Different spare tire diameter, auto locker (1 Viewer)

[flintknapper]

Will too.

Hah....!

Just waiting for someone to say "My Dad can whip your Dad"

Or pull a 'Double Dog Dare'.

 

[BILT4ME]

Hah....!

Just waiting for someone to say "My Dad can whip your Dad"

Or pull a 'Double Dog Dare'.

That was going to be my next reply!

 

[-Spike-]

That was going to be my next reply!

Double dog dares are for candyasses.
Triple is where it's at.

 

[DKW]

Some interesting discussion.....

But to summarize, in response to my question 'if you HAD to fit an undersize wheel temporarily and you had an autolocker fitted' a good solution is as follows:-

Fit undersize wheel, and run it anywhere on loose terrain. When you hit the tarmac, then fit it to front, engage CDL and remove front drive shaft.

Thanks all.

 

[jonheld]

I am talking about lowering the pressure significantly to get the rolling radius down.

I know exactly what you're taking about, and you know I understand the physics.
Tires are not balloons. They don't expand and contract in all directions evenly based on air pressure because the rubber is incased in a semi-rigid structural frame.
You can't make a 35 into a 37 by adding air pressure and you can make a 37 into a 35 by deflating it.
The OP was talking about 1.5 inch difference in diameter. You're not making up that difference with air pressure.
Will there be miniscule differences? I'll concede that.
1.5" difference? Cmon now, I know you know better.

 

[Heckraiser]

Haha, OP got more than he bargained for here

 

[hoser]

No. The radius and circumference of a circle are related mathematically by a ratio. Change one, you change the other. However, if you simply deform the circle, you are not changing the circumference. Take a string loop, for instance. When laid out as a circle, it has a radius, diameter, circumference, etc. Now stretch it so it is essentially two parallel lines. Has the length of the string changed? No, it obviously hasn't. In the case of the tire, the circumference can only be changed by the amount that the tread stretches if you overinflate it. This is a very minimal amount, as it's held together with steel belts, or other materials that don't stretch. You cannot shrink the length of the tread significantly. Certainly not by the more than one and a half inches you are postulating by suggesting you can make a 1/2" difference in the diameter by underinflating it. (32.5" circle has a circumference of 102.1", 33" circle circumference = 103.67") You are not going to magically shrink the tread length that much.

Once you deform a circle, it no longer has a radius by definition, and calculations that rely on radius don't apply.

On top of all that, from a mechanical perspective it's not the differentials that we're concerned with, it's the viscous coupler. It resists any variation in rpms between the front and rear driveshafts. Obviously it can deal with some variation there, as that's what it was designed to do. How much? Don't know, but Toyota is pretty adamant that you don't mix tire sizes. Short term, might not be a big deal. Who's to say what the limit is? This thread started out asking what the concerns are in running a different sized spare tire. One guy (jokingly) suggested running the other three tires at reduced pressure. We took the question at face value and explained why it's not a good idea. You went off on this for some reason, seemingly suggesting that it could be an acceptable solution. Well, it's not, for several reasons, the biggest of which is that it's not at all safe to run underinflated tires at high speed. I've seen secondhand why it's not a good idea, in the form of a completely disintegrated 37" tire on a friend's rig due to an hour or so at ~75mph at ~12psi. Just enough to look a little low. Maybe a half inch short.

In any case, if you want to continue, go right ahead. I just want to make every effort to make sure no one else follows your logic. And I'm bored relaxing today.

Your logic is too simplistic. If we were talking about a wagon wheel, where there is no air space and it has a rigid rubber tire/band, I'd agree with you.

In your logic, what part of the tire is the string? The steel belts or the outer rubber? You ever take a unmounted tire and flatten the tread.... similar to what happens when you air down. The void spaces between the tread blocks decrease, right? String doesn't do that.

Do you need a visual? Take a close look at the rubber tread and especially the void spaces between the tread blocks when the tread is "flat" versus the rest of the rotation. What happens? The void spaces get smaller! Now deflate the tires and make the "flat" part longer.



The area of the tire under the contact patch makes all the difference. It's complicated to calculate because the rolling radius is changing from the moment the tire first make contact with the ground and when it leaves. The tire tread is skidding in different directions. The tire is not in direct drive with the road.

Don't take it from me, search rolling radius or take a look here.

 

[Brimley]

Hey OP, if you have an auto locker in back (or front) only. You could drive to a flat spot and switch front/back to put undersized spare on unlocked axle for at least several miles on open diff.

Didn't read full gnashing of teeth thread.

 

[Grandlooser]

Your logic is too simplistic. If we were talking about a wagon wheel, where there is no air space and it has a rigid rubber tire/band, I'd agree with you.

In your logic, what part of the tire is the string? The steel belts or the outer rubber? You ever take a unmounted tire and flatten the tread.... similar to what happens when you air down. The void spaces between the tread blocks decrease, right? String doesn't do that.

Do you need a visual? Take a close look at the rubber tread and especially the void spaces between the tread blocks when the tread is "flat" versus the rest of the rotation. What happens? The void spaces get smaller! Now deflate the tires and make the "flat" part longer.



The area of the tire under the contact patch makes all the difference. It's complicated to calculate because the rolling radius is changing from the moment the tire first make contact with the ground and when it leaves. The tire tread is skidding in different directions. The tire is not in direct drive with the road.

Don't take it from me, search rolling radius or take a look here.

That article is amazing. Making me feel good about my initial comment.