ricohman

My Roubaix Elite has only 150km so the chain is pretty much new. My Ultegra 39/52 arrived today and I was going to swap it on.
But before I get in over my head, do I need a new chain as the large ring is now 2 teeth bigger?

Scorer75

Depending on how much slack there was, u may need a new chain.

To measure, put the chain on the big/big combination without running it through the derailleurs. If they touch, add one full link and you are good. If they don't meet you need a new chain.

timeedgevxr

2 teeth on/off a chainring should make little difference - its a 4% increase in circumference.

ricohman

Not sure I understand this but I will try it.
I am going from a 34/50 to a 39/52.

merlinextraligh

If the original chain was sized so it just worked in the big/big combination adding two teeth could be the straw that wrecks the drive train.

You need to make sure the bike will go in the big/big combination without jamming.

It's likely that the OP's chain is long enough that it will be fine with the 52, but I'd check it, before I made the assumption that it will be fine.

This may help.

https://www.parktool.com/repair/readhowto.asp?id=26

DaveSSS 

2T is 1/2 inch of chain. You can only change the length in one inch increments. The absolute best way to set the chain length is to shift into the little/little, routed through the RD and make the chain as long as possible, without hanging loose or rubbing on the chain guide tab on the RD cage.

If the chain can't be removed, shift to the big ring and next to largest cog. See if it appears to have much slack left in the RD cage travel. If not, then it probably won't be long enough to shift up to the big/big. If it's really too short, the chain will probably refuse to shift onto the largest cog. If done slowly on the work stand, it's not going to break anything in the attempt to shift to the big/big.

Homebrew01

If you need a longer chain you might be able to buy a new chain, take a link off it and add it to your existing chain, and save the new, slightly shorter chain for later.

DaveSSS 

There really is no good way to add 1 inch of length to a modern 10 speed chain. With Shimano, they do not recommend having two joining pins placed the close together.

Scorer75

Why do people perpetually give bad advice on the internet?

Every resource from park tool to Sheldon Brown (RIP) recommends chain sizing by routing around the big/big combo bypassing the derailleurs. Where do people get this other crap from???

https://www.parktool.com/repair/readhowto.asp?id=26

https://sheldonbrown.com/derailer-adjustment.html#chain

ricohman

Thanks for all the advice.
Here are a couple of pics. I can still push the RD forward to create more chain slack so I think its ok.
How does the FD clearance look? To much?
I had to move the FD up a bit for the larger big ring.

ricohman

I should add that I would never run big/big anyway. But I have a 14yr old who just might...

stedalus

Chain length looks good. FD looks like it could come down 1 mm, but some teeth are taller than others, so check the clearance at all points in the crank's revolution. And really, unless you have shifting troubles, it's probably fine as is.

Scorer75, if you actually read those pages you cite, you'll see that you are the one who has no idea what he is talking about.

DaveSSS 

The length is probably the minimum possible. Next time, start with a 1-inch longer length. I think you'll find it to still have tension in the small/small. As others noted, you should never use this combo, but I see dummies do it and not just for a short while.

DaveSSS 

I get his crap from 25 years of bike mechanics, plus a mechanical engineering degree. You're the dummy who doesn't understand chain length.

The big/big method will work, but it suggests the minimum chain length. Modern 10 speed bikes have ONE chain length that will handle any cassette the manufacturer offers, within the RD's wrap capacity. There is no need to change the chain length if the cassette is an 11-21 or 12-27 (Shimano's full range). The little/little method suggests this chain length. The same is true of all brands. My Campy 11 drivetrain uses the same chain length for an 11-23 or 12-27 (Campy's full range). This length is set using the little/little.

ricohman

Hmm. My cassette is a 12-27 but this Roubaix is only offered with a 34/50 crankset. Have I exceeded the design capacity?
I just came back from a quick test ride and all seems ok. I am not planning on running big/big or small/small anyway. I just like the fact that it won't jam if someone shifted it there.
Now while trying to tighten up the FD cable I have screwed with the RD cable by mistake and it's riding up on the rear cogs and double shifting.
Should I just loosen it up and start tightening in small increments until its shifting as normal?

ricohman

Went for a nice ride tonight. Same 30km after dinner run. Up a series long hills turn around and back down.
Why I didn't like the compact is very clear to me now. On the flat prairie the bigger rings let me get right where I want to be without a lot of shifting. It always seemed I was cross chaining the compact. Either way I just didn't like it with the upshifts needed after dropping from the big ring. Is this just me?
Anyway, I think its a decent upgrade for those that live in the flats.

DaveSSS 

AFAIK, all brands have short cage RDs that will handle a 50/34 with the largest cog or largest range road cassette that they sell. The 52/39 crank reduced your wrap by 3 teeth, compared to the 50/34.

If you tightened the RD cable too much, of course loosen it until the shifts all work correctly. Tightening the cable makes the RD move further to the left (toward the larger cogs). You've probably got the cabel so tight that you're half way over to the next larger cog.

As for the change you made, using a 52T big ring instead of a 50 is only half a cog's worth of gearing (4%), so it won't make much difference, unless you were constantly needing something lower than the 50/24. The 39T little ring will make a bigger difference. The 39/15 is like your old 34/13.

ricohman

Thanks for the tips Dave. The RD is shifting fine. I had the cable to tight.
And you are right about the 39 tooth ring. Thats where its making the difference.

ImaPoser

Hoping I can just bump this thread and get an answer. What about going the other way? It seems all the bikes I have looked at have 53/39 and I am wanting a 50/34. Coming from a triple, I know I can't do the hills with a min. 39/25 combo. Would I be needing a new chain?

DaveSSS 

You might need a 1 inch shorter chain, but it depends on how it was setup to start with. If the chain needs to be 1 inch shorter, that can be done to any chain, either with a new joining pin or a masterlink to rejoin the chain. If the chain has a masterlink, then it's a real no-brainer.

Shift to the small/small and see if the chain hangs loose of rubs the RD cage. If not, you're good to go.

Here's another way to think about it. The RD has enough capacity to wrap the big/big if the triple was setup correctly to start with. That wrap is smaller with a compact and the little/little is larger so there should be no problem. All you're doing is using less of the available wrap capacity.

Retro Grouch 

One of us is measuring wrong.