Full size spare necessary?

[TKL223/3]

A 5 tire rotation only requires 1 jack.

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[jeepoch]

Thanks for the info and help everybody. I'll go with 5 new tires, and rotate them over time.

B,

Good choice. Different diameter tires on the same axle will cause mechanical binding in the differential. Thus causing metal shaving, sludge and likely pretty extreme wear leading to premature gear failure. If different diameter tires are front to back (opposite axles) then it's the transfer case that will suffer the same kind of damage.

I sure would care much less about wearing rubber than gears. A different size spare may be appealing, right up until you need it.

This same type of damage can still occur (to a lesser degree) if you never rotate the spare. The more consistent the diameter of all the tires the happier your rig will be.

Jeep On...
Jay

 

[Dyolfknip74]

The manual states the factory spare tire, although it appears to be the same as the four on the ground, is a temporary tire. I'm aware the manuals are often used for different vehicles so I'm asking if anyone knows for certain if the spare is or isn't temporary.

Put the manual down and go look at your spare. Does it look temporary? It's a full size spare. You're good.

 

[GtX]

The manual states the factory spare tire, although it appears to be the same as the four on the ground, is a temporary tire. I'm aware the manuals are often used for different vehicles so I'm asking if anyone knows for certain if the spare is or isn't temporary.

Spare is the same. Usage is temporary because you should fix the damaged tire as soon as possible so you have a usable spare again.

 

[seanpublic]

Having TPMS sensors in all 5 wheels is a give away that the engineers designed all 5 wheels to be completely interchangeable and fully functional as regular use wheels. The sensor relearn procedure is fully automatic to assist in this regular procedure. That's different than a traditional pickup truck setup, where the full size spare is mounted on a basic, steel rim without a TPMS sensor. With a pickup, you don't lose capability to tow/haul on the spare, but the engineers clearly didn't intend for a pickup truck spare to be part of the rotation. At least that's how the older pickups with TPMS worked.

 

[J0E]

With all the computers in your JL, not to mention 2 differentials, if you try to drive your Jeep with 3 37" tires and a 33" tire, your Jeep is going to freak out before you get more than 10 feet.

Most modern vehicles will allow some discrepancy in tire size from front to rear (1-2% difference in height), but it typically has to be less than 1/4" or you'll start getting all kinds of warning messages or worse.

Not true. Lots of people run donut spares for months. Put the 33 on the front.

 

[BadAssBumbleBee]

Not true. Lots of people run donut spares for months. Put the 33 on the front.

If your vehicle is equipped with a donut spare from the factory, it's going to be the same diameter as your full size road tires, so you won't have a problem running it.

Running 3 37" tires and one 33" tire is going to create all sorts of problems and damage regardless of what axle you put the 33" tire on.

 

[rcadden]

Having TPMS sensors in all 5 wheels is a give away that the engineers designed all 5 wheels to be completely interchangeable and fully functional as regular use wheels. The sensor relearn procedure is fully automatic to assist in this regular procedure. That's different than a traditional pickup truck setup, where the full size spare is mounted on a basic, steel rim without a TPMS sensor. With a pickup, you don't lose capability to tow/haul on the spare, but the engineers clearly didn't intend for a pickup truck spare to be part of the rotation. At least that's how the older pickups with TPMS worked.

Fun fact - the Gladiator is the same way. The spare is on a basic steel rim.

I learned/confirmed when I was shopping for take-offs as I *really* like the Gladiator Rubicon wheels, but couldn't find a 5-pack listed anywhere. Would've had to buy a 4-pack and then find a way to just buy 1 for my JLU.

Gave up and just got JLU Rubicon take-offs.

 

[jeepoch]

Not true. Lots of people run donut spares for months. Put the 33 on the front.

J0e,

Double not true to you. Yes some people may run dissimilar sized tires all the time, but they are still nonetheless braniacs who are hurting their vehicles.

Both your differential and transfer case (on a 4WD or AWD powertrain) have multiple sets of gears. These all must mesh properly in order to efficiently transfer torque (rotational energy). This transfer works great when both sides of the gearing can rotate at the same speed. With different diameter tires this IS NOT possible.

If the surface is forgiving such as dirt, then the tire with the smaller diameter (2 x radius) will spin faster at any given speed. Think of the circumference of the tire which is simply 2 x π x radius (2πr). If you were to roll the tire one complete revolution it will give you a specific distance.

Clearly, the larger the tire the greater it's circumference, hence it travels further each revolution. Now the engine is providing torque which is an angular force producing rotational energy as measured in revolutions per minute (rev / min, or just RPM). So at any given RPM the smaller wheel is traveling less distance.

