Tyre pressure calculator

[flybobbie]

Good starting point if experimenting with tyre pressures.

http://arumdevil.com/pressures/

And some tyre info;

http://www.auto-data.net/en/?f=tires&d=225,45,17

Car weights ,etc.

http://www.auto-data.net/en/?f=showSubM ... li_id=1992

[jadatis]

The first link leads to a verry simple calculator which uses , what part the load is of maximum load /is part of AT pressure.
Then also assumes the weight is equally devided over the 4 wheels.
And for normal car tires and XL/reinforced /Extraload the maximum pressure is given on sidewall and not the AT-pressure.

Registered to this forum , because found this topic , and cougt my atention because of calculator, with google .
I have been busy with tyre pressure calculation since 2007 when I got hold of the official formula and its addings for speed and alighnment for normal car tyres.
Declared this formula holy and learned myself Excell to make spreadsheets for it.
Translated a few to English ( From Dutch ) to go worldwide with it.
Later , together with an Englishman Gary I made a language sheet in Excell wich I now implement in my newer spreadsheets to make them multi-lingual.

In My newer spreadsheets I also user my own formula, wich takes better care, that the deflection of tyre stays the same over the whole range of pressure/loadcapacity.

Will give the link to my Motorhome -tyre-pressure -calculator wich you can also use for every car with some adjustment.
Also have an older one wich workes exactly to the rules of the tyre-makers, but so also the old formula .
Also for low Aspect Ratio tyres ( AR = Hight/Width division) maximum load has to be substracted, I discovered .
So its best to give here the data of tyres and car and I will do it for you. Then I can take every exeption into account.

https://onedrive.live.com/?cid=a526e0ee ... E6DC%21793
This is the map for motorhomes, but from that you can navigate my complete public map of One-drive ( former skydrive) like in a forum.
To use a spreadsheet , first download it to your computer by RIGHTCLICKING and then choose DOWNLOAD.
Other means go wrong , like leftclicking or choosing open in EXCELL or WEB-APP when rightclicked.
After download and eventual virus-check, open it in the Excell programm on your computer , but Open Office CALC can handle it too.

Then at last about that AT-pressure . Saw it called maxload-pressure/ Pressure needed for the maximum load/ and in the formula sheet Reference- pressure ( Pr).
It is not the maximum pressure of the tyre, but as says the pressure needed for the maximum load for up to 160km/h ( up to V speedrated tyres) and camber angle ( wheels like this on the imaginary axle /-\) of smaller then 2 degrees.
The difference between Pr and Pmax is used for highening up first for speed the Pr and then multipy for camber angle above 2 degrees.
On normal car tyres/standardload and XL // only Pmax is given on sidewall, From C-load/6PR and up Pr is given on sidewall and so are allowed higher pressure than that.
Standard load Pr is 36psi/2.5 bar with some exeptions to lower, and XL// 42 psi / 2.9bar exeptions 2.8/41 and 3.0/44.

[Mick70]

[flybobbie]

Wll BMW claim 50/50 weight distribution. But is that at max weight, fuel, hood up or down?
So just assume even weight distribution.
Could imagine a huge variation in a motor home.

[jadatis]

I already read once that BMW had practically 50/50 weight division, but assumed it to be for normal use.
And that is driver, co-driver, and a little load.
before 2000 car makers gave advice for normal use, and for that the car maker determined the axle loads for 3 persons and a little load, and gave for most cars front higher pressure advice than rear.

BMW has probably camber angle on rear wheels, for better roadcontact in curves. This needs higher pressure and would yustifie higher rear then front when 50/50 loadivision.

The simple spreadsheet of link in first post mostly filled in with MPVW ( max permissable vehicle weight) and the maximum pressure of tyre instead of Pr.
This gives a reserve when you use this advice for normal use. But better ( though more complicated) is to use the real weights and the Pr and take a reserve for things like next.
Pressure loss in time.
unequall loading R/L ( can even be crossed front and behind) , wich almost always is the case for motorhomes , but normal cars is also possible.
incidental extra loading. If someone ever weigh the car per tyre , then this information can be used for the same model and other loading can be calculated or estimated.
misreadings of pressure and weight-scales.
But if you take to much reserve things go bumping .
With the low Aspect ratio tires ( low hight widht division further AR) you have to put in high pressure for savety , so you dont risk blow out, but gives with lowest save pressure already bumping. so possible is damage and discomfort and bad gripp at the same pressure .
For motorhomes I determined by reactions about bumping, that if real weight on a tyre is below 85% of the weight the pressure is calculated, that then discomfort begins.
But for low AR tyres , probably below 110% bumping begins. 100% above gives tyre damage and risk of blow-out at higher speed.