Load Index, Wheel load, Speed ratings, Tire Aging, Tire Repairs And Speed Ratings, Tire/Rim width

Discussion in 'SRT Hellcat Wheels and Tires' started by Tire God 2.0, Jun 5, 2016.

  1. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    Load index
    The number after the tire size that says 106 on the stock tires is the load index. Elsewhere on the tire it says standard load. The load index determines how much weight a tire can carry per pound of pressure according to the load inflation tables created by the TRA (tire and rim association) in a joint effort with the tire manufacturers and written into law by the NHTSA/DOT. A tire size that has a "P" before it is a P-metric tire size, which is an American size. A tire size that does not have a "P" is a euro-metric tire size. The P-metric tires have a pressure ceiling for weight capacity of 35psi SL, meaning even though the tire may have a max inflation pressure of 44, anything after 35psi doesn't carry more weight. The euro-metric pressure ceiling is 36psi SL. You will find that in some tire sizes there will be two of the same tires but one is p and one is not, and they will have a slightly different load index. This is due to the way America adopted this and makes it quite confusing. It is confirmed that if you put a P-metric at 36, it's the same as a euro-metric and actually makes the load index the same. So from here on I will not refer to the P-metric. There are standard load (SL) and extra load (XL) tires. These are different construction types used for different vehicles/purposes. The pressure ceiling for a SL is 36 and XL of 42. The load index defines the weight the tire can carry per pound of pressure, and the construction type determines at what air pressures the weight is carried at. It is important to make sure that your tire application meets or exceeds what is on the door placard. So if your placard says 106SL at 32 psi, we know the pressure ceiling is 36, so that means if you read the load inflation tables, you can determine if your new tire size will be a safe fitment. The load index could be a 105SL and still be safe as long as you put them at 35psi(this is an example not an exact). But also remember your TPMS is programmed for 32, so if you need to run higher pressures, the light still won't come on until 28ish, so I would check the pressure regularly.

    Wheel load capacity
    The wheel load capacity requirement is determined by taking the highest GAWR(gross axle weight rating) and dividing it by the number of wheels on the axle. This can be found on the other sticker on the door, FMVSS. The load capacity of a wheel is stamped somewhere on it, usually on the back of a spoke or the mounting pad. Be careful with replica/off brand/Chinese wheels as I have ran into many that do not meet US standards. Always ask before you buy.

    Speed ratings
    The letter after the load index is the speed rating. For example (106Y). The speed rating was developed in Europe for the autobahn. It is not a "max speed", it is the speed the tire can handle for long periods just like taking a long trip on the autobahn. The tire manufacturers put the tires through a very strenuous testing process to determine this, and has to do with how well the tires dissipate heat and many different tire construction aspects. You might notice that most drag radials/competition tires don't have a listed speed rating. This is because they are not designed to be driven at high speeds for long periods. Drag racers see very high speed then brake hard, track guys usually don't see mph over 100 depending on the track, and if so it's for an even shorter period. Speed ratings of W,Y, and (Y) are all Z-rated tires. The W and Y are called a limited Z rating, while the (Y) in parentheses is called an unlimited Z. Meaning to find out what the speed rating is of the (Y) you have to contact the tire manufacturer. The Z rated tires usually have a ZR in the tire size description which means Z rated and radial construction. While speed ratings are indicators of how fast you can go, it doesn't stop there. As mentioned above the speed rating is assigned by testing, and the components built into the tire have to be stronger to handle it. This means in general, the higher speed rated tires will handle better due to reinforced sidewalls and shoulders which give crisp steering response at those high speeds, so it inspires confidence in the driver. You should always use tires that meet or exceed the speed rating that the vehicle manufacturer specified, depending on the intended use of the vehicle.

