Reply
 
Thread Tools Display Modes
 
Old 05-23-2013, 04:25 AM   #1
Senior Member
 
Pschitt's Avatar
 
Join Date: Nov 2012
Location: Switzerland
Posts: 1,005
Garage
Some facts about tires

I have noticed a few posts on tire information on the forum.
For many years I worked with Michelin, so I'm pleased to share some knowledge of this very technical subject. Hopefully this will answer some questions members have been asking.
Thanks to Daveb who have checked my poor English.




Chapter 1 : How to read tire information?

There is quite a bit of information on the sidewall of a tire:





Metric example: 195/65 R 15 91H M+S
1 – overall width of the tire [mm]
2 – height of side wall [% of global width]
3 – indicates that it’s a Radial Carcass (when Diagonal ? -)
4 – diameter of the rim [inch]
5 – load index
6 – speed index
7 – brand or type
8 – DOT : indicates outside sidewall of the tire and date of production
9 – Tubeless (TL) or Tubetype (TT)
10 – M+S : Mud + Snow, indicates that tire is appropriate for snow conditions

Imperial example: 35/12.5 R 17 125Q
Only numbers 1 and 2 change:
1 – overall diameter of the tire unloaded [inch]
2 – overall width of the tire [inch]

Certain tires show some letters directly ahead of first size indication, for example, LT245/70 R 17.
That indicates for which vehicle category they are designated.
P : Passenger vehicle (cars)
LT : Light trucks
etc.

Certain tires also show maximum inflation pressure allowed, which depends of tire construction. Be careful: NEVER ECXEED THIS MAX. PRESSURE ! The bursting risk is real..!


Chapter 2 : How to choose the right tires for my rig?

They are basically four criteria to consider:
- maximum speed of the vehicle
- maximum weight of the vehicle
- width of the rim
- diameter of OE tires


Two first criteria determine the minimal load x speed index of the tire. Legally, you must have tires with speed index at least equivalent to the maximum speed of your vehicle.
Here is the speed index list:

Speed Index - Mph - Km/h
L--------75 mph-----120 km/h
M--------81 mph-----130 km/h
N--------87 mph-----140km/h
P--------93 mph-----150 km/h
Q--------99 mph-----160 km/h
R-------106 mph-----170 km/h
S-------112 mph-----180 km/h
T-------118 mph-----190 km/h
U-------124 mph-----200 km/h
H-------130 mph-----210 km/h
V-------149 mph-----240 km/h
W------168 mph-----270 km/h
Y-------186 mph-----300 km/h
Z----->186 mph---->300 km/h

Important is to understand that these speed limits are for the tire at nominal inflation pressure which is pressure for maximum acceptable load on the tire.

So, if our SMBs’s maximum speed is 90 mph, we can mount every tire with speed index P or higher.


Second tire index is the maximum acceptable load per tire. Legally, you must have tires with load index adequate for the maximum weight of the most loaded axle at maximum admissible weight (GVWR) of your vehicle. For example, if the GVWR is 9,500 lbs with 4’000 lbs on front axle and 5,500 lbs on rear, minimum load index must correspond to 2,750 lbs.
Here is the nominal load index list which indicates maximum load at maximum admissible speed:

