How do i change a double to a compact?

shaun75

Is it as easy as just changing the front chainrings?

I have a 2006 Allez with Sora 53/39 crankset and 8spd shifters, a Sora front derailleur and a Tiagra rear derailleur.

Sorry if this is a stupid question but i'd like to tackle a sportive this year and most info packs say a compact is preferable. Thanks

topdude

Hi, i don't know if you can change the chainrings on a Sora chainset but for about the same money you could fit a Tiagra compact 34/50 chainset with hollowtech 2 bearing cups. These are excellent value and much lighter than the Sora chainset.
Your front and rear deraileurs should work ok and the chain might need shortening a bit.

andyp

You can't change the front chainrings as the bolt circle diameter (or BCD) of a normal road chainset is different to that of a compact. The lowest inner chairing you can run on your chainset would be a 38 I think.

Providing you're convinced you need a compact, and that's a separate debate, then you've two options - buy a new compact chainset or get a new rear cassette with larger sprockets to give you a lower rear gear.

peanut

andyp wrote:

You can't change the front chainrings as the bolt circle diameter (or BCD) of a normal road chainset is different to that of a compact. The lowest inner chairing you can run on your chainset would be a 38 I think.

.

yep a 38t chainring will fit the Sora chainset fine but I'm only aware of Mavic and Stronglight versions.
Well worth getting imo as it gives the equivalent of an extra cassette sprocket
i

Raph

There are lots of different bolt circle diameters - just check what yours is and check what the minimum chainring is that'll fit.

A different pair of cranks might not cost the earth. There are lots from various makers with a 110BCD which takes a 34 ring.

If your front mech is fitted to a lug on the frame (as opposed to a clip-on) and you're going to get a smaller outer ring then also check that the lug goes down far enough if you get a smaller outer ring. I had to file the slot down on one of my bikes to get it down to a 48T.

As mentioned above, consider changing the cassette instead. The choice is limited at the top gear end, i.e. very few cassettes start at 14 or above - I think if you're on 8speed the choice might be even more limited, most of what's available will be MTB stuff with 11 or 12T top gear. OK of you don't mind big gaps.

jpembroke

Cheapest way to do it is get a compact chainset that fits your existing bottom bracket. I'm assuming it's a square taper with your Sora chainset in which case you could try a Stronglight Impact chainset from Ribble for around £50, available in 50/36 or 48/34. Once fitted all you'll need to do is lower your front mech. I did exactly the same thing on my old winter training bike. Worked fine.

oh yes, as mentioned above, you'll need to take a link or two out of your chain.

Cajun

I changed 53/39 to a 50/40 without any trouble...plus I found that the 40 is a fun gear and the 50 can be used 10x more than I used the 53....
http://peterwhitecycles.com/chainrings.asp

aracer

...though not that useful as a way of getting yourself lower gears for all the climbs on sportifs.

I'm curious about what makes the 40 tooth chainring more fun than the 39?

Raph

"I'm curious about what makes the 40 tooth chainring more fun than the 39?"

Well... it's ONE bigger!

Have you seen Spinal Tap? The guitar amp that goes up to 11 cos it's "ONE louder"? If not then that gag's lost...

Phil Russell

All you need is a new chainset. I did just what you want to do on my 'Allez' some time ago, except I went from a triple to a compact.. but my compact, not the standard version everyone talks about..
I went for a Stronglight 42/32 chainset as I found I was not using the 52 outer ring of the triple. I used the Escape model and it just fitted as a direct replacement for the Sora unit. The 42/32 gave me a top gear of about 100 which was fine while the 32 gave excellent low ratios. Still, most of the time I stay in the 42.
Your double shifters will work fine. On my triple conversion I needed to swap the front triple to a double.
Cheers, Phil

gundersen

whats the stupid yank on about?
I thought a 40 teeth gave a higher gear than a 39 teeth

not exactly the point here.

shaun75

Sorry, which bolt does the BCD refer to? Is it the small ones that connect each chainring to each other?

If i get the Stronglight Impact 50/34 i wont need anything else will I? I've got a square taper 110mm BB already so i dont need the one ribble recommends. It does say its for a 9/10 speed though, will that matter as mine is an 8?

robbie the roadie

shaun75 wrote:

Sorry, which bolt does the BCD refer to? Is it the small ones that connect each chainring to each other?

Sheldon to the rescue again! far easier then me trying to explain it

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

steve arnold

BCD is abbreviation for bolt circle diameter. If you placed a compass on the exact centre of the crank and set to say 55 radius it would pass thro centre of bolts on most compacts ie 110 BCD. Sometimes called PCD same thing but pitch circle diameter. Its the biggest thing to get right if ordering replacement rings other than correct speed 8-9-10

Cajun

aracer wrote:

...though not that useful as a way of getting yourself lower gears for all the climbs on sportifs.
I'm curious about what makes the 40 tooth chainring more fun than the 39?

You are correct....if planning ride with lots of climbs, I use the other bike: 30/40/50 with a 12/25..and the 40t gives me just a little more 'oomph' than the 39t for cruising @ 90-95rpm

Steve I

Another option would be to fit a Shimano mtb rear mech. enabling you to run an mtb cassette with a 32 or 34 tooth largest cog. Your Sora shifters will work well with this combination. You would have to fit a longer chain, but you could leave the mech. and chain on permanently and swap the cassettes over as necessary for an event, with no adjustments to gears needed. A 39 front with 32 rear would give you the same gear as a 30 front - 25 rear. Not glamorous, but it would work.

jpembroke

shaun75 wrote:

Sorry, which bolt does the BCD refer to? Is it the small ones that connect each chainring to each other?

If i get the Stronglight Impact 50/34 i wont need anything else will I? I've got a square taper 110mm BB already so i dont need the one ribble recommends. It does say its for a 9/10 speed though, will that matter as mine is an 8?

I use the Stronglight Impact chainset on my 8 speed cross bike (also with 110mm square taper BB) with no problems whatsoever. It'll be fine. Definitely sounds like your cheapest and simplest option. Oh, and in case you are interested, I have also fitted a 11-28 MTB cassette on the cross bike and it works just fine with my 105 short cage rear mech. This gives me a very wide range of gears for climbing.

BCD stands for Bolt Circle Diameter, by the way. It refers to the diameter of the inner circle of chainrings, centre of bolt to centre of opposite bolt. You need to know this if you are replacing chainrings. Compacts are 110mm, Shimano standard doubles are 130mm, and Campag standard doubles are 135mm. Hope that helps.