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by Matt Wikstrom
May 17, 2017
Photography by Matt Wikstrom
TECH NEWS BROUGHT TO YOU BY BIKEEXCHANGE
Most riders appreciate the importance of tyre pressure but our understanding is changing. Where once high tyre pressures (>100psi) were believed to be faster and more efficient, now there is a new enthusiasm for lower tyre pressures. So what is the optimal tyre pressure?
The simple answer is that there is no such thing. James Huang discussed this issue with Jan Heine and Josh Poertner in a podcast last year and while there are many factors to consider, lower tyre pressures are likely to have more to offer than high pressures.
“We were absolutely surprised that the optimum tyre pressure didn’t exist,” said Heine. “We were thinking maybe 120psi. Above that, it gets too uncomfortable, and below that is too slow. But there was no too slow. The tyres started to collapse in the corners before they got slower.”
Poertner arrived at the same conclusion after working on the issue with a variety of professional riders and teams:
“We spent a week in the Arenberg forest and every time we lowered the pressure, they [the riders] went faster for the same input. There is no perfect pressure; it changes for rider weight, it changes for the road surface, and so it really allows for optimisation, which becomes strategically important.”
One of the biggest revelations from these studies is that the majority of riders interpret the extra vibrations associated with over-inflated tyres as extra speed. In the absence of those vibrations, most believed they were travelling slower, so were quick to dismiss the value of lower tyre pressures. However, once accustomed to the change in feedback, riders in these studies consistently found that lower tyre pressures provided extra comfort without sacrificing speed.
Both men also stressed that the only way to determine the ideal tyre pressure for any given rider is to experiment with different tyre pressures. And since most riders are over-inflating their tyres, Poertner’s general advice is simple: “Lower the pressure by 5psi, then stick with it for a week.”
Deciding on a suitable tyre pressure really starts with an appreciation of all of the variables that can have an effect on the performance of the tyres:
Weight: Perhaps the primary consideration. While the majority of the load on the tyres comes from the weight of the rider, it’s important not to overlook the bike and any gear that is being carried.
Terrain: Paved versus unpaved roads, smooth bitumen versus rough chipseal. Tyre pressure needs to be adjusted depending on where the bike is being ridden.
Conditions: Any kind of water on the road decreases the grip of the tyres, so it’s prudent to lower tyre pressure ~10psi. A passing shower or shaded roads that haven’t dried after overnight rain can complicate tyre pressure choice.
Tyre width: Lower pressures can be used with wider tyres (e.g. 28c) with a lower risk of pinch-flats.
Casing and rubber compound: These components have a huge impact on how the tyre behaves when it is inflated. The bicycle industry makes use of a variety of tyre casings and rubber compounds, but in the absence of a meaningful baseline, comparisons are difficult. Thus, while high thread counts generally produce suppler casings, the final result also depends upon the rubber compound and how it is incorporated into the tyre. The presence of puncture-proofing belts and/or material can also have an effect.
Tyre type: Tubular tyres have a reputation for being suppler than clinchers, but this is more a reflection of the construction materials employed rather than the type of tyre. Any given pressure that works well for a tubular will not automatically work as well for a standard clincher or tubeless tyre unless they share the same materials and construction methods (e.g. tubulars and open-tubulars).
Rim width: The width of the rim bed has a direct impact on the size and shape of a clincher tyre. Narrow rims essentially pinch the beads of the tyre together, creating a bulb shape, while wide rims allow the tyre to form more of a semi-circle that is vertically stiffer. Thus, the width (Figure 1A) and height (Figure 1B) of any given tyre generally increases with the width of the rim, and therefore, lower inflation pressures can be used.
Figure 1: The influence of rim width on tyre width and height. For any given tyre size (23c, 25c and 28c are shown), the width of the inflated tyre increases with the width of the rim bed as shown in panel A. This also applies to the height of the inflated tyre, as shown in Panel B. Data supplied by Josh Poertner.
Given all of these variables, it should be obvious that there is little point in two riders comparing tyre pressures unless they weigh the same and are using matched tyres and rim widths on the same roads under the same conditions. As already stated, riders will gain far more information and insight when they take the time to experiment with different tyre pressures.
Which brings us to the challenge of assessing the performance of the tyres. Road cyclists are generally pre-occupied with speed, but to concentrate on that alone is to overlook two other equally important aspects, namely comfort and grip. And as it turns out, there is no need to compromise on the latter in order to enjoy plenty of the former.
