Gear Inches

Gear Inches – what a curious term. Basically, it tells you how hard a bicycle will be to pedal: higher numbers are more difficult and lower numbers are easier. Specifically it tells you many inches the rear wheel moves when the pedals are rotated one complete turn.

The specific equation is: gear/inches = (front gear/rear gear) X rear tire diameter.

Two examples:

  1. front gear = 48T, rear gear = 11T, rear tire diameter = 25.4″, the gear/inch = 110.8
  2. front gear = 26T, rear gear = 34T, rear tire diameter = 25.4″, the gear/inch = 19.4

Why worry about gear inches, you ask? Whether looking at a new bike, changing your current setup, or comparing what friends are riding, gear inches provides an easy way to compare different bike setups.

When looking at a new bicycle, rather than saying, ‘I currently ride a 48T front gear, an 11T rear gear, and a 26″ rear rim with 1.75″ tire size. How does this bike compare?’ It could be simplified with a, ‘My current bike has an 111-19 gear/inch range. How does this bike compare?’ Then, if the gear/inches indicate that the bike will be too difficult (or easy) for you, you could discuss changes before purchasing it and discovering things are amiss.

In my case, recumbents come with many different rear rim sizes (700c, 26″, 24″, 20″) and many different tire sizes in both metric and English dimensions. I know my capabilities for pedaling a heavy load uphill, so knowing my gear/inches, particularly the lower number, helps me evaluate different bicycle brands and types.

Generally speaking, most bikes come with a higher gear/inch set up than the average cyclist needs.  According to many orthopedic doctors and professional trainers most bicycle related knee injuries come from pedaling a gear/inch that is too high. Using a setup that is too high results in a slower cadence and ‘muscling’ up inclines and rough roads. It may not be as sexy, but having a lower gear/inch setup will allow the average cyclist (like me) to have a higher cadence, resulting in less chance for knee injury. Seldom do I need my highest gears but my lower gears are used quite frequently.

On the fully loaded cross country bicycle ride I used a gear/inch of 111-19; the low end was just barely low enough on the steeper grades. I am now shopping for a different bike and am looking for something on the low end of 17-18. I’ll probably have to do my own gear conversion since ‘as advertised’ touring bikes don’t come anywhere near this low.

Rear Derailleur Capacity

Rear DerailleurI am not a small person and don’t resemble a Tour de France competitor in any shape, form, or fashion. As a result, I have had to change the gearing on most of my bicycles in order to get a reasonably high spin (cadence) going up hill in order to protect my knees.

One concern when changing the gear ratios on a bicycle is whether or not the rear derailleur will work with the new gears. Rear derailleurs come in 3 sizes: small, medium, and long (some manufactures may have a slightly different name). How do you determine what size fits a particular application, you ask? Good question, I answer!

Rear Derailleur Requirement

  1. Chainring Gears: Subtract the smaller gear from the larger gear. i.e. 52 – 30 = 22
  2. Cassette Cogs: Subtract the smaller cog from the larger cog. i.e. 34 – 11 = 23
  3. Rear Derailleur Requirement: Add the two number together. i.e. 22 + 23 = 45

Rear Derailleur Capacity

  1. For SRAM derailleurs
    1. Short Cage
      1. largest cassette cog: 34 tooth for mountain bikes, 28 tooth for road bikes
      2. maximum capacity: 32 for mountain bikes and 31 for road bikes
    2. Medium Cage
      1. largest cassette cog:  34
      2. maximum capacity: 37
    3. Long Cage
      1. largest cassette cog: 34
      2. maximum capacity: 45
  2. For Shimano derailleurs
    1. Short Cage (SS designation)
      1. largest cassette cog: 27
      2. maximum capacity: 29
    2. Medium Cage (GS designation)
      1. largest cassette cog: 34 tooth for mountain bikes, 27 tooth for road bikes
      2. maximum capacity: 33 for mountain bikes and 37 for road bikes
    3. Large Cage (SCS designation)
      1. largest cassette cog: 34
      2. maximum capacity:  45
  3. Check the manufacturers information for other rear derailleurs brands.

