The long awaited (since last week) sequel to the not so gripping (as I didn’t cover traction as an aspect of tyre performance) first instalment of tyre selection factors. This week I’ll focus on Rolling Resistance and the combine the factors discussed into an overall view.
Unfortunately I hit a snag with some rolling resistance testing last night which delayed completion of the article (some people sleep at night – I hop on the rollers to test tyres).
Quite simply – how much effort it takes to roll your tyres along the road. Without getting into the detail too much – factors that affect CRR (coefficient of rolling resistance) include:
- Size of road contact patch
- Thickness of tread
- Suppleness of sidewalls (wall thickness and thread count of the casing are factors here)
- Tyre pressure
You might notice that most of those factors are related to flexibility. A fast tyre is one that deforms around the road surface easily, so stiff casings, thick tread or too much pressure, which all decrease the suppleness of the tyre, make the tyre slower by preventing it from flexing.
Tyre weight comes back into play here – a lighter tyre has, quite obviously, less material – which gives it a better chance of being supple. There have been some amazingly slow light tyres (mostly made by Tufo) so it’s not a particularly strong method for tyre selection but it might help you choose if there is no other data.
Pressure is complicated too. You need enough pressure that the tyre is not wasting energy by flexing excessively, but not so much that you stiffen the tyre to the point that it becomes inefficient; Ideal pressure is determined by rider weight and road surface.
A couple of good resources for tyre CRR information are:
Al Morrison (hosted by biketechreview)
I also do a bit of testing but have yet to format it for presentation on the site.
Combining the Data
Having considered the major performance factors for tyres let’s now combine them for the holistic view. Because rider mass and speed are part of this calculation each scenario requires its own graph. For the Graphs I factor mass and road surface into the CRR calc, then speed is combined with the windtunnel data referenced last week and a yaw distribution to generate a single ‘total power consumed by tyres’ figure at each wind yaw angle.
Recall that the windtunnel data (at right) showed the tyres to be close in aero performance at low yaw with the GP4000S 23mm leading the way at higher yaw. You’ll notice in the charts above that there are few cases where the GP4000S 23mm proves to be faster than the 25mm – despite the 23mm being more aero at every angle.
And the narrow Attack tyre is the fastest for all riders, all the time (or at least the 4 example riders in the 3 scenarios above). In some ways this is not a particularly interesting comparison as the Attack has the lowest CRR of the group. Tests where there is a CRR sacrifice in order to have a more aero tyre tend to be more complicated to find the ideal.
The differences in total power are mostly on the order of several watts – which puts the tenths of a watt effect of weight (when accelerating) in context as not particularly important.
Essentially this testing winnows down to two choices – if you want the fastest option and are prepared to sacrifice a little durability – go for the 165gm Attack tyre. If you want a tougher tyre with a bit more volume to smooth out the road – go with the 223gm GP4000S 25mm. The GP4000S 23mm (217gm) serves little purpose unless it is a very windy day. As the 303 is a 45mm deep wheel it is mostly favoured for road racing and waterfront cruising – in which case the 25mm makes sense for the greater comfort.
The GP4000S 23mm has come to be viewed as the ‘go to’ tyre for racing with wide clincher rims. However, in the specific instances discussed here, it turns out to be a less good option than its fat sibling when we look at the full picture. I would note that other data for wide rims that I’ve seen corroborates the results above – so the figures will be fairly applicable to any of the new generation wheels.
Tyre selection is one of the most important performance choices you can make. The aero differences between tyres can exceed the differences among comparable wheels. And the CRR differences can be very significant. Aside from those pure speed factors – durability and traction are matters of personal preference that can make or break your race. So when you’re looking for ways of improving your performance – pay attention to your tyres.
Look for more on this topic soon.