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Friday, February 26, 2021

Excessive pressure increase?

 Another question from a reader with a "heavy 5th wheel trailer.


Hopefully, this isn’t a duplicate.  I tried to post a comment before I saw the email.  Thanks for all the excellent information on your site.  It is incredibly difficult to determine who is sharing good knowledge on other forums.  

I have read through many of your posts about hot tire pressures in particular, but I’m hoping to get your experienced input on a more specific example.  How much pressure increase is reasonable in load range H (4805 max load) tires inflated to 122-125 psi CIP?  I have tandem axles on a fairly heavy fifth wheel.  All weights are within limits (trailer, axle, tires, etc.), but with the wheel-position weights, one of the four tires is carrying c. 4,700 lbs, which is 53% of the total for that axle.  I know this violates your ‘85% of max load’ recommendation.  However, I’m seeing up to 32-35 psi increases (to about 157-160 psi) after 3-4 hours on the road when in the sun.  I have always pulled over for a break at that point, so I don’t know if it would have continued to climb.  According to the TPMS, the heavily loaded tire reaches these ranges first, but the other three aren’t far behind; accounting for shade, wind, etc. all four increase in psi and temps together fairly steadily throughout.  Generally, the temperatures aren’t much higher than 30 degrees above ambient, but I’m really wondering if there’s a point I should be concerned about the pressure increases.  I know it's been said the tires are engineered to handle pressures up to 100% over max CIP, but just trying to find the threshold of when one should begin to be concerned.  Once elevated to the point where I pull over to cool things off, I can only get about 30 minutes of drive time before they start touching that 157-160 psi level again. The tires are within specs, but close to the maximum.  I can add to the CIP as you recommend, but I’m limited to only a couple of PSI before I hit the max CIP of 125 psi.  My rims are rated to 130, but not sure I have the clearance for larger tires, and I’m not sure if I need to consider that.  The current tires are Goodyear G114 215/75R17.5.  Unfortunately, Goodyear documentation or customer support doesn’t provide much guidance on this.

Any thoughts would be greatly appreciated.



My reply:

Let's get a few things settled. The pressure on the tire sidewall is actually the MINIMUM required if you are loaded to the MAXIMUM load. Ya I know it's confusing but the reason is that increasing above that pressure will not give you any additional load capacity.

You gave ranges for your pressure. Why aren't you inflating all your tires to 125 psi? I am also wondering if you have had your pressure gauge checked at the 125 psi level where you should be running your tires.  Is the gauge a "stick" type or dial or digital? Digital is best as they are easier to read and I have found them to be more accurate too.

You didn't mention your travel speed but those tires are rated to a MAX of 75 and you should consider that max just as you do the "Red Line" for your TV engine speed. I would be limiting travel speed to 70 with 65 being better since you are loading your tires to their almost their max. Something to remember is that your tires are considered "regional" service not long haul so it would normally not driven at full highway speeds for many hours on end. Think of how a commercial trailer would be used. Most of the time I bet less than an hour loaded then an hour back to the warehouse empty. You are running at Max load for hours on end.

While I have seen reports from some people seeing a +25% inflation increase to me that suggests that the combination of load and speed is pushing your tires to their limit.

While a NEW tire might be capable of handling 100% over-inflation, static in a special test chamber, that would be way too high for a used tire running down the highway.

You didn't ask about tire life but with your conditions, I would suggest you figure on a 5-year life At that point you should be able to sell your tires for a good price. Maybe in the $100 range but the main consideration is to sell them before they fail under your operating conditions.
When you are operating at the limit, confirming all the details such as load, inflation and speed, need to be checked. 



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