Dual wheels on blocks problem

[NXR]

Here's a pair of tires that may fail in the not-too-distant future. This pic is from a post where the owner said the newly-paved site was angled the wrong way so he had to use blocks under his rear tires.

I posted back that he massively overloaded those outer rear tires by only blocking the outer tires, thereby making those tires support 100% of the weight on each corner instead of 50%.

Look at how flattened that tire on the board is. The tire on the other side (in shadow) looks like it's flattened out over almost the entire board but it may be a shorter board, of course.

That's a 15,000 lb GAWR rear axle using Michelin 235/80R22.5 tires. The dual pair can support just over 7,500 lbs so each outer tire is now supporting close to that weight instead of about 3,700 lbs.

I know him and he has a 2019 model year on a 2018 F53 chassis so his tires likely are from 2017 or 2018. I suggested he might want to accelerate his tire replacement timeframe a bit...

Ray

[Daxinarian]

Originally Posted by NXR

Here's a pair of tires that may fail in the not-too-distant future. This pic is from a post where the owner said the newly-paved site was angled the wrong way so he had to use blocks under his rear tires.

I posted back that he massively overloaded those outer rear tires by only blocking the outer tires, thereby making those tires support 100% of the weight on each corner instead of 50%.

Look at how flattened that tire on the board is. The tire on the other side (in shadow) looks like it's flattened out over almost the entire board but it may be a shorter board, of course.

That's a 15,000 lb GAWR rear axle using Michelin 235/80R22.5 tires. The dual pair can support just over 7,500 lbs so each outer tire is now supporting close to that weight instead of about 3,700 lbs.

I know him and he has a 2019 model year on a 2018 F53 chassis so his tires likely are from 2017 or 2018. I suggested he might want to accelerate his tire replacement timeframe a bit...

Ray

Since the wheel is not rolling, it isn’t flexing dynamically, therefore not generating heat. Isn’t it the heat that ultimately leads to an overloaded tire to fail?

[Larry-NC]

Originally Posted by Daxinarian

Since the wheel is not rolling, it isn’t flexing dynamically, therefore not generating heat. Isn’t it the heat that ultimately leads to an overloaded tire to fail?

Not always. How much heat is generated when the driver hits a curb or deep pothole and simply applies more stress to certain portions than they are designed for. It doesn't matter whether that overload is applied quickly (curb, pothole) or slowly (driving up ramp, applying overload).

[NXR]

Originally Posted by Daxinarian

Since the wheel is not rolling, it isn’t flexing dynamically, therefore not generating heat. Isn’t it the heat that ultimately leads to an overloaded tire to fail?

Heat will contribute but we're talking about a possible 100% overload in this case.

I don't know how much the axle design and the wheel bearings factors in but now just the outer part of the axle is supporting 100% of the weight as well. And only one wheel bearing is bearing all of the weight (no pun intended).

The problem is that if the failure occurs weeks or months later then the owner likely will not correlate that failure with the time they massively overloaded the tire. They may even blame "China bombs".

Ray

[TitanMike]

Originally Posted by NXR

Heat will contribute but we're talking about a possible 100% overload in this case.

I don't know how much the axle design and the wheel bearings factors in but now just the outer part of the axle is supporting 100% of the weight as well. And only one wheel bearing is bearing all of the weight (no pun intended).

The problem is that if the failure occurs weeks or months later then the owner likely will not correlate that failure with the time they massively overloaded the tire. They may even blame "China bombs".

Ray

When a tire is built and tested for certification it's subjected to a test that imparts far more stress on a tire than the tires pictured is being subjected to.

The test is variously referred to as V-1 or "Tire Strength Test. The test involves mounting a tire on a wheel, inflating to rated pressure and conditioning to ambient temp for a given time, then pressing a rounded steel rod (3/4" dia) into the tire at a rate of 50 mm/min. The pressure on the rod must meet a minimum for the given tire's rating but usually the rod will push the inside of the tread "package" against the wheel without breaking the tire.

There's a HUGE difference between a static load on a tire and the dynamic loads on a tire moving down the highway under load and with heat buildup.

In reality it's not as much the load placed on a tire that causes failure but rather the heat that is generated by that load as it rolls down the highway.

The tires pictured could well outlast tires that are sitting flat on a level surface as it all depends on what happens to the tire as it's driving down the road. The load on that tire, sitting on a block with 10+ times (or more) the surface area as the "V-1 test rod" is being subjected to a fraction of the pressure it was tested for.

Now if the owner had just come in off the highway with tire temps approaching 200 degrees, and hit the leveling block at 50+ mph, then their might be an issue.

Now would I do that with my tires? Nope. But mainly because it doesn't look all that safe/stable.

[Brandoni]

Originally Posted by TitanMike

When a tire is built and tested for certification it's subjected to a test that imparts far more stress on a tire than the tires pictured is being subjected to.

The test is variously referred to as V-1 or "Tire Strength Test. The test involves mounting a tire on a wheel, inflating to rated pressure and conditioning to ambient temp for a given time, then pressing a rounded steel rod (3/4" dia) into the tire at a rate of 50 mm/min. The pressure on the rod must meet a minimum for the given tire's rating but usually the rod will push the inside of the tread "package" against the wheel without breaking the tire.

