FlatMaster

I have a century coming up with a big climb at mile 80, so I need some lower gears. my current setup is...
52-42 Chainrings biopace
13...28 Uniglide 6 speed
Shimano 105 Shortcage mech from the 80's

I've found that the lowest I can on the chainring is 38. However, this isn't a huge difference.

My question is what is the largest I can go on the rear cassette? What would I want to measure on my RD to determine what size cog will fit?

Sheldon has letter codes for different shimano Casettes. Where is this code printed?

DannoXYZ 

There is a difference, going from 42 to 38t is roughly one gear in the back. It's just not as much as you want or need.

Unfortunately, your gearing may be at the wrap-limit of the short-cage RD. You can get cassette cogs up to 34t if you call around, but the RD won't be able to accommodate it. The easiest solution is a compact crankset with 52/34t. You'll end up with two lower gears than the 38t or 3 lower gears than the 42t. A significant reduction for those hills.

FlatMaster

I know it would be bad practice, but technically, could I put on a larger rear cog and avoid the extreme cross chaning that is the combination that requires the most chain? Do short and long cages both allow the same distance of travel for the jockey away from the axel?

fuzz2050

Lowest I can think of going on a conventional road double would be 27 gear inches (with 28mm tires, this is a century after all). To get that you'd need both a new cassette and a new crankset. With a compact double (can be had fairly cheaply, any 110 bcd crank will work) your smallest chainring is 34 tooth. Then you get a freehub transplant (uniglide is so 1990's) and get a new mountain cassette with a 34 tooth large cog. This gets you pretty damn low gears, without the need for a triple.

You probably would need a new rear derailer though, a short cage couldn't handle the range well enough

HillRider

It not only won't handle the wrap but it probably won't clear a 34T cog either. A low line Shimano MTB rear derailleur (LX or Deore) should do both.

FlatMaster

Well, part of the goal was to try to keep it vintage, but I guess that's gone. Anyway, I could always change it back. I know I want it to stay a double and uniglide.

By the way, it's not 90's, It's 80's. Pink, white, and turquoise.

top506

Go with the 38 up front and a 30 in back as your low gear.
You'll have to re-size your chain, but that's it.
Top

meb

The 34T won't, and a 32T is borderline clearing the 105 rder.

If you want killer low gears, Mountain Tamer Quad can get you down to 17T rings:
https://www.abundantadventures.com/quads.html

Spicer makes (or maybe it's made since I can't find it on their website) a chainring-cassette adapter that lets you mount chainrings to your cassette:

https://spicercycles.com/sitesearch.c...pe=0&pageid=90

HillRider

Yeah, I know, but I said exactly that on another thread and someone posted that he knew two cases where a Ultegra rear derailleurs worked with 34T rear cogs. So, now I say "probably won't".

Gonzo Bob

If your 13-28 6-speed is really a cassette and not a freewheel, you could get a couple of 16 and 34 loose cogs
https://www.loosescrews.com/index.cgi...id=34154805759
https://www.loosescrews.com/index.cgi...id=34154805759

Your 13-28 is probably 13,15,17,20,24,28 so you could remove the 15 and 17 and replace with 16 and 34 to get 13,16,20,24,28,34 - or you could just get the 34 and replace the 28 with it to get 13,15,17,20,24,34 but that last jump from 24 to 34 is really big (although should work as Shimano's megarange 7-speed does the same thing with 11,13,15,18,21,24,34)

You would likely also need to use a Shimano MTB rear derailer. If you have an MTB with Shimano parts, borrow the derailer from that. Otherwise new MTB rear derailers are inexpensive.

superjohnny

Without side-tracking the conversation or stealing FlatMaster's thunder, what is the lowest gearing for a SRAM Rival equipped bike (50/34 chainrings & 12-25 cassette). Anyone know how big of a rear cassette I can run? Any idea if I can squeeze a Shimano LX/Deore cassette back there with a 30t ring and still clear the rear derailleur & shift?

DArthurBrown

I know the feeling. I have a couple old bikes that I've had similar issues with. The simple conclusions are, sadly, 1) new stuff is just better than old stuff, even the old good stuff, and 2) old bikes are usually quite expensive to modernize properly.

Even modern Alivio rear ders are quite superior to old Campy or higher-end vintage Shimano. The old parts look flashy and have nostalgia tied to them, but as is the case with old Ford Mustangs, they're actually not great compared to modern stuff.

The above post about switching out the crankset will likely be the easiest. A 34T chainring up front with a 25-28T cog in the back will get you up anything comfortably except the most absurd hills.

FlatMaster

Welp, I've decided to go with a new compact crankset. My spindle was wearing out anyway, so A whole new BB was a good idea. i'm going from 42 - 52 to a 34-50. I hope it clears the FD. I'm having the LBS do it because I don't have the BB tool to take off the drive side cup. I'll save on Labor by taking everything off before I take it in. A new crankset, chain, and wheel truing is around $150. This is in Alabama. Thanks for the input everyone.

FlatMaster

One more question. They said the newer modern chainrings are closer together, requiring a new, narrower chain. My question is where will my large chainring be? If the small ring is so small, I immagine I'll want to be on the large ring on the most of the time and use the small for large hills. Will I end up cross-chaining too much?

FlatMaster

I guess I'm asking if the new large chainring will be closer to the frame, allowing access to larger cogs.

noglider 

Get a longer derailler and a bigger cluster/freewheel. That's what I did. If you want, after the century, you can put the original parts back on. I think I have a 34T cog on my freewheel.

Al1943

The location of the chainrings shouldn't be much different than they are now. They will probably be slightly closer together but I doubt that you need a narrower chain. It's more important that the chain be compatible with the cassette (or freewheel). Since you now have a 6-speed you could probably go with a 7 or 8-speed chain. Measure the chain for stretch, you may need a new chain anyway.
For a century I'd want to add a water bottle cage to the seat tube.

Al

FlatMaster

It's supported. Water every 20 mi

Al1943

General rule about a century: Drink one bottle for each 10 miles.

DMF 

Unfortunately, you may be trading one problem for another. Dropping your gearing will drop your speed and that means longer in the saddle. I find that the limiting factor for me on long rides is more likely to be my ass than my legs.

RI_Swamp_Yankee

If the compact crank isn't enough, IRD sells a 2nd chainring with mounting points - you can turn your crank into a triple.

You can probably find vintage derailleurs that handle big cogs... accd. to Disraeli Gears, my old '70s- vintage Suntour GT can swallow a 34t. (Gonna find out - ordering a NOS Mega-range freewheel this weekend.)

StanSeven

????

Before advice like this is given, it needs qualified. Maybe that's the proper fluid intake when it's 100 degrees or very humid.

People die from taking in excess fluids. Too much water is called intoxication. When sodium is dangerously diluted, hypnatremia occus.

Even if that doesn't happen, 10 bottles on a moderate or cool day will have yoo peeing as much as you ride

DMF 

Note that the OP is in Alabama. Slightly different climate...

Al1943

That's why I didn't say anything about drinking water.
My comment was OT and I didn't want to get into details about what to drink on a century. But I couldn't help myself from saying something about an extra cage.
With 20 miles between stops one water bottle on the bike is not enough.

DannoXYZ 

Are you serious?