Tyres deflect at the point where they meet the road and this creates resistance that the cyclist must overcome to drive the bike forwards. There are, of course, other forms of resistance that are more significant (e.g. air resistance) but many road cyclists appreciate the fact that they can minimise rolling resistance through careful selection of tyres (and tubes).
It is relatively straightforward to measure rolling resistance under controlled conditions. A large rotating drum or a set of rollers can be used to reproducibly identify relatively minor differences in rolling resistance allowing different brands, models and sizes to be compared and ranked to identify the “fastest” tyres. The influence of other variables — including tyre pressure, different inner tube materials, and for tubular tyres, the method of gluing — has also been tested.
The results from these studies have provided a lot of insight on the parameters that can influence rolling resistance. These lead to some clear and long-held principles, such as rolling resistance tends to decrease as tyre pressure is increased, as well as providing fresh data on new trends such as wider tyres. However, there was always a risk that such a heavily controlled lab test would overlook other issues at play in the real world.
It was Tom Anhalt that first raised the possibility that there was more to rolling resistance than friction alone. By comparing his “lab” data with real-world data, Anhalt noticed an unexpected increase in rolling resistance when high tyre pressures were used on the road (Figure 2A). Jan Heine and Josh Poertner subsequently confirmed these observations, ushering in a fresh view on rolling resistance and renewed appreciation for lower tyre pressures.
Figure 2: When a tyre is tested on rollers, rolling resistance (Crr) decreases as the inflation pressure is increased. However, that does not hold true for the road, as data from Tom Anholt shows (panel A). Recent testing by others has confirmed this phenomenon, and it is most obvious for rough road surfaces (panel B, data supplied by Josh Poertner).
Current thinking now holds that there are two components to rolling resistance: first, hysteretic losses, which occur as the tyre flexes and are highest when the tyre is too soft; and second, suspension losses, which act on the rider and are at the greatest when the tyre is too hard. Each effect appears to be largely independent of the other, so the net result depends upon the sum of the two.
Every bike rider has a healthy respect for the kind of impact hysteretic losses can have, hence the willingness to over-inflate the tyres. In contrast, most accept (and even revel in) suspension losses when it takes the form of the buzz and rumble of firm tyres, choosing to interpret this kind of feedback as a measure of speed. While it is true that this kind of vibration increases with speed, any rider that experiments with lower tyre pressures will discover that it is possible to go just as fast without it.
The amount of energy that is wasted due to suspension losses increases significantly as the surface of the road gets rougher. Jan Heine measured huge losses when riding the rumble strips that border some roads while Josh Poertner found that even a small amount of over-inflation (10psi) could produce an obvious penalty (Figure 2B). He also found that stiff tyres (e.g. training tyres) were more susceptible to suspension losses than supple racing tyres.
The implications for these studies are clear: riders that tolerate over-inflated tyres are wasting energy. Importantly, these losses appear to be significant at the kinds of tyre pressures that are commonly used (100-120psi). Thus, a large number of riders will likely enjoy an immediate decrease in rolling resistance (and concomitant increase in efficiency) by simply lowering their tyre pressure 10-20psi, even if they ride on smooth bitumen roads.
Perhaps the biggest objection to lowering tyre pressure is that it will increase the risk a pinch-flat. This is a common fear that is mostly unique to conventional clinchers, however riders using tubeless or tubular tyres can’t ignore the risk.
A pinch-flat results as the tyre bottoms out against the rim and the tube is caught between the road surface and the sidewalls of the rim. Most riders intuitively understand that tyre pressure has some bearing on how readily a tyre will bottom out, however studies on the forces involved by Damon Rinard, and more recently Josh Poertner, have revealed some interesting observations.
Both men used an instrumented press to measure the amount of force required to compress different-sized tyres (23-28c) up to 20mm over a range of inflation pressures (6-8bar/87-116psi). The wheel was secured in a jig so these tests were static rather than dynamic, however different anvils were fitted to the press to mimic a variety of insults. Thus, an anvil with a 80mm radius represented a cobble while a 8mm radius was used for a sharp bump.
Figure 3: Determining the average and total force for displacing a tyre at different sizes and inflation pressures. Two anvils with different diameter faces (80mm versus 8mm) were used to displace the designated tyre sizes at three inflation pressures, as shown, over 15mm to determine the average amount of force required (Panels A and B). These values were combined with measurements for the height of each tyre to calculate the total force required to bottom out the tyre (Panel C). Data supplied by Josh Poertner.
