Tire Pressure Verification
I am looking for verification from some of you more experienced owners of my interpretation of the data I have collected concerning the appropriate tire pressure in my fiver.
First, my fiver is a 2019 and has the original Sailun S637 ST235/80R16 Load Range G tires. The sticker on the side of the RV says that the tire pressure should be 110 psi, which is what I have been running for the 2 months that we have owned it. The ride is very rough and I have experienced a couple of divots/impressions in the vinyl floor from the slide out rollers. Thus my research into this subject.
On our last trip, I weighed everything at the CAT scales while fully loaded for a 3 week long outing. The weight at the rear axles was 11,640 lbs. That comes out to 2,910 lbs. per tire.
I looked up the tire pressure chart from Sailun and found the information shown below.
Based on my interpretation of this chart and the weights that I got at the CAT scales, it appears that I can be running pressures as low as 85 psi, although my gut tells me to not go below 90 psi.
Am I interpreting this data correctly?
The rules police will say the minimum is the sticker value, regardless of reality. Because rules are rules, varying from the sticker is on you, even though you have actuals and good calculations to back you up. Whereas FR just slaps on "the min is the max" sticker to save time and energy of doing any engineering. I have the same problem with my little A-frame where FR slaps "the min is the max" sticker calling for 65PSI on my LR D tires. According to the charts, I could go to 55PSI at max gross weight of the camper with a 20% reserve.
2019 Flagstaff T21TBHW A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time
Filled in in my made traveltrailer- tirepressure calculator it gives 86 psi. This is using 90% of the loadcapacity for the pressure.
And because maxspeed of tires 75mph, I lowered loadindex by 4 steps to 125, to get max pressure without bumping. Then it gives 97 psi.
So I would start with 95 psi, and if then comfort is OK , keep it at that, for fuellsaving and livetime of tires.
Or are we talking about a motorhome?
Calculation the same, but what is the front tires psi ?
And weigt on front axle , and same tires?
Thanks for everyone's input.
If I had not wanted it, I wouldn't have created the post.
For now, I am going to install a spring kit on the rear, which should help out with the way the truck sits and rides when towing. I know that this is not the permanent fix and does not increase any of the weight specs of the truck.
When it is time to replace the truck in a few years, I will upgrade as others have suggested.
However, based on the actual CAT scales weight data, I am not anywhere near the GVW rating of the fiver when packed for a long trip so that is why I was considering lowering the pressures to 90 or 95 psi.
Obviously, I am responsible (and hopefully smart enough) to know when I add weight to the fiver (such as full fluid tanks, which I never travel with) that I will need to increase the tire pressure accordingly.
Only thebofficial institutes dont agree with it, and so it can give problems with insurance if something happens .
And I think that topicstarted realises , what is important for the pressure, as he writes in last post.
I respect Airdale's knowledge of the rules.
But those rules are determined by the official institutes, to cover the rules of nature.
But rules of nature always rule above those of officiall institutes.
In this case TS can, knowing the weights, safely go lower without overheating the tires at any moment, and riding quality is better.
Safety first, but enaugh is enaugh.
I know youíre a tire expert in your country but I wonder; do you have RV trailers 40í long weighing over 18,000#?
To compound that, all of our RV trailers are known to be very unbalanced across the axles and from one to the other. Consumers/owners donít do much to insure there are even loads on their axles.
The FMVSS regulation for axle loads on RV trailers only requires the tires to provide a load capacity equal to the trailers GAWRs. Recently a large RV membership organization (RVIA) has enacted a recommendation that all of their participating members provide a minimum of 10% in tire load capacity reserves above the trailers certified GAWRs. About 98 % of all of our RV trailer manufacturers are members of RVIA and their response to the recommendation has been very favorable.
Some of the RV trailer tire people that write posts in these forums are tire engineers. They all agree that 15 Ė 20% in load capacity reserves may be adequate.
The Federal Motor Vehicle Safety Standards (FMVSS), to which all or our trailer manufacturers must abide, are written to insure minimum safety standards have been followed in the manufacturing of the trailers and that any/all OEM materials used also met the same standard. The trailer manufacturer MUST certify that they have met all of those safety standards that were in effect on the day of vehicle certification.
When disseminating our standards, the reader CANNOT read anything into them. They are specific and mean what they say.
In FMVSS 571.120; paragraph S5.3.1 Tires. The size designation and the recommended cold inflation pressure for those tires such that the sum of the load ratings of the tires on each axle is appropriate for the GAWR.
Itís the one that most consumers do not understand or want to comply with. Vehicle manufacturers accept advice from tire manufacturers for the fitment and load capacities of Original Equipment tires. However, the vehicle manufacturers have the sole responsibility to set the minimum standard for those OE tires.
The minimum standard for OE tires dictates the minimum standard for replacement tires.
And that is my mission , when reacting on American fora, to make users realise that those official standards sometimes even aint enaugh.
I am not a tire-specialist in my country, but a " pigheaded Dutch SELFDECLARED tirePRESSURE-specialist" .
ST- tires with 10% reserve is not enaugh.
ST is calculated in maxload for 65mph, and so TT or 5th-wheel-maker can use smaller and cheaper tires and rimms, and still follow the minimum standards, and now + 10% recomandation.
When I calculate pressure with my made spreadsheet, automatically 11% is added to the determined axle-load, to make 90% used of loadcapacity for the pressure, to cover unequall load R/L. But for ST-tires I lower the maxload by 6 steps, to give tge tire a deflection it would need for 160kmph/99mph.
This together I determined to give still no bumping, and gives maximum savety-reserve,but also longer live of tire, and good for fuelsaving.
If you then calculate the needed maxload, so you dont need to go over the maximum allowed cold pressure, you roughly need 6 LI-step= 15% higher maxload + 11% for the R/L inbalance gives 27.65% reserve needed for ST tires.
And for LT tires only 11% for inbalance, because theż are calculated in maxload for 99mph.
But I read from TM9 that RV tires have maxspeed of 75mph, and that is different from European C-tyres.
Will attach a document about 50 years ETRTO ( european TRA) for your interest, in wuch you can read that all those standards are determined by nacociations between tiremakers, and TR&A is mentioned a lot, and a lot of ISO.
On page 6/18 about how the loadindex-system is determined.
This last on " nice to know " basis, for who is interested.
I dont chare the opinion of tire and TT makers, that RV ers all are yust dumb, if you can buy an expensive RV , you have somw inteligence.
Only risk is that you ( yust as I) think to be more clever then the tire and RV makers, and so forget for instance to add the persons and water to the weighed loads.
Calculated the reserve needed for ST acurate.
6 LI steps is maxload x 1,1885
So 1.1885 x 1.11 = 1.3192.
So ST need even 32% reserve to prefent you have to use higher then " max pressure" of tire, if you want maximum reserve without bumping.
Now going for instance from D load ST , with yust enaugh maxload to be OK with the old rules of no reserve for GAWR, to E load, you probably need the 80 psi, to give the max reserve with no bumping, to my system.
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