You are not logged in or registered. Please login or register to use the full functionality of this board...
Newell Gurus would like to thank Boomer for joining us.

Impact of Steer Tire Pressure on Straight Line Stability
#1
First, many of you know I am constantly in search of the self driving coach. 
Second, I am NOT recommending how to set the air pressure in your tires. 
Third, tire pressure is a highly opinionated subject, and I am not trying to change yours. 
Fourth, this applies to the wide 365 tires, you may not have any flexibility with pressures with other size tires and coaches. 

I want to tell you about an experiment I ran, and the results. I have consistently run my front tires 10 to 15 psi above the minimum inflation pressure recommended in the Michelin inflation tables. I have weighed the coach three times fully loaded and fully wet, so I am pretty confident about my axle weights. 

I have been processing several observations over the years which led me to this experiment. One, my coach required significantly less steering correction if the road surface was heavy as opposed to glass smooth. Two, I am aware that more castor (increasing the trail) of an axle results in the tire wanting to self correct and go straight. That is true, but with caveats, too much castor and the coach is hard to turn/ Too much castor can result in the front wheel acting like a shopping cart and giving shimmy in the steering wheel. Three, in reading a engineering text on suspension design, there is engineering reference made to the drag of the tire generating a pseudo castor in the system. 

A ha ! That’s why the rough surface made the coach track better. So let’s use a bicycle fork as the example because that is easier to visualize. The castor is the angle of the fork compared to vertical. So if you draw a line through the fork, that line will intersect the ground in front of where the tire touches the ground. That distance between where the imaginary line intersects the ground and the tire contact patch produces enough force to keep the tire in line. Proven by all who have ridden a bike no hands. Now the more rolling resistance the tire has, the more force it will generate to keep the steering in line. 

Sorry for dragging you through that, but the explanation puts the experiment in perspective. 

I started out with my steer tires 10 psi higher than the minimum inflation in the Michelin truck tire tables. I used an IR gun to measure their temperatures during a multiday drive across the sweltering Midwest. Each morning I would lower the tire by 3 psi, drive all day, and measure the tire temp at each stop. I was comparing the steer tire temp to the drive tire temp. I drive 65 mph, never faster for any significant time.  I finally reduced the pressure to the recommended minimum inflation level. I did not detect any rise in the tire temp at all compared to the drive tire temp. I DID detect an appreciable increase in straight line stability, and a decrease in steering wheel correction. I further detected better stability in cross winds. I cannot put a number to it, but it is appreciable to me. I did not detect any decrease in the harshness of expansion joints or potholes, so lowering the tire pressure by 10 psi did not soften the ride. 

Again, I am not suggesting you do this. After all, I do not know how you are loaded or how fast you drive. If you are intrigued by this, and want to see how your rig responds, I strongly recommend you know your weights, and you evolve into a lower pressure while monitoring the temperature of the tires. And in NO WAY am I suggesting that you go below the minimum inflation pressures listed in the inflation tables supplied by your tire manufacturer.
Richard and Rhonda Entrekin
99 Newell, 512
Subaru Outback Toad
Inverness, FL (when we're home Cool )
#2
Wow, very interesting. Not the results I experienced on mine.
Maybe the starting caster / camber / toe-in makes a difference along with tire size ? ? ?

A couple of years ago when I put new Michellins (11R24.5 - G) steer tires on my classic, I weighed the coach & inflated them according to the weight chart. The result was not good. It felt like the tires were squirming on the road. It was a real handful to keep centered in my lane.

I upped the air pressure to the max listed on the sidewall and the difference was clear - very solid & tight connection to the road and easier to track in a straight line.

I do know that some cars are sensitive to small changes in alignment settings, where others don't seem to care where the wheels are pointed LOL Tongue
1987 classic #159
8V92 MUI , Allison 740
#3
Not sure about the 1987's but on the 1992's Newell recommended always running H rated tires on the front to get some extra carrying capacity. I wonder if the G vs H would have affected the results since the H load rated tires have a stiffer side wall? I run mine at 120 psi which I need to accommodate the load. I run 305/75R24.5 J load rating when I can find them.
Michael Day
1992 Newell 43.5' #281
NewellOwner.com
#4
The Newell applied sticker in the engine compartment recommended 90 psi in all positions which the weight chart agreed for the steer axle. The sidewall max pressure for a G tire is 105 - that pressure yielded the best handling on mine.

I recently installed Firestone FS561  (11R24.5 - H) on the drive & steer (Michellins were moved to tag) I'm still running 105 psi.
I mounted them myself & put a 10 oz bag of balance powder in each tire. The result is a glass smooth ride - when road conditions permit . . . .  Tongue

I have no idea if any of my efforts will be repeatable . . . Undecided
1987 classic #159
8V92 MUI , Allison 740


Forum Jump:


Users browsing this thread: 2 Guest(s)