Bicycle Mechanics - A couple of my many questions

There are a couple of things I am trying to figure out. Here is goes...

--Would any 8spd derailleur (front or back) work with any 8spd shifter within the same brand? Or are bikes designed to take very specific crankset/derailleur sets? In other words, can I take a crankset off another bike (assuming both are the same speed) and put it on another without replacing the derailleurs?

--If one were to be building a bike from scratch, how would one know what derailleur/crankset combination to go with?

--What are the advantages to different chainring teeth count?

answer #1: it should.
answer #2: could you make the question a little more clear?
answer #3: different gearing.

Originally posted by Steele-Bike
There are a couple of things I am trying to figure out. Here is goes...

--Would any 8spd derailleur (front or back) work with any 8spd shifter within the same brand? Or are bikes designed to take very specific crankset/derailleur sets? In other words, can I take a crankset off another bike (assuming both are the same speed) and put it on another without replacing the derailleurs?

--If one were to be building a bike from scratch, how would one know what derailleur/crankset combination to go with?

--What are the advantages to different chainring teeth count?

1. 8-speed rear derailers of the same brand will be no problem; they all are set up to move the same distance laterally for a given amount of cable pull. Front derailers are a different story. Shimano MTB front derailers are NOT compatible with (indexed) road shifters, and vice versa; they don't move the same amount when shifted. The other difference with front derailers is the radius of the bottom curve of the front cage (that sits right above the outer chainring). Since MTB derailers are expected to work with smaller-diameter chainrings, they curve more. This can cause shifting/trimming annoyances and make them harder to set up when used with rings for which they were not desgned. Sheldon Brown has a good article on this on his web site:
http://www.sheldonbrown.com/derailer-adjustment.html

2. The defining choice for this is the desired gear ratios. The ratios determine which chainrings and crankset will be used, then pick a compatible derailer.

3. The number of teeth on a chainring, combined with the number of teeth on the rear cogs, determine the gear ratio between pedals and wheel. Changing either changes the ratio.

My 15-year-old road bike used to be set up as an 18-speed (triple chainring x 6 cogs on the wheel), with what's known as "half-step-plus-granny" gearing. Check the glossary on Sheldon Brown's web site for a thorough explanation of half-step gearing. It ran 26-46-50 chainrings against a 13-15-18-21-24-28 freewheel.

This bike is now a 24-speed, with 26-36-48 chainrings and a 12-13-14-15-17-19-21-23 cassette. If you do the math, you'll see that even though the largest chainring is smaller than before it has a taller/higher/faster top gear ratio because is uses a 12-tooth rear cog instead of a 13.

If you use a triple Crank, you should use a triple derailleur; a double crank, a double derailleur. On the rear it depends on the cassette whether you need a Long, Medium or Short cage RD. It also needs to be 8/9spd compatible.
Up to 28t =short cage: 30t= medium cage: 32/34= long cage. Aand you need to use an 8/9spd chain.

Another question...

Are crank arm lengths interchangeable? For example, on my Hardrock I am putting a 175 crank arm, but it did have a 170. The clearance is fine, but is there anything else that is different between a 175 and 170?

You should select a crank length according to your femur length, i.e., the 175mm cranks are generally for taller cyclists. The longer cranks will give you 3 percent more leverage, but also 6 percent more knee flex and a 3-percent longer pedaling circle. In rare cases, the longer cranks may cause overlap between your toes and your front wheel, but this is extremely rare on today's mountain bikes.

Interesting post above regarding 18-speed half-step plus grannie gearing -- my favourite road gearing scheme is a very similar 48-45-34 / 13-15-17-19-21-23, which works great with a SunTour Cyclone short-cage rear and a Shimano 600 front.

I'm 6'2", and a 175mm crank is just about right for me. If you are under 6', then it is too long for you.

cranks- some mfrs make 172.5s

8 spd shifters/ deraileurs- throughout line- not if Shimano yes if campy and some others

different shifters/derailleurs with different cranks- no problems

advantages different count on chainring teeth- depends ie: 53X45 or 42 good for flats and time trials, 53X39 better for hilly stuff.

Bike from scratch- triple chinrings, long arm derailleur; double chainrings, short arm derailleur

Originally posted by ljbike
Up to 28t =short cage: 30t= medium cage: 32/34= long cage. Aand you need to use an 8/9spd chain.

The important thing is the combined change from in gearing front and rear.

I.e a short cage might cope with a 28t cog, but not if you have a big jump at the front.

A derailleur's "capacity" is actually the total number of chain half-links it can wrap. The size of the largest permissible low-gear cog is a separate specification. My short-caged SunTour cyclone barely, but very successfully, copes with my 48-45-34/13-23 system, which is a 48+23 - (34+13) = 24-tooth drop, primarily because I avoid both cross-chaining.