DerKimbo, do you still have drivetrain shudder when pushing in clutch at low RPMs in 1st?
11-13-2006, 02:49 PM
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DerKimbo, do you still have drivetrain shudder when pushing in clutch at low RPMs in 1st?
I think I recalled you saying something about that a while back. I occassionally have this when the car is started up cold. I'll be in 1st gear, and at 3k rpm I'll push in the clutch. As I push it in, there is sometimes a shudder in the drivetrain, almost like the car is unbalanced and there is a shaking back and forth, the length of the car.
This seems to go away as the car warms up. It's not there any more later on, although it definitely is never there when shifting at higher RPMs or in gears higher than 1st.
Just wondering what this is, if other people have it and don't notice it.
This seems to go away as the car warms up. It's not there any more later on, although it definitely is never there when shifting at higher RPMs or in gears higher than 1st.
Just wondering what this is, if other people have it and don't notice it.
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11-13-2006, 09:10 PM
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Lighter flywheel in RS4 tends to accentuate this
With a lighter flywheel, the engine is more responsive and will accentuate low speed, low torque driveability. It is possible to get the engine and drive train into a bucking oscillation, which as others point out, can be resolved by applying a bit of throttle (more torque).
With a fixed throttle position, the ECU tries to keep the engine at constant torque, but when torque is low, driveline slop and load variations due to road bumps, actually cause the engine speed to change quickly. (you can't see it on the speedometer, because of lag and averaging). The ECU interprets this as a change in the engine operating point and tries to compensate. But at low throttle(torque) settings, it doesn't take much to over correct.
The ECU torque control system will actually overshoot and ring, causing oscillations that resonate with the drivetrain, which cycles from loading the engine to not loading the engine, for very brief instants. The ECU has an anti-bucking algoithm built in, but it doesn't seem to be able to totally compensate. It just keeps the engine and drivetrain from oscillating out of control. When you apply even a slight bit of extra power, slack is removed from the drivetrain, and the load on the engine is constant and predictable to the ECU.
With a fixed throttle position, the ECU tries to keep the engine at constant torque, but when torque is low, driveline slop and load variations due to road bumps, actually cause the engine speed to change quickly. (you can't see it on the speedometer, because of lag and averaging). The ECU interprets this as a change in the engine operating point and tries to compensate. But at low throttle(torque) settings, it doesn't take much to over correct.
The ECU torque control system will actually overshoot and ring, causing oscillations that resonate with the drivetrain, which cycles from loading the engine to not loading the engine, for very brief instants. The ECU has an anti-bucking algoithm built in, but it doesn't seem to be able to totally compensate. It just keeps the engine and drivetrain from oscillating out of control. When you apply even a slight bit of extra power, slack is removed from the drivetrain, and the load on the engine is constant and predictable to the ECU.
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