5%, 10%, 15% safety factor may be fine for drivers who carefully monitor tire pressures.
Car companies realize that lots of drivers do not carefully monitor tire pressures, so it is likely that their recommendations have much greater safety margins.
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Why 42 PSI ?
- Thread starter enemji
- Start date
Car companies realize that lots of drivers do not carefully monitor tire pressures, so it is likely that their recommendations have much greater safety margins.
Here is the additional info from below the calculator... note the highlighted bit:
Each tire is rated with a load range, load index number, and max pressure rating. Passenger tires will be rated Light Load (LL), Standard Load (SL), or Extra Load (XL). Extra Load tires of the same size will typically have the same load capacity at the same pressure as the Standard Load tire. However, they have added load capacity above certain pressures.
Light and Standard Load tires have a maximum capacity pressure rating of 35-36 psi and Extra load tires have a maximum capacity pressure rating of 41-42 psi. The maximum allowed pressure listed on a tire's sidewall can be higher however. Some tires list 44 psi and many higher speed tires will list 50-51 psi.
This added pressure does nothing for load capacity. Pressures above 35-36 psi for SL tires and 41-42 psi for XL tires will not increase the load capacity of the tire. Even if you put 50 psi in an SL tire the load capacity will remain what it is at 36 psi for the higher pressure levels.
Adding tire pressure above maximum load capacity is used mostly for high speed situations. A tires air pressure is increased to reduce tire deflection for the increase in rotations per second compared to typically city/highway use.
That's not quite correct. The reason why you want higher pressures for high speeds is to reduce deflections on the sidewalls, which reduces heat generation. Heat build up at very high speeds can cause tire failure.
Every racer knows if they want lower tire temp they can increase tire pressures and vice versa.
As for why 42psi, there are so many reasons and the forum will not be able to find out exactly why without a Tesla engineer chiming in. Rolling resistance, load capacity, tire wear, pot hole protection, ride quality, grip level in the wet, steering feel, safety margin plus more... are all factors.
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What I find most interesting, and what may (at least in part) help explain the 42-45psi debackle, is the info about the effect of tire pressure on effective load capacity and how that correlates with the load rating/index specs for a given tire.
Speaking strictly from a load capacity perspective, going below a particular pressure drops and will continue to drop the effective load capacity... while going above a particular pressure point will do nothing to increase the effective load capacity beyond the tire's load rating.
For OE size Model 3 tires, it appears that the max load capacity is first achieved at or close to 42psi.
Load Capacity for say 235/40R19 XL
@ 38 psi - 1,442 lb
@ 40 psi - 1,504 lb
@ 41 psi - 1,534 lb
@ 42 psi - 1,565 lb ('magic' threshold)
@ 43 psi - 1,565 lb
@ 45 psi - 1,565 lb
@ 50 psi - 1,565 lb
For our EV use-case, the benefit of exceeding the inflation pressure above the point where max load rating is first achieved and closer to max cold inflation pressure is beneficial only because of improved efficiency, and potentially a decrease in rolling resistance - at the cost of loss of traction, increased harshness, etc.
Speaking strictly from a load capacity perspective, going below a particular pressure drops and will continue to drop the effective load capacity... while going above a particular pressure point will do nothing to increase the effective load capacity beyond the tire's load rating.
For OE size Model 3 tires, it appears that the max load capacity is first achieved at or close to 42psi.
Load Capacity for say 235/40R19 XL
@ 38 psi - 1,442 lb
@ 40 psi - 1,504 lb
@ 41 psi - 1,534 lb
@ 42 psi - 1,565 lb ('magic' threshold)
@ 43 psi - 1,565 lb
@ 45 psi - 1,565 lb
@ 50 psi - 1,565 lb
For our EV use-case, the benefit of exceeding the inflation pressure above the point where max load rating is first achieved and closer to max cold inflation pressure is beneficial only because of improved efficiency, and potentially a decrease in rolling resistance - at the cost of loss of traction, increased harshness, etc.
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stopcrazypp
Well-Known Member
Yeah, this thread seems to be assuming a tire pressure is only chosen for a given load capacity (and then is adjusted to keep load capacity the same as the fixed variable) and nothing else plays a factor, when there are so many variables. They could even be keeping a 42 psi rating across the board simply to keep service easy without having to remember a bunch of different values.
