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Note: A lot of the information in this section is found in greater detail in the book Jeep 4x4 Performance Handbook by Jim Allen. If you are thinking about any of these modifications, buy the book -- it's a small investment and it'll provide a lot of expertise.
A body lift is a spacer that goes between the frame and the body of the Jeep. Generally, a body lift is used to obtain one or two inches of clearance between the body and frame. Anything more than two inches is generally frowned upon, because of the increased stress on the mounting bolts between the body and the frame. A body lift can be used to obtain a little more clearance for slightly larger tires or as the final bit of clearance when using a suspension lift. This FAQ at the 4wd.com forum suggests that most people use a body lift in conjunction with a suspension lift, not as the primary lift.
A spring spacer provides a small amount of extra lift by placing a spacer between the top of the coil springs and the frame. Typically, this ranges between 1/2 and 2 inches. This can be an inexpensive way to provide more lift, but not a lot more. If you combine a spring spacer lift with a body lift, you can get a moderate increase in tire size, without some of the installation difficulties of a larger lift kit. See also this FAQ at the 4wd.com forum.
A suspension lift provides extra clearance by replacing suspension components. A suspension lift includes at least springs and shocks. Larger lifts also include a dropped Pitman arm (for the steering system), a transfer case lowering kit, and/or replacement control arms. The larger lifts, with more components, also require more installation know-how. This is partly because a larger lift changes the steering alignment, such as the caster and pinion angles. Another difficulty is that you might need longer brake lines and you may need to relocate the Panhard (or track) rod, which is supposed to be parallel to the axle.
The Jeep 4x4 Performance Handbook has a complete section about the pros and cons of different lift sizes. The book advises, "The more lift you buy, the more money you spend. Don't buy lift for looks -- buy it for need, and buy no more than you need. The drivability and safety tradeoffs increase with more lift. Giving away your safety for looks is not smart!"
(copied from old FAQ section 4.4)
Many people ask, "How big of a lift kit do I need to fit a tire of a certain size?" I checked some manufacturers' sites and got the basic information on various lift kits.
S4T kit: 4 inches of lift, can accomodate up to 35" tires
S3T kit: 3 inches of lift, up to 33" tires
3 inch TJ lift: no tire size specified
2 inch TJ lift: no tire size specified
TJ 4.5" lift plus 1" body lift: up to 35" tires
TJ 3" standard or Superflex kit: up to 33"x12.5" tires
TJ 2" kit: 31x10.5" tires
Tire sizes not listed
ARB/Old Man Emu
Suspension systems are not advertised as a lift kit, though they provide one or two inches of lift
TJ Rockcrawler 2.5 inch lift: up to 33 inch tires
TJ 2 1/2 inch suspension kit: up to 32 inch tires
4" lift system: Tire sizes not listed, but the page shows a vehicle with 33" tires; another catalog I have shows this kit with a 2" body lift and 35" tires
1.75" lift: Tire sizes not listed
4" lift kit: Up to 32 inch tires
2" lift kit: Up to 33 inch tires (note: this looks wrong; it seems obvious to me that a 4" lift would take bigger tires than a 2" lift)
Note that your choice of a lift kit depends on several factors: the amount of on-road versus off-road driving you do, the type of off-road driving you do, the ease of installation, and the reliability and reputation of the manufacturer. If you're considering a lift kit, talk to some different manufacturers and get some technical information from them. Also, see if you can find someone who has the lift kit you're considering.
Note: I need to do more research here. In the meantime, I'll just link
to manufacturers' sites:
(copied from section 4.3, old FAQ)
You can fit tires up to 31x10.5 inches on a stock TJ without buying a lift kit. The stock YJ can only fit tires up to 30x9.5 inches. When you put larger tires than stock on a vehicle, you'll be slowing the engine down and losing some power. This may be a concern if you are thinking about putting 31" tires on a 4 cylinder TJ, but I have a friend who has done this and doesn't worry about it too much.
(Copied from the old FAQ section 4.3)
If you put 31" tires on the stock 7 inch wheels, you may experience rubbing on the control arms when you turn the steering wheel all the way in one direction. You may need to adjust the steering bump stops to compensate for this. Dave Willams wrote in the newsgroup:
Did you lose my e-mail, Todd? (I probably did.)
Here is a page, complete with photos, diagramming how to adjust the steering bump stops.