Something's got to give!

Again on forgiving surfaces, the smaller tire will always slip. On hard pavement this can't happen as easily. This results is slipping (binding) at the mechanics of the gearing. Either within the differential when the dissimilar tires are on the same axle, or the transfer case if different from front to back. Obviously both if running just one different size tire.

Mechanical binding is no fun. This causes shaving of the gears since they don't properly mesh. This is because the two sides of the axle are rotating at different rates. When the rubber is gripping the road (not slipping) the shaving of each and every tooth in the gear set WILL occur to some degree. Over time they will be shaved down to the point where zero torque can be transfered.

Until then, these metal shavings will collect in the diff fluid causing much higher friction and heat only exacerbating the gearings overall demise.

So yes J0e be my guest and drive around on different size tires. You'll end up paying a stupid tax with a large repair bill.

BUT PLEASE, don't pass this advice on to other potential mechanically challenged knuckleheads. Economics and inflation are already expensive enough. Ironically, which are likely the result of the same.

Jay

 

[J0E]

J0e,

Both your differential and transfer case (on a 4WD or AWD powertrain) have multiple sets of gears. These all must mesh properly in order to efficiently transfer torque (rotational energy). This transfer works great when both sides of the gearing can rotate at the same speed. With different diameter tires this IS NOT possible.

They never rotate at the same speed when not straight and level.

J0e,

Clearly, the larger the tire the greater it's circumference, hence it travels further each revolution. Now the engine is providing torque which is an angular force producing rotational energy as measured in revolutions per minute (rev / min, or just RPM). So at any given RPM the smaller wheel is traveling less distance.

Something's got to give!

That's what differentials are for. I'm not saying lock the differential when you have a different size tire. Put the small tire on the front and drive home in 2 WD. Then you have no problem.

Again on forgiving surfaces, the smaller tire will always slip. On hard pavement this can't happen as easily. This results is slipping (binding) at the mechanics of the gearing. Either within the differential when the dissimilar tires are on the same axle, or the transfer case if different from front to back. Obviously both if running just one different size tire.

Ridiculous. Even when locked that's not true. The tire with the least traction will slip. Even worst case with lockers, which I'm saying don't use, small tire on rock, large on loose dirt, which will slip?

Mechanical binding is no fun. This causes shaving of the gears since they don't properly mesh. This is because the two sides of the axle are rotating at different rates. When the rubber is gripping the road (not slipping) the shaving of each and every tooth in the gear set WILL occur to some degree. Over time they will be shaved down to the point where zero torque can be transfered.

You're obsessed with lockers. That's what differentials are for.

I know guys who commute daily and extremely winding roads. 50,000 miles and their differentials are fine.

You're going panic mode for putting a smaller tire on the front and driving home. If the OP doesn't want a full size spare, don't get one. He'll be fine driving home to anyone who understands how a differential works. I've seen it in action. Finish the run, drove home 45 miles. No detectable change in the front differential fluid.

I must assume you've never done any serious wheeling with lockers engaged. Rock crawling with front and rear locked put far more strain on the driveline than no front locker with different size tires.

Where were you when I was asserting never drive in 4H with the transfer case locked on the highway when you hit dry spots. Another guy said I was foolish to think some highway driving in 4 Hi (not full time 4 WD option) would be problematic on dry corners.

 

[J0E]

If your vehicle is equipped with a donut spare from the factory, it's going to be the same diameter as your full size road tires, so you won't have a problem running it.

LOL Your spare tire, be it a full-size non-matching spare tire or a compact spare, is typically a smaller diameter than your four normal use tires. It may be a slight half-inch up to a couple inches in diameter different and the width is usually significantly less than your factory tires.

Running 3 37" tires and one 33" tire is going to create all sorts of problems and damage regardless of what axle you put the 33" tire on.

How so on the front in 2 WD? Another person doesn't understand the FAD and how differentials work.

 

[jeepoch]

@J0E,

Of course differentials are designed for cornering. And you're correct that turns do indeed make one wheel spin faster than the other. That's after all the magic about open diffs.

Also I spoke zero words about 'lockers', never even brought up the concept. But you are again correct that wheeling dissimilar sized tires on a locked axle will indeed magnify the problem.

However, open diffs are designed to corner efficiently but not permanently. The two sun gears spin at the same rate and the planet gear does not when driving straight with no slip. Any turning or running with dissimilar sized tires will cause one sun gear, either the left or right depending on the direction of turn (or location of the smaller tire), to spin faster than the other which causes the planet gear to rotate, either clockwise or counter-clockwise.

