Bicycle Mechanics - Looking for advice with mods to my wife's "new" Trek 7.6fx
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08-20-12, 03:30 PM
My wife just became the proud owner of a very lightly used 2008 Trek 7.6fx. This model has the Shimano M443 triple crankshaft and a 9 speed rear sprocket with "105" rear derailleur. Because 90% of our riding is done in pancake-flat terrain and the other 10% is gently rolling, she obviously has no use for the triple up front and we would like to change it out to a single - the existing 48 big ring would be fine.
Question is, what is the best way to do this and maintain the best chain-line? Is it:
(a) Pull the crank, remove the two smaller chainwheels, and leave the 48 in the outermost position
(b) Pull the crank, remove all of the chainwheels and put the 48 back on where the middle ring was
(c) Pull the crank and replace it with a dedicated single chainwheel crank
(d) Something else
Logic tells me that (b) should give me a completely acceptable outcome, but I tried this same process with a triple Sugino on an old Cannondale H200 with an XT rear derailleur, and the chain kept juimping after a couple of miles for reasons I could never quite figure out, but assumed had something to do with the chain-line (what else could it be?).
Don't want to have this happen to my wife's bike - I can shrug it off when it happens to me; not so when it happens to her.
Thanks in advance - all advice appreciated.
P.S. - Solved the Cannondale issue by turning it into a single speed - great around town bike!
08-20-12, 03:38 PM
Why Change it?
I live in a flat area and ride two bikes with triples.
I just stay in the big ring most days.
When did your state become Flat?
08-20-12, 04:00 PM
Would like to eliminate the whole front derailleur array- the rings have a ridiculously wide range (48-36-26) and the trigger shifters are a poor match with the Tektro break levers, making shifting (esp the long throw front) awkward at best. She will never use it, and for the sake of simplicity, it would just be easier to get rid of a moderately complicated mechanical device which requires maintenance, adjustment and is just something else to break. if she decides to divorce me and move to the mountains, it can be her new boyfriends problem.
BTW - it doesn't get much flatter than coastal Virginia and Eastern Shore of Virginia. You can ride for hours and not have more than a 75 to 100 ft elevation change the whole trip.
08-20-12, 04:38 PM
I think I could be sort of ignorant here, but I am guessing that one could maintain a better chain line with a triple front x 9 rear as opposed to a single front x 9 rear. Is this true?
08-20-12, 05:01 PM
What's your crank/chainstay clearance? with the middle and small chainrings removed get a BB cartridge with a shorter spindle.
08-20-12, 05:51 PM
Theoretically, the best chain line is zero deflection, front to back; that is, the distance (in the rear) from the center of the chain to the center of the rear hub is identical to the distance (in the front) between the center of the chain and the center of the bottom bracket. "Big ring front to big ring rear" and "small ring front to small ring rear" give the most deflection (lateral stresss on the chain and inclination for chain to jump off of the chainring). "middle ring front to #5 rear sprocket (with a 9 spd rear)" should be just about ideal.
Problem is, when I put this into practice on my Cannondale (single 42t chainring in number 2 position on a triple front going to a 7sp rear) it didnt work, so apparently I know just enpough to be dangerous - hence the original request for assistance.
shimano engineers put a lot of time into making those teeth on those chainrings throw chains easily...obviously on purpose, with a shifter, from one ring to another. you can take out the shifter but you're not going to like the end result without a derailuer already atttached to the front to keep the chain on. unless you replace it with a dedicated single ring crankset- i.e., one without ramps and such.
I think you have a tinkering itch you're looking for an excuse to scratch....
If she's not using it, put in in the preferred gear and leave it there. Entropy will lock in place eventually. Shifting is only awkward if you use it - which you claim she doesn't do. Maintenance is only needed if you want the parts to move, so you'll reach your goal again through simple negligence.
Or move big ring to inside, lock derailer in place by adjusting limit screws, might have to fit longer ones.
Or use a short length of cable anchored wherever handy to set derailer position.
