Tyre pressure tuning

[Dev]

Oem rear 86.
Fitted 90 xl.

Ok so at 86 load rating for the OEM comes to 1058 pounds for 32 psi. This is taken from the standard load chart.
 The new tire is 90XL so we need to look at the reinforced chart for what matches 1058 or closest to it for the new tire pressure. Surprise, surprise its 32psi. 

 Much of the confusion is SL and XL tires use different charts. 

 You cant simply go by some rule of thumb of adding and subtracting without knowing what kind of tires you are dealing with otherwise you can damage your tires or worse.

[Dev]

@Dev refers to a "Chalk test" in the link.
The only chalk / tyre combination I can remember is the numbered marks a coach driver would put on the front tyre of the coach, iirc the passengers put a small coin ( before decimalisation) into a hat, when the coach stopped at the " Half Way House" the mark corresponding to 12 o'clock indicated the winning seat number.
My guess for Dev's version would be a chalk mark across the width of the tread to indicate over inflation/ chalk missing from middle of tyre, under inflation / chalk missing from each side of tyre and chalk evenly worn would be just right inflation.
The same way as gauging correct tyre inflation  with tread wear but a lot quicker..

 Yes that is spot on. I would say doing the chalk test is not necessary for most people as long as the load ratings are matched however if someone wanted to confirm the chalk test can be a fast way of seeing it with your own eyes.
You can also do pyrometry using a cheap infrared thermometer. It doesn't have to be spot on but it should be within range.

For a lot of us including myself we have been conditioned to using factory air pressure but that is not how it should be done. It really depends on what tires being replaced and matching load ratings. I have heard in some countries they will not sell you  tires unless they are matched and now in mine there is an effort by tire shops to be trained in setting air pressures by load ratings. 

[Joesson]

@Dev refers to a "Chalk test" in the link.
The only chalk / tyre combination I can remember is the numbered marks a coach driver would put on the front tyre of the coach, iirc the passengers put a small coin ( before decimalisation) into a hat, when the coach stopped at the " Half Way House" the mark corresponding to 12 o'clock indicated the winning seat number.
My guess for Dev's version would be a chalk mark across the width of the tread to indicate over inflation/ chalk missing from middle of tyre, under inflation / chalk missing from each side of tyre and chalk evenly worn would be just right inflation.
The same way as gauging correct tyre inflation  with tread wear but a lot quicker..

 Yes that is spot on. I would say doing the chalk test is not necessary for most people as long as the load ratings are matched however if someone wanted to confirm the chalk test can be a fast way of seeing it with your own eyes.
You can also do pyrometry using a cheap infrared thermometer. It doesn't have to be spot on but it should be within range.

For a lot of us including myself we have been conditioned to using factory air pressure but that is not how it should be done. It really depends on what tires being replaced and matching load ratings. I have heard in some countries they will not sell you  tires unless they are matched and now in mine there is an effort by tire shops to be trained in setting air pressures by load ratings. 

In my experience tyre fitters put some arbitrary pressure into tyres. The get out being that the receipt says to check tyres/ pressures within n period.
( As I have mentioned in another thread) In a similar way how is it that the air gun used to fit the wheel nuts is automatically adjusted to suit each and every vehicle that the shop works on!

[Ardent]

Don't start me on air guns. The answer is simple. They are all set to torque the living daylights out of everything. Unless it is hammered into next week, it's not tight enough.

First thing I do. Get home, crack the nuts and torque correctly.

[Dev]

@Dev refers to a "Chalk test" in the link.
The only chalk / tyre combination I can remember is the numbered marks a coach driver would put on the front tyre of the coach, iirc the passengers put a small coin ( before decimalisation) into a hat, when the coach stopped at the " Half Way House" the mark corresponding to 12 o'clock indicated the winning seat number.
My guess for Dev's version would be a chalk mark across the width of the tread to indicate over inflation/ chalk missing from middle of tyre, under inflation / chalk missing from each side of tyre and chalk evenly worn would be just right inflation.
The same way as gauging correct tyre inflation  with tread wear but a lot quicker..

