Ideal tire inflation practices on our cars - Page 3 - PeachParts Mercedes-Benz Forum

97 SL320 · #31 09-30-2015, 05:38 PM

If one digs into tire makers web sites, you will find a size / weight / inflation pressure chart for specific tires. Measure how much weight is on each corner and look to see what pressure is recommended as a starting point.

Prior to the 80's, most passenger tires had a max PSI of 32 , that jumped to 35 as radials became more common. Some passenger tires now have 45 PSI or higher as the limit.

This matters because if the old tire was rated for say 1,500 LB @ _32_ , replacement rated for 1,500 @ _35_ and owners manual says to inflate to 30, the new tire is now underinflated.

There is no way that an underinflated tire will become over inflated due to heat build up, there just isn't enough mass of _dry_ air in there. Now, if there is enough liquid water in the tire pressure will rise at greater rate than just _dry _ air alone. ( liquid water when it turns into vapor expands at a greater rate than _dry_ air. ) But still this isn't something we want to accomplish on purpose.

It is completely fine to fill a tire to the max pressure on the side wall as that is required to meet max load ratings. Pressures on the tire are specified cold and any pressure rise when hot is acceptable.

Underinflation relative to wheel load leads to more tire failures that overinflation ( with in reason ) When a tire is underinflated, the radius is reduced but there is still a lot of tire that has to make it's way round and round.

An example, if an _unloaded_ fully inflated tire has a measured radius of 15" , a fully inflated _loaded_ radius of 14.75" and we under inflate to say 14", we still have a full circle of tread rubber that needs to go round and round. In this situation tread is sliding across the road surface generating heat and will cause the tire rubber to soften and fail.

Drop pressure enough and the interior of the sidewalls start to rub against each other causing another point of failure. As a side note, this is why a tire that has been run drastically underinflated for an unknown period of time needs to be dismounted and have the interior inspected, damage won't always be visible on the outside.

leathermang · #32 09-30-2015, 06:06 PM

Anyone have their Wagon handbook ... provided with the car ? An interesting compromise which seems reasonable...

Drivers.com: Myths abound on correct tire inflation

dm says...

''My mercedes estate (station wagon) handbook say's something very interesting about pressures and backs up what I've found elsewhere.

The manufacturers stated pressures should be regarded as minimum inflation pressures.

The tyre sidewall should be regarded as maximum inflation pressures.

My handbook says (I paraphrase): if you have to carry a heavy load and so have your tyre pressures increased to the high load levels, when you unload, it is safe to drive on without reducing the pressures, although ride comfort and tyre wear pattern will be comprimised.

Note this difference in pressure is up to an additional 50% pressure (30psi to 45psi) on my car.""

mannys9130 · #33 09-30-2015, 06:07 PM

Guys, max load and max pressure don't always have a direct relationship. Sometimes the max load pressure of the tire is LESS than the max pressure it can handle.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=21

tjts1 · #34 09-30-2015, 06:22 PM

Quote:

Originally Posted by mannys9130

Guys, max load and max pressure don't always have a direct relationship. Sometimes the max load pressure of the tire is LESS than the max pressure it can handle.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=21

So what? We're talking about 40psi here. Most modern 15" tires have 45-51psi on the sidewall. Nobody is going that high.

I've seem some cheap chinese garbage tires with max 35psi on the sidewall. Avoid.

leathermang · #35 09-30-2015, 06:22 PM

Quote:

Originally Posted by vstech

I used to work at an old time alignment shop... He would measure the tire temp, align the car, then go drive his course and he always started with 30 front and 25 rear. After the drive he'd measure tire temp across the tread and adjust alignment to temp, then adjust pressure to overall temp.
Cars came to his shop from hundreds of miles for his voodoo.
He usually ended up with a hot pressure of 38 to 44 which is a cold of around 34.
I regret never getting training from him. He died stupidly a few years ago...

