Bicycle Mechanics - 39t --> 42t worth it?
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07-31-10, 05:46 PM
There is a recent thread about replacing a 42t chainring with 39t for better climbing. I have the opposite problem. I live in the Chicago area (pretty flat) and find the 39t chainring on my bike all but useless. I have a 10sp 12x25 cassette on the back and 53x39 on the front. Front and rear are both Ultegra (about four years old, 6600?). I had a bit of a hard time finding it but Lickton's has something:
0117-42 42t inner, fits -A outer (6500)
Will that fit what I have even though it says "6500"? What do the 'A' and 'B' designations mean? Are there other sources for 42t Ultegra inner chainrings? I take it that with a 10sp cassette there's probably much help coming from that end of the bike, though maybe 11x21 or 11x23 would help a little.
07-31-10, 06:45 PM
What really matters is the crank's bolt pattern diameter, and of course the number of bolts (yours should be 5-bolt, and probably 110mm BCD?)
If that's the case, then just about any 110mm 42T chainring will work, as long as it's ramped (made for shifting, not single speed)
EDIT: Just realized, it's a double, the inner ring doesn't need to be ramped. I'm used to running triples.
07-31-10, 06:54 PM
Actually you probably have a 130BCD. The 6500 should fit on the 6600, but not sure if you will lose any shifting performance. I would go with a 11-23 or 11-25 cassette.
07-31-10, 08:21 PM
I currently the same gears 53-39 x 12-25. I can easily pull 24 mph with my 39-13 ratio and can overlap down to 22 mph with the 53t up front so I have no problems on flatland and I, too, find my ratio all but useless. Just curious, where are you run out of gears? If its at the higher speeds I'd keep the same front rings and replace the cassette with 11-23.
07-31-10, 10:29 PM
I tend to ride with my 53t chainring and in the middle of the cogs on the cassette most of the time (17-20mph @ 80rpm). If I slow down a bit and am on the 19, 21, or 23 cog the jumps between the gears are kind of large for my old knees so I'd like to drop to a smaller chainring. With a 42t chainring the gears I want to move to are in the middle of the cogs (15, 17, 19, 21). With a 39t chainring they are well toward the small end of the cogs, so when shifting between 53t and 39t chainrings I find I need to march more or less from one end of the cassette to the other. Click to the other chainring. Now click three or four times to get back to the proper range on the cassette. Seems like I'd have less of that with a 42t chainring, maybe two clicks on the cassette.
In addition, when running 39-13 or 39-12 it seems to me that the chain hits the 53t chainring a bit. Maybe it's just extra noise from the sharp angle the chain makes (in which case a 42t ring wouldn't help).
Maybe an 11-21 or 11-23 cassette would be better, and if I needed it for some hills I could stick the 12-25 back on. I'll have to fool around some more with Sheldon Brown's gearing calculator, print out some options, and draw some arrows...
Thanks for the feedback,
08-01-10, 08:49 AM
Yep, you'll get much more use out of a 42 on flat roads. Many pros use a larger inner ring, like a 44 or 46, for tough flat races, such as the Paris-Roubaix
08-01-10, 09:53 AM
I note :there is a pattern for the shift pins and ramps machined into the outer chainring,
and that pattern is designed in relation to the tooth count of the inner ring.
which is why the outer chainring may be markeded with 2 numbers in it
thats the A outer, it would be marked X-42..
flat area with a 12 on the back you could probably lose both 39 & 53 chainrings
opt for a 42 and a 48, assuming you are not needing 40 mph finish line sprints
48-12 should be high enough, and the 42 fine for JRA.
A cadence of 80 rpm is pretty low, have you tried a higher cadence? Most experienced riders run 90 to 105 rpm.
08-02-10, 05:38 AM
I try to spin a bit faster, though I'm pretty sure I don't hit 90 very often. I just used 80 as a specific point from Sheldon's gear calculator.
My chainrings are 53/39 and my cassettes are 13-26. On group rides I'll ride up to 20 mph on the 39 without cross-chaining. Looking up the road I'll decide whether to stay on the 39 or shift up to the 53. Likewise if I'm on the 53 I'll ride it down to 18 mph and look up the road then decide whether to stay on it or go to the 39. Some of the steeper hills will drop me below 90 rpm but I always get back above 90 soon after, even when I'm tired. Many years ago I learned that a higher cadence helps develop stamina and keeps these old arthritic knees and ankles going.
No way would I trade my 39 for a 42.
08-02-10, 10:50 AM
Or possibly swap out for a cyclocross crankset that has a 46 or 48 big ring so you can stay on the big ring and avoid any front shifting. Sounds like it may do the job better. Or at least it would be worth looking at since it offers the chance of reducing your combined front and rear major shifts.
On hills I don't mind a faster cadence to reduce the leg load. But on flat portions I prefer to ride more at a 70'ish cadence where the max effort isn't required. I'm not saying if this is right or wrong but it's how I enjoy riding when I'm not being pushed for high outputs. When you're using the gear calculators it would pay to know what YOUR casual and performance cadence preferences are and to input those into the calculations. In that light you may want to get out and ride a few sessions with the present setup and get a better idea of how YOU ride at various times and factor that into the gear inch calculators before you splurge on new gearing. In fact it may well be worth the cost of one of the cheaper bike speedometers that has a cadence pickup to do this before investing in the new gearing.
When I find CL bikes that have standard road triples, the first thing I do is remove the granny ring, making it a 'tall double' (52/42). Even when you encounter hills, the 11-26 cassette is fine, and your legs get used to any increased resistance on the hills. Pure preference, but I agree with the OP - anything less than 42t doesn't really serve my cycling needs.
@BCRider - I also have a cross bike with the 46/36 and thought I'd hate the ratios, but they're not that bad. I spin out easily on descents, but I do not shift as often. I also have a hand built MTB frame built up like a cyclocross bike and runs a 44t front with 9 speed in the back. It is my favorite bike, although a 10 speed would be even better. The inner 32t ring is in case of chain drop since the chainline gets a little wacky on climbs.
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