How to calculate max velocity of a vehicle from torque and RPM?

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Discussion Overview

The discussion centers around calculating the maximum velocity of a vehicle using torque and RPM, with a focus on vehicle dynamics simulation. Participants explore various factors affecting velocity, including gear ratios, power losses, and the impact of drag and rolling resistance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant seeks to calculate maximum velocity from given torque and RPM, noting the challenge of accounting for losses at constant RPM.
  • Another participant suggests that drag and rolling resistance are significant losses that need to be specified for accurate calculations.
  • Power loss due to drag is described as varying with the cube of speed, while rolling resistance is approximated with a coefficient of ~0.01.
  • Multiple formulas for calculating velocity are proposed, including using engine speed, gear ratios, and wheel circumference, with some participants questioning the necessity of torque in these calculations.
  • Participants discuss the importance of correctly interpreting gear ratios as reduction ratios for speed calculations and the need to convert RPM to a per-second basis.
  • Concerns are raised about unrealistic velocity outputs in shared spreadsheets, prompting discussions about the accuracy of input values and calculations.
  • One participant mentions the need to adjust values for air resistance and tire resistance to achieve more realistic results.

Areas of Agreement / Disagreement

Participants express a range of views on the methods for calculating maximum velocity, with no consensus on the best approach or the accuracy of the shared spreadsheet data. Disagreements arise regarding the interpretation of gear ratios and the relationship between torque and velocity calculations.

Contextual Notes

Participants highlight limitations in their calculations, including potential inaccuracies in input values, assumptions about vehicle characteristics, and the need for real-world data to validate their models.

Who May Find This Useful

This discussion may be useful for individuals interested in vehicle dynamics, automotive engineering, or those looking to simulate vehicle performance based on torque and RPM data.

  • #31
Hello,

I have recently conducted a dyno run at a MAHA LPS 3000PKW dynamometer.
My car was equipped with a 6-gear manual transmission.
The optimum gear ratio 1:1 is achieved by the 5th gear. (due to car's official ratio for that transmission: 1st 4.46/ 2nd 2.61/ 3d 1.72/ 4th 1.25/ 5th 1.00/ 6th 0.84/ R 4.06)

However, the user of the dynamometer, made a dyno pull with 4h gear.

I would like to ask you,
1) would the horsepower and torque numbers that showed me as a result be different in case there was a dyno run with 5th gear??
2) A dyno run with 5th gear would have shown me better numbers or worse in comparison with the run at 4th gear?


thank you in advance
 
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  • #32
I would think a dyno run (with a BSFC (brake specific fuel consumption) map) would give you the engine RPM with the maximum power output (=RPM x 2π/60 x torque).
Calculation of the power required vs ground speed, including air drag and rolling resistance will give a monotonic plot of required power vs. speed. Find the max engine power on plot, then select best gear ratio to match engine to speed. I believe manuals are ≈98% efficient in the 1:1 gear, ≈96% in others.

Bob S
 
  • #33
Hi this may help

http://gti-vr6.net/library/engine/Schrick_howto/wheeldiameters.htm

I was doing the same thig only the other day

aussepom
 
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