Tuesday, May 25, 2010

More Wobble Diagnoses

Here are some responses to my previous post on wobble:

1. I'd mention how non-parallel drops will bow a hub axle & 'cone' out the bearing seat(s). I went through a couple hubs before I realized this. -WeedBMX

2. Some bearings just simply have play in them too. I've cured some wobbly hubs with new, higher quality bearings. I've also cured some hubs that are way too tight with once wobbly bearings. Although very very tight, bearing manufacturers also have a tolerance to work with. - Couch of Zodiac Engineering

3. It is very important to distinguish between bearing arrangements that may LOOK similar, but behave in different ways.

With a crank:-

The load acts in a relatively constant direction relative to the OUTER race, so the INNER race "sees" a load which acts radially but at a varying direction.

ie. if you mark a white dot on the inner race of the BB bearing, then when the dot is at the bottom, the load acts to push the axle INTO the dot.
But when the dot is at the top the load acts to pull the axle away from the dot.

So it is important that a crank axle is a fairly close fit inside the bearing to prevent the axle "rolling around" inside the race and causing wear.

With a hub:-

The load acts in a relatively constant direction relative to the INNER race, so the OUTER race "sees" a load which acts radially but at a varying direction.

So with a hub, it is more important that the OUTER race is a good fit in the hub shell and the inner race can be a looser fit on the axle (which helps allow the bearings to align with each other).

There are a number of standard bearing "arrangements" based upon whether load is radially oscillating or constant; and that "fully retain" different races. For the record it is "incorrect" to fully retain all the races. Bearing mountings should allow for differential thermal expansion of different components. - G of

Saturday, May 8, 2010

Diagnosing Wobble

What is wobble?

Wobble is play or movent in a bearing system. Cranks and hubs will experience it as latteral (axial; side to side) play, while headsets most often exhibit radial play. Although there is a difference in the tensioning mechanism of the two systems, some of the reason the play is different is due simply to the way we test for play: sitting attop our bikes shaking things around.

What causes wobble?

Even though the three bearing systems in our bikes (hubs, headset, and bottom bracket) are slightly different from one another, they all operate on the same principals. Essentially a bearing is placed at the interface of two tubes (headtube/steerer; spindle/BB shell; axle/hub shell) in order to minimize friction between the surfaces. In conventional threaded, poorly sealed systems, the ball bearings are held in place by positioning the inner race against the balls just enough to hold everything steady, but not so much as to add resistance to the system. This direct adjustment system is very simple and easy to understand; BUT it is finicky, and annoying to deal with under a time constraint.

We have moved, almost entirely, to sealed, cartridge bearing systems. These are bearings in which the inner and outer races entrap the balls or rollers in one assembly. These cartridges are easily installed and removed (except when the bearing shatters and the outer race gets stuck), have virtually zero maintenance, and reduce working time. In cartridge bearing systems, the inner race of the cartridge is fixed on both sides, while the outer race is only fixed at inside.

Image from George French's Ride UK article Three Piece Crank Assembly

There are a few main causes of "wobble" in a bearing system:
  • Poor inner race tension. This is the most common cause of wobble. On a normal, one piece axle system (conventional thread-on cone nuts), this can be the result of stripped threads, no spacer between driver and hubshell bearings, or poor bearing shoulders causing the cone nuts to run out of threads. On "sliding hardware" hubs, headsets, and crank sets, the tensioning bits (collars, spacers, stem, arm, etc) need to transfer force from the tensioning bolts while allowing the axle to slide beneath. What that means is that you need to add spacers so that the axle/steerer/spindle does not bottom out on the dropout/topcap/crank bolt. As you can see in the below photo, G-sport hubs require the collars to stick out farther than the inner axle (just as the crank arm and stem must be higher than the spindle and steerer). This is true of all female axle hubs that do not sit in the dropouts.

  • Poor bearing seats can also be a cause of wobble. Many people are familiar with ovalized headtubes, but the same thing can happen with bottom brackets and hubshells, usually from poor manufacturing. If the outer race is not secured properly, wobble will be virtually impossible to remove. To reduce a knocking sound, you can add grease or a retaining compound to the bearing seat; But a perminent solution is to change frames or hubs.
  • Poor interface between crank arm and spindle can lead to axial and/or rotational play. This only really happens on cranks because they are the only interface where rotational loads are constantly supported. Here is an article about interface wobble in 48-spline cranks, specifically Profile Racing cranks.
  • Blown bearings aren't as common as they used to be, but some times it happens. Try not to install or remove bearings by their inner races, as that puts axial load on conventional bearings that are not intended to handle such loads. The obvious exception is when dealing with angular contact bearings (such as most integrated headsets):


So the next time you find some play in your ride, before cursing some company with "******* Wobble", think about what could be causing the wobble. Maybe just adding some shims, or cutting a small bit off the axle or steerer is an easy solution.

