Bicycle and aerodynamics question

Summer’s here, and as any good cyclist knows hydration is very important for distance riding under the hot sun. So, I’ve taken a bottle cage off my old bike and attached it to my new one when I saw that there were two places to mount it, the front of the seat tube and on top of the down tube.

Is there a concrete answer on which place is better to mount the cage and bottle on a bike? Does it make any real difference?


Unless you compete at the world class level, you will do better to be well hydrated than any improvements in aerodynamics. Mount both bottles.

The difference is insignificant. Place it where it’s easiest for you to reach. Or get a camelback.

Surprisingly, wind tunnel tests have shown that having one frame-mounted water bottle actually reduces drag, with a downtube bottle being marginally better than one on the seat tube, because of the effects it has on the airflow around the rider’s legs. Two bottles do increase drag a little.

Whether you would notice it is another question. IIRC, a downtube bottle is worth about 30 seconds over a 40k time trial.

I would take the 30 secs any day of the week, are you sure its as much as that?

To get a 30 sec improvement over a short event like that, and maybe 1 min over a standard 50 miler is a good fortnight to a months work.

Okay, I found a cite here. They claim a 28-second advantage with a downtube bottle vs. a bike with no bottles over a 40k TT, and a 2:19 advantage over a 112 mi Ironman stage, assuming the rider is a decent age-grouper.

I think that figure is a bit optimistic, for a start, there are very few amateurs indeed that can keep 30mph rolling for 40km, and quite a lot of pros, probably most, could not do it either.

Once you come down to more realistic speeds, around 27-28mph for a top class amateur or pro, then that figure will come down, and when you start getting to mere mortals, in the 23-25mph regions, it will reduce still further.

Getting down to more average riders, 21-23 then it will be quite a lot less.

I wonder if disc wheels change the effect of the water bottle.

Well, they’re not assuming you’re rolling at 30 mph for 40k, that would be good for a sub-50 min time. The 30 mph is what they used in the wind tunnel to collect their data, because that’s the industry standard and they wanted to make their numbers correlate directly with what everyone else uses. For translating that into race times, they’re assuming a rider that’s capable of a 225-watt average for the ITT and a 150-watt average for the Ironman. That gives a bike with no bottles a time of 1:07:26 for 40k (about a 22 mph average) and 5:56:48 for the 112mi (a little under 19 mph), which IMHO are pretty mid-pack times.

FWIW, when I raced triathlons I had a bottle that mounted in-between my aerobars. That is supposedly the best aerodynamic configuration, but I used it simply because I could drink without getting out of my tuck.

Now I’m getting depressed. On an average cross-country run I am lucky to average 16, 21 on the flat is my flat out max.

And a new bike is out of the question at the moment. I just hope I get something from my new SPD pedals and shoes.


Ah, I am referencing riders here who are club riders, usually fit and not the average man, when I said average riders, I meant average club riders, who are used to doing around 140 - 200 miles a week.