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Fitting a Dual Chain Device

What you need

• Chain Device (We've done a Gamut P20/P30 Dual)
• Torx Wrench T30 & T40
• Allen Key Set
• Bottom Bracket Tool
• Small Phillips Screwdriver
• Degreaser
• Thread Locker
• Workshop Towel
• Grease

Optional tools

• Shimano XTR Crank Tool
• Fine File
• Torque Wrench

This guide covers fitting a bash ring (which is not specific to gamut devices) and also the chain guide which is similar to other brands of dual-guide devices.

P20 dual (32 - 34 tooth chainring) and P30 dual (22 - 36 tooth chainring) are dual chainring devices - converting a triple ring to a double ring setup. The P30 Dual featured here, is a chain guide which essentially converts a triple chain ring to double chainring and bash system with a chain retainer. It works with most cranks sets with a 4 arm spider system and a 104mm BCD (Bolt Circle Diamater) - basically Shimano and comparable cranksets like FSA, SRAM, etc.

 

Two items come with the P20/P30 Dual - the polycarb bashring and the bomeraning with the screw-in roller. It comes in 3 flavors: Ext BB mount, ISCG and ISCG5 mounts (only the BB mount requires the drive side bearing cup be removed).

Breakdown

This will be specific to your crank system but this has been covered in previous articles on the current Hollotech II (inc XTR M980 cranks) and compatible systems and also the previous XTR crank (M970).

Before you start, determine that you have the correct boomerang. Clean and inspect your cranks, spider, chainring and chain. Replace worn or any bent parts.

Bash Ring Fitting

Removal of the outer ring depends on the model of cranks you have. Most are based on a nut and recessed washer system either Torx fitting or Allen key fitting.

For XTR below you need a Torx T30 and T40 fittings (T30 outside bolt) and loosen anti-clockwise.

If the bolts are stiff or the inner nut spins you may need to use both together.

 

You do not need to remove the inner ring. You can then clean the spider and chainring bolt areas.

Chainring nuts (T30) and bolts (T40) removed - clean them and you may want to put blue threadlock on them before refitting.

For the XTR you may want to chamfer one of the fins down at an angle as it will foul on the inside of the cranks arm and push in the bash guard in making the chain intermittently rub.

Push on the bashring and align the bolt holes.

Replace the chainring bolts and nuts and tighten - hand tight (approx 5 N m). Take care not to overtighten as it's possible to split the acrylic around the bolt areas.

Fully assembled bash ring.

Guide Fitting

The boomerang below is a BB  fit. ISCG and the newer ISCG5 systems are similar but you need to fit 3 mount bolts to the ISCG around the BB shell. You do not have to remove the bearing cup for the ISCG mounts.

Firstly loosen the grub screw to remove the roller axle (1.5 mm Allen Key).

Turn anti-clockwise to unscrew the roller (5mm Allen Key).

Apply blue loctite (thread locker) on the roller axle thread.

Also apply thread locker to the grub screw. Then reassemble them in reverse order. This will help to keep everything in place when in use. Don't wind the thread in too much as it will stop the roller from moving.

Place the boomberang between the bottom bracket shell and the Beaing cup. Depending on your configuration you may have more than one 5mm spacer. Using the 5mm spacer will push the chainline further, This will improve the chainline going from a triple ring to a double setup - you may want to play with spacers to get your chain line optimal.

Screw in the bottom bracket, taking up most of the slack.

Rotate the boomerang backwards/clockwise and then tighten the bearing cup snugly but not to full torque (so the roller is just below the level of the bearing cup). Locate the chain over the top of the bearing cup before fitting the crank.

Alignment

Place only the drive side crank in at first.

Locate the chain over the roller and the inner chainring. On setups with a traditional chainstay, the boomerang should be turned as far clockwise as possible without allowing the chain to contact the chainstay; the bottom roller should also not be below the bottom roller limit line (not beyond the bash ring). For ISCG mounted bomerangs , if you can’t get the desired angle, flip the boomerang so the opposite side faces the frame (you'll need to swap over the roller).

Firstly rotate the cable adjust of the front mech so that when turning the crank the chain will shift up onto the outer ring properly (as you have changed the chainline installing the BB mounted boomerang). If you fitted the ISCG mounted boomerangs you don't have to do this.

Next, with the bike turned over, or on a bike stand, make sure the chain stays on each level of the roller when on the inner....

and outer chainring. In operation then chain should not sit anywhere outside the roller or sit on the midline for any length of time when in operation.

If it does not sit comfortably through the gear spectrum then the grub screw should be loosened and the roller axle wound out to the correct loaction (the grub screw should be re-tightened - hand tight (~ 1 N m approx) (picture with bike inverted). The thread lock, when dry, will stop the grub screw from loosening.

Once the chain sits happily on the roller then you can check if the chain tension is too much or too little. This is exactly the same principle as in the fitting a new chain. At the extremes of chainring and sprocket sizes (big to big and small to small) the mech should take up enough slack to stop the chain sagging and not too much in order for it to be too stretched.

The front mech should have the upper limit set to stop it over shifting by clockwise rotating the HIGH adjustment screw. On the setup above, this was the full reach of the screw.

Once this carried out the crank can be removed and the bearing cup tightened to the manufacturers torque setting, the axle greased, reinstalled and the non-drive side crank installed. And you're done...

Minor adjustments in the position of the roller maybe required after a few test rides.

Richard Fox - Features Contributor

Richard has been mountain biking for over 17 years,7 years on his local trails, Dartmoor. He also regularly rides away from home – usually riding each year (while doing the mechanics for others) in various events such as the Cheddar Challenge, Rough Ride, Soggy Bottom, Dyfi Enduro and twentyfour12.

All these years of riding (and breaking) bikes, has resulted in Richard acquiring the wealth of experience (and the tools) to keep his, and inevitably all his riding crew's, bikes on the trails. Richard is a Cyclist No.1 features contributor.

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