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Tire Pressure

Old 11-27-23, 10:03 AM
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rclouviere
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Tire Pressure

I’m confused about what air pressure to use for the most speed on tubeless tires. I’ve read a lot of info saying you should decrease the pressure a lot. However, the Silca guide says much higher (for me on smooth roads, 96 rear and 93.5 front).

Anyone have testing results/opinions?

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Old 11-27-23, 10:18 AM
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The Silca calcuator is a good starting point/guide, NOT gospel. Be sure you put in correct weight and actual measured tire width.

You probably can reduce the pressure by a few PSI and see how it feels.
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Old 11-27-23, 10:19 AM
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I think just like with tubed tires, you should ride the bike and find out what pressure you prefer to run in them and watch your metrics to see if they confirm your perceptions. Pick a number to start with and go from there. The Silca recommendation is as good as any for a starting point.

Your actual road conditions, smoothness or roughness as well as the frequency of bumps will make a difference that no calculator will be able to precisely figure for you.
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Old 11-27-23, 10:27 AM
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The Silca guide is a reasonable starting point. What width tyres and rider weight did you input?
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Old 11-27-23, 10:48 AM
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I don’t go above 80 on 25mm TLs. 80KG rider.
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Old 11-27-23, 12:09 PM
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Originally Posted by rclouviere
I’m confused about what air pressure to use for the most speed on tubeless tires. I’ve read a lot of info saying you should decrease the pressure a lot. However, the Silca guide says much higher (for me on smooth roads, 96 rear and 85 front).

Anyone have testing results/opinions?
I’ve used the Silca tire pressure calculator and the front and rear tire pressures have never been different by 11psi. What info are you inputting?
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Old 11-27-23, 12:10 PM
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Originally Posted by rclouviere
I’m confused about what air pressure to use for the most speed on tubeless tires. I’ve read a lot of info saying you should decrease the pressure a lot. However, the Silca guide says much higher (for me on smooth roads, 96 rear and 85 front).

Anyone have testing results/opinions?
If your tubeless wheels are hookless, you do indeed have to reduce your pressure by a lot to be safe per ETRTO. Make sure you also abide by the tire and rim manufacturers specifications -- ie. go with the lowest of the sources. See Silca disclaimer I pasted below

96R/85F -- that seems like a much larger difference than Silca's calculator usually returns between rear and front. Are you running different width tires in Front vs Rear?
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Old 11-27-23, 12:15 PM
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Originally Posted by choddo
I don’t go above 80 on 25mm TLs. 80KG rider.
Similar for me. 80R/75F in 26mm Vittoria Corsa Pros. I'm about 180lbs/82kg.
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Old 11-27-23, 12:46 PM
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I have inputed data into the Silca calculator the Rene Herse Calculator and the SRAM calculator and all are pretty different.
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Old 11-27-23, 12:59 PM
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Originally Posted by Biker Pete
I’ve used the Silca tire pressure calculator and the front and rear tire pressures have never been different by 11psi. What info are you inputting?
you’re right. My mistake. Front 93.5.
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Old 11-27-23, 04:53 PM
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I use the pressure calculator at zipp.com. The results aren't said to be the fastest, AFAIK. The issue is often how smooth or rough the asphalt is. One pressure is never perfect for all conditions. I ride tubeless with hookless rims. At 135 pounds, I never exceed 60 psi with 28-30mm tires. I have a lot of chip sealed roads, so I tend to use lower pressure.
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Old 11-27-23, 06:26 PM
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Originally Posted by jadmt
I have inputed data into the Silca calculator the Rene Herse Calculator and the SRAM calculator and all are pretty different.
I haven’t tried the RH calculator…..thanks for the info!

I have used Silca and SRAM and find them to be within about 2psi for the rear tire, SRAM being lower. And this is with using the actual measured width for Silca. For example I use 28mm tires, SRAM asks for that value (and the inner rim width) and when inflated the tires actually measure 29mm on my wheels which, if you input 28 vs 29mm in the Silca calculator, makes a significant difference in the calculated pressure. Also, SRAM always has a greater difference between front and rear tires than Silca.

