jonwvara 

I'm overhauling a "beta tourer" Bianchi Randonneur--a 1984 model, I think--and I'm scratching my head over the gearing. It has a nice half-step-and granny setup in front--a 50-45-28. That 17-tooth jump from the small ring to the middle is pretty big, but tolerable. I once set up a bike that dropped from a 42 to a 24, and it was okay, although it required a light touch on the upshift.

It's the six-speed Shimano freewheel that gets me. It's a 14-15-18-20-26-32. I can't figure out how to insert the gearing chart of that setup in this post, but it makes absolutely no sense. The steps between gears are just completely random.

The product managers who specified that must have been thinking something, but what? This would have happened soon after the switch from 5-speed to 6-speed freewheels. Maybe--given that there's no good way to get a perfect half-step setup with six speeds--they just threw up their hands and said the hell with it.

I may just add a spacer and install a 14-17-21-26-32 five-speed, which is my idea of half-step perfection.

Life as a gear nerd is just one compromise and disappointment after another.

Barrettscv 

Yes the 14-15 is odd. These normally have a progression to additional tooth count on the larger cogs, since the ideal interval is logarithmic, but the 14-15 doesn't work as a half step.

My 1972 Paramount that I purchased new in 1973 has 53 & 49 chainrings with a 14-16-18-23-26 freewheel. It made no sense.

T-Mar

That's not OEM gearing. According to my literature, the crankset was a Takagi AD with 52-47-34T chainrings, while the freewheels was SunTour Perfect with 14-16-18-21-24-28T cogs.

ThermionicScott 

^ That's a relief to this gearing dweeb. The freewheel in the OP is bonkers.

79pmooney

I calculated those gears and got (in gear inches) 96-90-87-81-75-68-68-61-54-52-50-47-42-38-38-29-24. OK, a pair of duplicates and 54,52 and 50, but not awful. No huge gaping gaps. 13 different gears.

I raced a 5-speed with 110-102-95-87-83-81-76-75-67-60. Almost exactly the same gears plus higher and no low ones. I was happy as a clam riding that. (I was a pure climber who knew that his only chance to inflict his will in races was to blaze uphill so I geared my bike so I had no choice.) Now I raced the then popular Campy standard 42-50something X a tight 13-19 FW. Very different shifting patterns and we never double shifted to fine tune gearing.

Ben

Salamandrine 

Replacement freewheel perhaps?

Irrational OEM gearing did happen. I remember one particular occasion of a customer bringing a new bike back. The complaint was that the gears seemed like they were all the same. Turns out they sort of were. I checked the numbers and every single half step was a duplicate, almost like they planned it that way. We had to swap a chainring IIRC. I gave the rep a hard time about it. They said they were trying to do a "half step kind of a thing." Clearly they didn't know how it worked.

That being so, as 79pmooney indcates, it's not that horrible. Could be worse. Having near duplicate gears in the 70" range where most people ride much of the time is a positive.

natterberry 

My ‘&2 Trek 614 came with a 50-45-28 and 14-30 6 speed freewheel. The steps were very terribly placed.

jonwvara 

Given your long experience with this kind of stuff, I'm happy to take your word for that. I just assumed the drive train was original because all of the components are have similar date codes. This one has an 86 BCD Sugino crankset and a Shimano freewheel. I'm probably going to replace it with a Suntour Perfect.

T-Mar

This isn't from experience. It's straight from the specs published in the July 1984 issue of Bicycling magazine. The frame's serial number should tell us the month and year of manufacture, which should allow us to determine if it actually is a 1984 model.

jonwvara 

Serial number is CS307225.

John E

14-15 makes sense with an alpine (1.5 step) up front. I set up my mountain bike with 48-40-28* / 12-13-15-17-19-22-25-28
For half-step it indeed does not make sense.
_________
* I have a 24T, as well, but I have to upshift very carefully/gently with the 16-tooth drop

I agree -- I have always hated and wondered about that gap right where I would want a good density of ratios.
It would have made perfect sense as a 52-49 (which I also saw spec'd in the Schwinn catalogs) with a 6-speed in back: 14-16-18-20-23-26. This makes a very pretty half-step, which converts to a 1.5-step if you replace the 49 with a 44 for hill work.
Been there ... done that. I run 50-42/14-16-18-20-23-26 on the Bianchi, replacing the 42 with a 47 to make a nice half-step for flatter rides. When I was young and immortal I ran a 5-speed -- same idea, but without the 26T granny in back.

