In cycling, aerodynamics is king and gets nearly all the attention... aero bikes, aero wheels, aero helmets, computer bike fitting... it's all high tech and "sexy" stuff for the geeky triathlete. On the other hand, tires are often simply considered a "wear item" that needs to be replaced periodically and pumped up before you ride.
Before you write off tires an unimportant, let me help you understand why tire choice matters. For a typical age group triathlete who generates around 3 watts/kg on the bike, you have far more to gain or lose through tire choice than any gains you can make with a set of Zipp wheels.
To demonstrate this, I compared two very popular Continental tires: Gatorskins and GP4000s. Gatorskins are a top seller for good reasons. They are known for being puncture resistant, they last a lot of miles and are considered a top training or commuter tire. The GP4000s are a top selling racing tire, known for it's low rolling resistance. But how much does rolling resistance really matter? I created a model in Best Bike Split and compared two sample riders (a hypothetical male and female) over Olympic and Ironman length courses to see just how much difference tires can make. I ran a total of six scenarios per rider, per course (total 24 runs) and the short answer is for your average age grouper in the 16 to 20 mph range... choosing a racing tire over a training tire provided more speed gains than paying $250 for an aero helmet and $3000 for a deep carbon front tire and running a solid carbon disc on back. For an Ironman length race, the GP4000s tires save around 16 minutes over the Gatorskins at the same power level and the full aero accessory package (helmet & wheels) saves only around 7 minutes.
I've listed the results below. From a modeling standpoint, I've assumed 3w/kg FTP, IF's of 90% for Olympic and 70% for IM, CdA's of approximately 0.31 (typical of non-optimized body position), 6' 175 lb male, and 5'-4" 125 lb female. The scenarios show below are 1) Gatorskin tires with standard road helmet and wheels, 2) GP4000s with standard road helmet and wheels, 3) GP4000s with Latex tubes (rather than standard butyl tubes), 4) Gatorskin tires with aero helmet, 5) Gatorskin tires with aero helmet, 808 front & disc rear, 6) GP4000s with latex tubes, aero helmet, 808 front & disk rear:
Because not everyone does long course racing, I also included an Olympic length race so you can see the impact there as well.
In terms of dollars spent for speed gained, tires are a great "bang for your buck". Although Gatorskins and GP4000s tires are on opposite ends of the spectrum in terms of rolling resistance, most manufacturer's standard tires are probably somewhere in the middle of these two... they don't offer the puncture resistance of a Gatorskin, nor do they offer the speed of a GP4000s. Depending on their actual performance, you may see half of the time savings above... which would still be comparable to the full aero accessory package. But what about wear and puncture resistance of racing tires? Everyone may have different results depending on the riding you do. I personally had a lot of pinch flats when I rode Michelin Pro Race 3 tires, but have been riding GP4000s for over 3 years and haven't had one. Your mileage may vary.
Want to compare some of the popular tires out there? This site does a great job of testing tires and showing how much energy they consume. The results are "per tire" so you need to double them for a set of tires.
Before running off any buying new tires, there are a few more important things to understand about tires:
- It is important to have the proper pressure in your tires. Your tire pressure is determined by the weight you put on the tires (bike + body weight + water bottles + etc.). This article does a fantastic job of explaining that ideally your tire will have a "15% drop" when optimally inflated, and has charts to demonstrate weight versus pressure with varying tire size. If you want the "easy button", here is a calculator that will figure out how much pressure you need based on your weight. For most triathlon bikes, you can use a 45/55 split of weight for front/rear (used the second calculator from the top... and don't forget to add your bike weight). I suspect that many heavier riders fail to use a larger enough tire (25mm rear) and many light riders tend to add more pressure than necessary (particularly in the front tire).
- Higher pressure isn't necessarily better. On smooth surfaces, higher pressures result in lower rolling resistance... this has been demonstrated in multiple tests and can be seen in the rolling resistance testing site I referred to earlier. But the real world isn't smooth like a stainless steel roller. And there are diminishing returns... going above 100 psi provides very little improvement in rolling resistance. In the real world, when you go over bumps there are "suspension losses" meaning some of the energy that is supposed to be pushing you forward instead pushes you upward as you go over bumps. Excess inflation makes for very bumpy and jittery rides, particularly on a Tri bike where your elbows are resting firmly on elbow pads with no suspension from your wrists. Way too many people put excess pressure in their tires on race day, assuming it will make them faster... when it could actually make them slower. I noticed a significant improvement in both ride quality and control when I lowered my front tire to around 90 psi, based on the weight recommendations. Here is a great article on suspension loss, where these people actually compared riding on the road versus rumble strips to see the impact on power loss. Not surprising... there was a huge difference.
- Wider tires have less rolling resistance than narrower tires. Again this can be seen on the testing on rolling resistance curves. Does that mean you should get the biggest tires possible? No, go with what fits in terms of pressure, under the first point above. Wider tires can potentially have higher aerodynamic losses than narrow tires. Most wheels made since 2014 are wider, making 23mm tires effectively 25mm wide which helps with rolling resistance and aerodynamics (the tires end up being flush with the wheels reducing turbulence and improving cross wind performance). The important point is that if your weight dictates that your need wider tires, they will actually be faster than narrow tires for you. There is also the option of mixing sizes, such as 23mm front and 25mm rear to optimize rolling resistance based on weight, while getting the benefit of having the rear tire tucked behind the frame on a Tri bike. Note that you do need to verify that a wider tire will fit in the rear with your frame. For some Tri bikes the frame is so close to the tire that you could potentially end up with tire rub with larger tires.
- Not all tubes are created equal. Because of additional elasticity, latex tubes have lower rolling resistance than standard butyl tubes. The downside is they are more expensive, lose air faster and they need to be installed with some talc powder to avoid having them stick to the tire and pop (I've blown a couple when I didn't use talc powder on the first inflation). But there are additional upsides as well. Besides having lower rolling resistance, latex tubes are very light weight, and some people feel that they have better flat resistance and have a smoother ride. My own personal experience is that bumps seem to be a little less "sharp" on latex. The other option is to consider using a lighter weight butyl racing tube rather than latex. They are between standard tubes and latex in terms of price and performance, but may not offer as good of flat protection. Light butyl tubes are available in 650 tires, and I've yet to find latex in the smaller tire size. Here is a test result showing the differences between standard butyl tubes, light butyl tubes (racing), latex and tubeless tires.
So... something as simple as bike tires do matter. And it's a relatively inexpensive and easy way to improve your bike speed.
- It's also important to understand that although bike tires trump aero in this example, that may not always be the case. In optimized body positions, top age groupers are typically in the 0.24 to 0.27 CdA range and are traveling at higher speeds where aerodynamics starts being a much larger portion of total bike power.