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Thread: Need help with ratio..center diff on my SM

  1. #1
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    Need help with ratio..center diff on my SM

    Hi guys,
    well recently i came across a guy who needed over 10 bodies painted. After i discussed my price he was a little taken back by how much it would cost. I told him that it was as low as i could go and if he had anything worth trading. This is when he tells me that he's a machinest and really doesn't have anything to offer. So i asked how difficult would it be to make a custom diff mount for my truck. He said it would be pretty easy. Now comes the hard part; the gearing.

    So i know most buggies use 13t bell and a 51 tooth spur. But the problem is you can't sit the diff high on top of the chasses. The Diff would need to be below the chassis and have a gear that spins it from the top. Similar to how the old E-Maxx Differentials worked. THe machinest says that he can make gears out of steel but i couldn't figure out what tooth count to make the first top gear. Also i imagine because of lack of space that i would need the smallest spur possible. ANyone have a clue on numbers for this?

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    UE ADDICT daparrothead's Avatar
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    Wouldn't this be real close to the RCM slipperential as far as gearing goes?

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    respi, the RCM slipperential uses a Revo 40tooth spur, but you can get it down to a Revo 36tooth. If remember correctly the 40 should be able to fit between the rails... now you just have to have the machinist make the mount so that the spur comes up far enough to engage the pinion. I might be able to do a little wrenching and mock up for you tomorrow and post up some pics...

    Since you're talking about the 'bell' this going in a Thunder chassis right?

  4. #4
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    How about a idle gear, then you could use stock revo spurs. The idle gear would be on a seperate mount by itself connecting the clutch bell and spur gear. Think of the gearing options with this setup.

  5. #5
    Veteran george16's Avatar
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    Here's a couple pics of the slipperential. You might get a better idea by looking at the picture.









    If your machinist can make a mount similar to the one pictured, shouldn't be that hard. Then use revo spurs like Jim stated for better adjustment and tuning. The gorilla maxx chassis has the same width as the thunder tranny rail. I have both chassis for comparison . I think something similar to the slipperential will be the best option.

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    George that is PERFECT and EXACTLY what i was looking for. Can you give me exact dimension? For example how thick are the two plates? Also how are apart are the top shaft from the bottom shaft (center of bearing hole opening to other center of hole opening)?

    1: what bearings on top part
    2: what bearings on bottom part----------8x16
    3: what tooth count is on the diff?
    4: is that a Revo shaft on the top with a revo gear (tooth count)?

    Basically i'm going to copy the slipperential and mod it slightly to work the Thunder. If it works out well then i would come back here and offer it up for sale to Thunder and Pred owners. Time to give Revos a smack down!!!

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    Where can I get the RCM slipperential?

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    Wow I've never seen a slipperential before but yea that's kind of what I was thinking. Only thing I don't get is why have a slipper and a center diff?

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    You would lock the slipper.

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    Veteran sweetride4me's Avatar
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    Quote Originally Posted by Tekk
    Where can I get the RCM slipperential?

    Rc-monster.com
    -Chris

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    respi, check out this thread...another way to do it
    http://www.rc-monster.com/forum/showthread.php?t=10233

    scroll down a little on this one.

    http://homepage.ntlworld.com/ryan.coombes/emaxx.htm

  12. #12
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    that first link is what i have in mind. Then i could just get an engine plate made maybe 3mm thick (shave 3mm off the top of a UE engine mount) and have the engine directly in the center of the chassis. I currently am running 16 toot bell with 51 tooth strobe. What kind of gearing would i need on this Thuder-Diff to make it the same as my current setup?

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    Quote Originally Posted by sweetride4me
    Rc-monster.com
    I have been through the entire site, can't find it anywhere...got a link?

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    UE ADDICT daparrothead's Avatar
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    Hey Respi,
    Have you thought about using the 3.3 trans with the Revo center diff? I talked with George about doing this months ago, someone over on the Traxxas forum tried it and it worked with a bit of modding. May be an easier & cheaper way to add the center diff and parts would be easier to find.

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    well the problem with the Revo tranny, is; i can't add a rear brake. This is where i lose a lot of time out on the track. Whil Revos can brake later i lose time because i have to slow down just a bit. Granted i am posting my fast lap times as fast or faster than most Revos but i'm more proned to crashing because i have the truck at its' limits. Just to give you an example: This past weekend i was .3 secs faster than the fastest MT's (24 BB MT racers). The revo's were killing me at the end of the straight. I would have to slow down 3/4 of the way down while they got to hit the brakes and throttle off the other side of the turn. So to compensate i had to increase my wheelbase to 17inches. Now i can hit turns hotter and faster but still am limited. I want to push it harder but when i do it will go up on two wheels. (I realy need to by a video camera to show you). SO with a center mount it will calm my truck down coming out of corners, give me a rear brake bias, and decrease number of rotational parts, centralize weight, and give me a really cooling looking outlaw BB MT.

