Interesting Bike Science

by Paul Pedriana
paul@pedriana.com

Here we present a bit of interesting technical bike information. Some of these items are the result of misconceptions, while others are simply curious facts. There is an Internet site devoted to technical bicycle discussions. It can be accessed via Usenet at rec.bicycles.tech or via your web browser at http://groups.google.com/groups?&group=rec.bicycles.tech. A couple other web sites with useful information are http://draco.acs.uci.edu/rbfaq/FAQ/ and http://sheldonbrown.com.

What causes tire tread wear?
Tire wear is caused by what is called "slippage." Slippage is what happens when the tire is rubbed against the pavement instead of simply rolling along it. It can be thought of as skidding at a micro level. Whenever you accelerate your bike during a sprint you are causing small amounts of slippage. When you turn around corners you are causing small amounts of slippage. Also, braking causes slippage. The harder the accelerations and the more you weigh, the greater the amount of tire that is rubbed away during slippage. Front tires wear so much longer than rear tires because they have much less weight on them (one third of your weight is on the front; two thirds on the rear) and because the don't have the drivetrain connected to them like with the rear tire.

What causes rolling resistance?
Rolling resistance of a tire is caused mostly by the flexing of rubber in the tire casing and tube when under compression during rolling. Other causes exist (see further below), but are less significant. Some rubbers are higher energy losing rubbers than others. In particular one of the most common tire rubber additives, carbon black, is a high energy loss material. But rubber without carbon black wears rapidly and has poor wet traction. A tire with lower pressure will flex more and thus give higher rolling resistance. A tire with a ridged tread will have lots of micro-flexing and thus have higher rolling resistance. Note that rolling resistance due to flexing applies to tubes as well as tires; this is because at high pressures the tube is pressed firmly against the tire and effectively becomes part of the tire. It can thus be concluded that thicker tubes will result in higher rolling resistance.

Which has lower rolling resistance, tubular or clincher tires?
The answer is that clincher tires generally (though not unilaterally) have lower rolling resistance at least at equal tire pressures. This has been shown in tests and is the result of energy losses caused by the elasticity of rim glue. There is a type of tubular glue made for track (velodrome) tires that dries harder than standard rim glue and lowers the rolling resistance of tubular tires to that of clinchers. Needless to say, the differences between tubulars and clinchers here is rather small and is perhaps only significant for highly competitive time trialing events. Other primary causes of increased rolling resistance are tire treading (as opposed to smooth treading), Kevlar belts (which add a surprising amount of resistance), lower tire pressure, heavier or thicker tire casings, and heavier or thicker tubes. Nevertheless, you will see offhanded reports and advertisements saying that tubular tires have lower rolling resistance than clinchers. These reports are only true in the sense that tubular tires can usually be brought to higher tire pressures than clinchers. Of course, none of this means that clinchers are better than tubulars. Both have their advantages and disadvantages.

What keeps a bike upright?
This is a classic trick question, as many people with some high school or college physics assume that it is the gyroscopic effect of the wheels that keeps a bike upright. But this is not the reason, and experiments with bikes that have counter-rotating wheels have demonstrated so. The reason a bike stays upright is largely two-fold: rider balance and "steering trail." Rider balance is simple enough to understand, but steering trail requires some explanation. Steering trail refers to the effect produced by the design and geometry of the front part of a bike whereby it tends to slightly correct itself when the front wheel is turned askew from the direction the bike is going. You can see this effect by holding a bike from its seat, pointing the bike north but pointing the front wheel slightly to the left or right and then pushing the bike forward. Notice how the front wheel adjusts itself towards the direction of bike motion. As for the gyroscopic effect of the wheels, only at very high speeds with heavy wheels do they begin to have any significant effect. A simple way to see how little effect gyroscopics have on bike balance is to prop your bike on a curb with the rear wheel off the ground while spinning as fast as you can get it. Then push the bike over. Notice that it falls over just as easily as when the wheel is still.

