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I performed a internet search on "Suretrac" differentials and found the following links. It appears Suretrac differentials were/are used in Peugeot rally cars, Caterham Lotus 7's, Mini's , and Honda and Suzuki ATV's. Below are some excerpts and some links. Some links were from google's cache and I don't know how long they may remain available.

I'm not knowledgeable on differentials, but I wanted to learn something about the unique Suretrac differentials since it is installed on the STi and has received decidedly negative comments from the automotive press.

Does anyone have any earth staggering observations about this application of the Suretrac in the STi to put it in context for me? If not, happy reading. Thanks.

http://www.wrxhawaii.com/misc/Impreza WRX STi Technical Description.pdf

The New Impreza WRX STi also comes equipped with front & rear "Suretrac" limited slip differentials. These LSDs respond to a torque differential between left and right wheels transferring the torque from the slip wheel to the non-slip wheel. The benefit is that they only activate under drive and do not respond to differences in left & right wheel cornering speed when coasting. This means that at the limit of adhesion torque is transferred from the inside wheel that is losing traction as a result of weight transfer to the outside that has traction. The effect is to noticeably reduce understeer under hard cornering under engine power. The 'Suretrac' LSD function, differs from a conventional differential in that it does not have pinion gears and side gears. In their place are two opposing face cams and nineteen cam followers. The face cams are formed with a series of inclined ramps resembling sharp teeth. One face cam has nine 'teeth' the other has ten 'teeth'. The cam followers have special asymmetrically shaped profiles to match the face cams, and they are splined to the differential case. This means that drive through the differential is via the cam followers to the face cams, and then to the axles. When moving in a straight line, there is no relative speed difference between the face cams, which are splined to the front axles. When cornering with no load, each of the face cams is free to rotate independently from the other. Under this condition, the cam followers oscillate freely from left to right. When torque is applied, the cam followers are forced against the face cams, transmitting driving force to the wheels. If a wheel begins to slip, the relative movement between the two face cams, combined with the angular forces of the cam teeth causes torque to be transmitted to the slower turning wheel. The Suretrac differential is lubricated by normal gear oil, with flutes on the side of the differential case used to direct oil into the cam followers and plates.

(Good Cut-Away Picture on this page)

One ingredient in the Foreman Rubicon's great handling is the torque-sensing differential up front. Not only did it keep the wheels from slipping no matter how much power was squeezed on in the mud, but it left us with the feeling that the Rubicon is a l ot more of a rear-wheel-drive than a four-wheel-drive ATV. The Foreman 400, a machine redone by Honda in '95 and heralded as a great ATV in its time, felt old and heavy when compared to the Rubicon. The steering required a lot more effort.
The Rubicon uses a newly designed, clutchless, limited-slip differential that employs a set of specially shaped teeth to intelligently transfer power, unlike the bevel gears that are used in conventional limited-slip differentials. In the heart of this new differential design is one wheel of teeth for each shaft going to the tires and a center set of teeth that transfers power to the teeth wheels from the final-drive gear. The interface of the teeth ensures that power is transferred smoothly and evenly from the engine to each wheel and creates a torque ratio between the two wheels when one is slipping. This wondrous differential is a proven piece of machinery that has made cameo appearances in a variety of applications under the Suretrac name, and variations on the design are in some Suzuki ATVs. In the Honda Foreman Rubicon, it virtually eliminates torque steer in the front end and brings a sheepish grin to your face when the guy you're riding with asks why you're trying to pull dirt-track-style turns and doughnuts on a 4wd machine

(Good Drawings on this page)

Suretrac Limited-Slip Differential
This torque-sensing differential employs a cage mechanism that houses a cam follower and two face cams, one for each wheel. The cam follower, a set of six-sided teeth that is driven by splines in the cage, meshes tooth-to-tooth with the face cams on either side so the drive force coming from the engine is transferred to the wheels. Power is transferred from the engine to the cage mechanism via the final drive gear that is attached to the outside of the cage. The brilliance of this whole setup is in the design of the teeth of the cam follower and the face cams. It's easy for rotation to cause the cam-follower teeth to shift positions on the face cams, but difficult for that shift to make one cam wheel rotate faster than the cage. This ensures a smooth and even transfer of power from the engine.

On the torque-sensing differential, the thrust bearings support the cam wheels on the hubs as the cam follower transmits force to the wheels. The whole shootin' match is kept together by a belleville washer, which is compressed flat under load.

