Some Spring News

We just added the German edition of Jobst classic Bicycle Wheel book. How about that? It sort of dropped off the map, but needlessly. Jobst was always proud of his heritage and took annual trips to ride the great mountains of the German speaking regions: Switzerland, Austria, Germany. This edition lacks mention of Wheelsmith in its acknowledgments. But who cares?

Gutes Buch.

Our price for the Ghisallo Medallion just rose. This, in spite of the Euro/dollar descent. New price is USD$31.50, up from $26. Please know that these can only be obtained from the Rector at the Ghisallo Chapel and the Church determines the appropriate price. We learned, for the first time, that these are silver plated, solid brass forgings.

Precious Medal

I was just sent a pair of hubs, two completely warped and wasted wood rims, and a set of spokes. They began in a shaft drive bike from 1901, now being restored by Steven Perelman. Thanks to the NY Times videographer, the capable Patrick Farrell, wheel restoration is now in Seattle. I'll need to manufacture duplicates to the spokes and nipples. Then we'll build them into contemporary (but period authentic) Ghisallo wood rims. The old rims have certainly bit the dust.

112 years later...

Why bother you with this? I am completely blown away by the hubs, spokes, and nipples. After nearly 40 years in this pursuit, how can I be seeing for the first time such unexpected beauty? This is the REASON to rise each morning, to meet the unexpected. Inspiration comes in unsteady doses, with unpredictable timing.

The spokes are chrome plated, ultra-thin 1.6mm (0.064") wire that ends in tiny, perfectly shaped spheres. Each sphere has a dainty pointed extension opposite the wire from which it's formed. These spheres engage the hub shell, rather than the familiar J-bend elbow of our times. The shafts are butted up to 2mm (0.08") right as they enter the nipple.

Direct pull with a difference.

The nipples are 25mm length. Ultra thin walled, they feature a close fitting neck at the spoke entry. This tapered girdle becomes a two-flat wrench surface before assuming a round shape up to its saucer head. Not so much a marvel as the product of an artist. Why design nipples so difficult to make? Well, artists can be forces of nature and this nipple, while it may be simple imitation, looks like a day dream.

Long and shapely.

The hub that receives these wonderful spokes is also extraordinary. In the front, it's a thin walled tube with special recesses to accept the incoming spokes at a tangential angle, Together with pressed in outboard bearing cups, the hub weighs 100g. The rear hub is an amazing coaster brake with similar features. It deserves a post of its own.

Hub simplicity.

Can't wait to make new spokes of polished stainless and connect them to Ghisallo rims. The result will be quite dazzling, especially because they will give "wings" to a special bike that hasn't been ridden for a lifetime. I'll report on the outcome. Stay tuned.

Elasticity

Few mechanisms are built at the edge of feasibility. It's expensive to make stuff and we require some measure of reliability. Hence, over-design. However, cycling and aviation seem to push those limits like no one else. For very different reasons, each fights an uphill battle to succeed. Human safety is a big consideration for both. For more on the cycling-aviation relationship, check here.

A flying sensation is common to both.

When you push structures to the limit you deal with materials: their strength, stiffness, endurance, hardness, thermal properties, elasticity, etc. With bicycle wheels, arguably the leanest and most efficient structure in general use, this is particularly true. Lately, I've been reflecting on elasticity.

A bike wheel consists of pneumatic tire, rim, spokes, and hub. The elastic behavior of the tire is easy to grasp. A rubber coated textile is soft in the hand until inflated. With substantial air pressure, a tire becomes firm and capable of supporting the vehicle at speed. If struck with high force, the elastic nature of the tire enables it to deform momentarily, absorbing shock and avoiding damage.

The rest of the wheel - aluminum, carbon fiber, steel - seems rigid and inelastic by comparison. No rubber here. If you closely examine the dynamic performance of a wheel you discover a world of elastic movement that is core to the structure's effectiveness.

Hub Flex
Hubs seem to be simple spools that rotate and connect spokes and frames. In fact, they withstand many forces extreme enough to change their shapes. The trusty quick release develops huge compression force, approaching 1,000lbs. The hub axle actually contracts with this force and hub designers need tricks to keep the bearings running free.

Spoke tension on an aluminum hub stretches the body, enlarging the bearing seat. A good fitting bearing can fall out loosely once spokes are tensioned. The bearing seat must be made undersized to compensate for tensioned induced deformation.

Lateral forces at the rim become magnified due to geometry once they reach the hub. Larger diameter axles are sought to help keep the hub from flexing.

Spoke Stretch
No bicycle component is regularly stressed so close to its failure point as spokes. The static load of a spoke can be 1/3 of its breaking strength. This requires spoke wire to be extremely strong and uniform. Tensile strengths and consistency are the highest in all of engineering for these materials.

Under the extreme tensile load of a bike wheel, spokes stretch. A straight gauge steel spoke will elongate more than 1mm during a wheel build. Butted spokes stretch more and it affects comfort, fatigue life, and nipple retention. The elasticity of spokes is part of the calculation of a successful bike wheel.

Rim Deformation
Rims, especially aluminum, are built to be rigid. Loaded with 20 to 32 tensioned spokes, they become an amazingly firm, almost brittle structure. But to look closely, rims elastically deform in dramatic ways.

When spokes are attached, a huge compressional force is applied to the rim hoop. It can be 600lbs, measured at a rim joint. Due to the small mass and density of the rim, the hoop actually becomes temporarily smaller. Clincher tire pressure pushes rim beads outward. You can use calipers across the brake track to see those surfaces change. With pressure, parallel brake tracks become angled more than one degree.

Inflation pressure spreads the rim at the brake which affects the rim "belly" below. For many rims, the change to the belly is enough to increase spoke length and change tension over 20%. Deflate the tire and brake track springs back and spoke tension rises.

With riding, all these elastic deformations combine to define wheel behavior. It's not just the tire that's in constant elastic motion. A structure as light as a standard bicycle wheel is a complex, dynamic structure. No wonder there are so many opinions and observations about the zillion combinations of components, spoke tension, tire pressure, and riding.

The more I learn about wheels, the more secrets are revealed. I discovered the elastic properties of spokes while studying them at Wheelsmith in the 1980's. Now it's clear that all wheel components have elastic properties that affect their neighbors and wheel performance. This elasticity is substantial and you should bear its contribution in mind whenever designing, building, or riding these magnificent structures.