Tire Pressure

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Originally Posted By: Craig in Canada
Capri:

My car is a 1998 528i with M Sport package. My dad's is a 2002 540iA with standard suspension. Both are/were also sold in the US as is.........


According to Tire Guides, the vehicle tire placard on your Dad's car with 225/55R16 95H should say 35 / 42 psi. Please keep in mind that Tire Guides only lists one pressure, but there might be 2 listed on the placard.

Originally Posted By: Craig in Canada
.......I just ran and checked my original factory spare which matched the tires on the car at delivery. Dunlop SP Sport 2000E 235/45 ZR17 94Y. The PS2s I'm currently running are also 94Y..........


Well, this would not be the first time that Tire Guides has been wrong. However, there are a couple of related items: It could be there is a difference between what was offered in the US and Canada. Plus Tire Guides doesn't particlarly delinate your vehicle as an M model - and there may be a difference there as well.

Originally Posted By: Craig in Canada
.......Other tires I have run in this size had slightly different load ratings (Toyo Proxes T1-S and T1R). Of course I don't need to tell you that we don't get much choice when it comes to load rating..........


There are some fundamentals at work here. Please read this and see if that doesn't help explain things:

http://www.barrystiretech.com/tirestandardizingorgs.html

You'll also need to read this:

http://www.barrystiretech.com/loadtables.html

Short Version:

Tire size pretty much equates to load carrying capacity within a type of tire. In the case of passenger car tires, there are 2 type: Standard Load (SL) and Extra Load (XL) with XL's requiring more inflation pressure to get the additional load carrying capacity. An important item of note is that the load carrying capacity of an XL tire is the same as an SL at SL level pressures.

Originally Posted By: Craig in Canada
.......So, if I understand correctly, you're saying that to go to the smaller tires the pressures actually go down?..........


Well, it depends on what you mean by "smaller".

When I use the word, I mean "smaller load carrying capacity", but you obviously mean "smaller rim diameter" and in your case, there is not enough information contained in the way you say that for your statement to be true.

For example a 195/55R16 has a Load Index of 86, but a 235/55R16 has a Load Index of 96. Rim diameter is not enough to define load carrying capacity.

To carry the same load, tires with lower Load Indices need to use more pressure and vice versa - and that's what my calculation was expressing.

Originally Posted By: Craig in Canada
.......Yes they have higher load rating (by 1 or 2) but they also have a different contact patch and the weight of the car isn't changing... Is that just for my current 99XL rated Hakkas? What about tires around the more typical 95 load rating?.......


It's a bit complex here, but trying to gauge the size of a contact patch has more variables than just tire size, inflation pressure and Load Index. In fact, I normally say you can NOT determine the size of the contact unless you actually measure it - and if you change even one variable, you have to measure for each change.

Originally Posted By: Craig in Canada
.......I'm shocked that the equivalent would be 26psi up front. While I don't have my dad's placard here, I know that's not what it says
smile.gif
..........


Yes, that is not what it says - which is why I postponed discussing it until you had a chance to post a reply.

There's a lot more involved in setting tire pressures than just load carrying capacity. But we haven't even discussed what small changes in the vehicle from year to year might result in - even though they are the same model vehicle.

Originally Posted By: Craig in Canada
....... You can probably look it up faster than I can find out, but it's something more like a 33/35 split......


I mentioned it above, but you need to look at it to see if there is a second set of pressures.


Originally Posted By: Craig in Canada
.......When dealing with some issues with my Toyo T1-S a number of years ago which involved their regional manager, I ran sticker pressures of 29/33 for a little while to satisfy them. It's a smoother ride, but soft for my taste. I also felt vulnerable to wheel damage from potholes etc... running less than 30 up front. I'm usually running 3-4 psi higher than placard but maintaining the split. I found that my Dunlop M3s were very sensitive to pressure and the only way I could even come close to balancing the car was 34/34. If I even went 34/35 the rear started stepping out in fairly normal urban left turns. I don't know my Hakkas well enough to feel any difference in tuning yet. I know that the high load rating seems to have given them a stiffer carcass than a lot of other winter tires. The tread is still squirmy but the tire is rigid. I think I filled them to 34/36 "cold" in my garage, which wasn't as cold as outside. Perhaps I should lower them...

