Tire pressure - Tire load rating question - PeachParts Mercedes-Benz Forum

ck42 · #1 01-24-2006, 01:40 PM

I know there's been quite a few threads on tire pressure....I've searched through the forumns. But, I can't seem to find an answer to the question I have though.

I'm looking for the connection between tire pressure and the load rating for the tire.
Hypothetical example:
Car has 50/50 weight distrobution. Front 50% equates to 2500 pounds.
So, each tire exerts a load of 1250 lbs.
Now, if the tire has a load rating of only 1100 lbs., what does that mean? What happens if you exceed this value based upon the weight of the car?

I understand that these loads can only be achieved at the max rated pressure. Correct? If this is right then this seems to indicate that if your car rates at the top of the rated load for the tire then you need to run the pressure that is required to achieve this load rating.

I've seen enough threads during my search to know that Luke pretty much says, 'stick with what the car manufacturer sticker says...the tire manf. doesn't know what kind of car it's going on....'

I can't find any good discussions regarding load and how it relates to the rated loads on the tire.

Luke@tirerack · #2 01-24-2006, 05:24 PM

here's a cut and paste I wrote for another website that will give you alot of the facts to pressure and load capacity

Quote:

The Facts:

A P-Metric tires max load is achieved at 35 psi and a Euro metric tires max load is achieved at 36 psi adding more air pressure will not increase the tires load capacity but, lowering it will reduce the load carrying ablility. When increasing pressure all you are accomplishing is a lower rolling resistance, quicker steering response, stiffer ride and contact patch reduction.

Based on a WRX:
the OE tire is a 205/55-16 89V
max load capacity is 1279 lbs.
load capacity at recommended pressure (which we will call PLC for placard load capacity at 32psi) = 1213 lbs.

so the minimum load capacity for a WRX is 1213 lbs. according to the PLC

now if you look at the optional tire "OK'd" by Subaru
215/45-17 87W
max load is 1201
PLC = 1135

so in reality the minimum load capacity is 1135 lbs.

now lets consider the idea of running a capacity that is too low .... what happens:
the tire is put into a condition where it can support the car at a static rest point but, when driven aggressively the sidewalls are so over worked they can not properly control the contact patch of the tire allowing for excessive sidewall roll-over, irregular wear tread block and contact patch flex which robs the tires performance ability and grip. also the added flexing of the tire will increase the heat it generates which can make a tire feel really slippery .... better gear ratio is achieved but if you can't utilize the power it doesn't matter ....

A good way to understand this is: let's say I just put a 100lb. back pack on you .... you can stand there for quite a while (that's your car sitting in a parking lot) now go run up and down some stairs with that back pack .... that's normal driving and you can't do that very well, nearly as fast or for very long before your legs give out .... after you've recovered enough to walk (let the car rest once you arrive at the venue for your autoX), take a 500 meter sprint through deep sand, that's your autoX and you will most likely not make it to the finish without falling atleast once if you can finish at all.

ck42 · #3 01-24-2006, 10:19 PM

Luke,

Good response.

Just to make sure I'm on the right page though:
Would it be accurate to then say that if the car's load at each tire is at or near the load rating of the tire, you should also be running the tire at or near 35-36PSI ? (If it's a P/Euro-metric tire).
...and if you don't, you would then begin introducing, to varying degrees, the detrimental effects that you mentioned?

This is all steming from a pretty bad outer edge wear problem on the front tires (especially the left hand side).
I've had the alignment shop compensate for the left side to reduce this, but they say that the wear I've been seeing is due to running too low a pressure. I normally run about 30-32PSI up front. He says I should be running more like 35-36PSI (coincidence).

But, after all the threads I found with everyone saying 'Run what the placard says!', Mercedes recommendation seems WAY too low. It may ride smooth and be comfortable, but it's seriously eating my front tires for lunch.

Chris

Luke@tirerack · #4 01-25-2006, 02:09 PM

the MB recommended pressure is typically low for the moderately aggressive driver because they are trying to improve ride quality .... I recommend higher pressure pretty routinely to improve steering respnse and tire wear

Matt L · #5 01-29-2006, 10:50 AM

All manufacturers recommend lower tire pressures than are recommended by the tire makers themselves. It's much cheaper to lower the tire pressure than to do anything else to improve ride quality and noise levels. Cheaper for them; more expensive for you.

Skippy · #6 02-18-2006, 07:43 PM

The Tire and Rim Association Yearbook contains information about running different tire pressures and what that does to the recommended loads and speeds. Keeping speed constant, a lower pressure will generally result in a lower recommended load. The generally part comes from the fact that many high performance car tires have a max of 44 psi on the sidewall (like the Potenzas on my 300D), but the max load rating applies from 35 psi on up.

Unfortunately, the yearbook only covers certain pressures for a given tire size, and they don't have values for tires running well under their sidewall load rating. For our cars, this isn't much of a problem, since the actual wheel loads are fairly close to the max sidewall ratings. I just went out and checked the sidewalls on my car and the rating is 1,312 lbs. Multiply by four and you get 5,248 lbs. I think the GVW on that car is around 4,800, so you can come down some from 35 psi and still be within limits. I don't have the book in front of me, but I'm guessing that the 28/32 psi Mercedes specifies for my car corresponds to the minimum allowable pressures with the wheel loads found at GVW.

On the other hand, I have a Suzuki Samurai fitted with LT235/75-15 tires. The max rating for these is 1,985 lbs@50 psi. Multiply by four and you get 7,940 lbs, but the Sammy with a full tank of fuel and me in it only weighs about 2,300. The TRA yearbook only goes down to 32 psi on this tire size, and the load rating is still way more than I need. So I winged it. I'm running 20 psi. So far, so good.

Oh, and some of the load charts from the yearbook are published online. I found them on a couple tire manufacturer's websites, but you have to dig for them.