Load Index - Pounds - Kilograms
71--------761---------345
72--------783---------355
73--------805---------365
74--------827---------375
75--------853---------387
76--------882---------400
77--------908---------412
78--------937---------425
79--------963---------437
80--------992---------450
81-------1019---------462
82-------1047---------475
83-------1074---------487
84-------1102---------500
85-------1135---------515
86-------1168---------530
87-------1201---------545
88-------1235---------560
89-------1279---------580
90-------1323---------600
91-------1356---------615
92-------1389---------630
93-------1433---------650
94-------1477---------670
95-------1521---------690
96-------1565---------710
97-------1609---------730
98-------1653---------750
99-------1709---------775
100------1764---------800
101------1819---------825
102------1874---------850
103------1929---------875
104------1984---------900
105------2039---------925
106------2094---------950
107------2149---------975
108------2205--------1000
109------2271--------1030
110------2337--------1060
111------2403--------1090
112------2469--------1120
113------2535--------1150
114------2601--------1180
115------2678--------1215
116------2755--------1250
117------2832--------1285
118------2910--------1320
119------2998--------1360
120------3086--------1400
121------3196--------1450
122------3306--------1500
123------3417--------1550
124------3527--------1600
125------3637--------1650
126------3747--------1700
127------3858--------1750
128------3968--------1800
129------4078--------1850
130------4387--------1990
131------4299--------1950
132------4409--------2000
133------4541--------2060
134------4673--------2120
135------4806--------2180
136------4938--------2240
137------5070--------2300
138------5202--------2360
139------5357--------2430
140------5511--------2500
141------5676--------2575
142------5842--------2650
143------6007--------2725
144------6172--------2800
145------6393--------2900
146------6613--------3000
147------6779--------3075
148------6944--------3150
149------7165--------3250
150------7385--------3350
151------7605--------3450
152------7826--------3550
153------8046--------3650
154------8267--------3750
155------8542--------3875
156------8818--------4000
157------9094--------4125
158------9369--------4250
159------9645--------4375

The speed index chart and the load index chart usually correspond to each other: for example 121P indicates that the tire supports 3,196 lbs at 93 mph.

You will also often see two different load index ratings on the same tire: 121/118P. The first one indicates load index when used on a single wheel axle and the second indicates maximum load on a dually axle.

A third criterion is the width of your rim. Each tire can be mounted on a rim width range indicated by the manufacturer. For example, a Toyo LT245/75R16/E 120R can be mounted on rims from 6.5’’ to 8.0’’ width. To mount the tire on smaller or wider rims won’t allow the sidewall to work properly and will degrade dynamic properties of the tire.

And fourth criterion is the radius of the tire under load. Every country law describes a maximum difference between speed indicator and real speed. So, if you change tire radius, your true speed will be off unless you change the transmission ratio of the speedometer to compensate the difference.

Speed index, load index and radius under load are generally legally enforced. Width of the rim is a producer specification.



Chapter 3: Which is the right inflation pressure of my tires?

Inflation pressure depends of many factors: tire characteristics, volume of the tire, load, speed, etc. And inflation pressure also directly affects dynamic properties of the tire, with influence on vehicle behavior and safety.

Here a reminder of basic principles:

- The more the vehicle weights and how fast the vehicle is traveling, dictates the tire pressure. Higher speeds and/or heavier vehicle weights require higher tire pressures. That’s why auto manufactures often show two recommendations of inflation pressure for a same car: the first for normal use (1 driver, one passenger), the second for heavy load and/or highways.

- Pressure applied by the tire on the ground doesn’t depend of the weight of the vehicle, but only of the inflation pressure! Many people struggle to understand this but you must remember that tires are flexible. So if the load increases, the tire(s) deform(s) and its contact surface with the ground also increases. But the pressure under this surface stays the same: always the inflation pressure. That’s why we deflate tires on soft grounds, reducing pressure under the tire to avoid becoming stuck.

- Inflation pressure depends of inflation volume of the tire. An example:
Original tire size of a Ford E350 is LT 245/70 R 16 on 7.0’’ rim. Supposedly they should be inflated at 60 Psi. If I replace this tire with a 35/12.5 R 17 on a 9.0’’ rim without changing the weight of the rig, inflation pressure in these larger tires will be lower.

- Maximum allowed speed decreases with lower inflation pressure. As I already said, it’s possible and recommended to deflate tires on soft grounds. But when done, you cannot drive as fast as with normally inflated tires. So when you leave soft ground, don’t forget to inflate your tires. If you don’t do it, your tires will warm up quickly and may burst. At a minimum, you can damage the integrity of the tire leading to premature failure.
Contrariwise, if you limit the speed lower than speed limit, you can exceed maximal load of the tire without risking bursting. There are specific charts indicating relationship between speed reduction and additional load allowed. Typically, a speed reduction of 10% allows a surcharge of 5%. But every tire manufacturer has its engineered ratings and specifications.

So, how to determine the right pressure for my rig?

