Change from standard to compact chainset

[thefensman]

Band on is good because you can adjust the height of the whole thing if you need to - e.g. if both chain wheels are smaller.

You need to take into account the total capacity of your rear mech.

Total capacity = (No teeth on big cog in cassette - No teeth on small cog in cassette) + (No. teeth on big chain ring - No. teeth on small chain ring)

At the moment your set up needs a capacity of (25-12(?)) +(52-42) = 13+10 = 23

If you put the other cassette on you will need a capacity of (34-11)+(52-42) = 23+10 = 33

If you use the other cassette and get a compact you will need a capacity of (34-11) + (50-34) = 23+16 = 39

The total capacity of your rear dérailleur should be marked on it somewhere - 33 might be too much if it's a short cage one.

hi and sorry to bother you again, been reading your post again and its like a little lights gone on in my head kinda like a eureka moment and things are much clearer regarding gearing, but when you say "short cage" is that a reference to the jockey wheel assembly?

 

[KneesUp]

hi and sorry to bother you again, been reading your post again and its like a little lights gone on in my head kinda like a eureka moment and things are much clearer regarding gearing, but when you say "short cage" is that a reference to the jockey wheel assembly?

You've got it.

 

[KneesUp]

Actually the article that picture is from (thanks, google images) is quite good and explains it a bit more clearly than I did

http://www.bike-advisor.com/bicycle-guides/long-or-short-derailleur-cage.html

 

[oldroadman]

Why not just change the chainrings to 50/34 (the standard compact size)? Then 34x25 will give you a bottom gear that will get you over anything. Keep it simple, just check that the rear mech can take a spread large enough. If you are running an 11-25 cassette, plus compact rings, this is 30 teeth total capacity. Quite simple!
50x11 (or even 12) is bigger than the gears we used to race on a few years back, so you'll be fine for descending at speed too.

 

[TheDoctor]

This is a suggestion that often crops up...
A standard road double won't take a 34T inner ring - you need a compact chainset for a 34T chainring.

 

[jay clock]

Why not just change the chainrings to 50/34 (the standard compact size)? .

because the spider size on the crank is a different size and you will be limited to what you can fit

A standard double chain set has a different BCD (google it) than the compact and will not take tiny rings.

 

[jay clock]

ok, beaten to it!

smallest on standard double is 38T.

 

[thefensman]

Hi guys and thanks for your suggestions. Changing chain rings was my first idea but as jay clock says compact rings wont fit standard crank spiders, due to BCD which at the moment I cant recall anything about but I did research it before my first post. The way I see it as things stand if I fit a new compact crank set and bb and retain my existing cassette will I see any benefits in my climb gearing or do I need to swap cassette also. It just seems to me things are becoming very complicated, and I mean no disrespect to all of you that have posted on this thread. In my head I keep returning to my mtb and the ease that I have climbing with that running 8 speed compact. And it weighs in like a tank in comparison to my road bike. Forgot to add, new chainset i,m considering is 50/34.

 

[morrisman]

BCD (google it)

Despite finding this new Google It thing bloody rude, and to save others the bother, I did and found all these and many more

BCD: Binary Coded Decimal
BCD: Buoyancy Control Device
BCD: Boot Configuration Data
BCD: Behind Closed Doors

After Googling 'Acronym BCD' finally 12th down in a list of acronyms BCD: Bolt Centre Diameter

 

[Sharky]

The Sheldon brown site seems to have all the answers to bike questions

http://www.sheldonbrown.com/gloss_bo-z.html

We can all get our coats ...

 

[uclown2002]

Despite finding this new Google It thing bloody rude, and to save others the bother, I did and found all these and many more

BCD: Binary Coded Decimal
BCD: Buoyancy Control Device
BCD: Boot Configuration Data
BCD: Behind Closed Doors

After Googling 'Acronym BCD' finally 12th down in a list of acronyms BCD: Bolt Centre Diameter

Sometimes you need to be more specific; google 'BCD bike' for example.

 

[morrisman]

Sometimes you need to be more specific; google 'BCD bike' for example.

Sometimes the original poster could just have shared his knowledge, though that was what we here for

 

[uclown2002]

Sometimes the original poster could just have shared his knowledge, though that was what we here for

Indeed, but my post was intended to help you with the googleman in future.

 

[oldroadman]

Here's a thought - which might be more economical - 38x27 is very close to 34x25 (1.41 v. 1.36). Amending the lower end sprockets would be cheaper and easier than a whole new chainset, and you could still run 38/50 rings.

 

[thefensman]

Just checked my rear mech and info is as follows. Shimano RD 4500 GS Medium cage. Does this help?