 Yes that is spot on. I would say doing the chalk test is not necessary for most people as long as the load ratings are matched however if someone wanted to confirm the chalk test can be a fast way of seeing it with your own eyes.
You can also do pyrometry using a cheap infrared thermometer. It doesn't have to be spot on but it should be within range.

For a lot of us including myself we have been conditioned to using factory air pressure but that is not how it should be done. It really depends on what tires being replaced and matching load ratings. I have heard in some countries they will not sell you  tires unless they are matched and now in mine there is an effort by tire shops to be trained in setting air pressures by load ratings. 

In my experience tyre fitters put some arbitrary pressure into tyres. The get out being that the receipt says to check tyres/ pressures within n period.
( As I have mentioned in another thread) In a similar way how is it that the air gun used to fit the wheel nuts is automatically adjusted to suit each and every vehicle that the shop works on!

I read an article long ago regarding the tire pressure monitor systems they put in cars at the turn of the century. They said that a large majority of vehicular accidents could have been avoided if people properly inflated their cars tires. Now its mandatory that manufactures have warnings but it still doesn't address proper pressures.

  I use to take with me my torque wrench when fitting new tires at the shop. Now I take just the wheels.

[Ardent]

Oem rear 86.
Fitted 90 xl.

Ok so at 86 load rating for the OEM comes to 1058 pounds for 32 psi. This is taken from the standard load chart.
 The new tire is 90XL so we need to look at the reinforced chart for what matches 1058 or closest to it for the new tire pressure. Surprise, surprise its 32psi. 

 Much of the confusion is SL and XL tires use different charts. 

 You cant simply go by some rule of thumb of adding and subtracting without knowing what kind of tires you are dealing with otherwise you can damage your tires or worse.

It is a curious game of many moving parts.
Load wise.
The fronts match the oem. But does not feel the same as as the yokos.
The rears are 90 vs 86 oem, but the rear feels great.

Have been running stock pressures. And with the info provided above. I will be leaving the rears well alone. They feel right and seemingly, they are.
Today, tried the fronts with 27. Cannot say that improved anything. Will return to 26. Drive again. And then try 25.
Probably down to construction characteristics. I am not lacking any grip that for sure.
More a feel/behavioral thing.

[Dev]

Oem rear 86.
Fitted 90 xl.

Ok so at 86 load rating for the OEM comes to 1058 pounds for 32 psi. This is taken from the standard load chart.
 The new tire is 90XL so we need to look at the reinforced chart for what matches 1058 or closest to it for the new tire pressure. Surprise, surprise its 32psi. 

 Much of the confusion is SL and XL tires use different charts. 

 You cant simply go by some rule of thumb of adding and subtracting without knowing what kind of tires you are dealing with otherwise you can damage your tires or worse.

The rears are 90 vs 86 oem, but the rear feels great.

 I just want to highlight something you might be missing. 90XL is exactly the same as 86SL.
  Two different numbers but the same load rating for the same PSI. 
  XL tires has its own chart. 

 This often confuses people and it has taken me awhile to figure out long ago until I asked a large tire distributer to clarify. 
 The main difference is the XL tires are reinforced to hold more air which supports  higher loads but are weaker than SL tires for its respective load rating.

[Ardent]

I use to take with me my torque wrench when fitting new tires at the shop. Now I take just the wheels.

Then sort the pressures when you get home.

[Ardent]

I just want to highlight something you might be missing. 90XL is exactly the same as 86SL.
  Two different numbers but the same load rating for the same PSI. 
  XL tires has its own chart. 

 This often confuses people and it has taken me awhile to figure out long ago until I asked a large tire distributer to clarify. 
 The main difference is the XL tires are reinforced to hold more air which supports  higher loads but are weaker than SL tires for its respective load rating.

I love this place. everyday is a school day.
Until this thread, and the last few posts, no I did not know there were two charts/tables.
I just took load rating to be load rating. Did not treat XL vs non XL as having different values. (just construction)
Thanks for clearing that up.
Seems a bit like centigrade vs Fahrenheit. Same temp diff scale.

I will resist the temptation to be lazy and ask for a link to the tables. Little project for later.

[Dev]

I use to take with me my torque wrench when fitting new tires at the shop. Now I take just the wheels.

Then sort the pressures when you get home.

