ginsoakedboy

I am restoring an old bike that takes a freewheel. Shimano has some nice 7sp freewheels, but large cog is 28 tooth on all of them.

For various reasons that I don't need to get into, I want to use current 105 or Ultegra rear derailleur, but they are both listed as max cog of 27t.

Can I fudge it by 1 tooth with the road rear derailleurs, or do I have to go to a MTB rear derailleur?

I'm looking for an answer to this very limited, specific question, and would very much appreciate hearing from you if you know the answer. I've considered and researched all other issues regarding compatibility of these parts so don't need general comments regarding my choices. Thanks.

SoonerBent

You'll have no problem. Shimano lists everything with a bit of a fudge factor. A 29 will even work.

ginsoakedboy

Thanks Sooner. Right after I posted this thread, I stumbled on another one about Shimano MegaRange 7sp Freewheel, which is the same freewheel I was looking at, but with a large "bailout" cog of 34t -- like what they used to refer to as Alpine gearing where the freewheel is normally graduated but with a single huge cog at the top for steep hills. Several replies to that post indicate that you can get away with up to a 30t large cog in combination with those Shimano road rear derailleurs. Thanks again.

TallRider

depends a bit on your frame - a 27t large cog is guaranteed to work with pretty much any road rear derailer from Shimano, but 29 and sometimes 30 will work on lots of frames.

Gonzo Bob

It'll probably work. I've run a 30T with my Shimano Dura-ace.

operator

Cheat.

ginsoakedboy

I'm not understanding how the frame would be a factor here? Please explain further if you would. Thanks.

DMF 

The chainstay lengths vary by frame, and thus the length of the chain varies. A chain has a two half-link minimum length increment but the chainstay variation doesn't, so one frame may have more slack to take up than another with identical gearing.

dzinehaus

oh what the hey, https://bikeforums.net/showthread.php...=1#post4743045

well biked

But they're talking about large cog capacity here, not chain wrap capacity. The variance timcupery is talking about would mostly be in the exact positioning of the derailleur hanger, it will vary some with different frames-

HillRider

The derailleur hanger lengths can vary somewhat on different frames and short hanger spacing could make the rear derailleur not clear a "too large" cog. This is unlikely to be a problem and almost certainly not with a 1 tooth cheat.

DMF is correct in his description of chain wrap differences due to chainstay length but it's not germane to this question.

Gonzo Bob

The thickness of the hanger can make a difference, too. A thin steel hanger will have the derailer sitting closer to the cogs. A thick aluminum hanger will give the derailer more room to drop down by the time it gets to the largest cog.

ginsoakedboy

OK, these are all the issues that ran through my head and caused me confusion when the frame compatibility issue was raised. If we're talking about the derailleur hanger not being long enough, then how would that issue be resolved by going with a MTB derailleur that would otherwise have enough range to accomodate a 32 or 34t large cog? It seems like this would be a matter of physical clearance between the derailleur body and the large cog and how would a MTB rd behave different than a road rd if that was the problem?

And I also read the response from "well biked" and said to myself "but wouldn't that just be a matter of getting a long enough chain".

And the only other frame clearance issue I could conceive of was if the dropout spacing was too narrow so that the chain would hit the frame -- but this seemed like it would only be an issue on the small cog, because the large cog would be well inboard of the seatstay frame member.

BTW - in practical terms, my question has been answered and thank you all for the replies -- now I'm just trying to understand on an academic level.

HillRider

The "parallelogram" of an MTB derailleur is longer so the jockey wheel sits further ftom the cogs despite the hanger geometry.

well biked

I'm not following you. What I was trying to explain in my earlier post is that the point where the derailleur is attached relative to the rear axle will have an affect on what size largest cog you can use without the upper pulley on the derailleur hitting the cog. And that will vary some from frame to frame.

Svr

I run Ultegra SS length cage rear derailleurs on all my mountain bikes with 32 tooth rear cogs. Works great as long as total capacity limits are observed.

DieselDan

I use a long cage 105 rear derailer with a 28T cog with no problems.

DMF 

It's the only explanation I could think of. So well written too.

I didn't know that hanger length is variable. I wonder why it isn't specified when the manufacturer specs the max cog size, then? Is there supposed to be a standard, more honored in the breach?

well biked

It's really not just "hanger length," it's really more accurate to think in terms of the distance from the derailleur bolt hole center to the center of the rear axle. Different dropouts have slightly different shapes, the hangers themselves have diffferent shapes, etc. Then there's the important factor of how far below the axle the derailleur bolt is, etc. 1 1/8" distance from center of derailleur bolt to center of axle is pretty typical, but it does vary some-

DMF 

Yeah, I know. But if I'd written "hanger radius" half the readers would be scratching their heads.