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Standard to compact gearing questions

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Standard to compact gearing questions

Old 09-15-13, 07:25 AM
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wirides
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Standard to compact gearing questions

Now that I am getting more serious about my riding, rather than just riding, I am finding that compact gearing would be a better fit for me. Right now I am riding a 2012 Supersix with standard 53/39 crank. My questions is what is all involved with making the switch to a compact 50/34 crank? Assuming I go rival to rival what role does the bottom bracket, chain length, and derailleur adjustment play? Long story short, is it as simple as swapping cranks? Thanks in advance!!
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Old 09-15-13, 07:33 AM
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It should be as simple as changing cranksets if both are Sram Rival. You might also shorten the chain by 3 links, but it might not be needed.
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Old 09-15-13, 08:03 AM
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...and lower the FD ~ 6mm to account for the smaller big ring. Chain length will decrease by 1/2 the number of the tooth difference, since the chain only wraps halfway around the ring. Going from 53 to 50 is three teeth so half of that is 1.5 links, but you can only change chain length in 2 link (1") increments, so it's likely removing 2 links will be appropriate, but you need to check to ensure that won't make it too short when in the big ring and big cog.
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Old 09-15-13, 09:19 AM
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Thank you for the responses. It's unclear to me...are all the cranks the same...are they all compatible with the same bottom bracket? I want to order online to avoid the huge shop prices but be assured I am buying the correct item.
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Old 09-15-13, 09:26 AM
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no .. good luck ..

The help you get going thru a bike shop is missing bypassing them ..
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Old 09-15-13, 10:06 AM
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In my case I swapped a standard Shimano Ultegra crankset for an FSA Energy compact. I bought a octalink version of the Energy crank so it was a direct replacement for the Ultegra. I lowered the front derailleur a few millimeters and shortened the chain. The only thing that I had to buy was the crankset and the swap took me less than an hour including readjusting the front derailleur.

I think that it's probably the best equipment change that I ever made on a bicycle. Essentially I traded a couple of high speed gears that I never used for a couple of hill climb gears that I desperately needed.
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Old 09-15-13, 11:31 AM
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@retro -- I am in the exact same boat regarding the unused vs climbing gears. I like climbing but not the gearing I have to do so. I was hoping my only outlay would be the crank itself.
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Old 09-15-13, 11:35 AM
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Not answering your question, but agreeing to the swap. Hard to believe that many riders need a 53 tooth chainring, especially given that the rear is usually an 11. Wouldn't you need to be doing about 30 or 35 mph to use that combination? I hardly even use my 50 (compact double).

I spun mine out once and that was at 55 mph (downhill of course). Don't want to go that fast on a bicycle again though
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Old 09-15-13, 01:48 PM
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Originally Posted by Ridefreemc View Post
Not answering your question, but agreeing to the swap. Hard to believe that many riders need a 53 tooth chainring, especially given that the rear is usually an 11. Wouldn't you need to be doing about 30 or 35 mph to use that combination? I hardly even use my 50 (compact double).

I spun mine out once and that was at 55 mph (downhill of course). Don't want to go that fast on a bicycle again though
On my just-built faster bike, I switched from 48 to 46T, so now 24-36-46 with a 12-28 cassette, because I am much more likely to use a smaller outer chainring in my riding. Although I do enjoy fast descents (35-40+ on the right roads), I found that trying to get another mph downhill took a lot of energy.
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Old 09-15-13, 04:09 PM
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How about just swapping chainrings? Why is there not much out there about that simple and seemingly cheaper option...to be correct, the crank I have now is SRAM S500, not Rival like the rest or the group.
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Old 09-15-13, 04:13 PM
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I just did this today. Ultegra standard to compact. I swapped the crank, lowered and adjusted the FD, and removed three links from the chain. It wasn't difficult at all.
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Old 09-15-13, 04:26 PM
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Originally Posted by laynlow View Post
I just did this today. Ultegra standard to compact. I swapped the crank, lowered and adjusted the FD, and removed three links from the chain. It wasn't difficult at all.
Any reason you could not just swap rings?
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Old 09-15-13, 07:18 PM
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Originally Posted by wirides View Post
Any reason you could not just swap rings?
The arms that the rings mount to are longer on a standard crank. They won't match up.
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Old 09-15-13, 07:35 PM
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Damn, I guess it is what it is...either way I think I will be very happy with the expenditure.
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Old 09-15-13, 08:04 PM
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Your bike has a BB30 bottom bracket shell. You'll need a compatible crankset. Cheapest SRAM-branded one seems to be this one: http://www.universalcycles.com/shopp...7&category=521 A brief google search revealed no BB30-compat Rival cranks, but that doesn't necessarily mean they don't exist. It just means I didn't linger on it. That s550 crank costs about $165.

You could save $65 and buy a 12-28t 10speed cassette. The 50/34x11/26 you're thinking of would give you a gear-inch spread of 34-119.5. If you stay with the 53/39 and run a 12/28 cassette, it'll give you a 36-116 spread, with tighter gaps between most of the shifts.