For extra points, replace with unramped ring.
Beware of chain recommendation for the ring though. A ring for 3/32" chain should work, while a dedicated 1/8" SS chainring might be too wide.
But 48 is a fairly big ring even on the flat.
I'd need to be on a fairly big sprocket to get a comfortable starting gear together with a 48 ring.
I find it easier to drop a gear on the front rather than traversing all the way into the big sprockets when stopping in an intersection for instance.
08-21-12, 05:50 AM
Large chainring to middle position, chain guard in outer position, chain keeper to help prevent chain being thrown. You may have to get a new BB to assure proper chainstay clearance while maintaining a decent chainline. As, without a lower front gear, your wife will be using the entire width of her cassette more (even in gently rolling hills) you want to keep your chainring lined up near the middle or just slightly to the outside of your cassette to prevent the ridiculous amount of cross chaining that would occur if you left it in the outer position. If clearance is a problem you can get a 44 tooth ring and something like an 11-28 cassette to give you a decent range of gears for fitness and cruising while still having fair climbing ability.
BTW, the chain guard is to prevent the chain from being thrown to the outside and also eliminates that "missing" chainring look. If your wife commutes or runs errands with the bike, she might appreciate the clean pants legs as well.
08-21-12, 06:09 AM
Classic case of a problem that doesn't exist and no solution to satisfy the OP.
08-21-12, 10:43 AM
1. Who's this "we"? If it's not the wife's idea your time might be better spent shopping for a more comfortable sofa.
2. After removing the rarely used chainrings, what do you think you will have accomplished?
3. I'd choose "b". The nice things about moving the big ring to the middle position is it's cheap and relatively easy to do (you'll need shorter chainring bolts), it'll work fine, and it's easily un-done.
Best to leave it as is. Any change made to the crankset will devalue the bike.
The road may be flat but the wind blows or sucks along the coast. Those low gears will be appreciated in a strong head wind. I wish my bike had 4 rings up front.
If the shifter are junk on the bike, then consider replacing them with something better. For me friction shifters work great for the front rings.
If you are sure your wife want this modification, then easiest solution would probably be to move the outer ring into the middle position and put a bash ring or chainring guard in the big position.
AN even better solution would be to buy a new ring that is somewhere in-between the current big-and middle ring - like a 42 or 44 tooth so that the bike is still relatively useful into strong winds or up hills (while you may not live close to any, bicycles can sometimes travel long distances where there are hills) and use that in the middle position and add a guard. A 48-26 ratio is a pretty rediculous low gear, and will make the bike useless in all but certain conditions.
If she just always uses the big ring while it is in the outer position then she wil be cross chaining (putting the chain at too severe an angle) a lot of the time, and this will cause excessive drag and wear ont he drivetrain, s moving the ring is a good idea.
Edit: Also - remve the shifter if you want, but leave the front derailleur in place and centre it over the chainring witht he limit screws or a peice of cable to stop the chain from jumping off. THis is a common problem and leaving the derailleur on the bike is the easiest and best solution/
08-21-12, 06:09 PM
PHT - Thanks - excellent point re the ramped chain wheel design - will look for a 42T or 44T track or single speed chain ring
Myosmith - Thanks for the great suggestions - and for staying on topic and resisting the apparently overwhelming urge to delve into the motives behind this evil scheme of mine.
Diesel Dan - What can I say! First prize for the most constructive, cogent and focused reply!
Honestly a 48t chainring is big enough for most riders in flat terrain.
My suggestion is to buy the wife a computer with a cadence meter and encourage her to ride in the middle chainring (36t) with a cadence, above 90 rpm. She may well be capable of doing that now. Even if she is capable of higher cadence the 36 will be a great gear for worming up or for climbing hills. After she is able to maintain a 100 rpm cadence for an extended time a shift to the 48 may make her hard to keep up with.
08-21-12, 09:22 PM
Diesel Dan - What can I say! First prize for the most constructive, cogent and focused reply!
Is that sarcasm? I stand by my point.
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