One for the gearing geeks :-)

vectra0

I have an issue to discuss....
Not wanting to spend a fortune (like most of us) i am wanting to give my self some better gearing options for ironman wales in 2 weeks
At the moment i have shimano 105, 53-39 on the front and 12-27 cassette on the rear...
I know that it is do-able on this ratio, but i want to make it as easy as possible over the constant rolling hills... saving the legs for the run...

1st question is: can i buy an inner chain ring size 34 and swap it over with the 39? will it fit straight on? or will i need to buy a full compact chain set to sort this? this would be the eaisest and cheapest solution to the problem...?

Next question is: I could get a 11-28 cassette and try this... I know this will also be fine. But i was wondering what are the sizes of each individual cog in between the 11-28 in comparison to my 12-27 which im not sure what this is either...? does it have the 26/27 etc

Any help with this would be great

Thanks folks

night_porter

1 - Nope
2 - Nope won't fit as BCD's will be different
3 - Yes
4 - When buying an 11-28 the seller will show you what the cogs are, they could be different for different cassettes

vectra0

BCDs?

night_porter

http://www.blackspire.com/site_assets/w ... 0Guide.pdf

redvee

vectra0 wrote:

BCDs?

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

jibberjim

You could get a 38 inner chain ring.

The 11-28 will have
11-12-13-14-15-17-19-21-24-28
your 12-27 has
12-13-14-15-16-17-19-21-24-27

so you get an 11 for the 16 and a 27 for 28, little real difference. You could also swap just the last 3 rings (you'll need to buy an Ultegra 11-28 not a Tiagra one) and just swap a 27 for 28 - that's not much of a change though and not really one worth buying IMO.

vectra0

Thanks folks you have been awsome!
Jibberjim: could I for example change the top three cogs and mix it with my 12-27 and have 24-27-28?

jibberjim

No, the top 3 cogs of both 105 and Ultegra are bonded together. It's also not at all logical - having both 27 and 28 is pretty pointless when you lose a 15/16 say? Remember going from 27 to 28 barely makes any difference.

vectra0

Ah ok. So it's looking like the compact is going to make the most differance....
Will I need to change my front mech for that?

Tom Dean

'rolling hills' doesn't suggest there's anything very long or steep. Are you really going to need lower than 39/27?

If so, a compact chainset is your only real option, but expensive. No need for a new front mech.

jibberjim

vectra0 wrote:

Ah ok. So it's looking like the compact is going to make the most differance....
Will I need to change my front mech for that?

No, just move it.

vectra0

Thanks guys!
Tom dean: I can get round on what I have. I just want to make life easier on the legs where possible. I know from experience the constant rolling hills will take their toll by 130k... The run is my strongest point so I want to be as fresh as possible... If possible :-)
Cheers all

#7rider

You can save the grief of changing chain ring and just Shimano Tiagra 12-30 cassette around £20 from Ribble Cycles.

You may need some adjustment but I have this running with short cage RD

buckmulligan

Changing to a compact is definitely the best option here.

As others have said, the biggest sprocket you're going to get to work without a whole boatload of hassle is a 28. Assuming 700C x 23mm wheels your current 39-27 bottom gear calculates at 39 gear inches, whereas 39-28 calculates at 38 gear inches, a 2.5% decrease. Going from a 39T inner chainring to the 34T of a compact means a decrease from 39" to 34", a 12.8% difference.

You'll lose a bit from the top end, going down from a 53T to the 50 of a compact but if you're trying to save your legs for the running you probably won't want to be pushing that hard at high speed.

As for the mechanics, changing over a Hollowtech chainset takes about 5 mins. You'll need the star-shaped tool to get the RH crank cap off (costs about £2) and have to adjust the height of the front derailleur but it's pretty easy. Only problem is, the FC-5750 chainsets cost ~£130 so unfortunately it doesn't fulfil your requirement of being cheap, but it's the best solution unfortunately!

jibberjim

#7rider wrote:

You can save the grief of changing chain ring and just Shimano Tiagra 12-30 cassette around £20 from Ribble Cycles.

You may need some adjustment but I have this running with short cage RD

For saving your legs ona rolling course though it's not really just a lower granny gear that the OP wants, he wants good close ratios at his typical speeds - so he gets to ride at his preferred cadence. For that close ratios. would be better.

A compact with 11-23 or 25 would almost certainly be better for most riders who like a reasonably high cadence at IM pace on a rolling course.

nickel

Switching to a compact with an 11-25 cassette would be best imo. That way you get a bottom gear that'll get you over pretty much any climb without having a big gaps between ratios. 50x11 is a bigger gear than 53x12 so you won't lose out on the top end either.

ba68

An SRAM 12-28 will give you low gears of 22,25,28',iirc, a small change but may be enough and personally I think the SRAM gives a better spread of ratios than a Shimano 12-27 or 12-28