Tire overloading on curves....

[Ky-N]

I know your focus is on tire overloading but I took time to check the gross on my single axle trailer, it is 3510 lbs. and the rims are rated at 1760 lbs. so there is only 20 lbs. more rim capacity than the gross of the trailer. Side loading of the tire is also side loading the rim and if the rim fails the result IMHO is worse than the tire failing though neither one is a picnic.

Really seems to me the manufacturers should be REQUIRED to install equipment with more capacity.

[Tireman9]

Originally Posted by Ky-N

I know your focus is on tire overloading but I took time to check the gross on my single axle trailer, it is 3510 lbs. and the rims are rated at 1760 lbs. so there is only 20 lbs. more rim capacity than the gross of the trailer. Side loading of the tire is also side loading the rim and if the rim fails the result IMHO is worse than the tire failing though neither one is a picnic.

Really seems to me the manufacturers should be REQUIRED to install equipment with more capacity.

IMO RV companies for the most part appear to have made the decision to cut corners and to only do the minimum required by federal regulation rather than spend a few hundred and deliver a much better, and safer vehicle to their customers. The requirement is that the GAWR be less than the total load capacity of the tires (and wheels). They have chosen to ignore the fact that side to side and axle to axle loads are not evenly split so they must, or should know that with load margins less than 50# in some cases one tire is almost guaranteed to be in constant overload even before considering side loading to curves or wind.
I will leave it up to you to decide if companies that cut things that close really are building the "quality RVs" they claim.

[Gyrogearloose]

I've changed focus from overloading on curves to looking at tire overloading due to cross wind. I've now learned how to calculate wind force using, among other things, an SAE technical document that analyzes tractor trailer instability in wind and on ice as a model.

An important question is how much wind is reasonable to assume under normal conditions. California and Oregon both issue wind speed advisories when the sustained wind is over 35mph. Other agencies seem to use different thresholds, some of which are higher. It seems reasonable for the moment to assume that if a wind advisory hasn't been issued, conditions are 'normal'.


If my math is right, when pulling my fully loaded trailer in a crosswind of 35mph, the leeward tires will be overloaded by 425lbs each. This could go on for a long time without my knowledge.


If my tires had the 20% margin recommended by Carlisle Tire, there would be no tire overload with a 35mph cross wind.


This math needs checking.
I'll provide my spreadsheet to anyone who's willing to check it.

[poorbuthappy]

I'm horrible with the numbers but, wind loading on rigging for overhead lifting is huge. Depending on surface area, you can see huge jumps in actual weight/ load cell.

[Gyrogearloose]

For anyone who might be willing to check it, or wants to try using it, here’s my math for calculating the increased load on leeward tires due to a cross wind. Don't glaze over. The math turns out to be just multiplies and divides. The numbers I use are for a 35mph wind and the dimensions of my trailer. You should be able to copy what I did using numbers for your own choice of wind speed and trailer.


1. Calculate the force of wind on the side of the trailer

A travel trailer has a lot in common with a tractor trailer.The following equation comes from “Wind Effects On Dynamic Stability of Tractor Trailers In Winter Conditions”, published by the SAE.

Fwind = 0.5 * Rho * Cd * Vwind * Vwind * L * H

Rho = 0.002376
This is the density of air and has the units of slug per cubic foot.
I had to look up ‘slug’.It’s mass in English units using feet and lbs for distance and force.

Cd = 2.0
This is the ‘Drag Coefficient’ that depends on the shape of the object.
The value of 2.0 is used for tractor trailers by the SAE article.

Vwind
This is the speed of the wind in units of ft/sec.
To get ft/sec, multiply mph by 5280/3600.
At 35mph, the wind speed is 51.33 ft/sec.

L and H
These are the length and height of the trailer box in units of feet.
Notice that they’re multiplied together in the equation to produce area.
For my trailer, I’m using L = 26ft and H=8ft.

So,Fwind = 0.5 * 0.002376 * 2.0 * 51.33 * 51.33 * 26 * 8=1302lbs



2. Calculate the tipping torque on the trailer

The force of the wind pushing on the side of the trailer creates a torque or moment that tries to tip the trailer over.The force from the wind is equivalent to a single force pressing in the middle of the side of the trailer. The higher the trailer, the more the torque.Multiplying total force times the height above the road surface of the center of the trailer side wall gives tipping torque.I’m using 5ft as the center of my wall.

