How Many Teeth in this Small Chainring?
 

Last April I replaced the small chainring shown in this photo. The new chainring has 38 teeth. I neglected to note down how many the old one had, though I recall it is greater than 38. The old one was so worn that I threw it away.

From this picture, taken before the replacement, how many teeth do you think there are?

http://i.imgur.com/hE4rjp4.jpg

I figure 39 (7.8 teeth between the bolts, times 5 = 39). This is a 1995 Specialized M2 Pro.

My money is on 39T as well -- it doesn't stand proud enough of the chainring bolts to look like a 42T.

1) wow, that is quite the stretched chain! and resulting shark-finned teeth on the small chainring. I take it the large chainring was less-worn and works fine with the new chain?
2) I can't confidently tell if the chainring pictured is a 38t or 39t, but am confident it's not 42t (the other common small chainring on a road bike). It's most likely 39 because that's much more common than 38. Plus you recall it was more than 38t.
3) It appears that your large chainring doesn't have pins/ramps/gates to aid shifting, and so it's less important to match it perfectly to a certain designation of small chainring. You'll be fine with the 38.

http://www.bikepedia.com/quickbike/B...el=M2+Road+Pro

says 42 for a 95. But I also think it looks like a 39.

You can "count" teeth my measuring how far above the rim of the bolt the tooth shoulder is. On a 130BCD ring, the shoulder just touches the bolt. Add 1 tooth for every 2mm or so of added clearance.

BTW- it's easier if you look from the back. but based on it not showing in the cutout, I'll join the 39t crowd

given 130 accepts a 38 or 39, and common sets in roadie world is 53 -39. and its probably
the original stuff , I'll vote 39..


Campag opted for their 135 BCD the 39 is their minimum.

Thanks for the confirmation.

Obviously you didn't want to get your hands dirty and just count them .

Are you guys are messing with my head? I count 8+ teeth between the bolts. Putting a right angle centered on the spindle center just outside the picture's frame I count around 11 teeth making the old chain ring at least a 42?

However many teeth it had, it was long overdue for replacement. If you had ridden it much longer, it wouldn't have any teeth left.

It's a '90's Shimano Ultegra "tri-color" crank. My money's on the standard 39-53 combination.

his small chainring is pretty bad, but it can get worse...
http://bikeforums.net/attachment.php...hmentid=343079

I remain curious about how the large chainring has fared with the new chain. I assume the cassette (rear gears) has been replaced, right?

guys, the bikepedia is saying 42.... and if we're seeing 8+ teeth, which looking again, I do.... 42 should be it.

It is the meaning of the universe, too, you know.

http://wallpoper.com/images/00/21/19...y_00211911.jpg

Don't panic.

I don't know about the meaning of the universe, but I do understand geometry. Every tooth adds 1/2" to the circumference. Therefore each tooth adds roughly 1/3" or 4mm to the diameter, or 2mm to the radius.

Now take a look at the photo, and note that the inner diameter of the chainring rim passes close to the line of the bolt circle, but certainly to within the rim of the bolts. eNow take a look at the inner rim of the ring, and compare it to one pitch. So since a 38t is possible, that means that if this is a 42t you'd have to take 8mm out of that rim. I don't see 8mm there to rob, so I'll bet a case of beer that this was not a 42t.

hmm, fbinny. what you say makes sense, and here's a 42....

http://www.tubeguru.hu/ebay2/shimano...5751-36328.jpg

which does look bigger. But I think the grease plays tricks on the eye. Stock is clearly 42.... but i think you're right.

This is how I see it:

http://i.imgur.com/EZyrsG3.jpg

And here it is today with a known 39 tooth chainring:

http://i.imgur.com/ijaIAOY.jpg

7.8 teeth from one bolt to the next.

And if those two pictures don't convert you to paraffin lubing... :)

He probably would have except for this quote from the OP; "The old one was so worn that I threw it away."

OP; Why and to what purpose... are you suffering from separation anxiety or something?

If you mean "Why do you want to know how many teeth were on the old chainring?" The answer is in this thread:

http://www.bikeforums.net/showthread...Skip-Woes-Help!

In summary: I had some problems with chain skip that, it turns out, were related to switching to a 38 tooth wheel. I wanted to go back to what I had.

Surly Stainless steel chainrings are long wearing .. I have a 38t on my R'off bike..


skeptical, but I cannot supervise and consult from a keyboard.

This is why BF is engrossing. I was hoping that someone would do this.

I think I was counting spaces.

Simply amazing.

Wait, amazing b/c I can't count teeth, b/c someone actually made this diagram, or b/c bikeforums is so awesome that I naturally expected someone to come up with it? :p

I know, I know--- probably all three.

You got it, friend.