Dynomometer measured HP limits
01-20-2006, 03:12 PM
#1
Dynomometer measured HP limits
What wheel Hp can I dyno?
Someone wants to run 1,000 Hp on 8” rolls! Foolish? No, its just inexperience regarding these factors:
1) Roller diameter is the major power limiter for chassis dynamometers. As roll (or tire) diameter decreases, the effective contact patch shrinks. The smaller the area, the higher the shearing loads on the reaming tread contact. Worse still, small rolls apply more PSI per pound of vehicle weight. That pressure deforms the tire’s sidewalls. Combine high Hp and speed with small rolls and this deformation can overheat and destroy tire integrity!
Approximate guidelines: 5" rolls top out near 60 Hp, 8” rolls handle about 300 Hp @ 120 MPH, 12” = 600 Hp @ 150 MPH, 16” is about 750 Hp @ 160 MPH, 24” gets to 1,000 Hp @ 180 MPH, 30" is good for 1,200+ @ 200 MPH, and for over 1,600 Hp... plan on 40+” diameter. Avoid rolls under 30% of tire diameter for high speed or Hp. Capacity improves significantly as a roll increases towards the tire’s diameter. Beyond that, expect less dramatic improvements.
2) Tire to roller friction is the next biggest factor. Street tires on steel roll can’t match the high coefficient of friction of race slicks on warm asphalt. However, by machining a special straight groove pattern into the rolls, it’s possible to greatly reduce slippage.
3) Inertial resistance provide some (or all) of the Hp load during dyno acceleration. For chassis dynos with an absorber, roll inertia should simulate about 30% to 100% of the actual vehicle’s mass (thus reducing demands on the dyno’s speed controller).
While inertia only dynos do not have an actual Hp capacity per-se, they need excess inertia to make tests last more than a few seconds! Unfortunately, inertia alone is inadequate for steady state tests, serious engine development, or troubleshooting. (Which is why you see inertia only dynos primarily at spectator “shows”)!
4) A dynamometer’s absorber handles the entire Hp load at steady state. Figure that 90% of the Engine Hp gets into the rolls. Water brakes dissipate the Hp heating water, requiring at least 5 gallons a minute for each 100 Hp. Air cooled eddy-current absorbers loose about half their capacity, as they heat up, during the first few minutes of high Hp loading (see the manufactures’ curves for each specific absorber’s characteristics).
Someone wants to run 1,000 Hp on 8” rolls! Foolish? No, its just inexperience regarding these factors:
1) Roller diameter is the major power limiter for chassis dynamometers. As roll (or tire) diameter decreases, the effective contact patch shrinks. The smaller the area, the higher the shearing loads on the reaming tread contact. Worse still, small rolls apply more PSI per pound of vehicle weight. That pressure deforms the tire’s sidewalls. Combine high Hp and speed with small rolls and this deformation can overheat and destroy tire integrity!
Approximate guidelines: 5" rolls top out near 60 Hp, 8” rolls handle about 300 Hp @ 120 MPH, 12” = 600 Hp @ 150 MPH, 16” is about 750 Hp @ 160 MPH, 24” gets to 1,000 Hp @ 180 MPH, 30" is good for 1,200+ @ 200 MPH, and for over 1,600 Hp... plan on 40+” diameter. Avoid rolls under 30% of tire diameter for high speed or Hp. Capacity improves significantly as a roll increases towards the tire’s diameter. Beyond that, expect less dramatic improvements.
2) Tire to roller friction is the next biggest factor. Street tires on steel roll can’t match the high coefficient of friction of race slicks on warm asphalt. However, by machining a special straight groove pattern into the rolls, it’s possible to greatly reduce slippage.
3) Inertial resistance provide some (or all) of the Hp load during dyno acceleration. For chassis dynos with an absorber, roll inertia should simulate about 30% to 100% of the actual vehicle’s mass (thus reducing demands on the dyno’s speed controller).
While inertia only dynos do not have an actual Hp capacity per-se, they need excess inertia to make tests last more than a few seconds! Unfortunately, inertia alone is inadequate for steady state tests, serious engine development, or troubleshooting. (Which is why you see inertia only dynos primarily at spectator “shows”)!
4) A dynamometer’s absorber handles the entire Hp load at steady state. Figure that 90% of the Engine Hp gets into the rolls. Water brakes dissipate the Hp heating water, requiring at least 5 gallons a minute for each 100 Hp. Air cooled eddy-current absorbers loose about half their capacity, as they heat up, during the first few minutes of high Hp loading (see the manufactures’ curves for each specific absorber’s characteristics).
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