How far would you drive... (1 Viewer)

[inkpot]

there is a thing i picked in the army in case of emergency
deflate the large one and over inflate the small one

So, how'd that work on 37" Hummer Run-Flats??

 

[BILT4ME]

I've heard folks on here argue that's a myth: Larger tire still has just as much tread/circumference; it's just that the contact patch is larger.

Haven't really thought it through myself...

I've read that argument, but disagree. Air one up to make the ROLLING RADIUS larger and air the other down to make the ROLLING RADIUS smaller. That is the distance from the center of the axle to the ground. The actual circumference will result in slip and tire flop and heat. If you keep it slow, it works with minimal tire damage or differential damage.

I do agree with what @Desert Dino said to get by.

 

[OREGON85]

I've heard folks on here argue that's a myth: Larger tire still has just as much tread/circumference; it's just that the contact patch is larger.

Haven't really thought it through myself...

It actually does change the wheel speed. On a sprayer (agricultural chemical sprayer) precise speed is very important. If the tires get under inflated only a little it can really effect the speed and throw off your calibration. Unless, of course, it has GPS speed control. The one I use at work doesn't have GPS speed control so I have to watch the tire pressure closely.

 

[Tools R Us]

It actually does change the wheel speed. On a sprayer (agricultural chemical sprayer) precise speed is very important. If the tires get under inflated only a little it can really effect the speed and throw off your calibration. Unless, of course, it has GPS speed control. The one I use at work doesn't have GPS speed control so I have to watch the tire pressure closely.

It does have some effect, but not as much as you would think, for sure not the same as the radius change.

Air pressure affects safe travel speed, to run at highway speed the tire needs to be inflated. If you are going to limp it with a flat tire, enough to make up 2" of difference, the safe speed is going to be slow, just use the small spare. In our tests, with a significant difference, can go ~25mph, much more starts sliding tires due to VC action.

 

[LINUS]

I've heard folks on here argue that's a myth: Larger tire still has just as much tread/circumference; it's just that the contact patch is larger.

Haven't really thought it through myself...

Pi * (r*r) - you are decreasing the 'r', byproduct is the larger contact patch & heat/scrub.
Or simple diameter * Pi, but then it gets fuzzy since someone will say "you aren't really changing the circumference" - meh.
My $.02

That said, I rotate 6 MT-R's so I have no issue, had a set & found a pair on Fleabay cheap.

 

[baldilocks]

What is your favorite color?

 

[retrofive]

What is your favorite color?

Flat....... black.

 

[BILT4ME]

What is your favorite color?

Blue


No, Wait......YELLOW!

 

[ppc]

Pi * (r*r) - you are decreasing the 'r', byproduct is the larger contact patch & heat/scrub.
Or simple diameter * Pi, but then it gets fuzzy since someone will say "you aren't really changing the circumference" - meh.
My $.02

That said, I rotate 6 MT-R's so I have no issue, had a set & found a pair on Fleabay cheap.

I guess that I'm one that falls into the "someone" group. The circumference doesn't change by reducing the air pressure. Your formula Pi * (r*r) is for area within a circle, I think you meant Pi * (2*r). What changes is that the tire is no longer a circle so radius and formulas are not really meaningful or applicable. What changes with lower pressure is tire flex with the tire scrubbing, more wear and heat buildup in the tire. The diff will still have to account for the difference.

 

[LINUS]

I guess that I'm one that falls into the "someone" group. The circumference doesn't change by reducing the air pressure. Your formula Pi * (r*r) is for area within a circle, I think you meant Pi * (2*r). What changes is that the tire is no longer a circle so radius and formulas are not really meaningful or applicable. What changes with lower pressure is tire flex with the tire scrubbing, more wear and heat buildup in the tire. The diff will still have to account for the difference.

No, I used the Pi * (r*r) format for 2 reasons:

1. I dunno how to type 'Pi times radius squared' as a formula. I'm not that smart
The way I read yours is 'Pi times radius times 2' -yours may be right, but I wouldn't know.
It looks like the equiv of "Pi times diameter" - if that was what you were going for.

2. It more clearly also illustrates the fact since by reducing the radius (it does) - the area of the circle is reduced when you flatten the circle, byproduct is a greater contact patch. Your axle centerline is lower, end result.

Using the Pi * diameter does get you circumference, but we all know the whole thing starts to turn into the "would a plane take off from a rolling runway"-type thing since the tire circumference stays the same, but the shape doesn't.

That's just my POV on it. <$.02 worth I'm sure.

 

[ppc]

I can appreciate your point of view and do realize you had the right formula using diameter for circumference. There are a lot of variables to consider, so like you, don't see it as exact science.

