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While watching a snow biking video, I saw that some of the bikes use a true swingarm design, which matches most motorcycles (as does much of bicycle technology). While this looks rather interesting, I'm wondering why it's so uncommon (and much more common are multi-linkage, seemingly more complex designs)?

I've found the bicycle he was riding, a Chumba F5, shown here:

Swingarm Design

Upon more searching, it's also used on Mountain Cycle Fury Frames and Motoped Kits as well, but an overwhelming majority do not use this design.

If I had to guess, I'd say it was solely weight concerns to not use this design with less moving parts. It also (seems to) allow for more travel, so is there perhaps an upper limit on how much travel you can get with the multi-linkage design?

Further, I've seen the Y-Frames (especially on BSOs) criticized for having poor flex characteristics. Does it again just come back to the weight issue in that more reinforcement is required for these designs?

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"Topologically" speaking, that design doesn't differ from the design used in about half the suspension bikes on the market. Eg: walmart.com/ip/… . The design simply compresses the mechanism into a smaller space, increasing stress and increasing expense. – Daniel R Hicks May 8 '12 at 12:00
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From my understanding, one of the major features of bicycle suspension is vertical travel. Having just a motorcycle-style swingarm means the wheel moves in an arc. I believe this has to do with efficiency for pedaling, but I'm not certain. – Jack M. May 8 '12 at 17:12
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What problem do you have to solve for which you need this question answered? – zenbike♦ May 8 '12 at 17:53
My next bicycle purchase is going to be a full-suspension one, which I have never owned before. (I usually buy the frame and build it from scratch). It is important in my decision process whether this is a design that is uncommon for a particular reason (i.e. too heavy, too expensive), whether it has certain tradeoffs that make it unpopular (i.e. more travel but insert some disadvantage), or is just new on the bicycle scene. – Ehryk May 8 '12 at 19:09

1 Answer

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This swing arm design has fallen into near complete disuse in the bicycle world for 2 reasons:

  1. First, there is translated suspension activation from pedaling. This design does not isolate pedaling or other rider generated forces from ground generated forces. Because of that, there is a loss of efficiency which makes it a poor choice for a bicycle suspension design.
  2. Second, the travel path of a single pivot swing arm carries the wheel forward as it moves through its travel path, because of the arc described by the wheel. If your rear brake is in use, the wheel is prevented from following that arc, and some or all of your suspension travel is lost until the brake is released. In a 4 bar linkage design, or similar vertical travel design, brake activation does not prevent suspension activation. Thus it has become the default design for high quality dual suspension bikes.
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I fully understand the pedaling power issue, and I also see why this is a non-issue with motorcycles since they don't use vertical motion to spin the rear wheel, and also why this design might then still be acceptable on a downhill/snow bike as seen in the video. Could you speculate (for my curiosity only) why the braking problem is perhaps a non-issue for motorcycles? I've yet to see even one motorcycle try to have anything but a swingarm design. I might postulate it causes problems for the chain length, and that perhaps at higher speeds it doesn't matter as much... – Ehryk May 9 '12 at 4:44
But certain motorcycles, such as small trials ones, are designed for about the same speeds as bicycles (sometimes even slower), and never even gave this design a second look (seemingly). If you don't care to guess, I'll understand. – Ehryk May 9 '12 at 4:45
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A motorcycle is powered by a motor, and thus has the torque to power through a brake lock if needed. The disc will slip before the suspension locks. Bicycle brakes are more powerful than our motors, and the traction to the trail, so it becomes an issue. Or so I understand. I'm hardly an engineer. – zenbike♦ May 9 '12 at 5:54
Necessarily, the wheel must move in an arc more or less centered on the crank, in order to maintain chain length. I just scanned through Google images of suspension bikes and I couldn't find a single one that didn't do this (though granted there are a few that are hard to make out). – Daniel R Hicks May 10 '12 at 3:38
Virtually all suspension frames are functionally like this one: sell.pakuya.com/upload/20110923/… The chain stay is pivoted near to the bottom bracket, and the wheel moves in an arc centered on that pivot. This is not, in terms of basic geometry and wheel movement, different from the "swingarm" bike above -- it's just the mechanism used to resist motion on the arm is different. – Daniel R Hicks May 10 '12 at 3:49
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