Any rotation of the planet gear causes friction. Any friction produces heat. This heat is produced by grinding the teeth between both the planet and sun gears since they are all now spinning. This obviously causes wear over time. The amount of wear is a function of the rate of rotation of the planetary gear and the amount of torque delivered by the engine.

Clearly, the most ideal and efficient outcome is to have zero rotation on the planetary gear. This can ONLY occur if travelling strait on similar sized wheels, or turning in the direction opposite the side of the smaller wheel.

Running with a smaller wheel anywhere, for any length of time will always generate more differential gear wear. Unless you're driving in circles in the appropriate direction.

My advice is to never use a smaller tire under any circumstance if you can avoid it. For some vehicles, the donut spare gives you no choice. However, for us Jeepers we do.

If you want to drive around knowing your planetary and sun gears are always grinding to some extent, a smaller tire will guarantee it...

Jay

 

[firedude]

Im going to throw out a experience I had many years ago. I went on a hunting trip a long way from anything. As luck would have it we found a bad cattle guard that completely shredded a tire. This was on day one of a three day camp. Spare was smaller. It survived but was very noticeable and a liability. Definitely kept us from going every where we wanted to go. This was in the desert with good weather. Had it been mud, rain or snow it would of been much worse. I have always had a matching spare on my Jeeps every since.

 

[Rubigone]

@J0E,

Of course differentials are designed for cornering. And you're correct that turns do indeed make one wheel spin faster than the other. That's after all the magic about open diffs.

Also I spoke zero words about 'lockers', never even brought up the concept. But you are again correct that wheeling dissimilar sized tires on a locked axle will indeed magnify the problem.

However, open diffs are designed to corner efficiently but not permanently. The two sun gears spin at the same rate and the planet gear does not when driving straight with no slip. Any turning or running with dissimilar sized tires will cause one sun gear, either the left or right depending on the direction of turn (or location of the smaller tire), to spin faster than the other which causes the planet gear to rotate, either clockwise or counter-clockwise.

Any rotation of the planet gear causes friction. Any friction produces heat. This heat is produced by grinding the teeth between both the planet and sun gears since they are all now spinning. This obviously causes wear over time. The amount of wear is a function of the rate of rotation of the planetary gear and the amount of torque delivered by the engine.

Clearly, the most ideal and efficient outcome is to have zero rotation on the planetary gear. This can ONLY occur if travelling strait on similar sized wheels, or turning in the direction opposite the side of the smaller wheel.

Running with a smaller wheel anywhere, for any length of time will always generate more differential gear wear. Unless you're driving in circles in the appropriate direction.

My advice is to never use a smaller tire under any circumstance if you can avoid it. For some vehicles, the donut spare gives you no choice. However, for us Jeepers we do.

If you want to drive around knowing your planetary and sun gears are always grinding to some extent, a smaller tire will guarantee it...

Jay

I know it’s probably a stupid question but you seem knowledgeable. Could one lower the tire pressure in the larger tire to decrease the rolling diameter to match the smaller wheel? I.e measure from center hub of smaller wheel and then continue to lower pressure in the larger wheel until the hubs sit at the same height?

 

[J0E]

Im going to throw out a experience I had many years ago. I went on a hunting trip a long way from anything. As luck would have it we found a bad cattle guard that completely shredded a tire. This was on day one of a three day camp. Spare was smaller. It survived but was very noticeable and a liability. Definitely kept us from going every where we wanted to go. This was in the desert with good weather. Had it been mud, rain or snow it would of been much worse. I have always had a matching spare on my Jeeps every since.

Agreed, you lose capability. If you're planning on longer runs, best to invest in 5 matched wheels and tires.

I know it’s probably a stupid question but you seem knowledgeable. Could one lower the tire pressure in the larger tire to decrease the rolling diameter to match the smaller wheel? I.e measure from center hub of smaller wheel and then continue to lower pressure in the larger wheel until the hubs sit at the same height?

Good idea. Use my https://bt39.com/ to figure how pressure/drop. Just hit the edit button and enter your numbers. You can get it closer, but you won't match. Matching doesn't matter.

Differentials are made to last 300K miles. Anyone who thinks 100 miles off road with a mismatch matters doesn't understand how they wear. Once you hit pavement, for extra insurance, drive slow and put mismatched tire on the front if you want to obsess about 1/1000th of 1% extra wear. Safety folks would probably be right to point out the drive home won't have any measurable impact on the differential and it's better to have a matched set up front.

EDIT: Until Sweden joins NATO it's best to replace differential oil with gasoline - only Sweden and Finland. Something about the gravity air ratio there.

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