I also worked in a front end alignment shop... and I am glad you used the word ' Voodoo' ... that is exactly what that was and I will explain the physics of my opinion on that....

The tires are a certain size at a certain psi.... the temperature makes no difference..... he should have put the tire at whatever psi he wanted to set the front end at.... and set the alignment...... then told the owners they needed to set the tire psi the next morning when the tires were cold...

He had no way except guessing as to the psi the tire was going to be the next morning when it was cold....

hopefully most of the settings he did were on cars which had been in the shop long enough to get front end parts replaced... thus cold enough to be accurately set that day and it be good when the tire cooled...
' he died stupidly' .... don't leave us hanging on which stupid means he fell victim to....

mannys9130 · #36 10-01-2015, 12:13 AM

Quote:

Originally Posted by tjts1

So what? We're talking about 40psi here. Most modern 15" tires have 45-51psi on the sidewall. Nobody is going that high.

I've seem some cheap chinese garbage tires with max 35psi on the sidewall. Avoid.

I'm pointing out that individuals are acting as authorities on info here and are not entirely informed on the topic themselves. I have had this conversation with customers in my shop about what tire pressure they wanted to run. The guys would either say: "Pump them up real high so I get good mpg" to which I replied that it's unsafe and leads to bad wear patterns. "Set them to the placard" to which I said nothing and just did it. "Set them to the max because I'm hauling loads/trailer and want max capacity" to which I replied that there wasn't a direct relationship between those numbers. Most had the good sense to understand that the people who designed the chassis knew what tire pressure was the best all around choice for the situation. Those who didn't got to talk about it for a bit and either realize what was being explained or persist. The small handful that persisted got a lengthy safety disclaimer printed on their invoice.

Jeremy5848 · #37 10-01-2015, 12:58 AM

The guy who aligns my two cars says the pressures listed on the Mercedes fuel door (or wherever) tend to be on the low side to give the old farts who traditionally buy Mercedes a softer ride. For my car that would be ~28 psi with a normal (low) load. The mechanic recommends 34 psi for my Michelins for best wear and handling so that is where I have kept them.

tjts1 · #38 10-01-2015, 02:25 AM

Quote:

Originally Posted by mannys9130

I'm pointing out that individuals are acting as authorities on info here and are not entirely informed on the topic themselves. I have had this conversation with customers in my shop about what tire pressure they wanted to run. The guys would either say: "Pump them up real high so I get good mpg" to which I replied that it's unsafe and leads to bad wear patterns. "Set them to the placard" to which I said nothing and just did it. "Set them to the max because I'm hauling loads/trailer and want max capacity" to which I replied that there wasn't a direct relationship between those numbers. Most had the good sense to understand that the people who designed the chassis knew what tire pressure was the best all around choice for the situation. Those who didn't got to talk about it for a bit and either realize what was being explained or persist. The small handful that persisted got a lengthy safety disclaimer printed on their invoice.

The placcard on the fuel door clearly says max 41psi. Nobody in this thread has said anything more than 40psi on an MB.

MBeige · #39 10-01-2015, 03:09 AM

Also keep in mind roads back then weren't as good as they are now. The lower psi requirements typically gave a comfortable ride through the rough roads. The recommended psi being a minimum and sidewall pressure being maximum makes perfect sense to me.

I keep mine at 34 to 35 psi. Same with my former 190E even if that car actually required less psi front and rear (27 front, 29 rear). The 300D, in contrast, requires 28 front, 32 rear.

On my non MB cars: the Subaru came from the dealer with 37 psi. I've kept it close to that, around 35-36 psi especially on long trips. It requires 32 front, 30 rear. So far the wear is pretty even.

On the other hand, the Odyssey has 36 front and 40 rear. It requires 33 front and 35 rear per door jamb tag. We got it used so the pre-existing wear on the tires invalidates any observation on wear since I never bought them new.

Skippy · #40 10-01-2015, 07:21 AM

Quote:

Originally Posted by babymog

What VS says.

