Carbon Down Tube Shapes (Custom versus non custom)

[nmrt]

So, it is around 12 am here and I cannot sleep because I cant figure this out:
How come most custom carbon bike builders (Crumpton, Alchemy, Argonaut, Parlee come to mind) have their down tubes in the ROUND shape? And why do most off the shelf manufacturers (Cervelo, Specialized, Focus, De Rosa, Pinarello come to mind) have their down tube in "NOT ROUND" shapes?

Is it because of tube to tube construction for the custom builders and the monocoque construction for the off-the-shelf companies? :confused:

[rnhood]

Yes, monocoque construction offers more opportunities to construct desired characteristics into the frame with less compromises, including cost of the model line. It affects all tubes, not just the down tube. If the custom builders started speccing shaped tubes their construction costs would escalate above their already high cost.

Having said this, we are already seeing some tube shaping from the custom builders like Parlee and their new aero ESX frame. Serotta led this effort when still in business with the aero Meivici frame. But these frames cost a lot and its debatable if they offer any advantage of monocoque construction.

[Black Dog]

The round shape is still inherently good and the layup of the carbon can be altered to give a round rude a wide range of characteristics. Most of the exotic tube shapes have more to do with marketing than performance.

[mike p]

$$$$$

Mike

[lhuerta]

Most of the exotic tube shapes have more to do with marketing than performance.

Oye!...if this was true we would all still be riding round tube bikes. The ability to shape a tube beyond round (be it steel, alum or carbon) as well as experiment with wall thickness, provides great advantages in tuning a bikes riding characteristics. Lou

[happycampyer]

The round shape is still inherently good and the layup of the carbon can be altered to give a round rude a wide range of characteristics. Most of the exotic tube shapes have more to do with marketing than performance.This is what I understand from conversations with Bob Parlee and others. The carbon layup of a round tube can be varied to produce a wide variety of characteristics, in a way that metal can't. In speaking with someone from Serotta not long ago, I was told that the tapered tubes on the original Meivici had more to do with maintaining the aesthetic of the Colorado Concept tubing that Serotta was famous for—perhaps Mike Lopez will chime in.

Shaped tubing allows a manufacturer to create a frame that is immediately distinctive visually—think of Pinarello. Notice, too, that as manufacturers look to shave weight, the rounder the tubes become... Of course, aero shapes are another story.

[oldpotatoe]

This is what I understand from conversations with Bob Parlee and others. The carbon layup of a round tube can be varied to produce a wide variety of characteristics, in a way that metal can't. In speaking with someone from Serotta not long ago, I was told that the tapered tubes on the original Meivici had more to do with maintaining the aesthetic of the Colorado Concept tubing that Serotta was famous for—perhaps Mike Lopez will chime in.

Shaped tubing allows a manufacturer to create a frame that is immediately distinctive visually—think of Pinarello. Notice, too, that as manufacturers look to shave weight, the rounder the tubes become... Of course, aero shapes are another story.

It's so they can have BIG decals!!

[Black Dog]

Oye!...if this was true we would all still be riding round tube bikes. The ability to shape a tube beyond round (be it steel, alum or carbon) as well as experiment with wall thickness, provides great advantages in tuning a bikes riding characteristics. Lou

Yes, very true for metallic tubes but less so for carbon where the material properties can be altered along the length of the tube. I did not say that shaping has no effect, just that most of it in carbon bikes is for marketing not performance (Aero tubes aside). I do think that tube shaping is good when done for the sake of performance.

[happycampyer]

It's so they can have BIG decals!!Of course, that too!

[kgreene10]

Bi-ovalized down tubes were all the rage for a while (and perhaps still are). The idea behind getting more torsional stiffness at the HT and BB junctions seemed sensible to me. I know next to nothing about this stuff, but the selling point was that you can't get the same characteristics out of a round tube regardless of layup.

[nmrt]

Bi-ovalized down tubes were all the rage for a while (and perhaps still are). The idea behind getting more torsional stiffness at the HT and BB junctions seemed sensible to me. I know next to nothing about this stuff, but the selling point was that you can't get the same characteristics out of a round tube regardless of layup.

This is the stuff that actually kept me awake at 12 am.
Really...? Are custom bike builder's round down tube shapes less stiff than the wierdo shaped down tubes from off shelf manufacturers. And I do not ask this as a theoretical question. I mean is their really a perceptible and a quantifiable difference for people that would actually matter in races/sprints/crits.

