Does rolling resistance make a notable difference on long steady climbs?
Crr is equivalent to slope (rise/run). If you are on flat ground with .004 Crr tires, then it’s just like climbing a 0.4% grade with .000 Crr tires.
Let’s say you are climbing a 8% (.08) grade. Tire A has a Crr of .004 and tire B has Crr of .005.
Climbing that slope with tire A is the physical equivalent of climbing a 8.4% grade and tire B a 8.5% grade. The great majority of your resistance will be overcoming gravity, so tire A will be a bit under 8.5/8.4 or 1.2% faster. Not trivial in by book. On a 1hr climb, that’s ~40 sec.
Ideal pressure increases with weight. And when you climb, more weight tends to shift to the rear wheel (on really steep climbs you have to work to prevent doing wheelies). So if you had perfect psi on the flat, it’d probably shift to a bit less in the front and a bit more in the rear for climbs.
Speaking of the rear tire, have you ever noticed that vulcanized clinchers are noisy (sounds like scrubbing) on steep climbs and “open tubulars” are not?
rruff is spot on with the analysis, and Robert Chung posted this image that makes it even clearer by translating Crr into equivalent weight, as luck would have it, he used the same two tires you are discussing, BUT these numbers would be for both tires having butyl tubes, so taking the 5000 to latex would take Crr to more like 0.0025 so on 10% grade would be more like carrying an extra kilogram of mass.
Don’t forget the descent. Rolling resistance will be hugely important on the descent.
While my road bike was getting major carbon repair done, I used my gravel bike as my daily road bike for a few weeks. I had no problem staying with people, except on descents. Tires made a huge difference on the descent. (And I’m fairly confident it was the tires, and not aero differences.)
I’m not so sure about this. It seems more logical that rolling resistance is more important on the climb, where you’re moving slowly and aerodynamics plays a much smaller role. I would think that the lower contribution of aerodynamics would make rolling resistance relatively more important. On the descent, aerodynamics plays such a dominant role that I would think rolling resistance becomes relatively less important.
Keep in mind that much wider tires can have low rolling resistance but they are likely to have significantly more aerodynamic drag, and if in fact it was your tires that was making you slow on the descent (and that’s a significant assumption) it’s more likely it was the extra aerodynamic drag rather than the rolling resistance.
I am of the same opinion that trail’s wide tires caused him to be slower on descents due to aero and not rolling resistance.
One more question on this thread. I did a hilly 55km loop that is all uphill or downhill (800m vertical total) yesterday and I chose to stand up for all the climbs (anywhere between 200m and 2km each). We were talking about more weight on rear wheel needing more pressure for climbing, but this assumes seated climbing. Standing way more pressure and deformation on front wheel (I would say much more given the side to side that happens with standing climbs). Thoughts on pressure and pressure distribution for standing climbs.
I’m not so sure about this. It seems more logical that rolling resistance is more important on the climb, where you’re moving slowly and aerodynamics plays a much smaller role. I
Don’t forget that the losses due to rolling resistance are proportional to speed. Not with the square of speed like in aero, but you’re still going to get more losses descending than climbing. Way less overall time, of course.
And I just don’t see that much of a difference in aero-ness between the two bikes. The the CX tires were 35mm vs. 25mm on the road bike. That’s about it, other than small frame differences. But I was seeing a ~10MPH or more drop in speed while descending. It sure “felt” like rolling resistance losses to me. Was in super tuck, whole deal. Just could never get moving very fast, and the tires made a ton of racket, like driving snow tires on the freeway.
Of course, can’t prove it.
Edit: Well I can prove it. Next time I wear out my CX tires I’ll throw on some wide, fast tires, and see if I can descend fast.
Edit edit: Looking at the CX rolling resistances on BRR. My tires aren’t on there, but I’m pretty sure they’re slow. Kenda. If I pick 30W/tire @ 18MPH, that becomes ~60W/tire at 36MPH (about as fast as I could go on descents where I can normally hit 50MPH super-tucked). Or 120W of losses. Easily enough to get dropped on a descent. I’ll throw in maybe 50W more of aero losses due to tire width and a little bit for wheels that are slightly less aero than the training wheels on my road bike. That’s my hand-waving guess.
But nevertheless, my point about Everesting should still hold: fast tires will matter on descents.
Our advice in the Everesting attempts we’ve worked on is to use the calculator to calculate front pressure per normal weight distribution and then use trig to calculate the rear weight bias during climbing and use that heavier weight for rear calculation.
The reasons here are two fold: 1. 80+% of the ride will be climbing, so rear tire should be optimized for that specifically. 2. The front tire pressure should not be lowered as you need optimal handling on the descents. Our calculator already accounts for this, but if using Berto or other methods, then you will have to increase front tire pressure for better handling during steep descending
For standing climbing, the rear tire load takes on a sinusoidal shape where the tire will actually see peak load that is higher than what it would be during seated climbing AND the tire sees additional lateral forces that allow for additional scrubbing losses. For this, the pressure should ideally be higher still by at least a few % but hard to really model without knowing your exact style and loads. In general, rolling losses will be a good bit higher for standing climbing than seated climbing, and this can be partially offset with higher front and rear pressures.
Josh, have you noticed a difference in rear tire sound on a steep climb? I suppose it happens all the time (when torque is applied) to a degree, but it’s usually impossible to hear due to wind.
IME vulcanized clinchers (even very good ones) make a sound that sounds like scrubbing with each pedal stroke. The “open tubular” tires do not. I first noticed this ~12 years ago with Challenge Criteriums, but didn’t think much of it. I primarily have been riding Continentals since then, but I’ve noticed the same thing with Veloflex and Vittoria tires with glued-on tread. Currently I have a Vittoria Corsa on the back and it’s silent. If I stand on a climb the front Attack makes a noise though! Makes me wonder if there is a difference between the two designs that contributes to a loss that isn’t captured by typical Crr tests.
Edit edit: Looking at the CX rolling resistances on BRR. My tires aren’t on there, but I’m pretty sure they’re slow. Kenda. If I pick 30W/tire @ 18MPH, that becomes ~60W/tire at 36MPH (about as fast as I could go on descents where I can normally hit 50MPH super-tucked). Or 120W of losses. Easily enough to get dropped on a descent. I’ll throw in maybe 50W more of aero losses due to tire width and a little bit for wheels that are slightly less aero than the training wheels on my road bike. That’s my hand-waving guess.
You could run the numbers through a calculator…
-8% grade, .20 CdA, 180lb total weight, .004 Crr = 51.0 mph
Now double the Crr:
A massive .04 Crr (10x higher!) drops the speed down to 37 mph.
If you go back to the 2x higher (.008) Crr, a CdA increase from .20 to .36 would get you down to 37 mph also. I think a controlled test might be in order…
Josh, have you noticed a difference in rear tire sound on a steep climb? I suppose it happens all the time (when torque is applied) to a degree, but it’s usually impossible to hear due to wind.
IME vulcanized clinchers (even very good ones) make a sound that sounds like scrubbing with each pedal stroke. The “open tubular” tires do not. I first noticed this ~12 years ago with Challenge Criteriums, but didn’t think much of it. I primarily have been riding Continentals since then, but I’ve noticed the same thing with Veloflex and Vittoria tires with glued-on tread. Currently I have a Vittoria Corsa on the back and it’s silent. If I stand on a climb the front Attack makes a noise though! Makes me wonder if there is a difference between the two designs that contributes to a loss that isn’t captured by typical Crr tests.
Most of what you are hearing is the rubber coated casing scrubbing against the bead of the rim… the cotton tires will do the same thing, but much more efficiently and quietly as they are relatively low friction in that area.
This is actually an area that we have been studying more and more as this Everesting thing has gotten so big and we are working with quite few athletes having a go at it… What I can tell you so far is that seated vs standing testing shows us that Crr can be quite a bit higher when standing, that the effect is different for different riders, and that it does seem to be affected by tire pressures with higher pressures reducing some of the losses when standing, but not enough to get to the seated numbers… the other interesting one to add here is the torsional forces/wind up in the tire casing under hard standing efforts on very steep grades…
The other area of inquiry for me for a while (though struggling to find the right place/time to test) has been the CRR increase in heavy cross winds when using a disc. Similar to standing, there are situations where you can quite literally hear the tire scrubbing from the side loading, so something is happening and it probably isn’t good, but have not had the time or bandwidth to really study this one very deeply yet.
This is actually an area that we have been studying more and more as this Everesting thing has gotten so big and we are working with quite few athletes having a go at it… What I can tell you so far is that seated vs standing testing shows us that Crr can be quite a bit higher when standing, that the effect is different for different riders, and that it does seem to be affected by tire pressures with higher pressures reducing some of the losses when standing, but not enough to get to the seated numbers…
Thanks for sharing this and I’m looking forward to more Crr data pertaining to seating vs standing, if and when it comes…
I’m going through tire selection for hill climbs/hill sprints myself, but I ride standing 80% of the time. I’ve followed all the conventional tire knowledge of the past several years, and setup my tires at pressures prescribed by the Silca calculator and make fine adjustments to the final PSI based on the course and road conditions.
However, when it comes to hill sprints I’ve always erred towards slightly higher PSI, or essentially prescribing exactly what Silca’s calculator recommends +0~3psi (because my current understanding is that Silca’s recommendation bias slightly higher for the average rider, unless this was corrected recently?)
Reasoning is, without higher pressure I can feel and visually see the sidewalls of the tire deflect with each heavy standing pedal stroke. This is most apparent when going around switchbacks with steep incline. Putting extra PSI helps, but does not always resolve it based on the course conditions. Plus, there is an upper limit when factoring break point pressures that needs to be managed.
Seems if identical wattage can be maintained for both seated vs standing, seated will be faster for not just aero but also Crr. For me, I produce more power standing. Question is, by how much more do I need if speaking strictly about Crr…
