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Gearing choices for 6 and 7 speeds?

Old 12-08-20, 11:28 PM
  #26  
83cannondale
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Had pretty good luck w 86 bcd

[QUOTE=bark_eater;21806937]Thank you all for the thoughts. I'm hoping to try a number of different set ups. As the bike is still a pile of parts I haven't found out whether I need to go with shorter 165mm cranks with the 650b. If 170mm cranks will work I'll be able to try a 46-30 up front. I hadn't realy considered running a half step and granny. That's how the bikes original gearing is set up. 50-45-32T and a 14-28t 5 speed. If I run a 7 speed cassette with a cog added on each end I can get a gear inch range of 24.4 to 118 which would certainly cover the range I would use locally.

......"I've had good luck with 6 speed 13-34 with 30,44,48 triple crank. Crank is 86bcd and will take down to 28 I think. These are half step with granny I think. Downtube, friction shifters. The gearing os good for pretty much anything though if I'm pedaling up a steep hill at 4mph or less I'll walk instead to stretch my legs. I really like the half step and the 30,34 is a nice bailout gear.. 48,13 for my big gear is pretty good and I seldom spin out.."

Last edited by 83cannondale; 12-08-20 at 11:29 PM. Reason: Separate from quoted entry
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Old 12-08-20, 11:49 PM
  #27  
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I was a huge fan of 53 (or 52), 42, 28 X fairly small and close freewheels forever until I aged, Now it is 50-38-24 X freewheels and cassettes getting as large as 28 teeth. (Basically the same thing, just easier.) Lots of redundancy and I love it. I spent most of my time before I went to the smaller chairings on the middle ring. Now, for the next couple of years I am relatively strong for the rings and I am about 50-50 large or middle and usually choose the one that puts the chain just to the right of center where the shifts are close and fine tuning is easy. (I have had 47 years to get tired of that chainring pattern and it hasn't happened yet. Everything else I have ridden has come and gone.)

I an likewise a huge fan of the old square taper 110-74 BCD cranksets. So versatile.
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Old 12-10-20, 10:05 AM
  #28  
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My Trek 610 now has a vintage Specialites TA 5 vis chainset, 170 mm, with 46-30. The great thing about this is you can further reduce that as deep as 24 teeth, if you can make sure to set up a front derailleur that can handle that. I also have a few 13-32 of 34 7s freewheels I stashed over the past few years. With such a wide front you don't have to worry too much about gear redundancy. And with remaining availability of TA spindles, it should not be too hard to tune the chainset offset for good chain tracking and stability and some options for chainline with 126 mm rear spacing. And with diligent periodic maintenance, lifetime of the BB bearing set could rival your own!

You won't get close gear spacing, but hey, Velocio lived with it (lol!)! The best you can do is one of two things. Shimano had a series of 6 and 7 speed freewheels whicn spanned 13/34 of 14/34. Those have the first 6 cogs relatively close-spaced, with a big bailout cog on the inside, and one of Shimano's good tooth designs. With the 26 to 32 tooth granny which are possible on a TA 5-vis, you can have a low/close running gear set until you need a stump-puller, perhaps 32/34 or better than 1:1. The high end would be 46/13 or 44/13. With a 27" wheel diameter, that gives you a top gear up to 95 or 96. The bottom would be about 22" with 46 front and 34 rear, rounging the wheel diameter to 27".

Or you can seek or make a freewheel with as close to an even ratiometric step between adjacent cogs, as possible. We can calculate perfect ratios until the cows come home, but real gears have only integer numbers of teeth, and well, the ratios will be the ratios. You can input the freewheel teeth into the common graphical gear calculator, graph it and eyeball the graph for evenness. You can also follow the math and optimize, using a good spreadsheet. I prefer Microsoft Excel because whenever I try this calc on the Google spreadsheet I cause it to crash. Excel can handle this bit of arithmetic and much bigger calculations.

Start by finding the total ratio of the freewheel. For a 13-34, 34/13 = 2.62. Now for example, the ratio between the first pair would be 14/13 = 1.08 if it's 13/14, or 1.155 if it's 13/15. To design we want to find 6 numbers which multiplied together equal 2.62, AND which are the ratios of real gear tooth numbers. In this case the ideal ratio is 1.174 (you can calculate this using logarithms), but obviously we cannot make a freewheel based on this number. We can work out an approximation, which would also result in a good solution for a wide range half-step.

For the first step we can choose 13/14 or 13/15. The ratios respectively have been calculated (above) as 1.08 and 1.174. We choose the second sprocket that gives a ratio closer to the ideal one. I also use a rule of not decreasing steps (monotonicity) - the first gap was 2 teeth so the second one should be at least two. So the next cog would be 17 or 18. For 15 to 17 the ratio is 17/15 = 1.17, and for an increase of three we'll have 20/17 = 1.1765, nearly ideal. So the design so far is 13-15-17-20. Reasoning similarly, the total design is 13-15-17-20-23-27-34. With this a step from the 5th cog to the 6th cog should feel just about hte same as from the first to the second. The big cog, 34 is a bigger step. If that last cog was 32, the step ratios would be about as consistent as possible given the first one is 13-15.

So if it meets your requirements, a slightly better design would be 13, 15, 17, 20, 23, 27, 32. With 46/30 in the front, it still gives you a gear range of 95.5 - 25.3.

If anyone is interested in the spreadsheet, I can send it to you email or maybe on PM here. I don't know if it wilt translate if imported, or remain functional. And you have to be able to open Microsoft Excel files.

Last edited by Road Fan; 12-10-20 at 10:08 AM.
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Old 12-10-20, 04:48 PM
  #29  
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Originally Posted by bark_eater View Post
Well I'm back at it with the gearing puzzles, and having started another thread over the summer:

50-34T compact double with 6 speed freewheels?

I figured I'd update this thread instead of starting yet another...

I ended up spending the spring and early summer on 700x28 bike with a 48-36t and a 6 speed 14-28t.

With current events I didn't do any century's or have the time to "train" but I did manage to get a good run of consecutive 100 mile weeks as proof of concept, for my riding at least.

Now I'm collecting parts for a 650b conversion of a 86 531 Trek 400T. I have a 165mm Sugino 110 BCD triple crank set. The plan is to run 48-34T chainrings and a 13-30T 7 speed cassette and 650bx38mm wheels.

The gearing is visually appealing on the calculator.

Gear-Calculator

Any thoughts? I'm making up the 7 speed cassette from available 8 speed cogs, so I could shift some of the cogs depending on what's in production right now.

Thanks, Woody
Looking at your gear calculator and javing just done my own calculation, I agree completely with what your example shows. Mine is only different in he two biggest rear cogs. Yours should work very well.
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