How many light bulbs can a cyclist power?
If you measure the power being generated by a cyclist by measuring the force upon the drivertrain, you will find that fit cyclists (e.g. Diablo Cyclists generally produce between 200 and 800 watts of power, depending on their strength and whether they are holding a steady speed or sprinting, respectively. Most cyclists average between 200 and 300 watts of power while doing an effort like climbing Mount Diablo, while top pros like Lance Armstrong would produce 400-450 watts while doing the same climb. This of course translates to anywhere between two and four 100 watt light bulbs. But any mechanism you use to convert bike power to electricity would likely be little more than 30% efficient, so in the end one 100 watt bulb is about how much a strong Diablo Cyclist could power continuously. For what it's worth, a watt is a measurement of energy over time (technically speaking: kg m2/s2).

How much does bike paint weigh?
Bike paint generally weighs 80 to 150 grams, depending on the paint and the thickness of its application. 80 to 150 grams corresponds to 2.8 ounces to 5.3 ounces or .17 to .33 pounds. So if you consider 100 grams significant, you might want to consider making your next bike paintless. On the other hand, 100 grams is about the same as a couple packs of PowerGel, so perhaps it really isn't very significant.

How much does wheel rotating weight matter?
It is a common misconception that the rotation of wheels makes their weight much more significant than non-rotating weight. The truth is that this effect only applies to wheel acceleration and even so is such a small effect as to be nearly nonexistent. Any steady-paced ride, whether it be on flat ground or up hill, does not involve acceleration and so in this case there is zero rotating weight effect. In the case of wheel acceleration, it has been shown that the weight of wheels is so small compared to the weight of the rest of the bike and the rider that the rotational effect is almost insignificant. Note that all the best professional and Olympic track sprinters today race with heaver carbon tri-, quad-, and penta-spoke wheels instead of racing with lighter weight spoked wheels.

Why is titanium expensive?
Why should titanium bike frames and components be expensive when titanium is such a common metal? Titanium happens to be the fifth most common elemental metal in the earth (behind aluminum, iron, sodium, and magnesium) and the ninth most common element in the earth. The reason is that titanium is difficult to extricate from the earth and difficult to manipulate in metal form once extricated. 6AL-4V titanium is more expensive than 3AL-2.5V titanium because it is even harder to work with.

Do you need to deflate a bike tire when going to high altitudes?
It is a common misconception that a bike tire at 120 PSI at sea level will explode when you take it to 8000 feet like at the Death Ride. In fact, the highest the pressure would effectively rise at 8000 feet is about 3PSI. Consider this: at sea level the air around us is at 15 PSI and in outer space the air is at 0 PSI. Thus, if you took a 120 PSI bike tire from sea level to outer space, it probably still wouldn't explode! Any stories you may have heard about tires exploding at altitude are almost certainly simply coincidental.

Are carbon wheels harsher than spoked wheels?
The answer is that carbon wheels are not significantly harsher than spoked wheels. In fact, probably no kind of wheel is significantly harsher than a standard spoked wheel. The reason for this is that the large majority of harshness of a wheel comes from the tire, and in particular the air pressure of that tire. A wheel made of solid metal (a heavy wheel indeed) with a tire at 90 PSI is going to be a softer ride than spoked wheel at 130 PSI. I can say from experience in riding carbon TriSpoke wheels in centuries that they feel no different from spoked wheels with respect to road feel and harshness.

What holds a hub in place at the center of a spoked wheel?
Does a hub hang from the spokes above it or is it supported by the spokes below it? The answer is that the hub is supported by the spokes below it both when the wheel is still and when it is rolling. This is a controversial topic, as one can think of reasons both for why it makes sense that the hub is supported from above and for why it makes sense that it is supported from below. If you think of the spokes above the hub as being part of a suspension bridge, then the hub seems to be supported from above. But if you think of the spokes as being like those of an old wooden wheel, then the hub seems to be supported from below. A long and detailed technical discussion on this topic and spoke issues in general can be found on the Internet at the rec.bicycles.racing Usenet forum here and here. Don't come to your own conclusions until you've read this discussion! The quickest explanation we can give here is that if you measure the tension of the spokes of an unloaded and loaded wheel, the only significant difference between them is that the spokes on the bottom of the loaded wheel happen to be compressed (lower tension due to weight-bearing) and the ones on the top stay the same. You can try this yourself by plucking the spokes and listening to the sound.