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Why four-arm cranks?

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Why four-arm cranks?

Old 09-14-16, 02:02 PM
  #1  
smontanaro 
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Why four-arm cranks?

I've got plenty of five-arm cranks (Campy NR, SR, Gran Sport, many clones), I've got one three-arm crank (also Campy Gran Sport). I've seen the Ambrosio six-arm crank made for (I believe) Colnago. There are plenty of new four-arm cranks out there. What do they do better than three- or five-arm cranks? Did four-arm cranks ever exist in the vintage era (say, pre-1985)?

Last edited by smontanaro; 09-14-16 at 02:46 PM. Reason: Spelling whoopsie
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Old 09-14-16, 02:44 PM
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"dramatically rigid power transfer"
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Old 09-14-16, 02:47 PM
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Marketing and ugliness
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Old 09-14-16, 02:55 PM
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I think the word you're looking for is 'baloney'.
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Old 09-14-16, 03:40 PM
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Originally Posted by Ex Pres View Post
Marketing and ugliness
The star is a dominant shape in nature. Car wheels went from four to five stud to combat loosening wheel nuts - not that generating massive amounts of torque is going to be an issue for me on a bicycle.
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Old 09-14-16, 03:51 PM
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Originally Posted by smontanaro View Post
I've got plenty of five-arm cranks (Campy NR, SR, Gran Sport, many clones), I've got one three-arm crank (also Campy Gran Sport). I've seen the Ambrosio six-arm crank made for (I believe) Colnago. There are plenty of new four-arm cranks out there. What do they do better than three- or five-arm cranks? Did four-arm cranks ever exist in the vintage era (say, pre-1985)?
As ugly as I find the new 4-arm cranks (especially the Campy ones, bleh), they do make structural sense. The chain loads are highest perpendicular to the crank arms, so material in line with them doesn't add as much value as elsewhere, and just having two arms wouldn't give enough support. Perhaps those 5-arm cranks where the DS arm served as one of the chainring attachment points could be seen as an engineering step toward the new ones?

That said, it appears that three arms can do the job well. Perhaps the industry as a whole will move there once they've convinced everyone that four arms are enough.
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Old 09-14-16, 04:13 PM
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My problem is aesthetics. Four-arm/bolt cranks just look ugly and wrong to me, whereas 3 (including 3-to-6) and 5 bolt systems look good.
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Old 09-14-16, 05:16 PM
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Originally Posted by ThermionicScott View Post
As ugly as I find the new 4-arm cranks (especially the Campy ones, bleh), they do make structural sense. The chain loads are highest perpendicular to the crank arms, so material in line with them doesn't add as much value as elsewhere...
I'm struggling with that.
With any number of arms, we have to analyze the worst case angle in the crank rotation. For instance, an infinite number of arms would be a contiguous disk, and loading is the same at all rotation angles. But for constant mass, the disk would be thin, and prone to tacoing.
For one arm, the worst case, I suspect, would be with the arm pointing downward to rearward (6 O'clock to 9 O'clock looking from the drive side). Then the chainload is trying to buckle, or taco the chainring.
Now add one arm at a time, figure out the worst case, and try to deduce what is the most efficient number of arms, analytically.
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Old 09-14-16, 05:17 PM
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Odd numbers in a circle. Good visual balance.
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Old 09-14-16, 05:19 PM
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Originally Posted by rootboy View Post
Odd numbers in a circle. Good visual balance.
Like a clock
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Old 09-14-16, 06:37 PM
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Originally Posted by smontanaro View Post
...There are plenty of new four-arm cranks out there. What do they do better than three- or five-arm cranks? ...
drive us old guys crazy???


I try to ignore the new stuff as much as possible.


Steve in Peoria
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Old 09-14-16, 07:15 PM
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For all those who say four arm cranksets are ugly. You have to exclude the C-Record.

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Old 09-14-16, 07:24 PM
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I'm convinced, I'll sell the crank



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Old 09-14-16, 07:47 PM
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I hope you read my post correctly. I said "exclude". The think the C-Record crank is Campy's third nicest component. Behind the NR rear derailleur and the C-Record high flange hub set.
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Old 09-14-16, 08:01 PM
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Maybe only four arms but...

it does have 5 attachment points for the chainwheels so I would really consider it a 5 arm crankset, looks too nice to be a four arm crank
Originally Posted by gearbasher View Post
For all those who say four arm cranksets are ugly. You have to exclude the C-Record.

