I’m not starting a new general discussion about aerodynamics and how much, but I have looked at the downtube. Many companies makes the downtube very sleek and aerodynamic, but I wonder if this makes any sense:
The top part of the downtube is hidden by the wider hed tube and fork crown.
A wide (standard) bottle mounted on the middle part of the tube.
The lower part of the downtube partly shelters the bb and lower part of seat tube.
Of course, without a bottle I can see the sense in the sleek tube, but with the bottle mounted on the downtube - does an aero downtube matter?
and lets not forget - there are two (relatively) huge great piston like objects moving up and down on either side of the tube, totally destroying the kind of laminar flow around it that you might see in a wind tunnel.
and lets not forget - there are two (relatively) huge great piston like objects moving up and down on either side of the tube, totally destroying the kind of laminar flow around it that you might see in a wind tunnel.
Umm…yeah, it’s not like anyone has actually tested a bike in a wind tunnel with a pedaling rider aboard……
I’m not starting a new general discussion about aerodynamics and how much, but I have looked at the downtube. Many companies makes the downtube very sleek and aerodynamic, but I wonder if this makes any sense:
The top part of the downtube is hidden by the wider hed tube and fork crown.
A wide (standard) bottle mounted on the middle part of the tube.
The lower part of the downtube partly shelters the bb and lower part of seat tube.
Of course, without a bottle I can see the sense in the sleek tube, but with the bottle mounted on the downtube - does an aero downtube matter?
Please define what you mean by “matter”…
Also, don’t forget to consider what happens when the airflow isn’t coming at the bike from straight ahead…
did i suggest that they only test without a rider? the biggest changes seen in wind tunnels when comparing round tubes with aero frames always come from tests done without riders. put the rider on, and the benefits come more from correct rider positioning than aero tubing. the two are not mutually exclusive, but one is a much bigger effect than the other.
did i suggest that they only test without a rider? the biggest changes seen in wind tunnels when comparing round tubes with aero frames always come from tests done without riders. put the rider on, and the benefits come more from correct rider positioning than aero tubing. the two are not mutually exclusive, but one is a much bigger effect than the other.
No…but you did imply that the legs of a rider negates any aerodynamic properties of the downtube.
Also, when comparing 2 different frames with a rider, doesn’t it make sense to use the same rider position on each frame? (which is what is typically done). How does one get a “benefit” from the rider position in that case? Isn’t any difference seen attributable to the frame differences, if that’s the only change made?
BTW, I know Andy Coggan has mentioned this before, but testing by Chet Kyle has shown VERY good correlation between testing frames without a rider and with…I believe Gerard may have mentioned something along these lines from their testing as well. That doesn’t mean there’s no reason to test with a rider, since there’s always the exceptions to watch out for…such as the old Trimble frame that tested great without a rider, but horrible with a rider.
Nobody is arguing that rider position doesn’t have a larger effect on overall drag…but the OP asked about whether or not an aerodynamically shaped downtube “matters”…well, as I pointed out to the OP, that kind of depends on your definition of “matters”, doesn’t it?
A full aero frame is supposed to give about 45-60 seconds over 40 kms. But there is aero tubing and there is aero tubing. Some “aero tubes” aren’t all that aero, including some found on some very expensive carbon tri bikes. AFAIK, out of the current mass produced bikes, Cervelo comes the closest to the “ideal” 3:1 aspect ratio, around 2.8:1 or so.
It’s my understanding, that a water bottle on a round downtube makes it a bit more aero with a sort of “sail effect” but has an opposite effect on an aero downtube?
Small round tubing is faster than big ovalized tubing btw.
Also, when comparing 2 different frames with a rider, doesn’t it make sense to use the same rider position on each frame? (which is what is typically done). How does one get a “benefit” from the rider position in that case? Isn’t any difference seen attributable to the frame differences, if that’s the only change made?
Of course. But without building special purpose test frames, thats a bit of a difficult test to do. I don’t know of too many setups that have identical rider position where all that changes is the frame tubing. Its probably been done, but I can’t think of any two comparable frames that would be trivial to do in this way.
Also, when comparing 2 different frames with a rider, doesn’t it make sense to use the same rider position on each frame? (which is what is typically done). How does one get a “benefit” from the rider position in that case? Isn’t any difference seen attributable to the frame differences, if that’s the only change made?
Of course. But without building special purpose test frames, thats a bit of a difficult test to do. I don’t know of too many setups that have identical rider position where all that changes is the frame tubing. Its probably been done, but I can’t think of any two comparable frames that would be trivial to do in this way.
All you need to do is recreate the “touch points” in space (i.e. saddle and bars) vs. the BB…for this, an ergostem is your friend
Small round tubing is faster than big ovalized tubing btw.
And there it is! Many of the old school round tubed steel bikes are still more aerodynamic thad the carbon wonders are today. Of course they were heavy and didnot look as cool. But facts are stubborn things.
The carbon bikes that are slower tend to have fat tubes. The current best C tribikes are more aero than round tube steel bikes. I am amazed at how fat the current head and downtubes are getting on many road bikes.
