Additional weight. It takes a bit more pressure to give the same footprint.
[Quote]Originally Posted by [B]bradthebard[/B]
I prefer puppies. Works equally well on irate drivers and motorcycle gangs. I tried kittens but they kept sticking to my hands when I tried to throw them...[/Quote]
The math formula works in getting the psi in the ball park then you gotta fine tune it with chalk. My dad taught me the math formula waaaaaay back when i was a kid and it has always worked for me. I never had a tire that wore more in the middle or outside.
That little chunk of iron and steel up there called an Engine.
it's not a Miata. F/R weight ratio is a little skewed to the engine bay end of things.
Meh, i guess with regular trucks and cars that run a bit higher psi it really doesnt matter if you run 2 psi more in the front...i really never gave it that much consideration until now while running lower pressures.
Forgive me for not reading every post but... the chalk test is designed to show what psi provides contact with the entire surface of the tire tread. THAT IS ALL.
Rim width is going to affect this greatly, for example I have (got em free) 10" wide wheels and 33x12.5 tires. Now recently I swapped to 31x10.5 tires on those same wide rims, of course it should be no surprise the pressures are way different to match that chalk test. So a little common sense says don't go so low that the sidewall is flexing to the point of creating heat and blowing out, nor should you run it so high you feel like you're driving a bias ply tire. Run your setup at 25lbs or so and call it a day. Remember, your tires will heat up during a long highway drive and you'll pick up a couple psi there as well. If you notice uneven tread-wear soon you can make changes in pressure accordingly however consider tread wear is second to SAFETY so don't drop below a reasonable pressure, which personally I'd say is 25psi generally. $.02
I usually run a % more pressure upfront based on the weight distribution of the vehicle. The jeep I figure is rather front heavy so I'd off the top of my head in this case give it another couple psi up front. However I just leave mine even as the tire's don't seem to bulge noticeably more in the front or anything. <EDIT> Looks like the jeep is more 50/50 than I thought so we can rule this out here, my pickup truck is another story though.
In wet conditions or snow conditions I also vary my tire temp slightly depending on what I'm doing. In the rain temps don't raise since the tires are being cooled, I give 2psi when I lived in Seattle since 80% of my days were wet conditions. Also a high pressure sometimes cuts through slush/snow better where a low pressure may allow you to float ontop of heavy powder if your tire is wide enough. Just some things to think about.
So here I was...on the flat, straight section of road by my house (dead end street). I get lined up and chalked the tires. No suprise that the chalk did not wear on the outsides of the tire. I dropped them down to 24 psi since the entire outside lugs showed no wear.
So for me...the chalk test is a sham. I did not even bother pulling the JK out to try it, and I'll be putting mine back up to 28 psi, maybe 26.
Just for a FYI...it was flat, dry and about 48 degrees using sidewalk chalk on 33" BFG A/T's and a stock TJ
If anyone is using the "chalk test" it should be taken with a grain of salt...I can't see how that would be safe, and from the searches I've done on it here most are running 24-30 psi on the street.
As one poster said it..."common sense must prevail"
How are you going to know how much tread is on the pavement IN A TURN if you only go in a straight line?
I use the chalk test to get the best wear possible. Have done it for years.
It also indicates alignment/wheel bearing issues when used correctly.
When I run a chalk test, I use a flat, empty parking lot. One with room to get up to city street speeds. Mark the tires, and do a slow figure 8.
This is to get into the balpark of how the edges are hitting.
Mark the tires again, and do a larger figure 8 at a slightly faster speed. check again and make adjustments as needed.
Then comes some slalom turns, then some "emergency" type swerves.
In a straight line, my tires have anywhere from 3/4" to 1 1/2" of chalk on the outside edges. after the figure 8s, slalom and swerves, its down to about 1/4" to 1/2" on them. I also check to see if the tires are getting unusually warm. Are the giving me good "feedback" while testing or are they getting squishy or mushy.
BTW, I'm running 31/10.50R15 GY Duratrac on a hardtop yj. I run them at 28psi, sometimes 30psi if I know there will be a drastic temperture change (warm to cold) in the next few days.
The chalk test is just a tool. If it doesnt work for you, then don't use it.
The chalk test isn't a sham.... your wheels are too narrow for your tire to put down a proper footprint that's all. My 10" wide wheels do it easily at high pressures. That said, I'd prefer a narrow wheel for off-roading because it doesn't rash and protects sidewalls more and also because I expect it may pop a bead less easily.... though I run 8-12psi with 12.5 wide tires and have no problems thus far.
I should mention, there's also a point where the tire tread footprint fully touches and where it applies even force, the point of even force is always slightly lower psi than when it touches of course. Just something to keep in mind, chalk touching enough to barely show chalk or enough to put real pressure down, in any case I can usually note the difference from how dark the chalk shows.
.... and I'm running 30 psi on my Cooper STT 33x12.5r15 ..
Would having a hardtop and a tire carrier makes any difference ??
I guess that I should lower the pressure to 26~28psi....
Yes all weight makes a little difference. You should be at slightly higher pressures due to your extra weight but it's not much weight in your case with just those items so I wouldn't hardly consider em even.
....then you have some tires which are not even designed to make full tread contact with the ground. Look at a General AT2 tire...it's crowned. Quite a few tires are made that way actually. If you chalk test that tire, you'll end up running WAY too low of pressure. So the chalk test, in my opinion, is a rule of thumb that is spread as fact and that is dangerous. There are WAY too many variables to consider to make it as simple as using a piece of chalk to set your tires.
'92 YJ - heavily modded
'96 ZJ - heavily modded
'05 KJ CRD Limited - 370 lb./ft. from a 4-cylinder and trail ready. Pure awesomeness.
'11 Mango Tango JKU Sport - Mods
'12 Dozer JKU Sport S - 6-speed - ACE rock rails - Max Tow package - Daystar spacers - 33x12.50 Duratracs