Re-gearing for 33's
- Thread starter JML
- Start date
I never measured hub height, I never did any math. What I did do instead was play what if with the calculator. I took several known vehicles we verified the speedo, gearing, etc. and plugged the dash numbers into the calculator and then kept changing the tire size until the calculator matched the vehicle. Works out to roughly the hub height doubled for a rolling radius.
Anyone else can do the same. Take the numbers you see on your dash using something easy like the most accurate RPM at an easy to remember speed, go plug that into the calculator until it matches by changing the tire size. It is pretty eye opening just what size tires really work out to be.
Even more eye opening is the next time you rotate your tires, roll one up beside one that is still on the ground supporting the rig. I just did that and while it wasn't something I was doing for a comparison, it did catch me a bit off guard when the loose tire was almost 2" taller than the one on the ground. I was going to get a pic but figured digging out a level and getting it set up to show the folks that want to argue that I did it wrong wasn't worth it.
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I agree, I’ve done the same many times and that method works the best. That was what I did to confirm either of the other two methods correct.
Why does that matter? Get your dash numbers, plug them into the calculator, change the tire size until they match, that is your tire size and that's all that matters.
It doesn't. Just a fun distraction to turn over. Do you know if radial tire treads lengthen in circumference enough to matter within usable pressures (say10-35psi)?
It takes so little effort to try this and once you see that difference it's instantly apparent that the entire "debate" is just all bs.
So for my build... I'm planning on stretching my TJ to an LJ with 115" WB with a WMS to WMS of 70", or about 84" wheel bubble to wheel bubble on Mickey Thompson 40" DOT MT's on 17" bead locks with 4-3/4" back space so I can keep a really good scrub radius, 5.38 gears in 1 tons single triangulated front, double triangulated rear... with outside coil overs so I shouldn't need sway bars, probably starting at 100 over 150 springs... sticking with the 4.0 5 speed manual initially but eventually an LS3 with a 4L80E with negative manual valve body... so long story short... I guess my question is... do I go ahead and get lights... or lockers...?
Hey Jerry, we can agree that the free length of a coil spring, and it’s length with the weight of the Jeep on it are very different, correct?
Interesting plan. Have you thought through how you will clear a triangulated front? Also, what about outboarded coilovers negates the need for sway bars?
deadbeat son
TJ Addict
I think the manufacturer knows how long the spring is; I blindly use their spec without thinking about the practical application of that information!
The manufacturer knows the free length of the spring and how stiff it is. They don’t know the weight of my Jeep and how much it will compress it. Therefore they don’t know the resulting compressed length on my Jeep. A tire is a spring. It squishes and is elastic. The manufacturer doesn’t know what the exact rolling radius (compressed length) of the tire will be when the weight of my specific Jeep is on it with my specific air pressure with my specific wear amount. Jerry doesn’t put the manufacturer recommended air pressure in his tires. Neither do I. That all changes the rolling radius (again, compressed length). The tire also wears with time which changes its free length and resulting compressed length.
Tires are springs. Manufacturer tire diameter specs are the free length (diameter), not the compressed length (diameter) when the tire is on a rig.
Btw I know you were being sarcastic and understand, you were just the first one to reply
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I don't care because I don't regear for when I am aired down. I regear for where I spend most of the time driving which is on the street so I tend to very much use street pressure. The way that I know what street pressure, my favored street pressure does to my tire circumference is I get the numbers off of the dash, I plug them into the calculator and when they work, bam, that is my tire size for all my important uses.
All Jerry would have to do to prove us wrong and verify the manufacturer as being the gold standard is just a very simple test.
Go for a drive, get an even number for RPM or get an even number for MPH and note the other. Look at the numbers on the tire, see what they are, go to the mfg website and see what that says. Then go to the calculator, plug in the dash numbers and then start changing the tire size to see who is right.
There is no need to debate it, just test it and prove it. We are right or we are wrong and the way to prove it is dead simple and easy.
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LOL... I just banged that whole thing out really quick! I needed to break that sentence up better... *a three link front and a double triangulated rear... is what I should have said...
...as for the coil overs... I am hoping that angling them out from the top of the tower to the absolute farthest I can put them on the axle and still clear the wheels will get me there... the attempt is planned... the execution is still yet to be seen
. I have heard that it can be done though and still have an amazingly stable platform... we shall see Sorry if I missed it but what RPM range is ideal for 65/70mph on the 4.0 with the 42RLE?
That isn't really how it works but good luck to you any way.I have heard that it can be done though and still have an amazingly stable platform... we shall see
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