First to do is to weigh your rig. Ideally, you should weigh it empty and then loaded (stuff and passengers). If you cannot do both, you must weigh it loaded. Important is to weigh each axle separately.
When done, you must consider tire size and type and refer to manufacturer’s inflation charts. Remember, it not only depends of tire size and weight, it also depends of construction type of the tire. So, even for the same size and same manufacturer, you can have different inflation pressures depending on tire type.



Chapter 4: When should I monitor tire pressure?

Recommended inflation pressure is always the cold tire pressure! Since a tire is flexible, when you drive, it warms up; more if you drive fast, less if you drive slow. So the only reference to check pressure is when the tire hasn’t been driven for two hours or more.
But what does it means?
1 – I only adjust pressure after the vehicle has set, (ideally in the morning before starting the vehicle).
2 – During cold weather, provided the vehicle spends the night in a heated garage with a large difference between inside and outside temperatures, you should take it outside and allow the tires to cool for half an hour before checking pressure.

If you have to adjust pressure after driving – for example when you drove a trail with deflated tires and you come back on a highway – overinflate your tires of about 5 Psi - 0.3 bar (it’s the estimating pressure increase due to the heat) and recheck cold pressure at the first opportunity.

And remember: it’s impossible to check tire pressure without a pressure gauge. It’s almost impossible to recognize a 10% underinflated tire from a well inflated one!



Chapter 5: Why is the right inflation pressure so important?

They are many reasons why inflation pressure is so important.

Security aspects
Too low of a pressure makes the tire warm up and can lead to rupture. Since a punctured tubeless tire loses air slowly, it’s important to check pressure regularly to detect a punctured tire. I’ve seen tire bursting after only 3 miles drive! An underinflated tire also causes a significant loss of road handling, steering precision and cornering stability since it deflects more as it rolls.
An over inflated tire has less longitudinal and lateral adherence. So breaking distances will increase with over inflation and traction will be lower. Ride will also be harsher and puncture risk is higher.

Longevity aspects
Many people fail to realize how much underinflating or over inflating a tire affects its longevity. A 5% over inflation can produce a loss of more than 15% in overall mileage of the tire, and 10% over inflation more than 30%. If the right pressure is 60 Psi, 10% is only 6 Psi..! Some people say “I overinflate my tires to provide better gas mileage”. They are right: by a 10% over inflation, you can hope to gain 3-5% better mpg. The problem is tire wear has also increased by 30% so the tires life is shortened. In the end is it worth saving a little fuel money compared to how long the tires last?

It’s about the same with under inflation even though it’s a little less severe regarding tire life: a 10% under inflation will affect tire life of about 20%. But remember: under inflation is very dangerous for your security!


I hope it will raise your awareness of regular monitoring of tire pressure… ;-)
__________________

__________________
Travelling in VivaLaVida (custom 2010 6.0 Ford E-350 Extended with U-Joint 4WD conversion)
Pschitt is offline   Reply With Quote
Old 05-23-2013, 07:29 AM   #2
Senior Member
 
ANZAC's Avatar
 
Join Date: May 2011
Posts: 1,005
Re: Some facts about tires

Good information!

Quote:
Originally Posted by Pschitt
So, how to determine the right pressure for my rig?

First to do is to weigh your rig. Ideally, you should weigh it empty and then loaded (stuff and passengers). If you cannot do both, you must weigh it loaded. Important is to weigh each axle separately.
When done, you must consider tire size and type and refer to manufacturer’s inflation charts. Remember, it not only depends of tire size and weight, it also depends of construction type of the tire. So, even for the same size and same manufacturer, you can have different inflation pressures depending on tire type.
The vehicle manufacturer's tire placard should trump everything, because it also accounts for axle capacities etc. The reason to weigh your vehicle is to check you're not exceeding any capacities (axle or tire) The difference between a full load and an empty load is going to be smaller for an RV which is already built out, you're not putting 5,000lb of cement in a cargo van, so my point isn't as acute for an SMB. But it is safer to inflate according to the loads you can derive from the placard, then you are good up to the max load of the vehicle.