I also have looked closely at the info on rollingresistance.com……so much data to digest in this ‘modern era’ of cycling……I love it!!!
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Old 11-27-23, 06:43 PM
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Originally Posted by Biker Pete
I haven’t tried the RH calculator…..thanks for the info!

I have used Silca and SRAM and find them to be within about 2psi for the rear tire, SRAM being lower. And this is with using the actual measured width for Silca. For example I use 28mm tires, SRAM asks for that value (and the inner rim width) and when inflated the tires actually measure 29mm on my wheels which, if you input 28 vs 29mm in the Silca calculator, makes a significant difference in the calculated pressure. Also, SRAM always has a greater difference between front and rear tires than Silca.

I also have looked closely at the info on rollingresistance.com……so much data to digest in this ‘modern era’ of cycling……I love it!!!
Never liked the SRAM calculator, because it is inferring how wide your tires will be for a given rim width, even though it's quite known that different manufacturers nominally label their tire sizes differently versus how they actually inflate. It also only has 2 road surfaces to choose from -- Dry or Wet. Nothing to do with actual texture -- eg. rocky, smooth, dirt, etc. And then of course, since they're heavily invested in selling the hookless variety of wheel through Zipp, I expect their bias on PSI recommendations is always going to be on the low side in order to satisfy the marketing side of their business.

For any calculator, what none of them seem to ask for is the inflated height as well. Even sites like BicycleRollingResistance, will show the inflated height as well as width. I presume a taller inflated height means more air volume than a lesser height (for an equivalent inflated width, and there are sometimes fairly significant differences between tire brands on inflated height.

Last edited by Sy Reene; 11-27-23 at 06:48 PM.
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Old 11-27-23, 06:56 PM
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As Sy Reene pasted, Silca's usually is race-focused and gives you the estimated breakpoint pressure -- meaning the maximum pressure before rolling resistance is expected to increase. Note that going too high increases Crr more than running pressure under that breakpoint, so it should be treated as an upper limit. The reasons the common wisdom is to use lower pressure for tubeless setups than tubed are 1) that it's more comfortable, 2) you don't have to worry about pinch flats with tubeless, and 3) you won't increase Crr much, so the trade-off tends to be worth it, especially for running narrower tires.

My recommendations would be to try lower pressures and see whether you like how the bike handles. Your goal should be to have your bike feels smooth, not sluggish or squirmy.
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Old 11-27-23, 07:02 PM
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Originally Posted by Sy Reene
Never liked the SRAM calculator, because it is inferring how wide your tires will be for a given rim width, even though it's quite known that different manufacturers nominally label their tire sizes differently versus how they actually inflate. It also only has 2 road surfaces to choose from -- Dry or Wet. Nothing to do with actual texture -- eg. rocky, smooth, dirt, etc. And then of course, since they're heavily invested in selling the hookless variety of wheel through Zipp, I expect their bias on PSI recommendations is always going to be on the low side in order to satisfy the marketing side of their business.

For any calculator, what none of them seem to ask for is the inflated height as well. Even sites like BicycleRollingResistance, will show the inflated height as well as width. I presume a taller inflated height means more air volume than a lesser height (for an equivalent inflated width, and there are sometimes fairly significant differences between tire brands on inflated height.
Great info. I love getting down into the weeds with all this stuff.

Regarding my own journey, I just recently came into the ‘modern era’ with the purchase of a used Kestrel from 2016. Still old fashioned rim brakes, cabled shifters, no power meter. But I did ‘upgrade’ the Zipp carbon wheels optimized for 23mm tires with new ‘aero’ ‘tubeless ready’ carbon wheels optimized for 28mm tires. Hence my investigation into the tire pressure calculators.
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Old 11-28-23, 02:51 AM
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Originally Posted by rclouviere
I’m confused about what air pressure to use for the most speed on tubeless tires. I’ve read a lot of info saying you should decrease the pressure a lot. However, the Silca guide says much higher (for me on smooth roads, 96 rear and 93.5 front).

Anyone have testing results/opinions?
I've seen a max. inflation of 72psi on some tubeless tires, so check the info on your tire against the Silca recommendation to make sure your tires can safely take 96 / 93.5 psi.

