New cyclocross tyre rolling resistance testing using virtual elevation analysis

A few weeks ago I did some additional tyre rolling resistance testing using proper 33mm wide cyclocross tyres, with foam tyre inserts installed in them.

The aim of my testing was to check how tyre pressure affects rolling resistance and in particular to see whether the installation of tyre inserts would negatively affect rolling resistance at low pressures, when there is greater deformation of the tyre (and potentially deformation of the insert).  The results confirmed my previous testing, that lower pressures are faster, and also showed that the presence of tyre inserts doesn't affect that conclusion, that lower pressure is faster.

Previous testing

2020 CX tyre testing: I first performed off-road rolling resistance testing in 2020, during the first Covid-19 wave, and described previously in this post.  That testing used Schwalbe X-One tyres (the blue curve in the plot above).  The surprising results of that first test was that on grass, even with dry & hard underlying ground, lower pressures were always faster, producing lower CRR values.

2022 MTB tyre testing: In 2022 I repeated the test method but using my hardtail mountain bike.  The results are described in this post here (and shown above with the red curve).  That testing used Schwalbe Thunder Burt tyres and also showed that lower pressures were always faster, albeit with a more subtle effect of tyre pressure on CRR values.

New testing

In previous cyclocross seasons I have done only a handful of the races, often picking the early season races that had the best and driest weather.  For this coming cyclocross season, I intend to do most of the races throughout the season, including the muddy ones, therefore I have bought some good quality 33mm cyclocross tyres that will be suitable for the later races in the season.

The tyres I bought are Challenge Handmade Tubeless Ready (HTLR) tyres, which are 33mm wide.  I have a Baby Limus on the front, and a Grifo for the rear wheel.

In addition, I installed these tubeless tyres with tyre inserts because from what I have read (for example, here), the use of inserts allows the tyres to run at lower pressures.  I ordered some Tubolight tyre inserts, as described in the article, but I was disappointed to find that they are simply closed cell foam cylindrical rods with a diameter of about 25mm.  

Considering the price of ~£50 for the Tubolights, I decided to instead return them and instead go the DIY route. I bought some 25mm 'backer rod' from Ebay, for ~£10 for 10 metres, and made my own inserts.

Having installed the inserts, I was keen to measure the rolling resistance at different pressures.  I'll be aiming for pressures around 20-24 psi probably in the races, and with such low pressures there is a chance that the compressed contact patch will touch the insert and cause a higher rolling resistance.

I was pleased withe results (below), which showed the similar trends to my 2020 CX tyre testing, albeit with high CRR values due to the softer ground.  In conclusion, lower tyre pressures are still faster, and the tyre inserts don't seem to negatively affect the rolling resistance.  A reduction of 5 psi, from 25 psi to 20 psi saves around 6 Watts of rolling resistance at 15 mph.

Calibrating my torque wrench

A couple of weekends ago I managed to damage the thread on the crank axle bolt of my XTR M9100 chainset.  It was really annoying, partly because I had a bike race the next day, but mainly because I felt it start to fail at a torque that was below Shimano's recommend torque setting of 45-55 Nm.  The bolt gave me that tell-tale sign of a problem, where it starts to turn more than it should, and without the resistance increasing in the way it should. This happened before my torque wrench was able to click when set to 42Nm, below the recommended torque setting.

I got a replacement bolt, but it made me wonder whether my torque wrench was badly calibrated.  It's a really cheap torque wrench, a budget Silverline torque wrench, bought from Toolstation, so it's quite possible that it's badly calibrated.

Anyway, I decided to use my workbench vice and hang a weight off the torque wrench, at several torque settings, then see at what distance (moment arm) the weight has to be at to cause the torque wrench to click.

The results, plotted below, shows that my torque wrench clicks out at a slightly lower torque than indicated on the wrench.  This means that it's safe, in that it won't over-torque a bolt. It means that the reason my XTR chainset bolt got threaded is a bit of a mystery, but it's probably because the Shimano bolt was faulty, rather than an issue with my torque wrench.