    Tire Aging
    This has been on the news and plenty of discussions have taken place, but just in case you don't know, I will go over tire aging. First, tires are a petroleum based product that have lots of oils, tires are manufactured by bonding rubber to fabric plies and steel cords. I'm sure you have taken an old rubber band and stretched it out and saw lots of cracks. This same principle applies to tires, and everything else rubber in a car. They dry rot over time. The main factor in tire aging is that air molecules don't stay inside the tire forever. They slowly penetrate the tire, there is a natural loss of air pressure of about 1 psi a month, this does not account for weather changes. If you leave a car sitting for a long period, you will return to find the tires flat or close to it even though they are not punctured. The air molecules slowly break down the rubber as they pass through and you will see small cracks on the tires sidewalls and between tread blocks and in the circumferential grooves. Other factors that accelerate this are, long term exposure to UV light, extreme temperatures/changes, poor tire storage before they are installed, poor tire maintenance. Tire manufacturers and the tire industry as a whole recommend replacing tires at the age of 6 years old from the DOT date on the sidewall, and that tires absolutely not be serviced or put into service at 10 years old or older. It doesn't matter if the tires have never been driven on, like a spare, or what the tread depths are. Porsche and other European automakers took it one step further and published in the owners manual, "under no circumstances should you drive on tires that are 6 years old or older". The last 4 digits of the DOT number is the week and year the tires were built. Worst case scenario is when the tire dry rots enough, you are on the highway doing 70 mph, you don't check the air pressure often so they are a little low. Tires do this fun thing called tire delamination, where the tread peels off the carcass of the tire and causes roll over accidents. I'm sure everyone remembers the Ford/Firestone nightmare with the Explorers. If not, look it up its a doozy. I'm sure you won't have to worry about it with Hellcats though considering they eat tires with all that power.

    Tire repairs, and the impact on speed ratings.

    Tire repairs should always be done by trained professionals that follow the RMA(rubber manufacturers association) repair guidelines. What's acceptable: a repair must be a cured plug and a patch, the injury no larger than 1/4" diameter, and the injury be within the tread area. Maximum of 3 repairs per tire, patches can't overlap. What's not acceptable: nylon, polyester rope plugs (sticky plugs), any type of plug with no patch, any patch with no plug. The sticky plugs that get shoved through the tire from the outside, take time to cure and in the mean time can absorb water and cause the belt package to rust, as well as swell up and stretch the belts out and cause tire failure. Same goes for a patch with no rubber filler/cured plug. It leaves an open hole for water/contaminants to enter the tire. Be very careful buying used tires, the mom and pop shops will "fix" anything as well as sell anything with no regards to safety. As far as speed ratings are concerned, each tire manufacturer has their own stance on repairs. Some retain the full speed rating, some say it lowers it by one or two, some say it voids it completely and they default to the lowest speed rating of "Q" which I believe is limited to like 80 mph. This is inconclusive due to lack of testing, but should still be followed as a safety precaution. With competition tires, I would just replace it. I have heard of plugs getting ripped out due to the extreme conditions on the track.

    Tire and rim width range safety.

    The TRA also created the tire and wheel width chart in a joint effort with the tire manufacturers. The chart specifies each tire size and the appropriate rim width range for each based off of the given tire size. Our chart is slightly modified to accommodate the actual measured tire size after production, due to there being up to a 5% variation in the tire size given and the actual measured size after production. For a 20x9.5 the widest safe size would be a 295/35-20 or 295/40-20. The tires are tested extensively for this as well. The sidewalls have to have a certain shape or contour in order to properly hold the weight of the vehicle. If the tires are too wide for the rim, the sidewalls are too close together and limits not only the tires performance but the tires ability to be stable and deflect properly under lateral loads ect. The best analogy is if you hold a gallon of water to your chest with one hand, you could hold it there for a long time. But if you fully extend your arm, you won't hold it for long. This same principle applies to the way tires are designed to operate and perform. The stress is not evenly distributed, and it works the tire like a coat hanger, back and forth until it snaps. There is conclusive data from a large amount of tires that failed prematurely, that this has a large impact. The days of "if it fits, it fits" are over. Everyone is a lot more educated now than ever before. This is a good thing. Safety first! There are groups out there trying to outlaw racing, and outlaw aftermarket parts in general. SEMA plays a big part in the aftermarket community and has a lot of say so in what goes on, and they are fighting for our right to mod and race. But if we are stupid and keep running misapplications and causing accidents and deaths, everyone's hands will be tied. The NHTSA wrote the TREAD act in which by law the vehicle manufacturers had to implement some form of TPMS on every passenger vehicle sold in the US from 2008 and later. They have the authority to take away our mods so let's not be stupid. Safety first!

