Bike Project: 2012 Tomac Flint 29 (Part 2) or: "DON'T THROW AWAY YOUR OLD DEORE XT M739 CRANKSET!!!" or: "A Tale Of Two Chainrings" or: "A Chance Meeting Yields Unexpected Results"

 

An eclectic mix of old and new working in perfect harmony.

BACKGROUND STORY:

I built up a 2012 Tomac Flint 29 XC frame into a dropper gravel bike a few months ago.  However, I didn't particularly like a few aspects of the end product.  The gearing was centred around a 46/34 crankset, and I really wanted to use a 32.  The 29 x 2.1" tires were great for floating over chunky gravel, but the tradeoff was a lack of agility on firmer ground.  A tire change is simple enough, but running a sub-compact crankset on this frame was tricky due to the wide (pre-boost) chainstays; no problems in installing one, but it was challenging creating an optimum chainline. The problem could have been solved by going to 1X, but I'm stubbornly sticking to 2x.

I was looking for something altogether unrelated, and came across a bunch of old and very dirty Shimano Deore XT M739 parts (circa 1995) in one of my parts bins.

So filthy; even the pivot bolt and internal spring were jammed with mud.

M739 CRANKSET MODIFICATION:

The M739 crankset was the version which integrated the big chainring with the spider.  The concept was ahead of its time, but once the big chainring was worn out (and this one definitely was), you had to replace the entire spider.  I was almost about to recycle it, when I got an idea: What if I could re-purpose the spider to run a sub-compact double inboard of the original triple configuration?

Deore XT M739 crankset. Integrated chainring & spider, 2 other chainrings removed.

Pop off circlip, remove lockring using Octalink BB tool or equivalent.

Cut off chainring, giving you the spider only.

Extended length 15mm chainring bolts, 5mm chainring spacers, M10 x 14mm washers.

Add beautiful Specialites TA / TA Specialties chainrings, made in France.

Re-assembled. Note the double vs triple inboard positioning of chainrings.

M739 crankset modified to 2X completed.

Circa 1995 Deore XT M739 crankset installed and ready to go for another 28 years.

NEW CLARIS BRIFTERS WITH OLD DEORE XT DERAILLEURS?:

I also wanted to return to an 8-speed system for its cheap, robust chains and mythical mud-shedding capabilities.  Shimano Claris R2000 brifters were used; not the most svelte, but 8-speed, no dangly shifter cables, and relatively cheap.  No worries in compatibility with the M739 rear derailleur, but I reasoned I had two options for the front derailleur: 1) Add a clamp-on pulley to allow the Claris bottom-pull front derailleur to work on the top-pull frame fittings; this would work for sure as as the R2000's longer leverage arm was a mate to the Claris shifter; 2) Use the old top-pull Deore XT front mech, but modify the cable mounting point to simulate the Claris's entirely different cable pull.  To my utter surprise, the Deore XT worked really, really well with the Claris 2x front shifter without any modifications at all!

So, the bike is a 2012 frame with 2017 brifters and 1995 derailleurs, and it all works perfectly fine.

Sometimes, you just take the win and run.

RD-M739 rebuilt and handling the 32T cog without even maxing out the b-screw.

Shimano Claris R2000 brifters.

Clement X'Plor USH tires. Tough to mount, hope their ride is worth it.

RIDE REPORT:

2003-08: Massive wildfires in British Columbia currently.  Will test-ride once the sky stops raining ash.

This time, N+0.  Okay, maybe N+0.25.