28 Nm on wrench

260mm with 10.15kg (99.57N)

= 25.88 Nm applied

36 Nm on wrench

320mm with 10.15kg (99.57N)

= 31.86 Nm applied

42 Nm on wrench

370mm with 10.15kg (99.57N)

= 36.84 Nm applied

46 Nm on wrench

402mm with 10.15 (99.57N)

= 40.03 Nm applied

Weight comparison for my mountain bikes

Weight is not the most important consideration when it comes to cycling performance, as discussed previously in one of my older blog posts.

Having said that, I still weigh the components of my bikes whenever it's convenient to do so.  By doing that, it gives me the information to know whether it'd be good value for money, or not, to upgrade a component, or swap something over from one of my older bikes.  Nevertheless, it's not something I pay a large amount of attention to these days, having understood how important (or rather how not important) bike weight is on cycling performance. Still, it remains slightly interesting to me, hence this comparion.

I recently bought a Specialized Epic Evo as a new full suspension mountain bike.  I already own a 14-year old first generation 26 inch-wheeled Giant Anthem full suspension MTB and a Scott Scale 29-inch wheeled hardtail MTB.  I thought it would be interesting to compare the weights.  For all of the bikes, I've made some improvements to make them lighter, prioritising the items that would provide the best value for money (best gram saved per £).

Perhaps not surprisingly, the carbon-framed Scott Scale hardtail is the lightest of the three.  Most of the difference is coming from the frame, obviously, but I'm also running SRAM XX1 carbon cranks on it, and these are a fair bit lighter than the alloy Shimano XTR cranks on the other two.

What's most interesting is that my Giant Anthem is lighter than the Specialized Epic Evo, despite having an alloy frame, alloy wheels, and a triple chainset (versus the Epic's carbon frame & wheels, and 1x transmission).  Although the Anthem's wheels are on 26", which helps, I think it still shows that alloy wheels can be very light. The 3x9 transmission of the Anthem is also almost as light as the Epic's, despite the hefty triple crankset, but this is helped by the 11-32 XTR cassette, which is about 60% of the weight of the Epic's enormous 11-52 cassette.

Finally, the other big difference comes from the Epic's dropper post, and the Epic is the only bike that has a dropper.  That alone adds about half a kg.

Despite all this, I'm happy with the Epic's build and 10.9kg (23.9lb) total weight.

Bespoke Garmin Varia mount

Like a lot of road cyclists, I really like my Garmin Varia, which I've owned for a year or two.  It's been one of my best bike purchases in recent years.  It gives me plenty of warning of cars approaching from behind before they become audible.

I've never liked the original Garmin mount that it came with though, which attaches to the bike seatpost using a rubber spacer and O-ring.  It looks untidy and doesn't really fit properly.  My seatpost has a teardrop profile, so I need to use the spacer that's shaped for aero frames, but it's not quite the same shape, and so doesn't fit properly.

This weekend I built a bespoke mount which attaches to the bike's seatpost clamp instead.   My seatpost clamp uses a 8mm threaded steel cylinder into which the bolt threads into.  I replaced this threaded cylinder with a 8mm diameter aluminium bar, which I drilled and tapped to create an M5 thread.  Into the lower end of the bar, I attached a spare Garmin mount I had, which came from a SuperStar Components TT Garmin mount.

I'm pleased with the result.  My new Varia mount feels solid and I think it looks much neater.  It's also slightly lighter than the Garmin mount too, at 22g for my mount versus 40g for the original mount, which is a nice little bonus.

The benefits of unlocking Zwift's rear disc wheel

Today I got to Level 35 in Zwift, which that means I was able to get the Zipp Super 9 rear disc wheel.

It looks better on any Time Trial (TT) bike, of course (if you care about such things for your Zwift avatar!).  However, I wanted to know how much faster it is.  This post explains my calculation to determine just that.  The quick answer, if you don't want to read the full post, is 2.5 Watts.

