A severe side (lateral) load on a bicycle wheel is bad news. Wheels are strong for radial (vertical) and torsional (twisting) loads but vulnerable for lateral. Fortunately, high lateral forces are uncommon with single track vehicles that lean through turns. Still, they account for most wheel failures and the actual dynamics remain poorly understood by many.

Let’s examine an extreme lateral load and discover how a wheel responds. How can a tensioned structure with balanced, triangulated bracing warp instantly into an unrideable potato chip shape?

Familiar shape with a bit of frost.

A lateral force usually arrives from the tire-ground contact, perhaps from skidding. The load moves the tire to one side which, in turn, pushes the rim to the side. The rim resists the force with both bending and torsional stiffness. The force is felt by spokes in the immediate region.

A spoke on the the force side tends to maintain its tension. A spoke opposite the force loses tension. The rim does not budge until the opposite side spoke reaches zero tension. At zero tension (all within milliseconds) all forces still balance out, and the rim is undeformed.

lateral load sequence

Once the opposite side spoke buckles, the rim begins moving. Would be helpful if the force side spoke, tight from the beginning, could increase its tension to resist the force. However, due to the low relative strength of bicycle rims, the rim begins moving away from the load in an arc whose radius is the force side spoke. The rim section swings at the end of the force side spoke as a yo-yo swings on its string going “around the world.”

Bent!

In practice, the force side spoke cannot increase tension to keep the rim stationary. The rim is too elastic and swings rather than remaining the same distance from the wheel’s theoretical center. In this example, the lateral force is extreme and our wheel now has a pronounced side bend at the site of impact. Neighboring off side spokes are unloaded as the rim moves their way. Their reduction in tension makes for a positive push that creates two bends that are reverse of the side load. Such “wings” also occur next to broken spokes.

Depending on the magnitude of lateral force, the flexibility of the rim and the amount of spoke tension; a sine wave can propagate around the wheel, driven by the reduction of spoke tension in half, or more, of the spokes. All energetic states seek a lower potential. The loss of spoke tension drives the sine wave, bending the rim into a potato chip shape. This has been triggered by the initial extreme lateral force, but the eventual collapse is an implosion of the tensioned structure.

© The Bicycle Wheel, J. Brandt

This potato chip shape, also known as a pretzel or taco, is technically a hyperbolic paraboloid. You can trace the edges of one on the Dumbo ride at Disneyland. When Dumbo swings low, its radial arm acts like a spoke that is maintaining tension. Swinging high is the same, but like an opposite side spoke.

Dumbo rises and falls as the ride rotates.

Hyperbolic paraboloid—Wikipedia

In wheels with extremely springy rims that do not easily take a permanent set, a potato chip wheel can sometimes be fixed by banging sideways on the ground. A reverse extreme lateral force can cause a sine wave that neutralizes the deformation. It’s a chancy strategy but if you witness one, you’ll know it was special. Steel and wood rims are better for this than aluminum or carbon.

If you travel around the rim like a bug (or Dumbo) it will be obvious the rim is deformed left and right but also twisted. It’s a torsional deformation, not simply left-right. Rims with great torsional rigidity resist taco shapes much better. Torsional rigidity increases as the 4th power of the radius of the largest inscribed circle that fits in the rim section.

The rim above (from 15yrs ago) can fit a large circle in its cross section, giving it extraordinary torsional rigidity. It is nearly impossible to twist into a taco shape. It’s not just the little dude inside making this rim strong. Many deep carbon fiber rims have high torsional rigidity. Pushed by an extreme lateral load they either bounce or break. Notice how many of today’s more enlightened rims feature large open space in their profiles? Good design!

A taco wheel often displays four lobes, two in one direction, two in the other; and one is usually larger than the others. Safe to assume the extreme side load was delivered at this spot and the other three lobes formed as a consequence. Trying to true this wheel is wasted time if the rim has taken a permanent set.

Loosening spokes will reveal how extreme the bend is. Once de-tensioned, the true rim shape appears. If you must use the rim again (think: remote location expedition) try and make it as straight as possible without spoke tension. Be very careful to not introduce kinks or dents as you lever the rim to flat. Once the rim is flat, it will support spoke tension again and have no residual tendency to return to a taco. Truing without first bending the rim makes an unbalanced structure prone to deform again.

