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Old 04-10-2019, 02:34 AM   #1
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Default Determining best tire pressure

Based on another thread, this may be more of an issue than one would suppose, as the very common front tire pressures used in the Chevies and Dodges (65psi) was presented as "over inflated" with a warning that there could be serious problems implied.


I think that there are some things that can be very confusing to many people about tire pressures, and in our vans one of the top ones is the door post recommended tire pressures. I can't speak for all vans, but on a Chevy single rear wheel 3500 van the pressures are shown at 50/80 psi front/rear. These are the GM specs and have to be safe for an empty to a fully loaded 1 ton van as delivered.


Consider: An empty van has 3331# on the front and 3075# on the rear.


A fully loaded van has variable weights front and rear, but

they can't total more than 9600# and or more thant4300#
front and 6080# rear so can't have max on both ends at the
same time.


This makes for a huge difference in the load#/psi on both the front and especially the rear.


An unloaded van will have 3331/50 66.2#/psi in the front and 3075/80 or 38.4#/psi in the rear. This is going to give the rear a contact patch that is barely above 1/2 the size of the front but carrying nearly the same weight, so handling is going to be poorly affected.


A fully loaded van like a Roadtrek 190 will have about 4300# on the front and 5300# on the rear. This gives 86#/psi in the front and 66.3#/psi in the rear.



If you just kick up the front pressure to 65psi on the last example you get the front at 66.2#/psi, which is essentially the same as the rear. both will have the same contact area and sidewall height so match well that way.


I think that this would point out that the 65/80 isn't over inflating the the tires as they are running right at the same #/psi as the stock front do on an empty van. and are both within the max inflation pressure for the tires.


I don't think it is a coincidence that the #/psi would be one of the indicators of a good choice of tire pressure, particularly in front/rear handling balance and especially in wind where sidewall flex and contact patch shape count. But you can't just go by #/psi by itself either as there is a very wide range of pressures from low to high that can be made to match that requirement.



This was mentioned in the other thread, but I have found the easiest way to zero in on best actual pressures and #/psi ratio is to get the tires warm on the highway and then drive over a damp spot or dusty spot, straight and slow, then stop and look at the tire tread areas. The water or dust should have coated the area it contacted on the tread so you can plainly see your running tread width. Normally, having that contact full width and just into the radius at the shoulder is a good spot to be in the rear and just to the radius in the front as turning rolls them over a bit and it is good to allow a bit for that. If they are the same or slightly different is usually not a big issue, however. If the water/dust does not reach all the way to the shoulder radius you would reduce the pressure a bit, and if it is too far into the radius you would increase the pressure a bit. I think most folks would find a fully loaded Chevy to be very close to 65/80 using this test although very stiff or very soft sidewall tires could move it up or down a little in the front (could go down in rear but not up as at max rated).


One thing that likely might get mentioned is the old Michelin truck tire pressure chart that many of us have seen over the years. I don't think they publish it anymore, but there are lots of places that still have it available. What most of us found using that chart was that they recommended pressures that were way low for good handling and seemed to be based on what the minimum pressure would be that maintained full load capacity and didn't have anything to do with driveability.


IMO, tire pressure tweaking is one of the first and easiest things to do to get improved handling in your van, to match your driving preferences. Different drivers may like quicker or slower steering response or other ride characteristics. In our vans the response if quite slow in stock form, so most drivers seem to like the faster response of the 65/80 over lower front pressures, but some don't. Neither choice is inherently more or less dangerous IMO.


In general, increasing tire pressure will make that end of the van have more traction so in the front increased pressure gives better steering response. Of course there is a practical limit of how high you would go based on the tread test mentioned earlier. Ride quality will also go down quickly with excess pressure. Lower front pressure will give less responsive steering. As an extreme example a BMW would have fast response and 1964 Buick would have slow response. The vans are closer to the Buick
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Old 04-10-2019, 03:12 AM   #2
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Aren't the upfitters supposed to add a second door-post sticker for use after the conversion? My GWV has such a sticker.
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Old 04-10-2019, 03:19 AM   #3
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Aren't the upfitters supposed to add a second door-post sticker for use after the conversion? My GWV has such a sticker.

I would certainly think they should, but haven't seen any with changed pressures on the Chevies, at least the one's in our age bracket. It has been mentioned many times that the 50/80psi was on the sticker. Ours does have a Roadtrek sticker on it, but all they did was copy all the weights and pressures from the Chevy sticker. Of course that may be a liability thing were they don't want to go off of Chevy specs and it is safe, just not the best driving.