Choice is yours, but I like the savings, easier install, and tighter ratios that come with keeping your cranks and getting a bigger cassette. No tinkering with the FD, or adding/subtracting links, or other needless worries. Compacts are trendy, but they're far from being the panacea that they're marketed as. (I do like compact doubles, for the record. )

Last edited by surreal; 09-15-13 at 08:11 PM. Reason: clarificationization
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Old 09-16-13, 03:48 AM
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Surreal, that is extremely helpful, thank you. I considered that but I was left with the impression the crank swap would give me a better climbing ratio and it appears it does. What I don't know is how the numbers translate to real life. Here's the thing, I am basically finding myself at or near the top of the cassette in the big ring or too far down the cassette while in the small ring. I am trying to accomplish too things: 1) better / easier climbing and 2) find gearing that will put me more in the middle of the cassette during my normal riding conditions.
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Old 09-16-13, 08:03 AM
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[QUOTE=wirides;16069954]Surreal, that is extremely helpful, thank you. I considered that but I was left with the impression the crank swap would give me a better climbing ratio and it appears it does. What I don't know is how the numbers translate to real life. Here's the thing, I am basically finding myself at or near the top of the cassette in the big ring or too far down the cassette while in the small ring. I am trying to accomplish too things: 1) better / easier climbing and 2) find gearing that will put me more in the middle of the cassette during my normal riding conditions.[/QUOTE

play around here: http://sheldonbrown.com/gears/ You'll find it offers you to play with the theoretical math without spending any money, but of course there's no substitute for actually riding a given setup. TBH, even if it weren't cheaper and easier, I think the closer-spaced ratios of the 53/39x12/28 would be preferable to going compact-- it obviates the highest and lowest ratios given by the 50/34x11/26, and gives you more to work with in the center of the cassette. Plus, if you're already spending a lot of time in smaller cogs while on the 39t ring, moving to a 34t ring will just exacerbate that problem.
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Old 09-16-13, 08:37 AM
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Originally Posted by wirides View Post
Any reason you could not just swap rings?
Bolt Circle Diameter (BCD), the theoretical diameter of the circles the chainring holding bolts are on, aren't the same. A standard Shimano or SRAM road crank has a BCD of 130 mm while the compacts have a BCD of 110 mm. The smallest chainring a 130 mm BCD will accept is 38 teeth.
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Old 09-17-13, 06:41 PM
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Thanks for the input. I really appreciate it.
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Old 09-17-13, 07:05 PM
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Originally Posted by HillRider View Post
Bolt Circle Diameter (BCD), the theoretical diameter of the circles the chainring holding bolts are on,
I always wondered why they reference the diameter instead of the radius - seems like that would be more intuitive and easier to measure... at least on the crank, not on a loose chainring. Must be a machining thing.
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Old 09-17-13, 07:16 PM
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Originally Posted by DiabloScott View Post
I always wondered why they reference the diameter instead of the radius - seems like that would be more intuitive and easier to measure... at least on the crank, not on a loose chainring. Must be a machining thing.
Maybe I'm misunderstanding you but I'm not sure how you think measuring a radius is any easier than a diameter. Neither is easy to measure on a five arm crank but you can fairly easily eyeball the centerlines on a four arm crank. I have some attachment arms with conical ends that I can mount on my calipers to get pretty accurate (within 0.10mm at least) center to center distances of holes. For a five arm crank you can figure out the bolt circle diameter once you know the hole to hole distance of adjacent holes. There's a formula somewhere for it.
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Old 09-17-13, 07:21 PM
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Originally Posted by joejack951 View Post
For a five arm crank you can figure out the bolt circle diameter once you know the hole to hole distance of adjacent holes. There's a formula somewhere for it.
Yeah, that's just simple trig.

But if you want to know if you have 130 or 110 BCD, and you can lay a ruler across the crank and measure 55 or 65 mm from the centerline to one of the bolts - seems like that would be easier. Although newer cranks don't have a crank bolt to speak of so there goes that idea too.
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Old 09-17-13, 08:22 PM
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So how many BCD are commonly used? 110, 130, 144, what else? Spend some time searching eBay for chain rings and you will see it is not hard to mix and match what you want. That said, you may discover you have some parts left over by the time you have exactly what you want.

Same for cartridge BBs, there is something to fit nearly anything. Sheldon will get you started and them just start measuring stuff. You can put together just about anything you want. Hell, I just bought the crankset I most liked the looks of and built it from there.
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Old 09-17-13, 09:15 PM
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for a long time, there were relatively few BCDs. However, it seems like the big manufacturers are going to great lengths to make these things even more complicated, especially in the mtb arena. Disgusting, and needless. These manufacturers can get the job done with 110/74 BCD rings, but they keep playing around with new "standards", including thoroughly obnoxious 3-bolt circles.

Just be thankful, wirides, that you've got a "normal" crank, and the replacement crank is "normal", too at least in regards to BCD. (I'll hold my tongue on the bb30 thing.)
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Old 09-17-13, 10:07 PM
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Campy OS UT cups for BB30. $30.

Campy Super Record Titanium 11 Speed Compact. Today at Wiggle for $528. No 170mm left.

That's the best solution. If you don't think so, at least buy it and flip it on eBay. Any American would pay $599 on the spot for it.
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