Author Topic: Gear increments  (Read 3072 times)

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Offline peterharris

Gear increments
« on: June 19, 2012, 09:40:14 am »
I had a hybrid, that I've since sold, that I bought off-the-rack and that had really smooth and even increments between gears as I shifted. What the bike lacked was a low enough granny gear for the hilly countryside I ride in. I'm getting up there in years and some of the hills in my area are short but pretty steep and I wanted a pretty low gear for doing those. I swapped out the cassette and got the granny gear I wanted but I lost some of the smooth, even increments between the other gears. I suppose the bike manufacturers try to select a chainring-cassette combination that gives those nice, even increments between gears.

I want to swap out the cassette on my new bike to get lower gears but I don't want to sacrifice the even shifting the bike currently has. Is there a formula or algorithm or something that I can use to determine how many teeth need to be on each sprocket of a new cassette so that I can keep those nice, even increments?

Offline dkoloko

Re: Gear increments
« Reply #1 on: June 19, 2012, 10:13:37 am »
Buy a copy of Berto's "Upgrading Your Bike"; tell everything you need to know. Out of print, but a few copies new, and number used are available.

Offline Pat Lamb

Re: Gear increments
« Reply #2 on: June 19, 2012, 11:48:06 am »
Work with the gear calculator at http://sheldonbrown.com/gears/ -- put in the cassettes and cranks you're looking at, and see how well they work out.

Don't expect a miracle.  Most of us are too lazy to meticulously plot out shift patterns and do double shifts (front and rear at the same time).  And unfortunately, most of the cassettes have ridiculously small cogs so the manufacturers can sell the high gears, which few people need or use effectively.  So to get your low gears, you pretty well have to accept bigger steps between gears.  (You effectively have to buy two cassettes to get Sheldon's 13-34 century special.)

Having said that, some crank and cassette combinations give you multiple gearing combinations that are nearly identical, while other cranks with the same cassette will give you a double shift that's intermediate between the single shift options.  It's worth looking for those if you're going to be replacing cranks, IMHO.

Offline RussSeaton

Re: Gear increments
« Reply #3 on: June 19, 2012, 05:20:07 pm »
A no longer used gearing pattern called Half-Step Gearing gives very consistent steps between every gear.  It has a unique, repeated shifting pattern for every shift.  It requires shifting the front derailleur on every shift.  And half the shifts require shifting both the front and rear derailleurs simultaneously.  But you do get that nice even consistent shift between every gear.  With modern 9-10 speed cassettes and triple mountain cranksets with limited choices for chainrings, and very expensive rings, the era of half-step gearing has probably passed.  Its more of a novelty to be different than a useful gearing pattern today.

I assume you want even steps between gears when shifting the rear derailleur.  You will more or less leave the chain on one of the chainrings in front, outer or middle, and shift up and down the rear cassette.  To get nice even smooth shifts on the rear cassette you want minimal jumps between cogs.  Something like 13-14-15-16-17-18-19-21-23 for a 9 speed cassette produces super smooth steps.  On small cogs you want small jumps.  On bigger cogs you want bigger jumps.  Its a percentage thing.  1 tooth divided by 12 teeth is 8.3%.  2 teeth divided by 19 teeth is 10.5%.  3 teeth divided by 28 teeth is 10.7%.  Close enough.  Even jumps in percentage.  Problem is you can't get exact jumps due to the fact cogs are available in 1 tooth jumps only.  Percent difference between 11-12-13-14-15-16-17 is not exactly the same.  Fairly close though.

For a touring bike you want 1-2 tooth jumps on the small cogs and 2-3 tooth jumps on the medium cogs and 4 tooth jumps on the large cogs.  For 9 speed cassette Shimano makes a XTR 12-34 cassette.  12-14-16-18-20-23-26-30-34.  SRAM makes a 1070 10 speed cassette in 12-32.  12-13-14-15-17-19-21-24-28-32.  Both of these cassettes provide smooth jumps between cogs in the back.  Shimano and SRAM also make 9 and 10 speed cassettes in 11-32, 11-34, 11-36 with sort of similar jumps in cogs to those listed above.

The problem with a touring bike is you want a wide range in the cogs.  From 11 or 12 or 13 or 14 on the small cogs to 32 or 34 or 36 on the big cogs.  That is about 20-22 tooth spread.  You have to have kind of big jumps in cogs to cover that range with 9 or 10 cogs on the cassette.  With racing bikes you have cassettes that go from 11 or 12 on the small cog to 23 on the big cog.  So you are covering 11 or 12 teeth spread with the same 9 or 10 cog cassette.  You are covering half the range with the same number of steps.  Easier to get the steps really close and smooth.  The original cassette on your hybrid bike was maybe 12-25.  The new cassette you have to get the lower gears is 11-34.  The range is about twice as big with the new cassette.  Yet you still only have 9 shifts to cover it.  Jumps between cogs are not as incremental.

