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Old 04-05-2019, 07:42 PM   #1
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THOR #6874
can't wrap my head around it

tire pressures, trying to figure it out and i just cant do it......i have a Challenger with Michelin tires on it..they're 255/80R22.5 XRV

the weights are as follows, driver side front-7300lbs
drivers side rear--4300 lbs.....passenger front--6700 lb.....passenger rear--4500 lbs..
i didn't have a chance to weigh the rig before so i just put 95 lbs in and hoped for the best.

my nephew work for the county road commission as a foreman and had access to the weigh masters scales... those numbers are what the dials rested at.
could someone PLEASE help me out ?
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Old 04-05-2019, 07:57 PM   #2
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This isn't "Rocket Surgery"... you can figure it out!
There should be a sticker: either in the door jam, or near the door on the inside. It'll list your tires, and their suggested pressures.
Rule of thumb: airing the tires up to the suggested maximums is never a bad thing...
...But running them without enough air in them is!
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Old 04-05-2019, 08:18 PM   #3
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THOR #6874
Quote:
Originally Posted by Bob Denman View Post
This isn't "Rocket Surgery"... you can figure it out!
There sh
ould be a sticker: either in the door jam, or near the door on the inside. It'll list your tires, and their suggested pressures.
Rule of thumb: airing the tires up to the suggested maximums is never a bad thing...
...But running them without enough air in them is!

found the sticker, it was over to the left of the drivers seat,,, 100 pound on all tires.

thank you
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Old 04-05-2019, 08:52 PM   #4
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First of all - all Challengers have six tires, but only four are listed. I guess I should assume the two inside tires support the same weight as the outer. Secondly, the coach should be load to traveling weight (full gas, water you carry, clothes, etc). Your front weights are way off. The front axle is rated for max of 8,000 lbs (22,000lb) or 9,000 lbs (24,000lbs). All tires are rated at 5203 lbs @ 120 psi. If you did actually weigh 14,000 lbs on the front your coach is seriously and dangerously overloaded. Just drive over to a CAT scale and weight the coach front and back separately. At least that will give you a starting point for your actual tire pressures. Secondly only to a correct front alignment, correct tire pressure effect the handling, stability and ride comfort of a coach.
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Old 04-05-2019, 09:56 PM   #5
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THOR #6874
we have no CAT scales anywhere around where i live, we used a pair of truck scales that the weigh master used to use.
now the million dollat question : why weigh the rig when the tire sticker inside the coach says the pressure to use ?
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Old 04-05-2019, 10:11 PM   #6
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can't wrap my head around it

You need to know your weight and adjust pressure accordingly
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Old 04-05-2019, 10:11 PM   #7
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and is there a chance i got the tire numbers from the front and rear backwards ?
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Old 04-05-2019, 10:17 PM   #8
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Originally Posted by Pete'sMH View Post
You need to know your weight and adjust pressure accordingly

i'm not reluctant at all...we weighed the rig today using two truck platform scales that were utilized by the local weighmaster,


i'm wondering is i got the front numbers and rear reversed because i wrote down 14,000 total axle pound for the front and 8,800 total pounds for the rear.
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Old 04-05-2019, 10:28 PM   #9
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Your posted front and rear weights are reversed. Use the chart and set pressure in all rear tires the same based on the heaviest weight on the rear axle. Same with the front. You may want to look at more evenly distributing the load. And depending on axle, you may be overloaded in the Front...

Michelin tire guide: (Minimum required pressure to support that weight)
Front tire -- minimum pressure for 4500# is 90psi
Rear tires -- minimum pressure for 7300# is 75#

Normally add 5-10 lbs over minimum as a cushion. 100psi in the front sounds good. If you have 90-100 psi in rear tires you may have a harsh ride.
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Old 04-05-2019, 10:39 PM   #10
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Originally Posted by rick kirby View Post
we have no CAT scales anywhere around where i live, we used a pair of truck scales that the weigh master used to use.
now the million dollat question : why weigh the rig when the tire sticker inside the coach says the pressure to use ?
You weigh the coach to the balance of the coach when loaded and adjust the tire pressures accordingly. The recommended tire pressure on the door stick is calculated by using the max axle loads. A summing you have the 8,000 lb front axle and 5203 psi per tire, that figures to 4,000 lbs per tire. It is the pressure in the tire that holds up the coach. The tire is just there to contain the air pressure. So, 5203lbs @120 psi; 4,770 lbs @ 110; 4,337 lbs @ 100 psi; 3,904 lbs @ 90 psi and so on. It is the same for the rear but the max tire pressure has to be adjusted for duals because duals run 10 to 20 degrees hotter than singles. I suggest that you will probably come close to the 14,000 rear axle load so that is where the 100 psi comes from.



