bastarddsm
20+ Year Contributor
- 5,583
- 1,409
- Aug 26, 2003
-
Mendota,
Illinois
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.
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.
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