I've been using Zwift since September 2015, which was not long after the launch of Zwift.  I'm not a massive fan of Zwift, I must admit, but I quite like it and I use it for a few months every year to help take some of the monotony out of winter indoor riding.  It's taken me that long time, about eight years, to get enough XP to get up to level 35 (approximately 5,700 Zwift miles).

Fastest Zwift Wheels

When talking about the fastest wheels in Zwift, it is worth clarifying that this applies only to wheels on TT bikes.  Why?  Because once you unlock the Zwift Concept Z1 road bike (aka the 'Tron bike'), then all other road bikes are slower, with a few caveats, regardless of the wheelset you put on them. See this Zwift Insider post for more information about that.

TT bikes are different though, and will always be faster than a road bike, including the Tron bike, when riding solo or in a no-draft TT event.

Zwift Insider's analysis

The guys at Zwift Insider do an excellent job of analysing bike and equipment choices within Zwift, amongst other things.  Their post here explains the fastest TT wheels, using their testing protocol, whereby they recorded the time required to ride the Tempus Fugit route (17.4km / 10.7miles) twice at 300W.

As shown in the table to the left, the faster wheelsets provide larger time savings.  However the fastest wheelset can only be unlocked at the higher levels within Zwift.

For some years, since Level 13, I've been using the Zipp 808 wheels on the TT bike, which are the fastest wheels for people at lower Zwift levels.  Now having the Zipp Super 9 disc wheel on the back, saves me 9 seconds on that 2 x Tempus Fugit route, versus two 808s, according to the Zwift Insider table above.  Given that the entire route takes "approximately 50 minutes" when ridden at 300W, then 9 seconds doesn't sound like very much!  In fact, the time/speed improvement is only 0.3% (9 seconds divided by 3000 seconds).

What I wanted to calculate is how much that time saving equates to, in terms of power savings and CdA reduction.

Important: As with all power saving values, it's crucial to keep in mind the associated speed, which in this case was 41.52 kph (25.79 mph), because any power savings due to aerodynamic changes are proportional to speed cubed.  Beware anybody that quotes power savings without also giving the associated speed!

CdA and Watt savings

I used my performance modelling spreadsheet (described here) to model the Zwift Insider test, where they completed two laps of the Tempus Fugit route at 300W, for a 75kg rider, in about 50 minutes, as shown below:

In addition to the parameters above, I had to make a few other assumptions to calculate the CdA, and these are simply guesses, because I don't know what assumptions Zwift makes.  I'm confident, however, that the power and CdA increments won't be particularly sensitive to this choice of conditions:

• Air pressure = 1012.5 bar (i.e. sea level pressure using the International Standard Atmosphere, ISA, conditions).
• Air temperature = 15 degrees C (i.e. sea level ISA conditions).
• CRR = 0.004 (which ZwiftInsider says here is the CRR that Zwift assumes for the road).
• Bike weight = 9kg
• Drivetrain losses = 2.5%

Using these assumptions, I calculated the baseline CdA to be 0.2710 m^2.  Then, by increasing the speed by 0.3%, giving a 9 second time saving, I calculated:

Either: 0.0025 m^2 CdA improvement (0.2710 -> 0.2685, or a 0.92% CdA reduction) at the same 300W power to achieve that 9 second saving.

Or: 2.5 Watt power saving at 41.52 kph (0.83% power saving) for the same 50 minute time, with that reduced CdA of 0.2685.

I think these numbers, especially the power saving, can be understood more intuitively by the average person.  A 2.5 Watt power saving is not large, but is not negligible either.

Are the numbers valid also for me?

Finally, I also wanted to check whether these values are valid for me, because I'm a slightly smaller and less powerful rider than was assumed by Zwift Insider.

Firstly, I calculated my CdA to be 0.2415, for my Zwift time trial bike using Golden Cheetah.  This is relatively easy to do (compared with real life), because in Zwift there is no braking, no wind and no other vehicles to mess things up!

