A. Inside rim lip (inside edge)
B. Dished center (wheel mounting surface)
C. Inside valley edge
D. Center weld point
E. Outside valley edge
F. Outside rim lip
G. Inside diameter
H. Outside diameter
I. Center of axle to outside edge of caliper
X. Back bell depth (measure from A to B)
Z. Front bell depth
There is also offset. Offset is the distance from the rim's mounting surface to the centerline of the rim. Positive offset means the mounting surface is moved towards the outside of the rim. Negative offset places the mounting surface closer to the axle. To figure a rims offset, meaure the rims overall width, subtract the backspace & divide by 2.
To measure backspacing, you need to remove the wheel and lay it face down on a flat surface. You will need to place a straightedge across the inside edge of the wheel then use a ruler to measure the distance from the straightedge down to the wheel mounting surface.
Here is a side by side comparison of the Factory rims vs Rock Crawlers. You'll also notice that there are BFG AT's tires on the Factory rims and BFG AT/KO's on the Rockies. Both are 31x10.5 & the only noticeable difference are the sidewall.
As you see, I have the tires mounted on all of the rims. This will not cause any problems for my straight edge because I managed to cut it perfectly from rim lip to rim lip. However...this is best measured with the tires unmounted.
Now keep in mind that my staight edge is a 1/2 inch think block of wood. So we want to look at the bottom of the straight edge when using the ruler...follow the red line! In the photo below you'll see that the Canyon rims have 5 inches of backspacing.
I use the same technique to measure the Rock Crawlers and as you can see they have 4.5 inches of backspace.
Here is a set of the ol' Saw Blades that are 15x7 & have 5 1/4 inch of backspace.
It's simple, the lower the backspace number is, the more the wheels will stick out and clear anything they might rub on. Let me tell you about my set-up.
I have a 2" Budget Boost on my Grand. At the time I mounted 31x10.5 BFG-AT's on my Canyons and they rubbed like hell whenever I made hardcore left & right turns. The rubbing got worse when ever I went off-roading. ( Duh!! I knew this and did it anyway.) If you're looking to go cruising around the mall then this set up will be fine. Just pray that your mallrat buddies aren't around when you turn those wheels. Shortly after, I mounted the 31's on the Rockcrawlers and with a 'lil bit of trimming to the underside of the front bumper, I never had any problems what so ever. Today I have 31x10.5 BFG MT's mounted on those Rockcrawlers and still have no problems. (Soon I'll ditch the 2" pucks for Clayton Long Arms.)
Spacers are another item that many off-roaders use as it creates the effect of having a rim with a negative offset. They're a good item for those who want to keep the factory rims with bigger meats and they are awesome for making up the difference in axle length, such as using the Ford 8.8 axle in our Grands. Both spacers or rims with less backspace will let you run bigger meats since the mounting surface is closer to the hub (Thus the rim is pushed outside the vehicle). However, many will tell you that this also increases the stress to the wheel bearings and such.
Just keep in mind that this is only the basics. Obviously the more meat you put on those rims the deeper the backspacing & higher the lift must be. Just use the search feature of this fourm and read about what everyone else is using, then use your own judgement.
More Info on Transfer Cases
# Chevy/GM New Process 203
This isn't a bad transfer case, but it could be better. Like other chain drive units, the 203 is susceptible to chain stretch, but it won't stretch easily. The low gear ratio is a not the best at 1.98:1. This unit has a right-hand drop. This transfer case, which was produced from 1971 to at least 1980, is a full time unit. There are kits available from various manufacturers to turn this transfer case into a part-time unit. Like the other New Process transfer cases listed here, there's also Dodge and Ford versions of this unit.
# Chevy/GM New Process 205
This very strong cast iron case has all gear construction. It first came out around 1971, and lasted at least until 1990. It was available in both married and divorced styles, both of which had a right-hand drop. The low gear ratio isn't the best at only 1.98:1.
# Dodge New Process 203
This isn't a bad transfer case, but it could be better. This chain drive unit is susceptible to chain stretch, but it takes a lot of abuse to do that. The gear ratio is a not-so-great 1.98:1 and a right-hand drop. This transfer case, which was produced from 1971 to around 1980, is a full time unit. However, there are kits available from various manufacturers to turn this unit into a part-time transfer case.