Thanks for taking the time to muddle through this.... I see three sides to things:

1/ load bearing
2/ tire "spring rate"
3/ tuning of the contact patch - setting a pressure which results in even wear and full utilization of the full width of the tread when pushing



There's actually more and you touched on one - personal preference.

So when someone says "What inflation pressure should I use?" I assume they know nothing about tires and always reference the tire placard. I would never recommend someone use something less than the placard (if they are using the original tire size.)

But when we start to talk about handling and how to better tune a vehicle's behavior to suit one's personal tastes - well, that's a whole differrent kettle of fish.
 
Originally Posted By: CapriRacer


According to Tire Guides, the vehicle tire placard on your Dad's car with 225/55R16 95H should say 35 / 42 psi. Please keep in mind that Tire Guides only lists one pressure, but there might be 2 listed on the placard.


That would be the max weight pressures, if anything. The "normal" is higher than my normal (29/33) IIRC though.

Quote:


There are some fundamentals at work here. Please read this and see if that doesn't help explain things:

Will read...
Quote:


Short Version:

Tire size pretty much equates to load carrying capacity within a type of tire. In the case of passenger car tires, there are 2 type: Standard Load (SL) and Extra Load (XL) with XL's requiring more inflation pressure to get the additional load carrying capacity. An important item of note is that the load carrying capacity of an XL tire is the same as an SL at SL level pressures.


Now here you're just referring to whether it carries "XL" or not, right? Not whether a given tire is 93 or 94, for instance?

Quote:

Well, it depends on what you mean by "smaller".

When I use the word, I mean "smaller load carrying capacity", but you obviously mean "smaller rim diameter" and in your case, there is not enough information contained in the way you say that for your statement to be true.


I didn't mean smaller rim diameter, actually, although the 225/55/16 is smaller in every way except circumference. I was thinking more about the reduction in width. Yes, it's only 10mm on paper, but in practice there's a heck of a difference in appearance between the two sizes. There's also a 1" difference in wheel width (16x7J vs. 17x8J). The E39 chassis also comes standard with a couple of staggered options with 18x8J front and 18x9J rear. Same weight, same suspension.

Quote:

To carry the same load, tires with lower Load Indices need to use more pressure and vice versa - and that's what my calculation was expressing.

So does a load index (I notice that it is an "index", not a "rating") relate strictly to the ratio of air:carcass which provides the support, or does it also directly determine the "max load" of a tire? Thus, as you say, for load purposes my 99 index tires need less air than 95s of the same size?

I guess to ask another related question - your formula resulting in a "load bearing" pressure of 26psi front - did that depend on the 99XL rating? If yes, what would the pressures be for 95 (more common in this size)?

Quote:

Originally Posted By: Craig in Canada
.......Yes they have higher load rating (by 1 or 2) but they also have a different contact patch and the weight of the car isn't changing... Is that just for my current 99XL rated Hakkas? What about tires around the more typical 95 load rating?.......


It's a bit complex here, but trying to gauge the size of a contact patch has more variables than just tire size, inflation pressure and Load Index. In fact, I normally say you can NOT determine the size of the contact unless you actually measure it - and if you change even one variable, you have to measure for each change.


I know that I've usually boiled contact patch down to the simple math of the air pressure. # of pounds on that tire / inflation pressure in psi = square inches of contact patch. Obviously that doesn't tell you the shape. Even alignment settings factor into that, plus the contact patch shape changes constantly depending on what you're doing with the car.

Quote:

Originally Posted By: Craig in Canada
.......I'm shocked that the equivalent would be 26psi up front. While I don't have my dad's placard here, I know that's not what it says
smile.gif
..........


Yes, that is not what it says - which is why I postponed discussing it until you had a chance to post a reply.

There's a lot more involved in setting tire pressures than just load carrying capacity. But we haven't even discussed what small changes in the vehicle from year to year might result in - even though they are the same model vehicle.

Originally Posted By: Craig in Canada
....... You can probably look it up faster than I can find out, but it's something more like a 33/35 split......


I mentioned it above, but you need to look at it to see if there is a second set of pressures.