Something to note is that my Zwift CdA is about 10% lower than the CdA for the Zwift Insider analysis, and this will be because I'm probably shorter (at 5 foot 10 inches) and slightly lighter (at 73 kg) than for the Zwift Insider study, and we know that Zwift adjust the CdA based on weight and height.

Next, I applied this CdA to my spreadsheet analysis.  Interestingly, for a 50 minute time to complete the 2 x Tempus Fugit distance, the power requirement was 270.6W, which is approximately what I could hold for 50 minutes, so I didn't adjust it any further.  Then, I reduced the CdA by the same 0.0025 m^2 value, and calculated the power saving in the same way as before:

The Zipp Super 9 rear disc wheel give the following benefits, both at 41.52kph:

                     Zwift Insider (75kg, 300W, 41.52kph)           Me (73kg, 270.6W, 41.52kph)

CdA              0.2710 -> 0.2685 (-0.0025 or -0.92%)      0.2415 -> 0.2390, (-0.0025 or -1.04%)

Power saving           2.5W saving (-0.83%)                               2.4W saving (-0.89%)

In summary, the Zipp Super 9 disc wheel will save me about two and a half watts at typical flat TT speeds.  This benefit is fairly small, but certainly not negligible.

Estimating my Anaerobic Work Capacity

My previous blog post showed how my critical power, how it compares to those of professional riders, and also to those of other amateur and recreational riders.

It showed that I'm relatively good at short duration efforts, less than a minute or so.  For those sort of efforts, my power numbers are in the top 2-3% of the 40-49 year-old amateur rider data stored on intervals.icu.  Furthermore, I'm actually better than the bottom 10% of professional riders (which is a nice little ego boost at this time of the year, post-Xmas, when I'm feeling fat and slow!).  I speculated that my good short duration power numbers might be a result of having a good anaerobic work capacity (AWC).  I've never analysed my AWC before, and this blog post describes a quick analysis I did to estimate my AWC.

What is Anaerobic Work Capacity?

Most serious cyclists will be familiar with functional threshold power (FTP), which is the maximum power that a rider can sustain for 60 minutes.  Riding at or just below FTP is an aerobic activity in which the build of lactate and other metabolic by-products reaches an uncomfortable but tolerable steady-state level, whereby the body is able to process lactate as quickly as it is generated by the muscles.

People will intuitively know that it's possible to cycle at powers above their FTP, but only for short periods of time.  At powers above FTP, the body is working anaerobically, because it is producing more lactate than it's able to process.  As a result, lactate and other by-products build up until they are painfully high and the rider must stop or slow down. 

The concept of Anaerobic Work Capacity (AWC), which is also called W prime or W', is that a certain amount of energy, a certain number of Joules, is available to allow a cyclist's power to exceed their person's FTP for a limited time.  AWC can be thought of as a 'battery' that has a limited number of Joules, that can be used in a number of ways.  High powers, significantly exceeding FTP (see the red rectangle in the figure above) will 'drain' the battery quickly and so can only be maintained for short periods.  Alternatively, if the power only slightly exceeds FTP (shown by the blue rectangle above), that anaerobic battery can maintain those powers for longer.  Note that the area of both red and blue rectangles is similar, because the area represents the AWC, which is calculated from the duration multiplied the the power above FTP (because power multiplied by time equals energy).

Anybody that cycles a bike will implicitly understand how these things work. You can push hard, but only for a short time, and the harder you push the shorter the time you can do that.  This AWC concept also explains the shape of a person's critical power curve, which tends to be hyperbolic, asymptoting towards their FTP value at higher durations.  At very short durations, the AWC model breaks down because a person's power will be limited by their sprint strength and their associated neuromuscular capabilities, instead of their anaerobic work capacity. This AWC concept is therefore a somewhat simplistic model of reality, but like all models it has some value.

Calculating my AWC

I calculated my AWC from my critical power curve that is shown by the black dashed line in the plot at the top of this post.

