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Back in the time... bikes did not have (rear) suspension and the most common frame design was the "diamond frame", with straight pipes. Today, mostly in full suspension bikes, there is mostly a bend in the down tube just in front of the bottom bracket. What for?

In some cases I can understand the reasons:

  • for frame layouts where the rear shock is connected at that point,
  • or where the rear shock needs that space during compression,
  • if that's the only space to accommodate for a bottle holder

but it seems so prevalent in all frames...

Can anyone point out a reason? Does it have a technical advantage (which one?), or is it just for the looks?

Bike with kink in down tube

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    I think in order to answer this question the entire bike design needs to be explained. You can't just isolate the downtube shape and pretend the rest of the bike is irellevant. For example: why is the crankset positioned like that? what adjustments to the geometry have been made to compensate for that huge fork height? where should the rider's weight be in order for the shock absorber to perform best? etc. I think there are multiple reasons for that downtube shape. We could perhaps compare it to another similar bike that has straight tubing and do some reverse engineering and observations.
    – Robert
    Jun 25, 2023 at 21:55

3 Answers 3

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This downtube shape provides extra space in the front triangle. For full suspension bikes, it's often required to fit the rear suspension and its range of motion.

While this particular design could maybe only-just-fit with a straight tube, many designs have it take up more space within the front triangle, and wouldn't work with a straight downtube.

Also, forget attaching a water bottle to the frame if the downtube was straight.

enter image description here

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    It doesn't help that top tube heights keep getting reduced (to provide more standover clearance), resulting in even less space in the front triangle. Good answer.
    – MaplePanda
    Jun 27, 2023 at 20:21
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The following are some excerpts from an internet search:

--"Some modern MTBs come with a curved downtube. The purpose of this engineering is to create clearance for long travel suspension forks.

When the fork compresses, the front tire gets closer to the frame. If there isn’t enough clearance, the wheel may get in contact with the frame."

--This comment from https://www.mtbr.com/threads/down-tube-shaping-in-last-few-years.790823/ initially refers to the curve of the down tube at the headtube junction but goes on to mention the bottom bracket junction curve. In between are reasons given that apply to both areas of curvature:

"The hydroformed tube shape creates a structural advantage at perhaps the most vulnerable part of the frame. When obstacles are encountered, the bottom of the head tube is driven back into the down tube. The more perpendicular these tubes are to each other at their intersection, the stronger the structure. The tube shape allows for a more perpendicular intersection and for much more of the downtube to absorb the force, instead of it being concentrated in one area (right where it meets the head tube). This eliminates the need for gussets at the down/head tube junction and saves weight.

All the frame manufacturers who use hydroformed tubes are doing that, often with a similar shape at the other end of the downtube where it meets the BB. Makes for a stronger/lighter frame by allowing the entire downtube to absorb the major forces."

--The one explanation I found to be the best and makes the most sense comes from our very own Stack Exchange. Check out: S shaped down tube (MTB).

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It's a manufacturing simplification. Here's a traditional BB shell:

enter image description here
From https://www.mtbr.com/threads/bottom-bracket-cluster-weld-sequence.843059/

Notice the weld that runs over the top - this is some difficult geometry to get right, and to get welded properly. Similar problem for someone brazing in that there's a lot of angles here. By simply having one tube to join to another tube, the intersections get easier.

By having the downtube leave the BB shell straight forward, there is no overlap between tubes and it's an easier weld especially for an automated welder.

Any other advantages are sales and marketting.

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    Wouldn't the same reasoning apply to pretty much any bike frame? Why generally only full suspension MTBs? Jun 25, 2023 at 19:38
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    Also, does this apply to carbon layup in the same way as it does to welded joints? Jun 25, 2023 at 19:44
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    With down tube diameters being as big as they are, is a 90° angle between seattube and dowwntube even enough to have independent welds?
    – Michael
    Jun 25, 2023 at 19:58
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    While I agree with this part being difficult to weld, the bike that is shown as an example isn't welded, but has a carbon monocoque frame (search for "RALLON M-LTD 2023")
    – sarnu
    Jun 26, 2023 at 6:55
  • @sarnu I was unaware - thank you. Image has been replaced.
    – Criggie
    Jun 26, 2023 at 9:43

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