Of all the scientific or pseudo/marketing scientific issues that divide cyclists, wheels must be right there at the top.
The story usually goes like this:
Someone asks what wheels he or she should “upgrade” to. This person is typically a beginner, who unfortunately read a bicycle magazine somewhere and got the wrong impression about wheels.
Several answers ensue but these can all be divided into two classes.
- Those who recommend XYZ wheels based on what they’ve purchased and “so far no issues, therefore...” and;
- Those who recite the old myths of rotational weight and aerodynamics and make recommendations based on wheels with the lowest of these two characteristics.
What you never find in these posts are rational answers based on realistic science. Nor can any amount of coaxing entice the advisers to justify their recommendations or opinions. So, let’s try and look at the issue objectively. I’ll make some assumptions.
- The OP is an amateur and probably Joe Average or Joe Beginner. The OP is not a time-triallist, tri-athlete, world hour record candidate or particularly talented and performs long solo break-aways at the head of a pursuing bunch.
- The OP does not have 8% body fat but typically double that.
- The OP is not a lightweight but probably average i.e. 70 kgs or above.
- The OP is not yet flexible enough to get very aero on the bike and doesn’t even feel comfortable in the drops.
- The OP’s average ride speed is 26 kph (http://tinyurl.com/jwnhr9n).
- The is nothing wrong with the OP’s current wheels.
Let’s deal with the recommendations based on ownership. The premise here is always that “these worked for me so therefore it will work for you.” They encourage the OP to make a meaningless upgrade without understanding why the OP even needs to change wheels. If there is a real need and a special one, what works for you won’t work for the OP. Sometimes these recommendations are pseuo-justified. “These XYZs roll nicely.” “These XYZs of mine are really responsive.” These are meaningless, unquantifiable terms. Often they even contain ridiculous conflicts. “Really stiff by very comfortable/Compliant.” Those properties are opposing.
Anyway, most of these recommendations are given with good intentions but that doesn’t mean they may not be questioned.
Let’s move on to the real pseudo-science – light, stiff, aero, fast etc etc.
Rolling resistance: Bearing resistance is an extremely small portion of the overall rolling resistance in a bicycle wheel. Drag here is in the order of one or two grams whereas drag from wind resistance could be a hundred times that. And then only at high speed. I again refer you to the nation’s average speed for competitive riders – 26 kph. A pro peloton goes 50kph. Aero drag increase with the square of speed. Going at 50kph requires four times more horsepower than going at 25kph. In other words, at low speed, aerodynamics is just not an issue at all. Nada.
A thin race tyre pumped at 8 bar gives you better improvements in rolling resistance than an expensive ceramic bearing.
Then, it is impossible to perceive differences in rolling resistance or aerodynamics between different wheels on a bicycle, so saying that “my XYZ wheels roll really fast” is nonsense. Obviously I’m excluding the outriggers in the bell curve but we’re not comparing cheap plastic BMX wheels with permatubes inside knobbly tyres here in anyway.
Aerodynamics: It is of course true that rims with larger profiles and wheels with lower spoke count have better aerodynamics but aerodynamics is just not important to Joe Average. Have a look at his statistics at the beginning of this article. He/she doesn’t go 50kph. Deep section rims are problematic too. They have to be made from carbon in order to bring them in at any reasonable weight. This increases costs, decreases braking performance significantly, reduces durability and is unsuitable for commuting and riding where there could be potholes. Carbon wheels are just not practical. Further, any aerodynamic wheel is almost certainly a proprietary wheel. Hub spares, spokes and rims are all a manufacturer’s part number and can only be obtained from the manufacturer itself. This increases repair costs and waiting time. A standard J-spoke wheel using 32-spokes on a common hub, laced with standard double-butted spokes on a common rim is worth its weight in gold when it comes to repairs.
Stiffness is never an issue in bicycle wheels unless the wheels are stupid to start off with. If they don’t touch the seatstays, chainstays or fork, then they are stiff enough for the job. No energy is lost from a little bit of flex (it is an elastic deformation and the energy is returned). Brake rub is an issue but not a problem. Most boutique wheels nowadays are not stiff enough to not flex when a powerful rider honks up a hill moving the bike from side to side. This manifests as little scraping noises from where the rim touches the brake. The noise, especially with carbon wheels is amplified and not indicative of the amount of friction it produces. However, this is a special case. Campagnolo acknowledges the problem by fitting single pivot brakes at the rear. Single pivot brakes have a low mechanical advantage and therefore the pads can be far from the rim and the brakes still have enough lever travel to lock the wheel. Some riders on Shimano/SRAM simply open the quick release on the brake a bit to prevent rub in this situation. Nevertheless, wheels with reasonable rims weighing 450 grams or more and 32 or more spokes do not rub. When the spoke count starts to diminish below 28, then wheel flex and consequently wheel rub become noticeable. Lateral wheel flex is not significantly reduced by stronger rims and stronger rims cannot compensate for fewer spokes. It is best dealt with by more and thicker spokes.
Claimed stiffness is almost always based on hearsay – what the magazines tell us repeatedly without any meaningful analysis. Stiff enough is stiff enough.
Rotating weight. The holy grail of bicycle wheels. The age old story is that losing weight on a wheel is twice as beneficial as losing weight elsewhere on the bike. Often called rotating weight, it is hailed as the solution to any wheel question. Whist it is based on science, it is not based on reality and the context is never stated. The irony is also never pointed out: rotating weight goes up as the rim profile increases.
Nevertheless, let’s look at the facts of “rotating weight.” We’re really talking of the moment of inertia (MOI) of rotating masses, in this case wheels. It isn’t easy to calculate the MOI for something as complex as a bicycle wheel because we have to mathematically define the transition from hub to tyre and where the mass is located. A rotating cylinder is easier to work with because it has zero mass on the inside and it is all concentrated on the outside at a known radius. Nevertheless, I’ll try and build a model to explain how it works.
There are a few concepts you have to understand about an accelerating bicycle. The first is an understanding of just how weak our engines are. For a bicycle to accelerate from zero to 30 kph is a lengthy struggle. Go and try it on your next ride and see how long it takes you. We accelerate inherently slowly.
Secondly, when we accelerate, we’re accelerating two bodies: 1) The bicycle and rider and pay load in a linear direction and; 2) The wheel in rotation. We are not only accelerating a wheel, like in a salad spinner. We are accelerating a heavy body and a light wheel. Very light, as a percentage of overall mass of the entire body.
Thirdly: We don’t really accelerate when riding a bike. Even so-called “surges” are slow affairs and few and far between. I can prove this. Those of you who do interval training and ride tactically know that you can shake of 90% of the wheelsuckers within the first 5 km of the race by simply surging for 5 or six times as you turn a corner. Half a dozen surges is the total capacity of most average riders, then they drop off. Just by doing a bit of interval training you can already shake off your competition. That’s before even training to ride faster! The accelerations and oft-called micro-acceleration we experience during rides are always over stated. We dont’ ride like that and we can’t accelerate like salad spinners.
I’ll use one method to demonstrate the negligent power requirements of accelerating a wheel over a heavy body. Some of you may come up with different models and we can play with these.
Here I want to to substantiate y argument that a reasonable weight saving of say 200 grams per wheel is no more important than a similar weight saving on the bicycle and rider combo. In other words, stop chasing the holy grail.
In order to avoid semantics, my term “no more important” means that the energy-saving in accelerating the two differently equipped bodies (but with same overall weight) is negligible compared to the total energy input to get those two bodies up to the same speed.
From now on I shall refer to the package of bike and rider simply as “the bike
”. Bike A has the heavier wheels.
I assume that 200 grams per wheel is reasonable, but if not, we can simply plug in another figure. I weighed a naked rim and tyre, but did not include a tube. Since both wheels need tubes, it will simply cancel out in anyway.
I start off by stating that the total energy stored in a bike travelling at speed is equal to its linear kinetic energy plus its rotational kinetic energy (the energy stored in its spinning wheels).
And by looking at the bike's total stored energy at the end of a constantly-accelerating run, we know how much energy was put in. The one who requires the least energy to get there wins.
The total package’s linear energy is calculated thus: 1/2 M*V^2.
The wheel’s rotational energy is also 1/2M*V^2. We have to add these to get the total.
Assumptions:
- The bike weighs 90kgs.
- We measure the energy at 30 kph.
- The heavy wheel weighs 800 grams.
- The light wheel weighs 600 grams.
- The bike has two wheels.
- All the wheel’s weight is concentrated in the rim/tyre combo.
The hubs and spokes weigh nothing. – but don’t break your head on these two statements, they simply take a bunch of calculations out of the equation that would have cancelled each other in anyway.
The two bikes reached 30 kph under exactly the same conditions in terms of wind, road, gradient etc.
I’m not converting to standard units since we’re only after a ratio. I do the same for both bikes and the effect is therefore nil.
Now for Bike A’s total kinetic energy at say 30 kph.
We know that it is 1/2 MV*^2 (one half mvsquared)
Plus 1/2 M*V^2. for wheels only.
Thus
1/2 x 90 x 30squared Plus 1/2 x 1.6 x 30squared
= 40 500 units Plus 720
= 41220. units of energy
Bike B’s total kinetic energy at 30 kph.
1/2 x 90 x 30squared Plus 1/2 x 1.2 x 30squared
= 40500 Plus 540
= 41040 Units of energy.
The summary:
***************************************************************************************************************
Bike A has
41220 units of energy stored after an acceleration from 0 to 30kph and
Bike B has
41040 units of energy stored after an acceleration from 0 to 30 kph.
The difference is 180 units of energy or 0,43 percent and that for an all-out acceleration from zero to 30kph.
Conclusion: it requires 0,4 percent more energy to accelerate a bike with tyres weighing 400 grams less than a bike of equal weight but with heavier tyres.
In conclusion:
Wheel upgrades for the usual reasons stated (drag, weight etc) are a waste of money.
Bigger benefits can be found elsewhere such as from training, aerodynamic positioning, nutrition etc.
Amateurs and commuters don’t need racing wheels.