Are the pressures and load ratings the same on your GWV sticker as the MB one?
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Old 04-10-2019, 01:09 PM   #4
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Quote:
Originally Posted by avanti View Post
Aren't the upfitters supposed to add a second door-post sticker for use after the conversion? My GWV has such a sticker.
My Coachmen/Transit has the upfitter stickers, but the tire pressures are the same as the base Transit (67 Front, 57 Rear).

If I could find a Load Inflation table for the stock Hankook tires, I'd use that to set pressure based on actual axle weight.
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Old 04-10-2019, 01:28 PM   #5
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Aren't the upfitters supposed to add a second door-post sticker for use after the conversion? My GWV has such a sticker.
I had the added upfitter sticker for tire pressure on my Great West Van and my Advanced RV. I probably had it for my Pleasure-way but that is so far back I couldn't say for sure now whether it was on the door post or the user manual. But I had the information.
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Old 04-10-2019, 01:34 PM   #6
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Are the pressures and load ratings the same on your GWV sticker as the MB one?
The pressures are different:

sticker1.jpeg
sticker2.jpeg
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Old 04-10-2019, 01:44 PM   #7
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Here is a screen grab of the Michelin light truck tire pressure/load chart





Looking at this chart for 245-75-16 it appears that a typically loaded Roadtrek 190P Chevy at 4300/5300# would be at 50/65psi, which most would find quite difficult to drive, I think. I think these charts should be taken literally to be what they are stating and that is what pressure should be to safely carry the load. What they never seem to mention is going with more pressure to improve driveability.



I think I need to see what DW's Honda CRV says on the sticker as it has tires that appear to be very large for the weight of the vehicle and call out 30psi all the way around.
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Old 04-10-2019, 02:24 PM   #8
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I just did a quick calc on DW's 2009 CRV which says 30 psi all around.



If loaded to max GVWR it would have about 2300# front and 1850# rear


102 load rated tires would be able to carry 1150# per tire at 26psi which may be the minimum on them, I think, so 2250# per axle.


Honda has increase pressure above minimum required on both ends to substantially above load rating, I would assume for handling and probably also for air pressure loss window.


My guess is that nearly all passenger vehicles do this, but it appears the light trucks may just be listing minimum pressures that will carry max rated load.
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Old 04-10-2019, 09:44 PM   #9
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The lists give maximum load for the pressure.
Motorhomes have always unequall load R/L.
And rear axle is often loaded to the max or even overloaded for a motorhome with relatively low GVWR, like the most on this forum , I suspect.
Best is to use as high possible pressure, at wich comfort and gripp is still acceptable.
This will come to the 80 psi rear, and most likely about 50 to 60 psi at front.
This " pigheaded Dutch selfdeclared tirepressure-specialist" registered to help you determine the best pressure.
Made spreadsheet for it.
Determining the weights is the most tricky part in it all. Only way is weighing fully loaded, per axle or better per axle-end.
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Old 04-10-2019, 09:56 PM   #10
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Originally Posted by jadatis View Post
The lists give maximum load for the pressure.
Motorhomes have always unequall load R/L.
And rear axle is often loaded to the max or even overloaded for a motorhome with relatively low GVWR, like the most on this forum , I suspect.
Best is to use as high possible pressure, at wich comfort and gripp is still acceptable.
This will come to the 80 psi rear, and most likely about 50 to 60 psi at front.
This " pigheaded Dutch selfdeclared tirepressure-specialist" registered to help you determine the best pressure.
Made spreadsheet for it.
Determining the weights is the most tricky part in it all. Only way is weighing fully loaded, per axle or better per axle-end.

Good information here, although I think I would say in our class b vans mostly applies to the rears and less to the fronts. Most of vans have tires rated at 3042# per tire on the front and have only a max of 2150# on them so way safe on capacity there. IMO just set for best handling that you like.


The rear is where the issues are, and we often see over the rated axle weigh their, and many of our vans rate right at max tire load at max pressure. As mentioned by the previous poster, even if you are within axle weight that doesn't mean that you are within the individual rear tire weights. Our Chevy routinely has 200-300# more on the left rear than on the right, so we could be good on the right and overloaded on the left. This seems to be typical in the Chevies and is definitely and issue for those that run right at max axle weight in the rear (we don't).


I built a set of individual wheel weight scales, so I have the capability to weigh the axle ends individually, which can be very useful. The scales are documented on another discussion here. We have a standing offer to weigh class b's in our driveway if we are around and you are in the area, as I think it would be good to get more data on weights.
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