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How to set your air pressure using the tire load rating chart .

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Posted by: @marsrock7

Well, done some reading and I haven't found evidence of many passenger vehicle folk who adjust tire pressures based on load index. 

I did find a second load index/pressure chart that supposedly was used by discount tire at some point. It is dated to 2005.

I also noticed in the Toyo file...

...there is this note:

Screenshot 20200504 181446~2

Both files provide multiple examples of changing to a different size tire with a different load rating but every single one of them only demonstrates an increase in pressure. Not one example of a decrease in pressure.

Then I found this calculator/site:

https://tiresize.com/pressure-calculator/

It's not perfect... It appears the specific load ratings that existed 15-20 years ago have been improved upon. 185/55/15 isn't 81SL, it's 82SL. 205/50/15 isn't 85SL, it's 86SL, etc.... But it gives you another ballpark. 

 

 I tried out the calculator and it works really well.  I was only off by one PSI for my rear tire calculation using the Toyo chart. Instead of 30psi for the rear it was calculated to 31psi. Fronts worked out to be 26psi and it was on the money but I will use 25psi since I have no spare and PS pump. 

A few more thoughts. I found OE Bridgestones load index. It’s 81SL for the front and 86SL for the rears. Off by about 30lbs and it’s probably made up in weight as the facelift cars weigh a tad more. 

 
Posted : May 5, 2020 2:47 am
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I think I am going to try 24F/30R on the MR2 and see how it feels. When I first out the new tires on the rear I had started with only 31, and did imagine they became a tad less sticky when I bumped up to 32.5. Hopefully dropping the front from 27 to 24 will make a difference with my understeer. The biggest annoyance I see ahead is I think my TPMS low pressure range only goes down to 24psi... I hear lots of beeping on cool mornings in my future...

I'm probably gonna drop my TSX from 32F/30R to 29F/28R to start and see how it feels. A bit skeptical of a 5psi drop all around so gonna start with half.... 

Edit: I'm willing to bet the recommendation to reduce tire pressure based on load index went out the window after the Firestone debacle. Nobody wants to get Firestone'd.

 
Posted : May 5, 2020 3:13 am
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I have had a friend have Firestone tire separate right in front of us.  It was his van and it happened just as he pulled into the neighborhood. This was when it was in the  news and we had a laugh about it. Those tires were just defective.  Keep in mind that the optimum tire pressures are not the bare minimum for safety. There is still some forgiveness if you run a little low. Before there was TPMS many commuters  were under inflated by a lot and tires still held up. 
Im also going to see what this does. Maybe this is one of the reasons I was never as satisfied with aftermarket tires compared to OEM.  

 
Posted : May 5, 2020 3:50 am
Petrus
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Thanks for the calculatoor Marsrock.

The rule of thump is that as the air chamber volume goes up, load capacity goes up.

As the pr goes up, load capacity goes up.

As load capacity goes up, tyre pressure máy go down.

As load goes down pressure shoúld go down.

 

Concerning weight, even helium will only be just over 1 gram per wheel.

 
Posted : May 5, 2020 8:03 am
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Posted by: @petrus

Thanks for the calculatoor Marsrock.

The rule of thump is that as the air chamber volume goes up, load capacity goes up.

As the pr goes up, load capacity goes up.

As load capacity goes up, tyre pressure máy go down.

As load goes down pressure shoúld go down.

 

Concerning weight, even helium will only be just over 1 gram per wheel.

Up to a certain point. 

If you add air pressure on an SL tire beyond a certain point 35-36PSI it will not increase the weight carrying capacity.  Therefore even if the max inflation is used (40PSI) there is no benefit.  

 For XL tires it seems to cap out at 41psi even though the max inflation is around 50psi. That is 9 PSI which is a huge safety margin. It is also just around 6psi difference between SL and XL where you can increase the load carrying capacity further.  This tells me that although air is what holds up the car it is the properties of the tires that ultimately determines the weight carrying capacity.  It also makes filling up beyond a certain point pointless keeping any rational of going dangerously close to the max inflation moot. Unfortunately Im sure there are many that go close to the max inflation pressure. I met an owner that did that. 

 

 

 
Posted : May 5, 2020 1:04 pm
NottaMiata
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Oh this explains a lot. I kept feeling rolling resistance and adding a few pounds to get rid of it. This helped but did not seem enough, so I kept going.

Once I got past 75 PSI, it got a lot worse. Not only was it a lot slower and the cornering just sucked, but the tires suddenly started making a "flop-flop-flop" sound. Now, no matter how much air I put in there, it just won't get any better. Help!

🐸, 2003, Electric Green Mica, (still !) not enough mods...

 
Posted : May 5, 2020 4:29 pm
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Posted by: @dev
Posted by: @petrus

Thanks for the calculatoor Marsrock.

The rule of thump is that as the air chamber volume goes up, load capacity goes up.

As the pr goes up, load capacity goes up.

As load capacity goes up, tyre pressure máy go down.

As load goes down pressure shoúld go down.

 

Concerning weight, even helium will only be just over 1 gram per wheel.

Up to a certain point. 

 

 

 

Obviously; I hope.

And thén there is the construction of the tread band.

As a rule of thumb though and the range of the Spyder... stiffer wall / more air volume / less weight = possible to go down a bit.

 

 
Posted : May 5, 2020 5:14 pm
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Adjusted and drove both cars today. Both felt better. 

The Toy is currently at 24/30 cold, turn it feels more crisp, understeer issue is not as bad, traction slightly improved. Rear break away is so much easier to correct... I grossly overcorrected the first time. 

The TSX is currently at 29/28 from 32/30, the charts recommend 27/25, I'm just not willing to dive that far down immediately. Similar improvement as above, but steering response and traction were greatly improved! I actually put this car through a cloverleaf faster than I have ever accomplished with the Toy! 