Before we leave the world of tacos, pretzels, and potato chips, let’s notice one further way a wheel can assume a multi-lobed deformation—excess tension. A rim can only support so much total tension, in proportion to its mass and shape. We used to build a favorite rim to 100kgf per spoke, with 36 spokes. The same rim with 48 spokes could not support 100kgf on each spoke.

An over tensioned wheel can “pop” suddenly into a wavy pattern. It’s the same principal as extreme side loads. Such a deformation will have many (not just 4) lobes, each small deflections. If you lower the tension total, with luck, the rim may behave as if nothing happened. It was just trying to reach a lower energy level. In its wavy state some spokes lost tension, others maintained. The total was lower.

Whether you ever undertake such truing, it is important to understand the forces involved. A wheel can suddenly become a taco shape with an extreme side force. In slow motion, this begins at one spot and a sine wave of deformation travels around the wheel often leaving a 4-lobed taco shape.

When it comes to metal bending and tension, most everything is reversible so, even if impractical, such wheels can be repaired. With these principles in mind, try your hand at it. You may not succeed but the exercise is always informative. Experience at the fringe of possibilities is what makes developing world bike mechanics so resourceful. Hone your own skills, don’t let them have all the fun!

As gravel beckons, where are we with disk road wheels?

Disk brakes have come late to road bikes and, until recently, have met a cool reception. Two questions arise:
1/ Will the feel and response match the best rim brakes?

2/ How well will road wheels support disk brake loads so fundamentally different than rim braking?

Read more →

As we northern climes head into damp weather and cyclocross, issues of corrosion deserve attention. Corrosion cannot be ignored but, for cycling, most of its challenges can be addressed.

Yum, yum!

Beware viewing extreme cases of corrosion on the Web. An incident only matters in terms of frequency. Being one of 1,000 witnesses to a wild case may not have statistical validity. Shape your practice around incidents that are representative.

Corrosion ~ oxidation ~ redox ~ rust
Beware, I’ll be interchanging these terms, most unscientific! To begin, corrosion is universal. Everything oxidizes under some conditions, life depends on this chemistry. It simply must be controlled and limited to an acceptable rate so we get good use from our wheels.

In general, corrosion is not a main driver of component failure, but is always present at the scene and often a player. Where a failure has occurred, note all the detail you can but be cautious assigning relative importance to each factor. A badly rusted hub bearing may show extreme wear but as it takes time and/or negligence to blow bearings, the coincidence does not define causality.

Corrosion is related to both material and environment. Warm, humid, coastal areas with onshore breeze and mist are hugely more corrosive than elsewhere. Components designed for average conditions need more care (rinsing) in FL or HI. In those areas, brass nipples can turn black with corrosion and crumble to dust. If you respond by cleaning and lubricating you might not feel aluminum is such a bad nipple material. Its corrosion is similar to brass and, maintained, can be reliable.

© bostonbiker.org

Hubs
Corroded hub shells are ugly. Chalky stains and pits can be impossible to remove. Anodizing provides a first line of defense. For hubs that are uncoated, a high polish is more corrosion resistant. Campagnolo’s early hubs were polished and builders cleaned and shined them before a build so subsequent maintenance was easier. Surface treatments like original WD40 go beneath water, clinging to metal surfaces. A light spray or wipe can minimize corrosion.

Spokes
Today’s better spokes are made of stainless steels, corrosion resistant enough to not require protective coatings. These steels can corrode so don’t be surprised to see a light haze of rust. Wipe it off with an oiled rag. The black color applied to many stainless spokes is decorative only and not as corrosion resistant as the underlying metal. A light haze of rust on a black oxide coating is easily wiped off with an oiled rag and will not rust again until the oil is removed.

Remember that multiple stainless alloys are used for spokes, each with different metallurgy and corrosion resistance. Likewise, there is no single black or color coating. Integrity of a coated spoke is difficult to predict without access to manufacturing details.

For non-stainless spokes, galvanizing is a common corrosion prevention. It begins as a shiny, attractive layer, quickly becoming dull, industrial gray. This is normal. On show motorcycles and antique bicycles we see chrome or nickel plating. Neither has much corrosion resistance without regular wiping and application of a protective polish. I’ve seen bad rust on non stainless spokes but rarely breakage for which the rust could be blamed. Of course, it happens but is rare on in-use wheels.