What if you've got one tire sunk in Mississippi mud? The one wheel that's spinning is going to be turning its cam very fast. Because of the interface of teeth angles, the fast-rotating cam has less friction force exerted on it from the cam follower. This is because the teeth have shifted and there is less contact between the fast cam and cam-follower teeth, along with the fact that the teeth of the fast cam move too quickly to transfer a good amount of drive force

The differential's cam follower in motion. Notice how it shifts to keep contact with the face cams.

Compare this to pushing a child on a playground carousel. When the carousel is at rest, it's easy to grab onto the push bars, run around in a circle and push your kid to the limit of nausea with some hard acceleration. When your child has become acclimated to the 2 g's of force being exerted and is screaming for you to push faster, however, it's difficult to transfer the same amount of force you did when the carousel was at rest. If you're lucky, you'll manage to get a couple of fingers on a passing bar sprinting at top speed and not pull a face plant.
The opposite happens to the slow cam. The shift of the teeth allows more surface area contact between the teeth of the cam follower and the teeth of the slow-face cam. Because the cam is slow, it's easier to transfer monster amounts of torque from the cam follower, which is moving faster. This is like accelerating the carousel from its resting state. The force exerted on the fast cam is going to be small, while the force on the slow cam will be great. Power is transferred to the wheels that grip, and everyone drives off happy.-J.G.
by John Griffin

Drive into a roundabout at speed and where the WRX would start to wash out into understeer, keep the power on however and the new car just grips and goes where you point it. This is even more noticeable in wet road conditions, where traction is quite brilliant. This is due to the inclusion of an AP Suretrac limited slip differential in the front axle. Its quite strange how the front initially feels as if it is going to slowly wash out into understeer, and then suddenly follows the steering input as if the front of the car had been tethered to a stake in the middle of the corner.

The car is definitely more controllable than the WRX, Subaru seem to have produced a four wheel drive car that is throttle adjustable, the front has more bite, the back end just sticks, lift off gently and the car just tightens its line, if you lift off very quickly, and then it steps out courtesy of its raised roll centre. This isn't a major drama however, and easily caught. It slowly dawns on you that the steering feel and weight of the new car are quite brilliant; this in part is due to the new higher ratio rack that has2.6 turns lock to lock.

On first acquaintance it seems a little heavier than the WRX, but greater exposure shows it in its true light, accurate, delicate and the total absence of kick back. This absence of kick back was quite extraordinary given some of the mountain roads in the English Lake District where I drove the car are akin to tarmac stages on a WRC event! The test car was fitted with the optional handling pack, which consists of progressive rate springs, and uprated rear anti-roll bar. These springs also drop the ride height by about 22mm at the front and about 27mm at the rear. The effect of these modifications is to make the car a little softer initially, yet less susceptible to roll when cornering. The absence of roll coupled with the ability to change direction with alacrity was nothing short of stunning!
- By JUSTIN LACY 14 December 2001

The introduction of a front limited slip differential is a significant change over the old car and a noticeable improvement.

When the LSD kicks in there is some initial torque steer, noticeable particularly when the turbo comes on boost, but it is really just letting you know that it is now working.

You have to adjust your cornering line slightly via the amount of steering lock that is wound on when the LSD activates, but it lets you get on the power earlier and harder, and helps dispense with much of the understeer inherent in four-wheel drive cars.

Peter Lyon

Acceleration times were not possible on our drive, but a conservative estimate puts the STi's 0-60mph sprint at about five seconds. And although straight-line per-formance is impressive, the vehicle's grip is simply astounding. Through long sweeping bends the car refuses to understeer, holding a line with real determination.

The extra stability comes from a significant redesign of the suspension and axles. Up front, the arrangement is moved 15mm forward to improve rigidity. Stronger components are used and there's more crossbracing, allowing engineers to add more negative camber. At the rear, the Impreza has beefed-up anti-roll bars and mounting points to stop it squirming during hard cornering.

With the new settings, more throttle input results in less understeer. In addition to the centre differential and front Suretrac LSD, Subaru also plans to offer DCCD (Driver's Control Centre Diff) in Japan. This allows the driver to choose how the transmission will deliver the power. The system can also detect wheelspin, and alter its set-up to maximise grip. As a result, corner exit speeds are higher than ever. Sadly, UK cars will not get this modification.

