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

mannys9130 · #1 09-27-2015, 11:34 PM

Quote:

Originally Posted by Skippy

I agree it's a great combination. However, it's less about the change in raw performance, which is small, and more a matter of the transmission going into whatever gear you want it in whenever you put it there, and stays in that gear until you take it out.

I never changed springs on mine. With the shocks and tires I have and properly adjusted tire pressures (I like 40/38), it handles quite acceptably.

I haven't weighed either, but I've moved both around. I'd say the manual is about 60 lbs and the auto is more like 160.

40 PSI and 38 PSI are WAY too high. They should be at ~28-30 PSI. Look inside your fuel fill door.

Skippy · #2 09-27-2015, 11:41 PM

I don't know how they came up with the numbers on the fill door, but if you go by them you get a general lack of grip accompanied by more understeer than I'd like. I've tried a variety of pressures and 40 or 41 in the front paired with about 38 in the rear gives me plenty of lateral grip which is neutral to slightly oversteering at the limit, which is how I like it.

leathermang · #3 09-27-2015, 11:45 PM

Since tire construction may have changed in thirty years...
I suggest reading on the side of the tire.... I think it lists both max load and max pressure.... recommended by the people who made the tire...

mannys9130 · #4 09-28-2015, 04:39 PM

Quote:

Originally Posted by leathermang

Since tire construction may have changed in thirty years...
I suggest reading on the side of the tire.... I think it lists both max load and max pressure.... recommended by the people who made the tire...

Reading the side of the tire will tell you the max pressure the tire can handle, period. You should NEVER inflate tires right up to their max. Always go by what the vehicle manufacturer recommends. Michelin makes tires for all kinds of vehicles and there is no way they can print one pressure on the tire and have it be true for all cars. That is very dangerous. Max load and max pressure also don't always have a correlation. Most of the time the tire can't handle max load at its max pressure.

Tire construction switched to radial a long time ago. The pressures inside the fuel door are valid. MB did a lot of calculation to come up with them, and they even provide high load/high speed pressures as well.

By running 40psi in a tire that should have 30psi in it, you round out the contact patch of the tire that is supposed to be flat and in total contact with the ground. You reduce the surface area of tread. That means less rubber gripping pavement, less channels to move water away when raining, and more wear on the small strip of tire. Instead of 60k miles, you only get 40k out of a set because the center goes bald.

vstech · #5 09-30-2015, 01:52 PM

Quote:

Originally Posted by leathermang

Since tire construction may have changed in thirty years...
I suggest reading on the side of the tire.... I think it lists both max load and max pressure.... recommended by the people who made the tire...

the information I've gotten from tire dealers and drivers and my personal experience with cars and trucks and racing over the years...

the tire max pressure rating is really for use at the max weight the tire is spec'd for... I would only use that pressure if I had the max weight on the tire.

for instance... most road car tires have a 1300KG max load rating... so... that's a LOT of weight.

high pressure should be based on how much mass the tires are expected to support...

if the tires are on a coupe with little in the trunk, and you're in fit shape, and the fuel tank is full... I'd set the pressure to the specs on the fuel door... notice that the specs CHANGE based on what's in the vehicle, and what speed it is driving at. higher speeds equate to higher sidewall loads, and the need for higher pressure to prevent bead separation.
the rubber compound has little to do with the air pressure recommendation aside from ply compound and strength for weight/speed. it's all about the air supporting the vehicle. heavy loads, higher pressure... high speeds, higher pressure. speeds exceeding the tire's speed rating need to be accommodated by higher speed rated tires, and set to the correct pressure for that speed.

