Are 4 arm cranks the future for road bikes?

What are the advantages?

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    – andy256
    Sep 16 '14 at 1:02
  • 1
    My view is that since Campag and Shimano have both done it, and they make the groupset more flexible, yes, they're here to stay.
    – andy256
    Sep 16 '14 at 1:03
  • 2
    Advantages? It's something to sell. Sep 16 '14 at 3:46
  • 1
    I'm not sure a flexible groupset is something I'd be looking for.
    – Holloway
    Sep 16 '14 at 7:57
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    I can't think of one benefit for the rider. Some for the manufacturer who locks you in to using proprietary chainrings. Sep 16 '14 at 11:57

Predominately a weight thing. The Shimano marketing spiel / hyperbole:

With a unique four-arm spider and Hollowtech II construction putting strength just where its needed, the FC-9000 chainset sets new standards for stiffness and low weight.


^^^ Shiny :)

That and because I think it looks distinctive and is seen as innovation. Some people don't like the look, but weight weenies will snap this up.

On the web I found the following comparison: (weights are combined chainset and bottom bracket)

DA-7900 725g  <-- previous Dura Ace version
DA-9000 683g  <-- latest

That's a whopping 42g, or 5.8%. I'd buy DA if I could afford it, but not for this reason. I'd be better off eating fewer pies.

As with other Shimano releases, the technology has already trickled down to Ultegra and 105.

I guess if you don't need 5 arms, why wouldn't you just have 4?

Shimano is just an example. As andy256 commented above, Campy also has this and maybe others.

  • Any reason the Shimano road crank spider looks so different from a mountain spider?
    – Aaron
    Sep 16 '14 at 13:11
  • Probably for marketing/aesthetics (though not to everyone's tastes!). I think perhaps they have moved some of the metal towards the crank to stiffen that area - the weight difference between 4 and 5 arm DA doesn't seem that much. It is quite interesting that MTB's have had 4 arm spiders for quite some time. I'm not sure if there's any reason why. Another question is why do road bikes often have a bold circle diameter (BCD) of 130 or 135, whilst MTB's often have 104 or 110? I guess because road chainrings are usually bigger...
    – adey_888
    Sep 18 '14 at 8:25

I guess Campy is just copying Shimano while it cannot afford proprietary innovations anymore.

Road groups always used to have 5 arm cranks since the 70s or so. Early MTB too but then in 110mm BCD (now compact road). Since MTB rings are smaller, it made sense to move to 104mm 4 arm spider, while larger road rings would be less stiff on smaller BCD. For compact road 50T rings smaller BCD makes sense and is needed for 34T inner rings. I guess aesthetics, need for granny (inner) ring and distance from middle line of frame are main reasons to have separate MTB and road compact crank designs.

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