Another ‘dadgum’ post re: tire PSI…please bear with me…

[goufgators]

I know and agree with posts relevant to ‘have your coach weighed’ and I have done so although not four-corner weights. Axle weights indicate 80 psi for all tires. In the title, I said this post was about tire psi and that is, at least, partially correct. But, it’s also about that age-old problem of steering wander. Let me explain, if I can. Wandering has been a problem since the coach was new (2017). I’ve added rear stabilizer, front stabilizer, steer safe and had alignment adjusted to 5 degrees camber (not sure that’s the correct terminology for the alignment). After all the add-ons and alignment, the problem still exists. Here’s the interesting part. As you know, as tires turn they build up heat and psi increases. When I start out on a trip, the tires are at the cold 80 psi and steering at that psi is beautiful (Cadillacish good)…well maybe not quiet that good, but no wandering. No wandering continues until psi reaches about 90 psi or slightly above. That’s when the steering wheel gets a work our along with the driver (that’s me). After all of the above, here’s the question: In order to keep the tires’ psi from increasing quickly to a point of ‘wander’, would it be safe to reduce the cold psi to say 75 (remember Goodyear says, based on weight, cold should be 80). In our Florida very warm weather, the tires, starting at 75, would rather quickly heat up along with the psi and exceed the cold 80. But, in so doing, the psi might not reach the wandering problem. At least not as soon. Comments welcomed!

[Octaneforce]

2bzy2c wrote:

Captain_Happy wrote:
Something else to consider is inflating your tire with Nitrogen instead of pain air. Nitrogen doesn't heat up like plain air does. You stated that you F53 handles pretty good with PSI at 80 PSI when you start out, but as your tires heat up the handling changes. I've owned 2 F53 and they both handled badly.

Bzzzzzt WRONG!

There is virtually no difference between N2, O2, Air and Co2 thermal expansion rates.

"I'll take Thermal Expansion for $100 Alex."


Geek speak on the subject -->

Ideal Gas Law: PV = nRT

This formula is the "Ideal Gas Law Formula." Although there is no such thing as an ideal gas the formula is pretty accurate for N2, CO2, and oxygen as we assume that the gas molecules are point masses and the collisions of the molecules are totally elastic. (A completely elastic collision means that the energy of the molecules before a collision equals the energy of the molecules after a collision, or, to put it another way, there is no attraction among the molecules.) The formula becomes less accurate as the gas becomes very compressed and as the temperature decreases but here "very compressed" pressures are well above even the highest tire pressures and "decreased temperatures" are extremely cold, too cold for tires. There are some correction factors for both of these factors for each gas to convert it to a Real Gas Law Formula, but the Ideal Gas Law is a good estimation of the way N2, CO2 and "air" should react through temperature changes. What does all this mean? It simply means that "air", nitrogen vapor, and CO2 vapor should all react pretty much the same within normal tire pressures (0-120 PSI) and temperatures.

In the hvac business we use nitrogen for leak down tests under the assumption that it doesn't fluctuate in pressure like compressed air will. However we use pressure way beyond that of a tire (500-600psi). I guess this is within the theory you provided.

Why do race car drivers bother to use nitrogen in their tires? Is it a myth?

[larry_cad]

The best thing I did to my coach to improve the wandering was the REMOVE the Steersafe that came with the coach. Before: wandered all over the road. After: steered straight down the road. I would never recommend them. Wore me out. I have a set in my garage if anyone wants it free!

[2bzy2c]

Captain_Happy wrote:
Something else to consider is inflating your tire with Nitrogen instead of pain air. Nitrogen doesn't heat up like plain air does. You stated that you F53 handles pretty good with PSI at 80 PSI when you start out, but as your tires heat up the handling changes. I've owned 2 F53 and they both handled badly.

Bzzzzzt WRONG!

There is virtually no difference between N2, O2, Air and Co2 thermal expansion rates.

"I'll take Thermal Expansion for $100 Alex."


Geek speak on the subject -->

Ideal Gas Law: PV = nRT

This formula is the "Ideal Gas Law Formula." Although there is no such thing as an ideal gas the formula is pretty accurate for N2, CO2, and oxygen as we assume that the gas molecules are point masses and the collisions of the molecules are totally elastic. (A completely elastic collision means that the energy of the molecules before a collision equals the energy of the molecules after a collision, or, to put it another way, there is no attraction among the molecules.) The formula becomes less accurate as the gas becomes very compressed and as the temperature decreases but here "very compressed" pressures are well above even the highest tire pressures and "decreased temperatures" are extremely cold, too cold for tires. There are some correction factors for both of these factors for each gas to convert it to a Real Gas Law Formula, but the Ideal Gas Law is a good estimation of the way N2, CO2 and "air" should react through temperature changes. What does all this mean? It simply means that "air", nitrogen vapor, and CO2 vapor should all react pretty much the same within normal tire pressures (0-120 PSI) and temperatures.

[Ductape]

FWIW the recommended inflation is for that weight at maximum rated speed. There’s no need to go that fast to test your hypothesis, drop the pressure five pounds and stay ten mph or more below the rated tire speed and don’t worry about it.

[Captain_Happy]

Something else to consider is inflating your tire with Nitrogen instead of pain air. Nitrogen doesn't heat up like plain air does. You stated that you F53 handles pretty good with PSI at 80 PSI when you start out, but as your tires heat up the handling changes. I've owned 2 F53 and they both handled badly.

[ArchHoagland]

What size are your tires?

