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Differential FAQ: Consolidated and made relevant for Legacy owners


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must be just for that year then cause i had a 02 BE and it had a open rear.


Was your car a GT?


Only the GT versions got the LSD. Of course it could be a Canadian thing.

Edited by camber
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  • 2 months later...

Bumping this up so I did not have to start a new thread..and sorry for the long post.


Today I was parked on an up-hill, ground was completely dry. Passenger front tire is on the edge of the curb almost on the pavement and rear passenger tire are on the sidewalk (very low curb), driver side tires are on the pavement. I turn the steering wheel almost full lock left to pull out of the parking spot and i ease into first gear. The passenger side front wheel spins and I go no where, actually I rolled backwards with a spinning front passenger side tire. Okay...so I try again, same result. I get out expecting to find oil or some other slick stuff on the ground or on the tire...there was nothing. Dry ground dry tire.. I am starting to panic now. So I decide to try to straighten out the wheel (instead of almost full lock) and same result. I let the car roll back straightened out the wheels and tried again and all was normal again.


After that episode I tried launching the car semi-hard to see if I spin that front right wheel and not the others. To my surprise car grabbed and took off in full awd fashion.


What the hell happened?! Is this the open front diff at play?

Edited by Spec B
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Open front diff gets my vote. You might think that the viscous coupling in the center would bind up and therefore save the day, but those things don't really bind up nearly as hard as we'd all like them to.
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Very good whitetiger! I'm one of those who really didn't "need" the front helical LSD, but went for it anyway (couldn't pass up the deal I found at RalliSpec). NO REGRETS on the LSD!!!


I love the low maintenance and seamless characteristics of the helical unit. And having stepped up to 225 PS2's, it shines through that much more.


....now I just need to wait for this flippin' humidity to pass through so my stage2 can let me push it that much more.


Thanks again whitetiger!

"If we are to guard against ignorance and remain free, it is the responsibility of every American to be informed." - T. Jefferson
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Pretty sad I think.


First time anything like that has happened to me in any car (fwd, rwd or awd).

its a combination of both the center viscous and front diffs failing. if one fo the front wheels has no traction, then its like one end of the center diff not having traction. the viscous unit cant transfer all the torque to the rear. so the rear wheels did not get enough power to move the car up the hill. a front LSD would prevent situations like this from happening.

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After a fair amount of searching I was able to find some real literature on the AP Racing Suretrac differential, which I just bought off Whitetiger. It is an interesting design for sure, and I've attached some images that I plucked from the mycaterham.com website to show off the cool internal geometry of the face cams and followers.


From reading the literature it does seem to me that this will act as an open differential when one wheel is in a near-zero traction condition (lifts a wheel or on glare ice) and during turns where there is little input torque. However, in power-on turns, the torque going into the differential will 'preferentially' go to the wheel that is slower (more grip) and act as a brake to the wheel that is slipping. The amount of torque transfer is a function of the friction between the cam and followers, which is directly proportional to the input torque. This means that the differential responds to how much torque you are sending through it, and not the speed differential between the left and right wheels (like the OEM viscous unit does).


Pretty cool stuff. :)


Oh, and why hasn't this been stickied yet?











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I've been trying to picture how the non-equal number of 'teeth' on the left and right face cams is critical to the operation of the diff, as noted on p.2 of the literature:


The number of teeth of the left and right face cams differs by one tooth. The reason for this is that, in order for the cam followers to transmit drive force to the face cams, the left and right face cams have to form wedge shapes, as shown in Figure 3. If the number of teeth were the same, all the hills and valleys of the teeth might in some cases arrange themselves opposite each other. The prerequisite wedge shape then would not be obtained, and the cam followers would slip down the helical surface of the face cams, zigzagging between the left and right face cams and making it impossible for drive force to be transmitted (see Figure 4). For this reason, the right face cam has 10 teeth and the left face cam has 9 teeth. If the number and angle of the teeth were the same, the force transmitted to the left and right sides would be the same and there would be no difference in rotational speed between the left and right sides. A slight difference between the left and right sides is created, therefore, by having a one-tooth difference on one side. Since this difference is slight and does not exert a marked effect, however, the torque transmitted to the left and right face cams is herein treated as equal for the sake of clarity.


But when I view the pictures of the torn-down diff on mycaterham.com, we clearly see six teeth on both cams... :confused:



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it would be ok, as long as the diff is meant for the 6speed. It would be better if you were to find a 05+ OEM USDM sti diff as they are a helical unit and generally operate faster and more predictably than the suretrac. but if you can get that suretrac on the cheap, go for it.


the 04 USDM STI came with a front suretac but it was changed in later years to the helical for the reasons stated above.

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