Ford Focus RS Forum banner
1 - 15 of 15 Posts

·
Registered
Joined
·
64 Posts
Discussion Starter · #1 ·
Hi guys, I'm regularly visiting tracks with my RS and participate in different local events. I have installed Ohlins suspension and RS is amazing in handling corners. One thing that I don't like much is corners exit. I would like to hit acceleration earlier but getting understeering. So, I'm wondering if front diff will helm me.

Does anyone one have real track times difference before and after installation an LSD?

Maybe some one has experience in comparing Wavetrac and Quaife on focusRS? I read a lot about those LSD's and it sound like Wavetrac doing better job but I'm afraid that such "hard" LSD can cause even more understeer.

Ford choose Quaife as LSD for their edition version, maybe there was some reason for that? or it could be just a business.

also there is some concerns that LSD may cause even more understeer. Can you guys share your real experience here?
 

·
Registered
Joined
·
1,002 Posts
Have you tried to reduce the corner exit understeer by making adjustments with your current setup - rear ride height, rear shock setting, and/or rear sway bar stiffness?

I don't have first hand experience with an LSD on this platform but, yes, concerns of an LSD inducing understeer are valid. Depending on application of throttle and corner layout, the LSD will promote some amount of understeer. It's the nature of the beast. I believe in the autocross thread, someone was actually datalogging wheel speeds and other data off the CANBUS and mentioned the improvements were negligible - don't quote me though.
 

·
Registered
Joined
·
365 Posts
This is either going to be a long thread or a very short one 😊

First, helical differentials like Quaife ATB and WaveTrac, and the original Torsen, are so called torque sensing or automatic torque biasing differentials and do nothing to limit wheel slip, they are not limited-slip differential where slip refers to left-right wheel speed difference.

The prevalent LSD technology used in racing actively couples the wheels using a clutch mechanism inside the differential. The clutches that couples the left-right wheels can be actuated electro-hydraulically (like the RS’s RDU) of mechanically using a clever mechanism, and controlling the clutches' slip is the slip limiting action of proper LSD’s. One can see the two multi-disc wet cutch packs, in gold inside the differential unit in the Drexler LSD cutout below. The principle is known as the ZF principle, after a famous German manufacturer of similar mechanisms.

353076


The range of control goes from no slip limit (like an open differential, or any of the torque biasing differentials) where nothing prevent an infinite speed difference between the wheels so 0% slip limiting, all the way to no slip at all where the differential is solid and both output shafts are mechanically coupled to rotate as the same speed (that would be 100% locking)

Proper limited slip differentials comes in a range of variants or setups: number of friction surfaces and friction material of the clutch packs, mechanical preload on the clutches so the diff is never at 0%, and various strengths of clutch actuation, that couples the shafts more or less in accel and coasting as a function of a ramp’s angles, the static preload, and the amount of torque applied at the diff’s input flange during acceleration, or output flanges when coasting. The driver has control over the amount of locking through the torque applied to the diff, so through the throttle.

Torque sensing / torque biasing diffs improve traction when there is good traction to begin with. They need grip on both wheels and bias the torque towards the wheel which has the most grip. If one wheel loses traction completely (spinning, bumping, unloading…) that wheel will spin freely as the diff is NOT limiting slip and the car looses all traction, at least momentarily.

This is why a torque biasing diff is of little help in “violent” situations like hard corner exit, or in case of massive torque input at high steering angles, such as when exiting a hairpin, when the inner wheel is unloaded and spins easily. The more slippery, the less effective are the torque biasing diffs at maintaining traction, doing literally nothing on snow or ice (offering little or no improvement over a completely open differential.)

This is to say, don’t expect miracles from installing a Quaife ATB or any other helical differential. It’s more of a feel-good mod that helps traction in everyday driving but is of little help when driving in anger. I had a pair of Quaife ATB's for a couple of years on my Escort Cosworth and they were terrible on snow, the car switching between 0, 1, 2, 3 and 4wd randomly and unpredictably at a moment's notice. I'd say they are great on any surface until you drive hard enough to cause any type of grip loss, where they suddenly become useless. I've long replaced both by Titan Traction Master ZF-style LSDs and never looked back. On the RS I went straight to a ZF-style LSD from Drexler.

WaveTrac makes an attempt at mechanical slip reduction using internal friction washers that probably help a little bit, but if you look around you will find videos showing the WaveTrac is of no real help either if one of the wheels is completely unloaded, or on a more slippery surface than the other. A Quaife would be just as useless as far as limiting slip.

Only mechanical action (clutch friction or, heaven’s forbid, brake friction) enables torque vectoring and slip-limiting. Then only torque vectoring enables driving techniques that promote faster cornering, but I save that part for another day.


