Tuesday, 12 August 2014
A Northern hemisphere winter too. Regular readers here will know this from my sojourn to Alaska, featuring the awesome titanium Muru Witjira fatbike.
Okay, not so obvious.
A winter in the southern hemisphere, at a latitude of 27.49 degrees S and 40m above MSL, is rarely one that brings with it precipitation requiring snow boots. More like a pair of thongs (or flip flops, jandals) Time to mod the bike for 'summer' mode in winter, which is like a summer.
In summery (not a typo), the titanium Swale fork is excellent as an expedition fork, but lacking in travel as a fun trail fork. I've long been a fan of Lefty forks - for reasons I'll explain soon - and I've had this project on the do list for some time.
The Lefty fork comes in many styles, with only a few suitable for a fatbike conversion. The upper needs to have non-bonded, removable clamps, the internals need to be re-jigged to suit the loading from a fatbike wheel and lastly the clamps and spacer kit need to be offset for a fatbike. This is the kind of techy project I love.
A kit like this gives me a LOT of scope for fine tuning and adjustment for handling performance. The front end of a fatbike has a lot more going than a regular bike, due to that fat tyre. For instance, when you turn that bar and corner, the tyre contact patch moves back and sideways off the centreline, changes the castor angle and increases the 'auto steer' - the bike oversteers into the corner and you have to fight the bars in the opposite direction to maintain line. Turn right, to go left, just like in the animated movie 'Cars'.
Fatbike 2.0 changes that, with frame geometries designed around specific fatbike suspension forks, Like the EYRE fork from Salted bikes, or the Rockshox Bluto. But for those of us with expedition geometry frames designed around rigid forks, installing a suspension fork can be a handling hit and miss affair if you don't know what you're doing. The travel on this fork has been set at 100mm, only a tiny bit longer overall in crown-to-axle length over the ti Swale fork.
My Witjira is a V1 - the first generation and it has expedition/overland geometry, ie no 'suspension correction'.
With the ti fork out, I leave the crown race installed so I can easily and quickly change forks to suit the purpose. A new crown race pressed onto the steerer on the clamp set and installed. A series of spacers packs out the top of the headtube, so the top clamp sits in the correct spot on the Lefty leg - there is a narrow range where the adaptor can clamp onto the Lefty upper.
Here the Lefty is installed in the adaptor clamps. For super long headtubes, there is a step down top clamp available. What I don't show in the geometry process is a complex series of measurements, detailed alogorithms, lasers and a kitten. What, you've never used a laser and a kitten?
The slotted brake adaptor mounts make wheel removal a simple affair, with zero loss of pad/rotor alignment.
The standard Lefty hub is fine for my purposes, light and strong, I was also able to use the same spoke length as the original Hope Fatsno hub.
Fine tune, change the rake, height check, measure, dish wheel, compress fork, measure, change the rake, laser point the kitten - it was all a complex merry go-round with many on and off points to get it right.
A change in one place affects other key areas. One constant - the tyre must remain aligned along the central plane of the frame. Here is about 10-12mm of clearance of the tyre and the upper fork leg, the boot is much lower than the pic would have you believe, it is level with the rim.
I like my stems low, this is not an overly aggressive stem, only around -6 degrees, but still room for a more aggressive drop.
Seeing as this is the V1 Witjira, it has the straight downtube. The latest 'Trail' Vx versions of the frame have a modified downtube to allow for fork crown clearance. No problem though, I've used the OEM Lefty rubber bumper on the downtube to absorb any sharp knocks.
At full lock, 80 somethin' degrees.
Many people are unsure of the Lefty technology, either the missing leg scares them or the hocus pocus of the one sided hub just defies their understanding. The parts below are the critical difference between a Lefty and a regular twin stanchion fork - these are the linear bearings that make the Lefty suspension action so velvety smooth with very little friction. There are four sets of these, along with the wiper seal and rubber boot.
But all this theory is useless without a rigorous testing procedure, and arbitrary shots of the bike leaning on stuff.
The conversion has radically transformed the bike. I'm still bracketing my fork pressure, rebound and tyre pressures - but that's the fun part! I can see some very big days on this bike in prep for Iditarod, and this fork has added a significant amount of versatility, comfort and performance to an already accomplished bike.