The pressure recommendation in the filler flap is for the stock size tire (listed) on the stock size rim, with the weight of the vehicle loaded. Mercedes-Benz did a fair amount of testing with this combination and found that this pressure in this size tire gave a good balance between handling, ride, economy, safety, etc.

You can add air pressure if you wish, but you will change the dynamics of the vehicle. You might reduce rolling resistance and increase cornering response, at the cost of worse ride and possibly worse handling on rough pavement, you might change the rolling tire patch on the ground leading to more wear in the center of the tire, you might change the understeer-oversteer balance of the vehicle.

Inflating less than the automaker's recommendation is always bad, unless you're traversing a sand-dune or otherwise trying to lower the planing speed of the tire for off-road use.

The inflation pressure will not change with newer tires, but it can change with different size tires. Playing with tire pressures should be done with the knowledge of what you expect to gain and typically we do it on a track, find an SCCA weekend on a local track to do your testing once the tires are warm.

Items in bold are why I go higher than what's on the placard. I don't care about ride quality unless it's really bad, and with the way I drive my tires are never long for this world anyway. I don't get excessive wear on the center tread. I actually wear out the outside shoulder first. It was worse when I was running closer to placard pressures and is closer to even wear now. People with more tame driving styles will have different results. Keep in mind I drive kind of like this: https://www.youtube.com/watch?v=afwWA0d0VlM

t walgamuth · #41 10-01-2015, 09:10 AM

Interesting discussion. I usually do what the car mfg suggests. I have used higher pressures before in search of better economy but the center of the tires wear out and the ride is jarring.

zu! · #42 10-01-2015, 09:50 AM

Quote:

Originally Posted by vstech

I used to work at an old time alignment shop... He would measure the tire temp, align the car, then go drive his course and he always started with 30 front and 25 rear. After the drive he'd measure tire temp across the tread and adjust alignment to temp, then adjust pressure to overall temp.
Cars came to his shop from hundreds of miles for his voodoo.
He usually ended up with a hot pressure of 38 to 44 which is a cold of around 34.
I regret never getting training from him. He died stupidly a few years ago...

Tire design absolutely affects tire pressure recommendations. .. but the overall starting point is for grip and handling. Changing pressure is a trial and error thing that requires measuring data.
Just feeling the car how it drives isn't enough.

THIS is the correct way to determine what is the right pressure for your car's tires. I first heard about this method on another forum I follow, for my other vehicle, which is an 87 Landcruiser. Now as you can well imagine, everybody in the 4x4 world thinks their tires are too small. So they get larger tires. How does one follow manufacturer's recommendations then? It's a long read, but I think it's worth it. Here's how:

First, you need to find the Tire and Rim Association load inflation table (LIT) for the size and type of tire you are switching from, and to. If you Google "Load Inflation Tables," you will find many sources of such info. The tire manufacturers will also supply this information directly, and tire shops often have it.

Step 1: Look at the tire placard on your vehicle (or owner's manual) and write down the pressures listed for the stock tires. In our case, they were 50 psi front, 60 psi rear for LT245/70-17 tires, which will be our example. Those are the pressures listed for the truck to carry a rated capacity load, but yours lists both loaded and unloaded specs; either will translate.

Step 2: Look up the old tire on the load inflation table and note the weight each tire is rated to carry at the psi rating you want to translate, in our case 50 and 60 psi.

Step 3: The LIT is broken up into 5psi increments, and your recommended pressure may fall between. Yes, it matters. If that happens, you will want to determine how many pounds of weight that 1 psi will carry. Let's say your pressure is 47 psi and you are going to do the same tire swap we are. The weight rating at 45 psi is 2,010 pounds, and at 50 psi it's 2,205 pounds. Use the following formula:

2205-2010 lb. = 195 lb . = 39 lb. per psi
50 psi-45 psi 5 psi

Subtract 45 from 47 and you have 2 psi. Multiply 39 x 2 = 78. Add 78 pounds (2 psi worth of weight) to the 45psi load rating and you get 2,088 (2,010 + 78 = 2,088). That's the load the tire can take at 47 psi.

