i don't think it's cheaper to use the current system vs. the abs system. the abs system just monitors wheel spin, as i understand it, and does not require any additional parts. and the abs system doesn't monitor actual pressure.

the system on the x5, as i believe, uses an actual sensor to monitor the pressure. the stupid part, is they don't TELL us the pressure they are monitoring, which i don't understand why.

In the M3 there is no additional sensor, it is literally counting the pulses given off of the ABS sensor that is already there and comparing the 4 wheels - in a way a free "intelligent" system.

The benefit of this is no additional weight in the wheels.

Cheers,
KEF

> ts far more accurate, reliable, less false alarms, and you don't need to purchase sensors each time you get a set of wheels.

Sorry, but that's not true. The ABS system is less accurate and you get more false alarms. The advantage of the ABS system is that it is cheaper, less complex, and avoids additional "stuff" in the wheels.

For example, tires over time all lose pressure. With the ABS system you will never be informed of low pressure as long as all the tires lose pressure at similar rates, e.g., if, over a period of months, the tires all lose 60% of their air pressure, the ABS system will think everything is just fine, since the relative rotational speeds of the wheels has not changed.

The ABS system does measure the overall diameter. Its a far superior system to the one that BMW uses on its cars now. I own vehicles with both, and he M system has been complete reliable.

Yes, I too owned both vehicles the E46 M3 and the E70 X5 and agree with you that the M system (the rotational speed system) has been flawless. I never once had an issue with the way it worked.

BUT the ABS system does NOT directly measure diameter, it is back calculated from Velocity = Omega x Radius (V=wr), Radius being one half of the wheel diameter.

What Penguin is saying is correct; I do not agree with him about reliability of the system, (based on my experience with it), but he is none the less correct. If all 4 wheels went down in pressure equally, there would be no wheel-to-wheel difference, and thus no alarm.

The ABS system is looking at a difference in Radius in the V=wr equation – since the car speed is constant, if there is a difference in w (rpm) there must be a difference in radius.

That is how the system is backing out a pressure difference – from the change in tire diameter, it is not directly measuring tire diameter.

Cheers,
KEF

Similar question to this thread:

I have OEM 20's, and am trying to determine what the most cost effective way of getting some snow tires.

I found some Pirelli Scorpion Ice & Snow tires that are supposed to work for the 20's, at about $250/tire. Do you guys think they'll still be useless since they're on these fat 20" wheels?

Or should I try and find some 18" wheel & tire combos? It seems like that's the more expensive route - putting me in the $1800 range?

HELP!!!

18" will be much more effective in the snow... and easier to change each year and you won't have to pay for remounting/balancing each time.