I'm not mechanically inclined at all so I may not even explain this right.

In an effort to improve my low gearing, I've been looking at going to a compact gearing system. Right now my front gears are 53/39 and the rear is 12/27. I'm looking to change the front to 50/34 which I think will be sufficient. Maybe someday I'd look into making the rear a 10-speed 11/27 if they ever offer it.

Anyway, I'm getting conflicting info. I have a dura-ace front derailer and an FSA carbon crank. So far everyone seems to say the front derailer is fine, even if I change later to a 10-speed too.

But one source says I merely need to change the rings to the ones I want while another says the compact setup has something (bolt holes maybe) at a different spacing and thus you MUST change the crank too which means lots more money.

Then the first guy tells me that is true only if you go with the compact setup, but I gather from him that it isn't the 50/34 that makes it "compact". I thought it was, so what's the deal here?

Can I switch to a 50/34 without changing to a compact crank too?

What makes it "compact"?

If it isn't the rings that make it compact, what is the advantage of a compact set-up compared to one with the same front rings that isn't compact?

If you want 50/34 chainrings, you have to replace your whole crankset.

Think of it this way. A chainring with fewer teeth will also have a smaller diameter. At some point that diameter becomes less than the crankset's bolt circle diameter.

Typical road bike double cranksets have a 130mm bolt circle diameter. I'm told that 38 teeth is the smallest chainring that will fit.

Compact road cranksets have a 110mm bolt circle diameter so smaller chainrings will fit.

I understand the reason why I would have to change - just trying to make sure it is always the case with these specific gear rings. I'm still getting conflicting reports. One shop agrees with you (and I suspect you are both right), but a friend says he talked to another repairman who says it may not be necessary. Since this particular repairman is arguably the best around, that is worth noting. When I get a chance, I hope to take my bike for him to see for sure.

My suspicion is that my friend either didn't pass on accurate info about my bike to him or he misunderstood what he was told! So while I wait for a chance to go there myself, I thought I'd do some basic checking with people here.

Thanks.

Take a metric ruler and measure your crank's bolt circle diameter. Chances are, it's at least 130mm.

Physically, your chain cannot tell what kind of crankset you have. A 39t chainring is the exact same on the 130mm standard crankset as the 110mm compact crankset. This measurement is the bolt-circle diameter of the 5 bolts holding the chainring to the crank.

Most likely you've got a 130mm BCD crank and the smallest you can put on there is a 38t. To figure out which you have, measure the spacing between two adjacent bolts:



A 130mm bolt-circle would measure 76.4mm between bolts and a 110mm compact-crank would measure 64.7mm. I run a 50/34t crankset with 13-24t rear most of the time. If I need even lower gears, I've got a 13-26t or even a 13-28t if necessary. A 50/34t crankset with 13-28t rear would effectively give you two lower gears than you have now.

Well, some cranksets have different bolt circle diameters, but 130 happens to be the most common for road bikes. Lots of mountain bikes, for example, have 104 bolt circle diameter cranksets but most of them have triple chainrings. I have to agree, however, that measureing your crankset is the place to start. Sometimes 1 measurement is worth 1,000 guesses.

Near as I can measure, it looks like it must be 130 mm or close to it. Tried to go by the shop with the really good repairman, but they opened two hours later than I thought and I couldn't wait until noon! I'll try again another day.

Thanks.

You might consider changing your large chainring only. Myself and others have done this. Many replace the 53 with a 50. You don't have to change your crankset. It's cheap & easy. Shimano makes a 50T riing but it usually isn't stocked. Mine cost around $30 and I waited maybe 10-12 days for it. Reducing the front ring by three teeth has the overall effect of reducing each rear cog by approxumately one tooth.

I've attached a spreadsheet useful for gear ratio planning and cadence.

"...the overall effect of reducing each rear cog by approxumately one tooth."
Please excuse the brain gas. I should have said, "....increasing each rear cog by one tooth."

Wouldn't that have the effect of increasing each rear cog by appx. one tooth?

Yeah, you're right. Take a look at my second post immediately following the 1st.

I'm curious as why you feel you need a compact. 39/27 is pretty good gearing for climbing. I've seen 220 lb dudes do 10,000 feet climbs on a 39/25. Not to be sarcastic, but training makes more of a difference than equipment.

Sound like money is not a concern for you, why not just go with a triple?

Seems odd that the only rings you can get for your cranks are 53/39. If you talk to your LBS I am certain that they will be able to source rings in the teeth size you want. On my Mountain bike gearing I can go a lot lower, but that is mountain bike gearing- or is it? I run a triple crankset- and use 48/36/24. I can get rings for my large ring up to 54- middle up to 42, and that is on an obscure Sugino crankset. The variety of rings made by Non original manufacturers is enormous. It could be that you have a crankset that cannot go much smaller, but have you thought about changing the rear Cassette? Could be opening another can of worms with new derailler being required, but Talk to your LBS and see what is possible.

Gently now. I am also looking at a compact. My road bike is great for rides around town, and I ride for hours on the flat without problems. But I can't make the hills on the Niagara Escarpment without putting in so much effort that I don't enjoy it any more. And biking for me is about having fun rather than proving to somebody that I'm strong enough to ride with the best of 'em. I don't want or need the gearing I have on my hybrid (which has a really tiny granny gear), but I would like to find those hills a little less painful. A compact seems an easier, and a more cost-effective way to go than a triple, although I'm intrigued by the idea of only swapping out one chain ring. Any more thoughts on that one?

Aaaaarrrrrgh! Aren't you guys paying any attention? Most road bike double cranksets come with a 130mm bolt circle, which means the smallest chainring you can get is a 38. A compact crankset is 110mm which lets you go down to a 34 tooth ring. Mountain bikes have much smaller bolt circles and thus are able to use smaller chainrings. Got it?

The other way to approach the problem is to swap the rear derailleur and cassette for a mountain setup. If you like really tight gear spacing, then you wouldn't like this, but it does allow you to go up to as much as a 34 tooth rear cog.

I just feel it would be mean to the road bike to put a mountain rear cassette on her. Don't ask me why. There is no logic in it, it just feels wrong.

That's what my wife has on her bike and the gaps between gears are too large. So I'm swapping it out and replacing it with a triple.

I had almost the exact setup on an old 12-speed I picked up at a garage sale. It had a 52/40 crank and a 14/28 cassette. My gear inches went from 40 - 100. I wanted the entire range lower. I spent considerable time researching gearing (thank you Sheldon!) and finally decided on keeping the rear since it was a nice, even spread and changing the front.

I installed (now don't everybody freak out here, it is my name after all) a 34/36 crank. This changed my gear inches spread to 30 - 70 reducing my low end by several mph and letting me use every gear. This set up is called "halfstep." The gearing is sequential and each gear is separated by a uniform measurement (in my case an approximately 8% difference in gear inches).

I don't know if this is anything you're interested in, but I am very satisfied with it and wanted to suggest it.

P.S. Nice spreadsheet, Don. I created one very much like it during my research and development.

To understand why you need a compact crank to go to a 34 tooth small ring, just go to the bike shop and hold 34 tooth ring next to your crank. Or hold a 39 tooth 130 BCD ring next to the 34 tooth. It is easy to understand once you see it. The bolt holes in the 130 BCD ring interfere with where the teeth are on the 34. You don't need to take anyone's word for it, take a look. Even a 38 with 130 BCD is pretty close.

The guy who says it may not be necessary may be thinking about swapping the cassette to get similar gear ratios.