Casing losses and suspension losses, seem to be too tightly intertwined to conveniently pull them apart but maybe I just haven’t seen the right research on it or know enough about it?
I say that partly because the paved stuff I test on is pretty consistently faster than steel drum, especially for mountain bike tires.
This is a good article explaining it in detail:
Your tests are probably the most accurate and representative of real world performance, but there’s one point a lot of testers miss: total rolling resistance vs pressure chart is U shaped, and you really need to find this optimal pressure, where crr is lowest. Otherwise the whole testing becomes just a lottery - whether you happened to run this particular tire at the right pressure. And Silca pressure calculator gives too high pressures, especially for narrower tires.
+1000 on this. I like BRR as a data point, but the pressures (and obviously the surface) are rarely going to align with real world use cases. Again, I’m not throwing rocks at BRR or the validity of their data, just how people tend to assume too much from it. Lots of “slow” tires could be fixed by running appropriate pressures and lots of “fast” tires on the drum at a given pressure could be very slow in the real world using terrain-appropriate pressure.
In addition to casing losses and impedance losses, there is a third category of rolling resistance loss that many people don’t realise, or think about, but which is relevant to any of us that ride off road. That third rolling resistance source is the energy loss you get from the ground deformation.
Ground deformation losses are independent from both impedance losses and casing losses.
Impedance losses are sometimes often called suspension losses, which I think better describes the mechanism that causes the energy losses. Essentially those impedance losses result from a rider’s flesh and organs jiggling around and absorbing energy, due to bumps that are transmitted through the tire to the rider’s body.
The best way to visualise ground deformation losses is to imagine some extreme cases where very high ground deformation losses occur, for example the situation when you ride though soft sand or soft mud. Those surface deformations are pure plastic deformation, with the front of the tire contact patch deforming the sand/mud, and with zero spring-back of that sand/mud at the back of the contact patch, so no return of that energy. Even if that sand or mud is perfectly smooth, the lack of bumpiness means the casing losses and impedance losses are not significantly different to the losses on a hard surface, yet the resistance is still very high in that sand/mud. On gravel surfaces, the ground deformation and associated losses might rather more subtle, but nevertheless most of us have experienced the speed differences between winter trail and a summer trail conditions whenever there is some underlying dirt and softness to the ground.
I think lower tire pressures might play a role in reducing these ground deformation losses, as well as helping with impendence losses. I think the increased tire footprint at low pressure probably helps. Anybody that has seen the ease with which a fat bike rides across soft sand or snow will know how the large tire footprint helps in those situations, reducing those ground deformation losses.
Practically, what this means for tire testing is that we might need to be careful when comparing tests from one day to another, in case the ground softness changes due to weather conditions and ground moisture content.
@ndart Nailed it.
Moisture content definitely changes things. For different areas I found different thresholds of soil moisture, where the rolling resistance stays the same once you are dry enough.
By this point I’ve tested enough on all these different areas I can usually predict a day where the rolling resistance will be off by even a small amount. The Delta of wet versus dry rolling resistance also depends on each course.
This is one reason why I think if you really want to do good off road tire testing you better really enjoy riding your bike!
Sounds like you’ve got things dialled. I think there’s no substitute for experience when it comes to testing, which you’ve got, because that gives you a better handle on the many potential pitfalls of testing. I’ve had lots of those in the past, either strange results or repeat runs that don’t repeat. Most of those dodgy results go unexplained and I just throw the data away, but in doing so it somewhat hides the fact that real world testing, aerodynamic or tire testing, is fraught with difficulties.
So currently for the top large gravel tires we have the Schwalbe rs/rx and Specialized Tracers with also the Tufo Thundero 48 mm (not quite 50)?
I’m pretty sure I can fit a Race king 2.2 but I do like not worrying about mud ruining my bike (UK based)
Should we now just call anything 2.2 down to 45s gravel tires? Everything above 2.2 is mtb, and 38-44s all road and below that are road tires?
Maybe. But there’s a lot of gravel bikes still spec’d with 45 and below. For example the crux comes with 38s. Even the Santa Cruz stigmata which is a “MTBr’s adventure gravel bike” comes with 45. Do any gravel bikes come with larger tires these days?
Seigla comes with 50s. Allied has Race Kings on their bikes in the showroom in the Meteor
The reason for the bike spec is likely two fold: One, bike companies likely have a pretty large stock of smaller tires on hand, as they make these purchases in bulk with a lot of lead time. Two, bike companies are also likely betting on a lot of users purchasing a gravel bike for both road and off road riding, so their tire choice is a compromise.
My tire choice is pretty specific based on a lot of factors. I wouldn’t expect to buy a stock bike and have it come with tires I plan to use regularly. I’m sure they get deals from manufacturers, and the amount of discount they get probably plays a part in that decision.
Items like tires, handlebars(width), crank length, saddle, and a few other similar items are all things that would have me at a bit of pause in buying a complete bike from a manufacturer since I’d be swapping them immediately.
Alternatively, some manufactures get such discounts on drivetrains, that it’s often still not worth getting a frame and piecing together a bike.
Yep, and RK 2.2’s are the tire spec’d on all of Allied’s standard builds. I think a big part of that is just highlighting the capability (since it’s a hot trend), but probably biased towards the gravel around Bentonville as well. Having done a couple races on 2.2’s on the Bentonville gravel, I’m convinced a MTB tire is the fastest setup on those roads. If picking a tire for all types of gravel and some road, a 50mm tire is probably the sweet spot where a lot of bike companies will eventually land for their standard builds. Pretty solid on road and smooth gravel and good enough on most chunky stuff. With Unbound week coming up, it will be interesting to see if any new gravel bikes get released with sub 50mm clearance. The Checkmate basically got bashed over 45mm clearance even though Trek also has the Checkpoint with 50mm clearance.
At least for today… a gravel tire is the fastest and best tire for MTB racing. 
So weird to see this in XCC. Inverted trend.
I’m shook but also the Tracer 50 is probably my favorite tire I’ve ever ridden so this is good confirmation bias 
There’s a 50?
Available for back order on the website. Looks like late June for us normies.
I’ve had surprisingly good luck getting specialized tires through my LBS even when they show out of stock on the website. Not sure about the Tracers, but good luck with scarce pathfinders in the past.
Some pictures from Nove Mesto: