My last race for the year is a mixed surface circuit on an 8 mile loop than it 60% paved, 20% hard pack dirt, and 20% loose, chunky gravel. In years past, I’ve been able to take advantage of my road fitness to stay in the lead pack on the paved and hard pack sections, but I consistently gap the leaders in the deep/lose gravel section. This isn’t a big problem for the first lap or two - I can bridge the gap without too much trouble, but by the 3rd or 4th lap, the extra effort takes its predictable toll, and I find myself in a solo chase looking for other stragglers to work with.
I have updated my setup and added a 40mm file tread tubeless front tire (CX frame only accepts a 35 on the rear), in hopes of improving my gravel performance enough to stay with the front pack and contest for the win.
I’m not sure how to approach air pressure. If I run the 40s at 50psi, they handle ok on pavement, but they still get a little squirrely in gravel. If I knock them down to 30-35, they handle much better in gravel, but they create a lot of extra work on the road.
Suggestions?
Weight?
50 psi for a 40mm tire seems like a lot….FWIW, I run my tubed 35’s around 50psi on the road and light gravel. I’m ~70kg / 155.
For me, I would gladly run mid-30’s on the road for a tubeless 40….but I also run that tire at ~25psi for gravel.
I use the SRAM Tire Pressure Calculator. I find it simpler than the Silca one and the numbers are very close.
That doesn’t help me with balancing the pavement and the gravel.
I would find the recommended tire pressures for your setup (tire width, inner rim width, etc) and run the calculator for both gravel and road, then split the difference.
Wrestling my annual question again. Should I run pressure for the road section, for the critical deep gravel section, or split the difference and be overinflated for the gravel and underinflated for the road?
Last year I split the difference and still struggled in the two deepest gravel sections.
What tires are you running?
Clement USH. 40mm in front, 35 on rear, tubeless.
Based on some recent podcasts, I would err to the gravel side of things…it may be less efficient on the road, but likely far less than being too high on the gravel sections.
Strange. I thought wider and lower pressure was always faster? 
Personally I would try to experiment and see if there a sweet spot where you can avoid risks of puncture, get decent rolling and at least some give in the chunk. Joking aside, I run the lowest pressure I can on gravel and dirt and haven’t found that it is much slower on pavement, especially if you can sit in on a draft.
I also tend to run lower pressure for gravel comfort and speed. I feel like I lose more asphalt speed based on my tire choice than my tire pressure.
I feel like you’ve answered your own question here
Probabaly makes sense to optimize for sections where moves are made. In your case biased towards gravel.
You can also optimize front towards gravel and back for pavement.
I recently heard an interview of Josh Poertner by Dylan Johnson, and he made the point that when you plot rolling resistance vs. tire pressure, the “bathtub” doesn’t have equal slopes. Having too low a pressure by a few pis can cost you 1/2 W, but erring on the high side by the same amount can cost you 5+ W.
That would suggest that you lower your pressure until your bike works quite well on dirt and loose gravel. Then check that it still works fine on road.
This is an insanely high tire pressure for 40 mm tires. I run my 28 mm/30 mm (width-as-measured) tires at 55–60 psi (I weigh 75 kg, my bike 8–8.5 kg). I recommend you have a look at Silca’s and SRAM’s tire pressure calculators, which have been linked above.
Going by SRAM’s suggestion, I picked 75 kg rider weight, 9 kg as bike weight, gravel, dry, hookless and 25 mm inner rim width, and got 33 psi (front) and 35 psi (rear). The road setting gave me 36 psi (front) and 39 psi (rear). Even on the road the pressure you use is way too high, you are wasting watts.
Changing the settings will lead to different tire pressures, but e. g. going from dry to wet will lead to lower recommended pressures, for example, 30 psi (front) and 32 psi (rear) for gravel and 33 psi (front) and 35 psi (rear) for road usage. Silca’s tire recommendations tend to be a few psi higher than SRAM’s/Zipp’s, but are in exactly the same ballpark.
Fair enough. Sometimes you just have to hear (read) someone else say it.
Fair enough. That was what I was running last year. For now, on my local gravel, I’m finding 35 to be faster and smoother, a few psi lower than that for especially deep or muddy gravel. It’s pretty dry, and MOST of the gravel should be dry and well packed, but there is one corner and one turnaround on especially large and loose gravel that eat a few seconds a lap. When I pre-ride the course, I’'m going to try dismounting and running around the turnaround to see if that is faster. I haven’t raced any CX in a few years, and it seems like a shame to waste that skill.