Quote:
If you have to adjust pressure after driving – for example when you drove a trail with deflated tires and you come back on a highway – overinflate your tires of about 5 Psi - 0.3 bar (it’s the estimating pressure increase due to the heat) and recheck cold pressure at the first opportunity.
5psi seems high, and it really differs per vehicle depending on the tire size and starting pressure. What I do is measure my tire pressures after driving and remember how much they increase. I've observed 2-5 PSI across different sizes/vehicles. Maybe this suggests ~10%.
__________________

__________________
Sportsmobile-less.

"A job worth doing is worth doing at least twice."
ANZAC is offline   Reply With Quote
Old 05-23-2013, 08:17 AM   #3
Senior Member
 
86Scotty's Avatar
 
Join Date: Jul 2011
Location: TN
Posts: 8,231
Re: Some facts about tires

Great stuff guys!

I've always wondered about the danger of inflating/deflating a warm or hot tire. Is it very dangerous to inflate or hot tire? If so, does it make a difference if you are doing it slow or fast (for instance, using a small portable air compressor which inflates the tire very slowly vs. a large shop compressor which inflates fast?)
Howbout deflating a warm or hot tire? Is there a proper method which reduces the risk of bursting?

Thanks
__________________
Currently vanless. Weird.
86Scotty is offline   Reply With Quote
Old 05-23-2013, 08:42 AM   #4
Senior Member
 
Join Date: Dec 2009
Posts: 582
Some facts about tires

Great write-up!
__________________
Current: 2014 15 Passenger V8
Former: 2009 SMB 4x4 6.0
TwoVans is offline   Reply With Quote
Old 05-23-2013, 09:15 AM   #5
Senior Member
 
shenrie's Avatar
 
Join Date: Nov 2012
Location: boise idaho
Posts: 2,072
Garage
Re: Some facts about tires

^^^ agreed! definitely picked up some useful info with that writeup. sticky worthy?

now, if someone could just explain to me why theres no industry standard for sizing and treadwear
__________________
"understeer is when you hit the wall with the front of your car, oversteer is when you hit the wall with the rear of your car, horsepower is how hard your car hits the wall, and torque is how far your car moves the wall."
shenrie is offline   Reply With Quote
Old 05-23-2013, 09:17 AM   #6
Senior Member
 
Pschitt's Avatar
 
Join Date: Nov 2012
Location: Switzerland
Posts: 1,005
Garage
Re: Some facts about tires

Quote:
Originally Posted by ANZAC
The vehicle manufacturer's tire placard should trump everything, because it also accounts for axle capacities etc. The reason to weigh your vehicle is to check you're not exceeding any capacities (axle or tire) The difference between a full load and an empty load is going to be smaller for an RV which is already built out, you're not putting 5,000lb of cement in a cargo van, so my point isn't as acute for an SMB. But it is safer to inflate according to the loads you can derive from the placard, then you are good up to the max load of the vehicle.
Safer yes but not to optimize tire life. But you're right: difference between empty and loaded RVis smaller. So you can weigh the van loaded and you will be good.



Quote:
Originally Posted by ANZAC
5psi seems high, and it really differs per vehicle depending on the tire size and starting pressure. What I do is measure my tire pressures after driving and remember how much they increase. I've observed 2-5 PSI across different sizes/vehicles. Maybe this suggests ~10%.
It's not exactly what you say. Pressure increase only depends of temperature elevation of tire (which depends of lot of parameters like tire structure, speed, load, etc), then you should not use percent to correct hot tire pressure.
But the way you describe is right. You can measure cold pressure, then drive at normal speed for half an hour and then check pressure elevation. You can then use this value when reinflating hot tires.



