Fuso tire pressure question

[kerry]

I'm a new member of the Fuso owner's club having bought a 99 FG in North Carolina and I'm driving it home to Denver. It has a service body on the back and some lube oil tanks (probably almost empty) so there is not much weight on the truck. It was giving a pretty harsh ride and I remembered a comment here by someone who bought one and reduced the tire pressure on the ride home to soften the ride. I checked the pressure in the rear duals and they were at 70psi. I reduced them to 60 psi and the ride improved pretty significantly. I'm wondering what the lowest safe pressure is with essentially no significant load on the back. The manual says 80psi lbs for load range E tires, which is what is on the truck. But even at 70psi it felt as if the back of the truck was bouncing up over the interstate cracks.
I've done nothing to the front tires, but I assume keeping them at spec would be appropriate, or could they even be reduced with no load on the truck? The manual calls for 65psi for load range D tires on an FE 639 with GVWR at 11,600 lbs. Mine is the 12,000lb GVWR.

 

[engineer]

I run mine at 55 psi, at 6000kg max speed 80 km/hr.
I've had no trouble over extemely rough corrugated roads, but it is fairly slow work. I don't see any issue with 60psi when empty.

 

[kdgreene]

Rough ride

I don't feel qualified to comment on the pressure, we keep ours at 80psi, but the first thing we did after picking up our 2004 FG 639 in New York and driving it to Arizona was change out the stock seats. We purchased two Mitsubishi driver seats with suspension shock absorbers and modified the one to fit the passenger side. I understand this is no long possible to do from Mitsubishi but there are other after market seats out there. It helped tremendously.

Kim Greene
Adventure Learning Foundation
www.QuestConnect.org
Americas Overland - The Driving Handbook
www.AmericasOverland.net

 

[kerry]

Yes, I planned to add some kind of suspension seats. I'm wondering if it's possible to get three suspension seats in there since there are three of us in the family. It looks like it might work. with some adaptation.
I was asking about the tire pressures mainly for the drive home but I do think the variation between the loaded and unloaded weight of the trucks does make a difference in tire pressures. In asking this question on another board, I have gotten answers from guys who run their tires at various pressures on their 3/4 ton trucks loaded and unloaded. I'm probably going to experiment with various pressures and take temperature readings of the tires with a non-contact thermometer to see what happens. If I had half a brain, I'd have taken my thermometer with me for the trip and I could have experimented on the way home.

 

[dhackney]

I do think the variation between the loaded and unloaded weight of the trucks does make a difference in tire pressures. ... I'm probably going to experiment with various pressures and take temperature readings of the tires with a non-contact thermometer to see what happens.

In researching tire options for our Fuso I learned that there is a multi-part relationship for tires.

Variables include:
- load (weight)
- ambient / road temperature
- road surface (paved/gravel/dirt/mud/etc.)
- speed
- wear rate (how fast the tire will wear out)
- wear state (remaining tread)
- age (tire age)

As I understand it, temperature is a real killer, so it is important to watch your speed when heavily loaded on very hot days/surfaces. For instance, if you were heavily loaded running across the desert at 110F, you would want to keep your air pressure and speed at amounts suitable for the conditions.

More weight = more pressure = lower speeds.

For a given load, if you reduce your pressure for a better ride on washboard or even more for sand, then you need to adjust your speed down as well.

For our weight, which is probably equal to several hours output of the world's largest copper mine, we have a very, very low speed limit at reduced pressures.

It is my understanding tire ratings are based on a set of standardized variables. Since wear rate is an important marketing consideration, other factors such as recommended inflation pressures and weight capacities may be adjusted accordingly to achieve a viable market position relative to wear rate.

 

[kerry]

Ran across this on RV.net. It's got some good info on tire pressures and loads.

http://www.goodyear.com/truck/pdf/databook/loadInflation.pdf

 

[kerry]

I just weighed the truck--6920lbs. Assuming 2/3--1/3 front back split on empty weight that means 2306 on the rear and 4612 on the front. So 2306 on each front tire and 600lbs on each rear tire.
So with LT 235/85 16's I could run 55lbs in the fronts and 35 lbs in the rears!!! That is according to the Goodyear tire pressure chart.
I couldn't get a per axle weight on the scale I used or I could be more precise.
Anyone have any precise data on the weight ratio front/rear on an FG?

 

[FusoFG]

There's no standard f/r weight split except for the bare chasis. It all depends on load and where it's mounted.

Go back to that scale and weigh it again with just the front axle or just the rear axle on the scale.

At 10600 lbs, I run 80 frt and 50 rear. I could go lower on the rear.

On wash board roads I drop at least to 50 front and slow down. Michelin has a chart for 7.50 x 16 tires that gives max speeds and reduced pressures for various off road surfaces that I use as a guide.

Dhackney is right, heat is the problem. But in most cases it's the heat caused by sidewall flex.

More flex in a given time means more heat. Less flex in a given time means less heat.

Things that cause more flex are lower air pressure, higher load and higher speed.

Things that reducs flex in a given time are higher pressure,lower load or lower speed.

Too much air will wear out the centers, too little will wear out the edges. Too much heat will cause failure.

 

[kerry]

There's no standard f/r weight split except for the bare chasis. .

That's what I had in mind since I'm not carrying much more than the bare chassis at the moment.
The guy running the scale wouldn't let me weigh just one axle. I don't know why but I asked him if he could and he said no.

 

[dhackney]

The guy running the scale wouldn't let me weigh just one axle. I don't know why but I asked him if he could and he said no.

Look in the Yellow Pages or do a search for a moving/storage company in your area. They usually have scales at their facilities and will weigh anything you want as many times as you want for a small fee.

It is a very good idea to do periodic weigh-ins of each axle and each corner during and after your build.

During the build it can help spot potential weight issues.

Once completed, load up the rig with everything you plan to take, fill up all the tanks (water, gray, black, fuel) and weigh the total vehicle, each axle and each corner.

If you end up with disproportionate corner weights you can get an extra leaf made for the heavy corner(s) or custom spring packs matched to your final, full - completely loaded with all that stuff - wet weight.