When I first started cycling, I was told to pump my tires to 100 psi. That is a nice round number and easy to remember. For years that is what I did. As I was training to Kona qualify, I looked for every opportunity to finish faster.
The bike portion of an Ironman offers the greatest opportunity for time savings because of the duration and the vast array of gear options to gain speed. I started asking questions about tires, tire pressure, and tubes.
There are a lot of choices in tires for a lot of good reasons. Finding the right tire for your race could make a significant difference in your finish time. The three main factors for deciding on tires include: goals, road conditions, and weather.
If your goal is to finish or to have the most comfortable ride possible, you are not really concerned with speed. But if your goal is to "shock the world", you are looking for the fastest tires for the road and expected weather.
What makes one tire faster than another?
To answer this question, we need to understand the concept of rolling resistance. Rolling resistance is the force of friction with the ground surface fighting against your tire rolling forward. Together with wind resistance (aerodynamics), these forces work against the power you put into your pedals to propel you forward. Reducing rolling resistance and increasing aerodynamics (decreasing wind resistance) are tricks of the trade to maximize speed.
As a rule of thumb, narrow tires are more aerodynamic. Wider tires have less rolling resistance and offer a smoother ride. On pristine roads or a velodrome, narrow tires with higher pressure are ideal. On unknown roads, go with wider tires and less pressure to soften the ride. Wider tires with less pressure move forward more and bounce less than high-pressured tires. If you are bouncing, you are not moving forward. Also, in wet conditions, wider tires with less pressure have more surface contact and are safer.
Is it risky riding low tire pressure?
We often hear that riding low tire pressure leads to pinch flats. And that is true, but the tire pressure needs to be really low to cause a flat or you need to hit a really hard bump. How low you can go depends on how much you weigh and the road conditions. Wider tires can tolerate lower pressure without increased risk of pinch flats.
Side note - I always recommend getting new tires a few weeks before a big race so they are fresh and so you can ride them a few times to make sure they are mounted properly.
My setup to KQ
Since I had dreams of Kona qualifying, I needed the fastest setup possible.
Considering the road conditions for Ironman Texas and later Ironman Kona, I decided that riding a 23 tire in the front and a 25 tire in the back would be best for me. The rear tire is shielded from the wind by the front tire, so aerodynamics isn't as important. Given my weight (135 lbs. at the time), I used the chart below to determine that my front tire pressure should be around 95 psi and my rear tire pressure should be around 85 psi.
(I do not know who developed this chart so I can't give credit where due. But there are a lot of sites and articles that have similar charts. This is the one I was first introduced to.)
By lowering my tire pressure, I was able to find the best balance of rolling resistance, aerodynamics, and comfort.
I started with 95 psi in the front and 85 psi in the back. This was in transition in the morning knowing that the pressure would increase as the day heated up. Given that this was well below the maximum, I didn't have to worry about the pressure increasing as the day heated up. This is something to keep in mind when you leave your bike in transition overnight. Don’t overpressure while it is cool and end up with a flat when the day heats up.
For someone stouter than me, higher tire pressure will be needed but be sure to consider temperature. Plan to stay below the maximum pressure as the day gets hotter.
Tubular, Clinchers, or Tubeless?
Your choice of tires is dictated by your wheels. Keep this in mind when purchasing a new bike or a new set of wheels. Be thinking about the type of tires you want to ride.
Let’s start with tubular tires. These tires have an inner tube sewn into the tire to form a single fabric. The tire is glued to your wheel. You then inflate them just like any other tire. If you have a flat, you change the whole tire. Tubular tires may offer better aerodynamics, lighter weight, and eliminate pinch points. So, you can ride with less pressure. But they are difficult and messy to change if you are unfortunate to have a flat. I recommend erroring on the side of caution. I prefer to sacrifice a little speed for the comfort of knowing that if I end up with a flat, I can change it by myself in a few short minutes. If you are a pro, riding in the Tour, no worries, your team car can give you another wheel in a matter of seconds. Tubular tires are very common among pro cycling teams. For the crazy and sometimes reckless mountain descents, tubular tires are preferred because if a flat does occur, the tire stays on the wheel (because of the glue). Whereas the tire may come off entirely on a clincher blowout.
Most of us ride clincher wheels - the standard wheels that require tubes. Recently tubeless has become popular. They work more like a car tire. Clincher wheels and tires rely on a bead between the tire and the rim of the wheel. With regular tires, air is contained in the inner tube. For tubeless tires, air is contained between the tire and the rim itself without the need for an inner tube. Tubeless tires offer improved rolling resistance over standard tires. When filled with sealant, they are more durable than either the tubular or regular tires. There’s something to be said for decreasing the likelihood and frequency of getting a flat. However, in the rare case you do encounter a large enough puncture that the sealant isn’t sufficient, changing to a standard tube is possible but will take some time.
Sealant is a liquid that you mix with the air in the tube/tire that can gum up a small puncture if one occurs. This is a good option for increasing speed without sacrificing reliability.
Butyl or Latex Tubes?
With your clincher tires, should you use the standard butyl tubes or go with a latex tubes? So many choices.
The butyl tube is denser and weighs a little more increasing rolling resistance. These are the safest option because they are easy to change.
Latex tubes are lighter, softer, and therefore faster. Some also claim they are less prone to punctures because they are more flexible. Many of the tubular tires have latex tubes sewn in. The downside to latex tubes are that they are costly, they lose air more quickly, and they are easier to pinch flat when installing them. More care should be taken when installing latex tubes. I've exploded more than one latex tube while replacing in my garage. It is quite a surprise when it explodes. Again, you must choose between speed, ease of use and cost.
Having a flat on course isn’t the end of the world but it can be costly from a time standpoint. One option is to add sealant to your tubes.
If you are really worried about reliability and avoiding flats, several vendors make extra sturdy tires just for that purpose. I rode the Specialized Armadillo tires some of the time while training. These tires are thick and have a high rolling resistance. It was very noticeable by my decreased speed on the road.
What about when riding on a trainer? Obviously, the front tire doesn't make a difference if it is not moving. Many trainers connect to your bike without the rear wheel. If that's the case, tires are irrelevant.
For trainers that rely on your rear wheel remaining on the bike, the tire makes a difference. Aerodynamics really isn't a factor when you are not actually travelling forward. But rolling resistance still plays a part. Sturdy tires designed for the trainer will likely have a higher rolling resistance versus a normal road tire. But the road tire will likely wear out more quickly. Again…so many choices.
What about weather conditions? If you've watched professional cycling, you've seen the peloton racing through all types of weather. In general, when the roads will be wet and slippery, wider tires and lower pressure provides more surface area for gripping the road. Even if you aren't navigating a switchback descent in a downpour, staying upright on a wet day may be worth sacrificing a little speed.
We've talked about riding on the road and riding on the trainer. Have you ever been to a velodrome? There is an outdoor public velodrome outside of Ft. Lauderdale in Brian Piccolo Park. Paula and I stumbled upon this while vacationing one year (without our bikes - bummer). I haven't had the pleasure of riding there yet. But someday, I'd like to give it a try.
A velodrome should provide a smooth surface so narrow higher-pressure tires may be ideal. I wouldn't recommend riding at Brian Piccolo Park in the rain. The banked curves are very steep.
We've covered a lot of material. The default 100 psi tire pressure is no longer the de facto standard. There are a lot of factors to consider to maximize speed or increase reliability.
If you are really into the science of cycling, check out Jarno Bierman's website that is all about rolling resistance.