Environmental Engineering Reference
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Solution: Estimate the tyre dynamic rolling radius, r w , as follows:
r w ¼ ð D rim þ 2 H sidewall Þ
2
¼ ½ 381 þ 2 ð 126 : 75 Þ
2
¼ 317mm
Therefore, r w = 0.317 m is the approximate dynamic rolling radius of this tyre.
Compare this to the tyre data in Table 1.10 for the mid-size car.
1.5 Calculation of road load
Hybrid propulsion systems are primarily targeted at the vehicle's longitudinal
performance. Tractive effort, fuel economy and braking performance top the list.
Dynamics about the pitch and yaw axes of the vehicle are secondary considera-
tions, but are important during the initial definition of the vehicle and the design
cycle. It is all too easy to sacrifice the hard won ride and handling characteristics of
a vehicle through improper hybrid component integration. This is why this topic
advocates a ground up design of any hybrid vehicle rather than a cut and overlay
approach.
1.5.1 Components of road load
The definition of propulsion system tractive effort by now is commonplace, but
worth reiterating. Tractive effort is the primary predictor of longitudinal perfor-
mance of the vehicle. The equation of motion along the X -axis is
m eq dx 2
dt 2 ¼ F trac F aero F rf F rr F grad
m eq ¼ f m m v þ N p m p
ð 1 : 5 Þ
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