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Gear Ratio Question for Road Bike

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Gear Ratio Question for Road Bike

Old 10-01-12, 02:37 PM
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SellOut
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Gear Ratio Question for Road Bike

I'm currently planning on switching my gearing on my road bike. I've got a 53/42 front ring and a 7 speed 13-21 freewheel. I am upgrading to an 8 speed with a 53/39 front and possibly a 13-23 cassette (its a C&V bike and I want to keep components from the era). I'm just wondering if that will be enough. I've got some 12+% grade hills around - they are fairly short but steep - and I can't spin fast enough and I've got more lung capacity on these hills than leg strength.

I looked up Gear Ratios on Sheldon Brown's wonderful website and I've got these ratios:
- Lowest Ratio Gear with 53/42 - 13-21: 3.9
- Lowest RatioGear with 53/39 - 13-23: 3.3 (with 3.6 and 4.0 incremental gear ratios).

So basically I will get two lower ratio gears with this new setup. I'd like to keep a 16T in the mix. Does anyone have any experience with this? Should I go for a bigger rear cassette or do you think the 13-23 will be enough.

I may just need to bite the bullet and go without a 16T and get a 12-25?

[Note that I was editing my post to correct wording about Lowest Gear (it originally said highest gear) when Scott was posting his reply.]

Thanks in advance.

Last edited by SellOut; 10-01-12 at 03:24 PM.
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Old 10-01-12, 02:40 PM
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I think your math is off -- your high gear stays the same (53/13). You gain a lower low with the 39/23, which should help climbing hills. Only you can tell what will be enough, though.
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Old 10-01-12, 03:15 PM
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I think you're on the money; bugger losing the 16t.

Still, it's more like a 17t when you swap the 42t for a 39t.

Scott, I don't see any mention of top gear.
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Old 10-01-12, 03:30 PM
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Originally Posted by SellOut View Post
I've got a 53/42 front ring and a 7 speed 13-21 freewheel. I am upgrading to an 8 speed with a 53/39 front and possibly a 13-23 cassette (its a C&V bike and I want to keep components from the era). I'm just wondering if that will be enough. I've got some 12+% grade hills around - they are fairly short but steep - and I can't spin fast enough and I've got more lung capacity on these hills than leg strength.
So your low is going from 42x21 to 39x23; well that's lower, but I gotta say 39x23 is not a low gear.
You could always go 11x28 8-speed, that one has a 16t.

Originally Posted by Kimmo View Post
I think you're on the money; bugger losing the 16t.

Still, it's more like a 17t when you swap the 42t for a 39t.
If he likes his 42x16, he should look for a cassette with a 15t: 12-25 and 12-26 both have this.

42/16=2.625
39/15=2.600
39/16=2.438

Last edited by DiabloScott; 10-01-12 at 03:37 PM.
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Old 10-01-12, 03:36 PM
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OP said the hills are only short... my low gear is 42x23, and that's low enough to get up some pretty steep stuff if it doesn't go on for long.

Guess it depends what you weigh. Modern stiffness features help a bit too (31.8mm, 1.125", 24/30mm).

Originally Posted by DiabloScott View Post
You could always go 11x28 8-speed, that one has a 16t.
Pretty sure there's nothing special about the 16t per se, especially when changing the chainring size. More a question of unwanted gaps, in which case advocating a cassette going to 11t will fall on deaf ears.

Last edited by Kimmo; 10-01-12 at 03:40 PM.
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Old 10-01-12, 03:52 PM
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Originally Posted by Kimmo View Post
Scott, I don't see any mention of top gear.
Neither do I, now.

Anyways, the OP could use a 13-26 7-speed cassette if his 39/23 isn't low enough. It loses the nice 13-14-15 at the top, but yields a solid 40" low gear.
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Old 10-01-12, 04:00 PM
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Not mentioned was the RD.. classics like N Record work with a 26t
26)39 = 1.5:1..
S Record later cage was OK for 2 more teeth
28)39 = 1.392..
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Old 10-01-12, 04:08 PM
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It is indeed hard to know as a 3rd party what will be low enough, but you could try this if possible. If you can locate a 12% grade and a 10% grade. If you lower your lowest gear as you've proposed the 12% now should feel like the 10% when you have changed, going purely by percentage gear ratio change (3.3/3.9*12).
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Old 10-01-12, 04:30 PM
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All - thanks for the great responses. I will be upgrading to either Chorus or C-Record - I believe they both support 26T. I think given all the feedback, it may not be a bad idea to try out a 13-26 or 12-25 8 Speed Cassette. I've got a bad knee and my lungs are usually in decent shape so its better for me to spin. There are many points that I had not considered. In particular, I did not consider that the ratio of my 16T will change when I change out my 42T front for a 39T - so the feedback was great.
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Old 10-01-12, 04:41 PM
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Originally Posted by SellOut View Post
All - thanks for the great responses. I will be upgrading to either Chorus or C-Record - I believe they both support 26T. I think given all the feedback, it may not be a bad idea to try out a 13-26 or 12-25 8 Speed Cassette. I've got a bad knee and my lungs are usually in decent shape so its better for me to spin. There are many points that I had not considered. In particular, I did not consider that the ratio of my 16T will change when I change out my 42T front for a 39T - so the feedback was great.
You could also move to a triple with a tight cassette in back where the Racing Triple derailleurs would be stylistically correct for the tail end of the C-Record era.

30x21 is the same low gear as 39x27 or a hypothetical 42x29 and with 8 cogs allows for 13-14-15-16-17-18-19-21 so you get one set of gears which are smaller than 42x28 for the mountains but act as a 7-speed corncob for flat rides; although you'd need to run something other than a Campagnolo cassette since they quickly discontinued all but three combinations after introducing 9 cogs.

I did that when I moved beyond 52-42 with six speed free wheels and never regretted the move.
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Old 10-02-12, 08:01 PM
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Your strong pushing that gear ratio. I run a 26t with a 50/34, use to be a 52/42/30 and I wish it had been a 50/39/30. I've done 17% grades with the 34/26t. I'm 60 yrs old, 200 lbs.
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