Author Topic: Trek 520 chainring  (Read 2281 times)

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Offline Old Guy New Hobby

Trek 520 chainring
« on: April 25, 2010, 03:02:22 pm »
I'm just starting out. I rode 2K miles last year on a hybrid and decided I am interested in touring. I bought a new Trek 520 this year and have about 500 miles on it. I'm still working up to my first tour, but I have noticed the gearing is too hard for some of the hills. (At least for me.) The new Treks come with a 11/32 9-speed cassette which I like a lot, and 26/36/48 chain ring. That gets me down to about 22 gear inches. I would like to swap the 26 T for a 22 T, which would get me down to 18 gear inches. But even with the 26 T, the chain starts dragging on the front derailleur 4 gears up from the bottom on the cassette. 

Can the front derailleur be adjusted to work with a 22 T chain ring and give me at least 5 gears on the cassette? Will it work dependably? Or should I settle on a 24 T, which at least gets me to 20 gear inches? Does anybody have experience with this?

I plan to take advice offered in several other posts and include a chain stop to prevent the chain from rolling off the inside of the cassette.

Offline whittierider

Re: Trek 520 chainring
« Reply #1 on: April 25, 2010, 04:34:16 pm »
Quote
I would like to swap the 26 T for a 22 T, which would get me down to 18 gear inches. But even with the 26 T, the chain starts dragging on the front derailleur 4 gears up from the bottom on the cassette.

You shouldn't be using the smallest four cogs when you're in the smallest chainring anyway.  The cross-chainging reduces the life of the equipment, and those gears overlap ones you can get in the middle ring.  The same goes for using the largest few cogs in the big ring.  Get those ratios in the middle ring to keep a straighter chain line.

According to Trek's website, the new 520 has the M543 crankset.  I can't find it on Shimano's website which is not as good as it was before they revamped it a year or two ago; but since the middle ring is smaller than 38, I guess it's not a road triple.  Road triples normally have a 74mm BCD for the inner ring which lets you get down to 24 teeth.  5 bolts is standard.  If this one is a 4-bolt one for MTBs, you can probably get down to 22 teeth.

If you want to get lower, you could use a cassette with a 34-tooth big cog.

Since my wife is anything but a climber, and we've done some super climbs like 8 miles of 10% on our road tandem, I put a 24T small ring on it and a 13-34 cassette, giving us a low gear of about 19" with 700c wheels.  So the crankset is 24-42-52, shifts fine (as well as can be expected for an inexpensive crankset whose chainrings don't have ramps and pins), and never drops the chain.  One secret to adjusting the front derailleur to work that well is usually missed:  The outside plate of the cage should not be perfectly parallel to the outer ring, but instead, the rear of the cage should be slightly farther out than the front is.

Offline Old Guy New Hobby

Re: Trek 520 chainring
« Reply #2 on: April 25, 2010, 05:05:28 pm »
Thanks Wittierrider. Sounds like the 22 will be fine.

Just to clarify: the 26 chain ring works with the 4 largest cassettes now (that's not cross-chaining). Moving from a 26 to a 22 would move the gears down by two cogs compared to what I have now. The overlap between the 26 and the 36 is 2 - 3 cogs. With a 22, the overlap would be 4 - 5 cogs. That's why I need at least half the cassette with the 22 T. (At least, that's what I'm thinking). The chain isn't rubbing against the side of the front derailleur. It's  rubbing against the bottom of the part that guides the chain back and forth. The problem is right now is small chainring - middle cog lowers the chain until it's riding the derailleur, if that makes sense. I looked, and the front derailleur can be lowered. I'm just not sure how much.

Offline whittierider

Re: Trek 520 chainring
« Reply #3 on: April 25, 2010, 06:46:51 pm »
Going to a smaller granny ring will give you less overlap between ranges.  But using the granny ring on my single (non-tandem) road bike with a 30-42-52 crankset, only the three biggest cogs (26, 23, & 21 teeth) on the cassette give lower gears than I get in the middle ring.  The fourth one (19) gives a gear that's nearly a perfect duplicate of the lowest gear in the middle ring, and even that is bigger than the middle cog (17).  It works fine in the next smaller one after that too (15) but there's no reason to use that combination.

Quote
The chain isn't rubbing against the side of the front derailleur. It's  rubbing against the bottom of the part that guides the chain back and forth. The problem is right now is small chainring - middle cog lowers the chain until it's riding the derailleur, if that makes sense
Yes; but the angle of the front derailleur as I was talking about is to improve shifting and keep from dropping the chain.

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I looked, and the front derailleur can be lowered. I'm just not sure how much.
Most front derailleurs should be put almost low enough to touch the big ring as the outer plate of the cage goes by it during shifting, but a few work better if there's a couple of millimeters separation.  Either way, it's closer than a lot of poorly set up ones are.

Offline Old Guy New Hobby

Re: Trek 520 chainring
« Reply #4 on: April 25, 2010, 09:14:53 pm »
I get it. Thanks again.