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A guide to aero wheels

Words by Kathryn Dickinson

on 02/07/2021 16:12:00

guide-to-aero-wheels

On the list of potential upgrades for your road bike, a shiny new set of wheels are arguably the best upgrade you can make. Swapping out your wheels can have an immediate impact on performance, with increased aerodynamics, lower rotational mass and a smoother ride. There's a plethora of options out there dependent on your needs, but if it's all-out speed you want look no further than a set of deep-section aero wheels. With optimised rim profiles for slicing through the wind, the right set of wheels can save you precious seconds against the clock, or on your local loop.

Why do aero wheels matter?

Wind resistance is the single biggest thing to slow you down on a bike. At speeds over 15mph, wind is the largest force against your forward motion when cycling. Both your position on a bike and wheels are the main causes of drag on a bike. Wheels have a large surface area and when they are rotating, this changes how easy it is to control airflow across the rim and over the whole of the bike.

The tyre is the leading edge and splits the air as it hits the wheel. Behind it, a deep rim controls the airflow, helping it to come together again smoothly. A shallow box-section rim which you see on most standard bikes can’t do this, so the air behind it is turbulent, causing more drag.

Deep section wheels aim to ‘sail’ on crosswinds by making the airflow follow the shape of the wheel as it passes over it at an angle, creating forward thrust. In this way the wheels’ own drag decreases as the wind angle increases, until they reach a ‘stall’ point at which the drag sharply increases again because the airflow detaches causing turbulence and an area of low pressure behind the rim. Wide rims enable much higher stall angles, as such there is less drag and less turbulence caused in comparison to a shallow rim.

Lightweight wheels vs. aero wheels

Many have researched whether the increase in weight, but increased efficiency of aero wheels, outweighs a light weight wheel. It would appear from most tests that the increase in weight doesn't matter as the aerodynamics add a larger benefit although the latest wheels from some of the biggest brands now blur the lines between lightweight and aero with developments in carbon fibre technology.

Are disc brakes aero?

In short, no one's sure! One company found a 16% increase in wheel drag between a disc and clincher version of the same wheel, whereas another company found no significant increase in drag using a disc brake.

The theory suggests that the extra drag comes from:

  1. The rotor
  2. Increased number of spokes
  3. The larger hub body

It is really up to the individual whether they want to use either type of break. More evidence is required to prove whether or not disc brakes have a significant effect on drag. One thing is for certain though - as bike manufacturers move towards disc brakes, frames are more optimised than ever to ensure that there's no aerodynamic cost to riding with disc rotors. With even triathlon and Time Trial bikes like the Specialized Shiv now becoming disc brake only, perhaps there is a shift towards disc brakes on time trial bikes due to the increased braking ability in pursuit of the ultimate all round speed machines.

What rim depth is best?

In previous years, 'deeper is better' has certainly been the prevailing opinion when it comes to aero wheels and what you sacrificed in handling ability, was more than made up for in outright speed. However, as technology has developed, rim profiles and wind tunnel technology means that aero wheels are designed for more all-round performance, with mid-depth wheels offering more aerodynamic performance than ever before without sacrificing on weight. When it comes to handling, for the everyday rider it would seem that 65mm is manageable, with some practice if you haven't ridden deep section wheels before, and more than enough to receive the benefits of having aero wheels. This depth will be perfect for racing, but if you're looking for something deeper then these are available, or you can go the whole way and get a rear disc wheel for your time trial bike - but beware the cross wind!

Reviewed: Zipp Wheels

Words by Josh Holman.

I was lucky enough to be hooked up with a new set of wheels from the guys over at Rutland Cycling, to test out as I embarked on a season of triathlon racing. In the front, a Zipp 303 and in the back, a Zipp 404 Firecrest.

The original 303's rose to cycling fame after winning the two most famous Cobbled Classics, Paris-Roubaix and the Tour of Flanders. Firecrest technology was engineered by Zipp back in 2010, but don't let that fool you into thinking the technology is out of date. Despite the deeper section rim of the 404, they still perform extremely well in crosswinds due to the Firecrest technology. With Firecrest, the magic happens in the back half of the wheel (this is in the deep section of the wheel underneath the tyre). Firecrest uses a mathematically derived shape made up of a series of complex curves, this basically means the wheels are more aerodynamically efficient and stable for the rider. Firecrest wheels feature a rounded rim which, as mentioned earlier, reduces the 'stall' as the air flows over the rim for a longer time, creating an effective airfoil. Rims which are pointed will have an earlier separation of the air flow and a larger 'stall'.

Still unsure what this translates to in the real world? Zipp have created a video explaining their Firecrest technology. Watch it below!

 

How did the Zipp's feel riding?

My first real test of the Zipp wheels came back in June when I raced in the Challenge Middle Distance Championships in Slovakia. A race which saw headwinds, crosswinds and hot temperatures. I can honestly say that in a straight line it was extremely easy to build momentum and increase top end speed, but the benefits didn’t stop there, during the crosswinds I was able to maintain control of my Giant Trinity whilst holding a decent pace, something that normally cyclists struggle with. I believe that this massively contributed to my fastest ever bike split over the middle distance, covering the 56 miles in 02:13:00 and averaging 40.1kph!

Another super cool benefit of deep section rims is the superb 'swooshing' noise that the wheels make! When you're riding on the flat, a quality set of deep-section wheels will make holding a given pace that little bit easier. This gives you the option to kick on, or keep something in the tank for the run and I definitely felt these benefits when starting the run, posting some of my best run splits ever! Overall, I was very impressed with the Zipp 303 & 404's. I believe that they complimented my Giant Trinity extremely well, giving me a far batter aerodynamic set up and I would recommend the upgrade to anyone serious about cycling or triathlon.

 

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