Tire Pressure Tools - your experience?

Equipment
1

Hi there,
Tire pressure is one of the easiest (or hardest to get right) measures to improve the feel of your bike.
The current trend is ever more toward wider tires, wider rims, and therefore lower tire pressure.

With various different tires, casings, surfaces, rim shapes etc. tire pressure tools can really come in handy.
There are many of those out there, with SRAM AXS (SRAM | AXS) and Silca (https://silca.cc/pages/sppc-form ) being the ones most prominent to me.

What sparked my uncertainty (and thereby this thread) is the vast difference between the results I tend to get. I have always used the SILCA tool for my TTing and clinchers on the road bike. All that has worked very well.
Now for the first time Iā€˜ll be running tubeless hookless on a road bike, and my results are all over the place.

What I know about these tools is:

SILCA: is based on data points. Josh Poertner stresses that one of the many data points is the pressure Sagan ran to win Paris Roubaix in 2018. He says itā€˜s most important to get the actual tire width right.
Also, Silca has a vast choice of surfaces, to really get this right.

AXS: here you plug the labeled width, and based on ETRTO, the calculator extrapolates what actual width that is. Therefore, their calculator also takes into account if the rims are hookless and will warn you if the recommend pressure exceeds ETRTO (72PSI).

So, my specific case is:
I am 71kg, ride a 7kg bike, with clothing and helmet and food in my pockets etc. I am circa 73kg. With bottles, spares and bike computer the bike is 9kg.
I run ENVE SES 4.5, that are hookless, 25mm internal, and will use GP5000s TR, labeled 28c, that come out to just under 31mm in actuality. I am looking to race these in the Alps, so there is no steady speed, but rather slow up the mountains, fast down. The surface is pretty fresh asphalt. I hope the roads are dry.

With all that data, the two calculators give me these results:

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I have tried different average speeds. The actual average speed will likely be high 20s (kph), but with 80+ high speed.
In either case, and even if you say this is not a thin but moderate casing or whatever, there is pretty large gap between the two.
7psi difference on the rear, and 8psi difference on the front are definitely noticeable.

So what to do? What is your experience. Go with the middle or disregard one of the two completely? The Silca one appears to be on the very high end of things. But maybe youā€˜ll know a trick to make this work for wide TL road tires.

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2

Iā€™d go with Silca. Josh seems to be one of the leading experts in this area, and I suspect the differences are largely down to the greater choice of road surfaces. If you put in cracked pavement or chipseal the numbers would likely be much closer to the SRAM calculator. Alps roads are generally pretty decent.

If it helps, I run a similar setup, hookless rims with slightly wider tyres (30mm) but also slightly higher total system weight (high 80s). Roads around here are often pretty mixed and day to day I run at about 60psi, but will go nearer to 70 if I know Iā€™m going to be on good roads, which includes trips to the Alps.

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3

Iā€™d say its given you a window to start in. Iā€™d take one of the two ends and then use bracketing to get to where you want to be as the +/- 10% is very individual and subjective.

There is no ā€œperfectā€ pressure :slight_smile:

4

Iā€™d say go with the higher pressure, feel the ride and if it feels too bumpy let a bit of air out

5

I just found it a little suspect, that Silca gives you such a high value (70.5psi rear), for a relatively light system weight, and that fat of a tire, when the upper limit of HL is 72.
If you go for a 27mm wide tire (which is the width of a 25c on a 21mm hookless rim) literally no one could ride this according to Silca (even at 35kg/ 80lbs System weight, the pressure recommendation exceeds the ETRTO standard of 72psi.
Kinda makes me think Silca is very retro here and clearly hasnā€™t adapted their calculator to HL.

6

I understood that Silcaā€™s calculator was based on performance. You could run lower pressure for comfort but it might not be quite as fast (probably a very minor difference). Their pressures correlate well for me for road racing and TTing

Hookless for the absolute fastest road riding doesnā€™t seem to be optimal really at the moment. The maximum pressures are just too low for those maximising speed on normal smooth roads. It might be great for gravel, or road riding where you donā€™t mind giving up a watt or two. I donā€™t know if theyā€™re just cautious in the max pressure, or if the tech will improve in future for higher pressures

7

Iā€™d go SRAM for sure. Those Silca values are high. Correct me if Iā€™m wrong but they donā€™t allow you to choose hookless, right?

Iā€™m thinking Silcaā€™s values are based on what the pro peloton runs (data points, as you said). Pros and pro mechanics are notoriously conservative, plus their speeds are much higher than ours even in races.

Thereā€™s no world in which I put 70psi in a hookless 31mm tire (I weigh the same as you).

8

I do wish SRAMā€™s calculator had measured width instead of labeled width. We all know everyoneā€™s 28s arenā€™t created equal.

I think I remember hearing Portner say that if in doubt lower pressure is better. Performance will gradually fall off as you go down from optimal, but if you go over optimal pressure performance takes a big dive right away.

9

Iā€™ve submitted this question to the Marginal Gains podcast, but so far they havenā€™t picked it. Maybe if you / more people also submit this question Josh will answer it

10

Not sure if the link will work, but Iā€™ve found this one helpful

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11

Ironically, Josh makes that joke about the inflation station, that they place at different road and gravel events throughout the country and he says ā€žWe hardly ever inflate, usually we just let air out of peopleā€˜s tiresā€œ.

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12

Hi,
On the Sram tire pressure calculator can anyone tell me what to input for tire casings if it;s 120 TPI?? Donā€™t know what thatā€™s considered. Thin?

13

TPI = Threads Per Inch

So, Larger Number TPI (like 120) is thinner thread which usually means ā€œthinā€. Thick stuff is usually around 60 tpi. Does that help?

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Thanks!

15

SRAM also has it set up to recommend to fewer people go above the pressure limit. I think that by nailing their colors to the hookless mast, they set up the recommendations to not recommend over 72 for most people. I also think that the sram ones are low for pavement

16

The interesting one is the Silca, which basically says 25mm tires are not hookless rim compatible as the recommended pressure is above 72psi even for a light rider (140lbs).

17

I have used SRAMā€™s tool to get recommended pressures on my road bike. They are pretty close to Enveā€™s official recommendations as well and it feels about right. The roads here in Japan are sometimes great, but sometimes quite rough, especially in the countryside. It isnā€™t about comfort necessarily either, on a broken surface you have more traction at lower pressures as your tire has an easier time to conform to the little undulations and surface defects. Your bike also becomes less ā€œjumpyā€ on rougher parts. Especially in corners, if your tires are too hard, you may lose traction.

Also, I noticed that you opted for ā€œNew Pavementā€ as surface condition in Silicaā€™s pressure calculator. Is that realistic where you live?

Personally, Iā€™d opt for the lower pressure first. Do a few rides and then up the pressure a little, do a few rides again and see whether that improves handling and the like.

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