Dink

Luke, I am running the normal setup for C32 with 19 inch rims with 235 in front and 265 in the rear. I have Goodyear F1 DS3, which I bought from you, on the back, and Pirelli Pzero nero's in the front.

What tire pressure should I run for the best compromise in handling and tire life?

Thanks

Luke@tirerack

32 front
35 rear

Eddie Knopp

Luke -

I wonder if you might go into a brief discussion of appropriate tire pressures - advantages/disadvantages of deviating from the MB fuel flap recommendations. I'd be curious as to what you recommend for my stock setup on hte '04 E55

Thanks

tjcMBE55AMG

/\ /\ /\ Bump /\ /\ /\

Luke@tirerack

Your tires support the weight of your vehicle, right? Well, they don't! It's the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tires are to provide all of the handling, traction and durability of which they are capable.

However, you can't set tire pressure...and then forget about it! Tire pressure has to be checked periodically to assure that the influences of time, changes in ambient temperatures or that a small tread puncture has caused it to change.

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.

Since air is a gas, it expands when heated and contracts when cooled. In most parts of North America, this makes fall and early winter months the most critical times to check inflation pressures...days are getting shorter...ambient temperatures are getting colder...and your tires' inflation pressure is going down!

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

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!

Additionally, the difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country is about 20° Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle's tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle's tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.

And finally, if the vehicle is parked in the sun, the sun's radiant heat will artificially and temporarily increase tire pressures.

We put some of these theories to the test at The Tire Rack. First, we mounted two tires on wheels. We let them sit overnight to equalize and stabilize their temperatures and pressures. The following morning we set them both to 35 psi. One tire and wheel was placed in the shade while the other was placed directly in the sun. We then monitored the ambient temperatures, tire temperatures and tire pressures through the day. 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.

Next we evaluated the effects of heat generated by the tire's flexing during use. We tried to eliminate the variable conditions we might encounter on the road by conducting this test using our "competition tire heat cycling service" that rolls the tires under load against the machine's rollers to simulate real world driving. We monitored the changes in tire pressure in 5-minute intervals. The test tires were inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the same load, the air pressure in all of the tires went up about 1 psi during every 5 minutes of use for the first 20 minutes of operation. Then the air pressures stabilized, typically gaining no more than 1 psi of additional pressure during the next 20 minutes. This means that even a short drive to inflate your tires will result in tires that will probably be "underinflated" by a few psi the following morning.

Add all of these together, and you can understand why the conditions in which you set your vehicle's tire pressures are almost as important as the fact that you do set it.

It's important to remember that your vehicle's recommended tire pressure is its "cold" tire inflation pressure. It should be checked in the morning before you drive more than a few miles, or rising ambient temperatures or sun's radiant heat affects it.

And by the way, if you live in the North and park in an attached or heated garage you will "lose" pressure when you leave its warmth and venture into the real world outside during winter. Add 1 psi "cold" pressure tire pressure to compensate for each 10° Fahrenheit temperature difference between the temperature in the garage and outside.

Correctly inflated tires receive appropriate support from the contained air pressure to provide an even distribution of load across the footprint and help stabilize the tire's structure. And while most drivers recognize that this has a significant impact on tire wear, rolling resistance and durability, only a few realize underinflation also has a noticeable influence on how quickly and precisely the tires respond to the driver's input.

In order to evaluate the influence of inflation pressure on response and handling, The Tire Rack conducted a Performance Test Track Drive, comparing properly inflated tires to purposely underinflated tires. We used 2003 BMW 330Ci coupes, and installed P225/50R16 tires on 16x7.5” wheels. We tested new, full tread depth tires.

We chose Goodyear Eagle GT-HR High Performance All-Season radials that were developed to blend good treadwear, responsive handling and dependable traction. One of the Eagle GT-HR's highlights is its internal construction that features Goodyear's RaceWrap Construction Technology developed for the Eagle Race tires used in NASCAR competition. RaceWrap Construction Technology brings a casing ply down the sidewall at a slight angle, wraps around the bead and returns it all of the way up the sidewall until it ends under the edge of the steel belts. This slightly angled, two-ply sidewall enhances steering response and handling stability.

The tires installed on one of our BMW 330Ci test cars were inflated to the vehicle manufacturer's recommended inflation pressures of 29 psi front and 33 psi rear, while the other car had its tires inflated 30% lower (20 psi in the front and 23 psi in the rear). We chose 30% underinflation because it was the percentage of loss initially established by the US DOT at which passive pressure monitoring systems must warn the driver of low inflation pressure on future cars.

