Derailleur Length & Gear Ratios

[un.hinged]

Could someone please explain why some derailleurs are the size of a baby's fist and others are the length of baby's entire arm?

Also, how do I calculate gear ratios - the theory and the practical side. The bike has a 40 front and 14/28 rear (6sp) with 20" wheels. It's used recreationally in England and Holland. I was thinking about putting a double chain ring on the front, a big cog for Holland, and a smaller one for England. I have seen some whooping rear cogs (34 teeth) on six speed freewheels. Is this like a crawler gear?

Cheers..

 

[ArDee]

A derailleur’s cage length is decided by how much spare chain it has to take up when changing gear. Its ability to take up spare chain is defined as its Tooth Compatibility or Capacity. The spare chain it has to take up is given by the difference between the number of teeth on the block plus the number of teeth difference on the chain rings.
As an example a road bike might have twin chain rings at 50 and 36 teeth giving a difference of 14 and a block of 12 – 24 giving a difference of 12, these 2 difference figures added makes a total tooth difference of 26 - so a rear derailleur with a capacity or compatibility of 30 would be sufficient – a short cage derailleur.
However, a mountain bike may have triple chain rings 44 – 32 –22 giving a difference of 22 teeth with an 11 – 32 block, a difference of 21 making a total tooth difference of 43; so a long cage rear derailleur would be required.
Gear ratios are calculated as an equivalent wheel diameter;
Diameter of the drive wheel x (number of teeth on chain wheel/number of teeth on the block)
So your maximum gear is 20 x (40/14) = around 57 inches
You minimum gear is 20 x (40/28) = around 28 inches
Both of which appear quite low, especially the maximum
The very large gear found on the block could be called a crawler gear; but, is more normally known as a granny gear – it’s for getting you up a mountain on a heavily laden touring bike or steep hill on a mountain bike.
Hope this helps

Sheldon Brown has a whole host of interesting stuff to help at http://sheldonbrown.com/

 

[un.hinged]

Thank you for your comprehensive explanation (and the link). That makes a lot of sense.

When I am using my bike I found I didn't touch the lower gears at all.. so if 28"-57" seems low what would be a more useful range? As I said I am thinking of putting a double chain ring on the front, but I am also aware that loads of cogs doesn't always equal lots of useful ratios. Either way my unhealthy and slightly obscure quest to replace every single part on the bike means that this stuff could do with some tinkering/replacing. There's nothing like a bit of bike tinkerage of an evening

 

[ArDee]

The range of gears you use will depend on your fitness, the type of riding that you do, how hilly it is in your area and the speed at which you want to turn the peddles over (known as the peddling cadence) measured in rpm; typical cadence rates are - Shopper 50-60rpm, leisure cyclist 60-70rpm, tourist/commuter 80-90rpm, fast road rider 100+ rpm.

On a single speed bike, depending on the surrounding area, hills, etc. the gear chosen will normally be in the range of 50" - 70"; although, like everything there are exceptions that prove the rule and somebody is likely to reply that they ride a 90" single with no problems. A typical middling gear would normally be about 70" for road use.

As an example mountain bike with 42 - 30 - 20 triple chain ring and an 11 - 28 block would have a gear range from 17" to 93"; whilst a road bike with 52 - 40 double chain rings and an 11 - 19 block would have a gear range from 51 to 115.

For the super fit, Tour de France riders, the top gears will be 125" or beyond.

 

[un.hinged]

Well I'm not totally unfit, but I am the sort of person who should avoid Lycra... IYSWIM. Perhaps the best thing would be to get out on her at a fairly steady pace and count

 

[the snail]

You could put the cog/wheel sizes of your current setup into Sheldons gear calculator to find what ratios you get from that setup, then compare the ratios from compact/double chainring setups eg 50/34, 52/39 etc
http://sheldonbrown.com/gears/index.html

 

[Nigelnaturist]

@the snail or just build a small spread sheet table, ratios easy front/back, gear inch same formula x 26.3 for a 700c wheel, different tyres will effect this slightly.
example 30/12=2.5x26.3=35.8" so my 30x12 (not a gear to use normally) has a ratio of 2.5:1 and a gear inch of 65.8" or the same as my 40x16 and close to my 52x21 I know which I would normally be on

 

[arch684]

you can get a gear calculator for your smart phone if you have one. very easy to use

 

[Nigelnaturist]

you can get a gear calculator for your smart phone if you have one. very easy to use

Still bet my table is easier to use plus i can include different wheel sizes.
My current set up

or ratios if you prefer

 

[arch684]

Still bet my table is easier to use plus i can include different wheel sizes.
My current set up
View attachment 55906

or ratios if you prefer

View attachment 55907

you can enter wheel size tyre size crank size sprocket size even cadence with the gear app and it will give you all you want to or need to know

 

[Nigelnaturist]

can do all that too, also sp and cad so you know what gear inch you were in at any point (data provided from a gps track), though to be honest the sp/cad/gear stuff is a little raw in layout.
My database works out my avg gear inch for any ride I do, so I can tailor my gear, though this isn't as easy to do with 10sp cassettes, but from the data I have I know I would do better with a 12/13/14/15/16/17/19/21/24/27 on my triple using a 38th middle ring front ring than the 40th I currently have because my avg gear inch is about 61" at @ 89 rpm = 16.16mph about my avg sp currently, the thing is i find the jump from 17 to 19 a little large on the 40th ring so a 38th front would make the 16/17 like a 17/18 on a 40th

or as near as dam it, this would be better for me, but it would entail a front chainset change really, the other draw back is i would lose about 5" on the 12th.

other things my table works out for you is how much the FD & RD would need to cope with, my current set up of a 52/40/32 front triple and a 12-27 rear cassette needs a FD than can do 22th and an RD that does 37th, all pretty basic stuff really
Also displays the gear curvature

 

[arch684]

can do all that too, also sp and cad so you know what gear inch you were in at any point (data provided from a gps track), though to be honest the sp/cad/gear stuff is a little raw in layout.
My database works out my avg gear inch for any ride I do, so I can tailor my gear, though this isn't as easy to do with 10sp cassettes, but from the data I have I know I would do better with a 12/13/14/15/16/17/19/21/24/27 on my triple using a 38th middle ring front ring than the 40th I currently have because my avg gear inch is about 61" at @ 89 rpm = 16.16mph about my avg sp currently, the thing is i find the jump from 17 to 19 a little large on the 40th ring so a 38th front would make the 16/17 like a 17/18 on a 40th

View attachment 55915

or as near as dam it, this would be better for me, but it would entail a front chainset change really, the other draw back is i would lose about 5" on the 12th.

other things my table works out for you is how much the FD & RD would need to cope with, my current set up of a 52/40/32 front triple and a 12-27 rear cassette needs a FD than can do 22th and an RD that does 37th, all pretty basic stuff really
Also displays the gear curvature
View attachment 55918

the app also gives you speed and cadence but just to let you know I don't use it I have been cycling for 47 years I let my legs tell me if I am in the right gear

 

[Nigelnaturist]

the app also gives you speed and cadence but just to let you know I don't use it I have been cycling for 47 years I let my legs tell me if I am in the right gear

so do mine, but a bit of a stats freak. plus I love making things work how I want, been build computers nearly 25 years.