Mon 4 Jun 2007
Note: this is a technical article
(Goat is a mechanic by inclination, and profession, not to mention a dedicated tinkerer, so, please forgive any lapses into the incomprehensible jargon of the bike obsessed)
As the regular visitors to this site are no doubt aware, there has lately been magic in the air, whiffs of melting bronze, tendrils of evaporating flux, and remnants of the arcane alchemy through which a bicycle is born. Since the snowy passes of Montana, a bike like no other has been slowly creating its self in my mind. Until — vision complete, it began to haunt me, whispering sweet nothings, and laughing as l pushed my bike through sand and snow. Eventually, it was clear that l would receive no respite until l took the final step: to make this monstrosity a reality.
I contacted Spencer Wright, chief alchemist of Traffic Cycles, and soon to be coconspirator. Fluent in the language of bikes, Spencer had no difficulty deciphering my cryptic and incoherent description, and quickly came to share my vision. The mythical creature of my imagination consumed him, and he put aside a myriad of obligations to bring it into the world.
We sketched, designed, and collaborated as the components began to pile up. Eventually, with the quirks finalized, I (isolated in flagstaff) returned to dreaming of effortless riding in sand and snow, and Spencer (faraway in Trukee) started melting metal.
It is difficult to describe something truly unique, for points of comparison are few, and technical jargon is of limited use. But with the aid of photographs, I hope my ramblings will attain some semblance of sense.
The Chupacabra is designed around itÂ´s wheels. And indeed they are perhaps itÂ´s most striking feature. Created by Surly Bikes for their ground-breaking, go-anywhere vehicle, the Pugsly (the ChupacabraÂ´s closest relative). The wheels sport the widest rims, and largest tires on the market. Measuring almost 4â€� the â€œEndomorphâ€� tires are twice the size of those on a normal mountain bike. Add this to the width of â€œLarge Marge Rimsâ€� â€“ more than twice the dimension of standard rims â€“ and you have an enormous foot print. Allowing bikes so equipped, to traverse with ease sand snow and mud that would swallow a more conventional bike whole.
Fitting wheels this large into a bike, is something of a logistical feat. In the front, the tires themselves are wider than a conventional front hub and thus incapable of fitting a normal fork. The solution to this problem is to use rear hubs (which are wider) for both wheels â€“ which makes the wheels interchangeable. My â€œfrontâ€� wheel caries a fixed-gear cog, and my â€œrearâ€� is equipped with gears, so that depending on the placement of the wheels l can have two very different bikes.
In the rear, the width of the tire is so great that if the wheel is built normally (hub centered evenly) the chain would hit the tire and render the bike un-rideable. SurlyÂ´s solution (which we adopted for the Chupacabra) is to build the wheel â€œunevenly â€˜â€™ with the hub offset 17.5mm towards the drive side. This means that in order to center the tire/wheel in the frame, both the rear triangle and front fork must be built to accommodate the offset.
The seeming lopsidedness created by the offset , is but the first of several unusual features of the Chupacabra frame. The frame is extra long, and equipped with mounts to fit Xtracycle bags and racks. Because it is one piece (rather than a bolt on extension like the Xtracycle kit) the rear end can be braced more effectively, creating a much stiffer, stronger, and more responsive bike. The rear rack support is further reinforced with heavy duty arch-like trussing which protects the vulnerable rear end, as well as keeping the bags from bulging in toward (and rubbing on) the disk brake and wheel.
Aside from the length and width of the Chupacabra, its most striking feature is the chain, or rather chainS. Indeed, there are two, and they interface on an â€œextraâ€� hub, which we call the â€œtransfer point.â€� This transfer hub is in turn supported by an extra set of dropouts; in this case long horizontal (PAUL) track ends. The transfer hub is moved in these dropouts to adjust the tension of the rear chain; while the front chain is tensioned with an Eccentric Bottom Bracket (BB). (The BB is the bearings and spindle which connect the cranks to the bike and allows them to spin) In an Eccentric BB, the cranks are mounted off-center in an aluminum cylinder which can be rotated within the BB shell. By rotating the Eccentric BB, the cranks can be moved forward or backward, thus tensioning the chain. As clear as mud no doubt; but with all that elaborate designing to tension the chainsâ€¦.. why two chains?
