Design Case Study : The Cherry Bomb MTB

Some of you may have heard that Naked Bicycles, owned by the Canadian speed demon Sam Whittingham, won the 2009 People's Choice Award at the North American Hand Made Bicycle Show early this year. This is the second year in a row that he's won an award at the show. The Canadian "A" channel had a piece on that story :

In case you don't know who Sam is, you can read a little about his background in bicycle design and his dabs over the years at setting speed records, recently one of which was a little over 1/9th the speed of sound. He is considered to be the fastest cyclist in the world as far as HPV's and short, flat distances are concerned.

MOTIVATIONS

British Columbia in Canada has a lot of mountain biking. Hence, Sam felt it made sense to build an MTB showbike for this year's NAHBS, which would be one of two bikes, the other being a "coffee cruiser" (which itself was badly sought after by Shimano).

However, his real motivation was to explore what a mountain bike would have looked or evolved into in the 1930's, had it first been built in the 1920's. That is a pretty interesting thought process considering that the mountain biking movement didn't start until the 1960's with the 'clunkers' of California. In essence, I guess he wanted to take his audience back to the age of Frank Sinatra and show what folks would mountain bike with then. As he told Velo Media, he wanted to 'throw everything over the top in a MTB and see what happens'.

Elsewhere on his blog, he stated somewhat differently :

"The idea with this one was to take the lines and modern technology of a modern rig and give it some old-skool building charm. I was delighted to see that many other builders where also not afraid to step outside of 1975 and let there mind wander a bit."

He also told Velonews :

“I wanted to take the modern technology of a mountain bike, the lines, 5 inches of travel, 29-inch wheels, but put it together in a very non-plastic way. Let’s take a nice modern mountain bike, but put some soul in it.”

SIX WEEKS AND 18,000 DOLLARS LATER...


Courtesy : Zack Vestal

From the above ideas, a mere six weeks of hard work and a hellova lot of cash resulted in the Cherry Bomb, a gracefully curvy lugged steel, single speed, dual suspension MTB with 29" wheels made of classic beech rims and a maple wood seatpost. Lacquered wood and nickel finishing was plenty. Its a medly of both old and modern design elements.

The curvy frame has a lugged design, which was polished and painted metallic red. The lugs were nickel plated.

The seatpost, used to represent Canada, was made from a piece of firewood, as Sam reported, which he split and turned.

The dual suspension has 5" of full travel. An interesting feature here is that the main upper and lower suspension pivot uses modified FSA headsets, which are designed to take pivoting forces and can be replaced if worn out. The headsets use angular contact bearings and are adjustable for bearing pre-load.




A Campagnolo down tube shifter was grafted onto a Fox Shox lockout lever. The linkages, which activate the shock, were fully curved, mitered and welded, and then nickel plated to a super shine. Nickel plating, as I said before, is used very liberally in this design, including for all of the lugs, suspension pivot and rear shock mounts, and even the entire rear triangle.

Courtesy : Dirt Rag

Then comes the "beefy" pedals which, beautiful as they look, were decorated with a mother of pearl inlay for the ultimate touch. There is plenty of wood here, an aluminum platform on top and what looks to be 'spikes' for grip (not sure if that was intentional). Sam likes to call them "Shin Burgers". Hey, who wouldn't want to stomp on these, provided they're given steel toed boots? (smirk)


The handlebars were made out of ash wood, turned down to fit with the one piece Al handlebar-stem combo and wrapped with leather grips. A Chris King In-Set headset was used in the front end for steering.

Another unique feature are eccentric dropouts which have a concentric pivot for clamping the rear wheel. This design also helps adjust the tension in the chain. While bikes like the new belt driven Treks use them, such designs are a staple for Sam's bikes. I pieced together a small section of a video from Veloo Media where Sam explains at the show how these dropouts work.