 

[mudgudgeon]

How far would you drive with a stock spare with 315/75r16's?

That's what a differential is for, right?!

As far as necessary to get home, or to a tyre shop, depending on the difference, until it's convenient to get to a tire shop

The 3" taller tire, 100mph, 15 minutes to pinion failure is an extreme example.

Most modern cars have space saver spare, and recommend maximum speed of 50mph. I'll happily use this as a guide.

I had 35" weekend play tires and 33" DD tires, but only carried a 33" spare for a long time. Not poor planning, but a conscious and educated (IMO) decision.

 

[ppc]

Most modern cars have space saver spare, and recommend maximum speed of 50mph. I'll happily use this as a guide.

Key point is a car means - 2WD no viscous coupler. Entirely different with Fulltime 4WD and viscous coupler.

 

[jonheld]

there is a thing i picked in the army in case of emergency
deflate the large one and over inflate the small one

Not this again. Tires are not balloons.

 

[jonheld]

No, I used the Pi * (r*r) format for 2 reasons:

1. I dunno how to type 'Pi times radius squared' as a formula. I'm not that smart
The way I read yours is 'Pi times radius times 2' -yours may be right, but I wouldn't know.
It looks like the equiv of "Pi times diameter" - if that was what you were going for.

2. It more clearly also illustrates the fact since by reducing the radius (it does) - the area of the circle is reduced when you flatten the circle, byproduct is a greater contact patch. Your axle centerline is lower, end result.

Using the Pi * diameter does get you circumference, but we all know the whole thing starts to turn into the "would a plane take off from a rolling runway"-type thing since the tire circumference stays the same, but the shape doesn't.

That's just my POV on it. <$.02 worth I'm sure.

The revs/mile remain the same. Tires are not balloons and the tread will deform the same way a tank tread deforms or a bicycle chain deforms. The length remains constant regardless of the shape.
Tires are not balloons.
If you under inflate 1 and over inflate another the rotations/mile don't change because the tread does not expand and contract like the surface of a balloon.
The contact patch changes but the revs/mile remain constant.
The tire carcass is made of many layers of steel and/or fiberglass and is designed NOT to expand or contract, but simply change shape and conform.
Tires are not balloons.

 

[60wag]

To save the VC from overheating, you could lock the center diff and pull the fuse. Then just drive it. Yes there will be some bind between the front and rear axles that would scrub the tires a bit. I'd keep the distance driven to a minimum but it shouldn't do any real damage, especially if its on dirt.

 

[LS1FJ40]

The revs/mile remain the same. Tires are not balloons and the tread will deform the same way a tank tread deforms or a bicycle chain deforms. The length remains constant regardless of the shape.
Tires are not balloons.
If you under inflate 1 and over inflate another the rotations/mile don't change because the tread does not expand and contract like the surface of a balloon.
The contact patch changes but the revs/mile remain constant.
The tire carcass is made of many layers of steel and/or fiberglass and is designed NOT to expand or contract, but simply change shape and conform.
Tires are not balloons.

I'm not sure I understand you. Are you saying that tires are not balloons or something else?

 

[LS1FJ40]

I run the same size spare on my 80s. However, I shredded two tires (only one spare) 9-10 years ago down in Mexico. I ran a spare WHEEL and TIRE off a chevy. After I drove 2.5 miles back to the road on basically a rim with rubber cushion. I drove it with the Chevy wheel and tire about 30 miles until I could buy two new tires. The new tires weren't the same size but they were close.

Bottom line is you do what you gotta do in the moment to save your bacon. I would never intentionally leave to go somewhere with a spare that I know is not the right size for my full time 4wd vehicle. Run smaller tires or make better accommodations for the bigger tire.

 

[LINUS]

The revs/mile remain the same. Tires are not balloons and the tread will deform the same way a tank tread deforms or a bicycle chain deforms. The length remains constant regardless of the shape.
Tires are not balloons.
If you under inflate 1 and over inflate another the rotations/mile don't change because the tread does not expand and contract like the surface of a balloon.
The contact patch changes but the revs/mile remain constant.
The tire carcass is made of many layers of steel and/or fiberglass and is designed NOT to expand or contract, but simply change shape and conform.
Tires are not balloons.

I'm a little hazey on where you stand here, hahaha.

It's cool & agree radial tires aren't balloons.

Potatoe-potato, pin 7 mod - 7 pin mod

I'm not any sort of tire expert & not out to 'die on this hill' so I'll simply refer back to my "I'm not that smart" statement.

I am barely smart enough to carry a spare in right sizing, and leave it at that.

 

[CJF]

There are a lot of variables to consider, so like you, don't see it as exact science.

It is absolutely an exact science. It can be theorized, tested, and verified or refuted.