4 Low Max RPM

[JDS123]

Hi All,
I read online that going over 2500 rpm while in 4 low, at least in a 4:1 T-case, would result in the planetary gear spinning at 10k rpm. Is this only for an automatic for something because I can only see a speed reduction in gears as you go down the line when I think about it. The engine would be at 2500. The transmission at 2500/4.01. The T-case at 2500/(4.01x4). Then the axle shafts at 2500/ (4.01 x 4 x 4.56). What am I missing here. What gear is spinning at 4x the engine rpm?

Sorry if this worded awkwardly, don't really know how else to say it.

Thanks!

 

[VirtualRussel]

The differentials aren't going to have any bearing on the Engine -> Transmission -> Transfer case RPMs. You're equation is right but you need to remove the 4.56 to get the output RPMs of the transfer case. Also, unless you've swapped transmissions your first gear should be 3.93 so 2500 Engine RPMs is transferring ~636 RPMs to the transfer case. At 4:1 this means you're having an output of 159 RPMs to the differentials. I'm not sure how this applies to the planetary gear RPMs inside the transfer case.

The nut of it is though, I can say that in Moab while rock crawling in 4:1 low you're definitely going to be exceeding 2500 RPMs at the engine at various points on the trail. I'm not sure what the author of your online note meant by not exceeding that limit.

 

[JDS123]

The differentials aren't going to have any bearing on the Engine -> Transmission -> Transfer case RPMs. You're equation is right but you need to remove the 4.56 to get the output RPMs of the transfer case. Also, unless you've swapped transmissions your first gear should be 3.93 so 2500 Engine RPMs is transferring ~636 RPMs to the transfer case. At 4:1 this means you're having an output of 159 RPMs to the differentials. I'm not sure how this applies to the planetary gear RPMs inside the transfer case.

The nut of it is though, I can say that in Moab while rock crawling in 4:1 low you're definitely going to be exceeding 2500 RPMs at the engine at various points on the trail. I'm not sure what the author of your online note meant by not exceeding that limit.

I have a NV3550 so first gear is 4.01 for me I think. I saw on the description for the tera low kit that exceeding 2500 rpm would spin a planetary gear at 10K rpm. Is that true? Should I keep the RPM low or just don't bother? I'm going to moab next week so...

 

[mrblaine]

I have a NV3550 so first gear is 4.01 for me I think. I saw on the description for the tera low kit that exceeding 2500 rpm would spin a planetary gear at 10K rpm. Is that true? Should I keep the RPM low or just don't bother? I'm going to moab next week so...

The big difference is Tera used a smaller set of planetary gears so the shaft speed is very high or high enough that you can burn them up by spinning them too fast. Jeep used a very large set of planetary gears so the shaft speed is much lower and able to be run at most any RPM the engine is good for. Double check the owner's manual from a Rubi to confirm but I've never seen a warning about an RPM limit.

 

[VirtualRussel]

I have a NV3550 so first gear is 4.01 for me I think. I saw on the description for the tera low kit that exceeding 2500 rpm would spin a planetary gear at 10K rpm. Is that true? Should I keep the RPM low or just don't bother? I'm going to moab next week so...

I'll qualify first that I'm not an expert in any shape or fashion on the Teralow kits but I did just read on another forum where Stu Olsen mentioned you don't want to exceed 3000 RPMs on the input shaft side of the Teralow. Going back to your 2500 RPM limit you should have plenty of room with your transmission. Sorry, I assumed the AX on my earlier note.

1st gear: 2500*4.01 = 10025 Engine RPMs (you're going to be fine)
2nd: 2500*2.33 = 5825 Engine RPMs (you'll still be fine)
3rd: 2500*1.39 = 3475 Engine RPMS

So, if Stu is right and you're more conservative than is 3000 RPM recommendation you'll be fine as long as you leave 4th out of the equation and don't hit it hard in 3rd.

 

[JDS123]

The big difference is Tera used a smaller set of planetary gears so the shaft speed is very high or high enough that you can burn them up by spinning them too fast. Jeep used a very large set of planetary gears so the shaft speed is much lower and able to be run at most any RPM the engine is good for. Double check the owner's manual from a Rubi to confirm but I've never seen a warning about an RPM limit.

So they best plan would be to buy a 4:1 transfer case instead of the tera low upgrade, right?
Also, would a 4:1 transfer case with a 2.72:1 underdrive be overkill? This isn't going to happen for many years but I like to plan ahead. Might be a dumb idea in the first place.

 

[Jerry Bransford]

4:1 is a great ratio for ultra-low speed crawling but many find that too low of a ratio for general off-road use. My 4:1 is great for rock crawling but it's not really optimal for easier trails.

But to answer your question, yes a 2.72 underdrive with a 4:1 would definitely be an expensive overkill, I assume you're asking about 3-speed transfer cases like Atlas makes.

And if the stretch of trail you're on is easy enough that the engine RPMs are high enough that you're worried they may be too high, it's time to shift out of 4Lo.

 

[billiebob]

this ^^^^ . Plus I have a TJR there is no reference in the Manual to rpm when in 4LO. Tera does some fabulous marketing/advertising/promotion but there are better choices when you dig into the engineering.

I would love a 3 speed transfer case. 40 years of wheeling with 2.72:1 transfer cases never left me wanting a lower ratio. I like the 4.0:1 mostly to smooth out really rough stuff but where I live there is no need to crawl. I miss the 2.72 ratio.

The Rubicon drive line can be run up to redline in 4H or 4LO..... but don't hold it there.