Tire Pressure for Partial Load

[r1sss]

I see that on the sticker of R1S 20" AT it says 48 psi but assuming it's for a full load and considering that this is a heavy 7 seat SUV with plenty of storage space, for a usage that is lets say just 2 average adults - is it recommended to asjust the tire pressure to a lower one?

I got the vehicle actually with 45 psi (on 3 wheels, and one was 48 ?‍) and the guide at the SC said that this is the recommended tire pressure but I didn't askhim why as I thought this will be the number on the sticker only later to find out it's actually 48.

Sponsored

 

[NY_Rob]

It's 48psi unless you need to air down for sand, etc... the Guides are useless. They spread disinformation more than any poster ever did on the forums.

 

[Dark-Fx]

48 cold. Should keep it at that through the season.

 

[r1sss]

Thanks for the quick replies but just for my knowledge, why it doesn't matter what is the load of the vehicle? 2 adults vs 7 adults and a lot of luggage - is it really negligible?
I recall until a few years ago most of the cars came with different tire pressure recommendation for partial or full load but maybe it is a thing of the past?

 

[ndmiller]

48 cold provides enough air pressure in the tires to support the weight of the truck and allow the tires to perform at their rated levels. For example the OEM Pirelli is A Traction B Temperature 116 Load rating etc (Max Load 2756 @ max pressure of 51psi specs off tire rack).

• Size: 275/65R20 116H
• RIV Rivian, Elect
• Style: Blackwall
• Eco Focus: ELECT
• Electric Vehicle Tuned
• Serv. Desc: 116HLoad Index 116= 2,756 lbs (1,250 kg) per tire
  Speed Rating “H”= 130 mph (210 kph)"
• UTQG: 640 A B Treadwear: 640
  Traction: A
  Temperature: B"
• 
  Severe Snow Service Rated

Tire pressure chart for more visual. Can't find the 116 one quickly sorry, but you get the idea. If an 85 load index tire is inflated to 36 PSI it can hold 1135 or 4540lbs for 4. Reducing pressure by 6 psi or add cold weather and load capacity drops to 3968 or 13% less. LT tires are different as their max pressure is 80psi with beefier and heavier construction, but they can never go below 35.

Air Pressure (psi)
Load Index	26	27	28	29	30	31	32	33	34	35	36
80	761	778	794	827	860	882	904	926	948	970	992
81	783	800	816	849	882	904	926	948	972	995	1,019
82	805	822	838	877	915	937	959	981	1,003	1,025	1,047
83	827	844	860	899	937	959	981	1,003	1,036	1,069	1,102
84	849	866	882	921	959	985	1,010	1,036	1,058	1,080	1,102
85	871	893	915	954	992	1,014	1,036	1,058	1,084	1,109	1,135

 

[r1sss]

48 cold provides enough air pressure in the tires to support the weight of the truck and allow the tires to perform at their rated levels. For example the OEM Pirelli is A Traction B Temperature 116 Load rating etc (Max Load 2756 @ max pressure of 51psi specs off tire rack).

• Size: 275/65R20 116H
• RIV Rivian, Elect
• Style: Blackwall
• Eco Focus: ELECT
• Electric Vehicle Tuned
• Serv. Desc: 116HLoad Index 116= 2,756 lbs (1,250 kg) per tire
  Speed Rating “H”= 130 mph (210 kph)"
• UTQG: 640 A B Treadwear: 640
  Traction: A
  Temperature: B"
• 
  Severe Snow Service Rated

Tire pressure chart for more visual. Can't find the 116 one quickly sorry, but you get the idea. If an 85 load index tire is inflated to 36 PSI it can hold 1135 or 4540lbs for 4. Reducing pressure by 6 psi or add cold weather and load capacity drops to 3968 or 13% less. LT tires are different as their max pressure is 80psi with beefier and heavier construction, but they can never go below 35.

