Online Tire Pressure Calculators

[oaklandhillsca]

I'm trying to establish a rough idea on what my tire pressure should be and I am relying on some online tire pressure calculators. Issue is there is a major variance on what is calculated.

Wheels are Zipp 202 FC - running Conti 5000 TL 25mm tubeless
Body Weight - 168lbs
Bike/Gear weight - 18.5

SRAM Calculator generates - Front 75 PSI and Rear 78 PSI
Silca Calculator generates - Front 91 PSI and Reat 94 PSI

I am leaning towards the SRAM amounts. Understanding that part of pressure is personal, I'm looking for a "safe" starting point and the above variance does not provide any comfort.

Thanks for your thoughts.

[yarg]

The SRAM values seem about right, Silica values are too high given your weight and going tubeless.

[rice rocket]

I'm roughly the same weight. The SRAM values seem idealized IMO.

With the state of the roads today, I would surely nuke a rim in short time.

[tomato coupe]

Different calculators give different pressures because there is no “correct” tire pressure. Start with something in the middle and experiment. You might find that one calculator gives results that match your riding preferences, and that becomes your “go to” source.

[yinzerniner]

Originally Posted by oaklandhillsca

I'm trying to establish a rough idea on what my tire pressure should be and I am relying on some online tire pressure calculators. Issue is there is a major variance on what is calculated.

Wheels are Zipp 202 FC - running Conti 5000 TL 25mm tubeless
Body Weight - 168lbs
Bike/Gear weight - 18.5

SRAM Calculator generates - Front 75 PSI and Rear 78 PSI
Silca Calculator generates - Front 91 PSI and Reat 94 PSI

I am leaning towards the SRAM amounts. Understanding that part of pressure is personal, I'm looking for a "safe" starting point and the above variance does not provide any comfort.

Thanks for your thoughts.

To get the figures you mentioned above I believe you had to include the following inputs:
Tubeless tires high performance
Moderate speeds
Worn pavement

Remember that the 25mm GP5K TL will probably blow up to an actual width of 26-27mm on the FC202s. If you use different inputs you get a different tire pressure - 26mm Tire
You get 86/88 Front Rear

That said, a reason for the numbers varying so much from Silca to Zipp is in what each calculator is trying to do.
Silca's intent is included at the bottom of the calculator page:

NOTE/WARNING: This calculator is estimating the fastest
possible pressure based on real world data taken with professional athletes
riding professionally specified and maintained equipment. This calculator
makes no claims to the compatibility of the rims/tires you have chosen, nor
does it read the internet deeply enough to know the limitations of your setup
as specified by ANY of the manufacturers of the various components in your
setup. If this calculator recommends pressures that violate ANY of the
limitations of your manufacturer's specifications, you must either change
components or lower your pressures to match those of the manufacturer.
Note: Tubeless setups, particularly those not adhering to ETRTO, OR hookless setups are VERY sensitive to pressure and surpassing manufacturers recommended pressures can lead to unneccessary risks including injury, equipment damage, and you guessed it: death.

In other words, the tire pressure calculator really gives the breakpoint pressure, and you should adjust down from there. And also, it's only based on their analysis, not what is recommended by the tire and/or rim manufacturer.

But I'm unable to get your numbers in the SRAM calclator. Using the following inputs, the output is similar to the Silca:
168+18.5 weight
25mm labeled (not actual) tire width
21mm internal width
Thin casing tire
Hooked Clincher
Dry conditions
81.4/86.6 front rear

If you change the casing to standard (which I don't think the TL qualifies as) then you get figures of 74/78.8

So sram/zipp goes by labeled size, and they probably use the inner width to then judge what the actual width will be, unlike Silca which just uses actual width. Also the sram/zipp is only for zipp wheels, so probably has some level of safety margin built in.

Another decent tire pressure suggestion is the Enve chart, which gives numbers similar to the above - 82-85 psi on 21mm inner rims with a 25mm tire:
https://www.enve.com/learn/tire-pressure/

[makoti]

Sram is about what I, at 165 + 19 bike, run. The other seems high

[GregL]

Originally Posted by oaklandhillsca

SRAM Calculator generates - Front 75 PSI and Rear 78 PSI
Silca Calculator generates - Front 91 PSI and Reat 94 PSI

I haven't used the SRAM calculator, but the numbers from the Silca calculator look quite high. For reference, my body/gear/bike weights are ~20 lbs heavier than yours. The Silca calculator recommends 85F / 88R for tires measuring 27mm wide (for racing on new pavement). Are you using the specified width of the tires, or the actual, measured width?

Greg

[Dave]

I've always used standard casing in my Zipp calculations. I use Michelin or Pirelli tubeless.
Using thin casing increases my 62 psi to 68. I used 60 on my last ride and all seemed fine. I've never felt the need to increase the suggested pressure. The Zipp calculator is not just for Zipp products. There are variables to cover just about any combination you can think of.

I did a test recently where I increased the pressure on my 28mm Michelin tubeless tires on 19mm IW hooked rims from my normal 62 to 80. Now I remember how awful the ride becomes with such high pressures, which some people still think reduces rolling resistance.

I've used 72 psi with 25mm tubed Michelin on 17mm rims. The ride wasn't terrible, but I did get a couple of pinch flats. Zipp recommends 77/82.