In the above example, the largest cassette cog is 34 tooth, and the derailleur requirement is 45. In checking the guidelines, for both SRAM and Shimano, the example gear setup would need a Long Cage rear derailleur.

In practical application, it is never recommended that the largest chainring and largest cassette cog(s) or smallest chainring and smallest cassette cog(s)  be used at the same time. Running a gear combination like this is sometimes referred to as cross chaining (see the diagram). Cross Chaining If you follow this recommendation and don’t cross chain then you can exceed the manufacturer’s capacity guidelines by several numbers, if necessary. When exceeding the guidelines make sure the derailleur is adjusted properly and test all gear combinations under little or no load.

Good luck and safe riding!

Changing The Front Crankset

FSA CranksetChanging The Front Crankset

Notes

  1. I am changing the front Crankset on my Bacchetta Giro 20 from the stock FSA-Tempo bottom bracket with 52/42/30 gears to a Shimano-Deore FC-M590L263648 with 48/36/26 gears.  I am doing this to provide better low end gearing for cross country touring on a loaded recumbent bicycle.
  2. I reference a number of Park Tool products.  There are other companies that make similar tools, and sometimes a standard ‘tool box’ tool will work.  I reference the Park Tools because this provides a starting place for assembling the needed tools.
  3. The various numbers and gears that I reference are relative to my bicycle.  They may not be the same on other bicycles.
  4. If your chain is really dirty you might consider cleaning it, or at least wiping it down with a solvent, to minimize getting grease and grime on everything.

Here is a chart I put together which shows the difference between the old chain set and the newer one.  Keep in mind that the circumference of my tires may not be the same as yours.