Might be time to put the G-meter back on my Torque dashboard. 

TPMS temps looked similar to before on both cars. Keep in mind these are external sensors so not accurate to what the internal tire temp actually is. In full sun heat of the day I saw 12-15° above ambient, and ~7° above ambient in the evening. Nothing different that I could tell. 

I also found it interesting, with these particular pressures I am currently using, both cars reached and held the recommended cold pressure temp while the tires were hot. 

All in all, very happy with initial results. Especially on the TSX... Now I'm really getting my money's worth out of these Conti DWS tires.

 
Posted : May 6, 2020 6:00 am
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 After reducing by a few PSI my car certainly feels better then the last drive. Im sure there is now a little more roll resistance which is fine as long as the contact patch is maximized. I couldn't be more pleased to actually feel more of the front end of the car.  

 
Posted : May 7, 2020 7:06 pm
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Tbh I am a bit surprised you guys don´t routine try out tyre pressures as an important part of tuning. The OEM spec is for average rider, average use, average circumstances. It´s like the camber. You think you need negative camber? Chances are tyre pressure is open for adjustment to you too.

It was THÉ first thing I did. When I got the car it was on very bad El Cheapo rubber, different front and rear to boot. On the way back home set the pressure to spec. Next day scared myself with the understeer and put 0.2 extra in up front. Etcetera. Have a well calibrated high pressure footpump at home. Works a treat.

Bought the car in spring so over the winter on cráp mismatched tyres got to know the car over the winter. Come spring opted for AD08Rs, tried tyres pressures and arrived at my preferred setting.

Hence the massive slap in the forehead moment that I had stopped messing with it despite adding lightness.

Faffing with tyre pressure is a fundamental part of suspension tuning as the air chamber is a fundamental part of the suspension.

Bummed that I can´t go out and try it now....

Btw. the AD08Rs are very good all year round save for near/sub zero wet mornings. So I take a bit more care on those few days. I will stick with these hoops as I am ever higher up on the learning curve and líke that. 

 

 

 
Posted : May 7, 2020 9:34 pm
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People do use air pressure for tuning. I would rather not. I do all of my tuning with my suspension settings and alignment. I want my tires to have the maximum amount of grip. I feel for a road car tire pressure adjustment is a corse way of making the tires worse to fix inherent problems with poor suspension setups and poor tire sizes choices. If there is understeer I will tune for it using my sway bar, alignment or burning down my bad suspension choices and starting over. My entire suspension is perfect, just the way I want it after a lot of trial. The only thing I want is to refocus it back if it drifts. After that I don’t want to mess with it. 
I have seen too many local friends never satisfied and always tinkering to the point where they end up selling and moving on. I’m fortunate that I got it right somewhere around 2008 and just want it to last.  

 
Posted : May 8, 2020 12:20 am
haloruler64
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No amount of suspension or car tuning can make the car perform right if your tire pressures are off. That being said, that's a track thing. Not a street thing. I have my pressures set to near OEM levels for the street, and tune them on the fly REAL low for autocross and track. On the front i have to run around 16 PSI to get my front tire to perform at its best. Full contact patch and some roll through turns. Completely eliminates all understeer. 

2000 Toyota MR2 Spyder, 2020 Mazda 3 Hatchback 6MT

 
Posted : May 8, 2020 4:53 am
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Posted by: @haloruler64

No amount of suspension or car tuning can make the car perform right if your tire pressures are off. That being said, that's a track thing. Not a street thing. I have my pressures set to near OEM levels for the street, and tune them on the fly REAL low for autocross and track. On the front i have to run around 16 PSI to get my front tire to perform at its best. Full contact patch and some roll through turns. Completely eliminates all understeer. 

Yup, it’s a track thing. From what I read the demands of the tires for track require adjustments to optimize the contact patch but it boils down to the same thing of optimizing the grip to compensate for heat that you don’t see on the street. 
Some street drivers do foolish things like overfill their tires for more response or under or overfill to change the behavior of the car by reducing the contact patch. It might work but this is not the way to do it. There are better solutions through other choices but if you have to diminish a tires contact patch to fix behavior then that is a clear indicator that bad choices were made that got you to this point. 

 
Posted : May 8, 2020 11:41 am
Petrus
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Posted by: @haloruler64

No amount of suspension or car tuning can make the car perform right if your tire pressures are off. That being said, that's a track thing. Not a street thing. 

 

I beg to differ as explained.

That being sad most users best simply stick to OEM specs for both pressure AND THE TYRES. My point was and is that a great many mess with different spec tyres, different size rims even and thus should adjust the pressure.

Which was the topic of this thread btw..... 😋 

 
Posted : May 8, 2020 12:09 pm
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I have read this article the other day and although its primarily  for the track, Im planing to use it to adjust my tire pressures. My starting point is based on getting me close using the load rating chart and then I will chalk my tires. Although he mentions not to use an infrared thermometer I did yesterday and found some interesting results that correspond to camber. But notice that he mentions correct pressure and maximum contact patch. Also notice that its not just about adjusting tire pressure. Its about the system as a whole which is what I have been emphasizing. If you don't have your pressures right you can forget about the rest.  This is where the majority get it wrong because they are only concentrating on one corse correction. If you have to adjust tire pressures to minimize grip to fix over or under steer you are doing it wrong.  

Once you have the tires working at the correct pressure and maximum contact patch, you can look at making adjustments in alignment, sway bars, shocks, springs, etc.

https://www.turnology.com/tech-stories/brakes-suspension/speed-secrets-with-kenny-brown-setting-tire-pressure-for-the-track/

 
Posted : May 8, 2020 1:58 pm
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