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Discussion Starter · #1 ·
Bike is more aero with a dead horizontal top tube. Almost all TT frames have a horizontal top tube. Yet, almost all so-called aero road frames have a sloping top tube. Why not make horizontal top tube for aero road frames? What's the purpose of sloping top tube anyway? so they can fit more riders of various heights?
 

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Discussion Starter · #3 ·
I read an argument where sloping top tube allows a greater fit of rider sizes, thus minimizing the number of different frame sizes manufacturers have to make, and that means more return on investment.

If a flexy seatpost is what they want for comfort, then they can certainly use a lesser stiff carbon layups, or even use space age silicone bushing on the top of the seatpost for the ultimate comfort eh.

But with all the emphasis on aero these days, I can't see that seatpost comfort takes a higher precedence than aero when we're dealing with an aero racing road bike, where its rider is willing to do anything for aero, comfort be dammed.

If this was for a roubaix or endurance bike, then I can maybe see a slight favor for seatpost comfort.
 

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Not entirely sure about your premise. Usually a horizontal top tubes means longer seat stays, which means more drag. I'm pretty sure that such a bike would have more drag in the wind tunnel riderless head on. It's anybody's guess how it would test with a rider and with cross wind.

Remember, the sloping top tube is leeward of the head tube and the down tube. the seat stays are leeward of nothing (riderless). With a rider they are leeward of the riders legs until the cross wind becomes extreme. Also, remember that added seat stay length is at a more acute angle to head wind than the top tube.

I'm sure designers have considered and tested all of this.
I read an argument where sloping top tube allows a greater fit of rider sizes, thus minimizing the number of different frame sizes manufacturers have to make, and that means more return on investment.

If a flexy seatpost is what they want for comfort, then they can certainly use a lesser stiff carbon layups, or even use space age silicone bushing on the top of the seatpost for the ultimate comfort eh.

But with all the emphasis on aero these days, I can't see that seatpost comfort takes a higher precedence than aero when we're dealing with an aero racing road bike, where its rider is willing to do anything for aero, comfort be dammed.

If this was for a roubaix or endurance bike, then I can maybe see a slight favor for seatpost comfort.
 

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...Usually a horizontal top tubes means longer seat stays, which means more drag...
In most TT bikes, the seat stays intersect the seat tube well below where the top tube intersects. A number of road bikes are also like this, though most are not. Where the top tube intersects does not absolutely dictate where the seat stays must intersect the seat tube. Agree that is very likely the designers have explored and tested these options and chose what was considered an optimized compromise between aero, ride and handling quality, fabrication cost, fit range, aesthetics, etc...
 

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I read an argument where sloping top tube allows a greater fit of rider sizes, thus minimizing the number of different frame sizes manufacturers have to make, and that means more return on investment.

If a flexy seatpost is what they want for comfort, then they can certainly use a lesser stiff carbon layups, or even use space age silicone bushing on the top of the seatpost for the ultimate comfort eh.

But with all the emphasis on aero these days, I can't see that seatpost comfort takes a higher precedence than aero when we're dealing with an aero racing road bike, where its rider is willing to do anything for aero, comfort be dammed.

If this was for a roubaix or endurance bike, then I can maybe see a slight favor for seatpost comfort.
I disagree with this.... I don't think the manufacturers are at all interested in building a bike that is "aero above all else, comfort be damned." These are still road bikes and they have to be ridden by the racers and the general public in a variety of situations. I think that a number of the manufacturers of the 1st generation aero bikes didn't build in enough "comfort" and they learned from that lesson. If you look at the send generation bikes... almost all of them have some sort of concession towards increasing comfort and thus increasing efficiency.
 

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Bike is more aero with a dead horizontal top tube. Almost all TT frames have a horizontal top tube. Yet, almost all so-called aero road frames have a sloping top tube. Why not make horizontal top tube for aero road frames?
Bike companies would make less money.

What's the purpose of sloping top tube anyway?
It lets them sell fewer sizes without stand-over problems, allows longer head-tubes which fit more people given carbon steerer spacer limits, and allows shallower stem angles with fewer spacers for better aesthetics.
 

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Bike companies would make less money.



