The roller diameters are so tiny, the tire probably deflects/deforms in a very unnatural way. Im not sure how valid these results are
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Good post, 20-28psi is about my window for 2.25/2.35 tires so itās nice to see more āreal worldā pressures compared to BRR.
Perhaps you already know or recall better than I do? IIRC the smaller roller diameter would overstate the crr by a greater degree for stiffer and/or thicker tires. Being that the Race King appears to be 30% thicker than the Super Race Thunder Burt this may explain the relatively larger difference given the BRR test is a 77cm drum?
I had thought Tom Anhalt had a post specifically discussing but couldnāt find it.
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This is something I think about often riding singletrack and rougher gravel. Rollers are what - 3 to 6 (8?) inches in diameter? I ride over quite a lot of roots around that diameter and although not much actual loose stone in that range there are quite a few braking bumps, embedded rock outcroppings, terrain features and so forth that are in that range and I would theorize produce similar deformation to the tire.
I think there is validity here, outside of pure comparison sake. Rough terrain and small obstacles require huge relative wattage during events and the marginal difference between tires, that is small on the road, or smooth gravel but very large on rough gravel can change the outcome significantly. Iāve been dropped by a few hundred yards of washboard and probably had a slower finishing time losing that group by 10-12 minutes over a 3 hour ride.
Youāre absolutely right, the tyre deforms more when itās in contact with rollers than it would if it was on a flat surface. However, that increased deformation is accounted for in the method used to derive flat surface CRR values from the roller CRR value. The method I use is exactly as described on Tom Anhaltās 2013 blog here. In fact I used the spreadsheet from Tomās blog as-is, without modifying the method. To me the mathematics described in his blog seem sound and the equations and relationships look logical. I also find it reassuring that the agreement Iāve got here and for my other tyre tests agrees fairly well with BRRās data (especially since BRR uses a drum which has a diameter thatās an order of magnitude larger than the my rollers). I admit that I canāt be sure the methodology is universally valid, and I canāt put an accuracy value on the CRR values. However, Iām personally quite confident that the method is good enough to determine whether one tyre is faster than another one.
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Thatās interesting. Iām not sure either. The method certainly doesnāt take that into account, but itās an interesting question, whether the method is missing something like that when looking at two different tyre constructions. Iāll have another look at Tom Anhaltās blog because itās been a few years since I read it all.
Just FYI for the threadā¦ Panaracer makes the Gravelking SK in a 29x2.1"
Scroll to the bottom for 27.5" and 29" sizes
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I know you guys keep showing examples but it is just blowing my mind that the MTB tires are faster. I may have missed it somewhere - but how does it correlate to the pointy end of the races where we are averaging 20-21mph?
I assume the narrow tire would be the route there or is data still showing a Thunder Burt as the winner?
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Those of you running inserts, whether with gravel or MTB tires, are you putting in extra tubeless sealant when setting them up? Does any of the sealant get absorbed by the inserts?
Standard Vittoria airliners will get coated in sealant and look like they absorb some. Air liner lights have a different coating and donāt appear to absorb but will get coated.
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I have a set of Thunder Burt super ground mounted up and a buddy has race kings. I hope to run them on my test lap at some point like I did with the kenda tires. I wonāt be able to get rolling resistance numbersā¦. But if one is more than 10 seconds faster per 14 minute lap I should be able to detect it. Itāll be fun to see if I end up with the same resultsā¦that the tb is faster.
Joe
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At what point should you just ride a hard tail mtb? 
Itās also crazy to me putting in inserts and tons of sealant that weigh down the wheels, yet spend thousands to shave grams elsewhere.
That would be great, if you could do some of your back to back testing. The results youāve shown so far, in this thread and also previously in the MTB tire thread have always been really interesting.