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    UE ADDICT daparrothead's Avatar
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    It would definately be cool looking and it should work great!! Oh the same guy that used the Revo center diff also made the Revo rear brake work on the 3.3 trans with some mods. If you pull off getting something machined to to make a center diff I would be interested in one or two.

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    There are a couple different rear-brake kits for the Revo transmission, Traxxas even makes one...that's exactly what you need!

  18. #18
    Veteran KiKiMaX's Avatar
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    Hi Respi

    Look this thread: http://monster.traxxas.com/showthrea...=417249&page=1
    3.3 t-maxx tranny with revo diff and rear brake
    HVL:VBS, Racer X, NEU 1521, 6S
    Pred: Predator, Mono, VBS , Racer X+setback, Rb SP10...

  19. #19
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    that thread looks good, but i'm concerned about those plastic gears.

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    as for gearing the Slipperential is a 2.2. Check out this site and you can figure out your gearing. The calculations are made for brushless but it lists nearly all the possibilities for nitro trannies and setups.

    http://www.scriptasylum.com/rc_speed/_top_speed.html

    maybe checkout Jordan's setup on the Gmaxx post as well for some ideas.

  21. #21
    Veteran george16's Avatar
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    Respi, I would be glad to give you dimensions and measurements if I have a slipperentiial. The pictures I posted was from a thread at rcmonster I saved last year because I was thinking of buying one for my truck. Unfortunately, Mike (rcmonster owner) havent't start selling them yet. Here's the thread: http://www.rc-monster.com/forum/showthread.php?t=4767

    The cd will work with the 3.3 tranny. Gotjeep? from the traxxas forums did it to his tmaxx. I was contemplating that before like Kenny stated but I got too lazy. I might revisit and have a machinist make me some stuff when I go back to the ship at the end of this year.

    Check this thread for the revo CD made by cowboy from rcmonster:
    http://www.rc-monster.com/forum/show...hlight=revo+cd

  22. #22
    Veteran george16's Avatar
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    Quote Originally Posted by Tekk
    I have been through the entire site, can't find it anywhere...got a link?

    It's not available. Mike is still working on version 2 and nobody knows when it's coming out.

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    Well i removed my tranny and measured the inside width. It appears that it is exactly 44mm wide. The smallest spur that i could find for a buggy size diff is 41t made by XRay for their buggy. I'm not sure what the diameter is of this spur so if anyone has one please feel free to chime in. I know the Xray 43t spur is 46mm in diamtere.

    What's intersting about the pics above is that the spur on the Gorilla Maxx chassis seems to fit in there nice and easy. I wonder what diff they used. Certainly isn't a revo diff but they do have a Revo spur.

  24. #24
    Mr Destruction wolfs4evr's Avatar
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    If one designed it direct drive like in a buggy or current truggy you would want a 65T spur or so. 13x65 is what I ran in my Razor most of the time with the Spider diffs.
    Tom...

  25. #25
    Mr Destruction wolfs4evr's Avatar
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    Their is also no brakes on the set-up above. Wouldn't work too well as posted above in a Nitro RC. Though a dam clean looking piece of work. A whole lot more thought would have to go into it to devise a way to add front and rear brakes to that diff and then the linkages from that point to the servo.
    Tom...

  26. #26
    Veteran george16's Avatar
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    41T mod 1 spur has a diameter of 43mm.

    Since you peeked my interest, I am thinking of ordering a revo tranny, CD, RR metal gears, and rear brake kit. I'm hoping the revo tranny has the same footprint as the old emaxx tranny (I fitted it on my thunder chassis before). If it does, it means the revo tranny will fit into the thunder chassis.

  27. #27
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    Quote Originally Posted by respirologyr
    George that is PERFECT and EXACTLY what i was looking for. Can you give me exact dimension? For example how thick are the two plates? Also how are apart are the top shaft from the bottom shaft (center of bearing hole opening to other center of hole opening)?

    1: what bearings on top part
    2: what bearings on bottom part----------8x16
    3: what tooth count is on the diff?
    4: is that a Revo shaft on the top with a revo gear (tooth count)?

    Basically i'm going to copy the slipperential and mod it slightly to work the Thunder. If it works out well then i would come back here and offer it up for sale to Thunder and Pred owners. Time to give Revos a smack down!!!
    Just for reference:
    1. Bearing on top is 5x10x4
    2. Bearing on bottom 8x16x5
    3. Diff has 44t
    4. Revo shaft has 20t giving you 2.2 ratio

    Respi, you got PM

  28. #28
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    what 44t spur did they use; do you know mtbnjim? Also i was thinking that if i could get rid of that top shaft entire (the one in the pics above), then i could mount the engine so that it touches the diff dirrectly. remember you can't let the diff stick out below the chassis braces because then you run into problems if you land wrong during racing. Also by having the diff partially half way up on the chassis you would now have room for front and rear brakes.

    SO what clutch bell would i need for a 44-41t spur? I think maybe a 16t-13tooth bell. Not bad at all in my oppinion.
    Please correct me if i'm wrong. This whole ratio gearing stuff is very new to me. Before i go having machine work done i need to just toss these ideas around a bit so that any issues that people can think off can be delt with now and not 2-3hundred dollars later

  29. #29
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    woops i was wrong on my gearing: Wolf said 65/13 ratio: 5
    with my maxx i'm running 51/16 ratio: 3.18
    BUT i'm using a 2 speed and not direct drive like a diff. So a spur of 44 with a bell of (16-13): ratios= 2.75, 2.9, 3.1, 3.3, (10t bell)--> 4.4
    Man that's a small bell!

  30. #30
    Veteran george16's Avatar
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    I think it was a kyosho 44T center diff, if I remember correctly. By the way, are you planning to gear it close to stock tmaxx gearing or more like a truggy/buggy?

  31. #31
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    Why has there never been a center differencial for the SuperMaxx, T-Maxx 2.5 and 3.3? Can you imagine the take off speed without the wheelies? There used to be a JT chassis that had a center differencial and had all the UE VBS, RacerX and all the trimmings!

  32. #32
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    Can someone help me with this ratio?

    The RCMonster unit pictured above has a diff gear of 44t and the gear directly above that at 22t then the spur is 40 tooth. How do i figure out the ratio for this?

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    It's 44t on diff and 20t giving 44/20= 2.2 with a 40t spur
    Typical Maxx:
    1st gear is 43/13=3.3...then most are using it with 51t spur then,
    51T spur/CBell 15T=3.4 = final gear ratio

    So you'd have to make up the difference somewhere. Maybe someone with better gearing knowledge will chime in

    But here's the pic with the 44t cd fitting between the thunder rails. It hangs below the rails about 6mm.



  34. #34
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    hanging below the rails by 6mm isn't a big deal. You can easily shim the lower plate to protect the spur, or you can cut into the plate and make a plate some what like an oil pan on a 1:1 car. You can easily hook up a brake system to that setup! See where the Revo SPur is. THere are 6mm in junk back there. YOu can remove the slipper off the Revo Spur and stick a buggy brake setup in there.

    Mtbnjim; what is that top shaft made of? Is it two seperate pieces or is that 20t gear all one piece with the shaft. After looking at various other CD pieces it looks like earlier models were using 22t pinion gears that were secured onto a custom shaft.

  35. #35
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    in either case it appears that we are stuck with the top shaft because we will need to use a spur to make up the difference in gear ratio.

  36. #36
    Veteran sweetride4me's Avatar
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    Will the diff gear fit between the thunder tranny opening?? Why couldent you make it so the spur and top shaft sat slightly off center and moved that center diff gear up 10mm??? Just throwing out ideas....
    -Chris

  37. #37
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    i'm guessing the top shaft is a standard tranny shaft since the Revo spur/slip fits on it naturally. It is an independent shaft, the 20t gear, is just a gear and has a hex pin holding it... if you custom make the mounts you probably could just swap out the 20t for anything else to get the ratio you need. Center to center between a 44t and 20t is 34mm.

  38. #38
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    Well swaping out the 20tooth would be great but then you run into problems with the gear braces. Reason being that the brace will be stuck at 34mm (or whatever number gear we decide). You would have to leave the 20t gear alone and use the spur and clutch bell as your tuning areas.

    Once the gear ratio numbers are figured out then i can most certainly get this project under way. i will be forking over all the money up front and making about 50 units. These units should fit Preds, Gorillas, Thunders, 3.3, 2.5s, original maxx, Hardcore,and anything else out there. Then it would be up to the buyer to buy and get their own drive shafts. This is still very much in the planning stages so keep your fingers crossed. I'm waiting for some stuff to come in the mail. I want to see how the diff fits. I also want to use a plastic spur because this will eliminate metal on metal problems. plus keep weight down.

  39. #39
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    Im sure everyone knew this, but if you go to the directory there is more pics. Thanks for the pics Lito

    http://rc.stuurmijmail.nl/slipperential/
    Drive a hybrid, I need your gas

  40. #40
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    So these are the ratios for this setup (44T Diff. and 20T top shaft pinion) with mod1 spurs and bells. These are all the Final Drive Gear Ratios.

    40T Revo Spur:
    16T Bell: 4.4
    15T: 5.9
    14T: 6.16


    Strobe Spur 51T:
    18T Bell: 6.2
    17T: 6.6
    16T: 7.0
    15T: 7.5

    Now if we were to use a 50T mod1 spur and the following clutch bells:
    18T bell: 6.1
    17T: 6.4

    Look like you can go either way with the Revo spur or the strobe spur. These calculations include the internal ratio and the primary ratio; which when multiplied together create a final drive ratio.

    Primary Ratio (PR)= Spur/clutch bell
    Internal Ratio (IR)= Diff/pinion
    Final Drive Ratio (FDR)= PR x IR

    It took me a while to find the calculations. So now i feel better knowing that Wolf was using a ratio of 6 on his razor. So anything around this ratio should do well on a thunder or pred that is using a 6 or 8 spyder gear diff. Not only that but it takes the guess work out of trying to figure out what will work and what will not work. Hopefully this week i can show you guys some pics of a mock up.

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