Took the 395s for a spin today

[chvss65]

looks good, Iam running the same on my bobber

 

[SasquatchSanta]

Ernie, what rims are your tires mounted on? I have mine on stockers and have the same trouble. I believe that it is because of the crown in the tire on rimm smaller than reccomended. I am working on fitting hemtt rims to my deuce.

My wheels have been modified to make them wider. I haven't measured them but my notes say (I think from a conversation with John Tennis) that they are 11" wide inside to inside.

 

[SasquatchSanta]

That is way too much pressure, I can tell you that shooting from the hip.

Figure something like this - I"m no math whiz, or at least not good at explaining it. This sound right to yous guys?

For a rough example, mine are rated for 5000# @ 80psi. 80 div by 5000 = .016 psi per #. My truck weighs about 1750 per tire. So 1750 X .016 = 28 psi, which works out pretty good from experience. I usually run higheri n the front and lower in the back since the front is a little heavier and has steering duty.


Take a chalk line and mark across the tread of your tire. Then drive it a bit. See where the chalk wears off. Adjust pressure til you get even wear across the tread. This may not work if you have alot of crown, probably end up too low.

Another way is to air them up, then air down until you get just little bulge in the sidewall. Too low and it will feel bouncy and balloon like at higher speed.

Thanks for the insight on this. Knowing that under inflated tires tend to bounce what do you feel the symptons are for over inflated tires. Actually, according to the side wall pressure specs I'm not over inflating them BUT I can see where since I am so light @ 10,000 lbs I probably don't need the max amount of air.

My wheels have been modified to made them wider. They are 11" inside to inside. I think that is about right for 395s

 

[DavidWymore]

Thanks for the insight on this. Knowing that under inflated tires tend to bounce what do you feel the symptons are for over inflated tires. Actually, according to the side wall pressure specs I'm not over inflating them BUT I can see where since I am so light @ 10,000 lbs I probably don't need the max amount of air.

My wheels have been modified to made them wider. They are 11" inside to inside. I think that is about right for 395s

Yeah, not really overinflated, but more than you need. Downside = poor ride and handling = tracking/wandering. Tread might wear out fater in the center leaving the shoulder lugs unworn. What style tire are you running? ZLs are pretty nice and flat, MLs are kinda round and XLs are real round. You wanna get the shoulder lugs touching the ground with a little sidewall bulge if you can. Try looking at your tracks = contact patch on dirt/soft ground. Drive on dusty ground and back onto pavement, then stop and look at your tracks/tires will give you a good idea of how they're riding.

 

[SasquatchSanta]

Yeah, not really overinflated, but more than you need. Downside = poor ride and handling = tracking/wandering. Tread might wear out fater in the center leaving the shoulder lugs unworn. What style tire are you running? ZLs are pretty nice and flat, MLs are kinda round and XLs are real round. You wanna get the shoulder lugs touching the ground with a little sidewall bulge if you can. Try looking at your tracks = contact patch on dirt/soft ground. Drive on dusty ground and back onto pavement, then stop and look at your tracks/tires will give you a good idea of how they're riding.

I just checked the load rating on the 395s. Wow! ... these things are rated for 9,700 pounds @ 95 PSI. I also took some pics --- attached. Hopefully letting some air out --- even though they don't appear to be crowned will make it stop wondering around the way it does. It doesn't do it (wonder around) all the time. It just seems to do so every once in a while. The front end seems to be solid with 1/4" of tow in.

 

[DavidWymore]

I just checked the load rating on the 395s. Wow! ... these things are rated for 9,700 pounds @ 95 PSI. I also took some pics --- attached. Hopefully letting some air out --- even though they don't appear to be crowned will make it stop wondering around the way it does. It doesn't do it (wonder around) all the time. It just seems to do so every once in a while. The front end seems to be solid with 1/4" of tow in.

The wandering is probably following "ruts" in the pavement from big rigs, etc.

Here's an interesting one I haven't seen before.


How to Determine the Best Tire Pressure

Another way I read was to measure from the ground to the edge of the rim with the tire at max pressure and with no weight on it, but touching the ground and then set it down and air it down til you get a dimension that is 75% of the first meas. I've never tried that one.

Different tires and vehicles behave differently. If you look at little fwd passenger cars, the front tires almost always look "squished" and underinflated but they seem OK like that. Most tires I would be hesitant to run them with that much bulge in the sidewall as bulge = flex and flex = heat and heat = excessive wear and possible tire damage. Most heavy duty tires will break up if run at too low pressure as they are built more stiff and rigid with more steel cords, etc. The Michelin info on our mil tires says they are designed to deal with low pressures, so I guess maybe they're OK, but they don't spec speed or exact pressure. Maybe they thinkg 25 psi is low, and on a heavy MV in stock setup I guess it would be, but your bobbed and my Dodge don't run near our tires designed weight capacity unless loaded heavy. I do run higher pressures in the summer on the freeway to reduce heat. (It was 114 here yesterday)

 

[DavidWymore]

Saw the pics. Looks like a good setup, maybe take some pics with the camera on the ground from the side as well as front so the contact patch is more easily seen.

Which reminds me of another thing to watch, the length front to back of the contact patch.

Kinda unrelated, but someone pointed something out to me the other day about why larger dia tires help in sand, esp when aired down because the patch gets longs and softer. In sand you are essentially contantly driving uphill because it's soft and you sink. Common sense tells you a larger softer patch will help you float and give you more traction but he also pointed out that it also makes the ramp you are climbing longer and less steep. I hadn't ever thought of it that way before.

 

[cranetruck]

Interesting discussion....
FWIW, the tires for the m656/xm757 trucks were designed to run in sand, mud and snow fully loaded (5 tons) with 10 psi. Image below shows the right front tire at 10 psi and as loaded as it gets. It's bias ply construction and shows very little bulge.

 

[DavidWymore]

Wow, very interesting cranetruck! That is very little bulge for that much weight and low pressure. Thanks.

As short a side note for those who may not be familiar, bias ply is whole different ball of wax. Tougher tire, but not as good for speed.