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Without getting crazy into the math....(7 posts)

Without getting crazy into the math....Geof
Dec 4, 2001 5:00 PM
In terms of cadence and climbing would a 42-53 with a 12-27 be basically the same as a 39-53 with a 12-25 or totally different? Which would be prefered overall for cadence and Colorado climbing. I know there are lots of factors here (power, strength etc.) I'm just looking for a 'generic opinion' on the gear crossover with these two setups...

Thanks...
Can't Help Itgrzy
Dec 4, 2001 5:47 PM
If you don't do the math on something this easy then you're soon going to be discussing the feelings of the gears and their hopes and dreams. The numbers say it all.

42/27 = 1.56

39/25 = 1.56

Multiply the above results by 27 (or your actual wheel/tire diameter) to get "gear inches".

So your lowest gear options are the same. Your top end is the same 53/12. It's the way that the in between gears are distributed. But given that there are still the same number of gears you can always find a higher or lower gear when in between the extremes. You need to do the same math drill in a table form for all gear combos to see if you have overlap or a good distribution with minimal duplication. A spreadsheet can do this in a snap for you. So really it's a wash - run what ever is paid for and don't expect to make the mentioned change and see any difference. Now if you run the 42-53 with the 12 -25 and compare it to the 39 - 53 with the 12 - 27 you will see a huge difference.
re: Without getting crazy into the math....collinsc
Dec 4, 2001 5:53 PM
using a handy program i found, GearCalcPro

42-27 is 40.94 inches
39-25 is 41.06 inches

and the min/max speed at 80/115 rpm is 9.74/14.01 and 9.77/14.05 respectively

minimal difference there, but would I be safe to assume that the decision lies between a double and a triple crank? Though Ive never to Colorado, I imagine it is quite hilly. a 30 tooth on the triple would get you up just about anything I imagine.

When I rode the triples on my friends bikes i found that the 30 really only slowed me down, and thats here in the pacific NW. I have a 12-27 cassette mated to the 53-39 and think i would be happier with something like 13-23 for the closer ratios for cadence reasons. Having the 3rd chainring, on the other hand, would obviously give you 9 more gear choices.

Depends on how big the hills are and how much you are willing to hurt to get over them faster i guess.
Interesting Observationgrzy
Dec 4, 2001 6:01 PM
Ii too find that having lower gearing avaible slows one down b/c you tend to use it. However, what I have noticed is that on a really long or nasty climbing ride the lower gearing allows you to keep going. It's the tortise and the hare thing all over. I have a bud who's lowest gear is a 39/23 and can climb anything in it - he's really strong. The difference is that after a short while he's toasted and we're still going. I'll cringe the day he lowers his gearing.
The story... the dilema...Geof
Dec 4, 2001 7:53 PM
Thanks grzy for the input. It tells me a lot. I could have done the math but wondered how it panned out in real feel. I'm sort of new to the road thing (mtb racer) and I recently purchased a dura ace crankset in the Cyclist bargain bin... Bought it, then discovered the inner ring was a 42. I currently run the 12-25 cassette, and yes, I felt a huge difference. Right now (winter) this is not a big deal because most of my time will be spent on my MTB in the snow or on my rollers to work on cadence etc. So I figured no biggy. However, I started to look at my options to get a 39 for the front for spring/summer and discovered my only options were both chainrings or a cassette. The 42-53 combo is an A Type setup and the 39 only comes in the B type rings. As usual Shimano goes specific, but, I'm wondering how much difference there is in the ramping of the two types... Hmmm... remember 9 speed cranks won't work with 8 speed stuff (right)...

Anyway, so the dilema was raised with the cassette option. A triple is not really nec as I'm a (slow) but strong climber. I figure my power should be increased by the 42-25 at this point and I'll do the 12-27 later in the year.

Thanks guys...
The story... the dilema...grzy
Dec 5, 2001 9:09 AM
Interesting - the devil is in the details. Somehow I thought that you could get away with just swaping the inner CR. Might as well try it, then if that doesn't work our well get the outer CR. I haven't really heard of anyone having problems doing this - FWIW. Some guys are running the TA chainrings with much success - I think the pair is around $100 - last time I saw them in a LBS. You can even get some colors if that's your thing - or maybe a discount on an unpopular color.

Ultimately you're probably going to be happier with the 53/39 up front - the 42 is tighter and preferred by many racers. Everything changes when you start looking at long grinding climbs as part of long rides. I'll run a 53/39 & 12-27 for the steep hills in NorCal, but then I'm more of a spinner and not a masher.
re: Without getting crazy into the math....brider
Dec 5, 2001 6:59 AM
Two things -- The 39/25 option will be slightly lighter, and won't necessitate a long-cage rear derailleur (I don't know for sure that the 42/27 option would necessitate it, but it increases the likelyhood of it), but it will increase chain wear. It's a trade off, but personally I'd go with the 39/25.