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Beginner's gear question

Catrin2011-01-20 17:23:39 +0000 #1
I do not understand the differences between different cassettes and the relation with the chain ring.

My rear cassette is an 11-34, and the head wrench at my LBS says that certain gears would be easier to shift into if my chain ring had a wider range.

I know about ratios, so I have at least an intellectual understanding of gears in that way. Is there an analogy that I can use that would make this more clear to me? I am just trying to find a way of understanding this that makes more sense to me
lunacycles2011-01-20 17:35:21 +0000 #2
Back in the old days, people talked about gears in terms of gearing "inches," which relates to how many inches the bike travels with one revolution of the crank.

To calculate, divide rear cog teeth# into front chainring teeth# and multiply by wheel diameter.

So, for a 13-tooth cog in the rear, 52 in the front, with a 27" wheel, you would get 108 gear inches (52/13 * 27). For a 26 tooth cog in the rear, with the other variables identical, you would get 54" (52/26 * 27).

When you analyze gear inches, you quickly discover that reducing or increasing the size of your chainring has a larger effect on the size of the gear than reducing or increasing the size of your rear cog by the same amount.

Does this help?

I don't know what your wrench means that certain gears would "be easier to shift into." In general, drivetrains run a bit "smoother" and there is less likelihood of dropping a chain when there is less discrepancy in size between the large chainring and the small chainring. In the rear, it doesn't make any difference.
JennK132011-01-20 17:58:57 +0000 #3
How did this come up in conversation? Where you telling him you had some issues or difficulties shifting, or have a hard time climbing or going fast?

That answered, what do you have up front?
JennK132011-01-20 18:39:49 +0000 #4
How did this come up in conversation? Where you telling him you had some issues or difficulties shifting, or have a hard time climbing or going fast?

That answered, what do you have up front?
Catrin2011-01-20 18:05:39 +0000 #5
This came up in the context of my wrench adjusting my shifter cables and derailleurs. I have had a lot of problems with things getting out of adjustment because of my lack of understanding of "trimming" when the chain rubs the FD. THere has also been some problems with the chain falling off in the front (rarely, but typically when going up a steep hill).

As far as climbing in general, to me that is more of a fitness issue than anything.

The crankset is 48-36-26t. The wrench showed me that in certain gears the chain has a problem hitting the right part of the chain-ring and will tend to rub the actual chain-ring rather than hitting the teeth. Is this the right way to put it? This seems to happen mainly in gears that I shouldn't be using, or duplicate gears. I just have to come up with some way of internalizing all of this so I can start to remember which gear combinations work, and which ones causes these chain-rub problems, and which gear combinations are duplicates and which ones are better to use. Note that I am not referring to the chain rubbing the RD, but the ring itself.

I do understand cross-chaining and why it is a bad idea. I do not do this - so I have at least learned that much. Things have improved since learning how/what/when to trim and also making sure my back gears are somewhere in the middle before shifting the front.

IndySteel has pointed out that I've a wide range of gears and that sometimes it would be better to shift the front to avoid a big leap from one gear to another. I hope that I accuratly reported what she said - it makes sense to me. It is just that looking at numbers on a chart isn't helping me much...

Am I over-thinking all of this?
indysteel2011-01-20 17:53:57 +0000 #6
That's close, but not quite what I was saying. Because you have a triple, you have a fair amount of overlap between your chainrings. Finding the next easiest or hardest gear among all the gears available to you may require a shift between chainrings, not just between cogs. If you look at a gear ratio chart using your bike's specifications, it's really use to see this. You need not memorize the chart; just be generally aware of it. As for what Scott at Nebo said, I'm not sure I understand what he was getting out. Beyond cross chaining, I would suggest trying not to overthink it. Hopefully with his recent adjustments, you won't lose your chain anymore.
JennK132011-01-20 19:52:31 +0000 #7
Yes, I'm also at a loss for what he means other than cross chaining. If everything's adjusted right, you should be able to go through all the gears other than the cross chained ones.....
carinapir2011-01-20 18:05:15 +0000 #8
Quote:

Originally Posted by Catrin

My rear cassette is an 11-34, and the head wrench at my LBS says that certain gears would be easier to shift into if my chain ring had a wider range.

My first thought when I read this was that he was referring to the fact that derailleurs have a harder time lifting the chain when the tooth difference from one cog to the next is more than 1 or 2. When its try to lift from a smaller cog to a much larger one the shifting might not be as smooth.

For example if you have an 8 speed 11-34 you might have the following:

11-13-15-17-20-23-26-34

But if you have a 10 speed 11-34 you would have:

11-13-15-17-19-21-23-25-28-34

You see with the 8 speed you have bigger gaps and there are more instances when your derailleur is changing to a cog with 3 more teeth rather than 1 or 2.

That's just what occurred to me when I read your question and that he meant the cassette rather than the chain ring. I could be wrong.

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