JKU Smaller Spare Tyre

[rjvines]

Originally Posted by samft

given that JK's have no center diff we get the same rotations sent to the front and rear axles.

this means 1 turn per wheel on the fronts (total of 2), and 1 turn per wheel on the back (total of 2) true in corners or in a straight line

the matching tyres will rotate on average once per wheel and will travel an distance of (2*pi*33)/2 or 103.7inches

the diff on the other axle will try and compensate for the miss-matched tyres but the average rolling distance will be less (pi*31.7)+(pi*33)/2 or 101.6 inches

i.e. the axle with matching tyres will try and move 2 inches further than the axle with the smaller tyre (which is bad obviously) this will create wind up in the diffs until the wheels slip (or until failure of the diff, i presume)

this means you can use a non matching spare if you are in 2wd, but not in 4wd

2 inches of slip isn't a huge amount though, and you would probably get away with it on sand where the tyres are slipping anyway.

You will also probably get an electronics error but should be able to drive still, i think traction control would be disabled.

I just made all that up based on my understanding of diffs and the JK transfer case, feel free to shoot me down anybody?

Your knowledge is clearly much more extensive than mine...but wouldn't the fact that the diffs are open negate any issues of slip?

I wonder if the bld would detect the difference in speeds and kick in?

[Hayesio]

I often see people driving around with a smaller spare and ponder this exact question. It would be an interesting experiment to put it on and see what happens (under control conditions of coarse). My guess is that the PTCM will see 3 wheels doing what it expects (with the steering angle sensor straight) then simply throw a fault code. Cruise will disable, abs, stability control, traction control will all disable, but the vehicle will probable go on fine. The other possibility is that the traction control and abs go nut and start applying the brakes! Be one or the other...

[MightyMouse]

Actually....... whatever differential is involved will operate ( that's what a diff does... compensate for wheel to wheel speed differences.... ) with the result that there will be no drive line windup, but the spider gears etc will be continuously operating and that's not a great idea if large distances are involved.

Driven sensibly / cautiously you'd likely get back to civilisation..... as in the scheme of things they wouldn't be all that highly loaded - but there's some risk.

The electronics however will be very unhappy as it will seem to be continually turning a corner with the steering straight ahead. Eventually they will give up and fault out.

All in all, probably ok if you must but not a perfect setup

[Banshee]

If you are in 4wd the front & rear turn at the same speed, the only difference will be in the smaller wheel turning faster. The open diff will accommodate it & there will be no front/rear slip.

This will probably be read as a sensor problem in RWD & the BLD etc will be shut off & you will get the ESP/BAS lights etc on, no cruise control etc.

I just keep the 285/70 on the stock wheels with the stock carrier on the stock hinges. I don't even have any rattles from it yet. Plenty from other places, but not the tailgate! And the only time the stock cast alloy carrier had a problem was when I backed into a tree that came up to the bottom half of the wheel, then went sideways, so I couldn't see it in the rear view mirror as I was reversing...

[Snort]

just get the spare the same size. What's another 400ish smackers when you've just dumped 52k on a special ops and another 2? On 4 tyres.
Checkout ntrubicon's review of his new cooper's (285/70/17) he has included photos of the spare on the stock carrier.

[samft]

I think you guys are forgetting the JK has no centre diff

which means in 4wd the 2 axle diffs cannot compensate for one axle having a smaller wheel

In 2wd it wouldnt cause any issues

the lack of centre diff is the reason we cant drive in 4wd on tarmac, because when you turn a corner, the rear wheels take a shorter path but are being forced to go the the same distance by the locked transfer case. this causes wheel slippage which can do a lot of damage on a sealed road.

the diffs can only compensate across the axle (there's no centre diff, so F/R power is always equal), both axles receive the same number of turns, but one axle has a smaller wheel so there will be wind up somewhere? (when in 4wd)

with differential, whatever one wheel is slowed down by, the other wheel is sped up by, always summing to a total of 2 turns

i.e. L:R wheels 1:1 turns 1.4:0.6 turns 1.9:0.1 turns or the problem situation of 2:0 turns where one wheel receives no power and one wheel is spinning

[MightyMouse]

As i see it..... the odd tire is always on a single axle so subject to differential action on that axle. Neither of the tires is positively connected to the tailshaft or transfer.... its always through a diff, which allows different relative wheel speeds by design

Really its the equivalent of continually turning a corner where the outside wheel rotates faster than the inside one ( why we use diffs ) the driveline doesn't lock up... the differential on the axle just does is job.

It certainly is interesting and a challenge to get your head around..... but I've had vehicles on trips use smaller spares after tyre damage etc and there just didn't seems to be any issues.

Perhaps its just a matter of "degree of mismatch"........ but 3 x 33's and one 31 seemed to be cool...... he finished the trip and drove it home.

A mathematician could work it out for us..... its a differential equation ( literally ) but when i run out of fingers its too hard for me.