That's a much better question with actual answers! Here, read this 37 page pdf for your answer!
Purely for load capacity, probably all the way down to 24 PSI for 18" and 32 PSI for 20" if your Model 3 is unloaded. But this isn't recommended of course, because that kind of PSI brings all sorts of other issues like pot hole protection, uneven grip level, sidewall folding over in corners, tires being too hot on highways etc etc...
Basically, the recommended PSI is a compromise for many reasons. You're probably safe to go +/- 3psi, but beyond that, other factors could come into play and ruin your day in extreme situations.
It's modeled pretty completely in the link I posted. You would be hard pressed to find a load configuration that requires more than 36-37 PSI.
stopcrazypp
Well-Known Member
That's a completely different question than your OP, it seemed you were asking why Tesla chose that value. Instead if you want use the lowest tire pressure possible for a given load capacity, others pointed out resources where you can calculate that for your specific tire and vehicle/load configuration. Obviously Tesla isn't choosing a value based solely on load capacity, as they compromise based on balancing various different other variables (for example as others mentioned they used to recommend 45 psi, but lowered it to 42 later, which many speculate is to improve comfort, even though 45 psi theoretically should result in lower rolling resistance).
"I always thought something was fundamentally wrong with the universe"
"I always thought something was fundamentally wrong with the universe"close ...
blackT3$!@
2021 LR Model 3
I replaced my MXM4's at 30k miles with Continental Extreme Contact DWS06's. They have 22k miles on them and have 50% life left. I did get my 3LR AWD aligned by Tesla after installing the new tires which revealed a bad alignment from the factory. Now the tires are wearing evenly.I wish I got 22k. I barely got 11k on my last set of PS4S. I switched to PS All-Seasons 4...hoping for at least 20K on these
Candleflame
Active Member
my sticker actually says 42 psi... and i could swear on delivery it was 45 and they put a new one over it.
I had mine aligned with the new tires. Alignment wasn’t that off. The 11k mile life was due to my driving…I take full responsibility
I’m hoping the PS a/s last longer. I hear the DWS06 are great tires though.
rpiotro
Active Member
A 2014 Infiniti Q50 hybrid? Pretty much instant acceleration. 0-60 in 5.0 flat. It was what gave me a taste of what electrons could do. ~4100 lbs. And before that a Pontiac G8 GT. 6.0 liter V-8 with a stump puller first gear. 0-60 in 5.1 ~4000 lbs. That thing really chewed up tires. Never got much more than 20K miles on a set.
What makes you think the tire manufacturer did not consider things like heat gain already? Why take their word as rule for things like wheel width requirements, then ignore their load calculations? Yes, the tire people say use the vehicle sticker, and there is a vehicle sticker by law, but consider which entity actually tests the tires and is ultimately responsible for meeting their suitability. If the sticker was the final say, there would be no need to for alternate sizes, no ability to get alternate sizes, etc.
I am not saying the sticker has no merit, but the sticker and forced tpms settings come from desires other than best ride and wear meeting load safety. Folks, search for Roger Marble’s tire posts on rv sites. Not only was he a career tire engineer (including examining failures) he was a racer. He explains how the load charts come about, heat gain, etc.
I am not saying run 26. I am saying the load charts have merit, likely more than any other source, and likely will not lead to failure.
comanchepilot
Member
I'm just gonna chime in and say that Tesla set the pressure for the least rolling resistance -
If you own a road bike - and this is a simple explanation - take it out and put the max pressure noted on the sidewall and go ride 10 miles. check your time - then - go ride the same route at the bottom recommended pressure - I'd be willing to bet your time will be longer at the lower pressure because of higher rolling resistance - its a real world easy test you can prove to yourself - and it will repeatable day after day to take into account you being stronger or weaker on any given day. . . .
If you own a road bike - and this is a simple explanation - take it out and put the max pressure noted on the sidewall and go ride 10 miles. check your time - then - go ride the same route at the bottom recommended pressure - I'd be willing to bet your time will be longer at the lower pressure because of higher rolling resistance - its a real world easy test you can prove to yourself - and it will repeatable day after day to take into account you being stronger or weaker on any given day. . . .
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