oversteer measurement formula in Motec i2
oversteer measurement formula in Motec i2
(OP)
Hi,
I had a look of the Motec i2 pro software. There is a maths function delivered with it, which calculates oversteer. According to my (very limited) knowledge, there is no low-cost way to get a reliable measure of oversteer/understeer. The formula in the software uses only the vehicle speed, LatG, Steered angle and the vehicle wheel base to come up with a figure. I am wondering if someone of you uses this and would be willing to comment on the measure vs driver feedback.
The complete formula is:
smooth(choose('Corr Speed'[km/h] < 50, 0, sgn('G Force Lat'[m/s/s]) * (('Vehicle Wheelbase'[m] * 'G Force Lat'[m/s/s] / sqr('Corr Speed'[m/s])) - sgn(stat_mean('Steered Angle'[rad] * 'G Force Lat'[m/s/s])) * 'Steered Angle'[rad])), 0.2)
Ignoring the filtering (smooth) and the condition to evaluate above 50 km/h (choose), this is just:
+/-(wheel base x LatG / speed^2) x (-/+ steer angle)
The +/- and -/+ represent change of sign (sgn in the function) in the first case in agreement with to the sign of the LatG. In the second case as the inverse of the stat-mean of the product of the LatG and Steered angle.
Does anyone have any comment about the deduction of this formula?
Thanks!
Nado
I had a look of the Motec i2 pro software. There is a maths function delivered with it, which calculates oversteer. According to my (very limited) knowledge, there is no low-cost way to get a reliable measure of oversteer/understeer. The formula in the software uses only the vehicle speed, LatG, Steered angle and the vehicle wheel base to come up with a figure. I am wondering if someone of you uses this and would be willing to comment on the measure vs driver feedback.
The complete formula is:
smooth(choose('Corr Speed'[km/h] < 50, 0, sgn('G Force Lat'[m/s/s]) * (('Vehicle Wheelbase'[m] * 'G Force Lat'[m/s/s] / sqr('Corr Speed'[m/s])) - sgn(stat_mean('Steered Angle'[rad] * 'G Force Lat'[m/s/s])) * 'Steered Angle'[rad])), 0.2)
Ignoring the filtering (smooth) and the condition to evaluate above 50 km/h (choose), this is just:
+/-(wheel base x LatG / speed^2) x (-/+ steer angle)
The +/- and -/+ represent change of sign (sgn in the function) in the first case in agreement with to the sign of the LatG. In the second case as the inverse of the stat-mean of the product of the LatG and Steered angle.
Does anyone have any comment about the deduction of this formula?
Thanks!
Nado





RE: oversteer measurement formula in Motec i2
Accelerate in a turn with some sideslip acceleration (the other major component of total lateral acceleration) and this gets bad/wrong very quickly.
I'd dispute your sorrow about a cheap way to compute understeer (gradient of rw_steer by Ayg minus Ackermann gradient). With just yaw velocity (r) and speed (u), you can compute understeer from r/u vs. u*r if you have enough room to run a speed sweep at several fixed steer angles (curvatures). The slope is 'understeer' (also need wheelbase). If you can sneak into a shopping mall parking lot, a constant radius test (100m is ISO4138) gets pretty simple. Use a protractor for Steer angle, your speedometer for speed, estimate your steer overall ratio from a protractor and a lock to lock test. Normalize steer angle to get road wheel angle. Fit data at speeds 10 up to max speed in 10 kph increments, use nice function, derivative is understeer. No Ackermann gradient to mess with in a const. radius test. If you have a sideslip transducer, it will read zero at some magic speed. This speed has as much value as understeer when used in computing the rear axle sideslip gain of the vehicle. All you need to have to complete this process is a measuring tape.
BTW: I used the first method (speed & yawrate) to compute the understeer of my bass boat. Its 6 deg/g (linear range) with a 3 bladed prop, 3.5 deg/g with a 4 bladed prop, and 2 deg/g with a 5 bladed stainless steel propeller. Had to estimate the 'wheelbase' though. You get the idea I hope.
RE: oversteer measurement formula in Motec i2
RE: oversteer measurement formula in Motec i2
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: oversteer measurement formula in Motec i2
Using that trace as an objective measure of understeer is probably taking it too far though.
Regards, Ian
RE: oversteer measurement formula in Motec i2
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: oversteer measurement formula in Motec i2
It will give a very rough guide to US/OS balance but it's so rought that any driver will be feeling it anyway. Magneti Marelli have had a similar thing in there software for years that they just call KUS. Details of the equation are hidden from view though.
RE: oversteer measurement formula in Motec i2
What do you mean by unworkable in a V8 context? Too noisy? Inconsistent answers?
Regards, Ian