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      07-28-2013, 06:55 PM   #23
pacoluca
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I totally agree with you, but sometimes people don't have time to play around with the air pressure, so the factory pressure is just the guideline for people to follow. But there are so many different "I assume they are correct" pressures people put in their cars, and u think the BMW dealers or car mechanics should know about the tyre pressures, but sometimes you find they don't really know what's the "right" information to advice customers which I find it is very annoying.
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      07-28-2013, 09:29 PM   #24
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To make it worse, the tire pumps at the service stations are highly inaccurate...
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      07-28-2013, 09:34 PM   #25
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that's true for our petrol stations, and sometimes the nozzles don't work, you have to keep pushing it hard enough to secure the connection. Better buy one of those digital air pumps and you can do it anywhere at time you want with more accurate measurement.
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      07-29-2013, 03:27 AM   #26
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The effect of road use on tyre pressures is quite interesting, so today I conducted an experiment. I had to complete quite a long trip - 630 km end-to-end. The attached pic shows this was completed at an average 94kph



Before I set off, I carefully checked and equalized all tyre pressures at 34psi front, 36 psi rear. These were set cold, in an ambient temp of 13C.

I measured them again, just then, at the completion of the drive. The last section of the drive was about 90 minutes at 110kph (GPS, on a heavily policed motorway). Tyre pressures were 35.5psi F, and 36.5psi R. Ambient temp is again 13C, although it was as high as 22C during the trip.

My conclusions:
  1. in day-to-day driving, ambient temps are as big a factor as road heat, although there is an effect (that's why you should check tyre pressures when there are big changes in ambient temps)
  2. Front (steering) tyres are more affected...makes sense...more friction, generating heat
  3. This is quite different to track driving, where there are big frictional effects at both ends, generating heat
.
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      07-29-2013, 03:47 AM   #27
ovekvam
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Quote:
Originally Posted by ttimbo View Post
This is quite different to track driving, where there are big frictional effects at both ends, generating heat
It depends on what type of road it is, and the driving style. On a twisty country road, the tires will heat up a lot more than on a straight road. A brisk driving style with lots of acceleration and braking also contributes.

New tires with maximum tread depth generate more heat than worn tires. If it is raining, the tires will generally stay rather cool.
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      07-29-2013, 04:34 AM   #28
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Quote:
Originally Posted by ovekvam View Post
It depends on what type of road it is, and the driving style. On a twisty country road, the tires will heat up a lot more than on a straight road. A brisk driving style with lots of acceleration and braking also contributes.
True, although I suspect it's extremely unlikely you'll generate a +10psi change, even on twisty public roads, unless there is also an accompanying increase in ambient temps during the drive.

As to tyres. I thought the opposite: new tyres generate less heat, relative to worn tyres due to differences in the contact area. It would be marginal, at best. Under inflated tyres also generate more heat.
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      07-29-2013, 04:36 AM   #29
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Few things and sorry if I digress a bit....
1. I follow the door panel for the tire pressure. Increase a little if there is a long trip on the highway, decrease a little if I am hitting the track. I try to do it when the tire is cold to establish a baseline.
2. For oil changes big manufacturers recommend 8000 km for synthetics, and BMW recommends 10000 km. In Asia when we suffer from high temperature and silly traffics BMW here recommends 5000 km, but not anything less.
3. 435i seems to oversteer easier compare to the M135i in Chris Harris' videos, is that down to the longer wheelbase of the 435i. A discussion with a friend he thinks with LSD M135i will snap away very easily. Well it is snappy alright, but I still think adding LSD helps, your opinion?
4. I used to drive irresponsibly in the mountains and just about everywhere as soon as I got the M135i. I hope people experience some track day to realize real driving can be improved there.
5. My trip computer tells me my oil change is December of 2014, and I have 12000 km left. I find the info useless, I wonder if that happens to anyone.
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      07-29-2013, 05:38 AM   #30
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Quote:
Originally Posted by westphone View Post
Few things and sorry if I digress a bit....
5. My trip computer tells me my oil change is December of 2014, and I have 12000 km left. I find the info useless, I wonder if that happens to anyone.
The condition based servicing uses gathered sensor data and time in it's scheduling, if you don't do the miles then it will be at the 2 year point so yes this is normal.
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      07-29-2013, 05:45 AM   #31
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Just a quick question. I haven't been on tracks at all. Why do we need to decrease tyre pressure? To make tyres have more contact with road surface? Or other reasons?
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      07-29-2013, 05:58 AM   #32
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Quote:
Originally Posted by pacoluca View Post
Just a quick question. I haven't been on tracks at all. Why do we need to decrease tyre pressure? To make tyres have more contact with road surface? Or other reasons?
During track driving, the pressure increases due to the heat. It is normal to decrease the warm pressure back to normal again, not lower than you use on the street. If it gets too low, you start wearing down the sidewalls.

Personally, I don't decrease the tire presssure on the track. I prefer running the pressure too high. It does cost me a little bit of grip and time, but it saves me from wearing out the outer edges of the front tires. I also avoid running very long stints, or else the tires and brakes can overheat. Many short stints are better.

Even better if it rains. Then you can just drive and don't worry about heat or pressures.
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      07-29-2013, 06:07 AM   #33
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Quote:
Originally Posted by ttimbo View Post
True, although I suspect it's extremely unlikely you'll generate a +10psi change, even on twisty public roads, unless there is also an accompanying increase in ambient temps during the drive.

As to tyres. I thought the opposite: new tyres generate less heat, relative to worn tyres due to differences in the contact area. It would be marginal, at best. Under inflated tyres also generate more heat.
I could make a 10 psi difference on a twisty road, but I won't! :-)
It is just a matter of driving in such a way that the tires are constantly screaming for mercy. You can build a lot of heat by doing 10 consecutive laps in a roundabout.

The heat generated in a tire comes mostly from rubber hysteresis. The ground deforms the tread pattern, and then it bounces back to the original shape once per tire revolution. New tires have more tread flex, thus more heat is generated. That is why new tires have horrendous tire wear during track days, while a well worn tire has a lot less wear. It runs cooler.

The tires that build the most heat, are studless winter tires. They have very soft and flexible tread.
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