Gear ratios

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Gear ratios

Postby jimboj » Thu Aug 30, 2012 6:11 pm

Hi! I'm new here so pleae don't bite!

I'm looking at buying a Giant TCR Advanced 2 (2012). I want to get a bike with a Compact Crankset, but this bike doesn't offer that option.

Is the gearing on this Giant equivalent to a Compact Crankset? If not, what component should I replace so that it is?

The set up on the Giant is: Crankset Shimano Ultegra 39x53T, Cassette Shimano Ultegra 10 sp 12-25T

Many thanks for your help. I'm keen to do 3 Peaks next year, and want to make sure I've got the right set up
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by BNA » Fri Aug 31, 2012 10:14 am

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Re: Gear ratios

Postby toppity » Fri Aug 31, 2012 10:14 am

Hi there Jim,
The gearing is not like a compact crack setup. Also the rear cassette is o.k. but still a little 'tight' for people like myself who struggle when the road goes up.
Gearing is easy to work out. The higher the ratio between the front chain ring and the rear sprocket the faster you go for each pedal stroke, but the harder it is to maintain.
The lowest gear on the bike you mentioned is 39(small ring on front)/25(biggest sprocket on back). Ratio is 1.56
On a compact like I have on my bikes its 34/27. Ratio is 1.259. To me that make a big difference when the going gets harder.
Crank length also comes into it. The longer the crank length the easier it is supposed to be for going up hill. However the compromise is that you can't spin a 180mm crank at the same speed as a 165mm one. For interest sake I run a 172.5mm which seems to work for me.
I ride several bicycles, but not at once.
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Re: Gear ratios

Postby craigg » Fri Aug 31, 2012 12:24 pm

You can purchase a compact crank to replace the standard. You'll need to lower the FD to accommodate the new chainrings and this adjustment should be available on the mount. You'll likely also need to adjust the throw on the FD as the rings size ratios are different, and finally may need to take a few links out of the existing chain. You lbs should be able to do it for you if you wanna go down this path. I wouldn't suggest changing the rear cassette to larger than 25 or 26 as the RD will be short cage and isn't really up to accommodating a wider range (say 11-27), especially in conjunction with the compact crank and the amount of chain it needs to handle when changing between 34/26 and 50/11, not to mention cross-chaining issues.
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Re: Gear ratios

Postby ozdavo » Fri Aug 31, 2012 2:59 pm

Any compact chainset (50/34) will work, and as noted you will likely need to shorten the chain a little and lower the FD (& retune). If you want to keep the same flavour, Ribble generally has the best prices for Shimano components in my experience (happy to be proven wrong). An Ultegra 6750 compact will cost you about $235 landed. http://www.ribblecycles.co.uk/sp/road-track-bike/chainsets-chainsets-road-shimano-ultegra-double-6700-6750-10-speed-chainset/shimchar222

As you get to choose the length of the new one (170-172.5-175mm), it's probably a good idea to get a good bike fit and crank length recommendation. Most bikes come with a 172.5, ( unless the frame is very small or large), but some people suit a shorter length as they prefer to spin, or a longer length as they are grinders.

You will be able to sell the standard chainset for $150 ( or a bit more) easily on these forums to offset some of the cost. It's probably a good idea to talk to your LBS about what you are planning if you're not comfortable in fitting it yourself. Maybe suggest that you will have the new compact when you take the bike in for it's 1st service at 4-6 weeks and ask what they will charge to fit. If they are a good shop looking to establish a good long term relationship with you, they may off to do it for free or only a small cost.

From there I'd ride/ train on the 11-25 until it wears out, or about a month out from the 3 peaks, then swap to an 11-28 (new chain & cassette) for the event.
Good luck
Always looking for new rides & ride partners in SE QLD area
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