# Dodge New Process 205
This is possibly the strongest transfer case ever put in Dodge light trucks, with its all gear construction in a cast iron case. It first came out around 1971, and lasted at least until 1980. It was available in both married and divorced styles, both of which had a right-hand drop for the front driveshaft. The low gear ratio isn't the best at only 1.98:1.
# Ford Dana 20
The Ford Dana 20 was produced from 1966 to 1977, and was used in the early style Bronco. Some appear to have made it into a few 1/2 ton (F-100) trucks, but this was not a standard. The very early 1966 model used a different shifter than the standard T and J shifters of the later transfer cases. From 1966 to mid 1973, the T shifter case was used. After that, the J shifter type was used. The J shifter type uses a shift pattern that looks like a backwards J. The T shifter uses a straight pattern, as does the early shifter. The early shifter (very rare) is rather hard to change gears with, and thus Ford/Dana changed the design.
The Ford Dana 20 design has a centered rear output, left-hand front output, and an all-gear design (read this as "strong and hard to break"). The case is made of cast iron, and a sheetmetal plate covers the bottom. This transfer case has two shift rails, one for low, neutral, and high on the front output, and one for low, neutral, and high on the rear output. A pair of metal lockout pins (or "pills") in tubes drilled between the holes for the rails work in conjuction with slots cut in the sides of the rails to provide only the gearing combinations of 2 high, 4 high, neutral, and 2 low. (These are put together by rear high/front neutral, rear high/front high, rear neutral/front neutral, and rear low/front low, respectively.) A linkage piece connects to the end of both shift rails, and then to a shift lever. This linkage rocks back and forth when the lever is moved, as the rails and lockout pins control the rail movement. The high range gear ratio is a straight 1:1, while the low range gear ratio is 2.48:1.
Twin-shifter setups on this transfer case put a shift lever on each shift rail. Most also recommend/require the removal of the lockout pins. This allows for some new gearing combinations, but the driver needs to be careful. The rear low/front neutral is sometimes nice and is the gear combination that most folks are after. If something's happened in the rear of the rig, the front high/rear neutral or front low/rear neutral might be useful. The gear combinations to avoid are rear high/front low and rear low/front high. If all four wheels have traction, these last two gear combinations will either kill the engine (at best), or break something in the drivetrain.
# Ford Dana 21
Definitely not one of Ford's better ideas, this was a single-speed transfer case (in other words, it has two high, neutral, and four high). This transfer case was used only in in some 1969 to 1976 F-100's. Its spline count and bolt pattern differ from the standard Ford stuff. The case on this unit is notorious for flexing and breaking gears and shafts. Parts are hard to find. This transfer case also uses a different input spline and bolt face that the rest of the Ford stuff. I'd recommend avoiding this transfer case; for less bucks and less trouble, there's better options.
# Ford Dana 24
This is a fairly uncommon transfer case. It's a two-speed, divorce-mounted transfer case that was used from 1960 to 1973. If you've got one of these, and it starts having problems, most likely it's the bearings. Some folks have a very high opinion of this unit, and others think that upon its breaking, it would probably be easier to replace the Dana 24 with a divorce mounted NP205 than to find parts for it.
# Ford New Process 203
This isn't a bad transfer case, but it could be better. It's a chain drive unit, so it is susceptible to chain stretch, but it takes a lot of abuse to do that. The gear ratio is a not-so-great 1.98:1, but it does have the Ford standard input and left-hand drop. This transfer case, which was produced from 1971 to 1980, is a full time unit. There are, however, kits available from various manufacturers to turn this unit into a part-time transfer case.
# Ford New Process 205
This is the ultimate beef for a light truck transfer case. It first came out around 1971, and lasted at least until 1980. The Ford version uses the standard Ford input and a left-hand drop for the front output. It's of an all gear design, with a cast iron case. The low gear ratio suffers a bit at being only 1.98:1. This unit will not be the weak link in your drivetrain.
# Ford New Process 208
The 208 is an aluminum cased, chain driven unit. It has the standard Ford input and left-hand drop. This unit has a nice low range ratio, at 2.72:1. This transfer case was supposedly used from 1980 to about 1982 (when the Borg Warner transfer cases supposedly took over), but I've got one of these in my 1984 Bronco.
# Ford Borg Warner 1345
This transfer case is chain driven, and has an aluminum case. It's pretty similar to the NP208 in those respects, as well as the fact that it has the same 2.72:1 low-range ratio. Ford started using this in 1980, and discontinued its use in 1988, when it was replaced with the BW 1356.