Yes. Its a much better experience this way. My car is on jack stands in my garage with no chance of anyone messing with it or the dangers of having it improperly lifted.
I drop off wheels with new tires and the shop does the rest on their own time without having to wait for it in the lobby. I drive off and go to a friends house and get a call when they are ready to be picked up.

The guy doing the work knows I mean business and  are going to do a good job with the mounting and balancing on my request because I am no ordinary customer that just hands over the keys and just drives away.

I recommend others to do the same if they have a second car, its much more convenient and less stressful.

[Dev]

I can post the charts. Although they are from Toyo tires they are universal. One chart is SL and the other is XL.

You cannot view this attachment.You cannot view this attachment.

[Dev]

I just want to highlight something you might be missing. 90XL is exactly the same as 86SL.
  Two different numbers but the same load rating for the same PSI. 
  XL tires has its own chart. 

 This often confuses people and it has taken me awhile to figure out long ago until I asked a large tire distributer to clarify. 
 The main difference is the XL tires are reinforced to hold more air which supports  higher loads but are weaker than SL tires for its respective load rating.

Seems a bit like centigrade vs Fahrenheit. Same temp diff scale.

Almost the same analogy but not quite.  There is a backwards rational for this if you want to get more confused.
It is because they try to match like for like between SL and XL for the consumer that is trying to match load rating blindly. 
 The difference is when you get the XL tire in the same load rating as an SL you have to put more air in it to carry the same load.
 
 How is that for confusion and a backwards way of thinking because the guy that is clueless will not know that he needs a higher inflation. 


So why do they do this thing with higher inflation of XL tires. It is because when people plus size their wheels (big wheel guys) they will need low profile tires that are weaker by nature  and therefore need to be reinforced to hold more air to carry the load. 

 Its kind of like having to use low compression pistons to run more boost to gain more power without detonation.  It doesn't make sense but it makes sense. 





 

[Ardent]

I can post the charts. Although they are from Toyo tires they are universal. One chart is SL and the other is XL.

You cannot view this attachment.You cannot view this attachment.

much appreciated.
I have (had) a load table. Just the one. But no mention if it is SL or XL. Big help that is.

[Petrus]

Its kind of like having to use low compression pistons to run more boost to gain more power without detonation.  It doesn't make sense but it makes sense. 
 

Nice one Dev.
Lóve it!
Thank you!!!

[Ardent]

So having printed out and looked over the tables kindly provided by @Dev I arrive at the following.

As Dev identified earlier, The OEM SL load index of 86 is  directly matched by the XL load index of 90. Pressure remains the same.
So what I thought might be the area to explore, isn't.

What has raised an eyebrow is the following.
OEM SL load index is 81. The tyres I have fitted are rated SL 82.
For the same 26 psi inflation. 81 = 783lbs 82 = 805lbs
That is a difference of +22.
Yesterday, I put 27psi in. And as noted above, did not improve things (handling) at all. The opposite in fact. Adding the 1psi now made a difference in load rating of +45. (783 vs 828)
But looking at the table. If I now go with the option I was going to take next and drop to 25. The difference is now -11. (783 vs 772) So perhaps 25.5. But will go 25 for a start just to see.

[Dev]

So having printed out and looked over the tables kindly provided by @Dev I arrive at the following.

As Dev identified earlier, The OEM SL load index of 86 is a directly matched by the XL load index of 90. Pressure remains the same.
So what I thought might be the area to explore, isn't.

What has raised an eyebrow is the following.
OEM SL load index is 81. The tyres I have fitted are rated SL 82.
For the same 26 psi inflation. 81 = 783lbs 82 = 805lbs
That is a difference of +22.
Yesterday, I put 27psi in. And as noted above, did not improve things at all. The opposite in fact. Adding the 1psi now made a difference in load rating of +45. (783 vs 828)
But looking at the table. If I now go with the option I was going to take next and drop to 25. The difference is now -11. (783 vs 772) So perhaps 25.5. But will go 25 for a start just to see.

 I would certainly try 25psi.  I bet it would feel right. 

  The interesting thing in your situation is you initially disregarded the what the SL chart was telling you for the 90XL on instincts alone which was a good move because you would have been under inflated and overtime you would have more heat in the tires and would have damaged them. Reading the chart the wrong way also has its consequences, the industry needs to fix this with a better rating system.
 