Road Fan

Let me try to analyze the existing gearing a little.

Your front pair is 50/45, setting the granny aside for the time being. The ratio between them is 1.111. The incremental ratio for the 6-speed should then be 1.222, or a 22% jump between adjacent rear sprockets.

For a 6-speed cluster you have 5 jumps. Assuming 22.2% that's a total ratio of 2.727 (raise 1.222 to the 5th power) across a 6-speed cluster. The small-big pairings (ignoring feasibility for the moment) would be 11-30, 12-33, 13-35, and 14-38. I think these are all problem gears to get, unless Pastor Bob can make you something special. But you don't really need a super wide range double because you will have a granny gear. So let's think of a more standard road set up then add in the granny thinking.

A more reasonable roadie setup back in the day was 14-26. let's calculate backwards to see what the chainrings need to be: overall rear ratio is 26/14 = 1.857. The cluster incremental is factor is 1.132, much smaller than the 1.222. We'll look at actual tooth counts in a moment. For a full step of 13.2%, we will have a half-step of 6.6%. The ratio between the big ring and the middle ring must then be 1.066, so for a 50 tooth big ring we want a middle ring of 50/1.066 = 47 teeth. So your chainset is 50/47. Two teeth make a big difference.

Now we use the 1.132 factor to get the cluster counts in raw form: 14, 15.85, 17.94, 20.30, 22.98, 26.01. Rounding those, we have tooth counts of 14, 16, 18, 20, 23, 26. This at least does not have any oddballs. The gear range (27"wheel diameter) will be 96 inches down to 49 inches. if you add a granny gear of 34 teeth, you can get a bottom gear about 35 inches.

The chainrings should not be too hard to find used. You would need a front derailleur that can raise the chain from 34 to 49, a 15 tooth jump. The total chain wrap is 50-34+26-14=76-48, so your rear mech needs to wrap 28 teeth. But if the granny shift is good you could make this and it would give you 10 evenly spaced gears to handle all the normal road riding tasks.

Other optimizations are possible. For example, the granny can get bigger if the rear big sprocket gets bigger. If you can get a log-spaced 14-30 made by Pastor Bob, you can make teh granny a lot bigger and easier to shift. A good target might be to assume a 10 tooth jump (easy front-end up-shifting!) down from the middle ring, for a 37 tooth granny. To get the 35 gear inches you need a 14-28 in the rear. Probably the rear cluster would then be something like 14, 16, 18, 21, 24, 28. Now the main gear range is 96" down to 45" roughly. This new configuration needs a rigorous numerical check. But still, 50/47/37 and 14-16-18-21-24-28 would be a pretty decent half-step plus granny. Chain wrap is 27 teeth. I'm not sure if the 47 middle ring should still be a 47, but that is one reason to repeat the rigorous numerical evaluation.

The other half steps suggested by JohnE would also be pretty good.

Ferrouscious 

This gearing makes sense to me. I would call it "cha-cha" gearing. You get double shifts less often if you plan the gearing carefully. The shifting pattern is a bit of a rhythm. Starting at the top end, it goes like so: (büm being the front shifts and cha the rear)

cha-cha, büm-cha-cha, büm-cha-cha, büm-cha-cha, büm.cha, büm.cha, drop.

jonwvara 

Cha-cha gearing! Brilliant! My problem is that I was expecting cold rationalism--I was deaf to the music. I can't dance, either. And thanks for figuring out how to post the gearing chart.

T-Mar

That would be a 1983 model, as the frame was built in Japan during March 1983. Freewheel specs were same as in 1984, but the chainrings were 32-46-52T.

jonwvara 

Thanks, T-Mar! I like the current chainring setup better than the original, and the freewheel is easily replaced.