 

·
Registered
Joined
·
64 Posts
Discussion Starter · #5 ·
I read this overview and was exited of it

You can now lean hard on the RS coming out of corners and start working the rear axle to get that subtle rear-wheel drive sensation that helps point the car’s nose into the apex to allow you to get on the power earlier

but now I'm not sure... I would like to try it but also don't want to waste about ~2000 USD for diff + installation
 

·
Registered
Joined
·
460 Posts
This has been discussed numerous times on this forum over the past 5 years and there is no quantifiable data supporting the addition of a front LSD. You'd be better off spending your money on "the driver mod" or basically more seat time on track. Preferably with a pro instructor sitting in the passenger seat at a HPDE event. Just my $0.02 worth...
 
  • Like
Reactions: nicklulu

·
Registered
2017 Black RS
Joined
·
451 Posts
At least our OE diff is decent. In the Contour, the stock diff had a hard time handling stock power (it would break) and had to be upgraded. I have a Quaiffe in that and it also made a big difference in putting the power down and reducing torque steer. I was hopeful one would help here... but looks like maybe not. Wouldn't one of these make it so the car isn't using the brakes to bias as much?
 

·
Registered
Joined
·
211 Posts
Main thing I found with wavetrac when driving hard on technical twisty tarmac or track is car feels less artificial. Things like eLSD brake rubbish don't happen as diff does it before threshold reached. Much less heat pumped into brakes. So car feels better, can go hard longer, and can be trusted more......less nannies trying to drive.

Same applies on dirt roads when sliding car and driving it like rally car, i.e. oversteer slide with understeer lock on front on power to get best grip vectors in corner. Unlike an old mk2 escort, the rs does not lift inside front wheel. In this situation again much more trustworthy and natural feeling. Without wavetrac it squirrels near the limit, with it calms it all down.

So do you go faster on track, maybe not as you know the track intimately. Do you go faster on dirt or technical tarmac... in my opinion yes. If the system hides the limit from you then your a passenger, so the wavetrzc makes it more readable too. Anyway, my 2 cents worth
 

·
Registered
Joined
·
211 Posts
Interested to know how you go. Incidentally, diffs like the Wavetrac have really sophisticated behaviour. If you are hard on the brakes and there is a lot of engine braking it locks to some extent, providing stability into a corner. As the car starts transitioning the diff ends up in a neutral almost unlocked state so allowing the car to rotate. Once the power comes on and post rotation it pulls the car into the corner allowing more power to be applied than otherwise possible without running wide as you relax the car coming out. Net result is you can brake harder with better control, rotate the car earlier, and then get on the power before the apex and start accelerating sooner.... but it is a different driving style. Or you can not do this and complain of understeer;)
 

·
Registered
Joined
·
365 Posts
Interested to know how you go. Incidentally, diffs like the Wavetrac have really sophisticated behaviour.
This is still a helical diff. With a small twist but what your describe requires multi-plate clutches that the WaveTrac does not have. The video above shows the wavetrack does very little in terms of actual locking and does not prevent an unloaded wheel from spinning. Note the engine power was very low in that demonstration. At full throttle the slipping wheel would spin freely just like an open diff.
 

·
Registered
Joined
·
211 Posts
In the end, it comes down to how it subjectively feels when driving it, and in particular how confident it makes you feel that you can push it and it won't do something odd. I've won a NSW state rally championship and my style tends to be a vectored approach with the car sliding, in what looks like oversteer, but with the front wheels almost straight ahead and vectoring into the corner. Most quick 4wd rally cars seem to be in this stance.

I use left foot braking, even in the RS, to set the balance so the car weight is evenly distributed. If I leant on the outside front wheel a lot then I'd imagine the diffs would have more trouble because that produces much more severe weight transfer and loads the outside front much more. The car certainly needs to be rotated and balanced before getting on the power but that can be done early in the corner. If you don't rotate and balance, and use the diff to pull you in with a heavily loaded outside then I guess a wavetrac setup will not be ideal for the reasons you have described. In the end, each technology setup demands an optimal driving style or else it doesn't perform to its best.

A friend of mine used to rally a turbocharged Datsun 1600 with a CIG locker with only a limited amount of rotation until it hit the welds. He was often fast but on occasion hit trees because he couldn't turn. As a result he drove it hugely sideways everywhere.

I'm wondering if my driving style means I don't notice the discrepancy. In the old Mk2 escort days we always were flying the inside front, but of course that's RWD. For me, I find that the wavetrac made a marked improvement but I guess that is coupled to my driving style. For other people, their mileage will vary.
 
1 - 15 of 15 Posts
Top