Bicycle Mechanics - 4 arm vs 5 arm cranks?

I noticed fubar5 's crank is 4 arm and I have been wondering about mine. My mtn bike has 4 arm and after 18 months the middle chain ring has bent 3 times. At first I thought I had somehow banged it on a rock or log. Last week it got slightly bent and I am pretty sure I didn't bang it. I am pretty good about not cross chaining. It is STX shimano and the chainrings are aluminum.

Are chainrings on four arm cranks prone to bending?

Can I bend them just by hammering?

Can i bend them by changing gear underload ( well maybe it has happened once or twice) ?

Fatigue on the chainring?

Slightly bent,
Mzungu

I was just looking at the all the cranks at Pricepoint, and the only cranks that have 5 arms are Race Face, not even the Shimano XTR had 5.


Just a little info bomb.

Because the chainrings on a 4-arm spider have longer unsupported arcs than those on a 5-arm of the same BCD, they may well be slightly more susceptible to warping. After one of my admittedly somewhat heavy friends bent a TA Professional (3-bolt) chainring, he replaced the crankset with a 5-bolt Stronglight, and I put the TA set on my wife's bike, a much less demanding application.

The new 4-arm designs are yet another stupid fad foisted on the bike-buying public by the manufacturers, to render their 5-arm chainrings and spiders obsolete, and perhaps to save 20 percent on the cost of bolts. Simple physics tells us that, for a given total weight, a wheel with more spokes is stronger than one with fewer. On my mountain bike and road touring bike, I plan to stay with a 5-bolt 110/74mm system as long as I can.

The wonderful thing about standards, such as BCDs and bolt counts, is that there are so many from which to choose! :)

Originally posted by fubar5
I was just looking at the all the cranks at Pricepoint, and the only cranks that have 5 arms are Race Face, not even the Shimano XTR had 5.


Hmmmm. I thought this bike was just trying to save money on the cranks. All the "old" bikes in my stable are 5 arm.

New question. Does this mean i am stuck forever and ever with 4 arm cranks?

four armed,

Mzungu

I just thought about something........once I was camping and I had my P.3 with me, and I went to ride it and the front rings were loose.......so I went down and grabbed them and found out that all the bolts were loose! dont know how that happend because I had been riding it the day before......maybe the underwear gnomes loosend the bolts while I wasnt with my bike!


Originally posted by fubar5
I was just looking at the all the cranks at Pricepoint, and the only cranks that have 5 arms are Race Face, not even the Shimano XTR had 5.


I bet that helps him alot! haha:D

Have you tried steel chainrings? I think Nashbar has some.

I guess I will try to find steel rings. Nashbar didn't have steel for 4 post rings just for five. It would be nice to know if others experience this or maybe i have another problem.

Erik

Originally posted by Mzungu
I noticed fubar5 's crank is 4 arm and I have been wondering about mine. My mtn bike has 4 arm and after 18 months the middle chain ring has bent 3 times. At first I thought I had somehow banged it on a rock or log. Last week it got slightly bent and I am pretty sure I didn't bang it. I am pretty good about not cross chaining. It is STX shimano and the chainrings are aluminum.

The ring will be weaker every time you bend it and bend it back. It does sound a little strange since you wouldn't keep striking the middle ring on something and not the big ring. How gradual or sharp is the bend and how did you come to notice it? Are you sure it's not a bent crank spider (of course this should affect the large ring also)?


Are chainrings on four arm cranks prone to bending?

Obviously they have longer unsupported spans but most impacts sufficient to bend chainrings in most cases would bend 5-bolt as well as 4. IOW, I don't think this is the root of the problem.


Can I bend them just by hammering?

I assume you mean pedalling hard (as opposed to the hammering you would do to get the ring back into the original shape ;-) )? I doubt it was what bent it originally, but who knows?


Can i bend them by changing gear underload ( well maybe it has happened once or twice) ?

Again, I doubt it was the initial cause but who knows?


Fatigue on the chainring?

The problem is that you have permanantly deformed the metal repeatedly and subsequently have weakened it - maybe enough for minor things to bend it again. Definitely get a new ring after you rule out a bent crank spider. Don't worry about getting steel, just get something! I just hope your chain doesn't 'skip' on the new ring!



Slightly bent,
Mzungu

Sounds painful!

Originally posted by John E
The new 4-arm designs are yet another stupid fad foisted on the bike-buying public by the manufacturers, to render their 5-arm chainrings and spiders obsolete, and perhaps to save 20 percent on the cost of bolts. Simple physics tells us that, for a given total weight, a wheel with more spokes is stronger than one with fewer. On my mountain bike and road touring bike, I plan to stay with a 5-bolt 110/74mm system as long as I can.

The wonderful thing about standards, such as BCDs and bolt counts, is that there are so many from which to choose! :)

I couldn't agree more. I still use 'standard' (how's that for ironic) spacing for my beater bike and my 'on-road' MTB. I also have picked up a few old forged 110/74 crank arms as spares for the future. I just hope I can get rings in 10 years.