All basses are different. But they generally fall into 2 categories, those with a lot of room to mount a trem and those with just a tiny thin area to mount a trem. But its much more than that. The area behind the pickup may have a lot of space, like a Fender P Bass does as in the picture below, but would still need a rearward facing roller configuration. The bass shown in the picture has a fixed bridge on it but it shows the "Intonation line" very clearly (marked with red). That is why we use it in the example below. As you can see there is plenty of space to mount a trem but look closely at the round saddle risers in the middle of each saddle housing....... see how if you "connect the dots" (saddle risers) they are generally in line (Click image to enlarge). THAT is the intonation  line which you must center the rollers of a trem over. So if you took a forward roller trem and lined up the forks after they are exactly halfway extended (so you have fork adjustment in and out equally for exact string intonation later during setup), over that intonation line, the body of the trem would hang over the back of the Bass. NOT GOOD! Now if you took a rear facing roller configuration and did the same thing, it would not hang over the end of the Bass because the rear facing rollers allow you to move the entire trem forward so now it will fit nicely. Now the second picture shows how much room is saved by using the rear (left trem) vs. the forwards (right trem).....

Note the inside forks of both versions is extended halfway out. This allows for adjustment both ways (in and out) for the correct intonation during final setups. There is approx. 1.200 inches (just short of an inch and a quarter) between the 2 extended forks of extra clearance when you compare the two in a tight situation. That is why you sometimes need the rearward version. To put it simply...   if you take the forward versions rollers and place them over the red line...well you can plainly see the trem is to long for that type of placement and would extend over the back edge of the bass. In this case...go rearward. It's as simple as that. No great mystery. Just don't over analyze the situation. FYI: you need to have 1.650  inches clearance or more from the intonation line to the back of the bass to use the rearward version properly. Anything less and our rearward trem is just to big to fit there.

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