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  1. #1
    RC Competitor
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    X-Maxx Diff Hardening and shimming The next Level

    I quickly realized my x-maxx constantly bites off more that it could chew. Learning how to jump the beast, I realized that any jump over five feet when landing on flat bottom, had the possibility of chewing the rear diff. It quickly became more than just a problem, blowing a set, before getting though the first set of batteries. I verified my process 100 diff fluid and lock everything down tight.
    At this time the spiral gears came out and other people having the same issues were leaning towards them for the solution. One issue is you are forced to buy another set if you just chip one. They turned out to be a flop from reviews and they were correct. My buddy, who knew I was having diff troubles, bought me a set for the rear last month. I installed them stock without pinion shims and burned them up in fifteen minutes in front of him on wet grass.
    Now several months after my purchase and on my tenth set of ring and pinion gears, I decided to make an effort to identify and resolve the weakness and move on. Inspecting the chipped gears was where I started. The first thing I noticed right off the bat is that the rings, when the chips exposed the metal, it didnít look like metal? It actually looks like pottery shard. Terracotta in color with large sandy looking grain structure and didnít really look right. Doing some research I found the gears are made created by heat and pressure, melting pellet metal medium in a mold and heat pressing. This is the weakest form of gear making and is usually done with low quality steel with low carbon content.
    Researching more on the net I found people were adding shims to the diff but not many people were explaining how or what they were doing. Most of the information and videos were talking about shimming the bearing on the opposite side of the gear which was supposed to push the gear towards the pinion. I found in my testing that shimming the bearing didnít do much for the durability of the gears but removed some slop or pull that the drive cups had on the stock diff. Fully locked down there was a millimeter or two of slip when you pull the drive cup away from the diff. Knowing that there is only 2 ľ teeth from the pinion contacting the ring gear at any given time any movement laterally or in any direction is a problem and the first to identify. Moving forward I do not shim on the bearing side anymore and now shave a millimeter or two off the stock drive cup screw which eliminates the slip.
    Still chewing gears left and right, I started to look at the mesh on the ring and pinion gear and found some surprising results. With the stock setup with stock gears, actually was worse with the spiral set, the wear from the pinion on the ring gear was only traveling half way up the ring gear and when matched left half of the pinion off the ring gear entirely. I realized that shimming the pinion was what was needed in a big way. I experimented with different shim size and material but was having difficulty finding the right shim sizes anywhere. I tried china but the washer sizes I purchased were not the ones I received. I tested the ptfe shims which come with the difs, plastic shims purchased, plastic shims made, soft rubber shims, bearing covers and finally came up with the perfect shim which I still use now. This is the point in the other guyís article where they would stop.
    The perfect shim which chewed up several sets of gears to get right is.
    Revo 3.3 copper head gaskets sold in packs of three from dollar hobby 20gram 40 gram and 60.
    Traxxas 2.5 copper head gaskets which are slightly smaller in diameter sold at dollar.
    I no longer test the shims anymore satisfied with the results and have been running them for several months now. This is an absolute requirement to the overall best solution and has made a significant difference. Both front and rear get one 3.3 60 gram shim which will fix itself to the bearing cover after running.
    For the front this is the only shim which is required and is a full shim. This is one factor why you blow more rear diffs than front. The front has a tighter fit. Anymore on the front and your pinion will hit the cup.
    For the rear one 3.3 large 60 gram gasket on the bearing and the two largest 2.5 gaskets, which are slightly smaller and fit tight to the pinion.
    A full shim on the front and rear was eventually determined by looking at the chewed pinions missing or shaved teeth and how far the shear traveled to the base of the pinion. Before shimming you will notice that the chip or shave doesnít travel to the base of the pinion leaving some unshaved material before the base.
    In my area we have several mountain bike pump tracks ranging from mild tracks with some hits to a track with extreme gap jumps and these tracks are primarily where I run.