 I have found when I used the charts exactly as intended for my new set of tires they were perfect.
 Now that others that are reading this thread I hope the information will be disseminated to others that are asking about air pressure.  We should no longer be quoting the factory air pressures without knowing what tires they have.

[Ardent]

As it turns out. Tried this out on the daily.
Same sort of thing.
I fitted oem to the front. 100 sl.
Then checked the rears. 104 xl

X ref'd the tables and they balance out.
Happy days. Just good to know.

[Ardent]

I would certainly try 25psi.  I bet it would feel right.

And I can now report. It does. For me, 25 is where it's at. Today. Conditions, atmospheric  pressure, temp etc.
Best run I've had with this tyre combo.

[1979scotte]

I would certainly try 25psi.  I bet it would feel right.

And I can now report. It does. For me, 25 is where it's at. Today. Conditions, atmospheric  pressure, temp etc.
Best run I've had with this tyre combo.

Warmest weather I would imagine

[Dev]

 I just remembered one of the discussions I had with a tire engineer when I called one of the manufactures.

 The reason why there is so much confusion and two charts is primarily because in some countries they have laws against fitting tires have a lower load index than what came with the car from the factory.
 
 If the 90XL tire were to instead rate the tires on the SL scale which would be 86 load rating all would be fine in the world. 
 Its not done because you would be limited by the number of tires in the XL category that can cary the load if you were to inflate the tire to a higher pressure. Since they are reinforced they can take up to 6PSI more pressure than a comparable SL tire to carry more load. 

  In order to solve the confusion at tire shops and for inspection purposes they make it simple so they can clearly see that tire matches or exceeds the load rating of the OEM tires to be safe.   

The problem is most people will not fill the XL tires to the correct air pressure to carry the load.

[Ardent]

Duplicate

[Ardent]

I would certainly try 25psi.  I bet it would feel right.

And I can now report. It does. For me, 25 is where it's at. Today. Conditions, atmospheric  pressure, temp etc.
Best run I've had with this tyre combo.

Warmest weather I would imagine

I certainly acknowledge can have an influence. But temps have been consistent between yesterday and today here.
Car was just overall happier on 25 than 26 and much happier than at 27.

Which in away, sadly throws up the question. Did I previously spend unesseaesrily on the the AD08R's.
I am cornering the same on these as before. I do miss the ad08r. But one wonders just how far from their limit was I. Because I'm now where near the limit of the rainsports.
The context being road legal speeds on the public highway. I am still all over everything else like a rash in the twisties.
As a bonus, the rainsports are more usable, more of the time.
Have fully  opened the torque taps and no fuss or nonsense from the rear. Just grip n go.

[Dev]

 lets have some fun and see if anyone can answer this riddle based on what we have learned about load ratings.

  You are a tire shop manager and a young rice boy comes to your shop asking to fit tires on his newly acquired 18" wheels.

 His previous tires/wheels had the load rating of 86SL and now his new low profile tires that he brought for mounting on his new wheels  have a load rating of 86XL.
 
   A. Mount the tires.
   B. You advise him that its unsafe.
   C. Kick him out of the store for having too many stickers.

 Your answer with the reason. 

[Petrus]

Which in away, sadly throws up the question. Did I previously spend unesseaesrily on the the AD08R's.
I am cornering the same on these as before. I do miss the ad08r. But one wonders just how far from their limit was I. Because I'm now where near the limit of the rainsports.
The context being road legal speeds on the public highway. I am still all over everything else like a rash in the twisties.
As a bonus, the rainsports are more usable, more of the time.
Have fully  opened the torque taps and no fuss or nonsense from the rear. Just grip n go.

At legal speeds trough corners you will not come néar the limit of AD08Rs bar wintery conditions. You will thus have traction surplus for the unexpected or mistake. Oh and stop shorter if needed.

[Ardent]

Assuming width/profile etc the same.

If the 86sl tyre had an oem inflation of 28psi. To obtain the same with the 86xl. The yoof should be advised to inflate to 32psi.

Dunce cap and flame retardant pants at the ready.