Drive train configuration

Noviss
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Drive train configuration

Postby Noviss » Tue Jan 23, 2018 7:46 am

I have recently changed my commute bike to a Cannondale CAAD5 that I picked up cheap. The commute is around 21 Kms each way with some short but challenging hills. The bike was a triple ring 9 speed that has been modified to two rings at the front - a 52 and a 42. The rear cassette is 12-25.

Generally, the bike is great until I hit the hills and I find the 42/25 combination just one or two gears short to ride comfortably - particularly if there is a head wind. Yeah, I know I could harden up but I really don't want to risk an injury. I like the 52/42 combination on the front because there isn't the big difference that I find on my 52/36, 11-25 set up on my other bike.

Am I better off putting a 39 tooth chain ring on the front or changing the rear cassette out to a 12-28? If I go with either option will I also need to change the rear derailleur cage?

Thanks in advance.

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bychosis
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Re: Drive train configuration

Postby bychosis » Tue Jan 23, 2018 8:13 am

If it was previously a 3x9 setup the derailleur may already suit a wider range.
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Noviss
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Location: Canberra, ACT

Re: Drive train configuration

Postby Noviss » Tue Jan 23, 2018 9:01 am

Thanks. I have taken a photo of the derailleur with the bike in the lowest gear (42 on front 25 at rear) which might help.

Image

There is still around 20mm of additional downward play in the derailleur.

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Duck!
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Re: Drive train configuration

Postby Duck! » Tue Jan 23, 2018 9:13 am

As above, if the rear derailleur hasn't been changed from when the bike was a 3x it will have the longer cage, which isn't actually necessary for *any* 2x combination. What you may find however, is that most of Shimano's derailleurs of that vintage had a 27T maximum sprocket capacity, so putting a 28 may cause rough running in the largest sprocket, especially if the spring in the main pivot is getting tired.

Changing to a 39T small ring will give a pretty good spread of gears, with a nicer ratio jump than the 36T. However, if it hasn't been changed already, which seems likely given it's still running the 42T ring, you will need to change the font derailleur from the 3x version to one for 2x, as the inner cage plate shaping will cause very rough running on the smaller ring.
I had a thought, but it got run over as it crossed my mind.

Noviss
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Re: Drive train configuration

Postby Noviss » Tue Jan 23, 2018 9:31 am

Sorry, I should have added that when it was converted to a 2x it also had a new rear derailleur installed (Shimano Sora) which is pictured above. This was a while ago and I haven't had any issues with the front at all.

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Duck!
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Re: Drive train configuration

Postby Duck! » Tue Jan 23, 2018 12:16 pm

The 3x FD is fine with the 42T ring because it's designed for that as the middle of a 3x. However the step on the inner plate to facilitate shifting from the 30T to the 42T will cause problems with anything smaller than 42.

There wasn't any need to change the rear derailleur unless there was something actually wrong with it, but anyway.... If the one on it now is a 3400 model the largest sprocket you can fit is 28T. If it's a 3500 model it will fit up to 32T. In both cases that's unrelated to cage length; the long version is only required if going back to 3x.
I had a thought, but it got run over as it crossed my mind.

Noviss
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Re: Drive train configuration

Postby Noviss » Tue Jan 23, 2018 12:24 pm

Great, thanks. Just checked the model on the rear derailleur and its a 3500. So, a new rear 12-28 it is. The rear derailleur was changed due to the last one shearing in half IIRC. Thanks again for your help Duck.

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