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Are 4 arm cranks the future for road bikes?

What are the advantages?

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  • Welcome to Bicycles. I have edited your question (you will not be able to see that immediately) to make it less opinion based. Please see the Help center about writing questions.
    – andy256
    Sep 16, 2014 at 1:02
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    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, 2014 at 1:03
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    Advantages? It's something to sell. Sep 16, 2014 at 3:46
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    I'm not sure a flexible groupset is something I'd be looking for.
    – Holloway
    Sep 16, 2014 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, 2014 at 11:57

3 Answers 3

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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.

Shimano_FC-9000

^^^ 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.

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  • Any reason the Shimano road crank spider looks so different from a mountain spider?
    – Aaron
    Sep 16, 2014 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, 2014 at 8:25
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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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4 arms can be made to be lighter than 5 arms.

Since there are only 4 arms it stands to reason that they are shorter since supporting a distant chainring with so few arms is difficult. The arms have to be fatter and or shorter. Shimano's four arm spiders are both fatter and shorter, though a lot of the "fatness" is due to the fact that they are hollow.

Shorter 110mm BCD spiders mean that compact (50-34) and standard 53-39 52-36 chainrings can go on the same 110 spider rather than needing a 110mm for compact and using a bigger 130 mm for standard.

Now that the spider is smaller, it means that the chainrings, especially in the standard sizes have a greater internal to external diameter difference (fatter doughnut). This means that in Shimano's design they are now no longer just plates of aluminum but three dimensional aluminium moldings with lots of splines on the back, since in a sense the job that the spider did, of holding the teeth rigid in the left right plane, is now shared by the chain rings.

This diffuse design, is closer to a sort of monocoque design, where all forces on the chainrings are shared throughout the assembly. The almost monocoque crank+chainring assembly is a marvel of engineering.

But really I don't think it was necessary to anyone outside of the pro peloton. However, the innovation means that Shimano can charge 60 USD or more for their molded, splined 3D chain-rings which are a part that inevitably wears out.

Additionally, on the economic front, while flat chainrings can be made by anyone, with the four arm design we are somewhat tied into purchasing a Shimano chainring every 10,000km (?). There are some flat 110 BCD standard chainrings but e.g. oval ones, but I am not sure about their flex/stiffness. They also do not match up to mate with the three dimensional 4 arm spider but sometimes they are sold with pieces of plastic to blend them into the cliff of Shimano's 4 arms.

The 4 arm plus three dimensional Hollotech '3' (They are Hollowtech 2 but, in sense the third ? iteration) are super stiff and super light.

I cycle a lot and am growing sick of paying 60USD for a replacement chainring, or leaving longer and getting chain slip (losing my chain off the outer worn chainring) so I think I am going to move back to 5 arm Hollowtech 2 Ultegras. I did not notice the flex when I used to use them.

I thought I would be able to just about tell the difference, but I could not when I tried them last night and this morning.

Flat 5-arm chain rings are very cheap. I am using an old bio-pace that was about 12USD. There are now quite a lot of 130 BCD standard chainrings on sale on aliexpress. 110 BDC standard chainrings are rare, but 50-34 are available both at about as low as 20USD for a pair.

Tim

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