Wednesday, 3 February 2010

Raven Nomad Rack Mounting Bosses

In the advertising literature for the Raven Nomad S&S that I downloaded, Thorn made several statements about their rack mounting bosses being 6mm, highlighting, to me anyway, a clear advantage over standard 5mm bosses commonly found on other bikes.

They also have 6mm stainless steel bosses for our own heat-treated Cro-Mo carriers... The frames have ... oversize (6mm thread) upper carrier bosses... The forks have ... oversize 6mm stainless steel fittings for lo-loader carriers...
Raven Nomad Brochure Issue 2 May 2009

Although the literature was accurate in this respect, I thought it slightly misleading after I received my bike. I had assumed all the rack mounting bosses would be 6mm when in fact, the rack mounting bosses on the Raven Nomad rear dropout are actually 2 x 5mm.

Two x M5 Bosses on Raven Nomad rear dropout

My Thorn Expedition rack was fitted to the rear dropout with one M5 bolt and four padding washers per side. I was surprised by this. It could be argued that, of all the rack mounting points, this point will be under the most stress when hauling heavy loads!

After speaking to SJS Cycles, I gathered this was a legacy issue concerning Thorn's Rohloff specific OEM rear dropouts. The same dropouts are used across Thorn's entire Rohloff range, apparently because they are very expensive to manufacture. The current dropouts have two 5mm bosses for rack and mudguard fitting per side. I suppose it must be considered too costly to produce a separate 6mm bossed version for the heavy duty Raven Nomad?

SJSC has some cast dropout adaptors that convert the twin 5mm bosses into a single 6mm rack mounting point, but I was told these were mainly used to move the rack outwards to prevent fouling on disc braked equipped bikes. They don't usually fit the adaptor to bikes without disc brakes because to paraphase the SJSC contact I spoke to: "it's debatable whether it makes the rack fixing any stronger, as the rack legs have to be 'sprung' outwards to fit the adaptor".

 Cast End 2 x M5 to 1 x M6 Stainless Dropout Adaptors
(with 5mm lathe reduction of face)

I could also tap out one 5mm boss to 6mm per side (if I felt brave enough) as the bosses appeared to have enough surplus metal.

I was finding this 5mm boss 'compromise' a touch frustrating. Either you needed 6mm mounting points for 'expedition' racks on an 'expedition' tourer (as Thorn's Raven Nomad literature suggested) or you didn't!

So the options were:
  • Continue with 1 x M5 bolt + padding washers per side;
  • Fit the cast dropout adaptors;
  • Tap out a pair of 5mm bosses to 6mm.

 Adaptor secured with two M5 cap-headed bolts

After some um-ing and ah-ing, I decided to fit the cast dropout adaptors. (I was sent them free of charge by SJSC). Crude reasoning suggested it would be better to spread the rack load, at the rear dropouts, over four x M5 bolts rather than two. I assumed the M5 bolt would continue to be the structural weak link under this setup. I didn't feel comfortable tapping out the boss threads on the bike's dropouts.

Rack secured to the adaptor with M6 bolt

In light of my conversation with SJSC over the adaptor's primary usage and the fact that I didn't run disc brakes, I asked my brother (who works in precision engineering) to lathe 5mm from the adaptor's face. This reduced the adaptor's width from 15mm to 10mm and meant that the rack legs needed to be sprung outwards by a smaller amount when fitted.

Rack now sits 10mm away from rear dropout

I don't believe I've compromised the adaptor (!) as it's still thicker than the bike's dropout bosses and I'm content with the final result. Time will tell...

(There was some discussion on this subject in this thread on the Thorn Forums during September 2009).

1 comment:

Stuart said...


I noticed when picking up my Nomad today that the adaptors are now supplied as standard and no charge was added to the invoice for them.

Oh..... and I'm obviously now the proud owner of a lovely matt black Nomad Mk2 ... about time ;-)

I'll get some nice shots for my blog on the weekend and then go and get it dirty!

Consider yourself partially to blame for this purchase, so if anything goes wrong I'll be pursuing you across Canada!!