At each duration, I calculated the excess amount of energy, having made an assumption about my FTP.  My FTP is quite stable throughout the season and is always in the region of 275-295W.  I calculated my AWC for three FTP assumptions of 275, 285 and 295W, to get a feeling for how sensitive the results would be to that FTP assumption.

The plot below shows the results for durations ranging from 1 second to 1200 seconds (1200s is 20 minutes).  For reasons explained previously, the AWC values calculated for the very short durations, less than 30 seconds, are not valid due to the neuromuscular component of the effort that limits the power.

At the other end, the 20-minute value (1200 seconds) is not a good representation of an anaerobic effort either, because of the large aerobic contribution for that duration. However, the shape of the curves at the 10-20 minute duration gives a clue about my FTP.  The very high AWC calculated with the 275W FTP assumption suggests that 275W is too low as a value for my FTP.  It's very unlikely that I could achieve an average power of 303W for 20 minutes if my FTP was only 275W.  Similarly, the FTP of 295W seems too high when looking at the AWC values for 10 and 20 minutes when compared to the short durations.  I conclude from this that 285W is the best FTP assumption.

The AWC values for the durations of 30s, 60s and 300s are, I feel, the most reliable values.  For those durations, assuming an FTP of 285W, my AWC is around 18-20kJ.

Is my AWC any good?

It's difficult to find good data about what a 'good' AWC value is, but I thought it would be interesting to calculate the AWC for 2018 Tour De France winner Geraint Thomas.  During a 2022 podcast, Geraint mentioned that he set his best ever 1-minute power of 730W during a stage of the 2022 Tour De France.

This impressive 730W number exceeds my best 1-minute power by 111 Watts.  However, when you consider that Geraint's FTP is significantly higher than mine, probably 130-150W better, the amount of power in excess of his FTP is actually lower than I achieved.  Calculating Thomas's AWC, assuming his FTP is 420W, gives an AWC of 18.6KJ.  This is actually slightly lower than my AWC.

Before I pat myself on the back, it's worth remembering that his effort was set up a climb in the middle of a stage in the middle of a grand tour, when he wouldn't have been fresh, whereas my power records have generally been set fully rested when going for Strava KOMs.  Furthermore, we shouldn't forget that AWC in itself doesn't achieve anything, and instead it's the power that can be achieve for a certain duration that's important.

Finally...

It's also worth bearing in mind that anybody's critical power curve will be a bit lumpy, because it's created from a number of discrete efforts of different durations.  Therefore, a better estimate of your AWC is achieved by selecting the durations at which the peaks in the power curve are seen.  For me, these are the durations of my efforts up certain hills and segments when I've been going for Strava KOMs.  If I calculate my AWC values for these best durations, instead of the 'standard' 60 seconds, 300 seconds etc, my AWC numbers are even better, as shown below. 

My best AWC values, 22.9kJ @75s and 25.1kJ @146s were achieved on two memorable occasions when I went especially deep, trying to get a good time up a couple of short hill segments.  I was also very fresh when I attempted those Strava KOMs.

Critical power curves of professional riders (and how mine compares)

I recently came across an interesting set of data, summarised on Twitter, which contains the critical power data from 188 professional cyclists between the years of 2013 and 2021.

I'm fascinated by this kind of data.  It's interesting and humbling to see the kind of numbers that the pros produce.  Until now, the best source of data of this type that I knew of is the chart in Table 4.10 of Hunter Allen & Andrew Coggon 2010 book 'Training and Racing with a Power Meter 2nd Edition' (Ref. 1).  That chart is also readily available on the Training Peaks power profiling blog page.  However, it has always been unclear to me, at least from reading the book, what data Allen & Coggon used to create that chart.

With this new data, from Pedro L. Valenzuela et al (Ref. 2), data has been analysed from 188 cyclists (144 of them male), from 7 teams.  It contained both Pro Tour and World Tour level athletes.  Furthermore, the 144 male athletes were a mixture of all-rounders, climbers, sprinters, time triallists and GC contenders.  The data, collected from ~130,000 race and training data files, was analysed to identify the best power values at standard durations.

