RTI Ramp

Well, our club finally finished making it's 30-degree RTI ramp. Staci did an excellent job fabricating the thing. At its inaugural meeting, nine of us took a run at it. The worst performances were put in by stock-ish YJ's (who shall remain nameless), ramping as low as 273. The best score was 635 from Aaron Kilgariff's CJ-5, with Greg Mattern's heavily-built TJ ranking second at 599.

For those not familiar with an RTI (Ramp Travel Index) ramp, here's the deal. It basically measures how well your axles articulate. This is important, especially for those without dual lockers, because if one of your wheels is airborne, you won't be getting full power to all your tires. The more your axles can articulate, the more likely you are to keep all four wheels in contact with Mother Earth, and hence the more uneven terrain you'll be able to cross.

You test your articulation on an RTI ramp by driving one wheel up the ramp while leaving the other three on flat ground. You then measure the distance that you traveled from the base of the ramp to the point on the ramp closest to the center of your wheel (NOT the point directly below your wheel center). You then divide that distance by the vehicle's wheelbase, since longer vehicles can generally go farther than shorter ones and we prefer to even the playing field a bit. Finally, you multiply that number by 1000 because somebody thought it would make the numbers prettier. The basic formula is:

(distance up ramp)

---------------- X 1000 = RTI score

(wheelbase of vehicle)

For example, Aaron's CJ-5 traveled 53 inches up the ramp. To get his RTI score, you divide 53 by his vehicle's wheelbase, which is 83.4 inches, to get 0.63549... You then multiply this number by 1000 and ignore any trailing fractions to get a nice, round 635 RTI score.

If you're flexible enough to drive all the way up the ramp until
your back wheel (assuming your front wheel is on the ramp) is
touching the base of the ramp, then you've ramped a perfect 1000.
It is possible to score even higher than 1000 by continuing on up
the ramp, but you must make sure that your rear tire runs *beside*
the ramp, not *up* it. If your rear tire runs up the ramp, then
the angle between your axles isn't changing any more, so neither
should your RTI score.

Of course, the angle of the ramp will make a huge difference in the score you get. The higher the angle, the lower your score will be. The original RTI ramps were only 20 degrees, but too many people were getting 1000+ scores, so folks decided to make it more challenging by raising the angle to 23 degrees or 25 degrees. Again, many of the more flexible rigs were still getting 1000+ scores. 30 degrees is the steepest ramp I've ever seen, and I'm pretty sure it'll stay that way for a while.

Many folks ask if it's possible to convert a score on a ramp of one
angle to an equivalent score on a ramp of a different angle. Yes,
it is. If you got a score **A** on a ramp of angle **g** and
you want to find your score on a ramp of angle **h**, then you
need to multiply your original score **A** by the **sine of
angle g** and then divide that by the **sine of angle h**.
Essentially:

sin(angle of original ramp)

------------------ X original score = new score

sin(angle of new ramp)

For example, we know that Aaron scored 635 on the 30-degree ramp. To compute his score on a 23-degree ramp, we'd divide sin(30) by sin(23) to get 1.280, then multiply that by his original score of 635 to get his new score of 812. Similarly, sin(30) divided by sin(20) is 1.462, which means his score on a 20-degree ramp would have been an impressive 928.

In layman's terms, this means that if you want to convert scores between ramps of different angles, you only need to multiply the original score by a fixed constant. Using the table below, you can convert score on a ramp with the angle listed in the first column to scores on a ramp with the angle listed in the second column by multiplying the original score by the number listed in the third column. To move the other direction, divide by the number instead of multiplying. It's really simpler than that sounds.

Original Ramp Angle | New Ramp Angle | Multiply Score By |
---|---|---|

30 | 20 | 1.462 |

30 | 23 | 1.280 |

30 | 25 | 1.183 |

25 | 20 | 1.236 |

25 | 23 | 1.081 |

23 | 20 | 1.143 |

Now, if you want to get picky, the radius of your tire also comes into play, since the angle off vertical from your hub to the closest point on the ramp (which will be a line perpendicular to the ramp) will also be equal to the angle of the ramp itself. This makes the conversion much hairier, and there's no longer a fixed mulplicative factor that converts any arbitrary score between ramps of different angles. Fortunately, this factor is generally small enough to ignore. For instance, taking this into account means Aaron's score on a 23-degree ramp would be lowered by 2.1 points down to 810. That's less than the error caused by a 1/4" difference in distance traveled up the ramp. I'm starting to sound like a math text book, so I'll stop now.

Back to the Flatlanders Jeep Club home page

last updated 16 Mar 99 Obi-Wan (obiwan@jedi.com)