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Old 09-14-16, 08:38 PM
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The current Campagnolo 4 arm crank is actually an 8 fastener crank.
I agree though that it's aesthetics are less than elegant.

I have a Ambrosio for Colnago 6 arm crank, it has taken a while for me to get used to its visual statement, looks a bit static, odd arm number cranks fare better visually. I do like the smaller bolt hole circle that allows smaller inner rings.

The 3 arm TA Professional was on my first road bike, works pretty well. With the chainrings having essentially 6 wide webs the stiffness under shifting load is good. Long ago when I was strong the Corsa Record would flex the big ring during a big "jump" and it would bring the chain to rub the derailleur cage. Visually nice, functionally challenged in my view because if you over shifted, the chain would drop right to the pedal, on the venerable Campagnolo Record crank, the chain would catch on the crank arm land and you could shift back and continue.
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Old 09-14-16, 09:22 PM
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Yeah, I've got one of those C-Records, and I'm counting 5 arms. One is just bigger than the others.
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Old 09-14-16, 09:25 PM
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Shimano's move to 4 bolt also made all their rings 110bcd so that means only one forging instead of a 130 and 110.
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Old 09-14-16, 09:52 PM
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Originally Posted by clasher View Post
Shimano's move to 4 bolt also made all their rings 110bcd so that means only one forging instead of a 130 and 110.
I'm probably never going to own one of these cranks (well, maybe in 20 years when they're considered vintage) but I love that idea. I try to use 110 bcd whenever I can and I love the idea of having that option even more widespread.
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Old 09-14-16, 10:15 PM
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@hairnet : Our bike coop renovates bikes, those are expensive, the spare parts they sell including cranks are cheap; so one might look into that.
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Old 09-14-16, 10:34 PM
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Originally Posted by CuttersRidge View Post
@hairnet : Our bike coop renovates bikes, those are expensive, the spare parts they sell including cranks are cheap; so one might look into that.
I have been a co-op volunteer mechanic for seven years. How else do I find good vintage parts at a reasonable price
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Old 09-14-16, 10:51 PM
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It's all style. We've been shopping strollers again lately, and we had a good giggle at the Mutsy brand website, where apparently only rail-thin fashion models in autumn runway clothes shop for strollers. They are served with a giant helping of Style. Check out the wheels. Can bike cranks be far behind?



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Old 09-15-16, 01:18 AM
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That is some certifiable 'WTF' stuff right there. The "fashion," the floating, the lack of enjoyment of anything/humanity, and walking the stroller at either the edge of a busy road OR in the freaking middle of it. Seriously...marketing.......guhhhhhhhhh

To four-arm cranks. Yeah, they say power transfer. Sure, no problem. Perhaps part marketing and part engineering. The aesthetics have been handled with less than preferable grace save for 9000-series Dura Ace. That was a nice way to do it. 9100 is a mess of lines and non-lining-up surfaces. Really ugly. The Campy stuff is a bit brutal, even in AL form.

Even arm/spoke counts do better at 6+, and odd counts at 5+. It's why we see 5 or split-5 spoke wheels on cars still--it still looks good! Heck, a split-3 spoke rim on a car can look alright (upscale Japanese aftermarket wheel company does one and it looks nice--I'd never buy it just because, but still, it's possible). I like 5 arm cranks, and I like their visual proportion and how they don't overwhelm a vintage frame.
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Old 09-15-16, 04:45 AM
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I think the main purpose of the four arm crank is to be visually distinctive so the ones that don't have the "modern look" (in other words, aren't really ugly) take on a quaint and vintage look. It won't be long before you take a bike with a five-arm crank into a shop, and someone will tell you "we can't get parts for that" and try to sell you a new one.
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Old 09-15-16, 05:07 AM
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Originally Posted by rhm View Post
It won't be long before you take a bike with a five-arm crank into a shop, and someone will tell you "we can't get parts for that" and try to sell you a new one.
God bless America.
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