Rusty spokes…

Wiping spokes with a rag lightly soaked in an oil of your choice is good practice. Pressure washing removes dirt but dirt doesn’t cause corrosion. Washing removes the protective coatings all components need. We’re in a modern world of materials and coatings but superficial oils and waxes still play a key role for weather resistance.

Nipples
The debate over brass vs aluminum nipples is eternal. The issue should be decided on mechanical attributes, not corrosion resistance (IMO). Both are quite similar and a well anodized aluminum nipple can equal brass with a nickel plated finish. Brass is more ductile and self lubricating. High grade (2024 and 7075) aluminum is stronger, can be brightly colored, and a bit lighter.

Check this chart for comparable corrosion. Notice columns for aluminum and brass. For exposure to sea water, aluminum is better. For distilled water, both are excellent. For vinegar, they are equivalent. For whiskey, brass is better. Two metals with different outcomes depending on environment. I’m for the whiskey.

In navigating the “controversy” remember that much unwelcome behavior by any single batch of nipples may depend on specific conditions. What, if any, lubricant is used? What process shaped the nipple (forged, cast, machined)? What plating and specification (depth, penetration) was used? Silver nipples are not necessarily anodized. Are yours? Notice also that many millions of aluminum nipples have been used on production wheels over the past 20 years; most outlasting their spokes, hubs, and rims.

Brass contains lead for formability. Recent law restricts the amount but the historic presence of lead in brass taints its otherwise classical charm. Regardless of nipple material, regular lubrication is key. If you soak nipples in heavyweight oil prior to building, the coating can last years. If your wheels see pressure washing, relubrication is required.

To lube a built wheel, touch a drop of light oil to the spoke as it enters the nipple. Watch it wick down the bore. A second drop belongs at the nipple-rim contact, applied from the same side. Spinning the wheel creates centrifugal force to drive lube into the assembly. Many of today’s chain lubes are ideal for this as they are thin for application and penetration but as their carriers evaporate the remaining formula becomes dense and long lasting.

Rims
Cracking of aluminum rims under tension at nipple holes can be accelerated by corrosion. After anodizing, rims are purged of liquids used in the electrochemical process. Inadequate flushing can leave seeds of corrosion deposited inside the rim. Anodizing can be too deep and hard, leading to surface cracking where the rim may be deforming from spoke tension.

Corrosion and cracking at once.

Tire sealants can also be corrosive agents and leak into rim interiors, especially those with ammonia.

Carbon rims are neutral for corrosion but can act as anodes when in contact with metals, stimulating corrosion. It is relatively easy to isolate and protect such galvanic combinations with anodizing, washers, and lubricants. For better or worse, the vast majority of carbon wheels are built with aluminum nipples, showing the combination is not automatically bad news. Enve’s recent switch from aluminum to brass for internal nipples does not indict all use of aluminum. Many of the problems they observed are owed to ammonia from tire sealants and poor anodizing on nipples. Substantial numbers of carbon wheels show no aluminum nipple corrosion. There is often more to the story than simply nipple material.

Storage batteries in use or awaiting disposal can create electrical fields that produce very corrosive conditions. When you see an “impossible” corrosion example, wonder about the context. With growth in e-bikes, e-cars, and home energy storage, we can expect more such cases.Electrical fields…?

In following corrosion, make careful observations and notes, utilize magnification and photo images, and use components with highest material and coating properties.

This upcoming La Niña winter should be fun, not corrosive!

Nipples aim from their rim holes in order to align with spoke angle. The goal is to have no bend in the spoke as it enters the nipple because threading there is a weak spot. Spokes can fracture in thread roots after many loading cycles. Nipple and rim shape determine the amount of spoke angle that can be accommodated. The angle is a product of hub and rim size, spoke number, and crossing pattern.

Today, rims are frequently smaller (650 instead of 700, 20” for folding, cargo, and e-bikes, etc.). Hubs can be larger (generator, internal gear, e-motors, etc.). These combinations produce spoke angles that are more concerning.

e-motor wheels are here

A perfect spoke angle is 90deg with no bend in the spoke. Side angles generated by hub width and dishing are rarely below 80 and most nipple-rim combo’s handle them. The angles we need to address are in the rim plane and a function of rim and hub size.