Step 4: Look up the new tire on the LIT. Find the pressure needed to hold up the same weight at the rated placard pressure. Our new LT285/70-17 tires can carry 2,105 pounds at 35 psi, so using the formula in Step 3, we know that to support 2,205 pounds, we need 38 psi. In back, our 60 psi load rating dropped to 44 psi with the larger tires.

The Michelin LTX M/S2 is the latest iteration of the LTX M/S line introduced in late 2009. It's an all-season light truck tire that comes in both P-Metric and LT versions. Sizes cover the majority of light trucks and SUVs. LTs come in Load Ranges D and E and, commonly, with a 13.5/32 tread depth. The tread compound is high in silica, which makes it outstanding for use in rain. The tread design also lends itself to good snow and ice performance. The LT versions are part of Michelin's Green-X program, which is their commitment to improve fuel economy by updating their tire designs to be class leaders in the fuel economy department.

So there you have it. No guessing, knock yourself out.

I also read a very interesting article by a guy who used to be some big wig at a tire company. He had a simple formula that involved inflating your tire to an amount you figured was right, driving around and THEN taking the temperature of the tire. The idea was, if your temps went UP, then your tire is underinflated. If they went DOWN, then your tire is overinflated. Somewhere in between is the happy medium where your tire temps don't change or something like that. I can't find the article now, but I'll post it when I can. So VStech, your guy is not doing voodoo. This actually is a known method for gauging the correct pressure.

FWIW, on my 1987 300TD, the manual actually states words to the effect of "driving at the maximum pressure listed would increase performance". I don't have it in front of me, but that's what it says.

Having said that, the pressure on the door lists at 45 psi rear and 40 psi front for max (IIRC). So I pump mine to 40 psi rear and 35 psi front. FWIW, my sidewall lists max at 44 psi.

Simpler=Better · #43 10-01-2015, 09:57 AM

I always had good luck with 35ish, 20ish on ice/snow(short jaunts)

babymog · #44 10-01-2015, 10:19 AM

This is starting to look like someone asked about synthetic oil.

Tire inflation pressures affect handling, ride, fuel mileage, tire wear. The pressure recommended on the fuel filler flap are the best compromise of these factors from Mercedes-Benz Engineers at the time your car was designed. I know some of those Engineers, I worked with them through the introduction of the 124, they didn't care much about cost and leaned toward the safety, durability, and ride of the car (in that order) from my recollection. Engineering ran Mercedes back then, not like the newer regime which took over in the following decade.

The tires that you buy today for your 1980s-on Mercedes are substantially unchanged from the original tires from the standpoint of construction and inflation, tire compounds have changed and construction materials have changed a little (original was steel-belted radial-ply, nylon plies in the sidewall (same as today) with aramid et al available at the time ex: Goodyear Wingfoot), roads have changed little (driven on a Michigan freeway lately?), legal speeds have changed but the speed for which your car was equipped has not (notice H and V speed ratings on the original tires?), therefore the only reason to increase inflation pressures beyond M-B recommended pressures is driver preference or a change to a smaller tire. Safety can be affected, but negatively as well as positively by increased pressure.

On our motorcoach I always run Michelins. These tires are rated at over 7,000lbs each, and that's where the sidewall pressures (130psi) are designed to create an equilibrium between the load and the road. Michelin cautions against running these max. pressures for lower loads, I have read that it can "bruise" the belts, I have no idea what that means but at over $1,000/tire I follow Michelin's load-inflation pressure charts. I have also overheated and blown these expensive tires from running hard and heavy on a hot day, once the heat begins to build up in the sidewalls (steel-belted sidewalls in these big babies) they flex more and generate more heat, leading to delamination and other irreversible damage in the sidewall where the tread separates and leaves itself on the highway (aka: Gator, remember the Explorer/Firestone underinflation issue and why tire pressure monitors are now required?). When 140lbs of rubber lets loose at 75mph you sometimes don't even feel it.

