compact gearing question

[d_douglas]

I am sure this has been asked already, so please redirect me if a reply is redundant.

Is it not possible to have the same gear ratio by using a compact crankset with a small (11-23 or so) cassette as by using a standard crnakset with a large (say, 13-26) cassette?

I just threw the numbers out there, so I don't want to get into the gear inch details, but just want to know if gearing can be compensated for that easily? (big chainrings + big cassette = small chainrings + small cassette)?


I have a 34t inner ring on my FSA cranks that is just too easy to spin, so I am thinking about gearing solutions. I would like to use a Campy carbon crankset and am wondering whether to just buy a standard crank and use the same size cassette or to get a compact 50 x 36 (or 38t) and use a smaller cassette (the cassette probably one more season left in it as well)

Next stop is the classifieds to find a great deal on a Campy carbon crankset - 175mm - in either standard drive or compact drive... Can be an older one as I am fine using a square taper BB.

Thoughts?

[Smiley]

I just ran the reverse numbers in Gear Inches for a client looking to mimic a triple wiith a compac and we we off by no more the 4 GI both high and low with a 50/34 and a 12/29 cassette . I think you can do it but somebody will need to do the quick and easy math.

[Richard]

Running the numbers will show that you can accomplish gearing coverage that you are talking about. The real issue is in gear ratio pattern. You find yourself having to make more multiple and simultaneous front and rear shifts than before. But you can find that you have a more useable range in each of the big and small rings.

[regularguy412]

-- about a compact is that the 'steps' between gears, both in the big ring and the small ring, are closer together. This is helpful to be able to fine tune your cadence when climbing, rolling along on the flats, etc. However, close gearing can become a pain in the arse if you ride in a lot of small, rolling hills. You find yourself constantly running up and down through the gears due to sudden terrain changes. You may want to consider borrowing a bike with the gear setup you desire and riding it a few days on your normal routes. In this way, you can see how various gearing affects your cadence and your overall riding comfort.

Mike in AR

[d_douglas]

Does this mean that the shorter the chain and tighter the combination, the more efficient it is, in spite of the ability to match gear ratios from standard to compact?

So that means that I could usea shorter cage derailleur as well, right?

This makes sense to me - is this correct?

[Richard]

There is no mechanical efficiency change with a shorter chain. Basically, a chain is a chaing. Certainly you can use a short cage derailleur if you were using long cage for a triple. There would be no change whatsoever from a 53x39 to a 50x34/36

[Dave]

-- about a compact is that the 'steps' between gears, both in the big ring and the small ring, are closer together. This is helpful to be able to fine tune your cadence when climbing, rolling along on the flats, etc. However, close gearing can become a pain in the arse if you ride in a lot of small, rolling hills. You find yourself constantly running up and down through the gears due to sudden terrain changes. You may want to consider borrowing a bike with the gear setup you desire and riding it a few days on your normal routes. In this way, you can see how various gearing affects your cadence and your overall riding comfort.

Mike in AR

Modern 10 speed cassettes have 1-tooth changes for 6 or 7 cogs. After that, the jumps increase to 2-teeth. It's not necessary to change cassettes at all, just because a compact crank is installed. It would be common for users of both to have a 12-25 cassette, although the compact user could opt for an 11-23. The only difference is the 11-23 has one more 1-tooth jump than the 12-25. Both cassettes have the first 2-tooth jump at the 17-19, but of course this jump occurs at a lower gear-inch value wiht the compact.

The big difference with a wide spaced compact is the additional 1-cog shift after every chainring shift.

To answer the OP's question, yes you can duplicate gear ratios quite closely with either a compact or standard crank. The simple equivalent gear formula is handy. For example, if you know that you don't need a gear ratio lower than a 39/25, then: 25/39 x 34 = 22. This tells you that an 11-21 cassette would not provide quite enough lower gearing, while an 11-23 would, since 23/34 x 39 = 26.4. This tell you that a 34/23 is a little lower than a 39/26.

If you want the closest match to a standard crank with a compact, you need a 50/36. Actually, the best match would be a 49/36, but no one offers one.

The other question is if a 34 is too easy, then why even consider a compact?

[regularguy412]

The other question is if a 34 is too easy, then why even consider a compact?

