The numbers suggested for me were about spot on from what i use. The zipp tyrewiz app suggests a bit lower.
Also surprised to see lower pressure recommended for smooth gravel than cobbles.
Edit: I read this backwards.
Yeah, bizarre.
I find it lower, but pretty much in line with what I previously evaluated from the “V shape” graph presented previously on their blog, for same tyre, wheel size and weight, on medium quality road.
From “V shape” graph (“official 25mm” GP4000sII on 17mm wheel - so probably wider - if I remember well ?) : optimal around 95 PSI
From calculator (based on 27mm measured “official 25mm” GP4000sII on 17mm wheels) : 86/87 PSI
Are people using the actual measured tire width or the stated tire size? It says measured, which you can pretty much add 2mm to the stated size if you don’t have calipers.
My complaint is there’s no rough & smooth chipseal option. That’s got to be somewhere between old asphalt and cobbles.** Also surprised to see lower pressure recommended for smooth gravel than cobbles**.
From the french side :
With cobbles, lower pressure is better (speed wise), but the problem is : you broke wheels. Or you need to have a really large tyre.
If tyre deformation is smaller than cobbles “height deviation”, with low pressure you broke the wheel.
This will not happen with smooth gravel, so you can really adjust the pressure lower for speed, not dealing with rim destruction risk.
See the “cobble” graph here : https://blog.silca.cc/asymmetric-effects-of-tire-pressure-optimization
There is clear indication that a lower pressure will be better for speed… but…
This is the reason for suspended bikes on Paris-Roubaix : increase flexibility to deal with cobbles “height deviation” above tyre deflection.
Maybe this is the statement we all seem to be missing:
“Surface Type: We recommend optimizing for the worst surface you are riding as you are generally better to be a bit low than to be a bit high.â€
Although we generally have good roads around here, they vary from new to sometimes “smooth†chip seal. I have found the old pavement pressures line up pretty close to what I run on a daily basis.
Ah yeah, I didn’t think about the rim damage aspect.
Ok, so let’s try and help provide a solution rather than just complain. How would everyone else like for the surface descriptions to progress?
I can only speak to roads, but my experience is this from smoothest to roughest:
Concrete
Smooth new asphalt
Old worn asphalt
Old smoother chipseal
New rough chipseal
Poorly laid chipseal that resembles gravel
Alright, sorry for being offline yesterday, we launched this and immediately had problems with the site and once the first people started sharing it, we spent the rest of the day trying to fix it and answer the emails and phone calls… 4,000 people got the notification, after 1 hour, 15,000 people worldwide were tying to access it… total cluster, so sorry to everybody who had a poor experience. The goal was to have a more interactive calculator with images and explanations etc, but once it was crashing like crazy we simplified and are now in process of putting it back together.
Answers for some of the questions and an idea of the future plan:
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Numbers seem weird - Try it today, we had a term in an equation meant to go active in a later version with an additional feature that wasn’t zeroed out, so pressures for the first handful of hours were probably trending a bit higher for narrow tires and lower for wider ones… not by much, but by a few% for sure. This is fixed
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Surface types - this has been the biggest question and we are working on providing images for each surface as our language is not consistent here. In reality, there are hundreds of options here and we are trying to make a gradient out of them. There is a blog post in the works on this for the official go live of the calculator that will have photos… also open to ideas on the language as everybody seems to disagree here.
‘New Pavement’ - think of this as having being paved a few weeks ago using high quality fine aggregate fill, super smooth, low void.
‘Mild’ pavement as having some visible imperfections, cracks, and undulations, could also be newly paved with large aggregate fill, visible voids.
‘old pavement’ as being most any road in the midwest, visible cracks, small-medium potholes, occasional damage, ‘alligator skinning’
‘Cobbles’ - Paver bricks to paver stone cobbles, also mild-medium aggregate chip-seal
‘Mild Gravel - packed dirt’ - This is packed dirt with mild-medium aggregate gravel over top. This is a deformable surface so more efficient to allow the tire to deform more rather than deforming/disrupting the surface. Could also be coarse chip-seal (uncommon)
‘Coarse Gravel’ - large aggregate or large embedded imperfections like Dirty Kanza flint
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Measured tire width - This is in fact the measured width of the installed tire at the casing (graphic pulled and coming back soon). It is hard even for me, dealing with this stuff every day, to keep this one straight as we continue to call the tires by their casing numbers, but I have some 23mm GP4000SII on some ENVE with 21.5 inner bead width that measure at 28.9mm wide, so would be 29mm in this calculator. Opposite of that, many of the gravel and mtn tires measure at or below the casing number which seems to be a combination of assuming wide bead seat rims and/or measuring to tread and not casing, for example WTB Riddler 29x2.25 (57mm) actually measure 51mm at the casing (I get 56-57mm at the tread) on the ENVE G23 rim… so that measured casing number is critical. We also have a piece on this that will make it’s way onto the calculator once we are stable.