# Ford Borg Warner 1356
The 1356 was first used in 1988 or 1989, replacing the BW 1345. Like its predecessor, it's chain driven. However, it has a stronger, magnesium case.
There are three flavors of this cast-iron transfer case in the Jeep line. They are designated by the size of the intermediate shaft. This all-gear transfer case was used from 1941 to about 1971, when it was eclipsed by the Dana 20. These cases are right-hand drop, and the rear output is also to the right, in line with the front output. These transfer cases all had two shift levers until mid 1968. One lever was for high/low range, and one for engaging or disengaging the front drive shaft. Overdrive units and PTO units are still available to bolt onto the back, directly behind the input shaft location. The input facing of these transfer cases is the 5-bolt "Texas" bolt pattern. One other interesting feature of these transfer cases is that they came with a drum-type emergency brake mounted to the rear output.
# Jeep Dana 20
This right-hand drop, centered-rear output transfer case was used from about 1972 until 1979. These cast iron cases can easily be converted to dual shifters.The case is so close to the Dana 18's that the internals of the 18 can be swapped into them (take heed that this changes the location of the rear output). Hoever, you cannot put a 20's internals into an 18 without using a dual-shifter setup (this is due to the difference in the cases; the 20's has the facilities for a lockout pin that allows the usage of a single shift lever). These cases can easily be converted to dual shifters. Like the Dana 18, the input facing of these transfer cases is the 5-bolt "Texas" bolt pattern. Despite the dissimilar cases, the internals of the Jeep case are close enough to the Ford Dana 20's that some folks swap the Ford gears in to get a lower low-range gear ratio (some minor grinding is required, however, to fit in the physically larger gears).
# Jeep Borg Warner Quadratrac/13-39
Used only from 1976 to 1979 in CJ-7's with automatic transmissions, this full-time unit has an aluminum case and a chain drive. An additional feature of this unit is the right-hand offset rear output. This unit isn't bad, but there's a lot of other choices available that don't have a chain or an aluminum case.
# Jeep Dana 300
This transfer case came on the scene in 1980, and replaced the Dana 20. It was used up thru the end of the CJ's, in 1986. This 23-spline case only comes in a right-hand drop, version and has a 6-bolt, circular input, that is bolt compatible with the NP231 transfer case that came in YJ's. This is a very stout case (all gear driven), and has great aftermarket support in the way of lower gear sets and dual-shifter conversion kits.
# Jeep New Process 207
The Jeep YJ debuted in 1987 with this as its transfer case. This case is was only used for part of the 1987 model year. This is a chain-driven transfer case, that has a left-hand drop for the front driveshaft and a centered output for the rear. This unit is fairly rare.
# Jeep New Process 231
The Jeep YJ started using this transfer case during the 1987 model year. This case is still used in the successor to the YJ, the new Jeep TJ. This is a chain-driven transfer case, that has a left-hand drop for the front driveshaft and a centered output for the rear. It uses a planetary gear system for low range gearing. (It is rumored that some of the planetary and shaft components from the stronger NP241 can be swapped into this case.) This case apparently only came from the factory with a slip-yoke rear output. This slip yoke is considered to be a weak point, and various manufacturers offer a variety of kits to convert the rear of this transfer case to the more traditional fixed yoke. The input spline count and bolt face is the same as the Dana 300, so if the owner is willing to swap to a front axle that has a right-hand pumpkin, the Dana 300 can be bolted up in place of the NP 231.
# Jeep New Process 242
This transfer case is the "bigger brother" of the NP 231. Like the 231, this transfer case is chain-driven, left-hand drop, and uses a planetary gear set for the 2.72:1 gear reduction. It has been around since at least 1979. In addition to being used in the bigger Jeeps (such as Grand Cherokees), it is the transfer case used in AM General's Hummers.
# Scout Dana 300
This transfer case was only used in 1980, on the last of the Scouts. (These vehicles are most often identified as being the only ones with rectangular headlights.) This transfer case is basically identical to the Jeep Dana 300, in that it's a 23-spline case with a right-hand drop. However, where it differs from the Jeep version is that the bolt interface uses the "Texas" bolt pattern, and thus this transfer case will bolt in directly in place of a Jeep or Scout Dana 20. These can be found, but not easily, as they're a much sought-after upgrade unit for the Jeep Dana 20, and they were only produced for one model year.
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