[goufgators]

Octaneforce wrote:
What do you mean by front and rear stabilizers? Sway bars? The best thing i ever did for steering wander was a supersteer rear trac bar and 4 bilstien shocks. I also did 4 centramatic balancing rings which is supposed to lower tire temps. And to ice the cake, i measured my front swaybar diameter (mine is 1 1/4) and ordered universal poly swaybar bushings from energy suspension to replace the mushy rubber ones. It made head winds almost non exististent. I have a p30.

I might have meant sway bars rather than stabilizers. Please forgive me! That aside, thanks for identifying efforts you made to control steering. I may be able to use some of them. Again, thanks.

[Octaneforce]

What do you mean by front and rear stabilizers? Sway bars? The best thing i ever did for steering wander was a supersteer rear trac bar and 4 bilstien shocks. I also did 4 centramatic balancing rings which is supposed to lower tire temps. And to ice the cake, i measured my front swaybar diameter (mine is 1 1/4) and ordered universal poly swaybar bushings from energy suspension to replace the mushy rubber ones. It made head winds almost non exististent. I have a p30.

[2chiefsRus]

In your first post, you seem to discount the value of a four corner weighing. It really can identify issues that you may not be aware of that could be causing your problem. Also I agree with some of the other posters, tire pressure set too low is one of the primary cause of tire failure, along with the tires being overloaded.

[way2roll]

goufgators wrote:

way2roll wrote:
Cold PSI means just that. Lowering PSI so the tires heat up to the recommended cold psi defeats the purpose of setting cold psi in the first place. They will be under inflated. As I am sure you are aware, low psi is the primary reason for tire failure - aside from a puncture. The F53 is notorious for poor handling. It's not going to handle like a car or a DP - ever. We had 2 gassers over the past decade and both were rather exhausting at the end of a long day in the saddle. Constant corrections. I would consider different tires, cheap handling fix or another model RV. Lowering psi below the tire manufacturers recommendation is not the proper way to handle the issue. You'd be borrowing risk. That of course is my opinion and I usually err to the side of caution. I am sure there are thousands of people out there driving on under inflated tires by 5 psi and either don't know it or don't care. I am confident however your tires will generate more heat.

I share your concern about under inflated tires but, knowing that they would be 'under inflated' for possible no more than 10 or 15 miles...I was just wondering if that would be enough time to damage the tires. I believe you're saying yes it would be inadvisable. Thanks for your comments.

I didn't mean to get all doom and gloom on you. Travelling all over the US, sure I check my tires prior to driving but if I am in a different elevation, dramatically different temp etc, if my tires are off by 5 psi I don't add air. Especially if I knew the conditions I were driving to would cause them now to be overly adjusted. Tires can survive a pretty wide range of psi and temps. The point I was trying to make is that I don't think knowingly adjusting PSI to lower than the tire manufacturer's chart doesn't seem to be the appropriate way to solve your problem, if it's a problem at all. Could be the nature of the beast. But no harm in trying and seeing for 20 miles. Do you have a TPMS?

And FWIW, I don't agree that the sticker in the MH is an accurate guide for setting PSI. Those stickers are put on at the factory and have no way of knowing how your rig is loaded for travel. Loading it and weighing is the only accurate way to know what they should be set to.

[goufgators]

Tyler0215 wrote:
Try it and see how the MH handles, 5 psi less is not going to kill your tires.

I was hoping for a comment like your's. Let's see if anyone else agrees.

[goufgators]

way2roll wrote:
Cold PSI means just that. Lowering PSI so the tires heat up to the recommended cold psi defeats the purpose of setting cold psi in the first place. They will be under inflated. As I am sure you are aware, low psi is the primary reason for tire failure - aside from a puncture. The F53 is notorious for poor handling. It's not going to handle like a car or a DP - ever. We had 2 gassers over the past decade and both were rather exhausting at the end of a long day in the saddle. Constant corrections. I would consider different tires, cheap handling fix or another model RV. Lowering psi below the tire manufacturers recommendation is not the proper way to handle the issue. You'd be borrowing risk. That of course is my opinion and I usually err to the side of caution. I am sure there are thousands of people out there driving on under inflated tires by 5 psi and either don't know it or don't care. I am confident however your tires will generate more heat.

I share your concern about under inflated tires but, knowing that they would be 'under inflated' for possible no more than 10 or 15 miles...I was just wondering if that would be enough time to damage the tires. I believe you're saying yes it would be inadvisable. Thanks for your comments.

[Bruce_Brown]

Try this; set them to 85 and see if they don't heat up as much. Heat is caused by rolling resistance, more pressure to begin with may equal less heat build up.

It probably won't work but it'd be free to try.

[CapriRacer]

First, caster is what contributes to straight line stability, so I assume you meant that the caster was moved to 5°. That much camber is only used in racecars.

Second, too high a pressure on the fronts tends to cause the tires to follow the ruts and grooves in the road. What you didn't tell us was the tire size and the specified inflation for that size - not what Goodyear's charts say, but what Winnebago's sticker says.

I am having trouble with the rear tire pressures being the same as the fronts. That would be a bit odd.

And lastly, while this isn't the time of year to really test this out, have you looked at how much the tire pressure builds up after an hour of driving at highway speeds? Rule of thumb say no more than 10%, and at 15% you need more.

I suspect you are hardly getting any at all.

OK, one more tidbit. When calculating the pressure buildup, you need to back out the pressure buildup due to ambient temperature change. 3% for avery 10°F. That is, if you start the day at 50°F ambient and 80 psi, and measure the buildup later when it's 90°F ambient, 12% of the buildup (about 10 psi) is due to the increased outside temperature, and not due to operating conditions (load in particular.)

[Tyler0215]

Try it and see how the MH handles, 5 psi less is not going to kill your tires.