greyghost_6

Had an interesting though. Does anyone actually like 5 bolts over 4 or 4 over 5? Other than 5 might be stronger, and possibly heavier? If you have any opinion please tell, and state the kind of riding you do, (road, mtn, trials, commute? etc)

cs1

It's not a matter of like or dislike. Each one has it's place. The 4 arm small BCD cranks work better on MTBs with their 44 or 42 tooth big rings. If you could get a 53 tooth big ring for a 104 BCD 4 arm crank it would be very flexy. You would have to increase the thicknes on it to much to compensate. That would negate any weight savings the small BCD 4 arm had.

That's why mfg's would rather use alternate materials, CF for example, on road cranks for lightness than changing the spider to a 4 arm small BCD.

Tim

bcart1991

5-bolt is prettier.

nun

+1 on 5 arm being prettier, 110/74 is the best BCD and the best crank ever is the TA Zephyr which has 110/74 or 56 BCD

HillRider

I prefer the look of the 5-arm crank too and consider the 110/74 BCD the most versatile as it can be set up for either road double or triple use (53/52 x 42/39 x 26), road compact (50 x 34) or a not too severe MTB (44x34x24) use.

For strictly road use I prefer the 130/74 or 135/74 configuration and for heavy duty MTB use (which I don't do), the 4-arm small BCD makes sense.

bcart1991

I'll see your TA Zephyr and raise you a Race Face Turbine LP.

top506

Ah, if only Shimano made 52t 110BCD Biopace chainrings!
Top

DMF 

Personal preference aside, there is an engineering justification for 5-armed spiders on road cranks.

The analysis is the same that dictates the use of 5 "feet" on a swivel chair. With 4 feet the chord between feet is 90°, placing the tipping point closer than the 72° chord of the 5-footed chair. (Try it; you'll see what I mean.) Historically, swivel chairs had 4 feet because until the advent of cheap casting and NC machining a 4-armed base was easier to manufacture. Now 5-footed bases are ubiquitous because they solve the problem better.

A crankset is different in three ways. First, mass (weight) is much more important. Second, there is a structure around the circumference - the ring. As the chord span* increases the ring itself must be made stiffer to oppose flex (as cs1 notes above). With more arms it is possible to make each slimmer and lighter. I don't know if (more arms x less arm mass) produces a net savings (doubt it), but adding the mass savings of lighter rings certainly does.

Third, with a chair, the diameter of the base is roughly fixed by the length of the human femur and 5 has been found an optimal number. Chain rings, OTOH, have a rough maximum diameter, but can be considerably smaller. As tooth count decreases the ring needs less strength (mass) to oppose flex, but a ring has a minimum chord mass because it also has to support the teeth against the (roughly constant) torsional force of the chain. Instead, fewer arms makes more sense. Thus, 4-armed spiders.

I wonder at what tooth count a solid disc makes more sense than crank arms?


* Chord span is the length of the chord at the rim; it is not the BCD. Chord span is a direct function of tooth count and arm count. The mounting flanges of the chainring act as arm extensions. For our purposes arm length = (ring diameter - hub diameter).

Wordbiker

^What he said.

I'd also add that I have seen quite a few 4-bolt MTB cranksets come into the shop with bent rings. Although I agree with DMF's theories, MTB drivetrains are more often shifted under duress than road, and should be stronger. The issue is a simple matter of distance between bolt centers. The chainring is spanning a greater distance with the 4-bolt design and can be distorted by the side loads of shifting under power. Sure, the chainring could be reinforced, but then you'd regain the weight savings over a 5-bolt design.

None of these would cause me to remove a 4-bolt crank from a bike just to replace it with a 5-bolt, but if I were choosing new parts, it would be worth considering.

HillRider

From shifting stress or from hitting things like rocks and logs?

nun

Which Race Face LP? One is more useful than the other, but neither look as good as the Zephyr, IMHO

bcart1991

The original. The personification of machined industrial strength. That Zephyr is purty though.

I've always been partial to the original XTR. That and the Cook Bros. E crank were lookers.

Bikedued

I was going to use a 4 hole crank from a Cannondale adventure 400 on a road bike, and it flexed so bad I could not even accelerate while sitting. It would buzz against the FD. It was my first 4 hole crank so it wasn't a good first impression. One thing that gets me is the chain ring price differences between 4 and 5 hole?? Why are 4 holes so much cheaper? IIRC one shop wanted $20-$25 for a set of 4 holes triple rings, and around $80 for the same in 5.,,,,BD

Plus, 5 hole IS so much prettier especially on a nice vintage steel bike....

Wordbiker

From shifting stress, always the middle ring. Granted, much of this is on entry level bikes used as rentals where the user is typically not very skilled with shifting, but I've seen it on customer bikes too and still see it as a weak design...and a bit silly since the tiny bit of "extra" weight matters little on a 30+ lb bike. I'd rather see a 5-bolt spec for less fleet maintenance and happier customers.

cs1

You said just said that so much more intelligently than I did. You must be related to Sheldon Brown. That was a great answer.


Tim

greyghost_6

Thanks DMF and everyone, that basically confirms my original thoughts in a much more mathematical manner. Oh yeah and +1 more to 5 bolts look prettier...

lymbzero

Yes to 5 Bolt because the stresses are distributed better.
Yes to 4 Bolt because finding a chain ring is much simpler.
But I have to say that the BMX 1 bolt is my favorite, assuming it doesn't loosen and you're using an extra small ring.

Sheldon Brown

The selection of chainring sizes for 4-bolt cranks is MUCH more limited than it is for the three most common 5-bolt patterns: 74 mm, 110 mm, 130 mm.

https://sheldonbrown.com/tooltips/taps.html

Sheldon "110/74" Brown

greyghost_6

Sheldon you crack me up! You are the first to answer my signature question....One of these days ill get around to running my kitchen sink tap through my bottom bracket....haha