Jeep plans (lift, gears, and lockers)

I've got this Jeep, and it's pretty cool. But, I want to make it cooler.
My plan from the start has been to build a not-flashy daily-driver with a moderate lift, that performs well on road and better than it looks like it should on the trail. Because it's a neat challenge.

To that end, I've been looking in to:
  • Lifts
  • Gear ratios
  • Transfer cases
  • Lockers
  • Armor
I actually just bought all the stuff for the lift. The rest of this is looking off into the future should I decide to do it. I wanted to understand everything I *might* do, to make sure I didn't do anything incompatible with it, just like designing a complex distributed computer system at a high-level, even if you're not sure if you'll actually implement some of the fancier features.

I should note that this represents a fair bit of research on my part. I'm only just learning about all of this. I've done one lift in the past, and it was quite a simple affair on a coil-over front and leaf-spring rear, and it came as a boxed as a kit. Once I threw out that option, I got in to building up my own kit from individual parts, which gets complex fast.

I don't know what I'm doing... this is just what I found. I'm happy to take suggestions, input, or questions.

Lets start by talking about lifts. I'm running 33" duratraks, which so far I like a lot. They have great on-road manners. I was playing in the mud a little last weekend and they performed great. We were on hard-pack, slime, and hitting mudpuddles. I'm running 33" because they are the largest tire you can easily run without damaging components, they also happen to be the largest tire that runs well with an unlifted Jeep... convenient. My stock height was 8.7" ground clearance, going from 29" to 33" tires I got another ~1.75 inches (tires aren't exactly the size they say), so now I have ~10.0" of clearance.

My Jeep is solid axle front and rear, with coil springs and independent shocks. Getting lift is easy, you swap out the springs for taller (and/or stiffer) springs. BUT if you lift a Jeep JK more than about 2.5" things start getting harry. Your steering geometry gets all mucked up and you end up swapping out half of the parts under your car.

What I think I want to do is to raise the vehicle another 2", so I get 12" total. At this point it's a matter of finding a set of springs that do that. I could do spacers instead (just blocks that sit on the coils), but I don't want to for reasons I'll explain shortly.

It turns out finding springs that will raise your vehicle 2" is a lot harder than it sounds though. A 2" lift maybe built for a heavier version of the Jeep, in which case it could lift it as much as 5", even if it's built for this model they are usually designed to get that lift over stock height, after also adding heavy bumpers and such. I plan to keep my Jeep quite light, so a 2" lift is likely to be at least a 3" lift. After a lot a lot of looking at various springs on the market, I don't like almost any of them. Most of the "triple rate" springs are a sham, and are also too short (I explain why this is bad later), and most of the good springs give 3" or more lift.

All of this is even further complicated by JKs having all different springs from the factory. While what springs they have is vaguely correlated with the model, it's only very vague and almost any spring will occasionally be found on almost any model. This means if you look at the current ride height that's based on some unknown quantity.

Luckily you can go look, I have 13's in front and 55's in the rear. As it happens this means I could get ~1.75" of lift in the front and maybe 1.5" in the rear by buying stock 19/60 springs someone else is swapping out. As it turned out I couldn't find these, so I got 18/59, we'll see how much lift I get out of it.

Now, the lift isn't just about getting the vehicle higher, it's also to increase droop, that is, the amount the axle can travel downward from it's normal position. I'd like to increase droop as much as is reasonable.
Droop lets you articulate more, meaning lets the axle pitch relative to the body of the vehicle more. The more you can articulate, the easier it is to keep your wheels on the ground in harry situations, surprisingly, more articulation also helps with stability and avoiding rolling the vehicle.

So, how do we get droop? The first limit to droop is usually your shock, which has a limited throw. If your axle is just hanging from the car, that's what it should hang by. So, to increase droop you want to swap out for a shock with a longer throw. The complication is that this usually also increases the minimum length of the shock as well. When the suspension is all of the way compressed, if you sit on the shock, you generally destroy the shock.


A good shock absorber will actually make my car much safer and better behaved on the road, as well as off the road. Right now when I corner hard the rear wheels frequently leave the pavement causing the car to oversteer. I don't like this, and better shocks should help.

After much research it turns out the Bilstein-5100 just barely fits on my vehicle with no modification, it has a significantly longer throw (4 inches in the rear, 2 in the front), and it's minimum length is only just smaller than my minimum length. As a bonus I had Bilstein 5100s on my last car, and I loved them, so I'm happy to use them again.

Oookay, but is that actually what we want, is it possible to let my no 33" wheel go ALL up without something else bad happening? To answer this I used a rock next to my driveway:


Below you can see that it's close, but at full stuff, my rear tire clears by maybe an inch and a half. This axle is sitting on it's bump-stop. I'll have to watch that if I ever run aired down though, since that can bulge a tire out a bit. This is with the current sway-bar, which I intend to leave.


And up front we see again that it clears fine. I'm actually about 2 inches from full stuff, but there's plenty of space. That's a good thing, because up-front I may decide to disconnect my sway-bar at some point, which would let it articulate further.


Now, if you think through the geometry. If the far wheel is drooping another 4 inches, this side is going to come up yet another inch (thus why the sway-bar matters). It might just barely clear, it might not... that'll be something to test. If it hits I'll probably rig some tiny spacer for the bumpstop, any bit of rubber will work. It's definitely in the ballpark, which is good enough for selecting parts.

