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Center Differentials

Posted by bastarddsm, Jul 28, 2016

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  1. bastarddsm

    bastarddsm Proven Member

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    Over the last few years, I've been steadily learning more and more about some of the shortcomings of our AWD system. One of the largest sore spots is the center differential. It's an all around bad design, and I can't recall any other differentials that just allow a cross shaft to "free float" Anyone who has been inside a DSM AWD transmission has probably seen the results of this - galled spider gears, and cross shaft.

    As any prudent DSM'r would do, one of the first things I did to my first "race trans" was to get a 4 spider center diff, as the 2 spiders are known to break. Being a budget minded DIY guy, I realized the cheapest way to get a decent 4 spider was send the housing off to be machined, and source the cross shaft from another supplier. Looking around I found TRE to have one of the best reputations, and seemed to speak the loudest about doing the nicest work. One of big points of TRE was how he spoke of how he squares up the housing so that all 4 gears are loaded evenly. So in June 2010, I happily ship him my parts and get them back ready to be assembled. I get my cross shaft from another supplier, and I put the trans together. I end up running this setup for the last half of the season, and end up breaking a 3rd gear shift fork. The car made a handful of 11.80's passes all with weak 1.8 60's on street tires. The 60's were weak from taking off soft, not spinning the tires.

    Upon inspection if found my newish center diff to be all galled up. The cross shaft supplier blames the heat treat on the cross shaft, and cd housing isn't machined square, as well as the backlash on the gears. The supplier end up replacing the cross shaft free of charge, and I get some new spider gears and put it back together, but I ended up not using it for several years.

    Fast forward to 2014, I put the center diff in with a brand new set of shot peened and detailed gears from TRE. This last about 10 12.0 passes, 10 11.0 passes and 1000 street miles. The center diff cross shaft broke in multiple pieces and went through the gear set, turning it all to trash. The remains were extremely galled. I contact the shaft supplier and am immediately told the housing was machined out of square..... How can that be if I had one of the best do it? I asked, how does that matter when the shaft is a sloppy fit in the case anyway, what does it matter if the housing is square, when the shaft doesn't run square to the body, and constantly gets worse?

    I never did get an answer to these questions.

    After having trouble getting a hold of TRE, I decided to do my own center diff. Here's a few things I learned.
    1. The stock design center diff will never survive, even with the best machined 4 spider and cross shaft. Here's why: The shaft floats in the housing. It is allowed to rotate in the housing. What's more is the rounded surface of the cross shaft contacts a flat surface of the body, leading to a very very small contact area. The contact stresses are huge, the body is dead soft, and the shaft beats into the body, allowing more rotational play. No amount of machining will fix that. When the shaft rotates, it is no longer parallel to the thrust surfaces of the spider gears, thus it is no longer parallel with the bearing surface of the spider gear. We then loose bearing surface area, and eventually the contact stress becomes higher than the film strength of even the best oils, and then we have metal on metal. A galled up mess follows soon.

    I've still not been informed of what "machined square" actually means.

    2. A 2 load point 4 spider has ~5 times more stress than a 4 load point 4 spider. In fact, with 500ft*lbs of torque in first gear there is ~228ksi of stress in the cross shaft, that's near the tensile strength of most metals used in this application. It also turns out that my quick FEA predicts that the peak stress is right where my cross shaft broke. A 4 load point shaft is down to about 38ksi.

    3. The housing being machined square is essential to equal gear loading, however the shaft must be centered and perpendicular to the housing. With a “floating” cross shaft, that all goes out the window, as the shaft moves to where it wants to go.


    After all that, here’s how I built my center diff. It’s very time consuming, as I have about 12hrs of labor in the machine time, hand fitting the parts and checking clearances. The reward is one of the toughest center diffs there is.
    The first thing I do is use a cross shaft out of a rear differential. This part is $70 from Mitsubishi. I then machine out the factory drive slots to fit a key that grips the new cross shaft and locks it in place with no rotational play. The new drive slots and the thrust faces are machined in 1 fixture to ensure that the shaft is centered, and perpendicular to the thrust faces. I also use doubled up thrust washers behind the gears.
    I installed this in my car at the beginning of May, and have had about 50 passes on it so far, including IFO –Joliet, and Street Car Takeover at St. Louis. I’ve also had a couple trips to my local track with no prep – this resulted in lots of front tire spin, and on slicks that’s very hard on the diff. It still looks perfect and I’m very happy with the results.
    In short if you’re in the market for a 4 spider, be wise on what you spend money on. One that utilizes a cut down stock cross shaft might not be any better than a good condition 2 spider. I would be weary of a 2 load point shaft, as well as a beat up housing that lets the shaft rotate a bunch.
     

    Attached Files:

    Last edited: Jul 28, 2016

    Showcar Build 5K  1

    1991 Eagle Talon N/T
    9.805 @ 144.00 · 1G DSM
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  2. DeadlyAKrunna47

    DeadlyAKrunna47 Proven Member

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    I'm absolutely amazed (.)
     

    Street Build 3K  8

    1995 Mitsubishi Eclipse GSX
    manual · 2G DSM
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  3. Vegas smith

    Vegas smith Proven Member

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    Where does a welded center diff fall into all this? Isn't it supposed to fix most of these problems? Thanks.
     

    Street Build 3K  10

    1993 Eagle Talon TSi AWD
    awd · manual · 1G DSM
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  4. DeadlyAKrunna47

    DeadlyAKrunna47 Proven Member

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    welded diff and 4 spider diff are total different options.
    I had a dsm with a welded center diff. Driving on one sucks!
    Welded diff is only an option for drag racing.
    Never an option for time attack, autocross ect..
     