I don't have access to the full journal article, but the key information was already published on Twitter and also analysed on the website wattkg.com.

I have further analysed the data to compare this new data with the older chart from Allen & Coggon. I also looked at my own critical power curve to see how I compare.

My Analysis

In the chart below you can see in the three red and pink lines the new data from Pedro L. Valenzuela et al, for the 10th, 75th and 90th percentiles of the 144 male riders.  The blue symbols and the error bars show the range of W/kg values from Allen & Coggon for 'World Class' and 'Domestic Pro' categories.

It can be seen from the plot that the two sources of critical power data agree very well for 1-minute, 5-minute and 60-minute durations.  For the 5-second duration, the Allen & Coggon data shows higher W/kg values than the data from Pedro L. Valenzuela.  However, the author commented on Twitter, in reply to one of my tweets, highlighting that they had only 11 sprinters in their database as a possible explanation why his data for the 5-second critical power might seem to be relatively low.

Nevertheless, I would say that the two sources of data agree rather well.

My Power Curve versus the Pros

The dashed black curve in the chart above shows my own critical power curve, for comparison against the pros.

At the lower durations, less than a minute or so, my critical powers aren't too bad. In some cases, they actually exceed the worst pro riders (the 10th percentile pro riders).  However, at the longer durations, my lack of aerobic fitness is clearly visible, with the pro riders having critical powers approximately 30-60% better than me.  This is further illustrated in the graph below, which shows how much better, as a percentage, the pros are compared to me.

What I conclude from this is that my power over short durations is pretty good considering that I am, at best, a very mediocre amateur racer.  However, the longer durations reveal my lack of aerobic capabilities.

This probably explains why I've been able to get and hold many local Strava KOMs during the last 5-10 years, over one hundred of them, mainly on segments lasting <2 minutes, whereas I've never won a bike race of any kind!  All this suggests that I have a relatively good anaerobic work capacity (AWC).  I will analyse my AWC as a next step - something I've never done before - and write a blog post to explain my findings.

My Power Curve versus other amatuers

Finally, I want to show quickly how my power curve compares to other amateurs, because I think this shows a broadly similar picture of my strengths and weaknesses.

The website intervals.icu provides an excellent and free set of analytical tools for your cycling data files that are stored on Strava.  Furthermore, the power curve analysis page allows you to see how your critical power numbers compare to other athletes in the same demographic, showing you graphically where you sit on the 'bell-shaped curve'.  For my 40-49 age range, that's a good-sized population of around 12,000 cyclists in that age range.  The people using intervals.icu are likely to be fairly serious recreational riders and amateur riders, so that's also worth keeping in mind.

The pIot below shows that I'm at about the 80th percentile mark for my best 5-minute and 60-minute powers, but my critical powers for 1-minute and 5-second durations are much better, where I'm in the ~95th and ~98th percentiles respectively.

Again, this goes to show that my strengths are with the relatively short duration efforts. 

References

1) Hunter Allen & Andrew Coggon 2010. Training and Racing with a Power Meter 2nd Edition

2) Valenzuela PL, Muriel X, van Erp T, Mateo-March M, Gandia-Soriano A, Zabala M, Lamberts RP, Lucia A, Barranco-Gil D, Pallarés JG. The Record Power Profile of Male Professional Cyclists: Normative Values Obtained From a Large Database. Int J Sports Physiol Perform. 2022 May 1;17(5):701-710. doi: 10.1123/ijspp.2021-0263. Epub 2022 Feb 21. PMID: 35193109.

Using virtual elevation analysis to find the fastest bike

 

Tomorrow I will be racing a local cyclocross race.

I pre-rode the course today on my cyclocross bike.  It's bone dry and very rough, and my feeling is that it would be quicker on my hardtail mountain bike, with its higher volume tyres. 

I wanted to properly check which bike was faster though, and I decided to check this by performing a quick Chung method virtual elevation test on the CX bike, with it's new tyres, and compare the results against the MTB, which I had tested previously.  The results, plotted above, show that the MTB is indeed fastest on the grass field that I did the testing on.

