Dev brought up an interesting point in the natasha thread.
Why does Cervelo, in their inerrant, infinite wisdom, choose to use relatively high bottom brackets? Doesn’t this result, at least, in a slightly taller seat tube and therefore more drag?
I want a bike with a 100mm bottom bracket drop…not the 60 on Cervelo or 72 on Kuota! Its fine if you are a big guy but for smaller guys, we want to be as low to the ground as possible (without having to compress into a pretzel)…the bike is already too large a percentage of our overall drag
You’re operating under the assumption that moving the rider as a whole up and down relative to the bike (in the amounts we’re talking about here) makes an appreciable difference in the overall drag.
I’m not so sure that’s a valid assumption…and I recall Jens either intimating that, or giving some results to that effect…
Having a higher BB makes for less of a need for crazy small HT lengths to get enough drop in the aero bars. A byproduct of not using the superior 650c wheels in smaller sizes.
But I don’t really get it.
Put the BB high in the wind and then fold the rider into a pretzel to get the same amount of “aeroness”. Why not just drop the BB and if you really want to be low, use a zero rise (actually minus 17) or negative rise stem? There is no need to make the HT smaller than 90mm…just drop the BB and we’ll put on a negative ride stem if we want to really get what would effectively be a 50 mm head tube.
And as I said in the other thread, don’t worry, I won’t pedal through corners…but even with a 100mm BB drop, my pedal axles is still 27 cm down from the BB or 8 cm off the ground.
Dev
But then 650 wheels have less moment of inertia which has a speed penalty in time trial efforts…Cervelo probably did that math too.
But you make a good point, that is probably why they do it.
Also, they do have track versions of the frames (no RD or FD mounts). You need the high BB for the track version. Perhaps to eliminate the need for a different mold for a track bike?
I am just spitballing, but I would imagine that we are dancing around all of the reasons.
yeah, and really, does 10cm of thin, aero, cervelo seat tube add much drag? probably not.
it probably begins as a UCI-legal thing.
** 1.3.015 **The distance between the bottom bracket spindle and the ground shall be between 24 cm minimum
and maximum 30 cm.
then design considerations may come into play.
just a guess. I’m sure Damon could elaborate 
Carl
Let’s be clear, its not about the aeronesss of the actual bike. It is where the rider sits in the airflow.
Put me on the same hill and sit me up 4 cm higher or lower and I will go down that same hill faster when my body is 4 cm lower in the wind. This is why I want a 100 cm BB drop vs 60 cm (or 72 mm)
yeah, and really, does 10cm of thin, aero, cervelo seat tube add much drag? probably not.Sure about that?
I recall reading some guy for MIT saying that 10cm of aero bottle straw negated the benefit of a 404 Zipp on the front of your bike…
I recall reading some guy for MIT saying that 10cm of aero bottle straw negated the benefit of a 404 Zipp on the front of your bike…
I’d wager there is a difference between a round straw in the clean air at the front of the bike and an airfoil shape in the already dirty air between a rider’s legs…
Well now, I recall Gregclimbs doing some experimenting with a drop of position around a cm or three when he went to Vista cranks and not finding any difference in CdA, theres probably more details but I’m not sure where the information is around the internet. I’m on your side Dev, there is probably a difference, if not in the body position change; but also in the fact that you can fair more of the rear wheel by dropping the BB more.
I think the much bigger problem is that Cervelo, a long/low bike company, make their bikes Longer and lower with a 6cm drop. If cervelo made their bikes with 70-72cm bb drop, I would have hopped on one and seen what it was all about when I was last shopping around but instead I went with a Felt without even trying the Cervelo; I didn’t want two cm of spacers if I didn’t have to have them. To be fair, I’d be a 51cm, which the P2c is particularly short in as compared with the other P2c sizes.
Dev-
I could be wrong, but don’t you or didn’t you ride for kuota?
if you did, what happened/
if not, ignore my stupidity
24 cm BB off the ground is 110mm BB drop for a 700 bike…this leaves 5 cm of drop to be used by Cervelo, 4.2 for QuintanaRoo (68 mm drop) and 3.8 cm for Kuota. For some reason, they probably think that trigeeks are riding their bikes at Crits or UCI stage race TT’s where if you don’t pedal out of corners you lose previous seconds…the question is whether a rider 3.8 cm lower to the ground can take all those corners harder and faster simply by not pedalling and being closer to the ground, not to mention the savings of being 3.8 to 5 cm lower to the ground for straight line riding.
Seriously at least for 51 cm and smaller bikes the lower BB drop should be considered. I have half the mind to find me a custom builder to build me a double diamond round tube bike that is 3.8 cm closer to the ground and I bet I kick the ass of all the aero tricked out bikes on EVERY descent while coasting (even at 140 lbs) not to mention pushing less air on the flats.
Anyone know Natasha’s BB drop on that Cheetah. Aside from her being small and with a reasonably low front end, she looked really low from the ground when I saw her riding around in Kona 2006.
EDIT: I guess it was just her who is low…went to their website and on the 650 bike, the drop is 40 mm or 65 mm equivalent on a 700 bike!
Well now, I recall Gregclimbs doing some experimenting with a drop of position around a cm or three when he went to Vista cranks and not finding any difference in CdA, theres probably more details but I’m not sure where the information is around the internet.
I never “published” anything yet since the HSCycle cranks went in for service… I do intend to lrevisit it one more time when the weather allows…
That said, I notice a few years back when I built up my “mula de prueba” to match my hooker that the bb drop was DRAMATICALLY different. And then the P3c was like the hooker:
Wow, that is kinda a memory lane moment since all three of those bikes are gone as is the carpet in that room!
I just wish someone who knows a local framebuilder would have them tack up two frames identical except bb drop and headtube length and compare those…even a round tube to round tube comparison should show the difference if there were one…
g
Yes, I still do ride the Kalibur. One age grouper riding a bike in support of a company is not going to change the BB drop. Kuota has the best BB drop of the ones I have been on in the past 5 years (Guru, Kestrel, QR, Cervelo, Kuota)…I just want more!!!
gotcha, I thought you were more than an agegrouper. You also mentioned some points I have never considered.
thanks.
I’m sure we had a thread about this not too long ago, but I sought and did not find.
My thoughts have always been:
- don’t need to make HT so short - slightly easier for manufacturing
- easier to pedal through corners and the corollary which is more relevant to triathletes - less likelihood of pedal strike if you’re a goober who can’t corner.
- high COM means that COM travels a shorter path through the corner thus you maintain speed more easily.
Not sure why you bring up up 10cm of extra? seat tube length. Isn’t the seat tube and seat post length remain the same for a give rider, regardless of the height of the BB from the ground?
Keep the BB high, but the top tube at the same height and what you lose in seat tube height gets added in seat post length. As the seat post is nominally thinner than the seat tube, (it fits inside the seat tube afterall) it would be a gnats cock more aero (less drag)anyway.
Just my 3 cents (inflation remember)