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Is there anything wrong with running big ol' cogs/sprockets?

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Is there anything wrong with running big ol' cogs/sprockets?

Old 11-01-08, 02:30 AM
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Bud_311
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Is there anything wrong with running big ol' cogs/sprockets?

My bike ends up with a 52t front sprocket, so I was planning on throwing a 19t cog on the back to give me a pretty moderate gear ratio. I notice most people run much smaller gears but end up with similar ratios. Is there any reason I should even consider upgrading my front sprocket to a smaller tooth?
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Old 11-01-08, 02:40 AM
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There is some great info on Sheldon Brown's website re: this exact thing

http://sheldonbrown.com/fixed.html#bigsmall
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Old 11-01-08, 03:26 AM
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I'm running that exact ratio of 52/19, and it's about perfect for my hilly rides. Fast enough to get down the hills without riding the brake, but I can still get up the hills without too much effort. Running a bigger chainring will give you less of an increment between cog sizes, so you can fine-tune your gearing a little more. Make sure it works with your chainstay though, I had to put mine on the inside mount of the crank spider to get my chainline right, and I have about 3mm clearance between my chainstay and the ring. Pretty much everything is covered in that sheldon article, but also 51/19 gives you 19 skid patches, if that's of any significance to you.
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Old 11-01-08, 03:49 AM
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I was under the impression that 52/19 also gave me 19 skid patches, since 52/19 can't simplify any lower.
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Old 11-01-08, 04:34 AM
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Running 48/17. Not so big big but F-it! What ever works!
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Old 11-01-08, 04:38 AM
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Originally Posted by Bud_311 View Post
I was under the impression that 52/19 also gave me 19 skid patches, since 52/19 can't simplify any lower.
That's right. As someone said, the biggest issue will be whether that huge ring can clear the chainstay when set at a 42mm chainline. The biggest i could fit on my own road conversion was a 49-50 or so.
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Old 11-01-08, 06:09 AM
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I just got a Campagnolo crank set with a 52T track chainring on it (fo freezy!). I was planning on running a 52/19 or 52/17.
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Old 11-01-08, 06:58 AM
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52/19 is what i literally started running this past week- it seems to be the magic gear. I can spin it up, but still do hills. I dont think there are any strength/integrity issues of the metal or anything since the same torque would be applied as a similar ration with a smaller gearing combo
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Old 11-01-08, 07:14 AM
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I run 53x19, no problems. Campagnolo chainring seems to last forever.
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Old 11-01-08, 10:46 AM
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53T chainring on front
20T freewheel and cog in the back
70.75 gear inches

I had to replace my bottom bracket to get a good chainline on my build since I used a flip-flop hub (not much wiggle room with spacers(if any)). Stock bb was 115mm. Replaced with a 102mm bb and my chainline is only off by 2mm. Quiet and the chain has stayed put so no worries.



I dig on the look of a larger chainring as well.



Upside, that I've read, is that they're more quiet vs a smaller toothcount setup.

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Old 11-01-08, 10:58 AM
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I'm glad to hear that other people are running this gear. I'm hoping it really is the magic gear.

Can someone please explain what they mean by 'clearing the chainstay'? I don't see where a problem could come up.
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Old 11-01-08, 11:09 AM
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well, chainstays go back at an angle to accomodate the rear wheel. if your chainring is too large, it could cause it to grind into the sides.
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Old 11-01-08, 11:15 AM
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Originally Posted by Bud_311 View Post
I'm glad to hear that other people are running this gear. I'm hoping it really is the magic gear.

Can someone please explain what they mean by 'clearing the chainstay'? I don't see where a problem could come up.
It can...

Modern road bikes have 130 mm spacing and because of that, the angle of the chainstay as it leaves the bottom bracket can interfere with the inner chain ring if it is larger than the bike was designed for... which is usually 39 teeth.

You can run a wider bottom bracket to give you that needed clearance but as said, that chain ring to chain stay clearance may be very close.

Chain stays can be be shaped (by design) to allow for a larger inner ring and older bikes often had a chainstay that was scalloped / indented behind the bb to allow room for a bigger 42 tooth ring... 5 and 6 speed bikes also had narrower rear stays and needed this little design feature to provide adequate clearance.

Touring bikes often come with half step gearing where the outer ring was 52, middle ring was 48, and the granny was very low... the chainstay also has to be designed to allow clearance for the much larger middle ring.

Tandems often run huge outer and middle rings... they have to have specially designed chain stays so that there is adequate clearance.

