Focus RS handling.... (diff)

[KSaiyu]

I was just watching an old Top Gear and saw them talking about the RS and the tricky handling it exhibits, which they blamed on the Quaife diff that is installed in the car. Thinking this was nonsense at first, I looked up an old Evo review of the car and there again it said it struggled with uneven roads and would dart off riding different cambers and under acceleration.

Now I've always thought limited slip diffs would prove the opposite, and reign in torque steer and provide more stability and resistance to tramlining when driving on cambered roads as opposed to an regular open diff setup - anyone know where I'm going wrong?

 

[niky]

The LSD helps keep the tires from spinning at different speeds (it doesn't lock them completely at all times, that'd be bad). It reins in torque steer by keeping wheel speeds similar. In cornering, it helps handling by, again, keeping wheel speeds similar... this time by making sure that the power isn't spun away by the inside front tire (turn your wheel to one side and drop the clutch from a standing start... the car will take off much slower than if the wheels are straight, because your inside front is getting most of the power).

Thus, with the LSD, you point it, and it goes... since the power is going to the loaded outside front tire. A camber will deflect your front wheels slightly, and thanks to the LSD, where your front wheels are pointed is where your car starts going.

There was an article in Evo not so long ago wherein an RS Owner explained it (blamed it, actually) on press-fleet Focus RS's having more aggressive Quaiffe Differentials than actual production ones (it was supposedly revised). The RS tested in that article had more benign handling over bumpy surfaces than press-fleet RS's, and the writer judged it as much better than the Astra VXR and Mark V Golf being tested with it.

Then on to the big best front-drive group test, where the same writer stuck the RS in based on that great drive, and the RS they get this time (also a private) is just the same as the press cars... edgy over bumps... and doesn't make the final cut... oh well...

 

[skip0110]

Isn't this a steering geometry problem?

There are three moving parts which locate the hub/knuckle: the upper control arm, the lower control arm, and the steering tie rod. Each moves in an arc as the tire moves up and down, and each arc has a different center. The weight of the car is going to try and make all these circles cross at a single point--and a well designed suspension will have them all be at the same point at ride height. When you hit a bump, they may misalign, and then the weight of the car and torque of the engine is going to try and line them up--usually by moving the tie rod and thus your steering wheel. You see: torque steer.

Why would a LSD worsen this issue? I don't know.


EDIT: Here is a diagram to explain what I am talking about. See how the three circles don't quite intersect at a single point? That means that at that point in the suspension travel, there will be slight bump/torque steer, as the weight of the car and the torque of the engine pushes the tie rod left or right until the circles all line up.

The dark gray is the control arms, the light gray is the tire, and the white is the steering tie rod.

 

[niky]

The LSD doesn't worsen torque steer... it just makes the car pretty darty as bump steer occurs while the car is driving over rough roads.

Glowing reviews for the Focus RS often come from racetrack driving or driving at 7/10ths below. At full chat on rough British roads, supposedly, it's a different story.

While bump steer is a problem on many grippy cars, it's enhanced by the aggressive differential, supposedly. Some cars with LSDs, like the Integra Type R, don't seem to suffer from this problem, due to better LSD design.

 

[KSaiyu]

See I can understand how the LSD would help it on track, as proved by reviews, but I don't see how it makes bump steer or torque steer worse?

 

[niky]

The LSD shuffles the grip back and forth to the wheel with the most grip. So, say you're on a bumpy road, driving at full chat. Your left wheel hits a bump, goes five millimeters in the air. The over-aggressive LSD, seeing no traction on the left wheel, suddenly shifts some 200+ hp to the right wheel. Suddenly, you're shooting to the left without knowing why.

Now imagine that happening at a corner... that's where the rep for "tricky handling" comes in.

Again, it's not torque steer. It's bump steer, but the effect is about the same. You only get torque steer from unbalanced power transmission to the axles (usually caused by one axle being longer than the other) which the LSD is supposed to cure.

Torque steer is when you storm off from a complete stop on a smooth road and the car pulls to one side or the other as one wheel gets power before the other. The LSD mitigates this somewhat, by the aforementioned power-shuffling.

 

[Keef]

I have personal experience with a high-powered, front-drive, LSD equipped car. The Chevrolet Cobalt SS puts down lots of power and torque, 205 and 200, respectively, and similarly to the Focus RS. The SS exhibits the same dartiness under power, though it probably isn't as bad because teh RS fleet cars were tuned aggressively.

The LSD allows the car to exit corners under power very well, like Niky has mentioned. But accelerating on rough, rutted streets is a chore. Hopefully you have strong arms.

 

[KSaiyu]

Gotcha, thanks guys 👍

EDIT: Something I found on the Quaife site -

The Ford Focus RS would not have been such a success without the use of a Quaife Automatic Torque Biasing differential as a standard part, fitted direct from the factory.

The initial "press" cars were not fitted with a Quaife ATB differential, and many journalists were less than polite after their drive!

Interesting!