Wednesday 29 June 2022

Testing MTB tyre rolling resistance using virtual elevation analysis

Mountain bike tire rolling resistance testing for optimum pressure
In a previous post from 26th April 2020, I described the rolling resistance testing that I did using my cyclocross bike, to determine the best pressure to run my tyres at, for CX races on grass.

The surprising result from that test was that there was no optimum pressure for my 35mm wide cyclocross tyres. The test instead showed that lower pressures are faster, even down to pressures that are impractically low.

Since then I've been really interested to see if the same trend holds true for other types of off-road riding, such as mountain biking.  Rolling resistance expert Tom Anhalt made an interesting comment on a Slowtwitch forum, in response to my 2020 results, saying that he remembered seeing results from the Swiss MTB Team who performed similar studies and they also concluded that lower pressure is faster.  I was keen to test this for myself, and finally had a chance to do it in June 2022.


The test and analysis method I used was the same as the one that I used previously for the cyclocross tyres, described here.  There were a couple of minor differences this time:
  • I performed a repeat at only one pressure on the grass surface, whereas for the previous cyclocross tyre test, I did several off-road repeats.
  • On the other hand, I did two road tests, before and after the off-road testing, to get a feel for the repeatability of my CdA estimate.
It's also worth noting that my MTB has a spider-mounted power meter, a Power2max power meter, which measures power from both legs, whereas my Cyclocross bike had a left-hand crank-based Stages power meter.  In theory, the Power2Max power numbers should be more reliable, as it records total power properly.  In addition though, I know there are some small differences between the power measurements from my Power2Max and Stages PMs from the comparative testing that I've done previously (see here and here).  All of this means that the rolling resistance numbers aren't strictly comparable between the two bikes, the MTB and the CX bike. However, rolling resistance differences for different pressures, for the the same bike, should be reliable.

Another thing to note is that I used the same grass field for the testing as I used previously.  A grass field obviously isn't particularly representative of a typical MTB trail, but I used it mainly because:
  • There's no need to brake.  Any braking would screw up the VE analysis.
  • It's quiet and free from other riders or people getting in the way.
  • I was able to ride a consistent line around the field each time.
  • Finally, I sometimes use my MTB for cyclocross races, so I was anyway interested in the optimum MTB tyre pressures on grass.
I've thought carefully about how I could use a more representative MTB trail loop. However, I've been unable to find a suitable local trail, that allows a consistent line to be ridden, and that requires no braking etc.  This, I feel, is a fundamental problem for performance testing of mountain bikes.  Facilities like the new Vittoria testing facility offer a possibility to overcome such difficulties, and I'm looking forward to what kind of testing might be done at this facility.


Optimum Pressure

The results shown in the plot below, which is the same plot as the one at the top of this blog post, show that the MTB tyres have a lower sensitivity of tyre pressure to rolling resistance than the CX tyres.  I would describe the MTB tyre red curve below as showing a flatter optimum, where the CRR doesn't change much between pressures of about 13 psi and 24psi.  The difference in rolling resistance across that pressure range is equivalent to less than a couple of Watts at 15 mph.  This is a convenient result, because I tend to run pressures between about 16 and 20 psi in these tyres, for comfort and grip reasons, in addition to rolling resistance considerations.  Therefore, I'll continue to run those kinds of pressures, as they seem to be best for rolling resistance too.

Other observations

Comparing the red and blue curves in the plot, for the MTB tyres and CX tyres respectively, shows that the MTB tyres are clearly faster tyres on grass, which is a conclusion I'm confident in, despite the power meter differences etc, because the difference we see there are so large.  The MTB tyre CRR values are about one third less (~40W) than for the CX tyres, which is more than the uncertainty coming from PM differences and differences in testing conditions on the day.  

The results also show that the rolling resistance of the tyres on the road doesn't change much between the two results at 34 psi and 16psi, which is a little surprising.  If I compare these data points against results of independent drum testing and my previous roller testing,  shown in the plot below, I see reasonable agreement with roller testing at the the lower pressure of 16 psi, but the 34 psi rolling resistance coefficient is much higher than the values from Bicycle Rolling Resistance obtained from their drum testing.  I can only think that this difference might be coming from higher suspension losses at 34 psi when riding the MTB over the fairly rough tarmac surface of my Aztec West road test loop.  I remember that AeroCoach's CEO Xavier Disley once said that he rarely sees long stretches of UK road surface that have a CRR less than 0.006 - and his comment was for the CRR of road tyres, not MTB tyres.  The CRR values from drum testing, which are less than 0.006 at 34 psi would not be achievable in real life based on this information from Xavier Disley, and this is my best explanation for the mismatch at 34 psi.