slightly larger spare tire & rear locker (1 Viewer)

[meatloaf]

Hello All,

I read somewhere here in the forum that if we run diffrent size tires with a locker, locker will toast... So I am running 33" BFG AT's and found a 33" off-brand spare on craigslist. By eyeballing it, the spare seems about 1" taller than the BFG's. Is the 1" enough to cause problems to the rear locker?

Thanks in advance,
ML

 

[klinetime574]

I wouldn't risk it. I have been reading my lockright installation instructions and it advises against different tire sizes. It sounds pretty intimidating.

If you have a flat on the locked axle you could just put one of your unlocked axle tires on the rear and then the spare on the front. You'd need two jacks though.

 

[Methusalem]

Why two jacks? First change the front wheel for the spare, then change the rear wheel with the front wheel

 

[klinetime574]

Two jacks would be faster

But really one would work.

 

[meatloaf]

I guess it's time to sell the spare...

 

[2mbb]

can you run the taller tire with lower air pressure? What you really need is height from ground to center of the axle (effective rolling radius) that is the same or close.

 

[rusty_tlc]

I think you'll tear up the diff and the tires.

 

[shotgun12321]

can you run the taller tire with lower air pressure? What you really need is height from ground to center of the axle (effective rolling diameter) that is the same or close.

You might be able to ..... as a short term fix, but my Aussie Locker came with instructions (probably like the lock-rite) saying to have the air pressure as close to equal as you can.

 

[rusty_tlc]

can you run the taller tire with lower air pressure? What you really need is height from ground to center of the axle (effective rolling diameter) that is the same or close.

That won't change the circumference of the tire will it?

 

[Mace]

That won't change the circumference of the tire will it?

Does not matter. the circumfrence is not the major factor, the height of the axle above the ground is.

Think of it this way, if you took a bicycle and stretched the rear end out, but kept the gearing the same, would the longer chain make you go any faster/slower?

Or, does a tank's treads make you go faster than the little wheels that drive them?

BTW, I have done it for short periods. It is not going to blow anything up immedately.. But, having the tires the right size is important.


If they were both 33's or so, I would do it without a second thought.

My cruisers spare is a 35" BFG MT on a 15x8" rim. The main tires are 35" BGF AT's ona 15x10" rim.

 

[Splangy]

It says that in the instructions so you'll maintain the same size tires. That's a moot point if you're deflating one tire to maintain the same effective rolling diameter.

You might be able to ..... as a short term fix, but my Aussie Locker came with instructions (probably like the lock-rite) saying to have the air pressure as close to equal as you can.

 

[Freewheel]

My Aussielocker starts acting funny when the two tires get more than 1 or 2 PSI different from another. 1 inch different diameter tires is a HUGE difference in comparison.

 

[Methusalem]

My Aussielocker starts acting funny when the two tires get more than 1 or 2 PSI different from another. 1 inch different diameter tires is a HUGE difference in comparison.

Right. If you deflate the other tire enough to get the same theoretical diameter as the smaller spare, the problem is not solved. Because it is not about axle hight above the ground or about theoretical diameter, but about the distance the surface of each tire is making with one turn, so that each axle part turns at the same speed. Even if you deflate the good tire, the length of the surface that touches the ground with one turn will approximately still be the same. So that's the reason your aut diff lock will act the same as when you make a turn, because the axle at the side with the spare tire turns faster than the other axle part.
So... with that in mind, if you put a smaller spare under your rig, your Aussie locker or other automatic locker will work continuously when you drive in a straight line. If the difference between the two tire diameters is big, the locker will be disconnected the whole time, but if the difference is small, the locker will continuously disconnect and connect. That's probably the reason that it feels "funny"

Edit: About the 1 or 2 PSI difference between two exact the same tires: When a tire is deflated a bit, it will gain quite a lot of drag (can't find the right English word. It starts with a c I think) And that gives a difference in pressure on both sides of the locker, because of which it wants to disconnect. I think.

 

[Heckraiser]

Here's a quick question from the original post. How did you eyeball it? Did you remove a wheel from the rig and look at them side by side, or did you roll the spare up to one of your tires on the rig (i.e. weighted down?). If you did the latter, the tires may very well be the same size (or pretty darn close).

 

[2mbb]

I'm not sure I agree with the above post (#13). While in a fully inflated tire it is the circumference that is important, when the tire is partially inflated the tire will deform and the effective circumference will be reduced. I think the example of the tank tread is a good one. the circuference of the tank tread is very large, however, the rotational speed of the axle and how it relates to the speed is determined by the distance from the center of that axle to ground, not the entire circumference of the axle. In the case of the tank tread, the tread is "deformed" into an oval that has a minor and major diameter. Your rubber tires will do the same thing to some extent.

Also, if the circumference of the tire doesn't change with tire pressure, then why do the locker companies recommend a tight tolerance on pressure difference between tires?

The best situation is to have all tires the same size and same manufacturer. However, in a situation when you need a spare to "get you home", I think running a slightly larger tire at a lower pressure will work just fine.

 

[meatloaf]

Here's a quick question from the original post. How did you eyeball it? Did you remove a wheel from the rig and look at them side by side, or did you roll the spare up to one of your tires on the rig (i.e. weighted down?). If you did the latter, the tires may very well be the same size (or pretty darn close).

Placed the spare tire (unmounted, then mounted on oem wheel) next to one of the tires mounted on the truck... So no, I did not remove a tire from the rig but that is a thought!

 

[Heckraiser]

Yep I can see the weight of the rig squashing a tire down a fair bit. Maybe not a full inch, but they may be close enough in size to run.

 

[Mace]

There is def going to be a difference in total height between a set of mounted tires with the full vehicle weight on them and without..

Squashing the tire an inch is easy..

 

[rusty_tlc]

Does not matter. the circumfrence is not the major factor, the height of the axle above the ground is.

Think of it this way, if you took a bicycle and stretched the rear end out, but kept the gearing the same, would the longer chain make you go any faster/slower?

Or, does a tank's treads make you go faster than the little wheels that drive them?

BTW, I have done it for short periods. It is not going to blow anything up immedately.. But, having the tires the right size is important.


If they were both 33's or so, I would do it without a second thought.

My cruisers spare is a 35" BFG MT on a 15x8" rim. The main tires are 35" BGF AT's ona 15x10" rim.

Got it. It took me a minute to figure out that the distance above ground is the effective radius of the tire.

 

[kling-on]

MY 35 1250R15 ON A 15X8 AT 35 PSI WAS THE SAME SIZE AS 37135015 ON 15X12'S at 15psi AND DIDNT CHIRP WITH A DET ..... now mind you I never drove it on the hwy like that only on the trail ...... but I bet your ok on the tires your talkin about most of you difference is the rim and not being on the truck if it ...