Wednesday, May 5, 2010

Rubber Side Down: A look at tires

"What tires are you running?" is a common question in all types of cycling. In BMX, we ride on widely varying terrain: smooth, slightly dusty concrete parks; rough asphalt roads; dusty, dry dirt; and even some rock infested trails. So which tires is best? In order to answer that, we have to look into what tires, compounds, and tread excel in which conditions.

Park: Smooth concrete, wood, metal

Taken from a simple perspective, the best grip at the park will come from the largest contact patch. So in a well cleaned park, treadless tires like the Primo Comet tend to work best. It's similar to car racing. The formula one cars zipping around at 200 mph do so on smooth tires. But as the conditions of the ground become more rough, or debris such as dust and water are introduced, the performance of smooth tires suffers.

When your car hydroplanes, it does so because the large contact patch of the tires distributes the weight well enough to not break the surface tension of the water it is resting on. This is where tread and narrower profiles come into play. Road bike tires are narrow enough that they slice down to the asphalt below. A similar thing happens with dust: the smaller the contact patch, the higher the pounds per square inch (psi) of pressure. Higher psi helps push the dust out of the way, getting down to the nicer ground.

Too much tread at the park can lead to an unstable contact with the ground. As one knob, or section of tread slips out of grip, the traction can suddenly change., which is never fun.

Street: a need for toughness

Two things happen when switching to street: the ground tends to be rougher and more diverse, calling for more aggressive tread than is necessary in park riding; and the rougher ledges and riding call for more sidewall toughness. I say toughness, because a stiffer sidewall is not necessary, just one that resists abrasion and tearing better.

Many "street" tires, like the legendary Primo Wall tires use heavy nylon sidewalls and casing. Although nylon provides good resistance to damage, it is relatively stiff, heavy, and has greater hysteresis (internal friction) than the Kevlar and cotton casing materials (at least in the low threads-per-inch count of many BMX tires).

Cotton is simply too easy to tear to use in a street tire. So a slight increase in hysteresis is necessary for the resilience of Kevlar casings. Perhaps this explains the long lifespan of Revenge Industries (S & M) tires. The low-profile, semi smooth tread, thin Kevlar casing, and now available in different sizes and with a folding (Aramid/Kevlar) bead make this tire a great choice for riders who find themselves riding street as much as park.

Dirt: where smooth tires hurt

I don't know about your trails, but all the ones around here (San Francisco) are covered in a layer of dust. As discussed earlier in this post, smooth tires are terrible in dusty conditions, they do not get down to the supportive ground underneath. This is why tread is the name of the game. How aggressive your tread is depends on the trails you're riding. If the dirt is relatively smooth, then a low/medium tread will give you the most solid feel (without the squirm-i-ness sometimes prevalent in higher knobs). As the terrain becomes more rocky and full of ruts and roots, you'll want to increase the tread. A tire like the Maxxis MaxxDaddy will give good grip in rougher terrain. But be warned, these tires have preferred channels to sit in when cornering. They will feel a bit on or off, but once you feel out where that channel is, you can rail any corner.

Some people, myself included, like to go with little grip in the back, but a nice grabby tire up front. This is because your front tire is leading, where it goes, everything else will want to follow. So if your front washes out, your down. But in the rear, it can be helpful to drift around corners, or slide the back when over rotating. The trick to making this work is to ride the front wheel. If you keep your back end light, you wont get sucked down when its sliding.


None: Good for smooth, clean park riding, and some street applications

Little: More tolerant of contaminants and rough terrain, so it works better as an all-around tire

Medium: Grippy on rougher asphalt and dirt conditions

Tall/Heavy: Almost exclusive to Dirt riding as it may feel unstable on more consistent terrain.

I'd like to talk about Durometer ratings, but only a few manufacturers provide ratings like Maxxis. In general, lower durometer ratings (below 60) provide a softer feel, but wear away faster, while higher ratings (65+) are harder. Interestingly, Maxxis uses higher rated rubber in their higher tread models.

Casing Material:
Cotton: Common, cheap, light, and relatively low hysteresis (Dirt/Park)

Nylon: More common, cheaper, much better toughness, but heavier and higher hysteresis (Low cost Park/Street)

Kevlar: Some variety, higher cost, very good toughness, lighter and lower hysteresis than nylon (Park/Street)

Higher threads-per-inch casings provide lower hysteresis, a more comfortable feel, and good protection agains tears, but are generally more expensive. Most folding tires are higher TPI (120+), while steel beaded tires usually run lower (60-100), but again, few manufacturers specify what they use.