(copied from the old FAQ section 4.3)
Tire manufacturers each have their own specifications for how large a tire can be mounted on a specific rim width. Some manufacturers' tables include:
BF Goodrich AT chart
Bridgestone Dueler AT chart
Goodyear Wrangler RT/S chart
To make things simpler, though, I'll just write out a table for common Wrangler tire sizes (with and without lifts):
|Tire Size||Rim Width||Average Diameter||Lift needed|
|LT225/75R15||6.0 to 7.5||28.3 inches||None|
|LT235/75R15||6.0 to 8.0 (6.0 to 7.0 for some manufacturers)||29.0 inches||None|
|30x9.5R15LT||6.5 to 8.5||29.6 inches||None|
|31x10.5R15LT||7.0 to 9.0||30.7 inches||None, but may need to adjust steering bump stops|
|32x11.5R15LT||8.0 to 10.0||31.7 inches||Yes: 2 inch suspension lift (Teraflex)|
|33x12.5R15LT||8.5 to 11.0||32.6 inches||Yes: 4 inch suspension lift|
|33x9.5R15LT||6.5 to 8.5||32.7 inches||Yes: 4 inch suspension lift|
|35x12.5R15LT||8.5 to 11.0||34.7 inches||Yes: 4 inch suspension lift plus 1 inch body lift|
Note that these tire sizes are just an average, compiled from the charts I've reviewed. The actual tires you buy might have different requirements, so check carefully.
Technically, your engine won't lose power if you install bigger tires, but your overall gear ratio will decrease. What this means is that your engine has to work harder to accelerate from a stop, and you'll have to shift at a higher speed than if you had the original tires. It's sort of like having to start from an intersection in second gear rather than first: it's possible, but it takes more work.
If your Jeep has a lower axle ratio, like 3.07 or 3:55 on the six-cylinder engine, then this effect will be more pronounced. If you have a higher axle ratio, like 3.73 (available as an option in certain years) or 4.11 (available on the Rubicon), this is less of a problem. Of course, if you have a four-cylinder engine, then power is already somewhat limited and bigger tires are even more of a concern.
It's possible to swap differential gears, but this is expensive (at least $1000). You'll need to swap both the front and rear differential gears, or chaos will ensue. Nevertheless, this may be a workable option for those who want really large tires.
The Jeep 4x4 Performance Handbook offers the following formulas for computing effective gear ratio:
"Effective" gear ratio: (old tire diameter) / (new tire diameter) x (original ratio) = effective ratio of new combination
Engine RPM at a given speed = mph x (total gear ratio) x 336 / (tire diameter in inches)
To obtain equivalent ratios: (new tire diameter) / (old tire diameter) x old ratio = new ratio
For example, the stock Wrangler X comes with a 3.07 axle ratio and P225/75R15 tires (28.3 inches in diameter). An upgrade to 33x12.5R15 tires (32.6 inches actual diameter) would give an effective gear ratio of 2.665. To obtain a gear ratio with an equivalent power output before the tire swap, the Wrangler would require a 3.54:1 axle ratio.
So, if you're ever thinking about bigger tires, here's why you want to look for a higher axle ratio when you're buying the Jeep in the first place.
(copied from the old FAQ section 4.3)
Also, bear in mind that if you change your tire size, your speedometer will not be accurate. To compensate, you can change your speedometer gears. Randall Groh wrote this handy table:
Mark Tener wrote:
Mark, I own a '97 TJ and ran into a similar question. It turned out the the dealer that I worked with had the speedometer gears in their computer based on tire size and axle ratio. Below is a chart that I created so that I would know the best gear to use if I change to a non-standard tire.
DS/RPM TIRE SIZE REVS/MILE AXLE 2,167 30X9.50R15 706 3.07 2,275 P225/75R15 741 3.07 2,315 P215/75R15 754 3.07 2,385 P205/75R15 777 3.07 2,506 30X9.50R15 706 3.55 2,631 P225/75R15 741 3.55 2,677 P215/75R15 754 3.55 2,758 P205/75R15 777 3.55 2,902 30X9.50R15 706 4.11 3,046 P225/75R15 741 4.11 3,099 P215/75R15 754 4.11 3,193 P205/75R15 777 4.11
The key columns are the driveshaft speed (assuming a 1:1 ratio), and the tire revolutions per mile. Tire manufacturers generally publish revolutions per mile for the tires. For example: a BFG All-Terrain T/A 33x12.50R15C is rated at 634 revolutions per mile. With a 3.55 axle ration this yields 2,251 driveshaft revolutions per mile. If I were to install this tire on my TJ I would tell the dealer that I need the speedometer gear for the P225/75R15 tire and 3.07 gear ratio. This is 2,275 driveshaft revolutions per mile for an accurate reading. My speedometer would only be off by 1%.
