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Old 06-09-2015, 06:09 PM   #1
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Tire pressure question

I have a 1996 Damon daybreak and the sticker on the wall says front GVW is 4880 tire pressure 65 pounds. Tire wall says 80 max. Is it correct that I divide 4880 by two and get 2440 Looking up the tire manufacture ratings for a 235/85/16 tire for 2440 also indicates to run at 65 psi. 80 PSI for 3042 load rating.

just wanting to know if I am reading all this correctly.
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Old 06-09-2015, 06:12 PM   #2
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In a nutshell: NO, you really need four "corner" weights and take the heaviest end of the axle and run that pressure across all tires on that axle.


But, if all you have is the totally axle weight I would do as you have AND add 5 psi as a safety measure.
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Old 06-09-2015, 06:37 PM   #3
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[QUOTE=Mr_D;2595887]In a nutshell: NO, you really need four "corner" weights and take the heaviest end of the axle and run that pressure across all tires on that axle.


But, if all you have is the totally axle weight I would do as you have AND add 5 psi as a safety measure.[/QUOTE]

This is what I was thinking also was going to run them at 70 PSI, until it will be a while before I load up for a trip and can get four corners weights. I just had steer rubber put on and ball joints replaced along with alignment. All is right and tight again.
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Old 06-10-2015, 03:02 AM   #4
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Googled for Damon Daybreak , because thinking it was a Travel- Trailer or 5th wheeler, because you only gave GVW.
Apeared thoug to be a motorhome, as you would expect in this part of the forum.
Sorry I am from Holland Europe so am not familiair with the different brands here used.

But to react on your question , dont use American made Pressure/loadcapacity lists, they are made with a formula that leads to to much deflection in the lower pressures. Goal of all calculations is to keep the deflection the same for the lower pressures as when maximum load and AT-pressure on tire.
The logical calculation most people tink is logica then is even better and comes pretty close to the ever to be constucted ideal formula to laws of nature.

I detemined my own universal formula , from wich official used and an alternative of an American IR J.C.Daws can be made by placing other construction load and power. Logical formula is same as using power 1 wich is the same as no power, and construction load zero . Also you can refine that formula by first substracting 100lbs a tire from maximum load of it and asumed real weight on it ( or GAWR/number of tires). That 100 lbs is average part of the maximum load that is carried by the construction of the tire, wich I babtised "construction load Lc".

Will include a picture of different calculations and at the bottom the "on the road methode, wich is also pretty simple and even closer to natures law.



But if you can give more data , I can calculate a save pressure with some reserve, using the GAWR's and GVWR of your motorhome.
Best would be weighing per wheel(pair) , second best per axle, but as long as you did not do that ,we can use those going from the idea that you are not allowed to go over that by law.

Copy text I used for other topic to show what I need from motorhome and tires. copyd it for general use so can be a bit off the subject here.

Tirepressure advice is all about load on tire and speed ( and sometimes about alighnment - camber angle).

So if you can give details of car and tires , I can calculate an advice pressure with some reserve for things like, pressure-loss in time, unequall loading R/L, incidental extra load, misreadings of pressure scales,and misyudging of weight, etc.

This is from tires next and can be read from sidewall:
Maximum load or loadindex.
Kind of tire to determine the AT-pressure/pressure needed for the maximum load up to maximum speed of tire, or if lower 160km/99m/h/reference-pressure, wich is not the maximum pressure of tire.
Maximum speed of tire, most given as letter ( Q=160km/99m/h,N=140km/86m/h fi)
If you have offroad or tires looking like that , with large profile blocs that cover a part of sidewall, also mention, they are allowed lesser deflection then a normal road tire, then the tire maker used to determine the maximum load (to my conclusion the case for the Bridgestone tires on Ford Explorer in the Ford/Firestone affaire).
If you cant find all of it give sises of tire and Loadkind, then I will google for it.

From car next and mostly can be found on same plate as the original pressure advices:
GAWR and GVWR ( Gross Axle/Vehicle Weight Rating)
But best would be to determine the real weights in your use on seperate tires or estimate it as acurate as possible, by weighing per wheel(pair) or axle.
Maximum speed , you dont go over for even a minute in your use, eventually different for different situations, for instance when towing or fully loaded.This apart from trafic regulations, if you drive faster then allowed give that speed. Nature punnisches with tire-failure, police only with a penalty.
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Old 06-10-2015, 06:59 AM   #5
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That sounds right to me.