Bead (almost independent of riding style):
Steel: Very low cost, heavy, and stretches with higher inflation pressures

Aramid/Kevlar: Higher cost, significantly lighter and stiffer, and does not stretch

Inflation pressure (depends greatly upon tire):
Lower: tire forms over rough terrain better, feels less solid or like a rail, more likely to pinch flat, greater give when landing, higher rolling resistance, and more likely to blow off the rim

Medium: Good rolling resistance, good pinch resistance, fair impact absorption, and good traction and stiffness

High: Slightly less rolling resistance, rare for pinch flats, low impact absorption, poor traction over debris such as rocks, ok traction over dust and water (due to a smaller contact patch), feel much more like riding on rails

Further Reading:

Sunday, April 25, 2010

Warranty Claims

I've had a number of cats come through the bike shop lately with various warranty claims. Some are legitimate (such as a new KHE tire's blown sidewall), while others are completely rider error. In this post I intend to discuss the purpose, scope, and effects of the warranty process...

A Little History

There was once a day when a manufacturing failure would lead to the rider swear off that particular company for life... or until they came out with some far too cool new part. At the same time, the quality of complementary that made its way into the BMX market was marginal. Tolerances were wide open, if specified at all; and materials were often very poor. What this did was leave an opening in the market for some quality.

Enter the high(er) quality manufacturers. Profile Racing began offering tight tolerance machined hubs; Primo introduced their cold forged Powerbite cranks (cold forging offers greater tolerance control than casting or hot forging along with good grain orientation in the outer limits of the structure). These components earned a reputation for being less likely to bend, break, or snap. But how could companies compete with high end, and high dollar, components?

In the late 1990's Odyssey took a turn from mediocre quality, fair variety components, toward a different business model. The idea was to introduce two concurrent and complimentary ideas to their product line: post weld heat treatment for their forks, and a no-questions-asked warranty.

41 Thermal, as they call their heat treatment regimen is designed not only to reduce the heat affected zone inherent in welded components, but also to make the component bend noticeably before breaking. From this, Odyssey forks developed a reputation for being extremely strong, while staying cost, and weight competitive. Even though there have been quite a number of bent components over the years, the easy, and fairly quick warranty process has reduced the amount of negative feedback on their components.

Today, it is quite common for riders to suggest purchasing Odyssey components because of their amazingly lenient warranty process (Nuno even offered to warranty my cracked Polycarbonate Twisted pedals!) Picking up on this, many other manufacturers have eased and streamlined their warranty systems. They have learned that supporting the rider, even to some immediate detriment, can have long term beneficial effects.

Purpose of the Warranty Process

You can ask around and I am sure you will get various responses to this question. My response is from the perspective of a shop manager, rider, and prospective manufacturer. The warranty process should be designed to induce a positive company image in the mind of the rider. What this means is that a component should not fail without due cause (i.e. riding hard will inevitably damage parts, especially when we get into lighter weight components). If a component fails due to manufacturing errors, it should most definitely be covered under warranty. A common example of this is a crack directly on a weld, which indicates contamination in the melt pool, and subsequent weld.

There is another purpose, as the Odyssey example shows: to increase customer base simply because of the streamlined warranty process. Although I do not list this as a primary reason for purchasing a particular component, it is certainly beneficial to the rider; even more so if they are on a budget.

When a claim goes too far

I had a cat come into the shop with corroded nipples on a machine built (pre-built) wheel. He claimed it was a warranty issue because he had never see that happen before. Perhaps unfortunately for him, I had. The corrosion was due to excessive sweating seeping onto the aluminum nipples. The electrolytes in sweat increase the rate of oxidation (and on some materials nitrogenation) of materials. Often these oxides are much more brittle than their parent materials, leading to sudden failure or cracking. The point is manufacturers have no way of knowing, nor a reasonable way of preventing all of the outlying conditions that may lead to failure.

So the next time a part fails on you, ask yourself a few questions:

* How did the component fail (break? bend? where? how much?)?
* What lead to the failure?
* Could it have been a manufacturing issue?

Of course you could send in the product in any case, but if they deny your claim, don't give them a bad wrap if they conclude that it is not a manufacturing issue. If they do deny and you feel it is unjust, post it up on line with the question of whether it is a manufacturing fault.

Wednesday, April 7, 2010

Cold Forging at Shimano

Primo Powerbites, and now Hollowbites are advertised as being manufactured using cold forging technology. Effectively, they are smashing a blank piece of metal into the shape they want. Cold forging offers the advantage of precise control over dimensions, no need for heat treating, and aligns the grains along the surface of the part, perpendicular for forging direction. Hot forging, on the other hand, works better at aligning all of the grains of the part (up to a certain thickness, based on material characteristics), but cannot control dimensions as well.