Keith Calver Oct 17, 2000
(Interesting Drawing/Picture on this page)

AP Racing. AP's 'Suretrac' LSD is a relatively recent addition, mainly targeting the road user. The design uses cams and pawls. Consequently, despite AP's claims that it's a 'novel' and 'unique' concept, the design and 'modus operandi' is acutely similar to the established Jack Knight pawl-type, just a little more refined. The cams have a series of 'lumps' on them (1). The pawls have 'lobes' on each end (2), and are positioned horizontally between the cams, carried by fluted internal and external drums (3). When running in a straight line, no movement of the pawls takes place as there's even traction on both wheels. Once a difference in traction occurs, the pawls slide in the flutes, engaging the cams. The amount of slip allowed is controlled by the frictional force developed between the drum flutes, pawl lobes and cam lumps, the resultant torque bias being applied to the axle with most traction. Pro's - very 'gentle' in operation. Con's - Comes as a sealed unit with 'factory' devised settings, so isn't user tunable or serviceable. Won't work with one wheel off ground - result, no drive. Usage - Road, drag racing, circuit racing with well-developed chassis (absolute minimal body roll, and stay off the kerbs), anywhere that won't experience air-born wheels. Bill says - "Never seen one, but theory says similar experience to Jack Knight diff". Cost around £485.00 plus VAT.

Jack Knight - Their pawl diff has been around a long time, developed from the original 'ZF' type. Design and operation very similar to the AP Suretrac except the cams are in the drums, the pawls held radially in a cage and have rounder 'lobes'. Pro's - relatively gentle in operation. Serviceable by user. Con's - Amount of slippage is a designed-in parameter so isn't tunable. Design creates a 'ratchetty' feel when working. High wear rate when used in most Minis/Metros, so regular maintenance needed to maintain performance, increasing running costs. Won't work with one wheel off ground - result, no drive. Usage - As AP Suretrac. Bill says - "Not a contender. It's twitchy and darty, personal doubts as to whether it was designed for front wheel drive. I've driven rear-wheel-drive racers with 'em fitted with no notable problems. Bust a CV or axle and it free-wheels". Cost around £570.00 plus VAT.

First Hand Expereince with Suretrac in Caterham Lotus Seven cars (possible not the same since it is discontinued)

I described the critism of the Suretrac in the STi to an actual Suretrac users and received the following reply:

> IF (Big IF) the Subaru uses the same differential (I think it is just the
> same name since the ones we have are no longer manufactured {{for Caterhams}}) they get
> better with use. New, the sliders are not well seated to the side members.
> With use they seem to get happier and provide a better, more progressive
> lock-up. We don't use any synthetic gear oils until the diff gets to this
> point and run regular old EP90 for the first 100 hours on track.
> I doubt it's the same diff though. Hope this helps.


The Suretrac Diff. is made by AP Racing <or Automotive Products UK??>
- Proven durability on road, road simulation and race track.
- Senses torque, does not require prior wheel slip
- Operates in conventional lubricants including ATF.
- Constant performance with respect to speed, torque, and temp.
- ABS compatible
- Supplementary to, and compatible with, electronic traction control

- Richard Meaden

January 2003

<article doesn't say if it has Suretrac>

Through the tightest corners the STi's front diff would tug and fight under full power, yanking the Impreza onto the straight with noticeable torque-steer. Through clearly sighted corners you could overcome this by braking deep into the corner (or even left-foot braking) to get the tail to slide out of line, then use the weight and power transfer to encourage the front and rear ends to work in unison. It was much easier and more reassuring through fast corners but the STi's dynamics were generally prone to yo-yo untidily between extremes in a fashion alien to the more malleable and ultimately more effective RB5 and P1. Experience and ability played more of a role in guiding the STi rapidly, but you rarely felt truly at one with it. And in the wet, even the most expert hands moistened at the prospect of pushing hard. No longer the great leveller of abilities, the STi was more than capable of biting the unwary or over-ambitious.


Suretrac® /Detroit LSD

4. Ratchet type LSD's

These are the most extreme of the LSD. These come on and off, with no
in between settings. These include the detroit locker types which are
used in RWD race cars (NASCAR) and the AP-Lockheed Suretrac ($2000+)
used on FIA homologated Touring cars.


Full-time 4WD operation is handled by a shaft final drive system that drives the rear wheels and a unique limited slip differential that drives the front wheels. The front differential is a torque sensing limited-slip design trademarked as SureTrack, and in automotive applications it is sometimes referred to as a "ratcheting cone" system. Compared to conventional limited slip systems, the SureTrack is a relatively simple and compact design that offers better performance on unequal traction surfaces and relatively light steering effort in tight turns.