    So now the maxx can handle all of the jumps consistently at the mild track but still didnít graduate me to the big park which would only last five minutes or so blowing the rear on the second or third gap jump all with good landings. Once in a while I would blow the front on a botched jump ending with a mild nose landing but the overall majority were rear ring and pinions gears.
    At this time I was probably at 20 gear sets and would consistently chip or crush both pinion and ring gear each time blowing the set and sometimes coming home with all four chewed. Doing some research I found a guy who was hardening the stock gears himself, heating them up red hot and quenching them in dirty motor oil then in the oven at 500 for a half an hour to temper and relieve some of the hardening stress to the metal. He had mentioned that he noticed some improvement in their durability.
    Surprised I hadnít thought of this myself I started testing the process on some burnt gears. I started by checking the overall hardness with a file scrape on the gear before and after. After heating up to cherry red and quenching I noticed a slight difference in the metals hardness but still was able to gouge the piece with a file. Watching some videos I found that not all steel works quenching in oil and some require quenching in water to achieve hardness. I found that the water quench made the steel much harder that the oil quench and I started there. Heating them up to cherry red a quenching those several times. After I was satisfied I would torch them up to 600F and try to keep them around that temp for fifteen minutes then letting them air cool.
    I noticed right off the bat that maxx could take a bit more pounding which gave me some encouragement to continue. The next thing which was apparent was that I wasnít chewing ring gears as much. If I stopped within a few minutes after it started clicking take it home and eighty percent of the time the pinion was either shattered teeth, too hard, or it shaved / re-shaped teeth on the pinion, pinion too soft. But overall I was able to reuse the ring gear with was a nice change. And the biggest difference was that I was able to hit the big park with an average of 15 to 20 minutes before it would crush or shatter the pinion teeth. Knowing that I was dealing with low grade steel to start with hardening the gears might do some good but they would do better if I could get some more carbon in the steel. The process of quenching in dirty motor oil is to infuse some of the carbon from the dirty oil in the metal giving it more durability, which is different, that hardness. Not having any dirty oil around I turned to filthy shock oil and diff oil which I would heat and quench several times before the final water quench to hopefully pick up some carbon. It seemed to work and started testing the results with bobby pins. Checking the original state, then heat and quench with a mix of combinations and checking whether too soft or too brittle or looking for the middle, not too hard or soft. I found that the shock oil quench first full heat cherry red followed by a water quench next full heat cherry red followed by a quench in the shock oil again but only heating the part up to 600 F roughly half the cherry red temp. Silicone shock oil has some interesting qualities when used for quenching. Itís not actually quenching to a cool quickly like water or oil but rather the heat of the part is insulated by the shock oil and it remains at a high temp as long or longer than letting the part air dry. So I use the last shock oil quench for my tempering / stress relieving process
    With the above processes I can now hit the park for a solid 30 minutes and If I donít go crazy large the max seems to be ok with ten foot to flat bottom jumping and is able to take a considerable amount of pounding. Still chewing up pinions and the occasional ring gear at this time but going much larger in the big park taking an average of 20 jumps but now most of the failures are from learning how to hit the jumps in series or going large off the larger ten foot lip jumps.
    Somewhat satisfied with my efforts and looking at the new China Gears now picked up by GPM, china. The same issues arise, they are too expensive, you are forced to buy the set instead of just the gear you need. They are spiral gears which are prone to slip and chip. They are the wrong color for being sold as hardened gears. If they were hardened they have been fully tempered back to the metals original state due to the yellowish color which is the lowest on the hardness scale which can be estimated by the color of the metal. Temper the metal all the way back and it becomes light grey then yellow. For the price of one gear set I can buy like five full sets of stock gears. And lastly, if Kershaw canít get theirs right and are being reported as failing, I doubt the china ones are better. Most likely they are a copy or knock off of Kershawís, you know how they roll. I chatted with the seller when these gears first showed up on ebay for 160 for both sets. He claimed that he has not burnt a set in his xmax. I have seen videos of peoples so called diff testing on flat smooth ground. Not good enough. My last post to him I mentioned the gears are the wrong color for being sold as hardened. I got no further response.
    Good luck waiting for traxxas to sell their new diffs outside of the power up package. They will keep selling only the items in the power up pack to soak the customers for 300 bucks as long as they can. I called support to ask if the items would be available for sale individually and the rep said of course. That was a month ago. Not likely. So what if I buy the power up pack and burn up the diff in the first week, currently I would have to buy another power pack just to get the diffs. Just like the metal gear server they have on the traxxas site which is 120 dollars has no image and is currently not available. Sure would be nice for the customer if they would just sell the serve gear upgrade instead of soaking us for 120 dollars when really the only gear you need is the metal spline gear which gets stripped a bit if you have aluminum up front and plastic in back. 300$ all for the esc which I donít need and the drive shafts which they only upgraded the pins in, donít need already done. And their new rear bulkhead which looks identical and apparently supports a larger bearing but needed a full workover. Personally I believe that skinny piece of plastic supported by the steel brace under the diff which they call a rear end is one of the core weak points of the truck.. Frankly I donít need their diffs either Iím sure mine are better.

