Time to put the snow tires on my '00 300m

In case anyone wants to know a good combination of snows for a 300m:

Front: Cooper Discoverer M+S 215/70 R 16
Rear: Bridgestone Blizzak WS-50 205/70 R 15

This will be the third or fourth season for that combination. The front
shock tower guard clears the Cooper's by about 1/4 inch. The Coopers
are technically an SUV/light-truck tire.

Yea, they put the speedo off by a little, but they've also pulled me
down some roads where the snow was so deep it was scraping the bottom of
the car (and I took my front air-dam off a few years ago).

Still running on the original battery. 10 years 1.5 months old and
still crank'n.

No snow on the ground around here yet. Been really lucky that the
lake-effect snow has totally missed us.

 

Not needed here, even though I climb mountains to ski.
Good all seasons do the job, the 300M traction control makes it a bit
easier.
BTW I wouldn't use a different tire on the rear, the same all around is
recommended for best handling.

 

Yep, it's winter time, and it's the time when I get used to habitually
pressing the big round traction-control button to cancel it ever time I
pull out of my driveway. The most useless feature on my 300m (the
next-most useless feature being auto-stick).

My tires can't act like mini-snow-blowers as I turn corners and get up
to speed with TC turned on.

Since I don't exactly drive like Mario Andretti during the winter,
handling is not exactly my primary concern.

Going to 15" rims is the only way I'm going to put 205-width tires on
the 300m (and it can only be done in the back). In the winter, having
wide tires is a liability. Few people seem to understand that...

 

It may be more trouble than it's worth to you, but you could put a
power-up one-shot timer with contact closure output on that TC wire to
automatically turn it off at startup.

 

I'd be more worried about mixed-sized tires confusing the ABS, personally.

 

For one thing, ABS systems don't compare wheel-speed differences between
wheels or use that info to modulate the braking of individual wheels
(there are too many variables that could cause rotational speed
differences between wheels during an emergency or panic braking
situation for that information to be useful). Only the wheel-speed data
of individual wheels are used as feedback to those same wheels.

Second, if the over-all tire height is kept close to the original OEM
height, the wheel sensors won't know if you're riding on 15, 16 or 17"
rims.

Third, it's not so much that ABS is relying on the fact that your tires
should be a specific over-all diameter for a given car as much as it's
looking at the rate-of-change of rotational speed during braking and
attempting to prevent that rate from reaching a point where lock-up is
imminent. When you're working with measurements such as
radians-per-second, the tire diameter doesn't even factor into that.

 

That I did not know - I always thought they compared rotational rate of
the wheels - kind of determining an average speed of the wheels and
looking for variations of an individual wheel outside of a moderately
wide tolerance band (to allow for normal variations during turning,
etc). Your rate of change explanation makes sense.

Actually they won't know if you vary from the overall OEM height. Maybe
you mis-stated what you intended to say. Your next paragraph seems to
correct that.

 

If your OEM rim is 17", and the OEM tire-diameter is 26 inches, then you
can also get 26" with 15" rims and an appropriately-sized tire. In
either case, the hub-mounted wheel speed sensor wouldn't know if you had
changed the rim size because it will still measure the same
rotations-per-mile in both cases.

Some cars do (or did) make use of differences in rotations-per-mile
between tires as a way to indicate to the driver that a specific tire
might be losing air pressure. If a given tire is generating a higher
R-P-M reading than it's counterparts, then it's likely that it's losing
air pressure.

It would be nice if the computer inside the 300m would use that simple
technique. Even the 300C does not use that method (relying on more
expensive sensors built into the wheel).

 

wheels or use that info to modulate the braking of individual wheels

I don't believe that is true. The software allows for rotational speed
differences up to a certain limit, but the only way it can tell if one
wheel is locked is by comparing that wheel's speed to the other wheels.
Using mismatched tires means that you're closer to the allowed limit
even when the wheels are rolling normally.

True, but the DIFFERENCE in tire diameter does matter. If the overall
circumference is within a few percent, then you're fine.

 

AIUI, ABS braking systems check each wheel's rotational speed
separately, looking for a sudden rotational deceleration that indicates
that that wheel has locked.

Perce

 

Unless the car's computers are looking at GPS data (maybe there are some
cars that do that, but the 300M definitely does not), there is no way
that it knows how fast the car is going. It's *ONLY* info. about how
fast (in mph) the car is going is based on what it thinks it knows about
the tires size (i.e., whatever pinion factor is programmed into it by a
human).