So inflation pressure in car tires affects:

Rolling Resistance.
The higher the pressure, the less sidewall flex and the smaller-narrower the tire patch. This can affect tire temperature also as the sidewall flex turns rolling energy into heat, usually reducing fuel mileage (slightly, at highway speeds over 70% of your parasitic losses is aerodynamic drag). So higher pressure can improve fuel mileage, ... slightly.

Tire Temperature
Mentioned above. The higher the speed, the greater the tire temps. Regardless of the extra airflow over the tire the heat can only dissipate slowly so you need to increase pressure to reduce flex at higher speeds. Also higher loads require higher pressure, a higher load will increase flex and thus internal temperatures, the higher pressures offset this by reducing the flex.

Vehicle Ride
The softer the tire, the smoother the ride. Your tires are the first spring in your suspension, isolates the entire wheel-end mass from mostly high-frequency hits like highway expansion strips or broken pavement. This was engineered as an entire suspension system from tire to chassis. The W124 was a game-changer with a chassis natural frequency of (IIRC) 54hz, which was well above anything else at the time. This entire chassis to tire suspension was tuned to give the best ride and handling available.

Tire Wear
Lower pressures have more heat, more heat is softer rubber, softer rubber wears faster, ... EXCEPT: higher pressures can put more pressure on the middle (lower on the outside, also bad), and regardless of the wear on the outside of the tire if the middle is bald, ... you're finished. Think of using all of the tire for acceleration/deceleration/cornering vs just the center, you see the issue. This is again addressed by the increased pressure to keep the tire patch at a fairly even pressure across the tread with increased loads. The wear vs normal inflation is for normal driving, if you like to corner hard a lot (as I do) you will wear the shoulders of the tire faster, if your car is not properly maintained (sagging suspension, worn bushings, caster issues) your tires won't wear normally anyway so this won't help you.

Traction
This one is a bit complicated, and segways into the next category:
Handling, so although separate they are interrelated and I need to cover them together.
When you increase the tire pressure, you decrease the tire patch. You also change the shape of the tire patch. As this affects the factors above and below, the tire patch is something that you want to understand vs traction.
Many people don't understand traction, but think that they do. Same with handling.
If you want more traction on dry roads, you want a good wide even tire patch. When we take a race-car out to the track tire pressures are set methodically and carefully as they directly affect cornering traction.
Cars are run until tires are at operating temperature and pressure, then run into the pit where a pyrometer is inserted into the many small divots in the "tread" and temperatures logged across the tire. Too high and the center will run hot, too low and the shoulders will run hot. This is because the high pressure puts more load on the center of the tire and low pressure more on the outside/shoulders, just right and the tread loading is even and thus evenly distributed heat (from friction with the road/track surface). Suspensions are tuned also using this data and individual scales under each tire. You can tune so many parameters on a race car that this complete suspension from tire to chassis is always balanced and re-balanced even as track temperatures change (which affects tire temperature/pressure).
You can't do this on your car, but Mercedes already did.
A higher pressure passenger car tire does not mean more traction. Engineers understand Coefficient of Friction, which basically means that with more pressure you reduce your tire patch, but you increase the pressure on the remaining patch so the available traction (friction) largely remains the same. Wide tires are not there for ultimate friction, wide tires create more area so that the heat and wear is distributed over a larger surface and the rubber lasts longer (in a race for example). Street tires are not racing tires but many of the principles are the same.
Higher tire pressure does increase the hydroplaning speed in rain, slush, other conditions. This can help you if you go through a puddle at highway speeds, it is a fairly direct correlation of double the pressure to double the hydroplaning speed (close enough for this discussion anyway). In other conditions like snow its more complicated but many of us have winter vehicles and winter tires anyway, ...
Traction and handling are also affected by how much time the tire is in contact with the road. Compare a Corvette and a Mercedes sedan on a skidpad or nice track, Corvette can outhandle your sedan by a huge margin right? Now try that on a broken-pavement curve and feel that Corvette chatter its way sideways around the turn, the suspension is so stiff (especially in track mode) that the tires can't stay in contact well enough to use all of that grip. As you increase tire pressure in your car the same thing happens, you lose grip on rough surfaces.
The next thing that can be affected by tire inflation pressures is weight transfer, typically higher pressures lead to slightly less body roll and pitch, which is weight transfer, so higher is better for this cornering force (on smooth pavement). Higher pressure also can reduce the sidewall flex in a turn which changes the way the tire tread is contacting the road under lateral loading, flex can lift the inside of the tread. Both of these can be addressed by a different wheel & tire combination, if handling is your primary focus get larger wheels with lower-profile tires and go with the maximum recommended rim width for the tire size.
Handling is also affected by tire inflation via roll-coupling. If you increase your front tire pressure, you will reduce the flex in those tires which is somewhat similar to increasing the front sway-bar stiffness. This usually leads to understeer as your front end is preventing the body roll in a turn more than with a softer front suspension, so the rear tires are able to maintain a more even left&right pressure/traction. Conversely increasing the stiffness in the rear (springs, swaybar, tires) will lead to more oversteer as the rear will now try to control roll more and there will be more pressure on the outside tire / lift on the inside tire leading to more rear wheel slip/drift and less front wheel slip/drift.

