Primer on tire pressure .....

[Red_Bean_Bun]

Decided to share some personal experience on the simple task of inflating the tires. The thing is I tend to get picky so I decided to delve a little deeper before pumping it ....

Having an appropriate tire pressure would bring the ride closer to what BMW had in mind when the tires and suspensions were specified. And so I decided to work out how I should inflate my ride's tires tonight.

It is a good thing most of the figures were worked out for us. Only thing is how do we use this information to determine the air pressure for our tires.

First off - BMW based its specs on the ETRTO standards. So with that I started to work this out.

First look at the ride's manual on the tire pressure

- I do occasionally travel above 160kmh
- I do not lug luggages around
- I have 19" tires
- I have a sports package
- The OE tires had "extra load" embossed on the side wall

View attachment 70892

That would mean the recommended spec from BMW

Front - 36 psi with a load index of 88
Rear - 39 psi with a load index of 91

So if you refer to the ETRTO reinforced load inflation table this would mean the load is

Front - 1091lbs
Rear - 1279 lbs

View attachment 70893

So should I inflate my tires to 36/39 ? I almost did because it's in the manual. But then I noticed something different .....

Here are the specs of my tires - see pic attached labeled tire information if you are not sure what I am looking at ....

- 19"with Extra Load embossed at the side
- front load index is 91
- rear load index is 94

View attachment 70894

Compare the load index of my tires versus the load index of what is specified in the manual ..... It's different. Load index tells you how much weight the tires can carry and so it has an influence on tire pressure.

This means that I should not inflate my tires to 36/39 because the load index is different from the tire BMW had in mind when the manual was printed.

The correct pressure would be derived as follows .....

Look up the load index row for load index 91 on the pic labeled reinforced load inflation table. Then cross reference for 1091 lbs.
Do it again for load index 94 and cross reference for 1279 lbs.

The following is found by using the closest approximate .....

Front - 32psi
Rear - 35psi

Why the lower pressure ? Because the load index of the my tires are higher than the ones BMW mentioned in the manual.

To validate that I got this right - I looked that the page on weights in my manual ... See attached pic - weights.

View attachment 70895

Look at the front and rear axle weights - add both and then compare

Front - 2182 psi (derived) vs 2172 psi (manual)
Rear - 2536 psi (derived) vs 2348 psi (manual)

So this means the load index and tire pressure of the tires more than able to meet BMW's expectations.

Then I tested how it feels at my regular road humps I frequent as well as the crappy parts of the PIE on my going home route .... Feels much better with less frequency of breaking traction and at the same time it's much quieter.

Enjoy .....

 

[Ahbengdriver]

Re: Primer on tire pressure .....

* clap *. This is better than the GE results.

 

[Viper Warrior]

Re: Primer on tire pressure .....

But after few weeks the tire pressure will slowly leak and not accurate again ...
I admit I'm a lazy person, only check tire pressure once a month

 

[topfuel]

Re: Primer on tire pressure .....

Cheem.....In a nutshell what would be the right or recommended pressure for front 245/35/19 and rear 275/30/19

 

[Red_Bean_Bun]

Re: Primer on tire pressure .....

But after few weeks the tire pressure will slowly leak and not accurate again ...
I admit I'm a lazy person, only check tire pressure once a month

If under inflate it will overheat.

If over inflate it will be susceptible to impact damage and braking issues.

Checking once a month is a good thing ....

By using load index to determine the tire pressure reduces tyre flex which results in accelerate wear and tear.

 

[Red_Bean_Bun]

Re: Primer on tire pressure .....

Cheem.....In a nutshell what would be the right or recommended pressure for front 245/35/19 and rear 275/30/19

Got a pic of your tire pressure card at the door and a pic of your tire (F+R) sidewalls ?

 

[MSportTune]

Re: Primer on tire pressure .....

Got a pic of your tire pressure card at the door and a pic of your tire (F+R) sidewalls ?

Hi Tony, I am also on this set up using Goodyear F1 tyres. After experimenting for a while vs how my MSport suspension behaves vs tire pressure, I settled on 260 front/290 rear. However on PIE, the ride seem choppy. I have to try again. I attended an advance driving course few years back by Aussie pros and they recommended tire pressures to be 36-42psi for optimum tire contact area on the road. Ya, I only pump N2 though it is very inconvenient. I find with years of monitoring tire pressure via Tpms system that normal compressed air need weekly TOP up while N2 can last around 3-4 weeks before TOP up is needed. On the door of my F30, there isn't this staggered tire combination so what to do?

 

[Red_Bean_Bun]

Re: Primer on tire pressure .....

This is the reason why I decided to use loading to determine the tire pressure. Each tire has the ratings embossed - Size, speed and load. Try to make full use of the data - Speed and data. Use the door card info to determine the original loading then use that info to find the correct pressure for your tire.

So far it seems to help improve the traction issues I have experiencing .....

Hi Tony, I am also on this set up using Goodyear F1 tyres. After experimenting for a while vs how my MSport suspension behaves vs tire pressure, I settled on 260 front/290 rear. However on PIE, the ride seem choppy. I have to try again. I attended an advance driving course few years back by Aussie pros and they recommended tire pressures to be 36-42psi for optimum tire contact area on the road. Ya, I only pump N2 though it is very inconvenient. I find with years of monitoring tire pressure via Tpms system that normal compressed air need weekly TOP up while N2 can last around 3-4 weeks before TOP up is needed. On the door of my F30, there isn't this staggered tire combination so what to do?

 

[topfuel]

Re: Primer on tire pressure .....

Got a pic of your tire pressure card at the door and a pic of your tire (F+R) sidewalls ?

Yes sorry will take a picture but I suppose there's for stock 18" tyres or somewhat diff from the 19" tyre specs I'm using