Load Index	26	27	28	29	30	31	32	33	34	35	36
Air Pressure (psi)
80	761	778	794	827	860	882	904	926	948	970	992
81	783	800	816	849	882	904	926	948	972	995	1,019
82	805	822	838	877	915	937	959	981	1,003	1,025	1,047
83	827	844	860	899	937	959	981	1,003	1,036	1,069	1,102
84	849	866	882	921	959	985	1,010	1,036	1,058	1,080	1,102
85	871	893	915	954	992	1,014	1,036	1,058	1,084	1,109	1,135

Unfortunately, most of it doesn't mean anything to me
So you'd recommend to keep it 48 psi cold even when driving with very light load (1 adult)?
No expected issues with comfort or tread wear in such case?

 

[Tr4ckD4ys]

Uh… I’m running mine at 51psi cold on the gauges page… as I understand that is off by 2psi, meaning the 51 is really a 49. When warm, the tires go beyond 51, for sure. Most of the time, they run at around 53-55psi when warm.

 

[SANZC02]

Uh… I’m running mine at 51psi cold on the gauges page… as I understand that is off by 2psi, meaning the 51 is really a 49. When warm, the tires go beyond 51, for sure. Most of the time, they run at around 53-55psi when warm.

The max number is cold, the increased pressure during use is calculated in there.

 

[R1Thor]

Pressure is pressure... doesn't matter if there is no weight on the tires (as in: if you removed it from the vehicle and aired it up) or 100,000 lbs sitting on the tires. The change is in force, and force (in this case by way of mass) will dictate contact patch based on the pressure.

In fact, a fun science experiment is to measure the contact patch and use that to determine the actual weight of your vehicle when compared to the psi of the tires.

P = F/A (in this case, the area is cross section of the tire).

The ONLY reason to air down is to INCREASE contact patch (like others have mentioned, in soft terrain like mud or sand). Otherwise, the vehicle driving dynamics are engineered based on contact patch and friction, which ultimately dictates (as long as within the operating limits of the tires) the pressure they should be run at. Things like efficiency also play a role in this.

It's also a self-equalizer. That is: in a scenario where you might need more friction (like for lateral turns) because there's a higher load, the P= F/A, where P is constant means that more force acting on the tires will increase the contact patch for you, ergo you'll have more friction to compensate for the additional load. Physics is great!!

 

[NY_Rob]

I also just keep my 20's at 50PSI cause it's easier to see on my analog pressure gauge.

I fill with78.08% nitrogen too....

 

[jimmyb2]

LT tires are different as their max pressure is 80psi with beefier and heavier construction, but they can never go below 35.

I have run 80 psi rated tires on many different vehicles over the years, but have never heard this.

Please explain why, and what happens if you do go lower.

 

[ndmiller]

The lowest PSI that the Tire and Rim Association (TRA) recommends for LT tires is 35 PSI. That 35 will support a certain load, but even if you don't need to support that much of a load, it is still recommended to run 35 PSI.

Some important considerations: (1) LT tires are not designed for pressures less than 35 PSI. (a) Below 35 PSI, LT tires MAY rotate on the rim under load and result in unbalanced tires. Anyone who airs down to drive on sand has likely experienced this issue.

On pavement driving, the lower the pressure, the more deflection, and thus more heat buildup. There is a point at which the amount of heat produced is outside the parameters of what the tire was designed to handle and it will start deteriorating.

 

[Cycliste]

I found this

https://tirepressure.org/lt-metric-tire-load-inflation-chart

(I love tire pressure calculators for road bicycles.)

 

[Zoidz]

You can run a few pounds lower pressure, and you might notice a slightly softer ride and/or less road noise, but probably not. You will notice a reduction in range because the lower pressure increases the rolling resisitance. Here's an example for an ICE.

 

[RexRemus]

Alongside this subject has anyone found a good quick-connect tire chuck with a gauge and bleeder valve for the stock Rivian air hose?

I know you can set the PSI but I've found it to overshoot at least one tire every time I use it, sometimes by like 6psi. I'd like a quick easy way to bleed it off and watch the pressure vs having to drop it way down and try filling again

Sponsored