Different spare tire diameter, auto locker (1 Viewer)

[-Spike-]

True. However tire DO fold and distort when under inflated no?

No, the tread does not fold if the pressure is sufficient to support the truck. If it did, the tire would be off the wheel. The tread is a fixed length, it doesn't stretch or contract more than a fraction of an inch.

Altering the pressure does change the tire's radius. The tire's tread will squirm and wear quicker to accommodate. Take a look at the early style, 1st generation, indirect tire pressure monitors. They measured a difference of tire rotation via ABS sensors. There was no tire pressure sensor. The under inflated tire will have a slightly smaller diameter (and present as more rotations to the ABS/wheel sensor).

Not enough to make up for a significantly smaller tire. The computer can measure fractions of RPMs over many miles and determine a difference, however slight. The difference in circumference of a 31.5" vs a 33" tire is over three and a half inches. You're not going to make up for that with some tire scrubbing.

 

[hoser]

Agreed, you can't make up 1.5" of diameter difference via tire pressure. The point is, there IS a difference and it is measurable. And it is not over many miles as even indirect TPMS could detect a single tire blowout quickly.

 

[-Spike-]

Agreed, you can't make up 1.5" of diameter difference via tire pressure. The point is, there IS a difference and it is measurable. And it is not over many miles as even indirect TPMS could detect a single tire blowout quickly.

Nobody is denying that. This is a lot of arguing over semantics. And now you're arguing that a blowout changes the tire diameter significantly. Duh.

I don't think it works that way. The full circumference of the tire still needs to travel over the ground. It might help a little, as much as the fully inflated tire might stretch, but that ain't much.

 

[jonheld]

Altering the pressure does change the tire's radius. The tire's tread will squirm and wear quicker to accommodate. Take a look at the early style, 1st generation, indirect tire pressure monitors. They measured a difference of tire rotation via ABS sensors. There was no tire pressure sensor. The under inflated tire will have a slightly smaller diameter (and present as more rotations to the ABS/wheel sensor).

A tire's tread can be thought of like a series of flexible steel cables encased in rubber. The tread remains flexible and can change shape, yet fixed in length. If it was able to expand and contract, it would tear away from the carcass. Again, not a balloon.

If a tire is inflated to 30 psi and you suddenly add 400 pounds of weight to that corner of the suspension, the tire will appear to be underinflated but it will still rotate at the same speed. The tread will deform just like a bulldozer track will change shape when it goes over a rock, but the track remains the same length.

There is confusion regarding radius. Yes, the distance from the center of the hub to the ground will decrease, but the length of the tread remains constant.

 

[Grandlooser]

So add a touch of overinflation of the spare.

 

[thatcabledude]

Thanks for the replies. I have to decide whether removing front drive shaft and halting any wheeling activity I am doing is more\less desirable than making space for new size tire. Saying that I have only had 2 flats in 30+ years.

If you have a unrepairable flat (on the rear) while wheeling don’t let it end the trip. You’ll be fine. Once you get to where it’s extended highway time throw it on the front. Pull the shaft if you want. I think you would be fine though.

 

[flintknapper]

A tire's tread can be thought of like a series of flexible steel cables encased in rubber. The tread remains flexible and can change shape, yet fixed in length. If it was able to expand and contract, it would tear away from the carcass. Again, not a balloon.

If a tire is inflated to 30 psi and you suddenly add 400 pounds of weight to that corner of the suspension, the tire will appear to be underinflated but it will still rotate at the same speed. The tread will deform just like a bulldozer track will change shape when it goes over a rock, but the track remains the same length.

There is confusion regarding radius. Yes, the distance from the center of the hub to the ground will decrease, but the length of the tread remains constant.

^^^^

This correct folks. Let's put it to rest.

 

[hoser]

@flintknapper Are you saying if I lock up the three differentials and then deflate any two tires by half the pressure.... I should get no binding, as long as I drive in a straight line?? Try it.

