BMWCurves
01-21-2016, 11:41 AM
After a somewhat lengthy discussion on tire inflation over in The Random E46/ZHP Question Thread (http://www.zhpmafia.com/forums/showthread.php?11957-The-Random-E46-ZHP-Question-Thread&p=468901#post468901) and doing some cursory research, especially in this old Mafia thread on tire pressure (http://www.zhpmafia.com/forums/showthread.php?1579-What-is-the-recommended-tire-air-pressure-for-the-E46-BMW-330-ZHP), I figured I would try and create a central thread on tire inflation info.
First, why is proper tire inflation is important?
To answer this important question, I turn to Tire Rack's expertise:
Advantages of Correct Tire Inflation
Maintaining correct tire inflation pressure helps optimize tire performance and fuel economy. Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles. Tire deflection (the tread and sidewall flexing where the tread comes into contact with the road) will remain as originally designed and excessive sidewall flexing and tread squirm will be avoided. Heat buildup will be managed and rolling resistance will be appropriate. Proper tire inflation pressure also stabilizes the tire's structure, blending the tire's responsiveness, traction and handling.
Disadvantages of Underinflation
An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.
Disadvantages of Overinflation
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
Source: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1
When should I check my tire pressure?
Air is a gas, and gas expands and contracts with temperature. Note: this isn't technically correct since gas fills the volume of any vessel it's contained in, but the pressure of a gas does increase with increasing temperature if volume stays the same. As a result, the pressure in your tires in the morning will be lower than after a long drive in the sun.
So when should you check my tire pressure?
According to Tire Rack, you should test your tire pressure in the morning, before you drive. This is so the air in tires have had sufficient time to reach ambient air temperature if the car has been driven the previous day, and the air be at the lowest temperature for the day.
Tire Rack has some other interesting information in their article, discussing ambient air temperature, heat due to driving, effects of filling your tires with air at different times of the day. The article is linked below. I'll include some of the important excerpts:
"The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle's recommended cold tire inflation pressure. This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it."
"In most parts of North America, the difference between average summer and winter temperatures is about -50° Fahrenheit...which results in a potential loss of about 5 psi as winter's temperatures set in. And a 5 psi loss is enough to sacrifice handling, traction, and durability!"
"As the day's temperatures went from 67° to 85° Fahrenheit, the tire that was kept in the shade went from our starting pressure of 35 psi to a high of 36.5 psi. The tire that was placed in the sun and subject to the increase in ambient temperature plus the sun's radiant heat went from our starting pressure of 35 psi to a high of 40 psi. In both cases, if we had set our tire pressures in the afternoon under the conditions of our evaluation, they would have been between 2 and 5 psi low the following morning."
Source: http://www.tirerack.com/winter/tech/techpage.jsp?techid=73
So when should I fill my tires?
This is similar to checking your tire pressure. It should be in the morning. For those of thus that prefer to drive to a gas station to fill our tires, you will want to keep the driving distance down to minimize the pressure increase in the tires.
Then what is proper tire inflation for our cars (ZHPs)?
ZHP stock tire sizes:
225/40R18 - Front
255/35R18 - Rear
BMW recommends the following tire pressures for these tire sizes on the ZHP:
Normal driving/cargo (4 passengers or less)
Sedans and coupes - 32/38 psi front/rear
Convertibles - 32/36 psi front/rear
Fully loaded (5 passengers and cargo)
Sedans, Coupes, and Convertibles - 39/46 psi front/rear
Note: as previously mentioned, these values are for cold tires.
These pressure values are found in the user manual for a Model Year (MY) 2004 325i/330i, MY 2005 325Ci/325Ci, and MY 2005 325CiC/330CiC. Pictures of the tire inflation spec pages in the manuals can be found here:
Sedans - http://i.imgur.com/zVJZzYP.png
Coupes - http://i.imgur.com/GWACJjF.png
Convertibles - http://i.imgur.com/1y4XW6d.png
I would like to see if any other values are found in other model years, so if any members have a ZHP from a different model year than listed above, feel free to check your user manual and report back!
Why are the pressure values different between the front and rear tires?
That's a good question, especially given that the E46 has 50/50 weight distribution (actual values for a 330i: 50.1/49.9 (manual), 50.6/49.4 (auto)). Although there are several possibilities as to why BMW recommends a higher pressure in the rear tires, the most likely reason is similar to why the ZHP comes in a staggered tire size: promote understeer. BMW may be in the business of taking your money, but they're not in the business of killing its customers (intentionally). Understeer is safer and more manageable than oversteer for the novice driver. Logically, biasing the car toward mild understeer will keep BMWs customers safe and out of trouble. Higher tire pressure in the rear as compared to the front tires promotes understeer. For a nifty, quick and dirty guide on ways to alter your car's handling parameters, see UUC's chart below, paying attention to the first two rows.
http://i.imgur.com/6ZMpxUm.png
Source: http://store.uucmotorwerks.com/articles/swaybar_setup.htm
Well understeer is for gutless commies, what tire pressure should I use?
