ultraclyde
ultraclyde PowerDork
1/3/18 8:27 a.m.

I'm about to replace the 235/70-16 street tires on my 1997 Grand Cherokee 4x4 with a good set of AT tires.  I'm probably going to a 255/70r16 - 1" taller.  I'd like to keep one of the street tires as a spare. They're new tires, they're the largest that will fit in the stock spare location, and it will save me $$. I'm seeing very mixed info about running a smaller spare on this vehicle, even for short distances. It has the QuadraTracII 4wd system - that is a NP249 viscous T-case and open diffs front and rear.  I know if you have a LSD or locker that mismatched sizes on an axle is BAD, but I'm not sure how this affects the T-case with open diffs.   Is this a concern because the rear prop shaft speed is increased with the smaller tire, effectively simulating a slipping rear axle and engaging the T case?

IF that's what's going on, could you put the smaller tire on the front and be okay? How does the T-case react if the front prop shaft speed is higher than the rear?

 

rslifkin
rslifkin SuperDork
1/3/18 9:15 a.m.

With the viscous t-case, you cannot run a different size spare on either end.  It'll cause slippage (and heat) in the viscous coupling regardless of which end is spinning faster.  In the immediate term, it'll cause the driveline to start binding up after a few miles (as if you had a standard 4wd setup in 4wd on pavement).  With the different size tire, that binding will cause noticeable drag when you try to move.  And it'll shorten the life of the t-case.  

Basically, like most AWD or fulltime 4wd systems, a different size spare isn't a spare at all, it's just decoration that can't be used.  

A 255/70R16 should fit in the stock spot (especially with the cargo cover removed), the cover just won't fit.  

ultraclyde
ultraclyde PowerDork
1/3/18 9:24 a.m.

Okay.  How much different does the size have to be to cause a problem? Can I get a used tire in the same size and be okay? I know buying 5 and doing standard rotations with the spare is the recommended procedure, but...

 

I'm also weighing the possibility of just carrying a big can of fix-a-flat.

GameboyRMH
GameboyRMH MegaDork
1/3/18 9:26 a.m.

It's not a viscous T-case by the looks like it, mechanically it's more like a modern active diff with hydraulically actuated clutch packs, although it's a passive system:

https://en.wikipedia.org/wiki/Jeep_four-wheel-drive_systems#Quadra-Trac_II

Any mismatch in axle speeds beyond a certain threshold will begin locking the front and rear axles together. If one undersized spare is enough to trigger this, it will wear on the clutch pack just like running mismatched tires across a clutch-type LSD.

ultraclyde
ultraclyde PowerDork
1/3/18 9:28 a.m.

In reply to GameboyRMH :

My bad, it's a QuadraTrac, not the II.  It is viscous but the end result is the same.

 

EDIT: This is another reason to put a 242 or 231 in when the 249 dies I guess. Then I could at least run it in 2wd.

GameboyRMH
GameboyRMH MegaDork
1/3/18 9:34 a.m.

Ah the NP249 is indeed a conventional viscous diff. A different sized spare will indeed cause some heat buildup that could eventually lead to binding. You might get away with one slightly undersized spare, since the axle speed mismatch will be very small, but it's a gamble.

rslifkin
rslifkin SuperDork
1/3/18 9:43 a.m.

A used spare of the same size should be ok.  Basically you're going for "as close as humanly possible".  Personally, what I did when I still had a 249 was buy tires in sets of 5 and rotate the spare in with the others.  It meant the spare was always worn the same, never older than the other tires and it wasn't really more expensive as the whole set lasted a bit longer to offset the higher initial cost.  

ultraclyde
ultraclyde PowerDork
1/3/18 9:45 a.m.

Interestingly, after a quick search on car-part.com it seems I can buy a NP242 for the same cost as a matching new spare tire.  

ultraclyde
ultraclyde PowerDork
1/3/18 9:47 a.m.

In reply to rslifkin :

well, I hadn't thought of increased wear decreasing cost in that manner. And it does greatly simplify tire rotations too.

rslifkin
rslifkin SuperDork
1/3/18 9:56 a.m.
ultraclyde said:

Interestingly, after a quick search on car-part.com it seems I can buy a NP242 for the same cost as a matching new spare tire.  

If you do the swap, I'd say go with a 231.  The fulltime mode in the 242 is an open center diff, so it's relatively useless IMO.  And the 231 shifts in / out of 4wd a lot better than the 242.  Plus, the 231 is 20 lbs lighter than the 96 - 98 249 or any 242, so it gains more racecar points.  

ultraclyde
ultraclyde PowerDork
1/3/18 10:05 a.m.

