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Tire pressure suggestions?

Old 05-24-18, 12:19 AM
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cvictorh
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Tire pressure suggestions?

Im riding 700x40c tires over various grades of gravel and am kinda new to this so just looking for a good starting point for tire pressure. Me + the bike is right around 200-210 lbs.
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Old 05-24-18, 12:20 AM
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Running clinchers not tubeless.
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Old 05-24-18, 03:02 AM
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The pressure indicated on the sidewall is a good place to start. You can then experiment from there to see which pressure gives you the best combination of comfort and rolling resistance. Tire design plays some part, some are better at different pressures.
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Old 05-24-18, 05:04 AM
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I haven't used tubes in years, so I forget how that affects things. With tubeless tires in that size, I would start with 35psi rear/30psi front and adjust from there.
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Old 05-24-18, 10:22 AM
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At the Bike Exchange we tell volunteers to fill tires on road bikes to 70 lbs. and mountain bikes to 40 lbs. which is a good starting point.
Never fill the tire to the max recommended pressure. During our work days someone will invariably do this and then after all afternoon in the sun the tire will blow out as the heat raised the pressure too high.
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Old 05-24-18, 12:27 PM
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This will get you in the ballpark: Bicycle tire pressure calculator

Calculator only goes up to 37mm but that's close enough to 40 for starting.

I run about 50 front and 80 rear with 1.5"/40mm tires on a recumbent bike with a rearward weight bias, compared to upright bikes. 30 pound bike with 185 pound rider.
Less pressure for lower weights. And lower pressure for riding on gravel than on pavement.
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Old 05-24-18, 12:47 PM
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some trial & error may be in order. you can start with max in back & knock off 10psi for the front & see how that feels. if it feels harsh, knock both down by 5 psi & try again. for better quality surfaces higher is good, for nasty large gravel, lower is better. I've adjusted pressure mid ride. not ideal but sometimes the lower pressure for a long rough stretch is just too sluggish for the rest of the ride

was surprised last year, when I wound up using approx 1/2 the listed max pressure on some new tires!

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Old 05-24-18, 04:14 PM
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Try 40 F / 45 R.
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Old 05-24-18, 08:00 PM
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37 psi rear / 32 psi front
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Old 05-24-18, 08:07 PM
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Originally Posted by cvictorh View Post
Im riding 700x40c tires over various grades of gravel and am kinda new to this so just looking for a good starting point for tire pressure. Me + the bike is right around 200-210 lbs.
I'm a little bit lighter but not by much, and I run my 38 mm tires at 40 psi front / rear. I actually went from 32 mm to 38 mm specifically to run a bit lower PSI for gravel and rough pavement. Lots of gravel opportunities where I live.
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Old 05-24-18, 08:19 PM
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700x43 tires. 35 front, 40 rear. In contrast to the sidewall recommendations which is 50psi to 80psi.
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Old 05-26-18, 07:12 AM
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As someone said, go with the manufacturer's recommendation.
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Old 05-26-18, 08:51 AM
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pump them up with a tire pump and ride. after a while you will figure out what you like. its not complicated.
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Old 05-26-18, 03:18 PM
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65/70
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Old 05-26-18, 08:40 PM
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Originally Posted by JanMM View Post
This will get you in the ballpark: Bicycle tire pressure calculator

Calculator only goes up to 37mm but that's close enough to 40 for starting.
.
This^^^

I find that calculator to be a great starting point. On stiff slow tires I might go a smidge higher, on some really supple/fast one I go lower.
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Old 05-26-18, 10:44 PM
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Originally Posted by JanMM View Post
This will get you in the ballpark: Bicycle tire pressure calculator

Calculator only goes up to 37mm but that's close enough to 40 for starting.