    Street Build 3K  8

    1995 Mitsubishi Eclipse GSX
    manual · 2G DSM
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  5. bastarddsm

    bastarddsm Proven Member

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    A welded diff does eliminate these issues, but creates others. A welded diff makes a car more likely to break axles and tcases. It also makes the car kinda suck to drive. It causes more Driveline noise and it also causes the car to understeer the car also will slow down when you go around corners as well as extra tire wear
     

    Showcar Build 5K  1

    1991 Eagle Talon N/T
    9.805 @ 144.00 · 1G DSM
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  6. 94allwheeldsm

    94allwheeldsm Proven Member

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    I always love seeing yours posts! Maybe market this? I made a 4 spider for an auto. So far so good but I haven't opened it up in about 2500 miles. Im half tempted to now and check it out.
     

    Street Build 1K  0

    1994 Eagle Talon TSi AWD
    automatic · 1G DSM
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  7. Kapok6

    Kapok6 Proven Member

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    What kind of price point are we talking to have a "proper" 4 spider done? I have a welded in my auto, and while I can easily manage it, I wouldn't say I enjoy it.
     
    Kapok6

    Street Build 2K  0

    1991 Mitsubishi Eclipse GSX
    11.3 @ 130.75 · 1G DSM
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  8. bonecollector

    bonecollector Proven Member

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    Why wouldn't the Quaife torsen Center Differential be a good alternative?
     

    Showcar Build 767  9

    1991 Mitsubishi Galant VR-4
    awd · manual · Galant VR-4
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  9. bastarddsm

    bastarddsm Proven Member

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    I don't know on an auto ID have to see one. Figure $5-600 plus parts at the least. it's very very time consuming. I don't expect many will spend the money on it. And I'm sure I'll get plenty that will tell me there jacks trans CD lasts just fine.
     

    Showcar Build 5K  1

    1991 Eagle Talon N/T
    9.805 @ 144.00 · 1G DSM
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  10. bastarddsm

    bastarddsm Proven Member

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    They like to loose teeth and junk the rest of your trans.
     

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    1991 Eagle Talon N/T
    9.805 @ 144.00 · 1G DSM
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    DeadlyAKrunna47 likes this.
  11. Kapok6

    Kapok6 Proven Member

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    Racecars...y u so expensive?
     
    Kapok6

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    1991 Mitsubishi Eclipse GSX
    11.3 @ 130.75 · 1G DSM
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  12. 1990dsmkid

    1990dsmkid Proven Member

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    Because of the time and intense labor it takes for him to do it.
     

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    1990 Eagle Talon TSi AWD
    manual · 1G DSM
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  13. Kapok6

    Kapok6 Proven Member

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    It was a rhetorical question/statement. I've been into these things for like 12yrs. I know what goes into them to make them fast and "reliable." I wasn't saying his pricing was out of line. I was merely acknowledging the true cost involved in making these things fast and keeping them on the streets.
     
    Kapok6

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    1991 Mitsubishi Eclipse GSX
    11.3 @ 130.75 · 1G DSM
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  14. 1990dsmkid

    1990dsmkid Proven Member

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    My bad man
     

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  15. keltalon

    keltalon Supporting VIP

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    So I assume that this one is still going strong in the tranny?
     

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    1998 Mitsubishi Eclipse GSX
    manual · 2G DSM

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    1998 Mitsubishi Eclipse GST
    11.1 @ 137.3 · 2G DSM
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  16. DSMReviver

    DSMReviver Proven Member

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    Thank you for sharing your knowledge and the advancement of dsm platform! :)
     
    DSMReviver

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    1998 Eagle Talon TSi AWD
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  17. redrkt

    redrkt Proven Member

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    Here is some info from TRE's website about it from their center diff page:http://teamrip.com/CHEAP_4_SPIDER_CENTER_DIFF.html

    "The center diff is lathed to square up the housing so it'll get machined properly. How are they indicating the center diff housing? Are they finding true center from the I.D. or just finding one wall and "going for it"? Are they averaging the two to make sure that Mitsubishi didn't machine the housing off center? Are they machining just two walls or are they taking a skim-cut to make sure that all 4 walls are perpendicular to one another? "

    Not sure if that is what the shaft supplier is referring to though.

    Thanks for sharing your work. I saw some pics on Facebook and was wondering if you were going to elaborate on it. That is pretty slick using the cross shaft out of a rear diff.
     

    421  0

    1992 Mitsubishi Eclipse GSX
    11.988 @ 115.020 · 1G DSM
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  18. bastarddsm

    bastarddsm Proven Member

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    I think it all depends on the use. My car is poorly suspended, and it spins the front tires a lot. Thats real hard on the diff. If you aren't beating it up like that I think most diffs will last a long time.

    I took this idea from tractor pullers. Lots of them have 4 pinion rear diffs, and the only thing they do is needle roller the thrust faces and they survice 2500+hp, with lots of independent wheels speeds. They clamp the shaft tight in the diff housing, not let it float like a dsm.
     

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    1991 Eagle Talon N/T
    9.805 @ 144.00 · 1G DSM
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  19. ThunderChild

    ThunderChild Supporting VIP

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    Thays pretty impressive bastard, I really like seeing that. I always wondered what might be different betwewn the rear and center diffs, and if some parts could be cross-polinated.

    I just had to throw a welded in mine today to get it back on the road.

    Out of curiosity, are you going to market this piece at all?
     

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    fwd · manual · 1G DSM

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  20. Tiki2777

    Tiki2777 Proven Member

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    Finally someone came up with a fix to the sh** CD engineering. Cheers, hopefully you'll market it, I can imagine there's some drag strip queens that want to ditch the welded diff.
     

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    1998 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM

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    1995 Mitsubishi Eclipse GSX
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