Why should one perform a chalk test on the cold tires?...

[taram]

Hello everyone,
I have read carefully many threads regarding tire pressure here before posting a new one, and obviously a chalk test is a good way to determine whether non-stock tires are set to a proper pressure - no doubts about this.
But my question is - and what I cannot explain to myself - why it should be done on a cold tires? My logic: we try to avoid an uneven wear of the tires, and they wear out the most on high speeds, being warm - correct? Why then wouldn’t we want to set a proper pressure on the warm tires, when we need the full contact the most - what am I missing? Could anybody clarify with arguments? Thanks!

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[Shots]

Consistency.

If you test on a "cold" tire you take away a variable. Keeping in mind a "cold" test does NOT mean winter cold. You want to do this on an average temperature day.
When you drive, the tires warm up as you mentioned. However they typically heat unevenly. For example turning one way or the other, super-elevation of the roadway, variable speeds, etc. all effect the tire temperature. Not only from tire to tire, but also inboard/outboard on each tire. It's minor, but it's still a variable. By testing a "cold" tire you have the most consistent tire with which to determine the ideal contact patch.
Since none of the variables will be the same for every time you drive it, setting at a cold temperature eliminates any inconsistent variables from affecting your test. That said, if you live in an area with extreme temperature differences from season to season, it's not a bad idea to determine your ideal pressure setting for each season's average temperature.

 

[taram]

Ok, now that makes sense, thanks. Also noticed that properly inflated warm tires show max. 2-3 psi more than cold ones - this probably doesn’t influence much the contact patch

 

[Arterius2]

Ok, now that makes sense, thanks. Also noticed that properly inflated warm tires show max. 2-3 psi more than cold ones - this probably doesn’t influence much the contact patch

I also realize that most modern tires (especially mud terrains) are not designed to be completely flat in cross-section, it bulges out in the middle and curves back on the sides, probably for improvements in on-road handling, noise reduction, better fuel economy etc. And thus if you abide by the chalk test rule you will definitely under-inflate the tires.

These MT tires simply are not designed to make FULL contact edge-to-edge all the time with pavement when driven on-road, doing so may severely damage your fuel economy and provide unnecessary friction when you don't need them. They are off-road tires and are designed to provide grip on uneven surfaces. Trying to meet chalk test with your off-road tires on flat pavement will always invariably under-inflate your tires more than you should.

https://www.intercotire.com/using_chalk_method_determining_psi

 

[Shots]

Ok, now that makes sense, thanks. Also noticed that properly inflated warm tires show max. 2-3 psi more than cold ones - this probably doesn’t influence much the contact patch

You are correct that the difference is very minor. It's not exactly a straight forward formula and there are a lot of variables such as vehicle load, weight distribution, tire compound etc, but I can give an example for a rough idea. Say you have a 285/70/17 on your Wrangler with roughly a 60-70 square inch contact patch at normal pressure. By increasing/decreasing the tire pressure 2 psi from normal the contact patch changes roughly 3-4 square inches.
That's not a huge difference, but it does have an effect. It's also worth noting that the difference isn't entirely linear either. For example increasing toward the high end may change the area by 4 sq-in, but decreasing the same amount of psi while near the low end may change the area 8 sq-in (those are completely random numbers for example, not calculated).

All that to simply say, a difference of 2-3 psi will have a small impact on the contact patch. How small that difference is depends on the pressure at which you are starting, the tire and the vehicle (weight/load/etc), but yes it's not much of an influence.

 

[Uhdinator]

Bottom line is find your ideal psi with the chalk test. Lets say 30 psi is what you have determined to work best first thing in the morning. Then knowing its normal for tires to warm up and increase in psi about 10% or 3 psi. Monitor psi increase. If psi increases more than this you may be a little under inflated.

Driving on a 95 degree day the psi may increase 5 psi because the hot tar is hotter than the ambient temp. On a 20 deg winter day the psi may rise only 2 psi.