If you check with your dealer you should be able to find out what tire and axle ratio options are available for the '97 Cherokee and build a similar chart.
Rob Chaput (rchaput at frontier.net) mailed me the following, which seems easier to deal with:
Old tooth count Old tire diameter Old axle ratio New tire diameter New axle ratio
The basic relationship is:
The 'old tooth count' can be obtained from a Jeep dealer's parts catalog (Catalog "97-98 TJ" GRP 21P.) The following table will provide some common configurations:
Axle Tire Size Tooth Part Ratio Count Number 3.0 30x9.5R15 28 52067-628 3.0 All P225 R15/R16/R17 29 52067-629 3.0 29x9.5R15, P225/75R15 29 52067-629 3.0 All P215 R15/R16/R17 30 52067-630 3.0 All P205 R15/R16 31 52067-631 3.5 30x9.5R15 33 52067-633 3.5 P225/75R15 34 52067-634 3.5 All P205 R15/R16 35 52067-635 3.7 30x9.5R15 34 52067-634 3.7 P225/75R15, P215/75R15 36 52067-636 3.7 P205/75R15 37 52067-637 4.0/ 4.1 All P225 R15/R16/R17 39 52067-639 " All P215 R15/R16/R17 40 52067-640 " All P205 R16/R16 41 52067-640
Tire diameter for metric tires is determined by the formula:
(((Size in mm./25.4) * aspect ratio) * 2) + rim diameter. For the P225/75R15 the calculation is as follows:
Size in mm. = 225, aspect ratio = 75, rim diameter = 15
D = (((225 / 25.4) * .75) * 2) + 15 = 28.29"
In my case, I wanted to upgrade to a 31x10.5R15 tire. Using the table, I determined that the 'old tooth count' for P225/75R15 tire and a 3.73 axle ratio (Dana 44-3) was 36. Plugging the other numbers into the 'ratio' formula one gets:
new teeth count = 36 * (28.29 / 31) = 32.85 = 33 (rounded)
The standard differential is a completely open differential. When you're going straight on a hard surface, it supplies equal traction to both tires. When you turn, the inside tire slows down. This is good for on-road driving, but if you're stuck, an open differential allows one tire to spin freely while the other is stuck.
A limited slip differential is built to alleviate some of these problems. Like the name implies, it limits the slip by sending some of the torque to the wheel that has the most traction. The Jeep Trac-Lok differential is an example of a limited slip differential. Without getting too much into details, a limited slip differential uses friction between the differential case and a series of clutch plates to achieve this goal. A limited slip differential provides civilized on-road performance and reasonable traction in off-road situations, but it may accelerate tire wear and require special lubricant for the differential.
A locking differential is useful for really complicated off-road driving situations, like rock crawling or lots of mud. An automatically locking differential works using a clutch that is engaged most of the time, but disengages when one wheel is traveling faster than the other (like when turning). This provides the best traction for off-road driving, but can cause squirrely handling for on-road driving, especially in slippery conditions (such as icy or snowy roads). Certain lockers are more forgiving than others, so it pays to check out reviews and ask people's opinions.
Finally, there are on-demand locking differentials. The best examples of these are the ARB Air Locker (an aftermarket item) and the Tru-Lok differential (standard on the Rubicon model). When they are disengaged, they behave like an open differential (or a limited slip, in the case of the Rubicon). The locking is actuated by a pushbutton on the dashboard. Then, the locker is engaged via an air hose or by other means. These are the most reliable lockers for mixing both civilized on-road driving and good traction for off-road driving. Unfortunately, they're also the most expensive.
This article from Jeepforum.com discusses differentials in more detail.
A winch is a valuable recovery mechanism for getting stuck in off-road situations. (Just look at what happened to Dennis Nedry in the movie Jurassic Park. Dinosaurs never had an easier meal.) Most winches sold these days are electrical winches, though the MileMarker winch is a hydraulic design that gets its power from the power steering. An electric winch takes its power from the Jeep's electrical system, which means that a heavy-duty battery would probably be a good investment. You will need a winch with at least 6000 pounds of pulling power to pull your Jeep out of a stuck situation -- smaller winches just won't handle the load, and they could prove to be dangerous. If you plan to get stuck really seriously, or fairly often, an 8000 pound or higher winch might be a better choice.
If you get a winch, a roller fairlead is a good idea, because it keeps the cable from binding as you are pulling it in. A remote control is also useful, and you will probably also want a pair of leather gloves, a tree strap, and a pulley. Pay attention to safety warnings when winching, because cables could snap. Keep bystanders away in case this happens.