My signature car has 16x8 wheels vs the original 15x5.5 with the offset balanced around the steering axis, carefully selected, they are 215/55-16 LRH Extra Load tires because it's a wagon, and I do use the high-speed rating as well as occasionally carrying home a load of landscape supplies or other heavy stuff. The car handles and rides better than new on this setup due to a good touring tire design with a wider patch and 34psi pressure front/rear (sidewall max. is 50psi). I go to great pains to find the sweet-spot in pressure and have found that through some track time and a couple of sets of tires that this works for everything I do on the street, including the need to dodge an errant raccoon on the way to work at 65mph without the rear of the car passing me after this typical double-lane-change maneuver. If you want to mess with the pressures in your own car, and feel confident with your ability to assess the handling and safety vs your mileage and comfort, go for it. As far as recommendations for others on the forum, I say read your manual, stick with the recommended inflation pressures until you have actual test data of your tires on your car (including the condition of your suspension bushings etc.) unless you really understand the dynamics involved and are comfortable with it. If you're comfortable with the data from a similar car (same model, same shocks, same tire size and type, same condition) to use their experience / data / information as your guide this is probably a good start. For example I would not recommend to a 240D owner that he run different tire pressures because it works on my 124/300TD.

That all having been said, it is usually better to err on the high side rather than the low (ex: the Ford Exploder / Firestone Tire debacle), if you're not sure go with the higher recommendation provided it doesn't exceed the max. pressure on the sidewall.

babymog · #45 10-01-2015, 10:28 AM

Zu:

I like your postings, but your references in this posting are IMO flawed.

In the first example, it is easy enough to weigh the vehicle at a CAT scale (truck stop), get front and rear axle weights (with normal load, can do it with a trailer also if you tow often), divide each axle weight by 2 to get approximate load on each tire (should be fairly even side/side), then look at the tire manufacturer's pressure recommendation for that load. Always round up.
How many pounds each PSI will hold? That one is simple: Each square inch of a tire holds 32lbs at 32psi, ... as someone posted earlier it is the air holding the car up (vertical loading), not the tire, the sidewalls hold the air in and transfer lateral & longitudinal loads.

The second reference is even worse, no matter what your tire pressure you will generate heat driving the car, with his method you will continue to increase pressure looking for the equilibrium. I'm guessing that big-wig was an accountant.

Sorry, just my take.