Does your test lap require any braking? If you donāt need to brake, you can do a virtual elevation analysis (Chung method analysis) using something like Golden Cheetah to determine CRR values, if thatās of interest. You could probably also get a crude estimate for CRR value differences from the difference in average speed. I guess though, that the CRR values are only useful to calculate power savings, or time savings, to give us something more tangible. So if youāre getting time savings already, itās not too important to know the CRR values I suppose. Perhaps the CRR values are useful only for comparing against other sources of data.
This is the direction that I went in, and also high level racers like Dylan Johnson have gone this way too, except we fitted drop bars to the hardtail MTB to get back the biggest chunk of the aero benefit of using a gravel bike.
As for weight, it doesnāt make as much difference as a lot of people think. I was one of those people when I started riding more seriously about 15 years ago. I spent hundreds to make my bike a kg or so lighter. Then I did some performance modelling 8 years ago, realised the relative effect of weight, aero, and rolling resistance, and kicked myself for spending all that money in the wrong place. Reducing weight helps of course, but for anybody with limited time/budget, your attention and budget is better spent elsewhere.
BestBikeSplit is an excellent tool for seeing the relative effects of weight, aero and rolling resistance. You can load your own gravel route and use the sliders to see what happens when changing weight, power, Crr or Drag. For example, for a virtual gravel route like Zwiftās Jungle Circuit, the BestBikeSplit model (click here) shows that a 10% improvement in rolling resistance is equivalent to a whopping 10lb weight saving, in terms of time saving/speed improvement.
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Aerodynamics has to come into play at those speeds (20-21mph). The narrow tire would be more aero than the wider MTB tire and require less watts to push against the wind. This will negate some of the rolling resistance gains the Thunder Burt might give.
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Big tire are course dependent. They are not going to be faster at steamboat gravel, but on rough gravel, they will be better. There is no one size fit all on gravel as there is very different kinds of gravel. Dylan does not race mtb tires every course.
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At 170 watts I have to feather the brakes on one or two turns only soā¦not much braking. A few other turns feel like they are close to the limit so tire grip (probably) has a small impact on lap times. Hadnāt thought of virtual elevationā¦Iāve had poor results with it on my TT bike but I could def take a look at it, itās a great idea!
Iāve seen Robert Chung say that he often snips out braking points, which are identifiable by a sudden jump in virtual elevation, but Iāve not tried doing that myself. The thing I like most about a virtual elevation test is that you donāt have to chase a target average power, like with timed laps, or worry about different cornering speeds affecting the results, if trying to test for Crr. On the other hand, if one tyre is genuinely allowing higher cornering speeds, timed laps are good for also capturing the additional benefit of that between two different tyres.
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I have a couple things bookmarked. Thereās also data for 700cx23 vs 700cx32 so we have a good idea of scale of difference just not actual numbers [that I could find].
Dylan Johnsonās wind tunnel video:
And this from Swissside
The measurement data was very consistent with both the Schwalbe and Continental tyres tested. The differences per 10mm increased tyre width, were around 3.6W at 30km/h, increasing to almost 12W at the higher speed of 45km/h.
Swiss Side Aero Tip - Gravel Tires Width
A quick search hasnāt shown much else (that hasnāt been posted earlier like Torsten Frankās excellent analysis) for larger MTB tires in gravel frames/drop bar MTBs.
I do wonder if there is anything interesting going on with frame/fork clearances wrt tire width. Say a gravel bike frame and fork that fits 700cx57 and optimized for aero at that tire size, is going to have a much different aerodynamic profile fitted with tires 700cx42 - which would probably be frame/fork/tire(s) specific. This is tangential but I think helps understanding the overall model.
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No, but he does always race a wider tire than almost anyone else.
I am tempted to experiment with some 650b wheels and MTB tiresā¦.but the old school rider in me is having a hard time getting over my decades of ālearningsā.
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Does he on smooth courses? I dont think he did at gravel locos, nor anything on the tamer SE Gravel series races? His āhome races ā are the same as mine, so ive been at the same local races quite a bit. Definitely on rougher races, or one with a solid DH he uses bigger tires, but i dont think on the smooth ones.