Tuning Tire Pressures

[IEales]

Originally Posted by CWSWine

I have been told and believe the tire placard is what you should run if the coach is at it's max weight

Our placard lists 110PSI front and rear for OEM installed tires.
Current Michelin Load Chart max is 110PSI.

Michelin 110PSI GCVW 31,200

Coach Chassis Spec:
GAWR front - 8,500
GAWR rear - 13,500

Total - 22,000

October 16, 2022 @ Coos Weigh Stn
7750 front / 13450 rear / 21200 total

Front 90PSI Rtg 9,210 or 118% of axle load
Rear 80PSI Rtg 15,800 or 117% or axle load

115% is about 5PSI less, but ride is just a tad more plush at the higher pressure.

Originally Posted by Old-Biscuit

Here we go again.

How many are OCD about the tire pressures on their daily drivers?

Not enough, obviously. That's why TPMS are mandated to keep idTENts from killing the rest of us.

If OCD is checking pressures before and after a trip, then guilty.

[jadatis]

If you now give specifications of tires, I will calculate with my pigheaded systen.
Then we will see if that comes close to what you determined in your practical test.

Need next.
1. Maximum load or loadindex
2. Loadrange, most likely G-load , because you gave AT 110 psi.
3. Speedcode, I think L for max 75mph.

Sizes would be nice to.

[IEales]

Originally Posted by jadatis

If you now give specifications of tires, I will calculate with my pigheaded systen.
Then we will see if that comes close to what you determined in your practical test.

Need next.
1. Maximum load or loadindex
2. Loadrange, most likely G-load , because you gave AT 110 psi.
3. Speedcode, I think L for max 75mph.

Sizes would be nice to.

Michelin 265/70R 19.5 X Multi Z Specs

Load Range G
Speed Range 140/138 M @ 110PSI [130kph / ≈81mph]

[jadatis]

Made list to look back in using loadindexes 138/136 to give also some reserve for speed, and also gave 90% of calculated loadcapacity, again to build in max reserve.
All to find max pressure with still acceptable comfort and gripp. Putted your weights in it behind the needed pressure.

In TOYO list I found 2 265/70 R 19.5 AT 110 psi
One Loadindex 140/138 as you have, and the other LI 137/134.
Most likely they did the same as I did, and 140 is for mayby even only L 120kmph/75mph, and the other for 140kmph/87mph.
But they did not give speedcodes in that list.

Here my list in wich pretty close to your determined pressures.
6149 lbs/ 70 psi / 11672 lbs
6230 lbs/ 71 psi / 11827 lbs
6312 lbs/ 72 psi / 11981 lbs
6393 lbs/ 73 psi / 12135 lbs
6474 lbs/ 74 psi / 12289 lbs
6555 lbs/ 75 psi / 12443 lbs
6636 lbs/ 76 psi / 12597 lbs
6717 lbs/ 77 psi / 12751 lbs
6798 lbs/ 78 psi / 12904 lbs
6879 lbs/ 79 psi / 13058 lbs
6960 lbs/ 80 psi / 13212 lbs
7041 lbs/ 81 psi / 13365 lbs
7122 lbs/ 82 psi / 13518 lbs/ rear both
7202 lbs/ 83 psi / 13671 lbs
7283 lbs/ 84 psi / 13825 lbs
7364 lbs/ 85 psi / 13978 lbs
7444 lbs/ 86 psi / 14130 lbs
7525 lbs/ 87 psi / 14283 lbs
7605 lbs/ 88 psi / 14436 lbs
7686 lbs/ 89 psi / 14589 lbs
7766 lbs/ 90 psi / 14741 lbs/ FR 7750
7846 lbs/ 91 psi / 14894 lbs
7927 lbs/ 92 psi / 15046 lbs
8007 lbs/ 93 psi / 15199 lbs
8087 lbs/ 94 psi / 15351 lbs
8167 lbs/ 95 psi / 15503 lbs
8247 lbs/ 96 psi / 15655 lbs
8327 lbs/ 97 psi / 15807 lbs
8407 lbs/ 98 psi / 15959 lbs
8487 lbs/ 99 psi / 16111 lbs
8567 lbs/ 100 psi / 16263 lbs/FR GAWR

These sizes are not always clear for wich speed the maxloads where calculated for.
Found sizes that gave same maxloads for M speedrate of one brand and gave L for other brand.

Then who is right? Is the lower giving extra carefull, or the higher being to optimistic, and tires will overheat driving 81mph?

So in your case it could be in my made list 2 LI steps lower is for 10kmph/6.3mph higher speed max. So if M true 87mph, and if L true 81mph. You always need also some reserve for speed.

Then for LT tires, wich are pretty the same given in maxload/speed, you only come to 5 to 10 psi adding before you are happy, and for these tires 10 to 15 psi.

What I am trying to achieve with my theoretical system, is that the given pressure is directly spot on, to your experimenting system.

But in this I can finetune my system, with your data, determined in practice.

[wildtoad]

This is going to be fun to follow!

[wildtoad]

Fun info: In my case the placard from Newmar and the one from Ford says 92/92. Tire chart for GAWR 87/80. Tire chart for actual weight 80/80. Goodyear tells me these are commercial tires and by convention not to go below 80% of sidewall which is 110 so 88/88. Goodyear told me the Sweet Spot (ride/wear) for my tires is 95, which I think is what Ford says in their chassis guide. Goodyear also said for liability reasons, mine not theirs, not to go below federal placard numbers in case you get in a wreck and someone sues because you set the temps too low. BTW tires are G670 245/70R19.5’s.

No freaking wonder the average guy, newbie, etc can be confused.

[Crasher]

Is there any other topic that is so simple to understand, receive more complicated in depth explanations than "Tire Pressure"? How did we ever survive for decades using just the load inflation tables or the RV placard? I guess we just didn't know what we don't know.

[Mudfrog]

Originally Posted by Crasher

Is there any other topic that is so simple to understand, receive more complicated in depth explanations than "Tire Pressure"? How did we ever survive for decades using just the load inflation tables or the RV placard? I guess we just didn't know what we don't know.

I agree completely!

Weigh the rig, look at the load/inflation chart for the particular tire, fill (tires cold) to 5 or 10% above what it shows for the load, and go.

Having each corner weighed is more precise but many only have the front and rear axle totals. If this is the case, then dividing the front axle weight by 2 will give an approximate weight of what each front tire is carrying. The rear, usually with 4 tires, is divided by 4. This is one of the reasons to add the 5 or 10%, to help cover an axle that might be carrying more weight on one side.

It's probably best to go with what the vehicle's placard indicates if the vehicle has not been weighed.

A person might experiment some to try and better the ride but they should not run a lower pressure than the minimum the chart indicates.

[IEales]

Originally Posted by Crasher

Is there any other topic that is so simple...

REAL simple:
Too Low pressure: ride on the edge and overheat

Optimal pressure: Individual for every vehicle, load, speed and road.

Too High pressure: wear out the center and hammer

Understanding Vibration and Resonance

Around 16 minutes, the system approximates an RV with its multiple compliances and resonances.

[twinboat]

Originally Posted by IEales

REAL simple:

Too Low pressure: ride on the edge and overheat



Optimal pressure: Individual for every vehicle, load, speed and road.



Too High pressure: wear out the center and hammer

Around 16 minutes, the system approximates an RV with its multiple compliances and resonances.

Simple, how do you account for the everchanging road surface tempeture changes, hot, cold, wet, sun baked....

PSIs vary by to to 15 percent depending on many things whole driving.

[IEales]

Originally Posted by twinboat

Simple, how do you account for the everchanging road surface tempeture changes, hot, cold, wet, sun baked....

PSIs vary by to to 15 percent depending on many things whole driving.

The pressure is the cold speed. Adjustments are made by adding or subtracting from the measured hot temperature while tuning. Or make multiple tests starting from cold.

Obviously, there is no 100% solution, but over many roads and many miles, it's possible to dial in a null where the coach handles best.

It's like a baseball and the bat: There is a sweet spot where the ball is out of the park. Either side of it and it's an infield fly.

[Ray,IN]

Am I the only one who remembers 50-60 years ago a piece of white chalk was the way to determine adequate tire pressure. Use the chalk to make a broad white line across the tread. Drive a few miles and check the chalk mark.
It it wore off evenly all across the tread, tire pressure was correct. Wore off in the middle, too much pressure, wore off on the sides, too little pressure.

[twinboat]

Originally Posted by Ray,IN

Am I the only one who remembers 50-60 years ago a piece of white chalk was the way to determine adequate tire pressure. Use the chalk to make a broad white line across the tread. Drive a few miles and check the chalk mark.
It it wore off evenly all across the tread, tire pressure was correct. Wore off in the middle, too much pressure, wore off on the sides, too little pressure.

That was when tires were stretchy and bias ply.

[IEales]

Originally Posted by twinboat

That was when tires were stretchy and bias ply.

I remember when the first customer drove into our garage with radials on an American car. They all looked half flat and I offered to fill them

We always checked oil, water, tires and the customer left with a spotless screen.

Sadly, I don't recall how we ascertained correct inflation pressure. Maybe customers were smarter then...