np20412

What keeps my car from rolling backwards on a hill when I am D? I have been in many cars where the car will roll backwards when on a steep hill even when the car is D. My 1994 Lexus SC doesn't roll backwards ever on a hill when I'm in D, despite the grade of the hill. What is the reason mechanically? Is the differential the cause for this, or is it the use of a one way clutch?

GFerg

Idle speed?? The fact that your car moves forward as soon as you let go of the brake??

np20412

Well the reason for that I know is the that the torque converter generates just enough torque to move the car when it is in gear, but it does not generate enough torque to overcome the brakes when they are applied. But when I am on a hill, my car will neither move forward nor backwards. I can take my foot completely off the pedal while still in D and the car will just sit there. I know the torque converter is not generating enough torque in this case to overcome the weight of the car and the force of gravity, hence no forward motion, but why no backwards motion either (while some cars will roll backwards if the brake is not applied) ?

rominl

iTrader: (4)

even my 92 camry, when it's on a not-so-steep incline, it doesn't roll back. just imagine when you are on a flat surface, if you don't hold your brake the car will roll forward right? you can think of that opposing the downhill down in "holding" your car's position.

np20412

ok I can understand that......but then why do some cars still end up having to be held when on an incline? Is it just poor quality in parts? Usually they are the cheaper cars that will roll backwards even when in D. My friend has a Cavalier, rolls back everytime. Another friend of mine has an automatic 94 Integra, rolls back every time.

mmarshall

Several factors affect this besides those already discussed.....torque converter stall speed, type and viscosity of transmission fluid, transmission fluid temperature ( cold fluid has more drag than hot fluid ) , amount of brake pressure, type and efficiency of brake linings, transmission gear ratios in first and reverse ( you may have the lever in " D " but at a dead stop, in gear, the transmission itself will actually be in first ). Additional factors are vehicle weight and weight distribution, the torque curve of the engine ( especially at idle ) and the fact that some newer vehicles have " grade logic " sensors in the transmissions that can determine if the car is nose-up or nose-down and not only adjust the transmission's shift and gear-hold characteristics but also adjust the idle speed accordingly.

np20412

Didn't know it was that complex! Thanks for the insight mmarshall, I'm gonna do some googling to satisfy my interest now

sha4000

iTrader: (1)

i always wondered why my LS didnt roll either

LexLaw

I never fail to learn something here at CL. You guys amaze me.

rominl

iTrader: (4)

like marshall said it depends on a lot of factors, and some cars put up a bit more "power" and "torque" to keep the car on incline. some cars they can't hold that much so...

AJ

This stuff is the reason why i love CL

mmarshall

Yes...there is almost nothing simple about a modern car or how it works. That is why auto engineers get paid big bucks.