All an RTI ramp is for is to compare different vehicles. The degree of the ramp is not material except that it should be such that all or most vehicles cannot reach the top. From that point on it is simply a tool to compare different vehicles.
Why a ramp? Well, it is easier to drive up something than to lift it with a jack, fork lift or big neighbor.
There is nothing magic about a particular angle, but remember it is easier to establish the angle than to keep up with the distance a wheel is off the ground. Here is an example of why the height is not important.
EXAMPLE (same ramp):
1) Veh 1 drives up the ramp til the wheel is 5 feet off the ground - no lets make it 7 feet off the ground. WOW!!! Big time flex, right?
2) Veh 2 didn't do so good. The wheel was only 2 feet off the ground.
Which one do you want to take on the trail???
A little more info:
Veh 2 is a Jeep CJ. Just kinda so-so on RTI, right? Veh 1 is looking better and better. Well, Veh 1 is 83 feet long. Now you see the importance of the relationship that the RTI index actually exemplifies?
The angle is really not important except that to make any comparison between vehicles worthwhile, the angle must be the same for each veh.
Actually you could get good comparative numbers using just the height and wheelbase of each veh, but then you would have to explain your numbers (give wheel base, height of lift etc) OR report your numbers in terms such as, "My rig ramps 1000 on a 27 degree ramp." Then someone else could compare his to yours by saying, "My rig ramps 1000 on a 29 degree ramp."
If you try to compare using numbers like, "Veh 1 ramps 845 on a 23 degree ramp and Veh 2 ramps 754 on a 25 degree ramp," can you make a reasonable comparison in your head? What you need is a constant. It can be 1000 or it can be the degree of the ramp. Using either of those you would come up with, "Veh 1 ramps 845 on a 23 degree ramp and Veh 2 ramps 884 on a 23 degree ramp," OR "Veh 1 ramps 1000 on a 19 degree ramp and Veh 2 ramps 1000 on a 20 degree ramp."
If you opt to use the 1000 constant, then it is easy to make comparisons when you don't even have a ramp. Just lift a wheel with a jack or a fork lift or whatever. Measure the height off the ground and the wheelbase of the vehicle. Then do theTrig (sine, cosine etc) and figure out the RTI as if the ramp was exactly as long as the wheelbase. You will need the Trig to figure out the angle of the imaginary ramp. Then you can say, "My veh ramps 1000 on a xxx degree ramp," and we can all ooh and aah over what flexy veh you have.
I didn't check the formula in the other posts, but the Trig is relatively simple and it is quite easy to find the formula if you have to go looking for them. Basically what it says is that if you know any two bits of information about a right triangle you can compute all the other numbers, because you always know a third number - the right angle - 90 degrees. There are 6 pieces of info about a triangle. The length of each of 3 sides and the size of each of 3 angles. If you know 2 of those - again actually 3 because of the 90 degree angle - you can figure out the remaining 3 measurements whether it is sides or angles or both.
The sine of an angle is the only Trig formula you need. The side of the right triangle opposite the 90 degree angle, the hypotenuse, is ALWAYS the longest side. The sine will ALWAYS be less than 1. To compute the sine you divide the side opposite the angle you are trying to determine by the hypotenuse. You will have to look at a sine table to find out what the angle for that particular sine is.
As a side note you might find the following interesting. When doing machine work, lathe, mill etc., a machinist offen has to machine metal at a particular angle. Say you have an angle that is critical to the half thousandith. You don't take a protractor and draw the angle with a scribe. Not nearly accurate enough. You use a sine plate. Sine plates have perfectly parallel sides and are either 5 inches long or 10 inches long and have a perfectly round bar mounted on the underside of each end exactly 10 inches apart. The sine of the angle you need to make will be carried out to 4 decimal places - 1/10,000 of an inch. If the sine is .4597, and the sine plate is 10 inches long, that means that the side opposite the angle you are trying to create will be 4.597 inches long. You put guage blocks under the end of the sine plate that are exactly 4.597 inches tall, and wallah you have produced the angle and to a much greater degree of accuracy than any protractor could ever make. Attach your metal to the sine plate, bolt the entire thing down to you mill table, and cut the angle.
With guage blocks capable of .0001 measurement (standard blocks, nothing fancy), you can create an angle accurate to 100ths of a second of a degree. In the above example, sine .4597, that is an angle of 27.36775074 degrees or 27 degrees, 22 minutes, 3.9 seconds, and an average or below average machinist can set this up and produce this angle using a sine plate and guage blocks quite easily. Try that with your protractor. If you are good enough to get within a half of a degree, the sine plate will produce results that are 18,000 times more accurate if you carry the sine out to the 5th place which is what standard guage blocks will duplicate.
Doug '97 TJ
Creator of the CBrack
CBrack.com