Tipping Torque = Fwind * CenterHeight= 1302 * 5 = 6510 ft-lbs



3. Calculate the restoring force on the tires

The trailer doesn’t tip over because the forces on the tires create a restoring torque that keeps the trailer upright.Force transfers from the windward tire to the leeward tire.This works until the windspeed is so large that the force on the windward tire is reduced to zero, at which point the trailer rolls over.Up to rollover, the increased force on the leeward tires, and reduced force on the windward tires creates a torque that balances the tipping torque.The wider the trailer, and further spaced the wheels, the less force it takes to do this.Like the SAE article, I use the leeward tires as the pivot point.So, to calculate restoring force from tipping torque, I divide by the distance between wheels, which in my case is about 7.5ft.


Restoring Force = Tipping Torque / Wheel Span = 6510 / 7.5 = 868 lbs

4. Calculate the force due to wind per tire

The total Restoring Force is spread among all the tires on one side.Divide by the number of axles to get the added load per tire due to wind.

Tire Wind Load = Restoring Force / #Axles = 868 / 2 = 434 lbs per tire


So, if my math is right, a 35mph wind will cause an increase in load of 434 lbs on each of my downwind tires. This is approximate because the drag coefficient in the original force equation is approximate.


If someone finds a fundamental error in this, I'll redo it.

[Mr Havercamp]

Originally Posted by Gyrogearloose

If someone finds a fundamental error in this, I'll redo it.

You forgot to factor in the rotational mass of the schwansoculator.

[Gyrogearloose]

Originally Posted by Mr Havercamp

You forgot to factor in the rotational mass of the schwansoculator.

I think you mean 'framistat', because it's what gets rectabularly extruded, not the schwansoculator.

[Springerdad]

Originally Posted by Gyrogearloose

I think you mean 'framistat', because it's what gets rectabularly extruded, not the schwansoculator.

I thought it was the dual focal metering valve.

FWIW, Gyrogearloose, I haven't run that formula against my TT but those load numbers you have look about right to me.

[Gyrogearloose]

Originally Posted by Springerdad

I thought it was the dual focal metering valve.

FWIW, Gyrogearloose, I haven't run that formula against my TT but those load numbers you have look about right to me.

And, of course, the implication is that if there's no margin between the load on my tires and their load rating when the trailer is sitting still in my driveway, then the tires are overloaded by hundreds of pounds when I encounter a significant crosswind (but not strong enough to trigger a high wind advisory).

[OKI Marine]

So, given all that what is the solution? If the senario of it that being already overloaded or near over loaded before moving and given the force of rotation on curves and coupled in with wind velocity then what are the options? Take curves at a low speed, pullover and stop during wind conditions, switch out axles, tires and rims to larger, higher capacity?

[SuicideSaints]

Im not intentionally trying to stir the pot (troll) here but by this same logic wouldn't all these forces make the axle rating not high enough to handle the trailer. If I add in the wind force and down force of a turn aren't my/your axles overloaded now? What about the frame too? Doesn't the newly calculated stresses put the frame out of spec?

We are in the realm of calculating all these forces to stay within weight so the weight police dont pull us over and ticket us or we fear that pulling this thing down the road is like a nuke waiting to go ff. I think all this is unnecessary in real world application. Basically slap some higher load rating tires on your trailer (like the hundreds of other campers out there) and stay under GVWR and go out and enjoy camping.

It seems like I could do hundreds of calculations to say something is out of spec, something is obviously working because I see hundreds of campers being pulled down the road daily.

The fact of the matter is the RV business is just that, a "business". The definition of a business is an establishment to make money through commerce. If manufacturers were to put top of the line tires that built in a safety margin and put upgraded axles on and then beef up the frame that cost has to be absorbed somewhere and that somewhere would be the consumers wallet. Based on all the threads on here that ask how to negotiate and find the good deal, as well as when the best time of year to get the cheapest price I don't think the average consumer would want to do that. In addition that is why there are levels of campers, Entry level all the way to 500k dollar top of the line untis.

I am not saying that this is a perfect business or practice but it is a reality. Look at the auto industry... you get trucks that have P rated tires and so forth.

I apologize for the rant but I think at times we get so lost in the "what ifs" we all miss the enjoyment of getting out there and LIVING LIFE.

[Tireman9]

Originally Posted by SuicideSaints

Im not intentionally trying to stir the pot (troll) here but by this same logic wouldn't all these forces make the axle rating not high enough to handle the trailer. If I add in the wind force and down force of a turn aren't my/your axles overloaded now? What about the frame too? Doesn't the newly calculated stresses put the frame out of spec?

We are in the realm of calculating all these forces to stay within weight so the weight police dont pull us over and ticket us or we fear that pulling this thing down the road is like a nuke waiting to go ff. I think all this is unnecessary in real world application. Basically slap some higher load rating tires on your trailer (like the hundreds of other campers out there) and stay under GVWR and go out and enjoy camping.

It seems like I could do hundreds of calculations to say something is out of spec, something is obviously working because I see hundreds of campers being pulled down the road daily.