    If you made it this far in the thread, you are a trooper. This is a lot of info all in one spot. My fingers hurt! But it's worth it to help you guys.

    "A bad attitude is like a flat tire, you can't go anywhere until you fix it." :D
     
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  2. Slowhand

    Slowhand Gold Member

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    I think this belongs in the wiki.
     
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  3. graycat

    graycat Automobile Aficionado Extraordinaire Staff Member

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    Very good write up! (Anther one that is) ;)
     
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  4. PlumTexasCrazy

    PlumTexasCrazy HELLCAT LIFE MEMBER Hellcat Car Club HCC Charter Member

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    LIKE!!
     
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  5. theRealJA105

    theRealJA105 Silver Member

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    Tire God 2.0, would the 305/35R20 Pirellis at (104Y) be the cause for a pretty violent shake/sway that feels like the suspension is leaning side to side very quickly when under full power towards the end of a quarter mile run? I've had this feeling numerous times and it is pretty alarming. It does seem to go away at higher pressures so I think I already found my answer from this thread.

    To build on that though, I've read numerous posts saying that the Bondurant school uses the 285/35R20 Eagle F1s which only appear to be rated at (100Y). How can they run these without issues?
     
  6. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    The stock tires are a 106XL at 32 psi. You just have to meet or exceed the weight capacity of that. A 100XL would have to be set at 40 psi to do that. Which is perfectly fine. As far as the shake, I would have to feel it myself to see what that is. It could be too low of air pressure.
     
  7. RDLSRT

    RDLSRT Silver Member

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    Tire god 2.0, the safest tire width for the Hellcat OEM wheels is the 295/35-20 or 295/40-20?
     
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  8. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    That's correct
     
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  9. theRealJA105

    theRealJA105 Silver Member

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    My 305s are on 11" wheels just to clarify. The shake seriously feels like the left suspension compresses half way then unloads and the right suspension compresses and unloads in very rapid side to side succession. I did a search and found some references to a challenger "death wobble" but idk if they are experiencing the same characteristics.
     
  10. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    I suppose it could be death wobble. Jeeps have that problem. The fix for the jeeps usually is the steering stabilizer being replaced, or the wheels and tires road forced balanced perfectly. Any imbalance around the center point of the axle can cause this. I would get the suspension and axles checked, if it's not that try balancing. Also if the air pressure is too low, it can cause the tire to fold so to speak and cause wheel spin, wheel hop.
     
  11. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    The old 03 04 cobras also had problems with wheel hop due to CV axles. I think it also has to do with the axles being different lengths. Same reason FWD cars get torque steer.
     
  12. BULL

    BULL ________173.7mph_______ ___2016 Colorado Mile___ Gold Supporting Member

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    I experienced a deal with Discount Tires once, they refused to dismount a tire from a wheel that was "older than 10 years"...

    With their absolute refusal to even touch the tire/wheel assembly, I ended up going down the street to a Peerless where they happily let the air out of the tire, dismounted it and replaced it...

    I can understand material degradation and that there is potential for failure while in service, but refusal to depressure and dismount the tire such that a new tire could be installed???

    A step too far...
     
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  13. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    Yeah that's dumb. The point is to replace it lol.
     
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  14. theRealJA105

    theRealJA105 Silver Member

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    Tire God 2.0 I would like to revisit my question as I am finally reading through the Toyo Load Inflation Tables. You stated that
    My confusion is that it seems the standard load (SL) tires are actually stronger than the extra load (XL) tires at the same load index number. Is that correct? For example: 100SL @ 36psi = 1764lbs vs 100XL @ 36psi = 1565lbs and @ full 42psi = the same 1764lbs.

    Therefore in this case if I were to run the 100SL tire I would only need to be @ 34psi to get the load rating to 1676lbs.
     
  15. MABBRYFL30

    MABBRYFL30 NYS President Hellcat Car Club Gold Supporting Member

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    I always appreciate your knowledge on all of your posts. They are always very good and informative
     
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