Yes, there are two sets of pressures. I can't remember them but I do know that the 42psi rear rating must be the one for max vehicle weight.

Quote:

There's actually more and you touched on one - personal preference.

So when someone says "What inflation pressure should I use?" I assume they know nothing about tires and always reference the tire placard. I would never recommend someone use something less than the placard (if they are using the original tire size.)

But when we start to talk about handling and how to better tune a vehicle's behavior to suit one's personal tastes - well, that's a whole differrent kettle of fish.


Agreed. Although I'm quite interested in what my starting points should be for a different size tire. I could always start with the placard from my dad's car which is pretty similar and came from the factory with the size I use in the winter, but it's interesting to "do the math" and also solve for results based on my placard and original size.

I wonder why the 225/55/16 factory placard differs so much from the extrapolation from the 235/45/17 placard? I guess, clearly, no one likes driving around with 26 psi in the front despite the fact that it might support the load? This is a fairly stout, front engine car - I'm surprised that 26psi wouldn't be formally considered underinflated, frankly, driver preferences aside. I'm not questioning your calculations or table - just discussing.

Thanks,
 
Originally Posted By: Craig in Canada
Originally Posted By: CapriRacer
....
There are some fundamentals at work here. Please read this and see if that doesn't help explain things:

Will read...


It's obvious that this posting would be quite different if the links had been read BEFORE the posting was written.

So I'll wait until you have read them and give you a chance to revise your questions based on what you learn from the links.
 
Craig,

Enough time has passed that I am sure you've read the links I provided, so I will not go over what is contained in them. That should shorten the post considerably (I've noticed my explanations tend to be really, really long.)
Originally Posted By: Craig in Canada
.......Now here you're just referring to whether it carries "XL" or not, right? Not whether a given tire is 93 or 94, for instance?


Correct

Quote:
........I didn't mean smaller rim diameter, actually, although the 225/55/16 is smaller in every way except circumference. I was thinking more about the reduction in width.......


I think you will also find that compared to a 245/45R17, a 225/55R16 is larger in section height (sidewall height) as well and that is due to the aspect ratio. That results in a slightly larger air chamber volume and therefore a slightly larger load carrying capacity (Load Index).

Quote:
.....So does a load index (I notice that it is an "index", not a "rating") relate strictly to the ratio of air:carcass which provides the support, or does it also directly determine the "max load" of a tire? ...........


Load Index relates directly to the air chamber volume, not the ratio of tire to air. Load Index IS max load - or stated better, is a way to express the nmaximum load carrying capacity.

Quote:
........ Thus, as you say, for load purposes my 99 index tires need less air than 95s of the same size?........


You've got to be careful here. Your 99LI tires were Extra Load (XL) and that changes things. Your 95 LI tires were Standard Load (SL). What that means is your 99LI tires can carry more load, but they only do that if the inflation pressure is also elevated. If you use the same inflation pressure, your 99LI and 95LI carry the same load (rememember, we are talking the same size passenger car tire!)

Quote:
.......I guess to ask another related question - your formula resulting in a "load bearing" pressure of 26psi front - did that depend on the 99XL rating?..........


No, it depended on the size, not that fact that the tire was a 99LI (XL). The 99LI in an XL means that I can utilize the increased capacity if I use more inflation pressure.

Originally Posted By: Craig in Canada
.......Yes they have higher load rating (by 1 or 2) but they also have a different contact patch and the weight of the car isn't changing...........


If you read the links I provded you should realize that the differences between the tire standardizing organizations results in a slight difference. It's not a difference in how tires do what they do (Physics), it's in the way it is described mathematically. All the orangizations agree on how tires work, but they've come up with slightly different ways of expressing it. Some of this is differences in the formulae used. Some of it is differences in round off. Some of it is difference in measurement systems (English and metric).

For practical purposes, they can be treated as though those differences don't exist.

Quote:
.......I know that I've usually boiled contact patch down to the simple math of the air pressure........


It's worse than that:

Quote:
.... # of pounds on that tire / inflation pressure in psi = square inches of contact patch.......


is not true.

Quote:
......I wonder why the 225/55/16 factory placard differs so much from the extrapolation from the 235/45/17 placard?.......