I'd never recommend anybody driving their car with average weight and average traffic/speeds EVER set their tire to the max pressure rating on the side of the tire.

vstech · #6 09-30-2015, 01:52 PM

Quote:

Originally Posted by leathermang

Since tire construction may have changed in thirty years...
I suggest reading on the side of the tire.... I think it lists both max load and max pressure.... recommended by the people who made the tire...

the information I've gotten from tire dealers and drivers and my personal experience with cars and trucks and racing over the years...

the tire max pressure rating is really for use at the max weight the tire is spec'd for... I would only use that pressure if I had the max weight on the tire.

for instance... most road car tires have a 1300KG max load rating... so... that's a LOT of weight.

high pressure should be based on how much mass the tires are expected to support...

if the tires are on a coupe with little in the trunk, and you're in fit shape, and the fuel tank is full... I'd set the pressure to the specs on the fuel door... notice that the specs CHANGE based on what's in the vehicle, and what speed it is driving at. higher speeds equate to higher sidewall loads, and the need for higher pressure to prevent bead separation.
the rubber compound has little to do with the air pressure recommendation aside from ply compound and strength for weight/speed. it's all about the air supporting the vehicle. heavy loads, higher pressure... high speeds, higher pressure. speeds exceeding the tire's speed rating need to be accommodated by higher speed rated tires, and set to the correct pressure for that speed.

I'd never recommend anybody driving their car with average weight and average traffic/speeds EVER set their tire to the max pressure rating on the side of the tire.

leathermang · #7 09-28-2015, 04:57 PM

Manny, you are making up stuff again...
The primary danger with regards to tires is under inflation... because it is Sidewall flexing which causes excess heat....and the potential for blow outs... OR the situation you describe where due to starting out under inflated the air expands to where it is Over inflated when you are driving it...
Where you got the idea someone was going to put 40 into a tire where the side of the tire said less than that... I do not know... but the physics is that sidewall flexing causes excess heat... which means over inflation in actual practice...

mannys9130 · #8 09-28-2015, 05:19 PM

Over-inflated tires are a bad idea - WHEELS.ca

I'll just let you read this. It corroborates everything I say.

leathermang · #9 09-28-2015, 07:22 PM

Quote:

Originally Posted by mannys9130

Over-inflated tires are a bad idea - WHEELS.ca

I'll just let you read this. It corroborates everything I say.

You missed the point.... if you under inflate a tire you get excessive side wall flexing.... a major cause of heat in a tire...
THEN it becomes OVER inflated due to that heat....
yes, over inflation is bad... but you have to look at the bigger picture....

wstetson3 · #10 09-28-2015, 08:05 PM

I have to disagree. Underinflation will cause failure but I can't see how it causes overinflation. I understand the idea, heat causing the molecules to exert more pressure and the like but the heat is what kills the tire.

I will agree that the flexing causes fatigue but not over inflation.

Why Blowouts Happen

leathermang · #11 09-28-2015, 09:03 PM

Quote:

Originally Posted by wstetson3

I have to disagree. Underinflation will cause failure but I can't see how it causes overinflation. I understand the idea, heat causing the molecules to exert more pressure and the like but the heat is what kills the tire.

I will agree that the flexing causes fatigue but not over inflation.

Why Blowouts Happen

LOL

wstetson3 · #12 09-28-2015, 09:24 PM

I don't see any evidence to support the underinflation leads to heat then overinflation theory. Not really worthy of discussion though.

leathermang · #13 09-28-2015, 09:37 PM

Quote:

Originally Posted by wstetson3

I don't see any evidence to support the underinflation leads to heat then overinflation theory. Not really worthy of discussion though.

How old are you ?

mannys9130 · #14 09-28-2015, 09:43 PM

If you go by the numbers Mercedes Benz printed inside the fuel door, you won't be under inflating. It will be perfectly inflated to what MERCEDES thought it should be. Less is under inflated, more is over inflated. Both are bad, so both are wrong. The numbers in the fuel door is what you need to run to be safe.

leathermang · #15 09-28-2015, 09:50 PM

Manny... ever driven a car with polyester tires ?
that was common at the time these cars were made....
If you are still using the tires which came on the car fine... the MB engineers had no way of knowing what would be put on their cars in terms of tires thirty years later..
How much difference is there between the fuel door specs and the side of your tires ?