Here is an article on Cozy Beehive about the cold forging at shimano. You'll notice the following graph, which may explain why some of the newer forged sprockets (such as Eastern's Medusa Lite, and Stolen's Mood Ring) are forged from 6061. Kudos to Primo for using a stronger, albeit tougher material to work with:


Sunday, April 4, 2010

BMX and Hygiene

Ok, I accept that many, perhaps even most, BMX riders are in their teens. As such, they may just scoff at this post. But there are good reasons why all riders (BMX and otherwise) should care for their hygiene:
  • Some people like the sickly-sweet smell of fresh sweat; nobody likes the smell of old, bacterially digested sweat.
  • Calloused hands show you work hard, but don't feel so nice against soft skin.
  • Fresh smelling pads and riding cloths will keep you excited about going out for a ride.

Hand Care

First things, first, when you're done riding, working on your bike, or otherwise, it is good practice to clean off after. Soaps can leave your skin dryer than they began. Even "moisturizing" soaps don't quite do it for me. I highly recommend Phil Wood Hand Cleaner. It is an oatmeal based hand cleaner that leaves your hands feeling refreshed, clean, and supple.

tub of hand cleaner

But Phil's hand cleaner won't be enough to remove the grater that is some riders' callouses. For this, there isn't much better than a pumice stone. Properly shaped, and preferably with one side bonded to tough nylon bristles for cleaning out under your finger nails, these little gems will let you round off and reduce the callouses. To work properly, hop in a hot shower; once your skin has soften, gently round off the callouses. You'll be surprised how much callous skin can be removed.

Foot Care

You can use your new pumice stone to soften the callouses on your heals and toes as well. The nylon bristles work well as a quick exfoliant to remove dead skin cells on top of your skin. And remember to clean and trim those nails too! Removing dead skin cells will give bacteria less to eat, producing less smelly excrement to make your shoes smell.


Cotton sucks! Sure it is cheap and readily available, but it absorbs moisture, leaving a dark, humid environment for those little nasties to have a field day. Many synthetics, and animal fibers (such as wool) do a much better job at moving sweat off of your skin and out to be evaporated, and effectively cool you. So should you go out and buy some synthetics to help you keep cooler, and smell less? Sure, if you can find some you would be comfortable wearing at the skate park.

Many bicycle shops will stock a good variety of aesthetic options for jerseys that will keep you cool. Shorts are a different story. I'd look to MTB shorts, as many of them have removable chamois. Fox racing offers a full line of MTB gear that may match your style. Have a look through MTB catalogs on line or in your local bike shop to find something you like.

In some cases, their either isn't a non-cotton choice, or no choice you would be caught at the park wearing. This especially hit me with pads. I have a full set of leg padding, much of which is constructed with cotton or (even worse) polyester. I could toss them in the washing machine, but I worry the stitching will tear and the pads will begin to fail. A solution I heard from a cat while I was in Oslo, is to toss the pads (pants, shoes, etc) in a zip-loc bag, and put it in the freezer over night. The extreme cold temperature will kill many bacteria, leaving your cloths relatively odorless.


Wash it. But find some shampoo and conditioner that matches your hair (dry, regular, oily) in the current season. This is also a time to pick a scent for your hair. Trust me, its nice with a little decedent scent.

It can be expensive, but I adore L'occitane.

Bottom Bracket and Crank Standards

Browsing around the Cozy Behive, I found and article on the evolution of bottom brackets and interfaces in road and MTN bikes. Unfortunately, no mention of BMX bottom brackets. Here is the BMX-U page on BB's.

BMX has gone through the same evolution (loose ball, cartridge, press fit); the only difference being that BMX adopted the cupped, press fit US bottom bracket. Since then, we have moved independent, albeit parallel to the development of road/mtn bottom brackets. We have arrived at something very similar to the BB30 standard:

BB30: 42mm ID 68mm width
MID: 41.275mm (1 5/8") ID, 68mm width

Advantages of BB30

BB30 uses a standard metric ID, which makes off-the shelf bearings available for up to a 30mm spindle. Though it should be noted that to fit a fair sized bearing to take the extra side loads our bikes encounter, it may be best to limit spindle size to 25 mm. Even so, 25mm is larger than our existing 22, and 19mm spindles. With this increase comes the prospect of high strength 7068 or 7075 Aluminum.

Using an existing standard accepted by the rest of the cycling industry opens up the BMX market to larger manufacturers, higher technologies, and respectively, BMX companies' customer base. Hopefully, expanding their market will help smaller operations survive. Profile has a similar situation with their 48 spline cranks and dirt jumpers.