As for the pressure that will provide the most speed... there is no single magical number. It depends. And it will change based on weather conditions, road conditions, and the specific tires you are using. As stated by others, the Silca guide is a good starting point. Start there and feel free to experiment until you find what you like. At the end of the day, the best pressure is the one that works best for you.
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Old 11-28-23, 07:33 AM
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Originally Posted by choddo
I don’t go above 80 on 25mm TLs. 80KG rider.
Same here by accident when I realized the pressure gage on my very old pump was reading way low when I replaced the pump.
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Old 11-28-23, 08:29 AM
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What hasn't been mentioned is to verify the accuracy of your pump gauge. That's not easy to do unless you want to purchase a pricey gauge that's for calibration purposes. I use liquid filled Winters brand gauges for calibration, with the gauge attached to some pipe with a presta valve attached to it. My old silca 0-160 psi gauge was off by 8 psi, only delivering 72 instead of 80. I replaced it with a much larger diameter Winters gauge. I use a 0-100 psi gauge, since I never use more than 60. I can compare several gauges at once. My newest lezyne pump is accurate.
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Old 11-28-23, 09:01 AM
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Originally Posted by DaveSSS
What hasn't been mentioned is to verify the accuracy of your pump gauge.
Accuracy isn't very important for most people. Once you find a pressure that works best for you, the actual number on the pump is irrelevant. Day-to-day repeatability is all you really need.
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Old 11-28-23, 09:21 AM
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Most tires have a specific rim width that's used as a basis for the advertised width. It's usually listed on the packaging. Wider rims will result in a wider mounted width. I have some 25mm internal width hookless rims. The narrowest tire that can be used is a 28mm and not all of those are approved for that rim width. 28mm tires will measure about 31mm on a 25mm IW rim and 30mm tires measure about 33mm. I've only used Michelin and Pirelli tires. Both measure about the same.
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Old 11-28-23, 09:33 AM
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Originally Posted by tomato coupe
Accuracy isn't very important for most people. Once you find a pressure that works best for you, the actual number on the pump is irrelevant. Day-to-day repeatability is all you really need.
I have two pumps and want similar results. Those pushing the 73psi limit with hookless might want accuracy.
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Old 11-28-23, 09:58 AM
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digital Air Pressure Regulators with a zero-out feature tends to be a less expensive route to test tire inflation tools for accuracy.
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Old 11-28-23, 10:01 AM
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Originally Posted by DaveSSS
What hasn't been mentioned is to verify the accuracy of your pump gauge. That's not easy to do unless you want to purchase a pricey gauge that's for calibration purposes. I use liquid filled Winters brand gauges for calibration, with the gauge attached to some pipe with a presta valve attached to it. My old silca 0-160 psi gauge was off by 8 psi, only delivering 72 instead of 80. I replaced it with a much larger diameter Winters gauge. I use a 0-100 psi gauge, since I never use more than 60. I can compare several gauges at once. My newest lezyne pump is accurate.
"That's not easy to do unless you want to purchase a pricey gauge that's for calibration purposes." But there is something you can do that is easy and cheap. Every time you pump your tires with a known pump, squeeze them with your fingers. You will get to know what an 80 psi tire feels like. When a pump is far off, you'll know. (And you will start associating the finger squeeze with ride feel. Gauge no longer needed at all. I rode and raced for a decade before I owned a gauge. It was never an issue.)
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Old 11-28-23, 10:06 AM
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Originally Posted by DaveSSS
I have two pumps and want similar results.
Accuracy isn't very important for most people. You can do whatever you like.
Those pushing the 73psi limit with hookless might want accuracy.
The 73 psi limit is chosen somewhat arbitrarily and the actual maximum safe pressure varies with tire and rim combos. Inflating your tubeless tires right up to the 73 psi limit with a "calibrated" pump might make you feel safer, but it's not doing much for you.
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Old 11-28-23, 10:12 AM
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Originally Posted by easyupbug
Same here by accident when I realized the pressure gage on my very old pump was reading way low when I replaced the pump.
Exactly my thoughts. How do I know how accurate my gauge is? Probably close enough.

If I were going for the world hour record on a track I would worry. Riding to the next town with a bunch of 70-year-old for cake and coffee, not so much. As long as I don't get a flat.

That said, I run Conti 5000 tires at about 90psi. Or so my pump says.
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