which is most important. front or rear derailer

I don't understand why that could possibly be. Which do you use most often? If it's the front derrailleur, then perhaps you should be doing loops around the local velodrome vs. logging miles on a road.

tackstrp said:

Seems to me looking at gear sizes that the front gears on the sprocket or more important. Than the gears available on the rear wheel.

Is this true>?

Funny, seems to me my bike needs both front and rear to operate making them both pretty darn important.

Are you asking which you should change to get higher gears (for going downhill or with a tailwind) or which is best to change to get some real low gearing (like climbing a steep grade)? It's really not at all clear what your question is, but you can go back to the concept of gear inches or development to understand how your overall gearing increases or decreases with changes to front chainrings, rear cogs or wheel size. Some folks also take crank length into account as it affects overall leverage or gearing relative to your legs but starting with gear inches gives you a pretty good idea of the tradeoffs:

Gear_inches = front_chainring_teeth/cog_teeth*wheel_diameter
Development = Gear_inches*pi (3.14 is close enough)

So for a road bike with 53:39 chainrings and a 12:23 rear cluster.

• Your highest gear would be the 53:12 which gives you approximately 119 gear inches assuming 27" wheels (you could split hairs and try to estimate real wheel sizes based on tire size, etc. but 26" for a mountain bike and 27" for a 700c road bike are plenty close and let you compare differences in gearing)
  
• your lowest gear would be the 39:23 combo which gives you:
  
  39/23*27 = 45.8"
  

These gear inch results are the equivalent wheel size if you just had really big wheels with cranks mounted on them and no chain or switchable gears. Think of the old style pennyfarthings or high wheelers with the huge front wheel. Bigger wheels on those bikes went faster but were harder to ride up hills.

Development takes it a step further and tells you how far you'd travel with one revolution of the cranks. So the 53:12 with 119 gear inches would travel approximately 374 inches (over 31 feet) for each complete turn of the cranks.

Anyway my best guess at the meaning of your question is whether changing the same number of teeth on the chainrings(front) has the same gearing impact of changing the same number of teeth on the cogs(rear). Use the gear inches forumula to see:

• On the high end: 54/12*27 = 121.5" (~3 inch increase from 53 tooth ring)
  
• or: 53/11*27 = 130" (~11" increase from 12 tooth cog)
  
• On the low gear end: 38/23*27 = 44.6" (~ 1 inch lower than 39 tooth ring)
  
• or: 39/24*27 = 43.8" (~2 inches lower than 23 tooth cog)
  

So in the case of a one tooth change to either the chainrings or cogs, cogs have a bigger impact on both ends of the range.

If you're asking which shifter to use while riding, again it's typically the rear and the front only shifts when the grade changes dramatically or the wind picks up in one direction or the other.

Does this answer your question?

-Dave

Judging by the other thread you've started you've got a bike with a broken shifter for the rear gears and want to know if you'll be OK with just the front.

The rear derailer keeps tension on the chain so is essential, while the front derailer isn't needed except to change gears. If just the shifter is broken that shouldn't matter though. There are also more gears on the rear than on the front, so if I could only use one set I'd choose the rear, especially if I were on a mountain bike (with a wide range on the rear cluster) that I was riding on the road.

You might be OK if the conditions you're riding in don't change, ie it is pretty flat and you're not trying to switch between road and some other surface that is much harder to ride on.

Having the right cadence (ie pedal speed) helps a lot, so if it were me I'd want to get everything working. Some people like single speed bikes though.

The only way to know for certain if it'll be OK is to give it a go.