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

Discussion in 'Chrysler 300' started by MoPar Man, Dec 14, 2009.

  1. MoPar Man

    MoPar Man Guest

    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.
    MoPar Man, Dec 14, 2009
  2. MoPar Man

    Josh S Guest

    Not needed here, even though I climb mountains to ski.
    Good all seasons do the job, the 300M traction control makes it a bit
    BTW I wouldn't use a different tire on the rear, the same all around is
    recommended for best handling.
    Josh S, Dec 14, 2009
  3. MoPar Man

    MoPar Man Guest

    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...
    MoPar Man, Dec 15, 2009
  4. MoPar Man

    Bill Putney Guest

    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.
    Bill Putney, Dec 15, 2009
  5. MoPar Man

    Steve Guest

    I'd be more worried about mixed-sized tires confusing the ABS, personally.
    Steve, Dec 15, 2009
  6. MoPar Man

    MoPar Man Guest

    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"

    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.
    MoPar Man, Dec 16, 2009
  7. MoPar Man

    Bill Putney Guest

    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.
    Bill Putney, Dec 16, 2009
  8. MoPar Man

    MoPar Man Guest

    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).
    MoPar Man, Dec 16, 2009
  9. MoPar Man

    Steve Guest

    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.
    Steve, Dec 16, 2009

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

    Percival P. Cassidy, Dec 16, 2009
  11. MoPar Man

    Bill Putney Guest

    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

    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. :)
    Bill Putney, Dec 16, 2009
  12. MoPar Man

    Bill Putney Guest

    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)."
    Bill Putney, Dec 16, 2009
  13. MoPar Man

    MoPar Man Guest

    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
    MoPar Man, Dec 17, 2009
  14. MoPar Man

    Steve Guest

    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.
    Steve, Dec 17, 2009
  15. MoPar Man

    Josh S Guest

    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
    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.
    Josh S, Dec 21, 2009
  16. MoPar Man

    Joe Pfeiffer Guest

    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.
    Joe Pfeiffer, Dec 21, 2009
  17. MoPar Man

    Josh S Guest

    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
    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.
    Josh S, Dec 27, 2009
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