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Front Cogs Ratios

Old 01-30-17, 07:27 AM
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Bill1704
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Front Cogs Ratios

Hi,

Apologies to all purists, but I'm fairly new to this and may not use correct terminology - Is there a link between the cog sizes on the front Chainring? I.e. If I was to put a 50 tooth cog on as the big option, what is the minimum size I could have for the small one?
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Old 01-30-17, 07:32 AM
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dabac
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They're called chainrings, sometimes chainwheels. May be called sprockets if you speak BMX.
Biggest/smallest depends on if you're running a double, a triple and what front shifter you're using. 50/34 would be a reasonably common combination for a double.
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Old 01-30-17, 07:41 AM
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Some front derailleurs, due to the design of their inner plate, will require a minimum difference between the largest chainwheel and the next-largest, to avoid the plate hitting the next-largest one. A 10-tooth difference is common. If this is not observed it may be necessary to mount the derailleur higher than optimum in order for the inner plate to clear.
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Old 01-30-17, 07:59 AM
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What controls the minimum size chainring a crank will accept is the 'Bolt Circle Diameter" (BCD) which is the diameter of the circle the fastening bolts are place on.

Many road cranks have a 130 mm BCD and will take a minimum chainring of 38T although 39T are far more commonly fitted. Campagnolo uses a 135 mm BCD and 39 teeth is the smallest.

A crank with a 110 mm BCD, known as a "Compact" these days, will accept down to 33 teeth, but again, a 34T is more common.

There are smaller bolt circles like 74 mm (24 teeth minimum) and MTB cranks have bolt circles that will accept down to 20 teeth.
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Old 01-30-17, 08:18 AM
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I depends on the front derailleur. Usually the biggest difference is 16 teeth for a 50-34 combination, that is also very common. Often you a limited in your chain ring options too unless you swap the crank.
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Old 01-30-17, 08:36 AM
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Your question could be interpreted several ways.
Maybe rephrase it as to what you want to accomplish and WHAT you have, bicycle wise.
Prove you aren't a 1 post wonder.
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Old 02-06-17, 04:57 AM
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Thanks all.

I reply to Bill Kapun.... The bike is great on the flat, but hard work up hills. Was hoping to keep the Big front cog and reduce the size of the small front cog to make it better up hills. Checked and the current ratio between the two is 53 tooth and 39 tooth, so I think it is either compromise the flat speed to make it better up hills, or just work harder?
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Old 02-06-17, 05:22 AM
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What are you running on the rear? Is this MTB or road?
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Old 02-06-17, 06:01 AM
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Bill1704, Welcome to the forum.

Nothing to be done on the front except for a replacement crank set. What do you have at the rear (tooth count and number of cogs).

Brad
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Old 02-06-17, 06:20 AM
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Originally Posted by Bill1704 View Post
Checked and the current ratio between the two is 53 tooth and 39 tooth...
53/39 is a "standard" double. Most standard double cranksets have a bolt circle diameter (BCD) of 130 mm. The smallest chainring that'll work with a 130 mm BCD is 38 teeth. If you tried to go to a smaller ring than 38 teeth on a 130 mm BCD, the chainring bolts would get in the way of the chain on the inner ring.

To use a chainring smaller than 38 teeth, you'd need a crankset with a smaller BCD than 130 mm. "Compact" cranksets typically have a 34 tooth small ring, and to accommodate that, they have a 110 mm BCD.

If your crankset has a 130 mm BCD, you have two options for lower gearing: (1) replace the crankset so you can use a smaller ring or (2) swap the cassette on the rear wheel for one that has bigger cogs.
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Old 02-06-17, 07:21 AM
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Originally Posted by Bill1704 View Post
Thanks all.

I reply to Bill Kapun.... The bike is great on the flat, but hard work up hills. Was hoping to keep the Big front cog and reduce the size of the small front cog to make it better up hills. Checked and the current ratio between the two is 53 tooth and 39 tooth, so I think it is either compromise the flat speed to make it better up hills, or just work harder?
What cogs do you have on your cassette/freewheel ? Lower gearing can also be gained with larger cogs.

Various options to lower gearing..... depends on how much lower you want, and what you currently have.

A slightly larger cog & longer chain might be enough, and cost less. Replacing parts that may be close to replacement time anyway.
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Old 02-06-17, 11:55 AM
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Originally Posted by mcours2006 View Post
What are you running on the rear? Is this MTB or road?
+1
This may be the simplest, least expensive solution.
However, we need to know what you have on the rear.
Largest/smallest cogs & number of them.

If, for example, you have an 11-23 cassette, you should be able to swap in something like a 12/13-27ish.

We just need more details to give a best answer.
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Old 02-06-17, 02:58 PM
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You won't really compromise top speed by going smaller on the large chainwheel. Very, very few riders can push even a 50-12, let alone 50/11 except downhill. The 53 is pretty much useless in my opinion, except perhaps for a very fast time trial rider.
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