So you determined that 30 psi is where you want to be. After driving awhile you see you are staying at 33 psi after tires are warm. Now you know via your chalk test and verifying psi increase is normal and not under inflated.....Air them down to 30 psi so they stay in the range while driving. Let the cold psi fall where it may as long as they are 30 when driving.

increase psi slightly as temps get colder to keep your driving psi at 30. I don't mind that the TPMS always alerts me an comes up every time I start the jeep (since it doesn't chime) I'm always aware of it this way and it reminds me to keep an eye on my psi as the temps drop.

i also rotate my MT's at 4000 miles vs the recommended 6K that most tire shops recommend for cars to keep the tires even . The MT's on the front will get chewed up quicker than an AT tire and start to get noisier and rough riding if you don't rotate soon enough.

The chalk test is not designed to give you optimum MPG. It will give you the best tire wear, traction and ride with a little sacrifice in MPG. You can pay more for tires and air up a few psi if MPG is your priority. I would rather have a better ride and replace $300 tires less often.

Also rim width will effect psi. If you got a 7.5" stock wheel with a 12" wide tire...........the thread will be more rounded from a narrower wheel and require a little less psi than the same 12" tire on a 9" wide wheel to get the contact patch flat on the ground. (hence the reason you don't want to narrow of a wheel for the tire width as lower psi would be required to get even wear and then you could be overheating the tire as well as the bead not sitting right in the wheel.)

 

[Shots]

Exactly how I do mine, early rotations and all. I set slightly low when cold, so the typical driving temp is where I want it. I run mine (driving temp) at 34 psi. Using chalk 34 was actually just a hair high for mine, but It seems to be where the KM2's are quiet, comfortable and responsive (based on my jeep, with my size tires anyway). I considered going a little lower, but I don't want the to deal with the alert like Uhdinator does.

 

[Uhdinator]

Exactly how I do mine, early rotations and all. I set slightly low when cold, so the typical driving temp is where I want it. I run mine (driving temp) at 34 psi. Using chalk 34 was actually just a hair high for mine, but It seems to be where the KM2's are quiet, comfortable and responsive (based on my jeep, with my size tires anyway). I considered going a little lower, but I don't want the to deal with the alert like Uhdinator does.

Exactly........I started at 30 psi warm and noticed a little wander...........upped it to 32 psi warm and its just right.
2 psi too much or too little is definitely noticeable for me.

 

[taram]

Thanks for the discussion, it’s good to know there’s more to consider than just a chalk test and a contact patch as many people here I’d like to go with 315/70R17 KO2’s on a Rubi’s stock wheels 7.5” wide, so definitely will have some more rounded profile, and will need to go a bit lower with psi, BUT just like @Arterius2 mentioned - not too low, even if the chalk test shows a perfect patch... But the good news is that a lot of people have this setup and already have some time approved experience with the pressure, as well as there are not much THAT cold days here in Cyprus

 

[Uhdinator]

Thanks for the discussion, it’s good to know there’s more to consider than just a chalk test and a contact patch as many people here I’d like to go with 315/70R17 KO2’s on a Rubi’s stock wheels 7.5” wide, so definitely will have some more rounded profile, and will need to go a bit lower with psi, BUT just like @Arterius2 mentioned - not too low, even if the chalk test shows a perfect patch... But the good news is that a lot of people have this setup and already have some time approved experience with the pressure, as well as there are not much THAT cold days here in Cyprus

Most have mentioned 2 issues in this forum concerning stock wheels and 315/70 tires........
1. Stock wheels are too narrow for 315's and most tire shops may not even mount it for you. (8-11" wheel required) Many are doing it but still doesn't make it right.
2. 315's may rub with stock wheels as the wheel offset/backspace is too close to the frame and will rub control arms, and possible other things besides the fenders.

9" wide with less backspace is what most are running. Better do some more searching here before you do that.

 

[Shots]

Yeah, I've seen it a kinda hit and miss with shops that will mount a 315 mm wide tire on a 7.5" wide rim. Some shops will strictly adhere to the tire manufacturer's approved rim width, while others don't care. At a minimum I'd check the shop you intend to install the tires and make sure they'll mount them before you spend the money. Just call and tell them you're considering some 315/70/17 KO2's for you stock Wrangler rim, and ask if they're able/willing to mount those for you.