    Alright for those of you who stuck with this post I will share the new method.
    I started researching the process of carburization or the process adding carbon to the surface of low carbon steel to gain greater durability and hardness. The street name is Case Hardening and is commonly used for hardening low grade steel parts like gears and guns and is most likely the last possible option work with improving the stock gears.
    The process force carbon in which penetrate the steel and creates a hard shell 1 to 2mm in depth. There is really only one product on the market and it is called Cherry Red and can be bought on amazon for around 30 bucks and is tested to improve the hardness and impact durability of the steel from 25 points to 55 points on the Rockwell hardness scale 1-100
    I received mine the other day and have hardened my spares several times. Today I took both sets from my running machine which had been previously hardened with my standard process and treated them three times each with the magic powder. I wanted to test the results with gears that had been in the system and pre meshed with runtime on them getting that close fit before final hardening.
    The first thing I noticed with the wheels up was that the gears were quieter or a smoother sound, Iím running 8s geared up to 17 50 and the thing absolutely flyís. I took it to three parks today and came home with both diffs intact. I didnít take it as far as I did in my post below X-maxx on drone patrol but respectively had the longest run time at the big park. I noticed sparks coming off the gears with a wire brush today and had never seen that before and have a good feeling about this process.
    Process is to heat the part up cherry red 1500 F Then take the gear and mash it Into the cherry red power forcing it into the gears. Then directly hit it with the torch for another five minutes or as hot as you can get it then quench in water. My water is mixed with cherry red and the water temp works best at 120 F Most everything dances off metal at this temp but the product encases the part with like lava and doesnít roll off. Clean the part and repeat at least once more. Clean the part then torch it to half heat roughly 600 to 700 degrease and let air dry or what I did is to mix the cherry red with some dirty shock oil and let it temper in the oil which I have in a silicone container which substantially retains the heat much more than air drying.

    Xmaxx on Drone Patrol
    https://youtu.be/iomnY_y9tkQ
    Diff Workshop
    https://youtu.be/m9csVSbMboo

    Things which made a difference not in order
    1. Shimming and hardening
    2. Lots of grease in case I installed a zerc fitting on the upper right of my diff cases to accept the grease gun.
    3. Hot Racing Aluminum Diff Covers
    4. Locking down the bulkheads and verifying the everything is tight.
    5. Larger bearing on the rear diff. The bearings in front are wider than the rear but when you buy the kit they only come with the wider for both which are more stable in the case.

  2. #2
    RC Champion
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    > WoW <

    Good JOB
    fabricator

  3. #3
    RC Racer w8liftn's Avatar
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    I watched your video and don't understand how you could blow anything based on the jumps you were doing. I have jumped and jumped and jumped and much of what I do is 5-8 feet of air and have had no problems. Not saying I'm some kind of jump pro, but I've also seen hundreds of videos of jumps much higher and landing harder with no issues. Just wondering if you have a flaw in your truck that is causing this?

  4. #4
    RC Qualifier rtmike's Avatar
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    brockseals, thanks for taking the time to write that out. I actually understood it, lol.

  5. #5
    RC Competitor
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    I'm 25 packs deep with the power up kit, HR diff cups and the HR bulkheads and not a single broken gear yet. Multiple trips to construction sites, more jumps than I can count and it's still perfect. Perhaps it's the center diff and beefier diff gears, all I know is I'm not having any issues and it's proven to me to be extremely durable.

    OP when you land jumps you should not be on the throttle at all when landing, sounds like you might be doing this.

  6. #6
    RC Qualifier
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    I amongst others find that the HR diff cover helps ALOT. The edge on the stock cover is to thin and flexes around the bearing area making the diff move sideways under heavy load I think. Great write up!

    I have high hopes of the new v2 8s gears using alu covers and alu cups. Or the same setup with the rock passion/gpm gears .

  7. #7
    RC Competitor
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    X-maxx on drone patrol video is probably the largest jump I have done estimating 12 to 15 feet out of a ten foot lip to flat bottom and drove away. Not sure which video you watched i have several.

  8. #8
    RC Turnbuckle Jr. ReglarGuy's Avatar
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    Man, that was really good and informative. You did a terrific job with your post and videos. For me, however, I just don't have any real problems with my diffs, but I'm a 6s guy to, though. So, maybe that's the difference between us. I have to say with me running the Castle/Kershaw 54/29 setup, I'm very happy with my 6s setup.

    P.S. Super awesome post !!! As KSB would say...EPIC ! You are now the new Traxxas forum god.
    Last edited by ReglarGuy; 02-20-2017 at 06:55 AM.
    Better to die a man, than live like a girly-guy.

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