What causes my Mustang to buck when I let off the gas at a high RPM?

Question

I have a 2007 Mustang GT, 4.6L V8 with a 5-speed manual transmission.

This is the car I learned how to drive stick on, and the only other car I've ever driven with a manual is a 1963 pick-up truck... so I've driven these two cars a lot, but only these two cars. Experience is pretty limited. Anyway...

Let's say I'm in first gear above 2500 RPM, if I just let off the gas (leave clutch out), the car will begin to buck back and forth, pretty severely. The same thing happens in higher gears, but the bucking is much less severe and generally I have to be much closer to the redline.

I'm just a little confused here. Why would the car begin bucking with the clutch out? Where's the slack in the drivetrain that causes the bucking? Is it just as simple as the engine fighting the transmission?

Thanks all.

Answer 1

I think what you are experiencing is normal behavior for just about any rear-wheel drive vehicle (and probably most front wheel drives, but to a lesser extent). This is due to the loading/unloading of the drive train as the engine (as you suggested) is forced to catch up to the movement force which is being counter acted upon.

To put it out there a little more understandable I will try to spell it out for you ...

When you have your foot on the gas to maintain a speed, everything is in equilibrium. Everything (engine motor mounts, transmission mount, rear differential, suspension, tires) are coasting along having a good day. All of a sudden, you take your foot off of the accelerator and things get thrown out of whack. The engine begins the process of engine braking or using compression to slow the vehicle down. This is the loading up I talked about above. Torque is created throughout the drive train which binds up (twists) all of the drive train. This is excess torque which becomes elastic potential energy. As this potential energy reaches its maximum (maximum amount the drive train can store), it becomes kinetic energy and is released, allowing the vehicle to lurch forward. This is the "bucking" you talked about. As all of the kinetic energy is used up on one side of the cycle, it all starts over again, which means it's cyclic and will happen again until everything is back to a point where the all of the parts I've described can take the shock of deceleration without having the bouncing or bucking effect you realize.

You only really experience this on cars with standard shift transmission because the torque converter will alleviate most of this with an automatic (won't diminish all of it, but you'll not notice it near as much). Mind you every car is different. Every standard shift vehicle is built differently (between models), so will have more or less elasticity built into the drive train. Even within the same model, it won't be exactly the same, mainly due to wear and tear on the items which can store the energy. Even the overall state of the engine itself will have an effect on how much of this behavior you'll see.