Proper Tire Pressure

[sgt3966]

I am buying a set of Wheels and Snow Tires. It was suggested that the Snow tires be the same size all around as opposed to what is on the car now. The tires that are on the car now are the stock tires and wheels ( Front: 225/40 R 18, 36 PSI Rear: 255/35 R 18, 42 PSI). I was told to go with 225/40 R 18's all around. I will be getting:Blizzak LM-60 RFT - Size: 225/40R18 (Run-Flats). My question is since the different size stock tires call for different tire pressures, what would be the proper tire pressure for these snow tires? I was thinking that since the front tires, which are the same size as the snow's that I am getting (225/40 R 18) call for a tire pressure of 36 PSI, that I should go with that pressure. What do you think? FYI. Car is : 2015 Z4 35i (M Package).

[nyalpine90]

36PSI

[02Pilot]

Too many variables to make a blanket recommendation. When you start changing sizes you will probably have to do a little experimenting to figure out where they like to be. Start with your baseline and observe. You want the tread to be sitting squarely, so look for evidence of sidewall contact (too soft) or lack of outer tread block contact (too hard).

If you want clearer feedback than just eyeballing the tire for contact, try the old autocrosser's trick of putting a couple white shoe polish marks on the sidewall to see if the tire's rolling over in corners.

[Maynard]

Softer is better for winter (larger patch, more compliant). And check the manual, if you haven't already. Mine lists pressures for 225/40 r18 M&S that are also staggered (32f/38r), even though tires are same size. I personally run higher pressures in front for summer (35/38), and more even winters at 33-35 all around - but I'm in a 228.

[02Pilot]

Originally Posted by Maynard

Softer is better for winter (larger patch, more compliant).

This is debatable. The size of the contact patch doesn't really change (the weight of the car is unchanged), but the shape does (minimally). When dealing with snow (at least more than a couple inches of it), handling benefits from a contact patch with a narrower frontal aspect. This allows the tire to cut through the snow more easily, while still maintaining grip along the directional axis. Wider tires will have a wider contact patch (good for lateral grip) that tends to cause them to want to go over the snow rather than through it.

For compliance, it's preferable to minus size the wheels to allow for a taller sidewall. Take a look at rally cars set up for snow or ice racers and you'll see a lot of really tall, skinny tires.

[nyalpine90]

since your winter tires are same size on all 4, its same size thats indicated on door panel it should be 36psi.

[Viffermike]

Originally Posted by 02Pilot

This is debatable. The size of the contact patch doesn't really change (the weight of the car is unchanged), but the shape does (minimally). When dealing with snow (at least more than a couple inches of it), handling benefits from a contact patch with a narrower frontal aspect. This allows the tire to cut through the snow more easily, while still maintaining grip along the directional axis. Wider tires will have a wider contact patch (good for lateral grip) that tends to cause them to want to go over the snow rather than through it.

For compliance, it's preferable to minus size the wheels to allow for a taller sidewall. Take a look at rally cars set up for snow or ice racers and you'll see a lot of really tall, skinny tires.

This, plus one other, relatively minor factor that matters a little more for BMWs if the wheels are not minused: the metallurgic integrity of the alloy in the wheels.

Just like almost any other material, alloys become more brittle as the temperature lowers. If you don't use a tire with a taller sidewall for compliance, the risk of bending or cracking a wheel is greater at lower temps -- and cracks are generally more likely. This risk isn't quite the same today as it was a generation ago, when steel was preferred for winter setups partially because it doesn't lose much integrity at extremely low temps. But it is still there.

[sgt3966]

THANKS! You guys are the best. It looks like 36 PSI is the winner. Some points that you guys mentioned and I also read somewhere[I]"The size of the contact patch doesn't really change (the weight of the car is unchanged), but the shape does (minimally). When dealing with snow (at least more than a couple inches of it), handling benefits from a contact patch with a narrower frontal aspect. This allows the tire to cut through the snow more easily, while still maintaining grip along the directional axis. Wider tires will have a wider contact patch (good for lateral grip) that tends to cause them to want to go over the snow rather than through it. For compliance, it's preferable to minus size the wheels to allow for a taller sidewall. Take a look at rally cars set up for snow or ice racers and you'll see a lot of really tall, skinny tires" [/I]was the reason that I why with the FRONT tire size.

Also like the answer " And check the manual, if you haven't already. Mine lists pressures for 225/40 r18 M&S". Never thought of that. I will check that out.