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Speed - lastly, we are building in a speed component to further help the discussion and that will also bring a pinch flat risk calculator to the equation. As we often find with the pros, the final pressure is often slightly higher than what you might expect due to the increased energy at which they hit things increasing pinch flat or damage likelihood. Also tire behavior is related to dynamic, rather than just static stiffness, so breakpoint pressure will move higher with higher velocity and lower with lower velocity. As the data in this calculator is almost exclusively taken from ProTour and top level triathlon competitors, the speeds ridden in compiling this data are likely higher than what most of us are riding!
Best and please keep the feedback coming
Thanks
Josh
Awesome. Thanks for the explanation. In playing around with it a little more I’m starting to see some numbers I more expected to see. In particular at my weight and 26mm, I found the mild vs old pavement numbers interesting and more in line with what I thought: 90psi for mild, 83psi for old. Waco 70.3 last month would have definitely fallen in the old but Indian Wells 70.3 next month will be mild-good.
On a side note, I finally broke down and ordered a Viaggio Travel Pump today. Figured there wasn’t much point in knowing the right numbers if I’m not pumping up to the right numbers. No more sharing my pump in transition with the other pros now 
Gave the pressure calculator a try, but even for mild pavement (with system weight 172lbs), its recommending 93psi for 25mm measured width tires. I’m using Hed jet plus wheels with 23mm gp5000 tires, and Hed says their rims are not rated for above 90psi. Are the hed rims unique in pressure required, or is the recommendation too high? Very cool idea though, thank you for making available!
Gave the pressure calculator a try, but even for mild pavement (with system weight 172lbs), its recommending 93psi for 25mm measured width tires.
Are you sure you are measuring 25mm for your tire on a HED wheel? Are you running a tire spec’d as 21mm width? I would imagine that a 23 or 25 spec’d tire would be a good bit wider on a HED wheel.
Thanks much. I have been waiting for this for a while.
One question: Does the tire width and rim width make a difference here? For example, two tires measuring the same may have dramatically different volumes. Take a hypothetical wide tire on a skinny rim that measures 26mm actual versus a narrower tire on a nice, fat, wide modern rim that also meaures 26mm. Should those two tires measuring the same width but with different volumes get different pressures? Do we need a height and width here?
The rim is 25mm outer diameter, and the 23mm tire does not balloon out. I think when I checked it with calipers is was 24. something, its a pretty flush fit with side of rim
If both tires measure the same diameter at the widest point, then the volume of the tire above the rim ought to be nearly identical. But, there could be some difference in total volume, like with HED rims that have a deeper and wider interior channel below the tire.
Wow. I didn’t realize this was a thing
I just pump it up to what it says on the side and ride all day
You triathletes and your tyre pressures. 😂
Gave the pressure calculator a try, but even for mild pavement (with system weight 172lbs), its recommending 93psi for 25mm measured width tires. I’m using Hed jet plus wheels with 23mm gp5000 tires, and Hed says their rims are not rated for above 90psi. Are the hed rims unique in pressure required, or is the recommendation too high? Very cool idea though, thank you for making available!
That’s a suggested max pressure. HED jets are AL rim. You’re not going to hurt the rim pumping it up higher than the recommended.
The GP5000’s are sized for newer rims. I’d suggest going with 25’s on the jet+. The new 25 measures about like the old 23 GP4000.
Josh;
4,000 people got the notification, after 1 hour, 15,000 people worldwide were tying to access it… total cluster, sorry to everybody who had a poor experience
I guess the good news, is that alot of people are following you and your work !
Thanks for your work.
Thanks for the suggestion. Even with the change to a 25 though, the tire pressure recommendations (per rider weight) on the Hed site recommend me in the mid 70s for tire pressure. Its a pretty big difference
Pressures for my road bike are very close to what I run. A couple pounds higher for the rear and ten pounds higher for the front. I’ll try the slightly higher pressures.
For my gravel bike (both 38mm and 47mm tires), the recommended pressures are very low compared to how I race. I suspected they would be, though. I fear the pinch flat and, historically, have kept my gravel tires up around 50 to 55 psi. I’ll try slightly lower pressures, but I’m not sure I’ll ever get down to 32psi and 24psi. (I’m not saying they aren’t the right pressures. I’m just saying I’m probably too chicken to get down to those numbers.)