@jonheld Rubber tires aren't not like bulldozer tracks. Yes, 400 lbs of weight, or weight transfer will will change the effective gearing. Top Fuel dragster count on it. There is a lot going on with dragster tires.... weight, weight transfer, centrifugal force... but in short, the effective tire diameter is changing throughout the entire run.

We Break down the Physics of Dragster Tires

No mere rubber donuts, dragster tires are active parts of the driveline. A single tire runs for roughly $917 and is expected to last only 1.5 miles.

www.caranddriver.com

 

[jonheld]

@flintknapper Are you saying if I lock up the three differentials and then deflate any two tires by half the pressure.... I should get no binding, as long as I drive in a straight line?? Try it.

@jonheld Rubber tires aren't not like bulldozer tracks. Yes, 400 lbs of weight, or weight transfer will will change the effective gearing. Top Fuel dragster count on it. There is a lot going on with dragster tires.... weight, weight transfer, centrifugal force... but in short, the effective tire diameter is changing throughout the entire run.

We Break down the Physics of Dragster Tires

No mere rubber donuts, dragster tires are active parts of the driveline. A single tire runs for roughly $917 and is expected to last only 1.5 miles.

www.caranddriver.com

Apples and oranges. We're talking about passenger car tires. Drag racing tires are a completely different manufacturing process using different materials. You're not running drag racing tires on a passenger car and you're not running passenger car tires on a dragster.
Not a valid comparison.

 

[-Spike-]

@flintknapper Are you saying if I lock up the three differentials and then deflate any two tires by half the pressure.... I should get no binding, as long as I drive in a straight line?? Try it.

@jonheld Rubber tires aren't not like bulldozer tracks. Yes, 400 lbs of weight, or weight transfer will will change the effective gearing. Top Fuel dragster count on it. There is a lot going on with dragster tires.... weight, weight transfer, centrifugal force... but in short, the effective tire diameter is changing throughout the entire run.

We Break down the Physics of Dragster Tires

No mere rubber donuts, dragster tires are active parts of the driveline. A single tire runs for roughly $917 and is expected to last only 1.5 miles.

www.caranddriver.com

I'm saying that deflating 3 tires won't make up for a 1.5" diameter difference in the fourth. Try it sometime, on a freeway for a significant amount of time, with the three lockers engaged. You will get binding, and you should be prepared for one or more of the tires to disintegrate from the heat. This is a stupid and pointless argument.

 

[hoser]

Tirerack agrees with me. Revolutions per mile are not fixed.

"Revolutions Per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate."

 

[-Spike-]

Tirerack agrees with me. Revolutions per mile are not fixed.

"Revolutions Per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate."

I can't believe I'm replying to this, but I can't help myself. Here we go...

Your reference doesn't say that lowering the air pressure will affect the 'absolute rolling circumference'. It simply says the exact rolling circumference of any tire isn't calculable. They are probably talking about differences of a fraction of an inch. But just for argument's sake, let's say it does. How much air do you have to let out of 3 tires to make up for a -1.5" diameter difference in one?

Is any part of your argument in any way relative to the original question?

 

[hoser]

Like I already said, you can't make up for 1.5" but you can make up for some.... maybe 1/2" diameter difference. Differentials are also pretty forgiving... 1/2" or even up to 1" should be fine, short term. So there's your 1.5" right there, happy?

Unloaded radius is half the tire's diameter. Loaded radius is less than half because it has the weight of the vehicle on it. Adding weight would indeed reduce the loaded radius as would reducing the tire pressure. Driving at freeway speed would increase the radius due to centrifugal force.

 

[Grandlooser]

I'll be more careful about comments made in jest from here on out.

 

[-Spike-]

Like I already said, you can't make up for 1.5" but you can make up for some.... maybe 1/2" diameter difference. Differentials are also pretty forgiving... 1/2" or even up to 1" should be fine, short term. So there's your 1.5" right there, happy?

Unloaded radius is half the tire's diameter. Loaded radius is less than half because it has the weight of the vehicle on it. Adding weight would indeed reduce the loaded radius as would reducing the tire pressure. Driving at freeway speed would increase the radius due to centrifugal force.

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.