Quote:
Originally Posted by 86Scotty
I've always wondered about the danger of inflating/deflating a warm or hot tire. Is it very dangerous to inflate or hot tire? If so, does it make a difference if you are doing it slow or fast (for instance, using a small portable air compressor which inflates the tire very slowly vs. a large shop compressor which inflates fast?)
Howbout deflating a warm or hot tire? Is there a proper method which reduces the risk of bursting?
If the tire is in his normal temparature range (below than about 170°F or 80°C), there is no danger inflating a hot tire. Contrariwise, if a tire has or had overheated, espacially after running underinflated at not reduced speed, it's very dangerous to inflate it without unmount it from the wheel and control that the inner surface isn't damaged. A damaged inner surface indicates that the tire structure as suffered from eat and can burst at anytime. Then the dire must be replaced.
__________________
Travelling in VivaLaVida (custom 2010 6.0 Ford E-350 Extended with U-Joint 4WD conversion)
Pschitt is offline   Reply With Quote
Old 05-23-2013, 09:22 AM   #7
Senior Member
 
Pschitt's Avatar
 
Join Date: Nov 2012
Location: Switzerland
Posts: 1,005
Garage
Re: Some facts about tires

Quote:
Originally Posted by shenrie
now, if someone could just explain to me why theres no industry standard for sizing and treadwear
I don't know. For example, indicated width of Michelin race tires is treadwear width as it's overall width for other tires...

Not sure it's the reason but I can imagine overall width is more practicle to verify that tire isn't touching any part of the body in any suspension/steering position...
__________________
Travelling in VivaLaVida (custom 2010 6.0 Ford E-350 Extended with U-Joint 4WD conversion)
Pschitt is offline   Reply With Quote
Old 06-20-2013, 12:01 PM   #8
Senior Member
 
Join Date: Apr 2008
Posts: 191
Re: Some facts about tires

Thanks for this thread. Its become a big issue for me as some may have seen on another thread or two.

I have a question the answer to which may well be "depends" but...if the tire is rated at a maximum load at its maximum inflation adn up to a maximum speed: is there anything to be gained by raising the inflation further and/or curtailing the speed? Put another way, if the cold pressure of an E rated 245X75X16 was raised to say 84 (5%) and speed was cutailed to a maximum of 60 mph, does that raise the potential loading above the 3040 or 3086 of its nominal E or UTQ number?
I know the manufacturers would never sanction this but if for example one had to subsitiute a "spare" D for an E or an E for an F would either of those actions (pressure/speed) give peace of mind?
__________________
Hanging Out
vlamgat is offline   Reply With Quote
Old 06-20-2013, 12:33 PM   #9
Senior Member
 
ANZAC's Avatar
 
Join Date: May 2011
Posts: 1,005
Re: Some facts about tires

Max cold is just that: maximum. If you overload the tire beyond what it is designed to carry, it is still overloaded even if you up the inflation.

(edit) but reducing speed is always a good option if you are having issues with a tire.
__________________
Sportsmobile-less.

"A job worth doing is worth doing at least twice."
ANZAC is offline   Reply With Quote
Old 06-20-2013, 12:52 PM   #10
Senior Member
 
Join Date: Apr 2008
Posts: 191
Re: Some facts about tires

I agree that the Max Cold is the appropriate Liability limit and no one is representing anything feasible, legal or safe above that. But it also implies that the load limits are "cliff vesting" meaning that if you carry 1 lb over the tire limit at any speed the tire will fail immediately. And we know thats not true so it implies there is a deterioration in capability at some (probably increasing rate) as you overload that could be offset to some limited extent by slightly higher inflation and probably much lower speed.

Put another way, does a tire fail at 60 mph, and 84 psi at 70 deg F carrying 3500 lbs (7000 lb rear axle weight) in 100, 1000, 10000 or some other mileage?

I would argue because I have done it that ist safer to fit a D load tire, over inflate it, and limit speed to get off an interstate rather than waiting 4 hours for a service truck at 11 pm on a Saturday night. But I alos argue we ought to know what the consequences of that action might be. And one of those could be to scrap the tire used as a spare because its iretrievably damaged by that action.
__________________

__________________
Hanging Out
vlamgat is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

» Sportsmobile Registry

Dusty

capnkurt

PolarVan

PolarVan
Add your Sportsmobile
Powered by vBadvanced CMPS v3.2.3
Disclaimer:

This website is not affiliated with or endorsed by Sportsmobile SIP or any of its affiliates. This is an independent, unofficial site.


All times are GMT -6. The time now is 09:24 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2019, vBulletin Solutions, Inc.