My '62 Peugeot has run a 53 tooth ring on the inside of the spider (and could run even more) with no issues although I normally run a 52:18 gearing... the chain stay is scalloped behind the bb.
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Old 11-01-08, 11:18 AM
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The benefits of running a larger chain ring and bigger rear cog is that is puts less stress on the drive as the force is distributed over many more teeth, the drive tends to be much smoother, and the life of the drive can be greatly increased.

The lifespan between a 15 tooth rear and a 19 tooth rear cog can be considerable.
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Old 11-01-08, 11:35 AM
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Couple pics of my bike showing about 1cm of clearance between the chainring and the chainstay:



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Old 11-01-08, 12:47 PM
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Okay, I see now. The 52t sprocket was already connected to the cranks before I started converting. I should be fine once I'm all set up then, right?
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Old 11-01-08, 12:58 PM
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Have you ever mounted the cranks on the bike, with a 42mm chainline on the 52t ring?
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Old 11-01-08, 03:33 PM
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Ohh, I had completely forgotten how I had removed my smaller chainring. Would there be any way to test my clearance before I reassemble the bottom bracket?

ALSO, the dudes at the bike shop were strongly urging me to get a smaller front chainring, saying that even if the ratio is similar to a smaller set up, it'll still take more power to start off on a bigger chainring. Thoughts?
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Old 11-01-08, 04:44 PM
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If noticable, it'll take less power to take off
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Old 11-02-08, 07:06 PM
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you'll need a big ass chain 140 links
 
Old 11-02-08, 07:34 PM
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Originally Posted by Bud_311 View Post
Ohh, I had completely forgotten how I had removed my smaller chainring. Would there be any way to test my clearance before I reassemble the bottom bracket?

ALSO, the dudes at the bike shop were strongly urging me to get a smaller front chainring, saying that even if the ratio is similar to a smaller set up, it'll still take more power to start off on a bigger chainring. Thoughts?
The force needed to take off from a start is not relative to the size of the chainring. It is relative to the gearing combination.
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Old 11-02-08, 09:27 PM
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[QUOTE=s
I dig on the look of a larger chainring as well.

[k[/QUOTE]

As do I. I had a Sugino messenger that came with a 42t ring. I wanted a 48t ring but got a 144 bcd crank and 49t ring.
It looks good but not sure it rides any different with similar gear inches. I would have to switch back and forth and compare right away.
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Old 11-03-08, 02:42 AM
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Originally Posted by Bud_311 View Post
Ohh, I had completely forgotten how I had removed my smaller chainring. Would there be any way to test my clearance before I reassemble the bottom bracket?
It's hard to tell for sure without just putting it on there. It definitely might rub though, since (assuming you're converting an old road frame) the people who built your frame would not have been thinking about putting rings that big that close to the frame.

Are you planning to use the original bb? If so, your inner ring position will be just about at 42mm on the old bb if the crank is anywhere near shimano's standard for road doubles. You could just install the big ring in that position, mount the cranks and have a look.

What kind of cranks are they?

ALSO, the dudes at the bike shop were strongly urging me to get a smaller front chainring, saying that even if the ratio is similar to a smaller set up, it'll still take more power to start off on a bigger chainring. Thoughts?
Bull shiat. Identical mechanical advantage=identical work, besides a contribution from weight that is too small to be noticed by an ordinary street rider. I would be hesitant to take any further advice from that shop.

Last edited by mander; 11-03-08 at 02:45 AM.
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Old 11-03-08, 02:55 AM
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I was definitely planning on using the original bottom bracket. I'm hoping it will reinstall in the same position, however, since I don't think the smaller ring was adding any spacing to the cranks. The small ring was floating from the larger ring with some rivets.
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Old 11-03-08, 03:15 AM
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Originally Posted by Bud_311 View Post
I was definitely planning on using the original bottom bracket. I'm hoping it will reinstall in the same position, however, since I don't think the smaller ring was adding any spacing to the cranks. The small ring was floating from the larger ring with some rivets.
Hrmmmmmm.... I'm not sure if these cranks will work out. The problem you are going to have is roughly that your big ring has to go where the small ring used to be, in order to line up correctly with the cog that's in back. If there is nothing to mount the ring to then they wont work.

I could be wrong but since you mentioned rivets, i suspect that what you have there is a cheap crankset that will just cause headaches. I would recommend finding some beat up old Shimano road double cranks (105/ 600/ etc) and getting a 107mm square taper shimano cartridge BB, which will allow you to run a ring on the outer position at 42 mm. The whole shebang shouldn't cost you much more than $40 (could be much less if you luck out at a bike co-op) and for your purposes, it will be just about as good as bike components get.

Last edited by mander; 11-03-08 at 03:21 AM.
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