This is a very delicate area only because your options are very limited doing it the cheap way. I do agree with 1bolt here that the weight of the roof line is very light. I wouldn't be too concerned with that if your XJ is a 2door early model with the fiberglass tailgate. Like with 1bolt says about the engine is true also.
The only thing is that lowering the engine might not do as good as lowering the chassis is because of clearance issues you will get with lowering the suspension. Your not going to be able to get the body to go as low. Now I'm going to break down this post hopefully to make it more understandable.
1. Engine
Lowering the engine or raising the engine is a viable way to adjust handling characteristics of any vehicle. The engine being the only large mass that is the highest on the chassis itself has a profound effect on weight distribution in scenarios of turning, less so in forward and backward but still a large effect. The problem with this method is simply because of the type of vehicle we have here to use, a vehicle that has a solid front axle or beam as the same with the rear. Lowering the engine like I mentioned above can cause positive benefits, obviously lowering the center of gravity of the vehicle overall but also reducing the ability to lower the chassis.
Now I believe in my understanding of physics that this option will hurt you more in the long run only because of this; It's a little hard to explain, the actual center of gravity is not the geometrical half of the vehicle, It's actually the geometrical center of the wheels. The wheels center is the load baring plane of the weight of the vehicle. In this case being a Jeep Cherokee what you actually want to do is raise the engine. Bare with me on this, I'm assuming that your going to end up using a 4WD XJ rather than a 2WD XJ (If your going to use a 2WD XJ You will be far, FAR better off converting your front suspension to independent suspension.) to do this project of yours. This is why, when you lower a Cherokee or a Comanche you can only go so close to the engine anyway with your axle, which would be about 2", or 1-1.5" if your brave. Which will allow you to lower the chassis more because the raised engine just allowed more travel.
2. Chassis
Now with the above paragraph in mind, I would like to explain the benefit of a raised engine in this application according to the chassis. Raising your engine will allow for more room to lower in the relation of the chassis to the wheels. This is the important part, like I said before, you want to get any weight of the vehicle as close to the wheels as possible. Being the fact that the rest of the car weighs more than engine itself, it and the fact that the chassis has a more direct influence on handling would benefit you more to have the most weight as close as possible to the ground. So in a simpiler terms raising the motor will allow you to lower the overall weight of the vehicle more than lowering the motor or leaving it.
3. Suspension
First off if you remember reading, I said that if a 2WD Jeep is being used it will be a lot better to convert to independent suspension. A few Jeeps did come with an independent front. So that would be best if that's your route.
The suspension on these things are ancient, no denying that BUT if done right it can come close to competing with independent and can be better than older independent designs. First it is known that solid axle driveline works a little different with suspension than independent does. Clearly as it's name states its independent, each wheel doesn't effect the other wheels in direct relation to each other aside from the indirect relation of weight over the chassis. The axle on the other hand is the opposite, what one wheel does the other is trying to do the opposite. Now the tricky part, the way the chassis sits on top of the axles has a much different physics involved. The best way to describe it really is that it's like driving on a mono spring.
The fact that the front suspension has a 4-link setup helps a great deal in stability rather than leaf springs like the rear setup, I'd say the front is more like a semi-independent system. To take advantage of this you must stiffen it up as much as possible that you can bear because it gets more rough the lower you go because of the lack of suspension travel you will have. The best I find is finding a pair of rising rate springs. These are good for driveability and performance when you want it, you'll won't feel the little bumps but the big ones though you'll FEEL them for sure. On top of that get stiff shocks, not too stiff but a little more than stock would have.
The rear in this case it would be suficiant to do the axle overleaf conversion. The only work really is to weld up shock mounts closer to the center of the axle to allow room for the axle to go on top. The shock mounts are basically in the way. I think I had a post a while back showing pictures of what I mean and what needs to be done. Another option is a 3-way or 4-link (like the front) setup in the back. Lots of work to retrofit but can introduce more tuneability and create a more stable base.
4. Wheels
Wheels and tires obviously is going to do most of the work where grip is concerned and choosing the right ones is important. You can go with smaller wheels to reduce the front profile of the vehicle further and reducing the drag coefficient overall. The problem here is that you can only go so small because of the brakes and also major readjustment of gears and also the availability of good tires. 14 inchers with donuts is not going to help you at all and steam roller tires to makeup for bad quality rubber just introduces more drag. For the latter, too big will increase your coeff. drag and also raise your center of gravity in relation to the track width of the wheels.
For simplicity sake I'd say stay within the 16-18" diameter range and 6-8" in width of the wheels. With these sizes you are more in range of higher quality road going tires. Also, pick a tire size appropriate for the wheels you choose, for a Jeep 45 series would be bare minimum. It would probably be best to get 50 series tires as it will help with absorbing small bumps better. As you get higher in the numbers like (60 65 70 series) the more tire roll you will get because the carcass is taller. Lastly is the offset, this is where people get it wrong a lot of the time. What you want is a offset that is as close to center as possible. Forget the super deep dish wheels there no good for serious on road as well with radical +50 offset wheels too. Take a look at race cars, 99% of them are running wheels with lots of offset but not too much, especially touring cars and GT GT1 GT2 GT3 where most of there tuning is very dependent on taking corners as fast as possible without ruining the tires. Even Indy and Formula1, they don't have deep dish wheels to make the cars wider. Deep dish wheels are no good two main reasons:
1. Weight applied by the vehicle itself is not equally distributed over the entire wheel and tire.
2. It increases the turn throwing arc of the wheels. Which causes chassis instability. (Which we are trying to get rid of as much as possible, cause it's a Jeep)
Overall on this post that I wrote hopefully when you or anyone trying to go the road route for a Jeep will keep this in mind in whatever you make modifications to your Jeep for road racing or want to make a better road mannered Jeep for your daily driver. The best option is to research the matter and make decisions on what is best, try not to go into 4x4 boards or similar try to find racing sites and boards where people do there own racing.
Any questions about this just ask, I hope I was clear enough for the rest of you guys, my mind is all over the place today, little hard to think. I woke up not to long ago.
:cheers:
