Hi guys, I know it's not the correct forum for tires, but as a C32 owner, i think it's better to ask fellow owners for opinions....

I upgraded to 18" 225/40/18 and 255/35/18, i am just wondering, do i still use the same tire pressures on the door ajar that is for the stock 17"?

I have been pumping 36f/39r as the label said this is for high speed performance or something.... I thought of using the C55 spec, although the C55 is just a little bit heavier..... but still sticking with C32.....

any c32 owners upgraded to 18" put in anything different?

btw, tires are Falken 512, they are quite good......

 

what are the tires rated at?

 

they are 360AA, W - rated... not too bad for an all-season tire.....

 

lol, i mean the tire pressure...

 

LOL.... i don't know what they are rated for tire pressure, you mean the max tire pressure? haven't look @ the sidewall for that... but my guess would be 52psi or something...

 

ok, checked with their website, said the front is max 50psi, and rear 51psi....but i should not put in that much right? i put in 36/39........

 

i would switch what you are running right now...front to rear...you have more weight over the front and it will give you more traction for the rear along with better wear...

i run 42 front/ 40 rear

 

you also running 18" right?

ok, i would do the same then, but why would factory use 36/39 though?

 

I have 18" 225/40/18 and 255/35/18, PS/2
cold tire pressire
Front 33psi
Rear 35 unacceptable bumpy on non smooth roads
Rear 34 max.
Rear 32 is recommended by my tire shop.

I would take the rear as 33 and front 32 for day to day commute
Front 35 Rear 37 are for passengers fully loaded with max. speed

 

I used to put 32/35, but found that the fronts are actually wearing too fast on the edges, so i changed back to 36/39, which helps a bit, but i can clearly see the rears are not using the full tread width, as the outer edge do not have any dirt on it (like 1/2 of the outer tread block)......I just changed it to 42/40, huge difference!!! extremely bumpy...especially at the front, rear is the same, but finds handling and acceleration suffers a bit, it seem to be slower and not as responsive in cornering.... (don't know why, maybe just myself), so thinking of changing back to 36/39 or 35/38.... still playing with it, but does any one has official #s? Like from an C55 owner?

 

you have to remember that the factory uses tire pressure ratings based on the original factory tires and the pressure ratings are designed for comfort, it is still a mercedes...

if you want the comfort, then run the factory settings...if you want better wear/performance, you have to run a compromise between the factory settings and the rating of the current tires on the car...also depends on the road conditions as well, you will feel it more with a higher pressure if the roads are not as good in your area....

 

- Caution long winded excerpt from an article I wrote many moons ago Alert! -


Before I tell you how I go about determining the optimum pressure, let me just say if anyone is just using their car for regular commutes back and forth to the office, use whatever pressure is stated in the owner's manual. If they do that and remember to check it every month they'll probably be fine. Also, if someone's car handles poorly because it's got 10-year-old blown struts, the sway bar mounts have rusted off, or the tires are bald, they've got other things to consider.

I know of two ways to “read” a tire and determine the optimum tire pressure. The best method that I know of requires the use of a pyrometer. The first reaction of some people may be, ‘Now I have to buy a pyrometer, just to air up my tires?!?!?’. I can understand that response. Buying a temperature sensing tool just to put air in a tire does seem like a bit much, but it’s the best way I know to insure that I’m getting the most traction that I can out of a tire.

If anyone spends six hundred dollars every eighteen months on street tires, but still feels that a $100 or so for a pyrometer is too much, here is the two-dollar chalk method.

At the track, or on your favorite curvy stretch of road, take some chalk and mark all four of your tires. Make the mark across the tread portion of the tire, around the edge, and about an inch down the sidewall (about where the lettering starts on the side of the tire).

After making a few passes through both left and right hand curves, pull over and check the tires. What you hope to see is all the chalk rubbed off the tread on the tires, and almost all of it still on the sidewalls. How much is almost? If you've only rubbed off one sixteenth of an inch of the chalk on the sidewalls, that's good. If any chalk remains on the outer edges of the tread, you have too much air pressure. If the chalk is rubbed off down to the lettering on your tires, you've got way too little.

That's as close as the chalk method gets. It lets you know that you're not at the extreme end of either spectrum.

Pyrometers work a little differently.Accelerating, braking, cornering and even driving down a straight road will cause the tires to heat up. The tread heats up based on how the weight of the vehicle is spread out across the tires. You will find that the tread typically will not heat up evenly. If you know how the temperatures are spread across the tread, you can come to conclusions about the alignment settings, whether you are running the proper tire sizes, your sway bar settings, and whether you’re running the proper amount of air pressure.

To read the air pressure with a pyrometer, I would take the car out on that same track or curvy road and do a little spirited driving. As soon as I made that last turn, I would bring the car to a stop and immediately take some tire temperature readings. Cool down laps or continuing to drive after the last curve will throw the readings off. I record the temperature of the tread in three places; the outside (about an inch from the edge), the center, and the inside (also, about an inch from the edge).

Now that I have recorded all the numbers, what do they mean?

• If the center of the tread is hotter than edges, the tire pressure is too high.
• If the edges of the tread are hotter than the center, the tire pressure is too low.
• If the inner edge of the tread is hotter than outer, the suspension has negative camber. This is not uncommon. Sports cars typically have some negative camber dialed in. If the difference in temperatures is greater than 20 degrees, you may want to have the alignment checked.
• If the outer edge of the tread is hotter than inner, the suspension has positive camber or toe-in. For me, that’s an indication the car may benefit from a trip to the alignment rack.
• If you have called the tire manufacturer and determined that your tires are running below the ideal temperature range (and unlike me you actually have the driving skills to bring a tire up to that range), then the tire pressure is either too high, the tire is too wide, the springs are too stiff or the sway bar is too thick at that axle.
• If the tires are above the ideal temperature range, either the tire pressure is too low, the tire is too narrow, the springs/sway bars are too soft at that axle, or you really know how to drive!
• If the front tires are hotter than the rears, then the car is under steering. That can be a result of front springs that are too stiff, a front sway bar that’s too big, rear springs that are not stiff enough, a rear sway bar that’s not stiff enough, front tire pressures that are too low, the front tires being too narrow, or rear tires that are too wide.
• If the rear tires are hotter than front, the car is over steering. That can be from rear springs that are too stiff, a rear sway bar that is too thick, front springs that are not stiff enough, a front sway bar that’s not thick enough, the rear tire pressures being too low, front pressures too high, the rear tires too narrow, or the front tires too wide.

What are the goals? The ultimate setup would result in all the tires having the same temperature readings in all three spots. Rest assured, that's not going to happen. What I hope to settle for is having the center of the tread average between the inner and outer portions of the tread. Secondly, I want all four tires as close as possible to the same temperature. Thirdly, I want the temperature range between the three areas as close as possible. Achieving these three goals usually results in all four tires working as equally as possible to achieve the maximum amount of grip as I negotiate the turns.

Once the tire pressures are such that the temperatures are where you want them, there will be some leeway for tuning. For example, if the car pushes (understeers) in most corners, you will be able to reduce the front tire pressures a little without throwing the temperatures off. If you can’t lower the front’s enough, you can increase the rear tire pressure. The reverse is true if the car oversteers. I like to adjust the pressures on the axle that is having the problem before making adjustments on the other axle.

I really recommend that you buy or borrow a pyrometer. The first thing that you’ll have to decide when buying a pyrometer is which type to get. There are probe type pyrometers and infrared (IR) pyrometers. If exacting tire temperatures for a track car are the sole purpose for this purchase, a probe type pyrometer is the best choice. IR pyrometers only measure the surface temperature, and don’t reveal the temperatures of the rubber at the cord. The temperatures at the cord can be as high as 40 degrees F higher than the surface temperature. When a tire manufacturer supplies the optimum temperature range, that temperature is typically taken with a probe type pyrometer. Engine heat, brake heat, and air temperatures can also influence IR temperatures. That doesn’t mean IR pyrometers are useless, it only means that probe types are better for this specific application. For simply determining air pressures on a street car, using an IR pyrometer is fine. If you’re going to buy a pyrometer for a wider range of tasks, an IR pyrometer is usually the better purchase. IR pyrometers can be used to determine the following:

• If your AC is blowing ice-cold air, or if it’s just cool.
• If and where your radiator is clogged.
• How much cooler the input side of the oil cooler is than the output side.
• When thermostats (coolant or oil) begin to open.
• The health of your catalytic converter.
• If your high flow/open element air filter is sucking air in a 200-degree environment.
• If an aftermarket intercooler really works better than the factory one.
• ****pit temperatures
• Brake disc temperatures
• Track temperatures

Using a pyrometer to determine optimum tire pressure won’t necessarily result in the kind of grip that pulls chunks of asphalt from the road in your wake, but it does allow amateurs like us to make educated guesses about suspension tuning - which is a lot better than just buying parts and hoping things get better.

 

right now, i am still playing with 42/40, i will try them for a few days and see if that's good...because when i was using 32/35, the edges are half gone after 4k miles, and my tires are rated for 30k!!! (however, the centre trends still have around 80-90%).... ...

 

very good write up! after working at a tire store for a few years i quit wasting my breath with this level of knowledge about tires and wear performance indicators...most people just did not care enough to listen or take the advice so that their tires would last for the recommended mileage...i agree with what you are saying though....

i personally think the chalk method is more fun, but owning a pyrometer is definitely the more accurate method...lol

 

yup, that sounds about right concerning your tread wear...keep monitoring the wear and report back as to what you find, i will be interested to hear what tire pressure you settle on...

 

I will try out the chalk method, but the question is, isn't our car has kind of negative camber at the rear to aid handling? so the outer edge of the rear tire will rarely have a chance of touching the ground? or am i just completely wrong?

 

anyone know what i should be pumping into my stock 17" pirelli p-zero's (rosso)? i think they're running super low right now at 26/27...seems a bit low... i can't find the max psi anywhere on the tire....

 

you are definitely running too low with 26/27, might even driving on the sidewall.... will cause damage to the tire......especially the rear....

as mentioned in the previous post, the stock rating is 32/35 for normal (please at least pump to this level),
36/39 for heavy load/max speed.......

max psi is 51psi.... listed on the sidewall.....

 

it does not have as much rear camber as some cars currently on the market, not enough to worry about anyway unless you are lowered with springs...it would be interesting to see what the chalk reveals...

 

For your reference, Michelin Spain has made a engineering test pressures on my car.

Here is the data:

225/40ZR18 92Y PS2 2.7 BAR or 39.16 PSI

255/35ZR18 94Y PS2 3.0 BAR or 43.51 PSI (if you have a lot of load and highway the 3.4 BAR or 49.31 PSI).

Best Regards.

 

that's very interesting....thanks for the info, but does it apply to your specific car, or specific tire (ps2) or specific tire size?

 

I don't know, since the test are done on my car with this tires, but they are similar cars.

But a good general pressure for this size is 39psi for all wheels if you run 2 persons in the car with few lugage.