But I felt the cake of the entire design were the wooden rims, a big step back if you will from our modern world of varied alloys, carbon fiber composites and other unobtanium. These rims were obtained from Wheel Fanatyk, a U.S distributor for Cerchi Ghisallo, the original Italian producer.

Let's talk a little about the design and manufacturing of these wooden rims.

DESIGN AND MANUFACTURE OF WOODEN RIMS

The wooden rims made of beech were provided by FSA employee Ric Hjertberg, the man behind Wheel Fanatyk, a workshop that distributes these rims for Cerchi Ghisallo.

Cerchi Ghisallo produces wooden rims, mudguards, chaincases, and beechwood packs and carry racks.

Rims like these were the bread and butter of bicycle racing for over seventy years and it made sense to rig the Cherry Bomb with the classic, lively ride of the yesteryear. However, rim brakes introduce the challenge of rim wear due to grit sticking onto the brake pads, although wood's co-efficient of friction with such brakes are superior in dry riding, as claimed by Ric here. Traditional brake pads would also melt due to the localized heat produced during braking. So it was decided that the Cherry Bomb would feature disc brakes for practicality and safety.

Ric has interesting things to teach us. I thank him for this writeup, where he talks in detail about the functioning and other design considerations for wooden rims.

"To understand how a wood rim functions, we need to talk about density and the stiffness of shapes and materials. A bicycle rim resists bending according to the stiffness of the given material and shape. However, material near the rim's exterior does most of the work. Why? When the rim bends, this exterior undergoes the greatest deformation. For example, with a bend to the left, compression is felt on the left and tension on the right. These forces are greatest on the surface, furthest from the rim centerline. As it bends, the magnitudes of compression and stretching are greatest on the surface and this area puts up the greatest resistance. If the rim were solid, material in the center would barely detect the bending. For every degree of bend, internal deformation is smaller than that on the surface.

Wood is much lighter than metals or composites, and this low density is what it leverages as a wheel rim.

Density (g/cm3)

• carbon fiber = 1.7
• aluminum = 2.7
• wood (beech) = .7

Because wood is so light, its resistance to bending is necessarily less than metals. Compared to the other materials, wood needs more frequent spoke support. So, we use traditional spoke numbers like 32 and 36 per wheel. In fact, wood's long reign as premier high performance rim is a major reason for these particular spoke counts. Even three decades after switching to aluminum alloys wheel makers retained these numbers. In the face of aerodynamic evidence, spoke numbers have come down dramatically. However, research shows that the wind resistance of larger spoke numbers only becomes a liability at high speeds rare outside of competition.

So, given more spoke support, what kind of wheel does this solid but very light material make? First, the lower spoke tensions that wood prefers allow it to move around more. This additional degree of motion allows it to absorb shock, to attenuate the vibrations of the road; the same as a lower pressure tire. But the actual deflection of a wood rim during riding is tiny, so the bicycle's quickness is not impaired. What seems to disappear are the higher frequency vibrations of pavement that can tire the body over time and make joints ache. An aluminum rim, built to lower tension, would also move around. Unfortunately, aluminum does not absorb energy to the degree of other materials like steel, wood or composites. So the comfort benefit would be small.

In addition to shock absorption, wood is harder to dent. Its low density means that a pot hole will create only local damage: a nick rather than a generalized dent that might interfere with braking. So, wood rims are legendary for resisting dents; a valuable asset in a world of poorly paved roads. One further advantage is the heat resistance of wood. Rim braking dumps large amounts of heat into the brake caliper and rim, in order to slow the vehicle. Aluminum rims eagerly accept this heat which, when excessive, can melt the tire or tire cement, causing failures. Wood rims refuse to accept this heat preferring, instead, to burn superficially at their surface. A wood rim pushed to braking extremes will create a barely detectable burning odor, but its tires remain cool. The flip side of this tendency is higher heat that brake pads see. Unable to hand off the heat to the wood rim, traditional brake pads will melt on wood. This characteristic can be managed.