I run about 50 front and 80 rear with 1.5"/40mm tires on a recumbent bike with a rearward weight bias, compared to upright bikes. 30 pound bike with 185 pound rider.
Less pressure for lower weights. And lower pressure for riding on gravel than on pavement.
There's a spreadsheet here that attempts to interpolate and extrapolate the Berto chart data to other tire sizes. Works well for my 42mm tires, for example: Tire Pressure Calculator ? Bike Tinker
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Old 05-27-18, 05:26 AM
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Originally Posted by capnjonny View Post
At the Bike Exchange we tell volunteers to fill tires on road bikes to 70 lbs. and mountain bikes to 40 lbs. which is a good starting point.
Never fill the tire to the max recommended pressure. During our work days someone will invariably do this and then after all afternoon in the sun the tire will blow out as the heat raised the pressure too high.
x2.

Any idea how much the PSi may statically increase to?
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Old 05-27-18, 09:25 AM
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There is also this: http://www.compasscycle.com/wp-conte...BQTireDrop.pdf
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Old 05-27-18, 05:41 PM
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Originally Posted by Troul View Post
x2.

Any idea how much the PSi may statically increase to?
compliments of Wickipedia...

"Effect of temperature

Since the volume of gas and the gas itself inside a tire is not altered significantly by a change of temperature, the ideal gas law states that the pressure of the gas should be directly proportional to the absolute temperature. Thus, if a tire is inflated to 4 bar (400 kPa; 58 psi) at room temperature, 20 C (68 F), the pressure will increase to 4.4 bar (440 kPa; 64 psi) (+6%) at 40 C (104 F) and decrease to 3.6 bar (360 kPa; 52 psi) (-13%) at −20 C (−4 F)."
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Old 05-27-18, 06:05 PM
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Originally Posted by bobwysiwyg View Post
compliments of Wickipedia...

"Effect of temperature

Since the volume of gas and the gas itself inside a tire is not altered significantly by a change of temperature, the ideal gas law states that the pressure of the gas should be directly proportional to the absolute temperature. Thus, if a tire is inflated to 4 bar (400 kPa; 58 psi) at room temperature, 20 C (68 F), the pressure will increase to 4.4 bar (440 kPa; 64 psi) (+6%) at 40 C (104 F) and decrease to 3.6 bar (360 kPa; 52 psi) (-13%) at −20 C (−4 F)."
Compliments of my high school physics teacher:

The temperature should be in Kelvin:

20C = 293K
40C = 313K
-20C = 253K
58 psi in 20C will be 58 X 313/293 = 62 psi in 40C and 58X253/293 = 50 psi in -20C
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Old 05-27-18, 07:35 PM
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Originally Posted by GerryinHouston View Post
Compliments of my high school physics teacher:

The temperature should be in Kelvin:

20C = 293K
40C = 313K
-20C = 253K
58 psi in 20C will be 58 X 313/293 = 62 psi in 40C and 58X253/293 = 50 psi in -20C
Good on the more precise addendum. Either is well within my "good enough" for government work criteria.
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Old 05-27-18, 07:42 PM
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Temps & Inflation may = 8PSi per +/-30*F~.
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Old 05-27-18, 08:32 PM
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35 on front 40 on back

35 on front 40 on back
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Old 05-28-18, 06:23 AM
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The temp effect due to the Universal Gas Law is surprisingly large. If you really want to nail it, measure pressure (accurately) on-road and adjust it on-road. This allows for actual tire temperature rise but only partly compensates for the fact that ambient air temperature is lower than tire internal volume temperature. Pressure should rise a little as the new, denser air mixes with the air thats in the tire already. But the error due to this warming just might be within the error due to measurement technique!

Im a good enough for government work (GEFGW) guy!
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Old 05-28-18, 01:47 PM
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Originally Posted by capnjonny View Post
Never fill the tire to the max recommended pressure. During our work days someone will invariably do this and then after all afternoon in the sun the tire will blow out as the heat raised the pressure too high.
I doubt the tires were only filled to the max pressure. They were likely inflated way over the max pressure rating. Probably 150% or more.
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