Low Q factor chain set replacement suggestions

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Low Q factor chain set replacement suggestions

Postby SIL4 » 12 Sep 2020, 5:54pm

Any help on this would be appreciated.

I have an as-new Dawes Gran Tour 725 26" tourer which I bought a few years back for some weekend touring but which laid unused after a long period working abroad. I had tried to sell it recently, rather than leave it unused but, after retiring my Ultegra-equipped BMC with a cracked frame, decided to keep the Dawes for use over winter while pondering whether I want a replacement road bike.

It uses a mix of old series Tiagra triple 4703 l/h STI with 4703 triple front mech, with a Deore trekking chainset (FC T551, 48/36/26, 175mm, 50mm chainline, 181mm Q factor on a BB52 63mm BB) with a Deore M771 rear mech and 10sp. 11-34 cassette.

It's a nice bike and I've been using it on flatish, Cambs roads and unladen, and not as a loaded tourer and probably wont. Two gripes are the gearing (front shifting is functional but very noisy and needs constant trimming, the 11 rear and 26 front ring and never used, and the big gaps at the top end of the cassette are too big) and the crank tread. After a few decent rides and miles, my knees and hips have been in agony and I suspect (from previous experience) that its the crank Q, which is quite different to my Ultregra (141mm). I've tried subtle adjustments to saddle, cleats etc. but it still feels alien and very uncomfortable.

There's at least 15mm clearance from inside of the cranks to the stays (picture below) and an Ultegra l/h arm will clear the stays by 3-5 mm (the r/h arm spider fouls the stay).

The simplest solution, without the expense of replacing shifters, mechs, BB etc. would be to find a lower Q but otherwise identical hollow tech-type crankset to simply replace the current one but other Shimano MTB (e.g. XT) chainset Q factor only seem to come in a few mm smaller (175mm). An alternative would be to perhaps swap out the rear cassette to a 12-30 and ditch the front mech and triple altogether to get rid of the noise and constant ring swapping and just replace to with a lower Q single 44ish ring or a double with a chainsaver inner ring but I know there might be issues with a road type crank not clearing the r/h stay but don't have them any spares hanging around to try.

Any suggestions would be appreciated.
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Re: Low Q factor chain set replacement suggestions

Postby slowster » 12 Sep 2020, 7:28pm

A square taper triple like Spa's TD-2 should enable you to minimise the Q factor, since you can choose different axle lengths. I imagine that it would also improve front shifting, since your front derailleur is presumably designed for road chainline.

Brucey has tabulated the various stickout measurements of different sized UN55 bottom brackets here. Once you know how far apart the inside faces of the ends of the cranks are with one size of UN55 bottom bracket and what the clearance is from the chainstays, you can calculate the amount of clearance you would have with the other sizes, and so choose a size which minimises the Q factor while still having enough clearance.

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Re: Low Q factor chain set replacement suggestions

Postby peetee » 12 Sep 2020, 7:43pm

I have a few bikes with hollowtech cranks both triple and double. All require me to fit a pedal spacer to allow me to tuck my heel in sufficiently to ride comfortably. The only bike that doesn’t need this is the one fitted with a Campagnolo Chorus square-taper Triple crankset.
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