2 CdA difference). I know it's a hashed out topic, but wondering if I've missed anything.

[caad4rep]

In theory it's correct. In an extreme up and down route the aero bike is likely tougher to control on the descent which is why light weight bikes are still preferred in the mountains.

[Marin]

If your downhill speed isn't limited by aerodynamics a light bike will be faster up & down a climb.

[Nefarious86]

In theory it's correct. In an extreme up and down route the aero bike is likely tougher to control on the descent which is why light weight bikes are still preferred in the mountains.

Tougher to control?

[mr4fox]

In theory it's correct. In an extreme up and down route the aero bike is likely tougher to control on the descent which is why light weight bikes are still preferred in the mountains.

Not sure I buy this. I've taken several KOMs descending mountains on my bmc tmr01. It's handles crazy good and better than any other bike over ridden. Chris grooms managed to escape on a descent on a dogma f8 in the TDF and that's a very aero frame. I don't think it's fair to label aero bikes as bad handling anymore. Any bike can be good or bad handling. It just depends on the individual bike. Not it's tube shapes.





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[TheKaiser]

I don't think it's fair to label aero bikes as bad handling anymore. Any bike can be good or bad handling. It just depends on the individual bike. Not it's tube shapes.

Yes, precisely correct. Downhill handling is a waaaaay more complex a topic than simply aero vs. lightweight. It would take all sorts of bike characteristics into account, angles, wheelbase, trail, etc...plus it would depend on the individual descent, it if was buffeted by crosswinds, how much stiffness or flex was optimal for the pavement quality, etc....

If we are including aero wheels in the topic, then a deep section wheel could either hinder descending if there are a lot of gusty crosswinds, or on rough pavement the additional gyroscopic stability could help by stabilizing the bike and making it less twitchy.

[Calnago]

I'll say I think the biggest difference in handling while descending is going to be determined by the amount of crosswinds and gusts, and how much the cross section of your bike, be it from wheels or the frame, is exposed to the crosswinds. Geometry can be identical, look at the Trek Emonda and Madone for example. Same geometry, but very different feeling in crosswinds.

Also, I think the gyroscopic stability of modern wheels plays quite a factor as well, but I would argue that because of their very lightweight, the stability (forget about crosswinds for the moment) of light aero wheels is much less than say, a standard 32 hole wheel with a shallow profile box section alloy rim. For one, all the weight of the shallow alloy rim is at the very outermost perimeter of the wheel. With the aero carbon wheel, it is distributetd over say 50mm or more of depth, and, it is likely less weight overall. You can experience the effect by taking two front wheels, one with a low profile alloy rim, and the other a deeper profile carbon rim. To demonstrate this to myself I used a standard wheel (Ambrosio Nemesis rim laced with 32 spokes to a Record hub and a Campy Bora Ultra 50 wheel for a somewhat more "aero" and much lighter offering). When holding each of the wheels by their axles in your hand, and spinning them as fast as you can, the Nemesis wheel was far more forceful in its attempt to right itself when tilted or twisted off the vertical plane, in other words more stable. It's a good experiment to try if you have a couple of wheels to experiment with. And in practice, that's exactly what I experience while descending. If rock solid stability was my sole concern while descending, I'd be on the box standard Nemesis wheels all day long.

But at the end of the day, you get used to whatever bike you are riding and just naturally adjust your riding style to it's idiosyncrasies either way I think, within reason. Ride what you like and feel good on.

[mattr]

I had some ideas, wrote them down, went to look for a couple of numbers to justify it, found them, then found this, that sums it up in much better language......
And with the numbers that i was trying to find...

https://www.cervelo.com/en/engineering- ... ht-vs-aero

[Calnago]

That link in the above post is a typical Cervelo kind of marketing white paper blah blah. In their extreme example, they're using a rider of 50kg, putting out 400 watts. That's a rider of 110lbs. Lol. Ok... how many of us are 110lbs? Let alone putting out 400 watts for an entire climb. In their example they claim the grade has to be 8% or more before the heavier bike is at a disadvantage. However, as weight goes up, so does the effort required to overcome the effect of gravity. So, in contrast to Cervelo's analysis using a featherweight rider, here's another chart showing in what is supposed to be an unbiased form... using a rider+bike weight of 75kg.
Make what you will of the two contrasting analyses....



