LBS made a mistake
 

I took my bike to the LBS to have the middle chainring replaced I asked for a 39 tooth on the phone because that is what was on there. When I picked it up he didn't say anything about having to go with a different size ring now I have a 42 tooth. So I am running a 50/42/30.
1 Is there any reason I would want a 42 over a 39?
2 Should the LBS install the 39 no charge?

1. Entirely up to you, depends on what gears you ride the most. There is absolutley no way for us to tell.
2. If you spec'd 39 and they did otherwise then yes, or if you can tolerate the 42 a discount is in order.

Shifts between middle and big rings will be a little snappier.
Shifts between middle and small rings will be a little sluggier.
You'll be in the granny ring for an extended range before shifting to the middle.


If you insist... You'll be an annoyance but they're the ones who didn't hold up their end of the agreement.

Once I had a bike shop replace a stem for me - I told them I wanted a 12 and they put on a 14; I guess they just didn't have a 12 on the shelf... like I'm not going to notice THAT? I didn't take the bike out of the store, told them to do it right.

As cny-bikeman said there are two separate issues. The first is whether a 39t or a 42t is better for you. That's strictly your call.\

As to whether the LBS should correct their mistake, yes they should, unless there's a valid reason, acceptable to you, to leave the 42t in place. One possible reason might be that your outer was a phased match for a 42t, and they felt that shift performance would be worse with a different size. They should still have asked you first, but you might decide to keep it as is.

Should I take it for a ride and see what I think about it?

If its a chainring for a triple with index shifting intended, it will have ramps and pins on the left side , facing the 3rd ring
to help the chain climb up onto the chainring.

If it doesn't , it's an inside ring from a double

42t vs 39t?

you can figure out the ratio sets here: http://sheldonbrown.com/gears/

50/42 is an odd chainring combination and doesn't come OEM on any crank I know of so I doubt the combination is phased together. What I would worry about is will the front derailleur inner cage plate clear the 42T ring if it's height above the 50T ring is correct. Many front derailleurs have a minimum tooth difference requirement and it's often 10 teeth minimum so a 50/42 doesn't meet it. It can be made to work only if the front derailleur is set too high above the 50t for optimum shifting.

That wouldn't be fair to the LBS. The first step is to call and tell them about it. If they say come in and we'll fix it, that's what you do, If they try to talk you into keeping it, then ask if trying it would cancel the option to switch.

Otherwise if you ride it, you'll be asking to swap a now used ring for a new one, and might run into resistance.

Haha yeah dude, go ride it!!!, dont listen to all these guys technical phase factor nonsense (no offense) the 42T wont have any negative effects on shifting that you will really notice enough to want it changed, and you'll be a little faster in your primary gearing, have fun

take it back and get what you asked for. period. there should be no additional charge.

Called he's trying to tell me that the standard size for a middle ring on a shimano triple is 42 teeth. Everywhere I look even on the shimano website the triples that are offered in the ultegra group are 52/39/30.

Not sure what crank you have, but on a Shimano 130bcd triple a 42t is standard for the middle chainring. Could have been an honest mistake, or all they had on hand and used it. Either way they should put on a 39t for no charge if that is what you prefer.

Yes, the 42 is the standard middle ring on a 52/42/30 Shimano triple crank such as former Ultegra and 105. Have the LBS look at the current 105 triple crank. It's 50/39/30.

That's pretty interesting gearing.

Even more interesting, I have a 1983 Schwinn Voyageur SP with a Sugino TAT Triple 50/46/30. Chainring sizes seem to change with the seasons... or decades as it were.

Yeah Sugino has always made some really nice cranks that are more along the lines of touring. I recently purchased an RD2 130bcd with 48/38/26 gearing for my Trek 520. Just thought it a little different for Shimano's road lineup.

I have the 52/42/30 triple setup. I never use the 52 (not strong enough to need to be able to go over ~22mph sustained yet). If I had a 39, I'd be shifting all the time and have to use the big ring as I would find it gimp for most applications around here when I don't need the granny. There are some rides that I never leave the 42t chainring if there are no long hills (not common, but feasible if I plan it out).

I am kind of the opposite on the bike I have 52/42/30. I use the 52 most of the time and the granny on hills. The 42 is too much for hills and not enough for flats. I have been thinking of switching the 42 to a 39. Ive got an old Fuji double that came with a 53/42. Hate the 42 on it as well and switched to the more modern standard 39.

That was "half step plus granny" gearing that was common in the era of 5 and 6-speed freewheels. The closely spaced bigger two chainrings let you split the big gaps in the freewheel and the granny provided a bail-out gear for tough climbs.

Appreciate the explanation, and of course, that makes total sense the constant relationship between the chainring and freewheel. In this case, 6 speed freewheel. Thanks for the history lesson and terminology "half step plus granny". I just got this Voyageur for a vintage project and continue to learn more about it.

What crankset do you have? If it is a newer Shimano with 50-39-30, you should stick with the 39 middle. As mentioned above, the fds are different and one designed for 39t middle may not clear a 42t middle when set to proper height. LBS may have had an older 42t they wanted to move....

Probably a cadence difference. Sure I could use the 52 for teens mph and up, but the cadence would be too low for me, causing my knees to want to kick me off the bike for being mean to them :)

Cadence is what I'm worried about, not being able to keep it high enough in the 42 when shifting up a gear.

Dude, just ride your bike and see if you like it, it's three teeth! It'll make you faster and you'll get stronger riding it.. If you decide you hate it a week from today the bike shop did make a mistake and I'm sure you'll get a 39T installed for free.. Another thing, this is your Bianchi? you should ride a double on that thing anyway

your desire over-rules the LBS's notion of orthopraxy. i hate it when an LBS tells me what they think is the correct gearing on something, as though they rode my bike with my knees day in and day out. in a situation like this they have no excuse for not following your orders; and you have no reason to grin and bear it if you don't want to.