Bimmerfest - BMW Forums - View Single Post - SMG II violent jerking back and forth!
View Single Post
Old 02-12-2006, 09:50 PM
joema joema is offline
Officially Welcomed to the 'Fest
Location: Nashville, TN
Join Date: Aug 2002
Posts: 209
Mein Auto: 2003 330i SP sedan 5-spd
Originally Posted by Pinecone
This has NOTHING to do with shifting. And it can happen with any clutch based system. It is a throttle based driver in the loop oscillation. And most times does not occur when shifting at all.

Driver steps on the gas, car moves forward, quickly, and driver moves back in seat, lifting foot from throttle. Driver pushes more to keep accelerating, but by the time the foot moves, the car has slowed and the driver moves forward depressing throttle, combined with foot movement makes too much throttle. Driver lifts foot, but body is moving back due to acceleration and lifts, combined with conscious lift lifts too mcuh, and so on, and so on...
Just wanted to say this is a great description. This is the first time on any forum anywhere I've heard this properly described. My 330i SP 5-speed does the exact same thing. It feels exactly like there's something wrong with the engine controller -- an oscillating surging. However it's driver induced oscillation.

Anybody experiencing this can verify it by noting the conditions where it occurs. If it's most common when accelerating briskly in 1st gear and hitting a bump, that tends to confirm it. The bump jostles your foot on the accelerator, which then sets up the oscillation. It's normal and there's nothing wrong with the car. It usually doesn't happen in 2nd or higher gears. It probably happens on SMG because it's more like a manual, but it's not the SMG transmission causing it.

It happens on BMWs, esp. manual transmission sport/ZHP package 3-series and M3s, because the suspension is stiff and the engine is very responsive. The suspension increases the chance of your foot jolting the throttle. It doesn't take much to start it. It might not happen as much on an automatic transmissions, which may buffer the throttle or upshift.

There may be other additional jerkiness from the SMG under some conditions, but the above driver induced oscillation is very common.
Reply With Quote