What can we gleam from it. Well, as far as I can tell, the tipping point is only a 2% grade for a 75kg (bike and rider)... and thus even less for those of us who may be over say, 90kg. Well, aero be damned.... must put down this donut. Either that or stay away from mountains.

[mattr]

Yeah, it´s blah blah from Cervelo, but the physics stacks up (but not the physiology)

The RST table is far more detailed.

[wingguy]

What can we gleam from it. Well, as far as I can tell, the tipping point is only a 2% grade for a 75kg (bike and rider)

No, that conclusion is impossible to make from that graph alone. It depends on how much extra weight you need to carry for a given drag reduction.

[tilf]

the tipping point is only a 2% grade for a 75kg (bike and rider)...

Aero is no longer the predominant source of drag, but that's not to say that the aero bike for a pound or two disadvantage will be much slower. Also .300 CdA is probably optimistic for a typical non-aero, non-optimized setup, especially if you are not climbing in the drops.

I've been using Best Bike Split and found that I'd need to add quite a bit of weight to make up for Aero savings while climbing-- and my FTP is only ~275. The obvious thing to do is try to maximize lightweight AND aerodynamic, the next step is realizing that a lot of aerodynamic and weight saving gains can be adjusted with rider position and rider diet.

[Fiery]

Since the original question is about a circuit or loop track, I'm guessing the idea is also that since the overall elevation change will be zero, weight won't matter. This is not so simple, because time lost on a climb is hard to make up on the descent. If you are climbing 1% slower due to extra weight, you will have to descent more than 1% faster to make up the time because the descent simply lasts shorter - and at a certain point your downhill speed is limited more by traction in corners than it is by air resistance on straights.

[Clingon]

Yep, as per tilf and wingguy above, interpretation of the RST table can be misleading. Yes, on a 3% gradient gravity accounts for a greater proportion of resistance, but this doesn't mean that a light weight bicycle is suddenly more viable.

Consider the following example: two riders climbing a 3% slope as per the chart above.
One is riding a light bike that is one Kg lighter than than his more aero counterpart. This gives a weight difference of 1.33% (1/75*100). Multiplied by the resistance factor given in the chart above and we find that the lighter machine will make its rider 0.77% faster on the 3% slope (1.33%*.58).
A more aero bike (frame, wheels, and components) can reduce overall system drag by something like 5% (some claim up to 10%, frames are typically less than 5%, this is of total bike plus rider drag). NB: a decrease in CdA remains the same even at lower speeds, so even at low speeds a 5% more aero bike is still 5% more aero, it just doesn't matter as much. Again using the above chart we find that the aero bike saves its rider 1.5% (5%*.3).
So even on a 3% gradient twice as much energy (0.77 vs 1.5) is saved by the more aero bicycle; as you can see the actual tipping point is therefore somewhat steeper than 3%. Of course this won't be true for every comparison, there are some surprisingly aero light weights and some downright pig aero bikes *cough* venge *cough*.

To answer the OP's question, yes on almost all circuit courses an aero bike will be faster as long as all other factors remain equal. As others have eluded to, the comfort or handling aspects of an aero bike may be enough to outweigh its advantages in certain situations, although I think that many modern aero bicycles have succeeded in overcoming most of these. Also, if you are racing and are able to get a more aero bike down to 6.8 then it won't matter anyway.
It is possible to imagine a circuit course where an aero bike could potentially be slower, eg. a very steep climb followed by a technical descent with lots of braking. As described by Fiery, you can't always make up time on a descent, but in practice I think such circumstances are rare.

[Calnago]

Like I said, make what you will of the two charts presented.