So, it looks like we can use most of the uptravel. I don't want to get in to welding my frame and crazy stuff, so bilstein 5100 should work perfectly, getting me the maximum droop I can get without crazy modifications.

You have to extend brake lines and a few other things, at some point on the 2015 Jeep JK, it turns out the driveshaft hits the exhaust, and there's a couple of other issues. These can be surpassed though with a little work as they arise. Another example is the sway bar links, which I'll want to extend. If you don't extend them it turns out they can invert, and break things. Still, I don't know how long they actually need to be, so I figured I'd see it all in place first and measure. There's also disconnectable sway-bar links which give more articulation make the vehicle less stable offroad, but keep full stability on road.
I'll probably keep the non-disconnects for now, since as it happens I can likely replace the rears and move the rears to the fronts... I can also probably hack up a disconnect myself.

More about coils
Besides those things, the next thing to limit your droop is your spring length. At some point the axle droops enough that the coil just falls out. Oops! For this reason you want a nice long coil. When the suspension compresses though the coil can be the limiting factor on that end, so you want one without too many turns. Spacers just take up space that could be used by coils that will unfurl, so spacers are bad for your droop if coils are your limiting factor. For this reason, I want a pure coil suspension if I can get it. As it turns out, not many aftermarket coils in the 1-2" range are longer than the longest stock ones, so that's another advantage of going with take-off OE springs.

Gear ratios

A 4wd car has an engine, which drives the transmission, which drives a transfer case, which drives the differentials, which spin the axles, which are connected to the wheels. The final gear ratio between the engine and the wheels depends on the product of the gear ratios at each stage.

My transmission has 6 gears, and my transfer case has 2. The transfer case is also responsible for swapping from 2 to 4wd (engaging the front wheels or not). Usually the high gear of the transfer case is used for normal driving, and you just use the transmission to shift. You can swap to 4wd still in the high range, and drive like normal on slick roads. When you need lower gears for crawling up steep hills really slowly you swap the transfer case to low range, giving you a whole new range of gears to shift between on the transmission.

Now, my car had ~29" tires and now has ~33" tires. This affectively changed the gear ratio of the whole drivetrain by ~10%. This is okay, but if it gets to far outside a reasonable range, 6'th gear becomes useless, and it gets hard to get up hills in 1'st. Usually people compensate for this change in the differentials. My car came stock with 3.21 gears in the differentials. The JK is also available with 3.73 and 4.10 gears in the diffs.

Interestingly, the differential gearing, transfer case, and transmission, are all the same on JKs prior to 2012, but at that time they were mated to a significantly less powerful engine. Personally, I actually think the feel of my car on the street was *improved* by the whole thing being shifted up a little. Most Jeep people would call me crazy for saying that, but hey, I'm used to an underpowered Toyota on oversized tires. Anyway, all of this is to say that I want to keep my gear ratio in the differential (I think).

But, what about when I 4-wheel? When I'm wheeling I'd really like that torque back that I lost by increasing the tire size. Well, if I like it when the transfer case is in high (which, btw, is 1:1 in the transfer case), and I don't like it in low (1:2.73 I think?) then maybe the answer is to change the transfer case!

Transfer case

I'm not doing this any time soon, but I want to figure out every change that I think I might want to make eventually, so I can make sure they all work together well. As it turns out, there are some very nice transfer case swaps available that make for a much lower low range (higher gear ratio numerically). After doing a bunch of research, if I did this I'd probably install a 4-speed transfer case from Atlas. This gives you 24 gear ratios, instead of just 12. The advantage would be having good gears for climbing hills and the like, as well as super super low gears for slowly crawling over dangerous stuff. It's not that I care about torque, actually the torque is bad since it breaks components. But going slower makes it easier to not break things, and that I like.

Differentials are designed so that when you spin the input shaft, the output shafts spin. If you hold one output shaft, the other spins at double the speed. This allows cars to go around corners. A locking differential allows you to disable this feature and make both wheels turn at the same speed, this way if one wheel is off the ground, the one on the ground is still forced to turn.

My first Tacoma had a locking rear differential, and I *loved* it. Having a locking differential doesn't matter much most of the time though. I discovered with Jane (my last Tacoma) that after I lifted her, thus giving her more articulation so she could keep her tires on the ground, I didn't ever need lockers the whole time I was mucking about in California.

The exceptions though are snow, mud, and ice. California had very little of any of these, but as soon as I hit snow in Jane I wanted one again. In these cases having your tires on the ground doesn't guarantee that you have traction, so the locker is still useful.

So, I want to install locking differentials eventually. This ties in to the decision about what gear ratios I want in the differentials, since if I replace/rebuild my current differentials and add lockers, I'd like to do the whole thing at the same time.

My current plan is not to armor my vehicle at all. I like it being light, and I like my gas-milage. My hope is that with the lift I'll get enough clearance that I don't have to worry about it. If I armored up much I would definitely have to change my axle ratio to 3.73 for normal driving, and it's possible that will happen, but for now I'm going to try not to. If I did armor up, it would probably be to add rock-sliders to protect the quarter panels (the painted bit below the door, between the front and rear wheels). Some rubicon stock sliders would be about perfect if I do decide that I need them, and still keep things light. 

1 comment:

  1. This was... intense to read. That's a lot of technical data all quick. It was fascinating to read though. I hadn't known most of this before. It was interesting to think about in relationship to my vehicles (which will never be used like yours). :)