tires at 40 psi? - Page 6 - PeachParts Mercedes-Benz Forum

Diseasel300 · #76 04-06-2019, 11:39 AM

Quote:

Originally Posted by 97 SL320

Please explain how I am wrong.

I think I did a pretty succinct job in Post 74.

97 SL320 · #77 04-06-2019, 11:39 AM

Quote:

Originally Posted by Diseasel300

See above. Just because max load and max pressure are printed on the same line doesn't mean max load is at max pressure. Pull data from the manufacturer of the specific tire in question to confirm at what pressure max load is available.

This does present a new wrinkle and thanks for posting the chart.

Are the tires currently on your car rated at 93 load index? I'm suspecting that your current 44 PSI tires are rated more than 93 as what would be the point of having a tire that can be inflated to a higher pressure but load rating does not increase.

Diseasel300 · #78 04-06-2019, 12:14 PM

The tires on the car are the exact ones shown in the excerpt I posted. Read the document I linked. Do more research online. The max load is typically specified somewhere around 35PSI unless the tire is "Extra load" or "Reinforced" rated. Max PSI is the cold tire pressure that the tire should never exceed. It's right there in the book, and published in plenty of other places besides.

Finding exact data can be difficult, for example, Bridgestone doesn't seem to publish load tables for their passenger cars. My SD has Bridgestone Turanza Serenity tires on it, and it's like the best kept secret ever what the load/PSI table is for them. I DO NOT run them at the 27/32PSI that the data tag says, you can be sure of that, I run 35PSI all 4 corners like I did in the SDL (which never had wear issues in the >20K miles that I drove it).

Squiggle Dog · #79 04-06-2019, 02:07 PM

Were the tires on these cars supposed to appear like they were flat back in 1980 when running the recommended tire pressures? When I've put modern tires on my old Mercedes using the recommended tire pressures, they've looked like they were flat, so I would add about 5 PSI or more, and people were still telling me that my tires were flat and scolded me that I should do a better job of airing up my tires. I would also get heavy wear on the outer edges and little wear in the center.

Putting 10-15 PSI over Mercedes' recommended tire pressures (which is still within the pressure range of the modern tires) seems to make the tires look like they're not underinflated and keeps the wear more even. So that would suggest that tires made in 2019 would need higher pressures on a 1980 vehicle than what Mercedes recommended. They were probably fine for tires made in 1980.

It seems that if the tires look flat and wear like they are underinflated, that they are in fact underinflated, and the Mercedes engineers didn't factor in the composition of 2019 tires. More data is probably needed, but those two factors alone seem to give a compelling case.

I also have to wonder if even back then the recommended tire pressures were always on the lower side of the acceptable range in order to give a softer ride. It's speculation but worth considering.

hercules · #80 04-06-2019, 02:28 PM

My test method,20-30 minute drive on freeway warm day,take temp.
of tire tread outsides and middle if temps. equal your good.
Outsides of tire warmer,pressure too low.middle of tire warmer pressure too
high.
Hard driving,fast cornering, few lbs.higher.

t walgamuth · #81 04-06-2019, 04:28 PM

Quote:

Originally Posted by Squiggle Dog

Were the tires on these cars supposed to appear like they were flat back in 1980 when running the recommended tire pressures? When I've put modern tires on my old Mercedes using the recommended tire pressures, they've looked like they were flat, so I would add about 5 PSI or more, and people were still telling me that my tires were flat and scolded me that I should do a better job of airing up my tires. I would also get heavy wear on the outer edges and little wear in the center.

Putting 10-15 PSI over Mercedes' recommended tire pressures (which is still within the pressure range of the modern tires) seems to make the tires look like they're not underinflated and keeps the wear more even. So that would suggest that tires made in 2019 would need higher pressures on a 1980 vehicle than what Mercedes recommended. They were probably fine for tires made in 1980.

It seems that if the tires look flat and wear like they are underinflated, that they are in fact underinflated, and the Mercedes engineers didn't factor in the composition of 2019 tires. More data is probably needed, but those two factors alone seem to give a compelling case.

I also have to wonder if even back then the recommended tire pressures were always on the lower side of the acceptable range in order to give a softer ride. It's speculation but worth considering.

I visually inspect my tires for excessive bulge but don't depend on it for an accurate reading. If the pavement is sloping a little it will bulge more on the high side. Filling it until someone else says it looks right would not do it for me.

I think Mercedes designed the cars to run flat out on the autobahn not for a cushy ride.

In one post above someone observed that the tire said max load is "x" at 40# and then said max inflation is 44#. No increase in load for the extra pressure, but the extra pressure allows one to run hard with safety even though it might get higher than needed to carry the design load.

t walgamuth · #82 04-06-2019, 04:45 PM

Quote:

Originally Posted by 97 SL320

You are missing a vital piece of information, tire construction has changed since the car was made so the factory pressures are no longer valid. Many newer tires need higher inflation pressures in order to carry the same load as older tires.

