off road vehicle

Quite a bit has been written on steering geometry:
http://www.google.com/search?q=race+car+steering+geometry

And of course, there are numerous books available on race car engineering: 
http://www.google.com/search?q=race+car+engineering+books

I cannot see the problem. The track rod can be in front of the front wheels. But the cranks on the king pins still need to intersect on the rear wheel axis for ackerman geomety to be effective.

"I cannot see the problem. The track rod can be in front of the front wheels. But the cranks on the king pins still need to intersect on the rear wheel axis for ackerman geometry to be effective."

The problem is usually that the arm-to-track rod (terms vary) joint is too close to the tires, or even inside them. Extending them and curving around the tire will work, but that could make them vulnerable to damage. On the other hand, Ackerman geometry is not a true requirement. The Triumph TR-4 (ca. 1962 - 65, and the TR-3Bs before it) had the forward-pointing arms, but angled inward as though designed for behind-the-pivot use. This was "anti-Ackerman" steering. But if you're driving hard enough to lift the inside front wheel [assuming rear-wheel drive], it means that the outside wheel has been moved through a greater steering angle than would have been the case with Ackerman, so steering is quicker. I think a proper Baja drive should reach this condition often enough to adopt the same strategy; on loose surfaces, the issue is less critical anyway, I'd think, because the sand will move to comply with steering.

i m not getting some of ur ans. first wat is this arm to track rod joint . secondly the problem is the vehicle i m thinking of is a four wheel drive. even i know that there is no such problem in violating ackerman geometry , but i m not able to get how to initiate its calculation . the books i have gone through r not that helpful regarding specific calculations.my rack needs to be placed ahead of front axle which is through differential . now it seems like davis design but then how to calculate the terms like steer angle, stopping distance , turning radius, etc.. i think i need to fix one data first according to my need .

"First what is this arm to track rod joint?" In your case, it will be the joint at the ends of the rack extensions, where they connect to the levers [= lever arms] off the kingpin pivot (assuming that there is a ball joint between the rack and the extensions, and a conventional tie-rod type connection at the outer ends). Same as "tie-rod ends", except that you don't have a tie-rod, since the rack & extensions serve that function.

"Secondly, the problem is the vehicle I'm thinking of is a four wheel drive." In a way, this further supports the idea that you needn't worry about Ackerman; four-wheel-drive vehicles with locking differentials - common for Baja, I'd think - won't permit the wheels to turn appropriately for Ackerman when in locked mode.

I believe you are correct to assume that the way to start is to "guess" at a few dimensions, and begin with them; you can later revise the starting dimensions, and see how much this influences the results. If the results don't change much, you should be OK, but if they're very sensitive to input values, you may have to re-think the initial setup. Sorry, but I'm not familiar with Davis design, so can't help there. Good luck!

ok, 1st thing, you need the tie-rods to come as close together in the centre of the vehicle as possible, its no use grabbing a steering rack from a car and using it in the Off Roaders, having the tie rods coming from the ends of the steering rack are prone for problems, ie, short steering shaft, and long suspension arms, if the arms are longer than the tie rods that come out the end of the rack, you'll have steering issues thru the suspension movement, I think this is Ackerman angle too.

once you have worked out your rack mechanism (I know a few guys still running a steering box, worm & gear) how are you going to add hydraulics to make it powered? seperate ram, or included?

Seperate Ram meaning, a ram tied to the steering yoke and on to the body some place.

Included, you bolt the rack in place, plumb the hydraulics and its race ready.

turning radius, what are you looking for here? your radius could be limited to the amount of angle you get on the wheels, as against the length of the frame.

Why do you want something efficient for Off Road? its basically "Point & Shoot" type steering.

The steering tie rods can be leading or trailing, or infront of the front axle, or behind.

Some have 1 complete turn (or less) lock to lock... for me, this is crazy, imagine how little of a turn to make a big reaction, on a bumpy track, at well over 160km/h and a slight turn gives you a new direction, and the counter steering on corners (this bit in the cartoon "CARS" cracked me up, - To turn left, you turn right - )