Full size spare necessary?

[jeepoch]

@Rubigone,

Marvin,

To answer your question simply, yes different tire pressures can cause the same effect but dramatically less pronounced. On-road the difference will be due to a more flattened surface causing a small (but not zero) diameter change delta on the tire with the lower pressure.

Off-road, with both tires depressurized, any change delta would be even less pronounced. Yet, any wheel slip whatsoever always causes planetary gear spin unless the axle is 'locked'.

Recall, the less the planetary gear spins, the more efficient torque is transfered because all the rotational energy is then coupled between the drive shaft carrier and sun gears directly. The sun gears are mechanically fixed to each axle shaft. This distributes the maximum torque between both wheels equally.

When the planetary gear rotates at all, the wheel with the most slip gets almost all the torque. Off-road this spells trouble. You always want torque delivered to the gripping wheel (not the spinning one). This is exactly why lockers are so advantageous over open diffs on trail.

On a final side-note, never rotating in the 5th tire can also do the same thing. Especially as the other four near the end of their tread life. It's always good sound advice to routinely perform a 5 tire rotation to insure the spare's diameter wears as evenly as all the others. But again, in regards to the differential's planetary gear's rotation any tire diameter difference (no matter how small) will cause the planetary gear to spin. Unfortunately even when driving straight.

Hope this gives you a better understanding.

Jay

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[J0E]

@J0E,

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

How much friction and grinding with the small tire on the front in 2 WD and an operational FAD?

@J0E,

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

So are you now recommending only straight driving, no bumps, no corners. Any of that by your definition causes grinding.

Maybe it's just that I know too many poor knuckleheads who drive around with 4 mismatched tires and have never had a differential problem. They've had every other problem, but worrying about extra stress on the differential is not one of them.

 

[Mikester86]

From the owner’s manual in the section regarding tire rotations:

 

[jeepoch]

How much friction and grinding with the small tire on the front in 2 WD and an operational FAD?

J0e,

In all honesty, I'm not trying to debate differential lifecycle expectency.

With full disclosure, I used to be employed by Chrysler as a Powertrain Software Engineer developing all small car, large car and jeep/truck engine controllers. Nearly 50% of our software feature set was to ensure that the engine's torque is always delivered in a manner to reduce any powertrain damage. The perception of delivered torque was always of primary concern. Without software intervention, today's power plants could easily destroy differentials, transfer cases and yes even transmissions.

Whether your quoted longevity is accurate I don't know. Whatever it is however, it's nonetheless impressive not because the materials used were chosen to be nearly indestructible but rather all the physics and calculus involved in determining the force vectors are specifically delivered to minimize any harm to the powertrain in general. Typically, the rate of torque increase is delayed (almost imperceptibly). Enough for the driver's expectation, but rarely enough to incur damage.

And yes gear wear was always a top warranty concern.

Jay

 

[SouthCo]

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

Only a dumbass would run a donut spare for more than a day or two. Donut spares are usually recommended for under 50mph. A full size spare maybe I can see for a week. If you have the money to buy a Jeep and lift it and put bigger tires, Please buy a matching set of 5.

And how about you no longer have a spare when you ride around on your spare. Not smart on any vehicle....

 

[TheBirdie72]

Problem solved - just carry two of the same size spare!

 

[Rubigone]

@J0E & @jeepoch

Thank you for the replies, always interesting to learn something new from someone more knowledgeable on the subject .

And yes you might be onto something with the gas in the diff, maybe nato will be more interested in protecting my dear Jeep.

 

[J0E]

Only a dumbass would run a donut spare for more than a day or two. Donut spares are usually recommended for under 50mph. A full size spare maybe I can see for a week.

And how about you no longer have a spare when you ride around on your spare. Not smart on any vehicle....

Agreed, I just mentioned I know a lot of knuckheads who drive for months on a donut spare, on the axle with a differential. Not a Jeep.

I'm too OCD to go one day without a spare.

If you have the money to buy a Jeep and lift it and put bigger tires, Please buy a matching set of 5.

Not needed as I pointed out. Suboptimal for sure.

@J0E & @jeepoch

Thank you for the replies, always interesting to learn something new from someone more knowledgeable on the subject .

And yes you might be onto something with the gas in the diff, maybe nato will be more interested in protecting my dear Jeep.

@jeepoch is the expert on gears and most things. I'm the expert at trailer park hacks and that actually work. Knowing a lot of poor knuckleheads has advantages.

 

[J0E]

Problem solved - just carry two of the same size spare!

All kidding aside if I was going to Alaska or a long trip where I had to carry extra fuel, I'd figure out a way to carry 2 full size spares.

 

[Rubigone]

@jeepoch is the expert on gears and most things. I'm the expert at trailer park hacks and that actually work. Knowing a lot of poor knuckleheads has advantages.

And I’ve learned to never underestimate either, sometimes things work in ways they weren’t designed to do, and sometimes they don’t. But you never know until you try. ?

 

[Tethmes]

All 5 tires are exactly the same and that is why when you go in for an oil change at the Dealer...They ask...4 or 5 tire rotation( if you are having that done which you should every oil change). So it includes the 5th which helps to keep all tires evenly worn and there is a specific rotation to include the 5th.

I'm either over thinking this or under thinking this. So since I don't care about looking stupid, when I start spending more than 5 minutes over or under thinking something, figure it's best to just ask. Where does front passenger side go in this graphic? Driver side rear correct??

 

[Rico1111]

?

 

[BadAssBumbleBee]

I'm either over thinking this or under thinking this. So since I don't care about looking stupid, when I start spending more than 5 minutes over or under thinking something, figure it's best to just ask. Where does front passenger side go in this graphic? Driver side rear correct??

Yes, that's correct.

 

[Tethmes]

Many thanks!

 

[smokeythecat]

I'm either over thinking this or under thinking this. So since I don't care about looking stupid, when I start spending more than 5 minutes over or under thinking something, figure it's best to just ask. Where does front passenger side go in this graphic? Driver side rear correct??

It goes in the corner in your garage. After 5 tire rotations you will have a full set to fall back on in the future.

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