nimbler
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- Thread starter
- #1
The problem statement is: determine what is the best course of action (w.r.t regearing axels) if one desires to move to either 35" or 37" tires.
I apologize for the length of the question, it is to avoid any back and forth on the math, so we can stick to the question. I'll change the color of the boring math so that you can skip it, and only refer to it as required; feel free to skip over it
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From my (very limited) understanding, the math is relatively simple when it comes to the gear setup:
For the Jeep JLs we have three components in this mechanical circuit:
Now, the JL Rubicon was designed for that ratio on 33" tires. A 35" is 6.061% larger in diameter, and the 37" is 12.12% larger when compared to the stock/standard 33". Since torque is linearly proportional to the radius/diameter, that means that torque that the circuit needs to drive is also 6.06% and 12.12% more, respectively, for these tires.
In order to relieve the powertrain of this additional torment, one would simply adjust the gear ratio accordingly in order to maintain the original 77.244 ratio. And that would take place in the Dana 44 gears.
One thing to correct for before moving forward: the Dana 44's 4.10 gear ratio is calculated from the ratio between its 45 ring gear teeth, and its 11 pinion teeth, so it's more accurate figure is 45 / 11 = 4.091. Therefore the actual crawl ratio with that figure is:
Also note that for this calculation, the other two numbers (transfer case and transmission) do not matter as they remain constant here.
Now we can solve for the 35" and the 37" gear ratios respectively:
From my understanding, it's not important at all to get an exact match (many are in fact running 37"s without any issues), but if you do change gear ratios, it's better to go higher than calculated, rather than lower.
Assuming that we will remain with Dana for now (see http://www.sierragear.com/ring-and-pinion-gear-sets-dana/), then the best new ratios would be the Dana 44 4.55, and the Dana 44 4.89 respectively. Our new adjusted (account for tire change) crawl ratios (and percentage change from stock design) would be:
Note that the effective crawl ratio takes in to account the change in the tire, actual new crawl ratios would be obviously higher due to the regearing: 85.72 and 92.13 respectively.
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With all that crap aside, would you regear to maintain factory ratio? Yes, the Rubicon can run the 35" and 37" without any apparent issues, but surely over the lifetime of the vehicle, it would have a noticeable impact, no? There's considerably more torque request on the axels, the powertrain components, the gears all the time.
I'm a neub, no experience in off-roading, in jeeps, nothing. And, I'm about to spend a lot of money on a JLUR Automatic. I like the 35"s, I love the 37"s + the MOPAR lift. I want to keep this vehicle for a very long time, so longevity is important. I also do plan on doing some off-roading, so this won't be a vanity/Nordstrom build.
Looking forward to your thoughts!
I apologize for the length of the question, it is to avoid any back and forth on the math, so we can stick to the question. I'll change the color of the boring math so that you can skip it, and only refer to it as required; feel free to skip over it
---
From my (very limited) understanding, the math is relatively simple when it comes to the gear setup:
For the Jeep JLs we have three components in this mechanical circuit:
- a transmission (850RE AUTOMATIC, EIGHT-SPEED OVERDRIVE),
- a transfer case (NV241OR ROCK-TRAC),
- and an axel differential (Dana 44).
- 4.71 in first gear
- 4.0 in low range
- 4.10
4.71 x 4.0 x 4.10 = 77.244
Now, the JL Rubicon was designed for that ratio on 33" tires. A 35" is 6.061% larger in diameter, and the 37" is 12.12% larger when compared to the stock/standard 33". Since torque is linearly proportional to the radius/diameter, that means that torque that the circuit needs to drive is also 6.06% and 12.12% more, respectively, for these tires.
In order to relieve the powertrain of this additional torment, one would simply adjust the gear ratio accordingly in order to maintain the original 77.244 ratio. And that would take place in the Dana 44 gears.
One thing to correct for before moving forward: the Dana 44's 4.10 gear ratio is calculated from the ratio between its 45 ring gear teeth, and its 11 pinion teeth, so it's more accurate figure is 45 / 11 = 4.091. Therefore the actual crawl ratio with that figure is:
4.71 x 4.0 x 45 / 11 = 77.07
Also note that for this calculation, the other two numbers (transfer case and transmission) do not matter as they remain constant here.
Now we can solve for the 35" and the 37" gear ratios respectively:
35" tires: (35 / 33) x (45 / 11) = 1575 / 363 = 4.339
37" tires: (37 / 33) x (45 / 11) = 1665 / 363 = 4.587
37" tires: (37 / 33) x (45 / 11) = 1665 / 363 = 4.587
From my understanding, it's not important at all to get an exact match (many are in fact running 37"s without any issues), but if you do change gear ratios, it's better to go higher than calculated, rather than lower.
Assuming that we will remain with Dana for now (see http://www.sierragear.com/ring-and-pinion-gear-sets-dana/), then the best new ratios would be the Dana 44 4.55, and the Dana 44 4.89 respectively. Our new adjusted (account for tire change) crawl ratios (and percentage change from stock design) would be:
35" tires: 4.71 x 4.0 x 4.55 x 33 / 35 = 80.8 (4.38% increase in effective crawl ratio)
37" tires: 4.71 x 4.0 x 4.55 x 33 / 37 = 82.2 (6.11% increase in effective crawl ratio)
37" tires: 4.71 x 4.0 x 4.55 x 33 / 37 = 82.2 (6.11% increase in effective crawl ratio)
Note that the effective crawl ratio takes in to account the change in the tire, actual new crawl ratios would be obviously higher due to the regearing: 85.72 and 92.13 respectively.
---
With all that crap aside, would you regear to maintain factory ratio? Yes, the Rubicon can run the 35" and 37" without any apparent issues, but surely over the lifetime of the vehicle, it would have a noticeable impact, no? There's considerably more torque request on the axels, the powertrain components, the gears all the time.
I'm a neub, no experience in off-roading, in jeeps, nothing. And, I'm about to spend a lot of money on a JLUR Automatic. I like the 35"s, I love the 37"s + the MOPAR lift. I want to keep this vehicle for a very long time, so longevity is important. I also do plan on doing some off-roading, so this won't be a vanity/Nordstrom build.
Looking forward to your thoughts!
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