Sunday, August 14, 2011

Shop Notes -- Carbon Boom Installed

Yesterday afternoon after grocery shopping was done and the provisions stored away I had a couple hours free to work on the recumbent.  When I have new bike parts laying around I can't resist the familiar  siren song for too long and I just have to get my hands dirty.

HP Velotechnik with stock aluminum boom.
The first order of business was to cut the shifter cable and pull the front derailleur and chain out of the way.  The computer mount and cable also were removed and tucked safely out of the way.  The Truvative   crankset is held on to the bike by an 8mm self extracting bolt on the non-drive side.  My hex key is about six inches long allowing me to provide just enough leverage to loosen the fastener and free the crank arm.  A couple taps with a rubber mallet on the hollow steel tube that is the axle of the crank and the whole assembly slides right out.

Once the crankset is removed only the outboard bearing cups remain in the bottom bracket shell.  This is where my new Park Tool BB-19 proved itself indispensable.  The cups were pretty tight and I had to call on the services of one of my favorite tools the big 12" adjustable wrench to coax the cups from the shell.

Every so often I discover another advantage of the recumbent bicycle.  While straddling the main tube and facing forward the bottom bracket is in the perfect position for loading torque on the parts while at the same time holding the bike steady.  I found this experience much easier than working on the bottom brackets of diamond frame bikes stuck in the work stand.

Upon my initial inspection of the carbon boom while holding it up along side the bike I noticed that it seemed too long.  There is a slight bend in the main tube just forward of the head tube/steerer weld.  This bend only lets the boom be inserted so far.  Sure enough when I laid the two booms side by side the carbon tube was a couple inches longer.

The telescoping boom is a unique feature of many recumbent designs that allow the manufacturer to produce just one size frame and then the end user can customize the  reach by sliding the boom forward or aft.  Finally pinch bolts on the main frame tube are tightened securing the fit.

If I installed the carbon boom as is it would bottom out and place the pedals too far forward.  Now I had reached the point of no return and chucked up a cutoff wheel in the dremel.  I cut off the end of the boom just behind the tape as shown in the photograph.  I have to say I do like working with carbon fiber.  The high speed rotary tools cut through the material like the proverbial hot knife with none of the jerkiness, heat and exploding cutting wheels I experience when working with metal tubes.  I swapped the cutoff wheel with an abrasive drum sander and cleaned up the raw edges of the boom and it was then ready for installation.

Before I started I measured the distance the boom protruded from the main tube so I could set the distance of the new boom perfectly.  Notice the clever use of a V-brake style "macaroni" to guide the front derailleur cable up through the boom.  The work proceeded quickly downhill from here.  I attached the crankset in reverse order of dis-assembly, reattached the front derailleur, ran a new cable, screwed on the computer mount and was shortly off on a test ride around the block.

Good fun! 

1 comment:

  1. Nicely done; I can appreciate your use of the V-brake noodle!