There's a HUGE difference between a static load on a tire and the dynamic loads on a tire moving down the highway under load and with heat buildup.

In reality it's not as much the load placed on a tire that causes failure but rather the heat that is generated by that load as it rolls down the highway.

The tires pictured could well outlast tires that are sitting flat on a level surface as it all depends on what happens to the tire as it's driving down the road. The load on that tire, sitting on a block with 10+ times (or more) the surface area as the "V-1 test rod" is being subjected to a fraction of the pressure it was tested for.

Now if the owner had just come in off the highway with tire temps approaching 200 degrees, and hit the leveling block at 50+ mph, then their might be an issue.

Now would I do that with my tires? Nope. But mainly because it doesn't look all that safe/stable.

That's quite a test!!

How big is the testing INSTRON and what type of cage is provided for those tires that don't have the 3/4" penetrator depress to the rim??

(Is the penetrator perpendicular to the tire circumference?? ASTM # ??)

Thanks

[TitanMike]

Originally Posted by Brandoni

That's quite a test!!

How big is the testing INSTRON and what type of cage is provided for those tires that don't have the 3/4" penetrator depress to the rim??

(Is the penetrator perpendicular to the tire circumference?? ASTM # ??)

Thanks

The "penetrator" is, if memory serves, is 3/4" with a "rounded" tip and is positioned in the center of the tread (but not in a tread groove). "Points" directly at the axis. The rest of the machine is about twice the size of the largest tires tested. Rest assured there are safety shields in place for the test.

This test has been used in one form or another since ~1969 and is just one of a battery of tests to ensure tires are a lot safer than those that were sold up to that date. I wonder how many people here were around when tire cords were made from Cotton? (trivia----Goodyear,AZ got it's name from the cotton fields around it owned by Goodyear Tire).

[aircommuter]

I was around when cotton was being phased out and the rayon and nylon wars were starting. Although more prevalent in bias ply tires being parked like that for extended time could creat flat spotting. Not damaging but irritating.

[TitanMike]

Originally Posted by aircommuter

I was around when cotton was being phased out and the rayon and nylon wars were starting. Although more prevalent in bias ply tires being parked like that for extended time could creat flat spotting. Not damaging but irritating.

Flat spotting was primarily due to nylon. They eventually came up with a version of nylon ( which really is kind of a catch-all term for several polymers) some tire co's advertised as "NF Nylon". Supposed to mean non-flat spotting nylon. Was better and if you had a half mile or so to drive slow the ride was a lot better than the old nylon which could take 4-5 miles.

[aircommuter]

Yep, we had a bunch of cabover trucks that could give a pretty bumpy ride in those days.

[NXR]

Michelin's bulletin on the proper use of blocks when leveling an RV: https://www.michelinb2b.com/wps/b2bc...motorhomes.pdf

Extreme caution must be taken to ensure that the tires are fully supported when using blocks to level motor homes and/or RV’s. The load on the tire should be evenly distributed on the block and in the case of duals, evenly distributed on blocks for both tires. If not properly done, the steel cables in the sidewall of the tires may be damaged and could lead to premature fatigue of the sidewall.

While I found some of the observations here interesting, they may fall under that old saying of "In theory, theory and practice are the same. In practice they are not."

Ray

[Reverse_snowbird]

Ignoring the tire problem.... Is the front tire also on blocks so that there isn't added stress to the frame of the unit?

BTW. I agree that both wheels should be supported properly.

[NXR]

Originally Posted by Reverse_snowbird

Ignoring the tire problem.... Is the front tire also on blocks so that there isn't added stress to the frame of the unit?

BTW. I agree that both wheels should be supported properly.

Dunno.

[aircommuter]

Originally Posted by Reverse_snowbird

Ignoring the tire problem.... Is the front tire also on blocks so that there isn't added stress to the frame of the unit?

BTW. I agree that both wheels should be supported properly.

Huh? You mean trucks must level at all times?

[Reverse_snowbird]

Originally Posted by aircommuter

Huh? You mean trucks must level at all times?

I mean that you don't want to just one portion of a rig. You don't want to twist the frame. The exception would probably be if one wheel was in a hole and you needed to raise it to get it level with the rest of the rig.

[aircommuter]

Doesn’t that happen when one parks their rig and not planning on sleeping or running the refrigerator? Frame twisting is a normal daily occurrence.

[MNtraveler]

Originally Posted by aircommuter

Doesn’t that happen when one parks their rig and not planning on sleeping or running the refrigerator? Frame twisting is a normal daily occurrence.

Bigfoot for example warns that if you are going to 'manually' level the RV, you should raise two front jacks, two rear, two left or two right together. Never just one of either axle nor one of either side (same difference, really).

The stated reason for this is to avoid twisting the frame.

I guess it could happen in un-level parking situations, but only if you put the right front wheel up on the curb, for example. Typically these levelers would be raising one corner 3-5" or more.

[ilmor]

Both dual wheels must definitely be blocked when leveling, which is yet another reason I avoided them w/ my current RV (class B van).