On first glance, the thermal characteristics of wood seem similar to carbon fiber: neither readily accepting heat. However, the similarity is superficial. A carbon rim accepts heat slowly, a wood rim nearly not at all. During a demanding descent, brake pads can feel overheated with either material, but slowly and relentlessly the carbon rim becomes hotter and hotter. It dissipates the heat too slowly, so can reach melting temperatures. Wood, on the other hand, might burn a bit on the surface but as a bicycle rim will not reach elevated temperatures. Bottom line, no rim material is ideal for braking. Aluminum or carbon, wood or magnesium, dealing with thousands of watts and trying to protect an inflated tire is a tough and hazardous job."

Ric also explains how wooden rims are historically made. It is an expensive and involved process that takes time. First, thin and specially aged beech strips are soaked prior to shaping, and then coated with a 2-part epoxy to be bent into a spiral wound, hoop shape. Between each strip is a layer of cotton cloth. The spiral hoops and basic rim shape are securely glued and then fly cut on a horizontal routing machine, several cuts after which the rim assumes its basic shape. The rim is then precisely drilled to make spoke nipple holes, after which it is carefully sanded with many coats of marine epoxy.

From Ric's Ebay Page For Wooden Rims : This beautiful wood rim is artisan made by Antonio Cermenati in Magreglio, Italy. It is constructed of aged Slovenian beech wood, assembled in thin laminations that are joined by 2-part epoxy in a proprietary process that the Cermenati's have been perfecting for over 60 years. The Sport rim is available in 700C, weighs about 560 g., has 32 spoke holes, is designed for clincher tires, and comes with a set of extra long nipples and shaped washers. This rim is sold for restoration and historic projects, however, such rims were ridden by athletes and adventurers on the World's most demanding terrain for nearly a century. Due to the individuality of handmade wood rims and the skills required of the wheelbuilder, we cannot warrant the performance of these rims. All we can guarantee is our vast experience in rim making and wheelbuilding, and our passion for excellence that extends to a commitment to work carefully with each customer. The Sport rim carries a pressure limit of 4.5 bar (65 psi) and its beads do not have the "hooks" which are common on today's high pressure rims. This design is the same as all clincher rims prior to the 1960's and carry a tire reliably as long as it is mounted carefully and pressure limits are observed. The "hookless" bead is, of course, universal for automotive and motorcycle rims. The third image is, incidentally, a daily commute bike, travelling 19 miles each way in Seattle (wearing fenders most of the year). The tire is a 700X38C IRC "Metro" tire. This bicycle uses disk brakes, although wood rims are normally used with caliper brakes. The disk brake is a nice touch, enabling the rims to retain their new appearance for many years. Wheels made with wood rims have an unmistakable liveliness and exceptional shock absorption plus, surprising strength and damage resistance. Their beauty is simply awesome. Bicycles are transformed into artistic, nearly magical objects. If you've had the treat of seeing a contemporary bicycle fitted with classic wood rims, you know exactly what we're describing.

Precise drilling of the finished wooden rim. More of the rim manufacturing pictures here.

I guess it is now pretty obvious how all the costs to make this bike added up to 18,000 dollars!

While it was a bike made to impress no doubt, what I was simply amazed with were some of the 'think outside the box' characteristics behind Sam's designs. Too often people are complaining that the bicycle has been around for 100+ years and that design has reached a plateau. Well, that plateau apparently came about because we're seeing the market saturated with the same nonsense year after year. Seriously, I could hear a sentimental ballad from sailors in the middle ages and still not get this bored.

One only needs to take a visit to bike expos such as NAHBS to see the floor teeming with hundreds of fresher ideas, or ideas brought to life from the past. Thanks to all the folks who put up a great show this year and to all the others behind the scenes who made this possible. You can read about all the other award winners and their bikes here.


ADDITIONAL RESOURCES :

Design Of Vintage Bikes
Engineered Wood : The Renovo Bicycle



Hey, CHILLOUT TIME now! Bring some ol' tunes back....

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