[bluesea]

Talk to DK about round tubes. ;)

[nmrt]

Talk to DK about round tubes. ;)

If I remember correctly there has been a lot said about round vs non round for metallic bikes.

But you know what the funny thing is -- Litespeed make a non round down tube for their Ti bikes!

[bluesea]

If I remember correctly there has been a lot said about round vs non round for metallic bikes.

But you know what the funny thing is -- Litespeed make a non round down tube for their Ti bikes!


They took it so far as to turn non-round into a gimmick. And I believe Serotta may have been the first.

[kgreene10]

Bi-ovalized DT is (or at least was, as of a couple years ago) one of Firefly's (many) key attributes.

[Black Dog]

Bi-ovalised tubes were 1st needed to mate a larger diameter tube (=stiffer) to the smaller diameter bb shells and head tubes. IIRC.

[Black Dog]

They took it so far as to turn non-round into a gimmick. And I believe Serotta may have been the first.

The latest generation if litespeed's had the tube shapes determined by finite analysis at JPL (Jet Propulsion Laboratories; the people who design space craft for NASA). I do not think that this is a just a gimmick.

[bluesea]

Sorry, by Litespeed I threw in Lynskey as well.

[sg8357]

Sorry, by Litespeed I threw in Lynskey as well.

Lynskey is inspired by the late great Indian Larry, not JPL.

[nicrump]

Most Crumpton's shipping today have a Bi-oval DT and an oval-oval tapered TT.

I started with round straight tubes because of ease of obtaining custom layups from tube makers utilizing their stock mandrels in whats called "roll wrapping".

While I think round is still boss when I chose to move tube production into my own shop I decided on some ovalization for a few reasons. The in house process of bladder molding tubes allowed this. The primary driver was the advantage of a torsion lock from tube to tube with the wrap over an oval vs. a round. Second reason was the ability to work with desired overall diameters while maintaining tube to tube diameter(X axis) matching. Lastly the form which followed. I like it.

[Mark McM]

Bi-ovalized down tubes were all the rage for a while (and perhaps still are). The idea behind getting more torsional stiffness at the HT and BB junctions seemed sensible to me.

Actually, ovalizing increases bending stiffness, at the cost of decreasing torsional stiffness (compared to a round tube of the same circumference).

The only way ovalizing can increase the total stiffness of a down tube is if the down tube is much larger in diameter than the head tube and BB shell, requiring it to be ovaiized at the ends to mate with the adjacent tubes - but in this case, the extra torsional stiffness is gained in the middle of the tube, not at the joints.

[Mike Lopez]

Shaped tubes do act differently than round tubes. Even with the ability to vary the lay-up, wall thickness, and fiber properties of a composite tube the shape is important. As you all know the shape of a part helps determine its mechanical characteristics. That's why a 2 x 4 is stiffer/stronger if turned in its upright direction rather than the flatter direction. This is called moment of inertia and it's a very important property in statics & strength of materials.

Round tubes work great too and can be made to achieve desired characteristics in terms of bending & torsion quite well. Think golf shafts...these are probably some of the finest examples of "well tuned tubes" and are made to very specific mechanical properties to complement the club head speed and swing style of the end user. In this case the PGA dictates that the shafts are generally round and structurally symmetrical around their long axis.

In the case of a bike tube sometimes the designers like to be able to tune the tube to be structurally asymmetrical. Ever heard the term "laterally stiff but vertically compliant" ? Some of this can be done with lay-up alone but it sure helps if you can tweak the shape as well. Regardless of how we may lay up a round tube it's harder to have unique properties in two different planes if the moment of inertia is the same in both planes. Let's use a top tube as an example. If we flatten/ovalize the tube we can de-tune it's moment of intertia in the vertical plane more than we could do by changing the lay-up alone. Then, if we've gone too far, we can tune it back up by adjusting the lay-up. This gives us a much wider range of adjustment than can be done on a simple round tube. And..as my pal Nick implied..this is much easier to achieve using a molding process rather than a roll wrapping process...

All that said. A little bit of science has been blown way out of proportion in many cases but hey...More is better right? ;) Past a certain point it's like the old veggie guys says. A bigger billboard for the stickers!