What tire pressures do you, the members, use?
Post below or PM me your tire size, brand, and tire pressure and I will log it in this post below for everyone to see. I think it will be helpful resource for everyone!
User Table
http://i.imgur.com/X2Z4dq9.png
First, why is proper tire inflation is important?
To answer this important question, I turn to Tire Rack's expertise:
Advantages of Correct Tire Inflation
Maintaining correct tire inflation pressure helps optimize tire performance and fuel economy. Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles. Tire deflection (the tread and sidewall flexing where the tread comes into contact with the road) will remain as originally designed and excessive sidewall flexing and tread squirm will be avoided. Heat buildup will be managed and rolling resistance will be appropriate. Proper tire inflation pressure also stabilizes the tire's structure, blending the tire's responsiveness, traction and handling.
Disadvantages of Underinflation
An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.
Disadvantages of Overinflation
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
Source: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1
When should I check my tire pressure?
Air is a gas, and gas expands and contracts with temperature. Note: this isn't technically correct since gas fills the volume of any vessel it's contained in, but the pressure of a gas does increase with increasing temperature if volume stays the same. As a result, the pressure in your tires in the morning will be lower than after a long drive in the sun.
So when should you check my tire pressure?
According to Tire Rack, you should test your tire pressure in the morning, before you drive. This is so the air in tires have had sufficient time to reach ambient air temperature if the car has been driven the previous day, and the air be at the lowest temperature for the day.
Tire Rack has some other interesting information in their article, discussing ambient air temperature, heat due to driving, effects of filling your tires with air at different times of the day. The article is linked below. I'll include some of the important excerpts:
"The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle's recommended cold tire inflation pressure. This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it."
"In most parts of North America, the difference between average summer and winter temperatures is about -50° Fahrenheit...which results in a potential loss of about 5 psi as winter's temperatures set in. And a 5 psi loss is enough to sacrifice handling, traction, and durability!"
"As the day's temperatures went from 67° to 85° Fahrenheit, the tire that was kept in the shade went from our starting pressure of 35 psi to a high of 36.5 psi. The tire that was placed in the sun and subject to the increase in ambient temperature plus the sun's radiant heat went from our starting pressure of 35 psi to a high of 40 psi. In both cases, if we had set our tire pressures in the afternoon under the conditions of our evaluation, they would have been between 2 and 5 psi low the following morning."
Source: http://www.tirerack.com/winter/tech/techpage.jsp?techid=73
So when should I fill my tires?
This is similar to checking your tire pressure. It should be in the morning. For those of thus that prefer to drive to a gas station to fill our tires, you will want to keep the driving distance down to minimize the pressure increase in the tires.
Then what is proper tire inflation for our cars (ZHPs)?
ZHP stock tire sizes:
225/40R18 - Front
255/35R18 - Rear
BMW recommends the following tire pressures for these tire sizes on the ZHP:
Normal driving/cargo (4 passengers or less)
Sedans and coupes - 32/38 psi front/rear
Convertibles - 32/36 psi front/rear
Fully loaded (5 passengers and cargo)
Sedans, Coupes, and Convertibles - 39/46 psi front/rear
Note: as previously mentioned, these values are for cold tires.
These pressure values are found in the user manual for a Model Year (MY) 2004 325i/330i, MY 2005 325Ci/325Ci, and MY 2005 325CiC/330CiC. Pictures of the tire inflation spec pages in the manuals can be found here:
Sedans - http://i.imgur.com/zVJZzYP.png
Coupes - http://i.imgur.com/GWACJjF.png
Convertibles - http://i.imgur.com/1y4XW6d.png
I would like to see if any other values are found in other model years, so if any members have a ZHP from a different model year than listed above, feel free to check your user manual and report back!
Why are the pressure values different between the front and rear tires?
That's a good question, especially given that the E46 has 50/50 weight distribution (actual values for a 330i: 50.1/49.9 (manual), 50.6/49.4 (auto)). Although there are several possibilities as to why BMW recommends a higher pressure in the rear tires, the most likely reason is similar to why the ZHP comes in a staggered tire size: promote understeer. BMW may be in the business of taking your money, but they're not in the business of killing its customers (intentionally). Understeer is safer and more manageable than oversteer for the novice driver. Logically, biasing the car toward mild understeer will keep BMWs customers safe and out of trouble. Higher tire pressure in the rear as compared to the front tires promotes understeer. For a nifty, quick and dirty guide on ways to alter your car's handling parameters, see UUC's chart below, paying attention to the first two rows.
http://i.imgur.com/6ZMpxUm.png
Source: http://store.uucmotorwerks.com/articles/swaybar_setup.htm
Well understeer is for gutless commies, what tire pressure should I use?
What tire pressures do you, the members, use?
Post below or PM me your tire size, brand, and tire pressure and I will log it in this post below for everyone to see. I think it will be helpful resource for everyone!
User Table
http://i.imgur.com/X2Z4dq9.png