Feedback on coach weight and tire inflation analysis...

[widdershins]

Thought I'd post a tire inflation thread that isn't asking for help without necessary details. I mostly wanted to make sure my logic here is good and I'm not missing something.

Current tires, no unusual wear:

Goodyear Wrangler HT LT225/75R16 load range E M+S rated
115/112Q Load Range
Max Capacity: single: 2680# @ 80PSI, dual:2470# @ 80PSI

Loaded up our coach as typical for travel (full fuel, full fresh water, food, clothes, personal items). Had our 2dr Jeep Wrangler toad with full tank of gas hooked up and setup as normal and hit the cat scale:

Steer Weight 3860 LBS
Drive Weight 7440 LBS
Trailer Weight 4280 LBS

1,800# below GVWR, which is nice.

According to the inflation chart:

60psi in the front will support 4,380# (520# more than actual required)
60psi in rear will support 7,980# (540# more than actual required)

I think running 60psi all around is a good place to start. Seems like reasonable weight 'cushion' before hitting tire capacity, we travel light.

I think my main concern is the only inflation chart I could find that covered Goodyear LT tires is from 2010. The 80psi chart numbers match the numbers on the tire, so maybe it's fine.

Thanks for any feedback.. good or bad ;-)

[Lt Keefer]

The lower tire pressures will soften the ride, that's for sure.

[lwmcguir]

60 psig is to low inmh, for the steering axle and duals

Start with 70 steer and 65 duals

Monitor temperature and handling

[widdershins]

Originally Posted by lwmcguir

60 psig is to low inmh, for the steering axle and duals

Start with 70 steer and 65 duals

Monitor temperature and handling

with these pressures, according to the inflation chart I'm using.. the steer tires capacity would be 1,020# and drive 1,440# over actual weight. Capacity would also be over gross axle weight rating (280# on steer, 380# on drive).

I the ride at 75psi steer 65psi drive is very harsh, 70psi front was better but still very rough. I've been running 65psi steer and 60psi drive for about 1k miles, 100F + air temps and a good amount of winding mountain roads (and I don't baby in the corners ;-). TPMS temperature alert is 149F and has never gone off, so I think temps are okay at those pressures.

Is your recommendation Just an overabundance of caution? I can't imagine a way I could exceed weight at 60/60, even with full holding tanks I would still have some cushion.

[lwmcguir]

Originally Posted by widdershins

with these pressures, according to the inflation chart I'm using.. the steer tires capacity would be 1,020# and drive 1,440# over actual weight. Capacity would also be over gross axle weight rating (280# on steer, 380# on drive).

I the ride at 75psi steer 65psi drive is very harsh, 70psi front was better but still very rough. I've been running 65psi steer and 60psi drive for about 1k miles, 100F + air temps and a good amount of winding mountain roads (and I don't baby in the corners ;-). TPMS temperature alert is 149F and has never gone off, so I think temps are okay at those pressures.

Is your recommendation Just an overabundance of caution? I can't imagine a way I could exceed weight at 60/60, even with full holding tanks I would still have some cushion.

I have blown out many tires of that size and ply on heavy duty pickups before I went to 19.5 or 20 inch

Experience was the call, watch the sidwall when you go from 60 to 65 or 70

Place a square or any good straight edge and see how much taller the tire gets and the resultant side wall flex

Just got back in from 700 miles on I70 and at a minimum 50 tire gators, most from overheating

Over loading or under pressure is why sidewalls heat up and tires throw the tread

[widdershins]

Originally Posted by lwmcguir

I have blown out many tires of that size and ply on heavy duty pickups before I went to 19.5 or 20 inch

Experience was the call, watch the sidwall when you go from 60 to 65 or 70

Place a square or any good straight edge and see how much taller the tire gets and the resultant side wall flex

Just got back in from 700 miles on I70 and at a minimum 50 tire gators, most from overheating

Over loading or under pressure is why sidewalls heat up and tires throw the tread

Appreciate the feedback, and details.

[Pete'sMH]

Originally Posted by lwmcguir

60 psig is to low inmh, for the steering axle and duals

Start with 70 steer and 65 duals

Monitor temperature and handling

On what do you base these recommends? If the manufacturer recommends 60 that’d be good enough for me. And the ride will be a lot more pleasant.

[OldWEB]

From Goodyear:

The car manufacturer has provided the vehicle’s tire sizes and recommended cold tire pressures located on a placard somewhere in your car. The first place to check would be somewhere along the door frame around the driver’s door jamb. This tire placard lists the proper cold tire pressure for both the front and rear of your car.....

One place to not look for the vehicle recommended tire pressure is on the sidewall of the tire. There will typically be an inflation pressure listed on your tire – but that pressure is the maximum inflation pressure for the tire, and not the pressure recommended by the vehicle manufacturer.

[widdershins]

Originally Posted by lwmcguir

60 psig is to low inmh, for the steering axle and duals

Start with 70 steer and 65 duals

Monitor temperature and handling

Originally Posted by OldWEB

From Goodyear:

The car manufacturer has provided the vehicle’s tire sizes and recommended cold tire pressures located on a placard somewhere in your car. The first place to check would be somewhere along the door frame around the driver’s door jamb. This tire placard lists the proper cold tire pressure for both the front and rear of your car.....

One place to not look for the vehicle recommended tire pressure is on the sidewall of the tire. There will typically be an inflation pressure listed on your tire – but that pressure is the maximum inflation pressure for the tire, and not the pressure recommended by the vehicle manufacturer.

The inflation pressures listed on the Ford provided sticker cannot possibly be correct as Ford delivered a cutaway chassis only. Perhaps they assume the RV manufacturer is going to load the chassis to maximum (which isn't the case here), and even if they did the pressure Ford called for (75 steer, 65 drive) would support 520#/820# over GAWR which seems very unnecessary considering the actual carried weight of our coach.

Goodyear provides a tire inflation chart based on actual weight carried, which is what I'm basing my inflation pressure on allowing for a reasonable 'cushion' in excess carrying capacity.

[MLP]

Manufacture tire pressure stickers are set for maximum weight the vehicle was designed for be it car, truck, or RV. To state anything else would open them up to lawsuits. Look at the maximum weight you vehicle can carry front and rear and look up the tire pressure/load chart.

[widdershins]

Originally Posted by MLP

Manufacture tire pressure stickers are set for maximum weight the vehicle was designed for be it car, truck, or RV. To state anything else would open them up to lawsuits. Look at the maximum weight you vehicle can carry front and rear and look up the tire pressure/load chart.

65 PSI drive and steer is what Ford should have specified based on the inflation chart *IF* covering GAWR was the goal:

Steer GAWR 4600# Inflation chart 65psi capacity 4670#
Drive GAWR 8500# Inflation chart 65psi capacity 8600#

And yes, I'm using the correct chart for dual on the drive and single on the steer.

The Ford sticker is 65 for drive axle, but for some reason Ford decided the steer axle tires should be inflated to 75psi which support 5120#. That's not a huge difference, but it sure makes the coach ride terrible ;-)