It lets them sell fewer sizes without stand-over problems, allows longer head-tubes which fit more people given carbon steerer spacer limits, and allows shallower stem angles with fewer spacers for better aesthetics.
Any guesses why Colnago offers 9 sloping sizes without what I'd consider long headtubes for their C60?
 

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Not sure that there is any evidence that sizing options decreased in adult mens or womens road bikes when sloping top tubes became popular
Bike companies would make less money.



It lets them sell fewer sizes without stand-over problems, allows longer head-tubes which fit more people given carbon steerer spacer limits, and allows shallower stem angles with fewer spacers for better aesthetics.
 

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Discussion Starter · #12 ·
Any guesses why Colnago offers 9 sloping sizes without what I'd consider long headtubes for their C60?
Seems like Italians, i.e. both Colnago and Pinarello, like to offer every sizes imaginable to consumers, which is a great thing. Pinarello is known for having a size frame to fit anyone and everyone as they offer sizes in 42, 44, 46.5, 50, 51.5, 53, 54, 55, 56, 57.5, 59.5, 62? That's as customized as they go for a big manufacturer. But for most other manufacturers, you'd get something like 49, 52, 54, 56,..

Props for Pina for offering all sizes across the spectrum. I think that's more of their philosophy of offering frame that has to fit like a taylor suit for their customers. Again props to them. However, some of these smaller sizes like 42 and 44 are hard to get. I know 2 girls in my club who ordered the 44 and both had to wait a few months to get the frames as nobody had it in stock.
 

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Discussion Starter · #13 · (Edited)
Not sure that there is any evidence that sizing options decreased in adult mens or womens road bikes when sloping top tubes became popular
sloping may not decrease the size offering, but what sloping did was made the current sizes fit better for more people. Whereas before people who were in between sizes had to just live with the next size up or next size down, but sloping, they now could fit better without making the whole bike look out of whack

I also heard the argument that with sloping top tube, the frame is now compact, and so it would be stiffer, and this was probably important in the early days of carbon fiber frames where stiffness was an issue. Today, stiffness is no longer an issue for cf frames.
 

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Discussion Starter · #14 ·
Bike companies would make less money.



It lets them sell fewer sizes without stand-over problems, allows longer head-tubes which fit more people given carbon steerer spacer limits, and allows shallower stem angles with fewer spacers for better aesthetics.
Aesthetics is one thing that a sloping top tube has definitely improved for the smaller frame sizes.
 

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Discussion Starter · #15 ·
Cervelo did studies to show that a sloping top tube has very little effect on aero.
I haven't seen what their studies are, and i'm not disputing their studies. But then why are virtually all TT bikes have horizontal top tubes? Is this a mere coincidence that have little to do with aero, when all these TT bikes are wind-tunnel tested?

Cervelo's own S5 and P5 have dramatically different top tube angles. Little aero effect here too?

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I found the quote from Damon Rinard, who at the time was the Cervelo "Spokes engineer"........

Conveniently, we tested level and sloping top tubes in the wind tunnel. (Phil insists on data for nearly every level of design decision.)
Result: With the tube shapes we use, we could find no significant difference.

Why? A level top tube can have less drag *on the top tube*. But a sloping top tube "moves" the seat cluster down, eliminating some seat tube below and substituting an equivalent length of (slimmer) seat post above. Sloping the top tube changes drag on other parts of the bike.

There is less difference with a sloping top tube now, than we used to think back then.

So with aero a wash, we went with sloping for all the practical reasons most road bikes have sloping top tubes these days.
Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Since 2015
 

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Any guesses why Colnago offers 9 sloping sizes without what I'd consider long headtubes for their C60?
Bikes design is as much about satisfying marketing requirements as engineering ones.

I'd hope their VP of product or product manager determined that combination makes more money in their niche market, although it could just be sloping top tubes are modern, "quality bike companies make frames in 2cm increments, and tall head tubes aren't racy.
 

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Discussion Starter · #20 · (Edited)
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Color aside, which one looks better? Which one is more aero?
If the Look is more aero, I'm betting it'd be because of its more integrated headseat and stem setup (that is slick! but of course also highly proprietary parts).
 

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