Peterbilt 348 2017 - Page 169

OPERATINGTHEBRAKESYSTEM
YawStability
Yawstabilitycounteractsthetendency
ofavehicletospinaboutitsvertical
axis.Duringoperation,ifthefriction
betweentheroadsurfaceandthetires
isnotsufcienttoopposelateral(side)
forces,oneormoreofthetirescan
slide,causingthetruck/tractortospin.
Theseyaweventsarereferredtoas
either"under-steer"(wherethereisa
lackofvehicleresponsetosteering
inputduetotireslideonthesteeraxle)
or"over-steer"(wherethetractor's
rearendslidesoutduetotireslide
ontherearaxle)situation.Generally,
shorterwheelbasevehicles(tractors,
forinstance)havelessnaturalyaw
stability,whilelongerwheelbase
vehicles(straighttrucks,forinstance)
havegreaternaturalyawstability.
Factorsthatinuenceyawstability
are:wheelbase,suspension,steering
geometry,weightdistributionfrontto
rear,andvehicletrackwidth.
YawControl
YawControlrespondstoawide
rangeoflow-tohigh-frictionsurface
scenariosincludingrollover,jackknife
andlossofcontrol.Inthecaseof
vehicleslide(over-steerorundersteer
situations),thesystemwillreducethe
throttleandthenbrakeoneormore
ofthe"fourcorners"ofthevehicle
(inadditiontopotentiallyapplying
thetrailerbrakes),thusapplyinga
counter-forcetobetteralignthevehicle
withanappropriatepathoftravel.For
example,inanover-steersituation,
thesystemappliesthe"outside"
frontbrake;whileinanunder-steer
condition,the"inside"rearbrakeis
applied.
ARealWorldExampleofHowYaw
ControlOperates
Excessivespeedexceedsthe
threshold,creatingasituationwherea
vehicleislikelytospinandjackknife.
TheBendix®YawControlsystem
reducesenginethrottleandselectively
appliesbrakestoreducethevehicle
speed,therebyreducingthetendency
tojackknife.
YawControlExample
(03/17)Y53-6060-1C14-29
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