Austinite

Well, it's only been 4 years since I bought my current Aluminum-frame road bike and while I was poking around at some of the latest 2010 full-carbon bikes, I noticed something that makes no sense to me at all. Some of these bikes have super aero wheels that probably cost more *per wheel* than my whole bike but they have HUGE FAT DOWNTUBES. I swear I saw one that must've been 4 or 5 inches wide. Not deep, WIDE. Why do folks bother getting aero wheels and light frame and then ride around with a wall on the front of their bike like that?

Fashion?

thegunner

that and the TT would the the primary points of flex across the front of a bike. fatten up the DT and you increase power transfer. there's a trade off between aero and efficiency you know...

carpediemracing 

Someone at Interbike jokingly asked if maybe the wheel folks (or aero folks) and certain frame designers maybe missed some memos. As the OP pointed out you have these super aero rims just in front of big fat wide square etc etc unaero looking tubes.

I'm not an engineer, nor do I understand the intricacies of carbon fiber, but I hope there's a functional reason for such frames. My guess (and just a guess) is that it has to do with "laterally stiff, vertically compliant" phrase that seems so over used.

Personally I'd be curious to see how a twin downtube bike would work. This would get the width without building an aero wall. Kind of like the bi-titan etc by Colnago, but one that didn't crack after a few weeks. You could envelope part of the front wheel, have aero struts to tie the two sides together, and have a lot of vertical flexibility on an extremely wide frame (figure minimum 68 mm wide, max would be 90-100 mm, based on BB width).

Of course there's the problem of having 2 extra tube sides, more joinery, etc etc. But, hey, nothing's for free.

I have a downtube that's oval vertically at the head tube and oval horizontally at the BB. It's pretty narrow up top (compared to my SystemSix especially). I "feel" more aero riding the bike. And it's plenty stiff for me, and I really hate a frame that is marginally noodly.

cdr

thegunner

if you pair two tubes on the dt, yes you lose the 'giant wall effect' but the sheer force is also effectively doubled for the inner lining. couple that with increased turbulence of wind flow behind the tubes and i'm not sure you're gaining very much

Reynolds 

Aero wheels and fat downtubes provide space for big advertising.

patentcad

..

carpediemracing 

ah thanks.

Staggered tubes? Like that frame with the staggered seatstay attach points?

No, I must be bonking or something.

If I could weld and had access to odd shapes of aluminum tubing, I'd have all sorts of failed frames laying around.

cdr

tj90

Well it could be like my stupid looking XRL scattante. From the side, the DT and TT tube is big enough for a billboard and the advertising. The top and bottom are sculpted like a knife edge ready to bust your balls in an accident. I guess the tube is more aero than the other way, but I dont think this frame is going to win any stiffness contests...

DubT

The big fat down tube on the Trek Madone is the reason that I bought the Trek 9.5 Equinox. I am using the Equinox as a road bike and am very happy with it. I no longer race, only ride for pleasure and fitness.

I feel the same way as Austinite. Why have aero wheels and an extremely large diameter down tube, the Equinox with the seatpost reversed provides "almost" the same geometry as my old Bianchi and has a very rigid bottom bracket, no flex when standing.

Wayne

thegunner

the thing is, when you raise the profile of your rims on a wheel, you're basically creating a laminar flow profile for the wind... in effect cutting through the wind. no eddys form behind the wheel on rotation like with box section rims (this effect is even more amplified when you consider the wind profile has a rotational velocity profile as well as one from the front to the back of the bike). with the DT, it's stationary, the effect isn't nearly as great as that of the wheel...

DaveSSS 

The BB is 68mm wide, so that limits the DT width to 68mm or 2.68 inches. Big DTs have been around for a long time.
Even an old C'dale 2.8 from '92 has a relatively fat DT.

Jinker

Have you ever noticed that the trailing edge of your wheel is always a circular section? Yeah. Your tire.

Also, laminar flow has a specific definition. 'Laminar flow' wings on airplanes (which are substantially more aerodynamic than wheels wrapped in gritty bike tires) only achieve laminar flow over a portion of their chord.

Saying that 'no eddys form behind the wheel on rotation' indicates a hook-line-and-sinker swallowing of marketingspeak, and has little to do with reality.

Yes, aero rims can create less turbulent flow than box section rims. But don't overstate things here. On a bike, the first things to encounter the air matter the most. An aero front wheel, a skinsuit, an aero helmet. The bike frame is contributing a small, SMALL portion of the drag of the system.

coasting 

the really fat down tube just doesn't look very nice.

thegunner

i certainly wouldn't say that it's over statement, if you simply think of any fluid flowing around a rounded block with square tapered edges, vs. a rounded block ending in a teardrop shape, it seems reasonable that the turbulence behind that initial leading edge of a wheel is minimized. if you think i'm spewing nonsense because of marketing, that'd be an insult to my academic background... i'm a graduate student in chemical engineering, and for the past 2 years my research has been on flow profiles around... no lie... pendant drops of lipid particles in various solvents. << okay so that's not exactly the same

i think you also completely misread my post, i was implying the bike frame's DT has a negligible impact on overall drag... why are we arguing again?

Banzai

So it goes with most technical "truths" around here.

Best part is how genuinely angry some people get defending those "truths". Like it's a faith of some form...or a deep seated knowledge that they've been had, but want desperately to never confront that fact.

Bob Dopolina 

A larger diameter tube allows you to use thinner tubes but maintain the same strength. This means a lighter tube with the same strength. Cannondale used to use the formula for this in their advertising some time in the early 90's.

It's about weight, folks.

carpediemracing 

Nowadays, with integrated external bearing BBs, you can get wider - 90mm or so (Trek's "BB90" was one of the first ones). There are some other BB numbers being thrown around, with all but Look's BB65 having to do with the width of the shell, not the diameter of the BB spindle. I don't necessarily agree with Trek's BB shell idea but I have to admit that you can make your downtube wider than you could on a 68 mm BB shell.

cdr

thegunner

or maybe it's an understanding through other facets of engineering, you know... a lot of people here jump to conclusions about other people's knowledge without first considering the possibility that it might actually be their forte..

tadawdy

Doesn't the benefit of an aero wheel have to do with the fact that it's rotating, as well as translating? The so-called "egg beater" effect of spokes? If so, having a fat downtube doesn't negate the benefits of an aero wheel.

dizzy101

On small frames, yes. On large bike frames, small tubes don't look good

midgetmaestro 

hmmmm I disagree... scattante.jpg

But it is a matter of personal taste.

Reynolds 

^ Now that's a non - conventional backdrop!

DArthurBrown

It could also be that the differences in the downtube sizes, shapes and colors mentioned have negligible effect on performance, but might sell more bikes by convincing folks that they have a measurable effect on performance.

Dick Rhee

I am currently taking Fluid Mechanics, so it's interesting to hear people's take on what would matter most in terms of aerodynamics. Unfortunately, the class is very calculation-based, so I feel as though I have as much practical knowledge of fluids and aerodynamics as I did after taking freshmen physics. I would like this class a lot more it had a lab section.

Anyway, I agree that the downtube is placed in at least somewhat turbulant air, so it shouldn't matter as much as the front wheel (and neither even remotely as important as the rider's position/frontal profile). An oval DT (wider than taller) should help prevent pedal flex while maintaining vertical flexibilty flex for comfort. The OS tube shapes allow for increased stiffness while using less material. Therefore, OS tubes allow for a lighter frame that is still rideable.

dmp

That's either the biggest dog bed or the smallest bicycle I've ever seen!