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Old 01-18-07, 09:07 PM   #1
europa
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Trek520 gearing change.

Yes, I know this is a popular one and most people do it by changing to an mtb crankset. Well, I don't want to do that. I'm riding a 2007 Trek520 and am very happy with the two larger chainrings (52/40 they suit most of my riding perfectly) but as I'm running out of puff on some of the nastier hills around here, a smaller granny gear (from the current 30) would be appreciated. If I'm going to change the small chainring, I'd might as well change it for something suitable for heavy touring (not that I'm planning it but while I'm doing the change ...).

Any suggestions on what sized chainring to go for?
As I said, I'm keeping the Ultegra crankset and 105 derailleur, so the new ring has to be compatible with that. A mate has suggested going to a 24 (which looks good on the gear chart - nice overlap, a few more ratios) but the lbs reckons the 105 shifter won't handle the 52-24 range. Any thoughts?

Better still, has anyone done it?

Richard

Last edited by europa; 01-18-07 at 09:26 PM.
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Old 01-18-07, 09:17 PM   #2
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Need more information. What size chainrings do you now have? Do you know exactly what crankset you have? If it is a typical Shimano road triple with a 130mm BCD then I would suggest that the minimum is a 26. A concern that I would have is the difference between the middle and smallest chainrings. The bigger the difference, the more difficult the shift. You can probably change all three rings. What's best for you will also depend on the the gearing on the rear of the bike. How many cogs and what are the sizes.

Al
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Old 01-18-07, 09:27 PM   #3
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Check the edits in the original post Al - thought it better to put it there for new readers
basically though, Ultegra crankset, 52/40/30, 105 front derailleur (don't know exactly which one).

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Old 01-18-07, 09:50 PM   #4
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With the big ring at 52t, I don't think it's going to work with a granny as low as 24t. With your 130/74 BCD crankset the 24t ring will indeed bolt on, but you'd be looking at a 28t difference between the big and granny rings. Shimano's "official" recommended max. difference for the FD is 22t I believe. You can usually go beyond their "official" recommendations, but probably not as much as that. The other concern would be the rear derailleur's chain wrap capacity, which is figured like this: big ring minus granny ring plus largest cog minus smallest cog. A "mountain" rear derailleur usually has a recommended max. chain wrap capacity of 45t, and assuming you're running a "mountain" cassette on this tourer (11 x 32?), you'd be beyond the rear derailleur's capacity as well-

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Old 01-18-07, 09:57 PM   #5
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I'm going to assume that the middle chainring is a 42, since that is the most common middle ring pre 10-speed.
As an example, I changed the chainrings on my wife's Ultegra triple to 49-39-28, she's a good spinner but light weight and doesn't develope much torque. I used TA Alize chainrings because they are designed to run in a Shimano triple, but they are on the expensive side. Changing just the granny gear will be very inexpensive so experimentation may be a good thing. I'm sure you could use a 28, and maybe a 26, but the bigger the differential between the middle and the granny, the greater the chances of degrading the shifting performance. If you choose to also change the middle chainring try to use one with ramps and pins to assist shifting from the granny to middle. Shimano's new 10-speed triples come with 39 middle rings, but I don't know if you can buy them individually.
Another option may be to consider a change in the rear gearing.

Al
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Old 01-18-07, 10:00 PM   #6
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Hmm.

Thinking about it, losing a bit off the top isn't such a bad move. Trouble is, that will then lead to changing the middle chainring as well.

Okay, time to think carefully about gearing then.

Thanks

Richard
it's never straightforward is it.
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Old 01-18-07, 10:19 PM   #7
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Quote:
Originally Posted by europa
Hmm.

Thinking about it, losing a bit off the top isn't such a bad move. Trouble is, that will then lead to changing the middle chainring as well.

Okay, time to think carefully about gearing then.

