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Old 10-31-07, 09:43 PM   #1
Tat2Art
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15 speed bike really only a 13 speed?????

Thanks in advance for reading this and/or replying.

OK, I may be a bit anal (I've been called one of the synonyms to this, just not to my face). Anyway, I'm kinda new to the bicycle scene since I got my drivers license. I'm now in my mid 50's. Two years ago I purchased an X-Mart Mtn bike, a 15 speed. Now I'm not the sharpest tool in the shed but I figured it was called a 15 speed because of the 5 cogs and the 3 chainrings. Now where I'm confused is from reading where you should not ride using the largest cog and the largest chainring or on the smallest cog and the smallest chainring due to chain having to stretch across too much. So according to my math it turns out that I can only use 13 cog and chainring combinations. Why isn't it called a 13 speed? Secondly, being done with X-Mart bikes and doing my own wrenching now, I purchased a Schwinn Sierra 10 speed for $25 at a swapmeet/flee market. It has 5 cogs and 2 chainrings but is it really only an 8 speed? Why is the sky blue, err that's another post, why are they called 21 speed, 15 speed or whatever when you shouldn't shift through all the gear combos? Is this some kind of Bicycle Manufacturers propoganda/conspirecy?
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Old 10-31-07, 09:48 PM   #2
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Old 10-31-07, 09:50 PM   #3
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Actually... because of gear overlap you don't even have 13 unique speeds on a 15 speed.

Running in the small/small or big/big combination in a 3 by x drive is hard on the drivetrain... on a 2 by x there is less deflection so the small/small and big/big combos still add stress to the chain and drive but this isn't nearly as bad.

On a 15 speed or any 3 by x a good rule is to use the small cog with the 3 biggest cogs, middle with all cogs, and the big ring with the three smallest cogs... on a 15 speed you'd have 9 effective and efficient gear ratios.
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Old 10-31-07, 09:54 PM   #4
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Old 10-31-07, 11:05 PM   #5
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Quote:
Originally Posted by Tat2Art View Post
Thanks in advance for reading this and/or replying.

OK, I may be a bit anal (I've been called one of the synonyms to this, just not to my face). Anyway, I'm kinda new to the bicycle scene since I got my drivers license. I'm now in my mid 50's. Two years ago I purchased an X-Mart Mtn bike, a 15 speed. Now I'm not the sharpest tool in the shed but I figured it was called a 15 speed because of the 5 cogs and the 3 chainrings. Now where I'm confused is from reading where you should not ride using the largest cog and the largest chainring or on the smallest cog and the smallest chainring due to chain having to stretch across too much. So according to my math it turns out that I can only use 13 cog and chainring combinations. Why isn't it called a 13 speed? Secondly, being done with X-Mart bikes and doing my own wrenching now, I purchased a Schwinn Sierra 10 speed for $25 at a swapmeet/flee market. It has 5 cogs and 2 chainrings but is it really only an 8 speed? Why is the sky blue, err that's another post, why are they called 21 speed, 15 speed or whatever when you shouldn't shift through all the gear combos? Is this some kind of Bicycle Manufacturers propoganda/conspirecy?
The number of speeds on a bike is reported as the number of chainrings x the number of cogs in the back. Why? Because explaining how many gears a bike has that are really useful is too involved and confusing for most people. If you get out a gear chart and explain the way it's generated and how it works and how there are overlapping gears and the differences between gear inches, gear ratios and gain rates and why you should use one over the other and you really shouldn't use the big/big combination or the little/little one or even lower than about the fourth one on the back if you have a triple, most people are going to be comatose by the time you get to the gear chart.

Sheldon Brown has a gear calculator that allows you to calculate the ratios and figure out which gears to use. Go and count your gears (front and rear), run them through the calculator, look at the combinations and the gears that repeat, and apply the information. But, for the most part, you won't really know which one to use by number (without looking) and end up using the one that 'feels' best anyway. Just try to avoid those big/big and little/little combinations.

And the answer to your sky question is Rayleigh scattering
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Old 10-31-07, 11:38 PM   #6
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It's marketed as 15 speed because that's the largest possible combination of 3 rings and 5 cogs and the marketing department likes big numbers.

You should be able to get to all of the gears, but it's usually not a good idea to run big-big or small-small.
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Old 11-01-07, 12:53 AM   #7
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Most likely you've got around 8-9 unique gear-ratios. The rest are just overlaps. Use Sheldon's gear-calculator and develop a shift-pattern for your particular bike and gearing. On my 16-speed bike, I've actually only have 12 unique gears. The rest are duplicates:

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Old 11-01-07, 07:05 AM   #8
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Not that duplicates are bad. It can help with cutting down on shifting the front derailer, having some amount of overlap in the rear gears. Otherwise, you'd go through all 5 rear gears, then have to downshift all 5 and shift the front up 1--which would take a long time.

My road bike has just two chainrings; and a front shift is worth about two rear shifts. That means there is plenty of overlap in gears. But it's kinda like having two ranges of gearing: one for where I need more grunt, oen for when I need more speed. "Low" range has two distinct gears, then "Low" and "High" have 4 gear ratios that are "shared", then "High" has two more distinct gears. So, if I was riding along in high range and needed just a bit more gear to make a hill, I don't have to shift the FD, I would normally have a gear down (or two) available.

[My bike is a 2x6, that is, 2 chainrings and a 6 speed freewheel. About 8 different gear ratios.]
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Old 11-01-07, 11:49 AM   #9
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You don't need that many gears. A wide range is good, but having 400 distinct gears would be pretty pointless. On trucks they do that because of limitations on the operating speed of the engine- but legs are usually a bit more versatile.
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Old 11-01-07, 06:45 PM   #10
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Derailleurs are as crude as leather drive belts. Go internal!
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Old 11-01-07, 08:39 PM   #11
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My singlespeed has one usable gear combination. Not complaining, tho.
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Old 11-01-07, 09:26 PM   #12
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Well thank you all very much for the prompt replies. I've learned a lot of useful information, actually enough to get a good nights sleep. On the other hand the reply from colombo357 is going to drive my wife crazy. I love that song and now to be able to play it on the phone - WOW

@ - cyccommute & DannoXYZ you guys are scary smart.
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Old 11-02-07, 06:44 AM   #13
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Someone anal explained to me once how you could use all the gears, and that you could print out the gear charts to tape to your stem so that you could know how to go through them on the right order... from first to second, to third to fourth, then shift to the big ring on the front while shifting down twice on the rear, then up on the rear while shifting down on the front...

It was really quite complex...

I just ride.

By the way, you can typically use all of the gears. Some gears are just not optimal.

Also gears are rarely duplicated exactly, they are just sometimes VERY close to one another... For example a 50x36 compact double with a 12-25 10 speed cassette has no duplicate gears. For 50x34 or 52x39 rings with the same cassette each duplicates once... admittedly there are some ratios that are very close to one another...

So NOW who is anal???
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Old 11-02-07, 10:13 AM   #14
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I only care about the highest gear, the lowest gear, and if I have enough steps in between to ride comfortably.
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