The fact of the matter is the RV business is just that, a "business". The definition of a business is an establishment to make money through commerce. If manufacturers were to put top of the line tires that built in a safety margin and put upgraded axles on and then beef up the frame that cost has to be absorbed somewhere and that somewhere would be the consumers wallet. Based on all the threads on here that ask how to negotiate and find the good deal, as well as when the best time of year to get the cheapest price I don't think the average consumer would want to do that. In addition that is why there are levels of campers, Entry level all the way to 500k dollar top of the line untis.

I am not saying that this is a perfect business or practice but it is a reality. Look at the auto industry... you get trucks that have P rated tires and so forth.

I apologize for the rant but I think at times we get so lost in the "what ifs" we all miss the enjoyment of getting out there and LIVING LIFE.

I understand your point. IMO the problem with the "Quality costs too much" thought process is what almost put "Detroit" out of business when Toyota & Honda etc entered the US market. The consumer decided they wanted better quality.

But given the growth of Walmart and cheap imports being bought by American workers who at the same time complain about their jobs being shipped overseas. One has to wonder if the average person has any concept of the cost of poor quality.
This cost shows up in a majority of RV forum complaints about the failure of not just tires but of refrigerators catching fire or slide-outs that don't or any of the numerous other component failures. I don't buy the argument that owning an RV is like dragging your house down the road.
If car makers can design and build 12v systems that they can warranty for 10/100,000 why can RV companies only manage to get their 12v systems to only last 12 months and a couple thousand miles?

Why do tires on motorhomes last 7 to 10 years but only 3 - 5 on TT? Could it be that a good portion of the tires selection is done by companies such as Ford or Freightliner or GM?

The purpose of this particular thread was to offer a possible reason for the short life and high failure rate of tires in RV use. If tires are selected that only have a safety margin of 50# but the RV is built with a 300# bias to one side then you add another 100 to 400# bias due to wind you end up with some tires being overloaded by 30% or more, and people are suppose to accept this?

If so I suggest we stop letting people complain about tires and just explain that they got what they wanted, a cheaper RV.

Just like hand tools. I don't expect S_K or Craftsman quality from Harbor Freight and I certainly don't complain when a HF tool fails early.

[Gyrogearloose]

OKI Marine,

Slowing down won't help in crosswind. If there is a high wind advisory, definitely stay off the road.


And if there's zero margin between the load on the tires and their load rating, you can't win. There's no way to avoid overloading them when moving. You can't avoid turning, and you can't avoid wind.

My overload occurs with the factory tires. I've replaced them with Kumho 857's that have a much higher load rating. Old tires are 1760lbs; new tires are 2270lbs. The slightly larger diameter worries me, but isn't a problem based on a consensus that includes a trailer dealer. When I try my math with my new tires, the overloads for both wind and curves for the most part go away. If I don't use my full GVWR, then the math looks even better.




So, all the advice I've seen to change from LRC to name brand LRD tires seems good to me. This certainly helps. Each trailer will be different, so to know for sure one needs to do the math for his set of trailer and tires.

The Carlisle Tire recommendation for a 20% margin between trailer weight (including tongue) and combined load rating of the tires seems also to be good advice. This does a good job of eliminating the overloads in reasonable conditions, both on curves and in wind based on my math.




Warning: if you use tires other than those on the vehicle rating plate, you may void your warranty and create other problems. For me, this is the lesser of evils.




The underlying problem is created by the manufacturers, IMHO. I think it's reasonable to expect that factory tires on a fully loaded new trailer should not be overloaded in normal driving conditions like turns taken at the advisory speed (which most people seem to exceed), and winds that don't trigger a high wind advisory. So I think the manufacturers have a serious problem, and possibly a product liability issue, to deal with.

[Tireman9]

Given the Rv warranty is probably only 12 months and your TT tire warranty may be shorter why worry about that warranty? Especially since they will not stand behind most tire failures.

[Gyrogearloose]

Originally Posted by OldCoot

I don't see any benefit in the discussion. People can do all the calculations they want and it still isn't going to change the rv industry. Engineers have calculated that a humming bird can't fly, but it still flies.

What will change the RV industry, as Tireman suggested previously, is educating people about the problems and motivating them to report safety issues to the NHTSA.

I have done this. I've provided my math to the NHTSA and they have agreed to take a look at it. My impression is that they're serious about it. It would certainly help if others also do this, while being careful that any complaints have substance.

[SuicideSaints]

Originally Posted by Tireman9

I understand your point. IMO the problem with the "Quality costs too much" thought process is what almost put "Detroit" out of business when Toyota & Honda etc entered the US market. The consumer decided they wanted better quality.