Why Chung testing?

Chung testing, also called virtual elevation (VE) testing, is a method for determining the performance of bicycle using its power meter.  It's often used by time trialists and triathletes to determine improvements to their aerodynamic efficiency, which is characterised by the CdA metric.  The method can, however, also be used to determine rolling resistance changes though, and I have performed off-road rolling resistance tests in the past using this techniques, which are documented here.

The use of the Chung method for this type of testing is that is allows one bike to be compared to another, to quickly determine the relative efficiency, regardless of whether that efficiency improvement is coming from rolling resistance, aerodynamics or weight.

The traditional way of doing an VE analysis is to iteratively adjust the parameters, either the CdA or the Crr (rolling resistance coefficient), usually using the Golden Cheetah software, until the VE profile becomes flat.  When the VE profile is flat, you know that you have a combination of CdA and Crr that is representing the performance of the bike correctly.

The alternative method, which I have used here, is to keep those parameters (CdA, Crr and weight) fixed for the analyses for both bikes, then look at the relative flatness of the two VE profiles.

In the plot above, it's clear that Bike B, the cyclocross bike, has a rising VE profile relative to Bike A, the MTB, when using the same values for CdA, Crr and weight as used for the MTB.  This rising VE profile means that the analysis 'thinks' Bike B should be climbing, because more power is needed to propel the bike than would be needed for a flat profile.  This shows that Bike B, the CX bike, is slower on the grass field I tested them on.

The beauty of this method is that it doesn't care whether the benefit is coming from rolling resistance, aerodynamics or weight.  Instead, it only shows the net results of changes to those three.  Also, as with all Chung testing, there is no need to hold a fixed power, which is a method I often see athletes and journalists trying to perform a comparative test.  The Chung method allows you to ride at whatever power you want, as long as it's reasonably similar for one test and another.

A few caveats

I should add a few caveats, because it wasn't an ideal test, performed in perfect back-to-back conditions.  Firstly the two bikes were tested on different days, so potentially the ground conditions and wind conditions were different. Qualitatively though, the ground conditions were similar on both days and although I felt there was slightly less wind for Bike B, this should favour the Bike B apparent performance.  Therefore, as Bike B is showing worse performance, it won't change the conclusion that Bike A is the best one to use.

Secondly, the two bikes had different power meters.  This not ideal, and in a perfect world I would test the same PM on both bikes.  However, I mitigated this potential bias by applying a 10W correction (10W reduction in power) to the powers from Bike B, based on comparative testing that I did previously (here and here).  All results are shown with this correction applied.  hence it would require a 16W correction for Bike B to be as fast as Bike A, and I don't think they are that far apart.

Finally, in a perfect world I would have done some repeats, like an A-B-A-B type test protocol.  However, I didn't have time.

Barbell riser blocks for deadlifts

 

I read somewhere recently that when performing deadlifts, you should use a proper olympic barbell and plates, so that the bar is the appropriate height off the ground.  Apparently if the bar is any lower than that, it puts excessive stress on your back.

I don't have an olympic barbell, and didn't want to spend >£100 on a new set of weights when the set I have is otherwise perfectly fine.

As you can see from the photo, I decided to instead to make a couple of riser blocks to raise my barbell to the appropriate height.

Weight plates for Olympic bars are 450mm in diameter, apparently, meaning the centre of an Olympic bar will be half that, 225mm, off the ground.  My barbell set, on the other hand, has weight plates that are 310mm diameter (155mm radius), which is a difference in radii of 70mm.  I therefore needed to make my riser blocks 70mm in height.

I did this with some spare timber I had, an old fence post and some plywood sheet.  I used an off-cut of my turbo trainer foam mat to add a bit of cushioning on top. 

So it was all done without having to buy any new materials, and it took only about an hour to build.

Testing MTB tyre rolling resistance using virtual elevation analysis

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.

Method

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.

 

Results

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.