The 231 swap takes a little more work, but it is a better unit.  Speaking of tcases and since I have your attention here's another question: The 249 works fine right now at 160k miles. I've got 2 bottles of the correct, 249 specific fluid to do a fluid change. IF the fluid has ever been changed (which I doubt) what are the chances I add the new fluid and the fresh friction modifiers cause it to start locking all the time? I know that the mileage puts it near the end of its service life, but I'd like to avoid the swap for a while. I'm hesitant to change the fluid even though I'm sure it "needs" it.

GameboyRMH
GameboyRMH MegaDork
1/3/18 10:09 a.m.

You can't make a viscous diff lock more aggressively than stock with fresh (stock) fluids.

Streetwiseguy
Streetwiseguy UltimaDork
1/3/18 10:15 a.m.

I swapped a 242 into my old Commanche specifically to get the open full time position.  Living in a snowey land, it is fabulous to pull it into AWD in November and back into 2WD in April.  It does have the 4wd lock position too for the times you really need it.

rslifkin
rslifkin SuperDork
1/3/18 10:20 a.m.

The viscous coupler is sealed within the t-case, so a fluid change won't affect it at all.  The t-case fluid just lubes the gears, bearings and chain.  

There's also no 249 specific fluid.  You've probably got the stuff for the 247 (Quadra-Trac II in a WJ).  It's fine to use that stuff in a 249, but the spec-ed fluid is ATF+4.  Personally, I've been running Redline 75W80 MTL in my case as it's just a hair thicker than ATF and cuts down the t-case noise in 4wd a bit but is still thin enough for the wimpy little pump in the case (thinner than the M1 5W30 some t-case builders recommend).  

If you get a 231 from a 96 - 01 XJ (6 cyl), it's a direct bolt-in to a 96 - 98 ZJ.  No changes needed at all, other than the connector for the 4wd position display won't hook up (it's fine to leave it disconnected).  I'm running an XJ 231 in my ZJ.  

Furious_E
Furious_E SuperDork
1/3/18 10:35 a.m.

Does the 249 have the same style outputs as the 231 and 242? When I did the lift and slip yoke eliminator on my XJ with the 231, I remember crawling under a lot of different ZJs trying to find the right front driveshaft that I needed for the rear of my XJ (IIRC 5.2/242/94-95 MY was the magic combo). The 231 and 242 should both have a fixed yoke on the front and a slip yoke on the rear output, but I remember a lot of trucks having a different t'case with flanged output on at least one end. IIRC that was the 249. 

What I'm getting at is you might need to budget for new driveshafts as well if you're looking at swapping the transfer case. Not that that's a huge deal, by any means, just another cost to consider. 

Edit: started typing that before rslifkins post, nvm

bigdaddylee82
bigdaddylee82 UltraDork
1/3/18 11:04 a.m.
Streetwiseguy said:

I swapped a 242 into my old Commanche specifically to get the open full time position.  Living in a snowey land, it is fabulous to pull it into AWD in November and back into 2WD in April.  It does have the 4wd lock position too for the times you really need it.

Did the same thing for similar reasons in our XJ, though our snow is rare, the thought process was the same.

rslifkin
rslifkin SuperDork
1/3/18 12:46 p.m.

In reply to Furious_E :

The 249 is internal slip yoke while the 96+ 242 and 231 are external slip yoke for the rear output.  But the same driveshaft works on both.  Front output yokes are the same on all of them.  No ZJ or XJ left the factory with a fixed yoke on the rear of the t-case.  TJ Rubicon NP241OR cases are fixed yoke, however.  

ultraclyde
ultraclyde PowerDork
1/3/18 1:55 p.m.

Here's another question while i have the attention of the Jeep and off-road crowd, and no one answered it in my build thread - I need new sway bar to frame bushings. Urethane or rubber for this application? 

GameboyRMH
GameboyRMH MegaDork
1/3/18 2:00 p.m.

Rubber for quiet, urethane for longevity and a slight increase in effective bar stiffness at the expense of more noise. You can try wrapping the bar in teflon tape where it contacts the bushing to reduce noise.

rslifkin
rslifkin SuperDork
1/3/18 2:23 p.m.

Run poly for the sway bar bushings.  I've heard a few people say they squeak, but mine have never made a single hint of noise (up front, anyway, as I don't run a rear sway bar)

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