Post 46 has actual data from an original and current tire. Load capacity is directly related to inflation pressure: Higher loads need higher pressures. Post 59 has a load Vs inflation chart. Have a look at the files in my posts 21 and 23.

Please explain how I am wrong.

Post 46

I think it may be an oversimplification to say newer tires need higher pressure to carry the same loads. Rather i still think that the higher pressures relate to the fact that the new tires are so low profile and there is a need to protect the rims.

sgnimj96 · #83 04-06-2019, 07:03 PM

I ran my vw mk1 (stock) tires at 50psi on a long road trip that also had some rough roads

Car started pulling more and more to one side, went to a tire shop to check the alignment. turns out one of my front tires was ruined - bulging out so much it affected the camber... only a few thousand miles on the tire and it was now trash.

Tire guy said it was probably from over-inflation, maybe also from potholes/speed bumps, but definitely too much psi was a big factor.

Tire have the psi written on the side wall. You can go a little over, but also keep in mind that that tire is going to heat up on the road and the pressure will increase with the heat.

INSIDIOUS · #84 04-06-2019, 07:21 PM

Quote:

Originally Posted by Diseasel300

Since this thread has turned into a typical bickerfest, let's inject some facts directly from the manufacturer shall we? Since nobody else bothered to look up any data on their tires or to back up their claims about pumping up the tires to max pressure on the sidewall, or running factory pressures printed on the filler cap, I went straight to Toyo to find data on the tires I have fitted to my SL.

An excerpt from the PDF table they have published on their website:

This certainly seems to me that running the factory pressure of 32PSI is perfectly adequate for this tire on this vehicle, and further proves that the max load rating is at 35PSI, but the max pressure is 44PSI. The Toyo document also does a good job explaining the difference between a standard passenger tire and a "reinforced" passenger tire. Essentially they both have the same load rating up to a certain pressure, but the reinforced can handle additional load at higher pressures up to the max printed on the sidewall.

If you wish to read the Toyo document yourself, have a read here: https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf
The tire inflation table is on Page 13

See above. Just because max load and max pressure are printed on the same line doesn't mean max load is at max pressure. Pull data from the manufacturer of the specific tire in question to confirm at what pressure max load is available.

But when they bother to emboss the word "at" between max load and max pressure it is for a reason.

Diseasel300 · #85 04-06-2019, 07:29 PM

Quote:

Originally Posted by INSIDIOUS

But when they bother to emboss the word "at" between max load and max pressure it is for a reason.

That's nice and all, but read the manufacturer's data posted above. The embossing is max load AND max pressure. The max load can occur below max pressure. The design of the tire dictates where that pressure point is, most passenger tires are somewhere around 35PSI.

vwnate1 · #86 04-06-2019, 08:36 PM

Closed minds gather no facts .

Squiggle Dog · #87 04-06-2019, 08:57 PM

I don't recommend inflating the tires higher than the maximum pressure stated on the side of the tire. And even airing up the tires close to the maximum pressure might be too much. But, in my experience modern tires have seemed to show symptoms of underinflation running them at Mercedes' recommended pressures for older vehicles.

The tires currently on my vehicle are 195/75R14, which is very close to the factory size of 185/80R14. The maximum pressure indicated on the tires is 44 PSI, which I imagine is higher than what the original tires would take, but don't know for sure. This tire isn't a low profile tire. It makes sense that a low profile tire would need higher pressures, but even tires with the same sidewall height as the originals seem to be able to be aired up higher, and this could be due to stronger design integrity, but thinner sidewalls which aren't as supportive. It's an interesting discussion, for sure.

Squiggle Dog · #88 04-06-2019, 09:03 PM

What I take from this is that modern tires have thinner sidewalls (not talking about profile height) and are therefore not able to support as much weight without being aired up to higher pressures, however the belt construction is stronger which allows them to be aired up higher without being as prone to bursting.

INSIDIOUS · #89 04-06-2019, 09:50 PM

Quote:

Originally Posted by Diseasel300

That's nice and all, but read the manufacturer's data posted above. The embossing is max load AND max pressure. The max load can occur below max pressure. The design of the tire dictates where that pressure point is, most passenger tires are somewhere around 35PSI.

Are actually claiming the clearly visible word 'at' is 'and'? i am not sure how to address that case.

INSIDIOUS · #90 04-06-2019, 10:02 PM

Are you sure about all of this? Are you saying the 'factory tire pressure' recommendations exceed the max pressure you show ? are we sure that spare was the factory original tire?

Quote:

Originally Posted by Diseasel300

Just a tire datapoint from my 1983 500SL Euro:

Original tire (factory spare, never used): Michelin XDX Radial 205/70 VR14, Max load 1490 lbs @ 36PSI Max Pressure

Current tire: Toyo Extenza All-Season: 205/70 R14, Max Load 1433 lbs @ 44PSI Max Pressure

GAWR Front: 2057 lbs.
GAWR Rear: 2255 lbs.

Factory tire pressures: 32/36, or 33/38 for >100mph. I typically keep inflated to 36/38, higher and the ride gets excessively rough.