Different wheels, different spoke angles. Adapted from Grin spoke calculator.

Grin offers a great spoke calculator, among many valuable resources. It will determine the spoke-rim angle and, incidentally, works with paired spoking (another topic). Nipples can easily aim for entries of 80-90 degrees. Less than 75 may be accommodated. Below 70 is beyond most components and requires special attention. Solutions:

1/  Kink the spoke with a plier or wrench so it enters the rim at the nipple’s angle, a slow but effective process.

2/  Drill holes larger so nipples are freer to pivot, not an option or advisable on many rims.

3/  Reduce the lacing cross number to make an angle closer to 90. Despite sub-optimal torque geometry, many builders are doing it. As Grin says, “In spite of the popular wisdom not to use radial lacing on drive wheels, empirical experience has been that this isn’t really an issue with the large hubs in small rims.” The burgeoning e-bike scene cannot be slowed down even though appropriate rims are not available.

Radial motor wheel, they are in use!

4/  Rims could address this issue, for example, with a bulge at each nipple. Motorcycles figured this out 100 years ago. Drill the rim to accommodate the required angle. Here is a solution with optimal spoke angle and torque transmission. Cycling will certainly figure this out soon.

Rim drilled to match spoke angle.

Anticipate spoke angles and plan accordingly. An engineered solution to the situation requires initiative from rim makers. Let’s hope it is sooner than later so I can stop envying motorcycle wheels!

Let’s talk rotating weight. Rotating weight is a big issue for wheel builders. Why? We make choices that determine wheel weight, its total and location. Builders must understand this topic.

Rotating weight directly affects inertia, so the topic is really inertia. Inertia is the resistance of a mass to acceleration. Moment of inertia (MOI) characterizes this resistance and depends on rotating and non-rotating mass. Builders should be measuring wheel MOI.

I’ll show you how to measure moment of inertia (MOI), plus I’ll share a spreadsheet to shortcut the math supporting MOI measurement and its effect on riding. Plug in numbers and get usable wattage estimates. Read more →

I once called tensioned wire wheels “the most ingenious contrivance in all of human engineering.”

Extraordinary claims must be based on extraordinary evidence (Carl Sagan) and tensioned wheels have it in spades. Think, more than two billion nearly identical wheels see daily use on our planet. There is no widely used structure with such astounding strength to weight, and they conquered gravel on the moon!

courtesy of Petar Milošević, displayed at the Moscow Museum of Cosmonautics

Read more →

One of the most memorable characters in our cycling scene of the early 1970’s was Phil Wood who, with his wife Vada and manager Bern Smith, pioneered the modern use of cartridge bearings in bike components. Here Bern recalls some of the special challenges:

Here’s an introduction to the complexities of specifying, testing, purchasing, re-testing, lubricating and assembling bearings. Some of this will be pretty pedestrian to folks with working knowledge of bearings, but there’s plenty other facets to it. 

In the earliest days of Phil Wood & Co. circa 1972, once Spence Wolfe had convinced Phil to manufacture some maintenance-free hubs, and they agreed that 50 pairs would be the most they could ever sell, Phil started testing bearing and lubricant samples. He settled fairly quickly on a single domestic manufacturer of bearings, but the grease samples were…disappointing…

Spence and Lillian Wolfe at their Cupertino Bike Shop, hugely influenced many aspects of the Nor Cal scene.

He found ‘waterproof’ grease that dissolved in water, grease that absorbed water, swelled up and forced itself out of the bearings past the seals, and other oddities. Eventually he found a product from a local (Bay Area) supplier that was something of an oddity itself – the manufacturer produced only very small batches of this particular grease, and only occasionally. It was somewhat inconsistent in texture and color. But after Phil consulted with them a little, they changed the mix, and after that it worked really well in full immersion salt bath tests. 
And it was green…

So we were off and rolling…

We specified that grease to the bearing manufacturer for lubing at their factory. Then the fun began.

A tangent (and a quiz) are required here.
• What is the highest rpm a bicycle bearing is ever likely to spin?
• How many rpm’s is a regular radial-contact industrial ball bearing typically tested to maintain without burning out?
• How hot do you think bearings get at max rpm? 
• What causes that heat?
• What exacerbates it?