The 65 lb tire pressure is all the tires need, to carry that weight and that is all of the weight that, that axle can carry.

If you come up with more weight, you are over weight, and you need to remove it, not put more air in the tires.

More pressure will make the ride harsher , and I doubt you will find much lighter front axle weights, when fully loaded.

All of this tire pressure management may be needed for the bigger tires, on big rigs, but there isn't a lot of wiggle room on the Class C 16 inch tires.

Someone mentioned adding 5 psi as a safety measure. If you need 5 more lbs, for safety, you also need a heavier axle and springs.

Then, there is the argument, of more pressure to protect the tires from impact damage. That impact will then, be sent into the chassis, possibly damaging something there.

IMHO, if the impact is that servere, let the tire take the hit.

I follow the manufactures, well thought out, advice. They want you to be safe.
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Old 06-10-2015, 07:09 AM   #6
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Better to read the sidewall max cold pressure and inflate to that pressure. LR C= 50# max, LR D= 65#, LR E= 80#
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Old 06-10-2015, 07:26 AM   #7
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ALL numbers molded into the tire or written on a sticker are WRONG for your RV (in most cases) save the MAX GVW Max CGVW and Max Axle weights.

ALL of them (Well if you read properly the one on the tire is correct, it is also a MAXIMUM like the GVW, CGVW and Axle but few read properly)

The PROPER inflation pressure is found by a very simple formula.

FIrst: Go to the tire manufacturer's web page and download the inflation chart for your tires.

Next GET WEIGHED,, pack for travel and either call Aweigh We Go (RVsafety.com if I recall correctly) or find a FLAT and preferrably segmented scale (not a bridge scale) where you can get the axle weight and then the wheel on just one side weights.

Find out the load on each corner and inflate per chart.
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Old 06-10-2015, 07:41 AM   #8
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two places you can get weighed (axles, not corners) are the local dump and highway weigh stations. Free if you talk nice to them.

Many truck stops have them, too, for about $8.
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Old 06-10-2015, 07:59 AM   #9
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Then there is the disclaimer by the tire manufacturers, to get past.

" Never go below the vechicle manufacturers recommended pressures ".

Ford recommends my Class C rears be at 80 lbs. The tire says 80 lbs max. No math there.
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Old 06-10-2015, 08:59 AM   #10
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A four corner weight is best, but you can do a reasonable estimate with gross axle weight, or even gross axle ratings if no scales are available. However, when estimating you should allow for unbalanced weight on the axle - motorhomes are often heavier on one side than the other. So, after diving by two (front axle) or 4 (rear axle), add back some weight (at least 10%) in case the one end is heaver than the other. Then look up the required psi, which is the minimum for that weight. Add another 5 psi to that, just so you don't have to worry about temperature and altitude changes.
For the rear axle, make sure you use the DUAL column in the weight tables. Tires are rated for less capacity when used as a dual pair and the psi is adjusted accordingly.

I totally disagree with Jadatis's contention that American tire inflation tables are wrong, but see no point in debating it here. Make your own call on that.
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Old 06-10-2015, 09:40 AM   #11
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Quote:
Originally Posted by Gary RVRoamer View Post

However, when estimating you should allow for unbalanced weight on the axle - motorhomes are often heavier on one side than the other. So, after diving by two (front axle) or 4 (rear axle), add back some weight (at least 10%) in case the one end is heaver than the other.

I totally disagree with Jadatis's contention that American tire inflation tables are wrong, but see no point in debating it here. Make your own call on that.
Only left the things I react to in your quote to make this post shorter.
About the fat printed, Already made my own post on other fora to refer to and discuss about this , to prefent from hyjacking those other topics.
Will see if I can do this in this forum too, when I opened it I will give link to it here.

The first quoted: Tireman9 did some collecting of seperate wheel(pair) loads and found often load-division side to side on an axle of 48/52%, but also as far as 45/55%, so this adding reserve to weighed axle loads is sertainly a good idea and I also use that in spreadsheets I made.
This unequall load is 9 out of 10 times for a motorhome crossed, so for instance rear Left more heavy then Right, and front Right more heavy then Left. Not important though because you have to keep same pressure on all wheels on the axle, but if you level the car by some means, this weightdiffence gets higher, because then compensation by torsion forces in the frame is lost.
This then would need a bit higher pressure .
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