The first part of the test was visual. We asked the drivers to look at the tires and decide which of the two vehicles was equipped with the underinflated tires. While perhaps this visual test might have been easier with taller tires of the past, today's low profile tires fitted to the BMWs demonstrated how difficult it has become. The drivers agreed that the tire appearance alone did not provide irrefutable confirmation of the tire pressure contained inside. You can't use your eyes as a tire pressure gauge.

The next test was run on our test course to confirm the influence of tire pressure on the tire's performance at its limit. While the drivers thought that the properly inflated tires provided responsiveness and predictable handling, they quickly realized that the same tires in an underinflated state left a lot to be desired. The underinflated tires required more steering input to initiate maneuvers and were slower to respond. The underinflated tires also felt out of sync during transitions; instead of moving in unison, the rear tires' reactions lagged behind the front tires, resulting in a detached sensation being transmitted to the drivers.

The underinflated tires delivered acceptable steady-state cornering force once they stabilized on our test track's skid pad, but the car was uncooperative anytime it was asked to change directions. It proved to be over 2 seconds slower around our test course (2 seconds represents about a 7% loss of handling performance).

In other words, the performance that tire manufacturers build in, low tire pressures can take away.

Adjust your tire pressures as indicated on the vehicle tire placard or in the owner's manual. Check you inflation pressures at least once a month and before highway trips.

Eddie Knopp

Luke - Thanks for the discussion - I do have a question - what about the converse - tire pressures higher than on the placard ? I think a lot of us here (myself included) are doing this. Also are there any advantages/disadvantages to specfically alter either the fronts or rears ?

Thanks

Luke@tirerack

while all of the previous information was taken from our website here is some additional info directly from me:

when driving my car on a daily basis I run 34 in the fron and 32 in the rear to make the ride comfy and soft while still maintaining good, if not great, handling. I like to have a predictable, stable car that is not harsh on the "not always smooth streets I drive on. I also track my car and have found significant improvements over OE recommendatin in terms of steering response and sidewall deflection rates at higher pressures. I run 40 in the front and 34 in the rear for track days. I do track tire info a little closer than most, as you can imagine, so I have alot of data regarding pressure increases and tire temps for both daily driving and track driving.

On a road trip, after setting my tire pressure and filling the car up, I embarked on a straight through drive to the Corvette National museum (about 8.5 hours). After exhausting the first tank of fuel with no stopping I checked both pressure and temp .... I found that the pressure rose pretty much as expected (which was still below the track pressure I typically see) but the temp rose to almost track temps because of the added tire flex caused by the lower pressures I run on the street. I always thought that the track driving temps would be higher because of the added friction created by the aggressive nature of that type venue but, after double checking my findings at the track, the fact is, since I run higher pressure at the track the tire does not flex nearly as much which is why the temps does increase in the same increments as they do on the street.

The OE recommedations are just that .... recommendations, designed to blend ride quality and handling to an acceptable level for the average driver. If you want better steering response and handling you can up the pressure with no issues providing you do not exceed the max pressure listed on the tire and for a softer ride let a couple pounds out.

You can dramtically affect the cars stability and handling characteristics by changing air pressure so, if you are making any changes .... make small adjustments several times until you reach your driving nirvana.

speed ratings also affect the heat generated in tires but, that is a discussion for another time

Eddie Knopp

Luke - it seems you have the fronts at a higher pressure than the rears - my OEM rec is the opposite - I presume this affects steering and handling.

Let me be specific with my situation - The E55 is my daily drive - to and from NYC from the burbs - the roads (as you can imagine) are far from pristine. I am an aggressive driver and will push short bursts of speeds in excess of 80 -90 and (if the road is open) 100 - 130. Most of my commute (home at least) is in heavy traffic - it's just the end where things open up. I'm running with the stock Conti's on the stock rims. What would you suggest as to pressures ? Presently I think I have 40 up front and 42 in the rear (don't ask me specifically why - I'm not really sure).

Thanks

Eddie

Luke@tirerack

as with most rear drive cars (yours included) running higher pressure in the front tires will reduce the understeer inherently designed into your car that's why I always run higher pressure in the front ... try running a few pounds higher in the front than the rear and you will find the handling much more neutral .... I want my car to be able to go through the twisties faster than most

Eddie Knopp

Thanks - I'll switch it over the weekend. interms of specifics - somewhere between the recomended and the max load rec ?

Luke@tirerack

try 42 front 36 rear

Eddie Knopp

Great - Thanks

Eddie

Sidd

Luke,

I am amazed at your knowledge. I did still have a couple questions for you, though. First, what about front wheel drive cars, would you still recommend higher pressure in the front than in the rear? Also, can you explain how you got the numbers 42 front, 36 rear for Eddie? Why the difference of 6 pounds and why 42 and 36? Thanks!