There are also difference between model years - and cars are constantly being tweaked - a slight difference in spring rate, differences in shock damping, etc. So trying to make sense of what the vehicle engineer's change becomes like trying to read a mystery with the front half of the book missing.
 
Originally Posted By: bobbob
Billy Bob says to put in 44 PSI because it says 44 max load
on the side of the tire. I tell him that is only if the
vehicle is Heavily Loaded!! I want to put in what it says
in the owners manual. Who is right?
Both of you are right. You can go up to the max inflation psi. Granted if the max psi is put in the tires, but you don't have the vehicle loaded to warrant it, the tires will wear out in the center fairly quickly. Some people run max psi to help with MPG because it decreases rolling resistance at the expense of shorter tire life and a rougher ride. Personally, start out with the sticker psi and adjust from there depending on how you drive and load your vehicle. Most people will end up with a air psi that falls somewhere in between the min and max psi.

All this gets thrown out the window if you go with differently sized tires than OEM spec because the sticker is for OEM spec sized tires. In that case, you have to experiment with different air psi settings until you find the one that works.
 
I will go with owner manual. my owner manual say to put 29 psi.. I put 33 psi in the winter. I check my tire pressure every 2 weeks.

craftsman digital tire pressure guage work perfect.
 
It's strange how everyone uses different numbers.

Just to add to the mix:

I run 38 PSI in the Accord and 39 PSI in my truck.

I have a friend that swears by overinflating the front, but underinflating the rear. No idea why?
 
This is a very interesting topic, and I myself have completed numerous tests using various brand of radial performance tyres (on 55 and 50 aspect ratio, and some with higher load index and reinforced).

So far what I found, that nowadays most tyres need at least 32 psi to perform well during cornering, except for ones with very hard sidewall.

During summer, the same hot pressure can be achieved using lower cold pressure to give slightly better grip, but this may cause problem during rainy days where the hot and cold pressure difference may only about 1 psi.

So for me, sacrifice a bit of comfort when the tyre is cold, I will set at least 32 psi on my tire and move up until there is no more traction gained or the ride become too uncomfortable.
Of course during winter, it will be different, it may be worth to try to increase the pressure until near maximum allowable pressure if the tire is not winter tyres.
 
Comfort is one issue, but so is MPG, safety, wear pattern, if the vehicle is FWD/RWD/AWD, etc. I don't know if there is a wrong answer unless it becomes a safety issue.
 
If you have "stock" tires on the vehicle, by all means go by the door placard.

BUT, if you have aftermarket wheels/rims, go by a combination of the sidewall, and the door placard. lol. Lesson learned there.

A friend of mine purchased 2 tires for his 94 Honda Accord right before a road trip....they mounted the 2 new tires on the rear, since he needed an alignment severly....(still does....lol...) - anyways, half way to Tampa, he hit one of those reflectors in the middle of the road, and THUMP, lol, we go [censored] was that, so we slow down and pull to the side of the road, and find the tire came off the bead going 75 mph. Mounted the spare tire, and headed back home.

The next morning on our way back to WM, hit another reflector upon lane change, THUMP, there goes that tire.....so we cramled to WM going 20 mph with the hazards on....no other "spare" to mount.



Turns out, they only put 32 psi in the "low profile" tires that were mounted on the vehicle (Goodyear Viva Authority Fuel Max), and the side wall of the tire calls for 52 if I recall....basically what happened (my theory anyways...) since the tires were so underinflated the rear tires were riding on the sidewall....rather than a "full" contact with the road.

So, anyways, they did fix the problem, got him 2 new tires since it wasn't even 24 hours since he bought the tires....but the one tire's inside had slices from apparantly running on the sidewall/rim contact....thanks to their 90 day/3k mile warranty
smile.gif


Long story short, experiment with the PSI. Personally, I'd start at the "MAX" psi and work my way down to what's comfortable....and remember it's "cold" psi - having not driven the vehicle for 3 hours, or over 1 mile. Or preferably in the early morning before the sun heats up the rubber - if you really want to get technical...lol.

I generally keep my tires at 38 psi. Max cold PSI is 40; and my car doesn't have a placard :P
 
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