…and what is the maximum percentage fill of grease in the bearing assembly cavities (those gaps between the bearings, their retainers, and the seals) that most any manufacturer will agree to?

Back to the early bearings. We found soon enough that our idea of how much grease a bearing should have in it was vastly different from what the bearing suppliers were willing to provide. Typical over-the-counter industrial bearings are filled ~25% full with grease. That’s because those bearings, in a normal installation (an electric motor, say) might run as high as 10,000 rpm – 10 times higher than a bicycle bearing will ever spin. A bearing running at 10,000 rpm gets hot – really hot – from friction, and hotter still if it has too much grease in it (retaining heat in the assembly). Too much grease meaning anything over about 25%.

We specified 95% fill because our tests showed that provided satisfactory water resistance in the extreme tests we put bearings through. But the suppliers refused, citing product liability, and other not-pertinent reasons. So we had to lift seals and add grease to each bearing. That probly doubled our bearing cost.

At least we were getting good bearings…until…batches started arriving from the factory with large grit, wood chips, crystals and other unknown stuff.  We rejected lots of bearings. It got so bad that, in the final batch we rejected from the original supplier, most of the bearings would not rotate. Now, there’s a handful of things a bearing needs to do, but above all…

Eventually we found a (foreign) company that produced consistently good bearings for considerably lower cost than the others. Were they eventually prosecuted for dumping bearings in the U.S. to put domestic factories under? Another story for another time…

Then the grease started to get weird…

Cartoon icon Alley Oop, enemy of corrosion and friction!

As you might have guessed, as we grew more comfortable with the grease we chose, we had an idea that maybe we could use that grease for other purposes, and maybe other folks might like it as well. In particular, we felt that the lovely deep green color itself could help sell the stuff, and we settled on a slogan – ‘It’s Green!’.  Anyway, we asked the manufacturer about making larger batches, that we could repackage from 55 gallon drums into 3oz tubes. They asked how much we might ever sell…

Each time we got ready to order grease for repackaging, I went to the lab at the lubricant plant and inspected samples. They had a few minor problems and we rejected some batches – turns out that the grease mixing vat was used for several different products and occasionally did not get cleaned out completely between product switchovers. Eventually the plant assigned a mixer for this grease alone, and things smoothed out. 

At one point, after about 3 years without a single problem, I asked Phil if maybe we didn’t need to go to the plant to check the samples first. We looked at each other for a moment and said simultaneously “Check the grease”…So I drove to the plant and met the project manager in the lab, where he pulled out the latest sample. It was a beautiful, deep black. Lesson learned the easy way for once.

Years later, after the project manager had retired, I related that story to his successor, who laughed, and said “Oh, yeah – he was colorblind!”

Phil and Vada circa 2009

In Part 1 we saw rims deform from tire pressure causing spoke tension drop and discussed the role of tire dimensions, noticing road tubeless clinchers have the most potential to amplify tension drop.

In Part 2 we established that design, not manufacturing inconsistency, makes for odd matches and extreme tension drop situations.

Here in Part 3, let’s ask what builders can do to minimize negative influences of tension drop. After all, builders invest spoked structures with tension and answer for wheel performance. Read more →

Part 1 showed how clincher rims can alter spoke tension when tires are inflated:
(1) Outward splaying of brake tracks, changing the rim shape, dropping spoke tension.

(2) Inflation pressure down on the rim bed, a constricting force that shrinks the rim causing measurable tension loss.

Tension loss can be confusing to wheel builders following tension recommendations from rim makers. We need to know why this happens and when to worry.

Here in Part 2 we explore how tires, independent of inflation, affect this dynamic.

First let’s review tire and rim sizing for those less acquainted. Why are they made to the sizes we see?

Read more →

There’s hardly a more multi-tasking structure in all of engineering than the bicycle wheel. All its components are mutually dependent and interactive. Then no surprise tire pressure affects spoke tension. Here’s first of a 2-part discussion of this phenomenon, that is reaching a worrying scale in today’s wheels. Inflate a clincher to 90psi (6 bar) and you may see tension drop 20-50%. Why? Is tension drop bad? Should wheels be over tightened to compensate?

Read more →