Sidd

Eddie Knopp

Sidd - as an update with Luke's advice the car handles significantly better !

Luke@tirerack

it's really based on what you are looking to achieve and as far as how I came up with the numbers ..... I drive alot in many different types of cars and experiment while doing that. I have a spread sheet that is well over 40 pages long with tire inflation info that I started about 20 years ago

Sidd

Eddie,

I'm glad to hear your car handles better with Luke's advice. Then again, did we ever have a doubt?


Luke,

Let's say my goal was to have my car ('95 E320) handle at its best, what would you recommend? I believe the recommended pressure is more in the rear than in the front. And also, I have a 2001 Passat, what would you recommend for that since it is front wheel drive? Both cars are driven agressively. Since most of us don't have your knowledge and experience, is there some quick tips or guidelines you can give us so that we could estimate/calculate the best tire pressure for our cars?

Sidd

Luke@tirerack

it's really based on vehicle dynamics, weight and what the car is doing


here is a way to find out what works best for your individual needs

#1.) pick a starting point (rear drive make the fronts 6-8 psi higher than the rears ... front drive make the difference 4-6 psi)

#2.) drive the car and take note of the characteristics

#3.) too much understeer add air to the front ... too much understeer and harsh ride let air out of the rear

#4.) over steering raise the rear or lower the front using the above guidelines

the manufacturer recommendations are for comfort and handling compromises ... and they always induce understeer

Sidd

Great, thanks Luke! One last question, what about all wheel drive vehicules?

Sidd

DavidNJ

On the race track (oval) there are two ways of determining pressure. One is tire temps across the tread (with some allowance for tire stagger) and the second is scribing a very shallow line across the tread and observing wear (the groove is so shallow wear shows very quickly). A tool that attaches to a drill is available to scribe the line.

On the S2000 board, people report a major difference in handling by adjusting camber.

Also, when considering handling, are you talking about corner entry or exit? I would have thought the higher front pressure would have made the car feel more agile on entry, however that it might have hurt the performance on exit.

Have you used either of these at Tire Rack? Have you tried adjusting camber? Doesn't using tire pressure to adjust handling lead to uneven tire wear (bald centers or shoulders)?

Thanks,

David

Luke@tirerack

yes camber adjustments are critical to optimize handling chacteristics but pressure adjustments can fine tune the setup for conditions ... I run 2.5 degrees of negative camber out fron and 1.75 in the rear but, it is not practical to adjust camber to compensate for conditions such as temperature and humidity

DavidNJ

Doesn't 2.5 deg negative camber cause some tire wear problems?

Does running a lower pressure in the rear alleviate some wear problems? From the tire pressure stickers, it seems that the rear pressure are more to enable high load capacity than for handling.

Sidd

Luke,

I did what you said, and man did the handling improve! However, can you also give us a guideline on how to "pick a starting point"? The way I chose it was inflating the fronts to their maximum (in this case 44 PSI), and inflating the rears 6-8 lbs. lower. This was for my '95 E320. For my '01 Passat I put the rears 4-6 lbs lower (max is 44 PSI also). My question is, am I correct in picking this starting point? I had another theory as well. I wanted to put the highest recommended pressure from the car manufacturer in the front and inflate the rear accordingly. I also thought about doing it the other way around. However, what would YOU do? Please enlighten us, Luke, thanks!

Sidd

Luke@tirerack

almost all cars have understeer designed into them for safety reasons. You can reduce or in somecases eliminate that understeer with different tire pressures. Front drive and all wheel drive cars have another thing to worry about though ... torque steer .... while I do recommend higher front pressure for better handling in almost all cars it is especially critical in cars which drive the front tires. The pressure differences need to be a little lower on those cars since the front tires will pull you through the corners somewhat

str8ridin

Luke, so, in theory, if the tire is under-inflated, it will cause more friction, consequently raising the air temp which will in effect raise the tire pressure back up? If so, how much increase is expected in general from heat/friction?

This is in respect to achieving even tire wear, not necessarily handling.

Also, if negative camber is desired like in your car, but inside edge wear is not, how can one alleviate, not eliminate, the inevitable inner tire wear? Increasing tire pressure?

This thread should be stickied!

Luke@tirerack

well all recommended tire pressure is cold pressure and you can easily see 4-8 psi gain ..... and there are very few ways to eliminate wear caused by negative camber except frequent rotation or dismount the tire and turning it around