I doubt you will ever come close to the 8,000 lbs on the front. My little coach with smaller tires only has 5,100 lbs on the front loaded to the max while the rear is maxed at 12,000 lbs.
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Old 04-05-2019, 11:25 PM   #11
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THOR #6874
another question ?
these scales had a dial showing the weight. we put a platform under each front tire.
do you add the numbers from each platform to get the axle weight or does the numbers from both give you the average weight ?
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Old 04-05-2019, 11:27 PM   #12
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Yes... and yes again...

Example:
3200 plus 3400 equals 6600 total weight
3200 plus 3400 equals 6600.. divided by two equals 3300 average
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Old 04-05-2019, 11:40 PM   #13
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Originally Posted by Bob Denman View Post
Yes... and yes again...

Example:
3200 plus 3400 equals 6600 total weight
3200 plus 3400 equals 6600.. divided by two equals 3300 average

ok, so if one dial said 4300# and the other said 4700# at the same time, the average axle weight is app 4500 ?
i may be catching on...not positive...lol
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Old 04-06-2019, 12:12 AM   #14
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isn't 75# and 85# awful soft for a 22.5 tire ?
because thats what the Michelin chart is saying...i was running 95# all the way around...wow.
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Old 04-06-2019, 12:45 AM   #15
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THOR #6874
actual tire numbers
front left 4300#( should carry 80#)

front right 4500#(should carry 85#)



left rear 7300#(should carry about 73#)

right rear 6700#(should carry about 70#) chart doesn't go any lower.


would you guys feel comfortable with these numbers ?
i reall don't want to annoy people with this any longer
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Old 04-06-2019, 03:07 PM   #16
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THOR #2812
Rick- You are making it too hard... Here is Michelin RV tire Guide. https://www.michelinb2b.com/wps/b2bc...s_Brochure.pdf

In your example remember tire pressure across an axle should be the same in all tires on that axle.. Where you can't balance the load use the pressure for the tire carrying the most weight. (see pg 4)

The average weight is used when individual wheel weights cannot be determined, like using a CAT scale. Using average weight, you usually add a larger "fudge factor".

Don't interpolate between weights or pressures in the chart. go to next higher value.
For XRV 255/80 22.5, for 4500#single use chart at 4620 and 90psi. Use 90psi for both front tires.
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Old 04-06-2019, 03:52 PM   #17
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When in doubt 90 to 95 pounds will work. May not be exact but close enough. Watch tire wear closely and adjust accordingly. You can stick your tread monthly with a tread depth gauge across the surface, side to side, and see uneven wear patterns. This is a sure-fire method to correct pressure for your specific vehicle. If they're wearing a bit on the outside...inflate a few pounds...if they're wearing on the inside...deflate a few pounds. You may find, due to uneven load dynamics, that one tire may require a tad more air than another. It's all about footprint.



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Old 04-06-2019, 04:44 PM   #18
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Thanks guys, I do tend to overthink things to the point of ending up completely mental.
Ill go 90# in the front and 85 in the rears. Ill just keep an eye out for the numbers.
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Old 04-13-2019, 09:01 PM   #19
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Quote:
Originally Posted by Bob Denman View Post
This isn't "Rocket Surgery"... you can figure it out!
There should be a sticker: either in the door jam, or near the door on the inside. It'll list your tires, and their suggested pressures.
Rule of thumb: airing the tires up to the suggested maximums is never a bad thing...
...But running them without enough air in them is!
Too much thinking involved here. Look at the sticker usually beside the driver's seat and see what the manufacturer wants. Hint....do your tire checks early in the morning before the sun has any chance to heat them up. Even when my tires are "cold" meaning I haven't run them, the tires on the sunny side run about 2 1/2 lbs above the shady side on tires requiring 82 all around ....Hurricane.
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Old 04-13-2019, 09:03 PM   #20
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Quote:
Originally Posted by rick kirby View Post
isn't 75# and 85# awful soft for a 22.5 tire ?
because thats what the Michelin chart is saying...i was running 95# all the way around...wow.
Ya since 19.5s are running at 82
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