Measuring the DistanceMaking The Change

  1. with the front derailleur set on the largest gear measure the smallest distance between the large chainring and the front derailleur (normally toward the top of the chainring – 4.3mm)
  2. position the chain on smallest gear front gear
  3. rear cassette – position the chain where the rear idlers are perpendicular (23T)
  4. remove the front pedals using a thin 15mm wrench (Park Tool PW-3)
  5. non-drive side pedal – reverse threadeddrive side pedal – normal threaded
  6. Perpendicular Idlerscheck the chain to see if it needs replacement (Park Tool CC-3.2) – For the modern 9 and 10 speed chains, replace chain at or just before the 0.75% readings. For the 11 speed chain, replace at or just before the 0.5% reading.
  7. count the number of links in the chain
    1. find the removable link
    2. put a dab of fingernail polish on it
    3. I also put fingernail polish on every 20th link to aid in keeping track
    4. my chain – KMC Z99 with 137 links
  8. locate the removable link, compress the chain on either side of the link (Park Tool MLP-1), separate the removable link and pull one side out (be prepared, the chain  will rapidly unspool and fall to the ground)
  9. remove the existing crank arms
    1. Park Tool has some instructions on this process
    2. use an 8mm allen wrench to remove the allen bolts on each crank arm (may need an extension bar or dead-blow hammer to loosen them up)
    3. non-drive side – clockwise threaded
    4. drive side – counter-clockwise threaded
    5. remove the FSA-Tempo cranks arm (Park Tool CWP-7)
    6. on the crank puller, follow the directions to insure the correct end cap is in position on the inner threaded arm (small cap), if not, switch end caps
    7. use a dab of lubricant on the threads when screwing the outer section of the crank puller into the crank arm (otherwise it tends to gall somewhat easily)
    8. screw the outer section of the puller all the way into the crank arm (if not screwed all the way in the threads may pull out of the aluminum crank arm)
    9. screw the inner section in until the crank arm pops off of the tapered square shaft (this may take some amount of force)
    10. repeat the process for the other side
  10. remove the bottom bracket bearing set (Park Tool BBT-22)
    1. Park Tool has some instructions on this process.
    2. unscrew the removable (non-drive side) ring – clockwise threaded
    3. unscrew the fixed (drive side) flange – counter-clockwise threaded
    4. remove the bearing set
    5. at this point I put the bearing set and end bolts in a plastic sandwich bag to minimize contamination
  11. on the bicycle, measure the width of the bottom bracket housing – 68.3mm
  12. install the Deore bearing set (the Shimano directions are well written and easy to follow)
    1. Park Tool has some instructions on this process.
    2. follow the directions closely for a 68mm bottom bracket width
    3. rub a small amount of grease on the bottom bracket threads
    4. all three spacers measured 2.5mm each
    5. drop 2 of the spacers on bearing housing with the inner cover tube, screw the bearing housing into the drive side of the bracket (counter-clockwise threaded), and use Park Tool BBT-19 and a torque wrench to tighten to 305-435 inch pounds
    6. drop the remaining spacer over the right adapter, screw the bearing cap into the non-drive side bottom bracket (clockwise threaded), and use Park Tool BBT-19  and a torque wrench to tighten to 305-435 inch pounds
    7. check to see if there is a light coating of grease on the crank arm shaft, if not, apply a light coat of grease
    8. insert the crank arm shaft into the drive side of the bottom bracket and press all the way through (may need to tap lightly with a dead-blow hammer)
    9. there should be some residual grease on the splines of the crank arm shaft, if not, apply a light coat of grease
    10. press the non-drive side crank arm on the splined shaft, 180 degrees away from the drive side arm (press all the way against the bearing housing)
    11. screw the cap into the end of the non-drive side crank arm
      1. use Park Tool BBT-10 and a torque wrench to tighten to 6-13 inch pounds
      2. if using your fingers instead of a torque wrench, just firmly tighten, there is no need to really crank down on it
    12. press the little stopper plate (on the non-drive side crank arm) all the way down (it may take firm pressure with the thumb)
    13. use a 5mm allen socket and torque wrench to tighten the crank arm bolts to 106 – 122 inch pounds
  13. adjust the chain length
    1. thread the chain through the front derailleur cage and wrap it around the small gear
    2. the chain should still be on the same gear on the rear cassette (23T)
    3. pull the rear section of the chain forward which will apply tension with the rear derailleur
    4. use Park Tool MLP-1 to hold the chain together
    5. install the removable link sections on either side and clip them together
    6. loosen the chain pliers, and the chain tension should lock the link sections together
    7. check the rear derailleur to see if rear idlers are perpendicular, in my case the were about one chain link section to far back, so I needed to remove one chain link section
    8. repeat the process to unclip the removeable link
    9. using Park Tool CT-5 remove one link from the chain
    10. repeat the to re-attach the chain per the above instructions
    11. check the rear derailleur to see if rear idlers are perpendicular (mine were)
  14. re-install the pedals
  15. adjust the front derailleur
    1. loosen the shift cable adjusting bolt with a 5mm allen wrench (the derailleur will close up due to internal spring pressure
    2. loosen the bracket that holds the front derailleur on the bicycle with a                     5mm allen wrench
    3. lower the derailleur to achieve the same spacing between the derailleur and large front gear as previously measured
    4. re-tighten the bracket bolt
    5. use the inner derailleur set screw to position the derailleur cage over the small chainring
    6. pull the shift cable up to remove all slack, and then tighten the locking bolt
    7. turn the pedals by hand and try shifting the front derailleur, you may need to adjust the set screws to allow sufficient movement without over shifting
  16. New Cranksetcheck the rear derailleur
    1. use your hand to pedal the bike and shift the front and rear derailleurs in all gear positions to insure the rear derailleur has sufficient travel
    2. note: when the chain is on the large front gear do not use the two largest rear gears, and when the chain is on the small front chain do not use the two smallest rear gears
  17. good luck and good cranking