 

[BILT4ME]

I'm going to have to argue with this statement.
Again, tires are not balloons. Altering tire pressure does not change rolling length of the tread, unless you deflate to the point where the sidewall is now in contact with the road.
The steel belts in the tread do not change length due to inflation, they allow the tread to flex, but the rolling length remains constant.

Does a W24 x 76 beam flex? Yes.
Do cables stretch and shrink? Yes.
Do steel bands embedded in a casing of rubber stretch and shrink? Yes.
How much for all of these? There are design parameters and failure points for all.

I am talking about lowering the pressure significantly to get the rolling radius down. However, you would be sacrificing three tires and safety to save the gears.
If you placed the one smaller (or larger) tire on the axle WITHOUT the auto locker would be best.
The difference in rotation will stiff cause the viscous coupler, the center differential, and the axle differential to rotate internally due to the different rotations of the tires.
You can do it without killing a busload of Nuns or Kindergartners if you do it SLOWLY AND FOR A SHORT TIME.

The flexing of the tire allows it to run more revolutions per mile. However, that generates heat.

I'm not talking about a 10 psi difference in tire pressure on equal sized tires.

See, there's a reason that new cars have TPS in them, in order to operate the ABS properly.

Jon: There are a few "limiting factors" here:
1) Yes, you can drive a truck with one smaller tire. Slowly and for a short time. The faster you go, the more heat is generated and the more problems are incurred.
2) Do you alter tire pressures across an axle to save gearing while reducing safety? Probably not, but it can be done if done slowly and for a short time.
3) Do tires flex? Yes. the more they flex, the more heat is generated in the tire and its materials.
4) Do you want the spider gears inside the axle to spin or to be in a "fixed" condition? Sometimes yes, sometimes no.

Different tire diameters cause spider gears to spin due to the different rotational speeds of wheels on the same differential. Minor tire differences will cause the spiders to spin a little with each rotation. Major differences will cause the spiders to rotate a lot with each rotation.
Both scenarios are doable if you do them SLOWLY and for a SHORT TIME. The faster you go, the heat builds up and that is what will cause the ultimate failure.

A low pressure tire absolutely has a different rolling radius than a fully aired up tire. The difference is taken up by the folding and flexing of the tire steel belts, rubber, and materials, causing heat to build up and eventually destroy the tire. If you do not believe this, you can drive your truck one mile with normal tire pressures and count the number of rotations of the driveshaft or the wheel during that mile.

Then, lower your tire pressures to 10 PSI and do the same test. You can do this at any speed you choose, be it safe or unsafe. There WILL be a difference because the ROLLING RADIUS is different.

Make sure to take the temperature of the tires before and after each test, as the temperature will be significantly higher on the second version.

Maybe remove the rear cover on any open differential vehicle, and make one tire larger than another. Then roll slowly across the yard. You will see how much the spiders turn.

ABS systems count the number of rotations of the axle shafts. They then base the computer on those numbers to determine which wheels to brake. However, they discovered that low tire pressures on one tire on one axle caused serious problems with the math and could cause the vehicle to lose control because the friction properties were now different than the designed plan.

My advise to the OP:
Buy (5) of the same tire. You will get 25% more mileage out of the full set if the rotation is done properly at proper intervals. I'm sure there is someone out there that wants to argue with me on that as well.


And yes, I have blown up a rear differential driving with one tire about 2" in DIAMETER difference from one side to the other. However, I did NOT do it slowly OR for a short time. I WAS a teenager and I know exactly why. So does my Dad. No, it was NOT a Land Cruiser.

 

[BILT4ME]

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.

Don't forget about slip.

Friction.

Heat.

Real world.

F'n Engineers........

 

[BILT4ME]

Again:

Here's the best solution:

Buy (5) of the same tire. Problem solved and all the a-holes arguing in your thread go away.



We'll argue more, just about some other 0.02 mm to 0.04 mm difference of something.

 

[-Spike-]

We'll argue more, just about some other 0.02 mm to 0.04 mm difference of something.

Will not.

 

[BILT4ME]

Will not.

Will too.