'97 GC Tire Dilemma

[skibrain]

I'm new here and could use a bit of tire advice related to the importance of all tires being the same diameter.

I've got a '97 Jeep Gr. Cherokee Laredo/75K miles. 4.0L Auto trans. Quadratrac full-time 4WD (4Whigh/4Wlow). It's driven (6-7K miles a year) as a commuter for my daugter and occasional tow vehicle for my boat (2,000 # w/ trailer)

I have two sets of wheels and tires. stock size 225/75-15. Kelly passenger car type on the alloy wheels for summer. Blizzak WS-15 snows on steel wheels for the winter. The Blizzaks probably have 25K miles on them, but are in pretty good shape(2-3 more winters left). The Kelly's were on the Jeep when I bought it, are nothing special and need to be replaced next spring.

This week the Jeep is in for some minor body work. The garage called to say they had cut the sidewall on one of the Blizzaks when taking it off the hoist (?) Whatever, this model tire Blizzak WS-15 is no longer made. The newer Blizzak DM-Z2 is the closest but slightly different replacement. The shop got me a pair no cost to me, which I thought was fair.

This vehicle tends to be sensitive to tires. It is happiest (less trailing throttle whine from the drive train) when my summer tires are mounted one way, noiser if I rotate fronts to backs. So I have not been rotating them. Yes, they are all the same size rubber my dad put on it at 36K miles.

When I test drove the Jeep after picking it up last night, (new DM-Z2 Blizzaks on the back, old WS-15s on the front) I was getting a new pulsing thrumming noise. Not a tire imbalance. Not freeway grooves. A little bit faster than 1x per second at 50-60 mph. Strong enough to make the coins in the console buzz. Seems like it was better (or masked by wind noise) at 70-75 mph. I got out a couple of framing squares and a long ruler when I got home. Best I could measure horizontally accross the Tire diam. measured (from center of tread to center of tread all tires 32 psi) on old is 28 1/16" on the old. 28 1/2" on the new(Bridgestone gives a 28.3" diameter spec for the new DM-Z2 tires - maybe that is deflated?). The decreased size of the old ones is not surprising given the DEEP and now 1/2 worn tread depth on the WS-15s.

I have no locking rear diff. or center lock transfer case. When I called today the Jeep dealer didn't exactly endorse continuing to drive it that way with different diam tires. Am I going to wind up the T-Case and ruin it? or is there enough slip to accomodate this on dry pavement? Possible I was experiencing some tire tread noise, but that is typically a much greater frequency.

Not to have a matched set of tires bugs me anyway so I will likely spring for two more new snows and throw out the 3 still OK WS-15s. GRRr. It is pretty much guaranteed to get us a snow-free winter in MN. Or I'm still considering some good all-terrain tires to leave on year around, if the shop would give me credit for the somewhat rare size, and now mounted DM-Z2s.

I'm not so interested in debating the merits of winter tires (I've had Blizzaks on another vehicle as well) but could use some insights into the problems running slightly different diam. tires.

Thanks.

[PlatinumZJ]

Originally Posted by skibrain

I'm new here and could use a bit of tire advice related to the importance of all tires being the same diameter....

...I'm not so interested in debating the merits of winter tires (I've had Blizzaks on another vehicle as well) but could use some insights into the problems running slightly different diam. tires.

Thanks.

the quadratrac hac a viscous coupler (VC) in the transfer case (TC) it acts like limited slip device (LSD) between the front and rear axles. When the tire size is different between the front and back alxes, there is added strees on the vc because the tires turn at different speeds. Provided that your measurements represent the diameter of the tire (as best as you can measure) the difference in circumference is about 1-1/3 inches. Thats probably not enough to cause serious damage in the short run, but probably gives you the thrumming sound out of the tc. long term it will likely wear out the VC quicker than normal (They are about $1000± to replace).

You can measure the circumference directly by either jacking up one side and with a cloth tape wrapped around the center of the tire (should be about 89.5 inches on the new tire) or mark a spot on the side wall and roll it one revolution and measure the distance rolled.

I would ask the shop for two additonal tires or ask them to share cost for them and replace the old ones within a few weeks.

[josephmartins]

Here's a supplement to SecondZJ's answer:

Tire Rack's Guide for Mixing Tires on a 4 Wheel Drive

Basically it's not a good idea and can cause costly premature failure of other driveline and/or suspension components.

Kind regards,

Joe Martins
Dublin, NH

[LJ_Cool]

The Dealer (parts guy) informed me that the tolerance for front to rear is 1/16 of an inch in diameter, (that causes enough drive shaft rotational varience to cause the VC to lock up)

I did question him as that seems pretty tight, but he was pretty convincing

I did a little math to determine how different the drive shafts would be rotating assuming the following
28.5 " tire
28.5625 (1/16" bigger)

Drive shaft Revs per mile with a 3.55 is
old 2512.17
new 2506.67

at the tires it's 707.65 vs. 706.1

so for every mile you drive, the vc has to allow for 5 rotations without locking and that's with just 1/16 of an inch,

if your tread is 7/16" different, then you're asking the VC to ignore roughly 38 additional rotations of the bigger tires every mile, sounds to me that it should engage

[Meerkat]

Now that the rocket scientists have weighed in (they are a bunch of smart fellers, aren't they?), it seems to me that if you like the Blizzaks, consider you got two new ones for free and head on down and get two more on your dime. Sure you'd be throwing away three tires that may last 2 or 3 more winters (barring accident), but you are getting a full set at half price. I think the garage showed their mettle when they: 1. notified you of their error vice waited to see if you would notice 2. replaced the tire with two new ones. Alternatively, perhaps the tire dealership will take back the two new Blizzaks and outfit you with one set of BFG Long Trail A/T's that you can leave on there year round at a significant discount. That way you will get the most tire (usage) for your dollar.

HTH

[Texas ZJ1]

I agree with WVD. And, If you don't want to throw away the three tires, keep them for spares. Yes, they are slightly different but, a spare is just to get you to a shop.

Hunter

[skibrain]

Good stuff. I appreciate straight talk and shared wisdom/experience.

Yeah, I think I am not doing too badly having to spring for a pair ($226 mounted shop's quote) of new Blizzaks to make a set - I think this again shows their good faith. In a previous incident where a guy rear-ended me in winter (I had Blizzaks, he didn't) they went above and beyond to help me out.

Or I'll hear from them today and see what they might do if there is a refund on the trading in the now-mounted DM-Z2s as credit toward a set of 4 all-seasons. (the 225 75-15 size is not real common on newer SUVs) I don't really need anything that agressive since it is pavement or more likely occasional intersection ice or compacted snow than rural deep stuff.

You mentioned BFG Long Trail A/T's.

Any other favorites? TireRack seemed to think pretty highly of the Firestone Destination LE or the Yokahama H/T-S G051.

(I don't know how legit the readers opinions/ratings are, but that is what I am looking at) It seems like some of the more aggressive tread tires that you would THINK rate highly in the snow are not as good as some of the all-seasons.

Thanks All!