Re: "RTI Revealed . . ."

["Bill Thebert" <bthebert@domain.elided>]

> Date: Mon, 10 Aug 98 10:00:05 PST
> From: jmbrodsky@domain.elided
> Subject: RTI revealed...
> 
> Curt, Darrel, et al,
> 
>         If this is truly the case, then Tom Mandera's 100WB ScoutII
ramped a 63 
> (63" on a 100WB) and my T'all ramped a 61.6 (74" on a 120"WB) so WOW,
> 
> How can these two trucks even be close?  

Joel:

I'm not sure I understand your question just right, but according to the
"established" RTI calculations, the two trucks *aren't* really close.

Ignoring the "25-degrees vs. 20-degrees" issue for the moment (and using
the numbers you provided),

Tom's Truck = 63/100 * 1000 = 630 RTI
Your Truck = 62/120 * 1000 = 517 RTI

If you arrived at the 74" number by "converting" your 25-degree score to
its 20-degree equivalent, then we should also do the same for Tom's score.
Your longer wheelbase actually works *against* you (somewhat unfairly, as I
suggest below) according to the conventional calculation method.

--------------------------------------------

All that said, I've given these calculations considerable thought, and here
are just a few observations that I have on the matter:

"Track Width" seems to me like it would play a bigger role in determining
one's score on the ramp than would wheelbase -- yet this isn't factored
into the equation at all.

Imagine for the sake of mental exercise lifting the left front tire on a
hydraulic platform -- straight up.  In the case of the RTI measurement, the
ramp is just a means to an end -- a convenient method of achieving a
specific vertical lift of one tire.

If we can accept as "given" that the ground is level, and that the rear
axle remains parallel to the ground (even though this is not *precisely*
true), then what we're *really* measuring here is the angular offset
between the front axle and the rear -- how many degees AWAY from parallel
we can force the two axles -- before the suspension reaches the limit of
its travel and a tire is forced to leave the ground.

If a vehicle with a 60" track width were driven up the ramp so that one
tire was 20.5" off the ground, the ANGLE of the front axle to the ground
would be approximately 20 degrees. The same 20-degree front axle
articulation would be present regardless of whether the wheelbase were 80",
100", 120" or 120 miles. I'm not sure I see why "long rigs" are
"handicapped" as they are, according to the customary calculation.

A vehicle with a track-width of 70", however, would need to be lifted (one
tire) nearly 24" in order to force the front axle into the identical
20-degree offset in relation to the ground (and in relation to the
earthbound axle). The wider truck *truly* has an advantage -- in that it
can go an extra 4 "vertical inches" up the ramp (regardless of ramp pitch)
before being forced into the same degree of articulation as the narrower
truck.

I've mused, therefore, that it might be interesting to "bag" all of the
linear measurements along the ramp's length, and merely put an angle
protractor on the front axle.  Measure how many degrees "apart" we can
force the two axles -- regardless of wheelbase, track width, tire pressure
inequalities, and any other variables -- before something leaves the
ground.  Measuring "axle angle" in this way would be a "great equalizer"
among rigs of different shapes & sizes.

Don't get me wrong here -- I surely don't lose any sleep over these
musings. <g>  But I do think that there's a fundamental "design flaw" in
the way Four Wheeler magazine started this whole "RTI Calculation" craze.

Regards,

"et al"
(Bill Thebert)