EveryManALion

SnapWidget | Today was gee ? #mycritical like the bike on the left, why is the bottom tube on the left bike thicker than the top tube, what does that accomplish on a chro-moly frame? does it maker it strong or it is only for looks? I have seen a few bikes like that and I have no idea why or what that does or benefits?


or like this, https://www.yesbikes.com.au/media/cat...akeless4_1.jpg

Why?

Scrodzilla

Some girls' mothers are bigger than other girls' mothers.

CliffordK

You could always go a bit minimalistic with your design.

SLINGSHOT news


All Portraits Photo Roundup | G & O Family Cyclery

TejanoTrackie 

By thicker I think you mean larger diameter. Anyway, the downtube on any bicycle frame carries the lion's share of the load and by making the tube larger you greatly increase its stiffness and strength. The top tube can be much smaller w/o affecting the frame's stiffness or strength.

CliffordK

My guess is there are a number of issues leading to the greater utilization of oversized tubing in bicycle frame manufacturing.

Part of it would be the evaluation of stresses in the frame.

As evident from the slingshot frames above, for the most part with general riding, there is downward force on the two wheels and a stretching or tension on the downtube... EXCEPT WHEN THERE ISN'T.

Browse around used bikes or frames enough, or the Wacky Thread long enough and you'll see a typical crimp in the top tube and downtube, just behind the head tube from a frontal impact.

For a road bike, these front impacts may be a fairly rare condition. For a MTB, it may be part of normal every day riding. Nonetheless, it is not fun to trash one's frame.

The slingshot MTB may be odd riding as it bumps over large limbs or over curbs.

The other issue that comes up is FLEX.

I think a lot of the flex issues are in the bottom bracket. One of the more recent innovations is to move from a T-shaped connection to the bottom bracket shell to a full width connection to the bottom bracket shell. Thus giving maximum support to the BB.

Many of the ovalized oversized tubes have a reverse ovalization at the point where they are welded to the bottom bracket for a wider weld point, or perhaps as in the case of the black bike posted by the OP, the downtube is mitered and welded to the bottom bracket shell and the seatube.

Ovalizing can also help with welding a tube bigger than the head tube to the head tube.

Around 2000, there were several aluminum frames made with teardrop tubes. Perhaps only a short-term fad, but the idea of teardrop tubes will return periodically. Plus, ovalized or teardrop tubes may be more aerodynamic for a downtube being exposed to frontal wind. The top tube isn't exposed to the same frontal wind as much.

Different parts of the frame may be subjected different stresses. The front part of the downtube likely is mainly subjected to a stress parallel to the frame axis. On the other hand, the bottom bracket would be subjected to stresses perpendicular to the frame.

You might request this be moved to the FRAMEBUILDER'S SUBFORUM for more technical insight.

EveryManALion

You rule for typing this out thank you

Regulatori

While we're on a tubing topic, what is everyone's thoughts on a wishbone rear compared to a standard setup? Back in my BMX days I had a couple of bikes with wishbone on the upper rear triangle...always thought it looked awesome. I've seen a few higher end road bike with a similar setup. Is there any disadvantage to using a thicker single tube above the brake bridge as opposed to the standard two tubes?

AlmostTrick

So there's more room for stickers.

Cynikal 

This is always the best answer.

The real answer is that it add stiffness to the bottom bracket area.

CliffordK

Attaching a bike rack to the two seatstays seems to give a good solid connection. A monostay might require attaching it to the brake mounting hole, giving one only a 3 point connection instead of 4.

bbattle

Cervelo started out with very large downtubes in an airfoil shape for strength and aerodynamics. They've since scaled it back as the Kamm style foil (truncated airfoil) has become popular. Cervelo bikes have very skinny seatstays; the company says they aren't required for frame strength, just to satisfy UCI requirements.


Klein had some radical bikes with only a downtube and it was huge.



The monostay is supposedly stronger than the "fastback" or "side tack" stays but I like the look of the fastback stays myself.



Cinelli came up with this style, also seen on 3Rensho


And there's the wrapped seatstay

LesterOfPuppets

That's way too high to be a downtube, I think downtubes have to touch the bottombracket by law.

This is closer but still a top tube, but they call it a boom tube.