Cycling, Gear, Podcast

Tire pressure, aerodynamics, and smart equipment upgrades with Josh Poertner | EP#235

 May 25, 2020

By  Mikael Eriksson

LISTEN TO THE EPISODE HERE:

In this Episode you'll learn about:

  • Optimizing tire pressure (and the potential cost of getting it wrong)
  • Equipment choices and watts-for-buck: bike frame, wheels, and nutrition and hydration setup
  • Equipment choices and watts-for-buck: helmet, clothing, socks, shoes and shoe covers
  • Equipment choices and watts-for-buck: drivetrain friction, ceramic bearings, oversized pulleys, 1x drivetrain

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Shownotes

Background

04:35 - 

  • I am currently the owner of Silca, a pump and tool company, which I bought six years ago.

    I also run a company called aeromind, which does a variety of consulting jobs within aerodynamics and tire pressure.

    Before that I have been working at Zipp wheels and SRAM.

Tire pressure

06:40 -

  • When I was at Zipp as we worked on developing the fastest wheels possible, we discovered that no matter how strong and/or resilient we made the wheel, if the tire pressure was too high or low, the rider would be slow, get a pinch flat or risk breaking the wheel.

    At that, no one really tested the rolling resistance of tires, and as we discovered how crucial it was in order to go fast, at first we decided to keep this a secret.

    During this time, the common belief was that the harder you pump up the tire (higher pressure), the faster you go/the lower the rolling resistance.

    What we discovered was that this is true but only to a certain point as we call the ”breaking point” and after this pressure the rolling resistance go up.
  • The relationship between tire pressure and rolling resistance is quite linear until the breaking point, on average ~1-3w per bar increased pressure.

    After the breaking point, however, this relationship becomes exponential in the other direction, the more you increase the tire pressure, the bigger does the rolling resistance penalty get.

    For some tires, we have seen an around ~10w per bar increase in rolling resistance.

    For top rage tires, however, the slopes are less steep both before and after the breaking point.
  • In general I think that most people, both amateurs and professionals tend to pump their tires up too much.
  • On the Silca webpage, we have developed a calculator where you put some parameters such as your weight, tire width, pavement quality etc. etc. and then it calculates the optimal tire pressure for you and your circumstances.

    The biggest uncertain parameter of this calculator is what road quality category you should enter in the calculator and how to know which category your road belongs to.

    If you are insecure about what road quality category your road belongs to, the best advice would be pump your tires to a pressure somewhere in the middle of the two road quality categories.
  • Factors that impact tire pressure:

    1. Weight (bike + rider), then higher the weight, the higher the pressure.

    2. Tire size, the larger the diameter of the tire, the bigger the tire, the lower the pressure.

    3. Road surface conditions, the rougher the road, the lower the pressure.

    4. Rider speed, the faster the rider, the higher the pressure.

    5. Wheel diameter, the longer the diameter, the higher the pressure.

    6. Weight distribution, the more weight you put on either the front or rear wheel, the higher pressure you need at this wheel/tire.

    These are the factors that are included in the calculator but there are several other parameters that affect the tire pressure choice, in particular the temperature gradient during the day, which we probably soon will include in the calculator as well.

Bike frame

40:20 -

  • The frame is for sure important but maybe not among the most important aspects of the bike in terms of aerodynamics.

    The most important part of the frame is that you have a frame that can support a really good position.

    In general, weight is not that big of a concern, even at the pro level we do not pay that much attention to weight in most cases.

    The bike frame is probably fourth or fifth in the hierarchy of important parts of the bike in regards to aerodynamics.
  • The most important aspects that influence aerodynamics would be position (by far the most important), helmet choice, wheels (disc wheel in the rear is recommended at all time, especially in rough wind conditions but it requires certain bike handling skills), skin suit (the fit of the suit is probably more important than the fabric itself, wrinkles on the suit do create more drag than you may think at first) and after that the frame.

Testing

50:05 -

  • When it comes to field sensors, I think that most people can get better test data by just applying the Chung method (outdoors field testing) than spending plenty of money on a field sensor.

    Sometimes in the future, these sensors will be worth the cost, but the technique is not quite there yet.

Socks and shoes

58:10 -

  • A good pair of aero socks could be 2-3w savings.

    Example of good aero socks are the Castelli aero socks and aero coach socks.

    For the Ironman distance I would say that it is definitely worth putting on aero socks, you will more than make up for the extra time it takes to put them on.
  • Shoes can also play a fairly big impact on the aerodynamics, you should search for a as smooth and ”wrinkle free” shoe as possible.

    In general, a shoe cover is more aerodynamic than the most aerodynamic shoe.
  • Together, aero socks and shoes could give an aero benefit of between 5-7w.

Hydration setup (from the perspective of long distance triathlon racing)

01:04:45 -

  • In general, all cylindrical bottles should be avoided on the bike as long as they are not placed behind the saddle or sometimes between the arms in the front (but this could come with a penalty).

    Aero bottles are usually really good and sometimes pro riders place empty aero bottles just because they are aerodynamically beneficial.
  • In terms of nutrition, I am a big fan of storage boxes that are either placed behind the saddle or are incorporated/configured in the frame.

    The other aspect you should consider when putting on your nutrition on the bike is how much you must break your position in order to excess your nutrition.

Drive train

1:15:50 -

  • The most important part of the drive train is the chain, which should be clean and waxed in order to be as affective as possible.

    I would like to emphasize extra much on the cleanness of the drive train, a bike that has been ridden without being cleaned for a month or two could easily have lost 2-3 % of the power translation of the drive train.

    Other important aspects of the drive train to consider is chain ring size, I would recommend going as big as possible in front as this reduces frictional losses in the drive train.

    The same thing goes in the rear, I am a bit skeptical to 10 cogs chain rings in the cassette as these comes with rather big frictional losses, so if you ride a 1xdrive train, you should really try avoiding riding on the biggest gear in the rear as much as possible.

Rapid fire questions

1:25:45 -

  • What is your favorite book, blog or resource related to endurance sports? I don’t have one, I read plenty of economics and financial literature and within that field one of my favorite is ”Marginal Revolutions” that is a great economics blog.
  • What is your favorite piece of gear or equipment? I bought a 1990 Eddie Merckx mx leader, which is my all time favorite bicycle.
  • What is a personal habit that has helped you achieve success? No television, twelve years ago I stopped watching television and it has freed maybe 2h a day of extra time.

​LINKS AND RESOURCES:


Mikael Eriksson

I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal. Please contact me if you have feedback on the podcast or want to make suggestions for improvement or send in a question for a Q&A episode. If you are a long-time listener and appreciate the value the podcast brings, please consider taking a couple of minutes for leaving a rating and review on iTunes/Apple Podcasts, or wherever else you can think of leaving a rating and review.

Mikael Eriksson

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September 14, 2020

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