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Tech Talk - about Roll Diameter vs. Tire Deformation |
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(603) 226-DYNO |
Read about how roll diameter is tied to tire deformation, which limits the maximum safe speed and power transfer capacity of rolling-road chassis dynamometers.“Highlights of 3D FEA modeling analysis”Dyno Tech Talk is a compilation or copyrighted material, however, relevant web sites and forums are welcome to link to the Dyno Tech Talk index page. For additional information on this topic, read about Goodyear Tire's Dynamometer Test recommendations. Also see major sources of rear wheel horsepower losses as well as tire slippage - and consider using larger rolls with machined-in deep traction grooves. |
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Are your tires getting overly depressed and stressed? We are not talking about the kind of depression a psychologist might help with! Rather, we are asking if you might be excessively deforming your tires during chassis dyno pulls. A quick look at the accompanying (above right) illustration shows you what we mean.
Tires are engineered to run on the nearly-flat earth. At over 500 million inches in diameter, the earth's radius dwarfs that of any chassis dynamometer roll. Running a tire on a roll decreases the contact patch (traction) area while simultaneously deforming the tires shape. Worse still, the location of this this added deformation works its way around the entire tire with each revolution. That continuous flexing and unflexing cycle generates heat (energy) within the tire which means it is absorbing power from the engine. This is power that will never be delivered into the dyno for measurement.
Fortunately, as the dynamometer's roll diameter grows to match (or exceed) the tire's own, the degree of deformation falls quickly. So, rolls over five-feet in diameter are rarely warranted. The series of 3D FEA modeling analysis screen captures (below) help illustrate the relative deformation for a typical automotive performance tire against a range of increasing roll diameters.
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3D FEA
image of Tire Deformation |
Analysis Comments |
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Flat-earth slab (as a baseline) with a 24" tire shows that only
negligible deflection occurs in attaining its 5" long traction contact patch.
This baseline and the analysis below were generated with 1,000 foot-pounds of forward direction axle
torque. |
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8½" diameter unlinked tandem rolls with a 24" tire requires over 3½" of
vertical "strap down" (tire indentation and deformation) travel to wrap around
the equivalent (flat-earth) 5" long contact patch – which must all
be on the forward (working) roll! Notice the extreme-stress
hotspots (in red). The high strap-down forces required to transfer any serious
power are terrible for tire life. Even then traction is poor, and much of the full-power transfer goes away as
tire damaging heat energy. The lost power, measured in BTU, KW, or Hp units, is
not free – this is power that will never make it from the engine to the
rollers (or pavement)!
Note: If tandem-rolls are not linked (front-to-back) the rear set of rolls offer no load sharing capability. Therefore it is counterproductive to use rear biased strap force to restraint the tires against the back rolls, doing so only serves to deform the tire – without contributing to power transfer. This is worse than no rear roll at all! This class of roller assembly was originally designed for checking speedometers and certifying taxi odometers (at zero-load). |
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12" to 13" diameter linked tandem rolls with a 24" tire
requires about 2¾" of vertical "strap down" (tire indentation and deformation)
travel to wrap around the equivalent (flat-earth) 5" long contact patch –
with roughly 70% on forward
roll! Notice the high-stress
hotspots (in red). We consider a pair of linked 12" diameter tandem rolls
the smallest set that can be used for seriously testing even moderate
performance vehicles (under full-power).
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16" diameter roll with a 24" tire requires about 2½" of
vertical "strap down" (tire indentation and deformation) travel to wrap around
the equivalent (flat-earth) 5" long contact patch! This is better than the a
single 12" roll diameter for tire life traction and full-power transfer, and
about equal to a pair of linked 12" tandem rolls.
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20" diameter roll with a 24" tire requires about 2¼"" of
vertical "strap down" (tire indentation and deformation) travel to wrap around
the equivalent (flat-earth) 5" long contact patch! This is better than the a
single 16" roll diameter for tire life traction and full-power transfer, and
about equal to a pair of linked 13" tandem rolls.
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24" diameter roll with a 24" tire requires about 2" of vertical "strap down" (tire indentation and deformation) travel to wrap around the equivalent (flat-earth) 5" long contact patch! This is much better than the above roll diameters for tire life. And, traction and full-power transfer pretty much match what is available on highway pavement. |
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30" diameter roll with a 24" tire requires about 1¾" of vertical "strap down" (tire indentation and deformation) travel to wrap around the equivalent (flat-earth) 5" long contact patch! This is measurably better than the above 24" roll diameter for tire life. In fact, with the addition of our machined in traction grooves, traction and full-power transfer exceed what is available on highway pavement. |
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44" diameter roll with a 24" tire requires about 1¼" of vertical "strap down" (tire indentation and deformation) travel to wrap around the equivalent (flat-earth) 5" long contact patch! This is almost as good for tire life as running on a stretch of smooth and level concrete. In fact, with the addition of our machined in traction grooves, traction and full-power transfer easily exceed what is available on highway pavement and begin to match what is available from a well prepped drag-strip starting line. |
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60" diameter roll with a 24" tire requires less than 1" of vertical "strap down" (tire indentation and deformation) travel to wrap around the equivalent (flat-earth) 5" long contact patch! This is essentially equivalent for the tire life to running on a stretch of smooth and level concrete. And, with the addition of our machined in traction grooves, traction and full-power transfer exceed what is available from a well prepped drag-strip starting line.
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