[Maynard]

Originally Posted by 02Pilot

This is debatable. The size of the contact patch doesn't really change (the weight of the car is unchanged), but the shape does (minimally). ......

Actually, contact patch size is a function of weight / psi = contact area, so lowering pressure DOES enlarge the contact patch. But on street cars this is probably too small a diff to matter - I think in winter the increased compliance helps more. You are right that shape matters most and narrower is better (for snow/rain), but I was assuming you were going to narrower wheels; I doubt a few lbs of air pressure would make a noticeable diff in width. If you have the option, go with the narrowest wheel that you can fit with a tire that is at the mid-size of it's fitment range (w/o going out of recc's for load limits or diameter).

[sgt3966]

Thanks again, while I am on the subject of tires and pressure, I also will be putting on new TPMS Sensors on the rims. I am assuming the 'System" will need to be "Reset"? Is that something I can do or do I need to take it to the dealership?

[WingZeroX5]

Originally Posted by sgt3966

Thanks again, while I am on the subject of tires and pressure, I also will be putting on new TPMS Sensors on the rims. I am assuming the 'System" will need to be "Reset"? Is that something I can do or do I need to take it to the dealership?

if you have idrive, go into the car info section. while your car is stopped, hit the TPMS reset option and drive off. System should auto relearn the new TPMS. If not, hold the button near the middle of the central vents

[sgt3966]

Originally Posted by WingZeroX5

if you have idrive, go into the car info section. while your car is stopped, hit the TPMS reset option and drive off. System should auto relearn the new TPMS. If not, hold the button near the middle of the central vents

THANKS!!!!

[cxp213]

I was gonna chime in but 02pilot pretty much hit the nail on the head so no need to repeat.

Originally Posted by Maynard

Actually, contact patch size is a function of weight / psi = contact area, so lowering pressure DOES enlarge the contact patch. But on street cars this is probably too small a diff to matter - I think in winter the increased compliance helps more. You are right that shape matters most and narrower is better (for snow/rain), but I was assuming you were going to narrower wheels; I doubt a few lbs of air pressure would make a noticeable diff in width. If you have the option, go with the narrowest wheel that you can fit with a tire that is at the mid-size of it's fitment range (w/o going out of recc's for load limits or diameter).

You're right, but larger contact patch at low mu conditions isn't what you want. You want more pressure per square inch so you can get past the water/snow/sleet on the ground and find traction. By having a wider contact patch, you're spreading that load across a wider area thereby lessening grip. That's why he was suggesting narrower tires.

[Maynard]

Originally Posted by cxp213

....You're right, but larger contact patch at low mu conditions isn't what you want. You want more pressure per square inch so you can get past the water/snow/sleet on the ground and find traction. By having a wider contact patch, you're spreading that load across a wider area thereby lessening grip. That's why he was suggesting narrower tires.

Had not thought of it this way, but this does make a lot of sense - these mechanical issues subvert the simple physics (a theoretically better or larger patch doesn't matter if it doesn't get through the snow to make true contact). Wonder if TR has any empirical testing on this, especially to see how much of a difference it would make.

[MightyMouseTech]

Originally Posted by cxp213

I was gonna chime in but 02pilot pretty much hit the nail on the head so no need to repeat.


You're right, but larger contact patch at low mu conditions isn't what you want. You want more pressure per square inch so you can get past the water/snow/sleet on the ground and find traction. By having a wider contact patch, you're spreading that load across a wider area thereby lessening grip. That's why he was suggesting narrower tires.

Exactly. On ice, you want a smaller contact patch. Just like going from a snow shoe that floats to a spike that will dig in (that is an extreme example of course).

Of course, lower pressures also tend to help acceleration with more sidewall flex.

Just like everything else, there are trade offs. The load rating of the tire will also make a difference.

Start at factory specs and play around with it and see what you like. I ended up running 36/36 on my 1er with 205/50/17 R2.

[sgt3966]

Originally Posted by MightyMouseTech

Exactly. On ice, you want a smaller contact patch. Just like going from a snow shoe that floats to a spike that will dig in (that is an extreme example of course).

Of course, lower pressures also tend to help acceleration with more sidewall flex.

Just like everything else, there are trade offs. The load rating of the tire will also make a difference.

Start at factory specs and play around with it and see what you like. I ended up running 36/36 on my 1er with 205/50/17 R2.

Thanks