Amongst the results from these studies is the clear demonstration that the average amount of force required to displace a tyre decreases significantly as the diameter of the insult gets smaller, as shown in Figure 3. Almost twice as much force was required to displace the tyres with a 80mm anvil (Figure 3A) than a 8mm anvil (Figure 3B).
This is something that most cyclists that have suffered a pinch-flat will already understand. What is more interesting is that tyre pressure only has a minor effect on displacement by a cobble (Figure 3A) that essentially disappears for a sharp bump (Figure 3B). Thus, if you hit a pothole hard enough to bottom out a tyre, then an extra 20psi is not going to make much difference (though the size of the pinch-flat might be a little smaller).
Similarly, the size of the tyre involved doesn’t appear to make much difference, either (provided the brand and model are otherwise equivalent), at least in terms of the average forces involved. However, larger tyres are taller (Figure 1B), and therefore, must be displaced further before a pinch-flat will occur.
For a 23c tyre, this height can be 22-23.5mm, depending on the width of the rim (Figure 1B), compared to 26-28mm for a 28c tyre. When applied to the data presented in Figures 3A-B, the total force required to bottom-out the tyre increases with the size of the tyre, as shown in Figure 3C. Put simply, the extra cushioning provided by larger tyres means that more force is required to produce a pinch-flat.
In fact, by using a larger tyre, it is possible to lower tyre pressure and still enjoy extra resistance to pinch-flats. Thus, for those suffering ongoing pinch-flats, there is more merit in changing to a set of larger (taller) tyres than over-inflating narrow tyres.
Compared to off-road riders, road cyclists don’t have the same kind of demand for the grip of their tyres, but it still is important for cornering, and staying upright in wet conditions. While our understanding of grip is far from complete, it’s clear that lower tyre pressures favour extra grip by allowing the tyre to better conform to the road surface. Supple tyre casings also help grip in the same way, as do soft rubber compounds and the broad contact patch afforded by wider tyres and/or rims.
Of course, there is a limit to how much the tyre pressure can be lowered in order to improve grip. Once the tyre starts to collapse under the weight of the rider, it becomes unstable, and that will undermine the grip of the tyres.
Lowering the tyre pressure for any given tyre will not change its suppleness. This applies particularly to robust training clinchers that are stiff with extra rubber. Such tyres are going to be more resistant to punctures, but the sturdy construction means that they won’t conform as readily to the road surface or rebound as quickly as a supple racing tyre. As such, they will always suffer extra rolling resistance and suspension losses on the road.
Thus, the most effective way to maximise grip is to use larger tyres made with supple casings that are inflated to relatively low pressures. Since this kind of setup is also going to minimise suspension losses, there is the promise that riders will enjoy extra grip and comfort without sacrificing their speed.
For riders hoping to maximise their marginal gains, it’s worth noting there is a limit to how much larger a tyre can be before it starts to detract from the aerodynamic performance of a set of high-profile wheels. The tyre will generally begin to interfere with airflow once its width exceeds that of the rim by 5%, but Josh Poertner has shown that it is possible to use a wider tyre (e.g. 25c) on a rim that has been optimised for a smaller tyre (e.g. 23c) as long as the inflation pressure is kept low.
There is no simple formula for determining the optimal tyre pressure for an individual. And as discussed above, there is little point in consulting your riding buddies. Instead, the best approach is to start experimenting with different tyre pressures, remembering that 5psi can make a difference.
While such experimentation may seem time-consuming, a detailed log will soon inform the rider about the kinds of tyre pressures that will work in terms of comfort, grip and rolling resistance, and those that won’t. This is the kind of information that can provide racers with a strategic edge, while recreational riders will find that they’ll be better prepared for their favourite ride.
With all that said, every rider needs a meaningful starting point. One place to start is to use Frank Berto’s data for 15% wheel drop. Yet another is to follow Jan Heine’s advice by starting with a high pressure (e.g. 100psi) and then let air out until the vibrations caused by a rough road start to disappear.
All of these are just suggestions, though. Much of the data favours wide (25-28c), supple tyres at lower pressures (60-80psi/4-5.5bar), but every rider should feel free to experiment with tyre size and pressure until they are pleased with the performance of the bike.