Thanks

Richard
it's never straightforward is it.
Planning is good, but don't give up too easy. Maybe try a 28 (74mm BCD), they're cheap.
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Old 01-18-07, 10:28 PM   #8
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With 52 at the top, you could go down to 26. By that, I mean that the cage is long enough to work properly. Maybe 24 would work, but that would be really close to the limit, if not pushing it. If you go that way, I would suggest you install a jump stop to prevent the chain from falling inside. Not necessary if you have bar-end shifters (the original configuration) tuned properly, but almost necessary with STI.

But the other important question is whether or not you might be missing a few important gears. Assuming you have a typical 11-32 cassette, the lowest gear available on the middle ring, 40/32 or 42/32 gives you 34 gear-inches, whereas the highest useable gear on the granny would be something like 26/17, or 41 gear-inches. This means quite a few double shifts in perspective.
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Old 01-18-07, 10:44 PM   #9
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I've used a 24 tooth inner chainring on a 52/42/30 Shimano crank and it worked. But it was an RX100 group with downtube shifters and 7 speeds.
It probably would work for you and it is not expensive to find out.
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Old 01-19-07, 07:22 AM   #10
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I've changed the 30T granny ring for a 26T on a bunch of 7,8,and 9-speed Shimano cranks for both 105, and Ultegra groups and they have always worked well. I've never tried a 24T but from looking at one of my bikes, I think it will work.

Of course, you will not be able to use the smallest one (or more) cogs with the smaller granny since you will have exceeded the rear derailleur's wrap capacity.
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Old 01-19-07, 07:43 AM   #11
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After playing with Sheldon's gear calculator, and your positive thoughts on the workability of 26, I'm going to try a 26 tooth cog - it's a good move for a lot of reasons.

Now, to buy one of the things (I'm in Australia and gave up believing such actions were simple a while ago ) ... then to work out how to put it on . A quick look at the grid suggests I have to remove the crank. Probably easy but I'm guessing that's another special tool to buy .

Richard
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Old 01-19-07, 08:57 AM   #12
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All you need is a crank puller. Swapping out the granny ring is really easy once you have the crankarm off - unbolt and then bolt the new one on. Grease the bolt threads. Make sure to really honk on the crankarm bolts when reinstalling the crankarm.
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Old 01-19-07, 10:55 AM   #13
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Quote:
Originally Posted by europa
Thinking about it, losing a bit off the top isn't such a bad move. Trouble is, that will then lead to changing the middle chainring as well.
I've changed my 52/42/30 down to 50/39/30, and I am basically thrilled w/ progress so far. I haven't figured how low to drop the 30 to yet, and meantime I've basically stopped riding that bike anyway. (I also switched from 12/25 to 11/28 cassette, so still have a top gear bigger than I started with that I don't use, or the next couple down from the top either)

You can also expand how you look at rear derailer capacity by considering the smallest rear cog *that you will use with the large chainring* , not just the smallest one on the bike. I gather one can accomodate a 24t chainring w/ 105 rear derailer if you only use the 3-4 largest cogs on the cassette w/ that ring.
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Old 01-19-07, 11:44 AM   #14
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You can also expand how you look at rear derailer capacity by considering the smallest rear cog *that you will use with the large chainring* , not just the smallest one on the bike. I gather one can accomodate a 24t chainring w/ 105 rear derailer if you only use the 3-4 largest cogs on the cassette w/ that ring.
Actually, it's when you're using the granny ring and the smaller cogs on the cassette that the rear derailleur has the most slack to take up. So when in the granny ring up front, if the smallest few cogs on the rear are avoided (which they should be anyway on a triple), you can get away with "theoretically" exceeding the rear derailleur's chain wrap capacity-
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Old 01-19-07, 05:56 PM   #15
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The Trek520 comes with a DeoreLX rear derailleur so I'm suspecting I won't have too many problems.

Richard
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Old 01-19-07, 10:00 PM   #16
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The rear derailleur will cover aplenty. On my tandem, I know it covers a 47 or 48-tooth range.
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