If you put tires on that are 27.5" in effective tread diameter vs. the
stock 26", it's "rotations per mile" data will be in error - it must
look at *relative* speed data.

Again, unless it is via some GPS info., that is an impossibility.

No - they compare one wheel rotational speed to the others, like Steve
was saying.

Correct. But the car knows nothing about rotations per mile other than
from the pinion factor programed into it (IOW, change the effective tire
OD to something different, and it doesn't know that). It can do
relative measurements (one wheel relative to another), but it does not
know absolute measurements.

What do these sensors on the 300C look at? Is it at all related to GPS?

If I'm wrong about anything I've said above, I will apologize.

 

That's what I was saying in a post from this a.m. when I said "I always
thought they compared rotational rate of the wheels - kind of
determining an average speed of the wheels and looking for variations of
an individual wheel outside of a moderately wide tolerance band (to
allow for normal variations during turning, etc)."

 

You can look at the RPM / RPS history (rotations per minute or per
second) of any wheel independently or individually to know if that wheel
is slowing down at a rate that would point to imminent lock-up.

The road conditions, traction, etc, can be too variable between wheels
for their information to be used as part of an algorythm to feed back
into the modulation pattern of individual wheels.

Remember that the RPM history of each wheel is knowable to the computer
during the few seconds prior to brake application. During those few
seconds, the computer knows the RPM of each wheel, and an internal
algorythm can theoretically know the allowable rate-of-change of RPM at
that specific RPM before it starts to modulate the brake pressure at
that wheel. It doesn't need to know what the other wheels are doing
during braking, and it's hard imagine just how you would use any
differential information in a reliable and effective way.

Once the RPM has reached some arbitrary low value, the abs must
deactivate itself - otherwise theoretically it would never allow you to
stop the car because it would never allow the wheels to stop rolling.

You are making a prediction that if my front tires were, say, 1" in
diameter larger than the rear tires, that my ABS system would either (a)
activate itself unnecessarily during non-skid braking, or (b) would fail
to activate and would allow some or all tires to skid. I don't buy that
argument.

 

I believe that's more or less exactly what the FSM for my (now sold) 93
Vision TSi said. Granted, that's older-tech ABS.

The reason I cannot believe that the computer looks *only* at sudden
changes in each wheel's rotation is that you could have a pathological
case where a wheel starts slipping and gradually drops its speed to zero
while all the other wheels continue turning. That situation never
happens with ABS, so it DOES look at the other wheels to know that
they're still turning, even if that's not used for the initial trip-in
of ABS.

 

Auto stick and TC are two features I like very much.
If you left TC on you'd get there with regular all season tires.

Sliding friction is much less static friction. My objective on slippery
roads is not to wheel spin, TC helps you to achieve that, but is is very
noisy.
You must have skipped your high school physics class.

True, but I've not found the 300M's tires to be that excessively wide,
that the wheels float up on snow. If speed is excessive that could be a
problem, but snow calls for much lower driving speeds anyway to be safe.

 

Interesting -- we got autostick on our Intrepid because we couldn't get
a manual transmission (we have gotten a manual transmission on every
vehicle we have owned on which we have had a choice. We even gave up
the 4.7L V8 on our Dakota in favor of the 3.7V6, so we could get a real
transmission). We've found we've almost never used the autostick in
practice -- the only use it's ever gotten has been when we've had cruise
control on in hilly country and the "hunting" has gotten obnoxious.

It's in freshman college physics you find out that "dynamic" friction
and "static" friction are incredibly crude models, and neither models
tire grip even vaguely reasonably. Especially when you're looking at
tire grip in the presence of a dry lubricant.

 

You must live in flatter country. Here in western Canada there are lots
of long hills, even in the city I live in. On a 15 minute drive to my
tennis club I downshift twice going and once returning. Perhaps since
about 50% of my driving life was done with a stick shift, I'm more into
downshifting on hills, rather than standing on the brake for a few
minutes.

They are crude for sure, but tell a basic story. Then there is the
reduced traction from water then ice that forms under a slipping tire.
The bottom line is don't slide on snow or ice if possible, either when
braking or accelerating. The engineers who designed TC and ABS are well
aware of this.