Gear ratio's

[word]

I was wondering, I am currently riding a 53/39 with a 12/25 rear cassette. Effectively this means that I have 20 gears but I never actually use all 20. When changing from 53 to 39, I will flick down from 25 to about 21 or 19. This gives me 3 or 4 more gears to use before I have to concentrate on spinning.

This means that I am losing (on the downshift) the entire range from 12 to 19.

 

The same happens the other way where if I go from 39 to 53, I might flick up on the rear cassette from 12 to about 14 or 15 (not alwyas though)

 

I read an article somewhere but cannot find it which helps you calculate what all the ratio's mean (i.e. all the combos).

 

Can anybody explain this please?

['Sop]

Read up all about it here: http://en.wikipedia.org/wiki/Bicycle_gearing , this gives a comprehensive explanation of gearing and ratios.

 

In practise we rarely use the full range of our gears, i.e. all 20 possible combinations, as engaging crossover ratios, that is for instance the large (front) chainring & largest (rear) sprocket, or the converse - small front, smallest rear, would lead to accelerated wear on all components involved.

 

Shimano has claimed that their chain is flexible enough to make this a non-issue, but then again, if they are being economical with the truth it would mean that they'll sell more parts, sooooo I'm sceptical of that claim.

 

In general, try not to use the top and bottom three rear sprockets with the corresponding crossover front chainring. Generally, don't overly worry if you do this every now and then, EXCEPT if you put in some real power, such as climbing or sprinting.

 

On my bicycle I have a compact crank, and I find that at around 30 km/h I enter this problematic area of crossover. When I shift the front d?railleur I then have to compensate  with two or three gears in the rear in order to maintain the same cadence and speed.

 

 

 

[Johan Bornman]

Gear ratios on bicycles are calculated in gear inches. This is a measurement of how many inches (or centimeters if you want to be metric) the bike moves forward with each revolution of the cranks.

 

To calculate it you divide the size of the relevant front sprocket by the size of the relavant rear sprocket (measured in teetch) and multiply that with the circumference of your wheel.

 

It'll look like this: 53/13 x 2085mm = 8,5 m.

Now do the same for all your gears and you'll quickly spot the duplications.

If you don't care for the forward dimension, simply leave out the wheel size and you'll still get a figure that can be used to spot duplicates.

 

[JasonW]

http://www.sheldonbrown.com/gears/ 

 

is also really useful. Great web site generally

[word]

JB, your answer was the one I was looking for. Using a std wheel size which travles 2085 m per revolution, I calculated the whole table and there is certainly extensive duplication. Below is the charts - the first table is unsorted and the second is sorted in terms of maximum travel:

	Rear cassette	Cranks	Metres
1	12	39	6.78
2	13	39	6.26
3	14	39	5.81
4	15	39	5.42
5	16	39	5.08
6	17	39	4.78
7	19	39	4.28
8	21	39	3.87
9	23	39	3.54
10	25	39	3.25
11	12	53	9.21
12	13	53	8.50
13	14	53	7.89
14	15	53	7.37
15	16	53	6.91
16	17	53	6.50
17	19	53	5.82
18	21	53	5.26
19	23	53	4.80
20	25	53	4.42

 

	Rear cassette	Cranks	Metres
10	25	39	3.25
9	23	39	3.54
8	21	39	3.87
7	19	39	4.28
20	25	53	4.42
6	17	39	4.78
19	23	53	4.80
5	16	39	5.08
18	21	53	5.26
4	15	39	5.42
3	14	39	5.81
17	19	53	5.82
2	13	39	6.26
16	17	53	6.50
1	12	39	6.78
15	16	53	6.91
14	15	53	7.37
13	14	53	7.89
12	13	53	8.50
11	12	53	9.21