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Old 06-02-2020, 05:38 PM   #1
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Smile Class A with Tag axle

Weve had the Ventana for 2 years now. My question deals with tire pressure. I have had our previous coach 4 corner weighed and understand how that all works in determining pressures. I have not had the opportunity to have this coach weighed 4 corner/ with tag. I have put it on a truck scale and have the front axle weight and the rear axle weight. I have not had the time to drive the coach over the scale and weigh the driver side and the passenger side however.

With the Tag axle, and the rear axle weights, how do you judge the pressures between the rear duals and the single tag on each side? Freightliner and Newmar said basically to run the tag pressures at 120 psi. That doesn't make sense to me.

Any suggestions as to when I take the coach and weigh the front and rear axles, on each side. I have seen the diagrams on how to do it, but with the weight results for the rear, there will be 6 tires to deal with, 3 on each side.
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Old 06-02-2020, 07:17 PM   #2
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You need individual position weights to get the correct pressures. If you can't do that, you need to weigh on a scale with multiple platforms so you can get the drive axle on a different scale from the tag axle. Very easy to do on the CAT scales at many truck stops. Once you have separate axle weights, refer to the load inflation chart for your tires, or come back here for help. Just be sure the tag suspension is not dumped.
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Old 06-02-2020, 07:47 PM   #3
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"benbom".....Email Newmar Service and they will send you the coach weights when the coach rolled out the door. They weigh them to make sure there is nothing wrong with the coach, as it would show up with different weights than what they have for each model.

This is what they will send you. You can get a good starting point on what each corner weighs.

607408 corner weights.pdf
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Old 06-03-2020, 07:26 AM   #4
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Thank both of you for your responses. I will ask Newmar for this weight spec sheet. I noticed that the weights on the sheet include fuel weight, but do not show water weight. I am assuming that water weight will add to the driver side weight, and then cargo weight will add to the passenger side weight more so than driver side weight.

Once you have the weights for the front and rear/side to side corners, how is the rear corner weights calculated? Do you use the combined passenger side rear weight and then divide by 3 to come up with the tire pressure? Or do you use the estimated Tag weight for the tag corner weight? Or do you position the rear wheels so only the drivers are weighed and then only the tag is weighed?

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Old 06-03-2020, 07:40 AM   #5
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Can't speak to individual coach mfg's but I went to the cat scale and was able to weigh all three axles at once - good info on multiple pages here: https://catscale.com/

And from the weight ticket shown below found the tags were so light I didn't want to go below 80 psi (in fact, more air in the tags made my front axle go over my 13k lb rating)
Our placard says 115 for fronts and 90 for rears and tag, but that's to support your full gross weight rating which I am not close to... so can drop the psi down to what my tires actually need to carry as show in the first pic... good luck, but if you want, just set it to the placard as a start... and imho, set left and right tires to same pressure and at highest psi called for on THAT axle.
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Old 06-03-2020, 07:54 AM   #6
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Originally Posted by benbom View Post
Once you have the weights for the front and rear/side to side corners, how is the rear corner weights calculated? Do you use the combined passenger side rear weight and then divide by 3 to come up with the tire pressure? Or do you use the estimated Tag weight for the tag corner weight? Or do you position the rear wheels so only the drivers are weighed and then only the tag is weighed?

Regards,

Brian
Till you can get the individual weights of all six positions, weigh each axle separately. An accepted rule is to divide each axle weight by two and add 5% for possible uneven side to side weight. Once you have those numbers, go to the inflation table for your tire size for the correct pressure. Never weigh the tag and drive together and divide by three to get their weights.
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Old 06-03-2020, 09:36 AM   #7
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The tire pressures on the tag will affect the amount of load carried by the other axles. It tips the balance of the coach enough to do that. Typically the tag should have a lower psi than the rear axle so that it does NOT try to carry a proportional share of the weight, but the psi should never be less than the minimum for the tire size as shown in the inflation tables.


Newmar or the chassis manufacturer can probably advise what proportion of the weight should be carried on the tag.
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Old 06-03-2020, 10:05 AM   #8
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What I have found on my Tags is that the pressure regulator compensates for the tire pressure

Tried lowering the tag tire psig to 90 once leaving Alvarado on a July day

Increased the pressure back to 110 after the tires were hotter than the other 6 100 miles down the road

Others may find it differently and you can adjust the ratio between the air bags on the tag and drive, compensating for lowering the psig in the tag tires

As always weights loaded tell the story
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Old 06-03-2020, 10:17 AM   #9
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110 on your tags not your drive axles ?!? Wow that Be a lotta psiís !
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Old 06-03-2020, 01:24 PM   #10
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110 on your tags not your drive axles ?!? Wow that Be a lotta psiís !
Placard states 110 so started there and came back there after seeing the temps rise at 90

I have as much weight on the tags as the drivers per tire

Unlike many weights I see
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Old 06-03-2020, 03:53 PM   #11
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Again wow ! I wonder which one of ours is the oddball ?!? As you can see from my cat weights posted, my tag is barely holding any weight.
I did increase the bag psi to push down on it more but then my front axle went way over it’s rating..
have to think about that some- maybe. Drop drive axle tire psi so it makes tag. Carry more ? But then that too would mess up balance .... oh well, been like this for 5 years so it’ll do
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Old 06-04-2020, 07:25 AM   #12
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Thanks all for causing an Excedrin headache!

Just kidding...
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Old 06-04-2020, 07:39 AM   #13
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Again wow ! I wonder which one of ours is the oddball ?!? As you can see from my cat weights posted, my tag is barely holding any weight.
I did increase the bag psi to push down on it more but then my front axle went way over itís rating..
have to think about that some- maybe. Drop drive axle tire psi so it makes tag. Carry more ? But then that too would mess up balance .... oh well, been like this for 5 years so itíll do
Dropping the drive axle tire pressures will NOT cause the tag to carry more weight. The proportioning valves will cause the tag to carry the same weight regardless of the drive wheel pressures. As an example, if the tag carries 6,000# with normal tire pressures, it would still carry just 6,000# with the drive wheels completely flat. The prop valves will limit what they carry based on the pressure they are set at. They will also do nothing to carry any extra weight should one dual tire go flat. All of the weight the dual tires were carrying will transfer to the remaining tire. The tag will continue to carry it's share of the 6,000#
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Old 06-04-2020, 09:25 AM   #14
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Originally Posted by Gary RVRoamer View Post
The tire pressures on the tag will affect the amount of load carried by the other axles. It tips the balance of the coach enough to do that. Typically the tag should have a lower psi than the rear axle so that it does NOT try to carry a proportional share of the weight, but the psi should never be less than the minimum for the tire size as shown in the inflation tables.


Newmar or the chassis manufacturer can probably advise what proportion of the weight should be carried on the tag.
Gary,

Hate to disagree but tag axle tire pressures have minimal impact on TPs at other positions. Too high front axle weights and too low drive weights are caused by improper pressures in the air bag suspension for the tag axle.

Tire pressures for the tag axles should be the same has the drive axle tire pressures per the tire experts.
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