Offline bogiesan

Re: Gear increments
« Reply #4 on: June 20, 2012, 08:41:46 am »
Peter you're making it too complicated. If you follow the advice given and do the math, you will see there is magic cogset. Only competent shifting. It only takes a few climbs to realize where the low end is and how to get there. On my recumbent, the granny on the triple is tiny but it only provides a very small difference in the middle ring/large cog gear. I only need it occasionally but I'm glad it's available.

The standard cassette may not offer the largest cog you want so add a few teeth. The style of shifting where you actively use the triple is how most of us find incremental inches. But I've got to say looking for a smooth shift pattern is only necessary if you're tracing. And you're not doing that.
I play go. I use Macintosh. Of course I ride a recumbent

Offline RussSeaton

Re: Gear increments
« Reply #5 on: June 20, 2012, 10:09:43 pm »
On my recumbent, the granny on the triple is tiny but it only provides a very small difference in the middle ring/large cog gear. I only need it occasionally but I'm glad it's available.

I'm curious to know what chainrings you have on this bike.  I have two bikes with triple cranksets.  One is 52-42-24 and the other is 44-33-20.  There is a very noticeable difference between the middle ring and the inner ring no matter which cog on the cassette I am using.  The 52-42-24 triple uses a 13-28 cassette in the mountains.  There is a world of difference between 42x28 and 24x28.  The 44-33-20 triple uses a 11-32 cassette.  There is a pretty good size difference between the 33x32 and 20x32 gears.

Lots of people for unknown reasons leave their triple cranksets as it came from the bike factory.  They are not wise enough to put on the smallest possible inner chainring.  Road bike cranksets come with triples with 53-39-30 chainrings.  That 30 tooth inner ring is idiotic.  All these cranksets use 74mm bcd inner rings and can accept a 24 tooth inner ring.  Why people use triples with 30 tooth inner chainrings that aren't much lower than the 39 middle ring instead of a truely low 24 tooth inner ring is a mystery.

Offline bogiesan

Re: Gear increments
« Reply #6 on: June 22, 2012, 12:21:52 am »
I have no clue what the tooth count is. I believe the big ring is 52 and the small is 24, it's a mountain package. The cogset goes under 1-to-1 so there must a 28 back there.

This is getting off the topic but I'll continue to explain what I mean by little difference. I don't feel it and that's really all I care about. Mathematically there may significant change. Well, let me rephrase that without going back and erasing stuff. My practice hill outside of Boise is 4,000 feet in 16 miles. I'm in the center ring and the largest cog for 70% of the climb. There are a few steep pitches that I don't mess with anymore and I just drop into the granny ring and the largest or next one or two cogs. The bottom end of the gear set, those three cogs on the granny ring, are, of course, the only useable  gears. The other six cogs/granny are all repeated in the middle ring's range, which, of course, overlaps considerably with the large ring's range. A 27-speed transmission only has about 15 distinct gear ratios with overlap and a few incrementals. I've got the shifting pattern memorized and don't think about the fancy front/rear combinations that allow a smooth, tooth by tooth progression from ultragranny to tallest.

Sorry for misstating my point the first time around. I still say there's no magic cogset, only competent shifting.
I play go. I use Macintosh. Of course I ride a recumbent

Offline Clem

Re: Gear increments
« Reply #7 on: June 22, 2012, 03:15:31 pm »
Compared to the ”good old days,” 9 spd clusters are heavenly compared to the old 5 speed stuff. With them it really was a matter of picking and choosing how to cover the desired range and the desired steps. Now with 9 sp. stuff I can have my cake and eat it too. As a certified old geezer, I live very comfortably with 20-32-42 rings and 12-36 9 spd clusters. In 700c I get 15” to 95” range with nice intervals. I am happy as hell with current gearing. I only wish I had it 40 yrs. ago.

Offline driftlessregion

Re: Gear increments
« Reply #8 on: June 22, 2012, 11:14:00 pm »
I'm with Russ in advocating for replacing the 30 with a 24 for loaded touring. Shifting is not the smoothest but hey, the view of the mountains more than makes up for that.

Offline mcparsons

Re: Gear increments
« Reply #9 on: June 23, 2012, 10:04:32 am »
+1 on going with the smaller triple. 

With the stock gearing on my bike I used maybe 5 gear combinations 90% of the time.  Most of that on the smallest chain ring.  And I could not pull the trailer up some hills even when standing on the pedals. 

I counted teeth and created a gearing chart, converted to inches, with the gear combinations that I used highlighted.  I took that to my LBS and he found a 22-32-42 triple that really opened up the power train for me.  Now I use 10 gear combinations 90% of the time.  Even better, the whole thing is arranged so that the first chain ring has all my climbing gears, the second is for flat or shallow grades and the third is for those rare, smooth downhills where I want to break 30 mph. 
My lowest gear went from 27" to 18".  I have not met a hill I can't climb with a 50 lb trailer load (yet).