AP Supertrack road/rally
A Torque bias diff - gives torque to the wheel with the most grip.
A new concept in differential technology for all road and competition applications (whilst stocks last.)

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I think that the plain and simple answer is that with a 300/300 hp/ft.lbs combo that the mfg EXPECTS people to drive all year, they wanted us to get the best way of puting as many horsies on the pavment as possible. :)

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Drove the competition

murley said:
I think that the plain and simple answer is that with a 300/300 hp/ft.lbs combo that the mfg EXPECTS people to drive all year, they wanted us to get the best way of puting as many horsies on the pavment as possible. :)
From the little that I read...It sounds like the LSD might be the reason why the STi got lower reviews in handling. Not because the LSD was makeing the car handle worse but because you have to drive differently with this kind of LSD. If you let off the gas in a turn the car will understeer...so...Keep your foot on that gas!!!

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That, and notice the one article says if you let off slowly, the line will tighten...yet if you let off quickly, the car will step out.

That sure sounds like what the mag guys had happen to em.

Oh well...I'm glad we have it. We'll appreciate the traction a lot more than we'll complain about the possible understeer problem.

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Keep your foot on that gas!!!
I have found this to be very true. My RS was very different handling, modulating steering with the throttle. But the STi bites in hard with the DCCD-A, you just have to learn to trust her. Keep your hands on the wheel and yer foot on the gas! I keep amazing myself with this car everyday. WOW :eek:

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Image uploading. Refresh page to view


Really good post, all feeble attempts at humor aside.
Sounds to me like the Suretrac actually addresses the AWD understeer issue at the root instead of suspension tuning it away. Usually when you push an Audi AWD car to its limit, you get understeer, at least stock. You can tune for oversteer or neutral, but it's still predominately understeering unless you make the chassis so stiff that the car isn't driveable.

It might take some learning because I'm so used to liftoff->oversteer, correct w/throttle, line straightens.

Should be fun, though...might need some swaybars regardless...

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I just hope someone is still watching this thread... :) Maybe someone has experience with suretrac and can answer this question: is this acting like a torsen (helical) differential and multiplies the torque that goes to the wheel with less traction and applies the result to the wheel with more traction? - in other words, in the one wheel in the air scenario, would a slight push to the brakes make more torque be dirrected to the the wheel on the ground? - because if torque is not multiplied from the wheel with less traction to the wheel with more traction then the push of the brake will do nothing. Thank you very much in advance?

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You just bumped a 15 year old thread. 15 years.
Good. This was his first post too. That shows a new member utilized the search function and asked a question in a thread that's already been created rather than creating a new thread.

Ya know... like everybody bitches about new members not doing.

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"If a wheel begins to slip, the relative movement between the two face cams, combined with the angular forces of the cam teeth causes torque to be transmitted to the slower turning wheel."

From the original post, 15 years ago.

What logically follows is -- if brake torque is applied to the slipping wheel (the one in the air), it will shift torque distribution to that wheel, not from it, as it will slow the correlated face cam.

Think of it this way, slip is the inverse of traction/friction/resistance. By applying the brake to a wheel, you are adding resistance and thus slowing the correlated cam, and shifting torque to that wheel. If the opposite were true, torque would always go to the wheel with the least resistance, which would functionally be an open differential.

Suretrac Differentials - Vehicle - Eaton

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Thanks for all the replies. Has anyone actually tried this? - I mean a diagonal test, maybe even going up a little bit. The videos I saw on youtube were showing that it either cannot be done or the driver didn't know how to apply the brakes...

8cd03gro - the link that you posted, is it for the suretrac that we are talking here about? - it seems to me that that is a suretrac developed by eaton and it's a clutch type, not the suretrac developed by AP Racing (?) which is the one this thread is about and some Subarus had it from factory.

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Thanks for all the replies. Has anyone actually tried this? - I mean a diagonal test, maybe even going up a little bit. The videos I saw on youtube were showing that it either cannot be done or the driver didn't know how to apply the brakes...

8cd03gro - the link that you posted, is it for the suretrac that we are talking here about? - it seems to me that that is a suretrac developed by eaton and it's a clutch type, not the suretrac developed by AP Racing (?) which is the one this thread is about and some Subarus had it from factory.
No, it is not the same Suretrac. I shared that link because the Eaton site has a lot of informational diagrams and videos. Though there will be some differences, the functionality is very similar to the unit in the '04 STi.
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