Calculated VSS (used to verify speedo calibration)

[joecar]

To calculate what the VSS should read at any RPM, create a calculated pid using this equation...
vss = rpm x tire_diameter / (trans_ratio x axle_ratio x factor)

where...
vss is in MPH or km/h,
rpm is SAE.RPM (engine speed) or GM.TIS (trans input speed),
tire_diameter is in inches or mm,
trans_ratio is GM. BOXRATIO (for A4),
axle_ratio is hard coded (e.g. 3.23),
factor is 336.135 (imperial) or 5305.16 (metric).

This calculated vss can be compared against SAE.VSS to see if the speedo is calibrated correctly.

To accurately measure tire_diameter, do this...
lift tire off ground, measure circumference, and divide by pi
(since tire circumference almost never changes by much...).

Questions regarding M6:
a. do GM.TIS and GM.TOS read valid values...?
b. does GM.BOXRATIO read valid values...?

[joecar]

Can also use:
vss = {GM.TOS} x tire_diameter / (axle_ratio * factor)

[joecar]

To calculate what the VSS should read at any RPM, create a calculated pid using this equation...
vss = rpm x tire_diameter / (trans_ratio x axle_ratio x factor)

Here's my calc_pids.txt file...

# ==============================================================================
# File details
# ------------
#
# This section defines various details about the file format.

*FILE

#Parameter  Value            Description
#---------- ---------------- ---------------------------------------------------
VERSION     7.1.1            File version
DECSEP      .                Decimal separator used in this file



# ==============================================================================
# Units
# -------------------
# See sae_generic.txt for more information on the *UNITS section

*UNITS

#Code     System     Abbr     Description
#-------- ---------- -------- -------------------------------------------------------------

None      None       ""       "No units"


# ==============================================================================
# Add slot definitions here
# --------------------------------
# See sae_generic.txt for more information on "SLOT" formats
#
#Units             Low          High      Fmt  Expression
#------------ ------------- ------------- ---- --------------------------------------------------------------
*CLC-00-060
mph                0.0          160.0      .0  "{SAE.RPM} * 25.66 / ({GM.BOXRATIO} * 3.23 * 336.135)"
kph                0.0          260.0      .0  "{SAE.RPM} * 651.8 / ({GM.BOXRATIO} * 3.23 * 5305.16)"


# ==============================================================================
*PRN - Parameter Reference Numbers
# --------------------------------
# See sae_generic.txt for more information on the *PRN section
#
#Code                      PRN  SLOT         Units            System           Description
#------------------------- ---- ------------ ---------------- ---------------- ------------------------------------------
CALC.VSS                   F060 CLC-00-060   "mph,kph"        Calculated       "Calculated VSS"

My car has tire diameter is 25.66" (651.8mm) and 3.23 rear.

[joecar]

Can also use: vss = {GM.TOS} x tire_diameter / (axle_ratio * factor)

This seems to get better results; here's calc_pid.txt for my Mrs' GAGT:

# ==============================================================================
# File details
# ------------
#
# This section defines various details about the file format.

*FILE

#Parameter  Value            Description
#---------- ---------------- ---------------------------------------------------
VERSION     7.1.1            File version
DECSEP      .                Decimal separator used in this file



# ==============================================================================
# Units
# -------------------
# See sae_generic.txt for more information on the *UNITS section

*UNITS

#Code     System     Abbr     Description
#-------- ---------- -------- -------------------------------------------------------------

None      None       ""       "No units"


# ==============================================================================
# Add slot definitions here
# --------------------------------
# See sae_generic.txt for more information on "SLOT" formats
#
#Units             Low          High      Fmt  Expression
#------------ ------------- ------------- ---- --------------------------------------------------------------
*CLC-00-060
mph                0.0          160.0      .0  "{GM.TOS} * 24.86 / (3.29 * 336.135)"
kph                0.0          260.0      .0  "{GM.TOS} * 631.4 / (3.29 * 5305.16)"


# ==============================================================================
*PRN - Parameter Reference Numbers
# --------------------------------
# See sae_generic.txt for more information on the *PRN section
#
#Code                      PRN  SLOT         Units            System           Description
#------------------------- ---- ------------ ---------------- ---------------- ------------------------------------------
CALC.VSS                   F060 CLC-00-060   "mph,kph"        Calculated       "Calculated VSS"

[Ira]

To accurately measure tire_diameter, do this...
lift tire off ground, measure circumference, and divide by pi
(since tire circumference almost never changes by much...).

I'm not sure that's valid. The rolling diameter is twice the distance from the ground to the center of the axle. On a modern low profile tire that might be close to circumference/pi, but it's still probably a bit smaller.

Ira

[joecar]

Can also use:
vss = {GM.TOS} x tire_diameter / (axle_ratio * factor)

[joecar]

To accurately measure tire_diameter, do this...
lift tire off ground, measure circumference, and divide by pi
(since tire circumference almost never changes by much...).

I'm not sure that's valid. The rolling diameter is twice the distance from the ground to the center of the axle. On a modern low profile tire that might be close to circumference/pi, but it's still probably a bit smaller.

Ira

Ira, you have a good point;

I was considering this: tire circumference depends on inflation pressure and is the same distance regardless of radius (distance to ground); this is the distance travelled in 1 tire revolution no matter what the tire deflection is.
I'll try it the radius method (car's weight on tire) and compare.


BTW: I spliced in the IAT connector I got from you, and all her problems (harsh shifts on Mrs' GAGT) are solved. Thanks.

Cheers,
Joe

[FRISKY]

Here is the best way I have found to measure a tires true circumference (rollout):

Stick a piece of tape across the tread allowing one end to stick out past the sidewall a few inches.

Move the car to where one the edge of the tape is flat on the ground directly under the axle centerline of the wheel.

Using a pointed pen, mark a thin line where the edge of the tape and the ground meet.

Move the car again to where the tire makes one full rotation and the same edge of the tape is once again flat on the ground directly under the centerline of the axle.

Make another line with the pen.

Measure the distance between the two marks and you have the exact distance (tire circumference) the tire travels in one rotation.