So on that note. I'm not sure if you are specifically looking for 12.5" wide tire, or if you're more interested in "35's". If it's the overall diameter you're after you can look at a few options that are still very close. Even though 315/70/17 is commonly accepted as a 35 the KO2 is 34.4" diameter, and many other manufacturers are close to the same size. If you look at "34's" or the 285/75/17 which specs at 33.9" diameter the difference is only 1/2 inch different. Don't confuse them with the stock 285/70/17 because the 1st and last numbers are the same. You can get different diameters with the same width, which is what the first number stands for.
Anyway, the narrower width reduces the risk of it rubbing, and more importantly the stock rim falls within its approved rim width. 285/75/17 is available in the KO2 if you're willing to give up that 1/4" of sidewall and you wouldn't even need to find a new tire.

FWIW, I've got 285/75/17 on my Sahara with stock Rubicon rims, and they don't rub anywhere even when articulating. It's worth mentioning that I do have a 2" lift which replaced the stock control arms, so I can't say 100% that the LCA clearance wasn't influenced from that.

 

[taram]

Hi everybody, thanks for keeping posting in my thread. Let’s go through quickly.
Yes, I realize there may be issues with installing 315/70 on stock rims. And there’s no doubt I will have them installed - my main concern is a safety of my family and me... BUT, there’re people here who post their reports after 20’000 miles run in such a setup - BFG KO2’s and stock rims with no lift - no problem at all, and certainly going off-road as well. So, to be honest - yes, I just like how 35’s/315 fit in the wheel well, also I had 315 mm wide tires on my BMW X6 and they looked just great. Rubi is a big car, and I definitely don’t like these chicken legs of EU-stock - 255/75R17...
More than that - I had several people telling me that the tire holds on the narrower rim even better than on the wider one, e.g. 9 inches, especially when aired down ... and even if these are just rumors- there are a lot of people enjoying this setup and I haven’t seen any threads like “I lost my 315/70 tire from stock Rubi rim while off-roading” so far.
Look, I appreciate your posts, really, made me think of many things now, but I’d stick to my initial idea - go with 315/70 BFG KO2’s with stock suspension/rims and then I will see. If they rub on inner fenders - I will get a minimal 1.5-2” lift (it will cost me almost the same as 5 good aftermarket rims, but IMO is a much better option in general); if they rub on LCA’s - I’d probably go with 0.5-1.0” wheel spacers instead of heavier rims, and so on... I’m ready for possible additional PITA and expenses , but would definitely start with possible minimum, and proceed only with crucial upgrades. Now I have just to have my Pumpkin delivered and we’ll start... you’re lucky to receive your custom ordered Jeeps in 2-3 months maximum, and it takes 6 months here in Europe...
Cheers everyone!

 

[anotherWS6]

I've run 80-90K miles on my 2 different sets of tires mounted on 7 inch stock rims on my Chevy. Started about 7 years ago, no issues. Truck weighs 5,500lbs and it's been weighed at just under 7,000lbs several times while making runs to my local garbage dump. I've also towed several thousand pounds for several thousand miles at speeds up to 75+mph. I've aired down to 18pisi on the beach. I wouldn't worry one little bit. I sure don't anymore.

 

[taram]

Great, thanks for sharing your story, but short question - what tire width you had on those 7” rims?

 

[Shots]

Nobody actually said "don't do it". You're correct that a lot of people run those tires on those rims. I can be done. The only words of caution were that it's not the approved rim width, in case you weren't aware.
If you know you can get them mounted, that eliminates one concern. I'd just hate to spend that much money on tires only to find out I couldn't use them because I couldn't find a shop willing to install them. If you don't have to worry about that, no big deal problem solved.
As for safety and/or stability. On a JL, that 12mm difference in the rim is not likely be a problem, and nobody said it would be. The tire's load capacity will be decreased, but not likely enough to be an issue on a Wrangler. I wouldn't say the width difference becomes a significant safety concern until the rim is much narrower than that.

It seemed that there was a bit of a hot button with recommending different sizes, and/or cautioning about clearance issues. Or maybe I just perceived the response wrong. In any case, we were just trying to make sure you didn't run into any unforeseen problems. Obviously you should do what you like/want on your Jeep. If you want 35's then go that route, there's nothing wrong with that, and you don't need any justification other than it's what you want. It's your Jeep, you do your own thing. It sounds like you have a plan for any clearance issues so you should be fine.

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