As usual the reason has to do with the wheels, whose hubs are offset, to allow the chain to clear the tire. For the chain to run straight (and not hit the wheel) the position of the front chain ring would have to match the offset, requiring, in theory, an exceptionally wide BB (which is exactly how Surly solves the problem on the Pugsly). The downside to a wide BB is that it increases the distance between the riders feet, known for reasons unknown as the â€œQ Factorâ€�. A wide Q has a similar effect on rider comfort/fatigue as walking with oneÂ´s feet further apart than oneÂ´s shoulders (awkward and tiring), and not exactly ideal for a touring bike. Thanks to the extra length of the Chupacabra, this problem is solved by offsetting two chains on the transfer hub. The rear chain is set wide enough to clear the wheel, and the front chain set in enough to maintain a comfortable Q. The double chain has some side benefits as well. Two short chains, have considerably less sag/slop/play, than does one long chain. And the extra set of gears on the transfer hub allows me to manipulate gear ratios and run a much smaller chainring, increasing ground-obstacle clearance.
In addition, the frame is customized to accommodate my body type and riding style. Which includes high handle-bars, low top tube (good crotch clearance) and the seat way back, to center my weight on the bike and put my legs in their preferred riding position (this is my dream bike after all).
In addition to the Xtracycle bags and racks from my previous steed, l kept the Brooks Saddle (which, the leather having worn in nicely, fits me like a glove), my custom built H-Bars (which provide multiple hand positions, while keeping the wrists at a natural neutral angle), my stem and seat post . Everything else just wasnÂ´t really applicable.
As usual, my choices are a little eccentricâ€¦
The cranks and BB are heavy duty steel, designed for BMX bikes, and arguably the strongest/stiffest/most durable on the market; made by Profile Racing. The pedals are my customary old road pedals with flip flops bolted on (Shimano 105Â´s this time around thanks to VO2Min from the Fixed Gear Gallery Forum). The â€œfrontâ€� wheel has a Surly single speed disk hub with a fixed gear cog and an old-fashioned BB lock ring. After mucking about with a piece of junk made by the company now called Sturmey-Archer, the â€œrearâ€� wheel is equipped with the legendary Rholoff speed hub. Truly a marvel of Tutonic Engineering, which manages to cram 14 internal gears, covering the normal range of a 27 speed bike, into a hub smaller than the SA 8-speed with which l first experimented.
I chose internal gearing, because of the nice tight chain – no derailleurs or slack – which resists freezing and clogging in snow and mud, and therefore less prone to wear and failure. The Rholoff advertising literature claims you can even ride it in ocean surf with no ill effects, and the only real downside (other than the price, which is prohibitively steep) is that it uses two cables, requiring some awkward routing, and their proprietary grip shifter.
These wheels, more or less require disk brakes, forcing me to overcome my luddite predispositions. I chose Avid mechanicals, being as far as l can tell, as simple and robust as possible with these finicky contraptions. So far l have no complaints, excellent stopping power and minimal fiddling required. But as with the rest of the beast, this assessment remains preliminary. It is after all a prototype â€“ one of a kind.
Thus far the Chupacabra has been everything l expected, and has served me well in all conditions. Even on pavement, where a skeptic might imagine that the larger tires would provide extra rolling resistance and slow us (The Chupacabra and I) down. On the contrary, we descend exponentially faster than the rest of the team, keep pace efficiently, and climb quite nimbly. Indeed the only thing l would change at this juncture is to add a short-travel suspension fork (like the FOX Vanilla we run on the rest of the bikes). While the massive tires do help, my wrists and arms take the brunt of absorbing rocks bumps and uneven terrain, which isnâ€™t unbearable, and wouldnÂ´t even be a problem, except that we ride all day, every day, and usually on rough terrain. But alas, no suspension fork exists that can handle these tiresâ€¦..
In any case the Chupacabra is alive and well, and l canÂ´t wait to try it in the snow.