Winter tyre pressure

[kw2010vish]

Hi everyone
Im new to mbclub uk. From tayside, scotland.
Just getting the car (2010 c220 sport auto) ready for winter and this being first rear wheel i have ever owned and my first rear wheel winter, i have a few questions-
1. Rears are avon ice touring st 225/45/17. What pressure should i set them to?
2. Fronts are Bridgestone blizzak lm25 225/45/17. Again what pressure?
3. I have read on forums about rear axles needing higher pressures and some extra weight to help. Anyone here with the same kind of spec? please shine some light into this.
Thanks in advance

 

[robtt]

I run mine at slighty higher pressures : not sure it has any advantage but was told it was right thing to do ! I ran with them through last winter in rural North Yorkshire and never got stuck once.

 

[Sp!ke]

what pressure does it say on the tyres?

 

[whitenemesis]

Fuel filler flap and/or the owner's handbook will list the recommended tyre pressures.

 

[kw2010vish]

I put something around 30-32psi light loads (upto 3 people) and around 36 psi for full load (5peoplel). but thats on summer conti 3s.

 

[theogeor]

Worth remembering that tyre temperature quoted is for ambient temperature which is 20-22C. When is colder the tyre pressure will show lower. For example 40 PSI will be 36-37 at 5C so keep that in mind.
So look at your tyres and car and try to adjust it based on the temperature. I have done a mathematical modelling of that which I can post if people are interested. It shows how temperature affect pressure both is cold but also when the tyres are warmed up. It was very useful to understand that in conjunction with my TPS in the recent past during high speed driving in Germany and during a track day..

Regards

Theo

 

[Chalpkin]

Worth remembering that tyre temperature quoted is for ambient temperature which is 20-22C. When is colder the tyre pressure will show lower. For example 40 PSI will be 36-37 at 5C so keep that in mind.
So look at your tyres and car and try to adjust it based on the temperature. I have done a mathematical modelling of that which I can post if people are interested. It shows how temperature affect pressure both is cold but also when the tyres are warmed up. It was very useful to understand that in conjunction with my TPS in the recent past during high speed driving in Germany and during a track day..

Regards

Theo

Hi Theo,
If you could post your model I'm sure most of us would find it useful.
Regards,
Chalpkin.

 

[theogeor]

Hi Theo,
If you could post your model I'm sure most of us would find it useful.
Regards,
Chalpkin.

Its an excel spreadsheet. (see zip attachment) The only thing you need to adjust is the reference pressure and that will give you the pressure in different temperatures. In my calculations I assume that the volume of air within the tyre does not change. In reality when you have very low or very high pressure you will have some differences but percentage wise are small and not significant for most of the cases

Would be interested in getting some feedback .

Regards

Theo

 

[grober]

Just use the standard pressures for your car with your winter tyres. Winter tyres do respond slightly differently to summer tyres at lower temperatures as they retain their wall elasticity to a greater extent than summer tyres. However as a simple rule of thumb approach the best way to check your tyres pressures in the winter is in the middle of the day preferably when the car has been sitting in the sun for a time to warm everything up and ambient temperature is as high-ish as its going to get. maybe take the car for a short run to warm everything up then let it sit for 15-30 minutes. That should give you a reasonable tyre temperature that normal pressure settings will suit. the one time not to check tyre pressures in the winter is when you have just rolled it out of the garage or its been sitting in the drive outside over night - they will be artificially low- resist the temptation to blow them up- as pressure will rise to normal levels as the tyre warms up. Only cause for alarm with cold tyres would be if any one tyre showed lower pressure than the others which might indicate a puncture.

 

[tode]

Its an excel spreadsheet. (see zip attachment) The only thing you need to adjust is the reference pressure and that will give you the pressure in different temperatures. In my calculations I assume that the volume of air within the tyre does not change. In reality when you have very low or very high pressure you will have some differences but percentage wise are small and not significant for most of the cases

Would be interested in getting some feedback .

Regards

Theo

Should you not be using the absolute pressure in the tyre for the gas law to be correct ? (Guage pressure < abs. press.)

 

[theogeor]

Should you not be using the absolute pressure in the tyre for the gas law to be correct ? (Guage pressure < abs. press.)

You are correct but if you are using a TPMS like in my case then I am measuring absolute pressure. If I try o make he model more accurate then I have to take into consideration other things like
1. Difference between tyre temp and external/ambient temp
2. composition and elasticity of the tyre
3. elevation as that affects atmospheric pressure
4. margin of error of th guage
Plus I a sure few other things that I have missed

Saying that what the calculations are trying to illustrate is the relationship between temperature and pressure within the tyre it self and from measures I have done the theoretical model is very close to the actual measurements. And this should be used as a guide by the way...

Regards

Theo

 

[kw2010vish]

thanks for the upload theo

grober- you are right. i have always done the same on my cars, but jus a bit worried with winters and rear wheels

jus had the car all washed and waxed ready for winter.
fronts are only like 4.5mms and rears on 6mms.
hopefully it would do me through this winter
read on a forum that winter tyres under 4mms arent really much useful, which didnt really much sense to me!

 

[grober]

Winters less than 4mm will suffer a slight diminution in performance over new but will still be superior to summer tyres due to their directional staggered tread pattern and rubber mix.