But given the growth of Walmart and cheap imports being bought by American workers who at the same time complain about their jobs being shipped overseas. One has to wonder if the average person has any concept of the cost of poor quality.
This cost shows up in a majority of RV forum complaints about the failure of not just tires but of refrigerators catching fire or slide-outs that don't or any of the numerous other component failures. I don't buy the argument that owning an RV is like dragging your house down the road.
If car makers can design and build 12v systems that they can warranty for 10/100,000 why can RV companies only manage to get their 12v systems to only last 12 months and a couple thousand miles?

Why do tires on motorhomes last 7 to 10 years but only 3 - 5 on TT? Could it be that a good portion of the tires selection is done by companies such as Ford or Freightliner or GM?

The purpose of this particular thread was to offer a possible reason for the short life and high failure rate of tires in RV use. If tires are selected that only have a safety margin of 50# but the RV is built with a 300# bias to one side then you add another 100 to 400# bias due to wind you end up with some tires being overloaded by 30% or more, and people are suppose to accept this?

If so I suggest we stop letting people complain about tires and just explain that they got what they wanted, a cheaper RV.

Just like hand tools. I don't expect S_K or Craftsman quality from Harbor Freight and I certainly don't complain when a HF tool fails early.

I understand the point of the thread and I understand what you are saying, I am just saying that this is a known problem and the only current solution is either roll the maxxed/cheap tires till they fail, negotiate at the time of sale for better tires or drive straight to your shop of choice and get new ones with a higher load rating.

A caveat though which is my main point.....

By the OPs own admission and math, his current tires (at the time of failure) were rated for 1760 each. The trailer is 7700#s - TW (700#s) divide that by 4 tires = 1750 per tire. This is within spec for static.

Now add in 500# of force for dynamic (turns) and add in 400#s for wind force. So that makes the load on the tire 1750 + 500 + 400 = 2650.

In addition to that 2650 it was stated that you "should" have a 20% margin of safety. 2650 + 20% = 3180 to have all my bases covered. Or even if you do 20% safety on the original 1750 tire that still isn't enough.

So this thread would lead me to believe that I need tires that can hold a minimum of 2650 lbs to 3180 pounds (for a 7k# TT) depending on the dynamics and "recommended" safety margin. That is at the top of the range for D tires almost to E tires.

I just dont buy it.......

[SuicideSaints]

Originally Posted by Gyrogearloose

My overload occurs with the factory tires. I've replaced them with Kumho 857's that have a much higher load rating. Old tires are 1760lbs; new tires are 2270lbs. The slightly larger diameter worries me, but isn't a problem based on a consensus that includes a trailer dealer. When I try my math with my new tires, the overloads for both wind and curves for the most part go away. If I don't use my full GVWR, then the math looks even better.

By your own admission and math, in a dynamic state of turn and wind you would need a minimum of 2650# capacity on each tire, even more capacity if you want a safety margin bigger than anticipated turn and wind. So those new tires are still overloaded by 400#s.

I hope you are seeing my point about just getting out there and living/enjoying life......

[Gyrogearloose]

Originally Posted by SuicideSaints

By your own admission and math, in a dynamic state of turn and wind you would need a minimum of 2650# capacity on each tire, even more capacity if you want a safety margin bigger than anticipated turn and wind. So those new tires are still overloaded by 400#s.

I hope you are seeing my point about just getting out there and living/enjoying life......

I've got a little over 40 years of experience designing products. This problem of worst case calculations comes up all the time.

Yup. If you take the worst case of multiple bad things happening all at the same time, and then try to design a product that survives, you will usually end up with something that you can't sell because it's too complex, or heavy, or expensive.

But this doesn't mean that a product shouldn't handle individual and reasonable stresses. Good designs will handle some simultaneous stresses, but not necessarily all possible combinations.

For me, having had an unexplained tire failure on a winding road, and having discovered that my tires are likely to be overloaded by hundreds of pounds when I tow on curves, doing nothing is not an option. My new tires should handle curves or wind, and if it's windy and curvy at the same time I'll slow down to reduce at least the portion of the load due to the curves. This is the best solution I can think of for my situation.

[wmtire]

The site team cleaned this one up somewhat. Any posts that were condescending, off-topic, or not that faintest bit relevant to the ongoing discussion were removed, and will continue to be removed.

If this topic isn't something you can contribute to, then please find one that you can. It is trolling and a disruption to forum harmony for those who want to interject themselves into a thread, just to state they don't feel the thread is relevant.

It has been a good discussion and the site team will make sure it continues as well as it's been.

[lbrjet]

Do all of the math you like. The only thing that matters is if your trailer came with 'C's and they are close to being maxed out, change them out with 'D's. It is really just that simple.


2010 F250 5.4L 3.73
2011 Flagstaff 831FKBSS
Equal-i-zer 4pt 12K