That's what I was trying to get at, but didn't do a very good job of articulating. I'll confess that I've never ridden a compact ring set, but I ride with several guys that do. We all come over the top of a little roller and I hear them shifting up at least 3 or 4 cogs while still in their small chainring. Now maybe they just don't know that they should likely just hit the big ring instead, but all the shifting suggests that the overall gear inch increase when upshifting across the rear cogs is smaller than with a standard bolt-circle style crankset. I typically ride a 42 in front, and find myself only upshifting one or two gears on the same rolling hills.

To me, it's just a lot of unnecessary shifting -- at least on that type of terrain.

Mike in AR

[RPS]

If you want the closest match to a standard crank with a compact, you need a 50/36. Actually, the best match would be a 49/36, but no one offers one.Dave, the 49/36 is very close but only when reducing the cassette by one tooth -- which is typical. However, for the few who use 13T cassettes, the numbers would be more like 45/33 if they wanted to go from a 13T to an 11T. :beer:

[Dave]

Dave, the 49/36 is very close but only when reducing the cassette by one tooth -- which is typical. However, for the few who use 13T cassettes, the numbers would be more like 45/33 if they wanted to go from a 13T to an 11T. :beer:

But in both cases, there's no good reason to use a compact. Might as well stick to a standard crank, if you don't take advantage of wider spaced rings.

[RPS]

Dave, I’m not a fan of compacts yet, but will disagree with you on one point. For those who are riding 13T cassettes and want very low climbing gears, going to a compact will give them greater options even with equal spaced rings.

You are absolutely correct if we compare 53/39 with non-standard 45/33 rings as long as the total ratios are equal -- like 13-29 versus an 11-25 (although there is a slight advantage to the compact in that the required rear-derailleur total capacity would be 2T less).

However, a better comparison in my opinion would be an 11-27; in which case the slightly larger gear spacing will add up over ten cogs to give the rider 10% more range.

53/39 with 13-14-15-16-17-19-21-23-26-29T

versus a similarly spaced

45/33 with 11-12-13-14-15-17-19-21-24-27T

This may not seem like much, but the compact above would give the same climbing ratio as a 39-32.

[Dave]

Dave, I’m not a fan of compacts yet, but will disagree with you on one point. For those who are riding 13T cassettes and want very low climbing gears, going to a compact will give them greater options even with equal spaced rings.

You are absolutely correct if we compare 53/39 with non-standard 45/33 rings as long as the total ratios are equal -- like 13-29 versus an 11-25 (although there is a slight advantage to the compact in that the required rear-derailleur total capacity would be 2T less).

However, a better comparison in my opinion would be an 11-27; in which case the slightly larger gear spacing will add up over ten cogs to give the rider 10% more range.

53/39 with 13-14-15-16-17-19-21-23-26-29T

versus a similarly spaced

45/33 with 11-12-13-14-15-17-19-21-24-27T

This may not seem like much, but the compact above would give the same climbing ratio as a 39-32.

All you're doing is comparing two cassettes with different ranges, since the rings have a nearly identical ratio: 53/39 = 1.359 and 45/33 = 1.363. You're not gaining any range with the 45/33.

27/11 x 13 = 32

Use a 13-32 cassette with the standard crank and you'll have the same range. 13-14-15-16-17-19-21-24-28-32

[RPS]

Exactly Dave; but apparently I'm not making my point clear. What I'm saying is that by starting with a numerically smaller number (i.e. -- 11 versus 13) a cassette with near-identical jumps (i.e. -- spacing) will end up with a larger range (total high-to-low spread). And in my opinion that's an "advantage" for the compact -- however small it may be. I'm saying that the advantage in this case does not come directly from the identical chainring spacing, but from the resulting improvement in the "matching" cassette.

Another way to look at it is that if we compared an 11-25 to the 13-29 (same range as you suggest), the compact would have better gear selections IMO (based solely on personal preference).

Once we get into the 13-32, we are out of normal road-bike area as well. With an 11-27, standard short cage derailleurs can be used; not so with a 13-32. Additionally, the jumps in a 13-32 are larger.

As as said, IMHO a compact in this case gives the rider more "options".


P.S. -- What I'm really saying is that I don't see any benefit to using cassettes that start with a 13T. I would personally go with an 11T or 12T and use smaller rings.

[palincss]

Does this mean that the shorter the chain and tighter the combination, the more efficient it is, in spite of the ability to match gear ratios from standard to compact?

Absolutely not. You take a big efficiency hit with sprockets smaller than 14T. If it's efficiency you care about, the larger the sprockets the better.

[RPS]

You take a big efficiency hit with sprockets smaller than 14T.Care to define big? As I recall, this issue is not that clear. :confused: