Aero v Weight

In the cycling world there is a regular argument between the weight weenies and the aero heads.  We thought we'd take a look at this and come up with some guidance as to what the science actually says.  

If you are a weight weenie, you might want to stop reading, the news is not good.  It looks like aero wins, and wins big!  (The table below gives an insight into what we found. Our full report is a 5 minute read.)

(70kg rider @ 200 watts)

We wanted to make this guide as comprehensive as possible, so these are the areas that we cover:

  • The forces of nature - what forces affect you when you ride.
  • The power to overcome - why those forces matter.
  • Why the obsession with weight?
  • Why the obsession with aerodynamics?
  • Yeah? So what? - Exactly what does this ACTUALLY mean? 
  • What does this mean for wheel selection?
  • Direct comparison weight v aero - just for fun! 

The forces of nature

When you ride you are really impacted by three different forces:

  • Rolling resistance
  • Gravity
  • Drag

All of these forces are very well understood, with well established methods of calculation.  The equations are as follows:

Force (total) = Force (gravity) + Force (rolling) + Force (drag)

Force (gravity) = G x sin(arctan g%) x W

Force (rolling) = G x cos(arctan g%) x W x Cr

Force (drag) = 0.5 x Cd x A x Rho x V^2

Where:

G = 9.8067 (Gravitational constant)
g% = gradient as a %
W = combined weight of bike and rider
Cr = coefficient of rolling resistance
Cd = drag coefficient
A = frontal area of bike and rider
Rho = density of air
V = speed in metres per second

Of all the factors in the equations, the ones we are really interested in are weight and air resistance (often referenced as CdA). CdA is made up of two components, one being the size of the object involved (frontal area to be exact) and the other being how well it slips through the air.

The power to overcome

You have to generate a force to overcome all of the above, and when we take into account the speed you are trying to maintain, that lets us calculate the power you are generating.

Why the obsession with weight?

Way back when bikes were made of steel and riders had a sly cigarette before a race, weight was the ONLY well understood physical way to obtain an advantage over your competitors.  When you talk in terms of large weight differences you can clearly see why that would be the case.  Below we show you the impact on rider speed (kph) for various changes in weight.  We are comparing to a 70kg rider holding a constant 200 Watts.

It is very clear that large weight differences make large performance differences.  For those heavier riders among us this explains a great deal!  If you think about competitive riders, it's not too much of a stretch to say a 5kg body weight difference is a realistic scenario.  That gives the light rider a real advantage on even the slightest gradient. NOTE - the benefit starts to reduce after around 5%.  This is due to the fact that the absolute speed has reduced so much, that even though the % gain is higher the actual speed gain reduces.

What about when you start to look at possible equipment differences?  You might think that a bike that is 1kg lighter than another would give you a big advantage.  That all depends upon your definition of "big".  If we run the numbers for a 1kg weight difference (graph above, dark blue line showing 1kg at 5%) we see that at a 5% grade you only get a 0.18 kph difference. If you are racing a stage in the TDF up a mountain, maybe 0.18 kph is a huge deal, but if you are racing in a flat criterium, is 0.03 kph (graph above, dark blue line showing 1kg at 0%) worth the $'000's of dollars for the light build?  If we look at weight differences of less than 1kg (as would be the case for possible wheel changes), we see the weight-dependent changes are even smaller.

A set of wheels that are 500 grams heavier will only cost you 0.01 KPH on a flat course and only a maximum of 0.09 KPH on a 5% gradient.  For the reasons stated above the difference then starts to decrease.

Why the obsession with Aerodynamics?

In the late 80's and through the 90's some riders started doing some pretty weird stuff to gain aerodynamic advantages - at the time everyone thought they were just nuts!  That was until they started winning and setting records....

You can google all of that later.

From that point on, people started putting everything in wind tunnels. Frames, riders, wheels, helmets, shoe covers, shaved legs and even electrical tape on the front of your fork to create vortexes???? EVERYTHING!

We started to understand the differences in drag that can be achieved. From that we were able to quantify the actual performance benefits of all these innovations.  Now we go back to our trusty formulas.

Below we show the benefit of both a 5% reduction in drag and a 10% reduction in drag.  Again, for a 70kg rider at 200 watts.  5% and 10% are indicative of numbers quoted for various items of aero equipment in the cycling industry.

As you can see this is a VERY different shape to the graphs for weight changes.  This is due to the simple fact that as you go uphill you slow down, therefore the aero benefit gives you a smaller speed benefit.

Yeah, so what?

Your bike wheels are a distinct point of drag generation. Using a more aerodynamic wheel can help reduce drag by reducing your CdA, therefore making you faster.

What is really interesting is when we start to compare weight and aero savings!

The graph below does just that.

Scenarios compared to the base case of a 70kg rider riding at 200 watts are:

70KG 0 CdA 300 weight - the weight is reduced by 300g

70KG 0 CdA 600 weight - the weight is reduced by 600g

70KG 10% CdA 0 weight - The CdA is reduced by 10%

70KG 5% CdA 0 weight - The CdA is reduced by 5%

We see a few key points here:

The 10% CdA reduction is better than a 600gram weight reduction up to a 6% gradient.

The 10% CdA reduction is better than a 300gram weight reduction up to a 7% gradient.

The 5% CdA reduction is better than the 300gram weight reduction up to almost 6%.

What does this mean for wheel selection?

Now we have done all the ground work we can FINALLY(!) translate this all into what it means for wheel selection.  We established the drag savings for each of our various wheels through our performance testing and have then produced the below data to compare our wheel sets to a set of Mavic Aksium alloy wheels.

We have highlighted our 88mm wheelset. Not the first wheelset that comes to mind when you think climbing!  We did this because they have almost the same weight as the Mavics.  For all the gradients we tested, you are better off with the aero wheel.

It's interesting to see that all our wheels converge at around the 4% gradient.

This shows the superior performance of our wheels, with the Aksium only winning out at all versus our rear disc wheelsets when the grade is above 7% (That would have to be a constant climb of 7% for your whole ride of course!)  This is due to the fact that only our rear disc wheelsets are actually heavier than the Aksium wheels. With all other sets they are both lighter and more aerodynamic.

This essentially informs us that we should all, in almost all circumstances ride, THE deepest rims we "can".  A number of things factor into that "can".  Most importantly you need to feel safe It's no good riding a set of 88mm wheels in a gusty cross wind, that will not be a great experience at all!  You may also not be that keen on riding around on super deep wheels just due to their look.  This all comes into play, and it is your choice.

But, the message is very clear.  If you are racing, or just want to ride faster - lighter IS better than heavier BUT aero wins. 

Direct comparison of weight v aero - just for fun

To round this all out, we thought it would be fun to look at a direct comparison between weight and aero savings at various gradients.  Specifically, what weight saving would you have to make in order to have the same benefit as switching from the Mavic Aksium wheels to the a set of ELEMENTAL 88mm wheels? (70kg rider @ 200 watts)

 

We think it about sums this up to say that on a flat road you need to reduce your weight by 26.5kgs to have the same performance benefit as the 88mm wheels.  

We hope you have found this all useful and informative, if so please share this with your friends and sign up to our eNews below - it gets you $50 off!  We also GIVE AWAY a set of wheels every three months to our eNews subscribers!

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Equations, assumptions and other calculations

Key equations:

Force (Total) = Force (gravity) + Force (rolling) + Force (drag)

Force (gravity) = G x sin(arctan g%) x W

Force (rolling) = G x cos(arctan g%) x W x Cr

Force (drag) = 0.5 x Cd x A x Rho x V^2

Where:

G = 9.8067 (Gravitational constant)
g% = gradient as a %
W = Combined weight of bike and rider
Cr = coefficient of rolling resistance
Cd = drag coefficient
A = Frontal area of bike and rider
Rho = Density of air
V = speed in metres per second
Baseline CdA - 0.3213

 

Assumptions

 

Cd changes related to each aero wheelset are constant.
CdA assumptions for our wheels have been based upon taking the results from our performance testing.
Rider power is a constant 200 watts.
These results are comparative, as such varying yaw angles have not been considered.  Given our rims will actually perform better at common yaw angles these results should be considered a worst case.