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Old 12-20-2006, 01:03 AM   #1
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Default Brushless & Diffs

I run my sedan (RDX) on carpet with foam tires in a class where we use the Novak SS4300 motor. For those of you that aren't familiar, the motor performs similar in top speed to a 19T, but has the torque of something much faster.

The problem is, with all that torque, I really have to tighten down the diffs a LOT to keep them from slipping. And I'm not talking about from a dead stop, I mean when I get on power out of the turns, if the diffs aren't tight, they'll squeal.

I can't help but think this is affecting the handling and setup of my car, since diff adjustment is often considered one of the most important factors, at least according to the books. What are people doing to get around this problem? Presumably, mod motors have enough power to cause similar problems? How are you guys running mod dealing with this? Am I overestimating the importance of the diff setting, or how much keeping it really tight will affect my car? Would one manufacturer's diff be less prone to slipping under torque than another?
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Old 12-20-2006, 01:25 AM   #2
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I'm running a MRE with a 3.5 Novak BL motor, I build the diff as usual. But I think the MRE can with stand the power of the brushless is due to the aluminium diff outdrives.

My friend runs a TA05 with plastic outdrives and it does slip and has to be rebuilded often...
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Old 12-20-2006, 05:59 AM   #3
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I had success using the expo feature in my raido to dial out the punch and reduce the squell in my RDX. That was before we switched to rubber. Now the problem is gone without the instant traction of foams
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Old 12-20-2006, 09:52 AM   #4
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I also run a 4300 and tightend my rear diff because I felt some slipping. For the longest time I was chasing set-up because the car felt tweaked.So I backed off the diff just a little and the car was dailed. So yeah a properly tuned diff makes a big difference at least in my case it did.




Quote:
Originally Posted by syndr0me
I run my sedan (RDX) on carpet with foam tires in a class where we use the Novak SS4300 motor. For those of you that aren't familiar, the motor performs similar in top speed to a 19T, but has the torque of something much faster.

The problem is, with all that torque, I really have to tighten down the diffs a LOT to keep them from slipping. And I'm not talking about from a dead stop, I mean when I get on power out of the turns, if the diffs aren't tight, they'll squeal.

I can't help but think this is affecting the handling and setup of my car, since diff adjustment is often considered one of the most important factors, at least according to the books. What are people doing to get around this problem? Presumably, mod motors have enough power to cause similar problems? How are you guys running mod dealing with this? Am I overestimating the importance of the diff setting, or how much keeping it really tight will affect my car? Would one manufacturer's diff be less prone to slipping under torque than another?
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Old 12-20-2006, 10:52 AM   #5
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Do mod motors have the same kind of instant, diff-killing torque? What about the faster brushless motors? What are the pro guys doing?

I feel like the tightness of my diffs is preventing me from truly knowing where my setup is at.
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Old 12-20-2006, 11:43 AM   #6
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Quote:
Originally Posted by syndr0me
Do mod motors have the same kind of instant, diff-killing torque? What about the faster brushless motors? What are the pro guys doing?

I feel like the tightness of my diffs is preventing me from truly knowing where my setup is at.
Brushless is a bit more brutal torque wise than even a really strong Brushed motor. A lot of the Pros I talk to have to rebuild diffs pretty frequently. Cars with bigger diffs (diff balls on a larger hole circle) have fewer problems than cars with really small diffs. The bigger diffs can handle the loads better. Really high quality thrust bearings and really flat diff rings help a lot too.
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Old 12-20-2006, 11:52 AM   #7
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Quote:
Originally Posted by AdrianM
Brushless is a bit more brutal torque wise than even a really strong Brushed motor. A lot of the Pros I talk to have to rebuild diffs pretty frequently. Cars with bigger diffs (diff balls on a larger hole circle) have fewer problems than cars with really small diffs. The bigger diffs can handle the loads better. Really high quality thrust bearings and really flat diff rings help a lot too.
Thanks Adrian, that's the kind of response I was looking for. I wonder if we'll see some additional thought given to diff design in the next generation of cars as brushless continues to come on strong.
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Old 12-20-2006, 11:54 AM   #8
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syndrOme,

While not the same car, my TC3 with 4.5 brushless DESTROYS plastic diffs in less than 1 run with any decent throttle punch. 16T did took a few dozen runs to do the same. 27T gives me a few months.

Metal outdrives are a must for TC3/TC4 in mod. Might not apply so much to your Corally, because I suspect shaft drive = more diff killing power transfer.

My experience on TC3 says unusual level of slip in my diffs = highly erratic, unpredictable performance, sudden pulls one way or another, especially in turns getting back on the throttle. There have been a few times when I had the car set up well on the board right before the heat but had weird handling problems. In several of those cases, the problem was a loose or failing diff, and fixing it fixed the handling. If you're chasing set-up, sounds like a likely cause.

Hope this helps some.
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Old 12-20-2006, 12:29 PM   #9
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Quote:
Originally Posted by syndr0me
Thanks Adrian, that's the kind of response I was looking for. I wonder if we'll see some additional thought given to diff design in the next generation of cars as brushless continues to come on strong.
Syndr0me,

Since rebuilding my diff with ceramic diff balls/thrust brg and polishing the diff rings I have not had any problems with my rear diff. This is with either the 4300 or the 4.5. I used AE black grease on the thrust and Corally lube on the diff balls. I occasionally take it apart to clean it and re-polish the diff rings but I have not had a failure in quite a while. YMMV....

P.S. the stock stuff (diff balls etc.) does not last very long under these conditions........

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Old 12-20-2006, 01:06 PM   #10
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Good advise. We fould sanding the diff rings like on a 1/12th car make the diff last longer too.

Becareful with ceramic diff balls though. Not all are created equal. You can run into problems with materials whos friction coefficients are too diffrent. If the balls are too hard they won't have enough bite into the ring and they will slip and dig a trench in your rings. I think Tungsten Carbide balls are still the best.
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Old 12-20-2006, 01:14 PM   #11
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Thanks for the advice guys. I've found the Corally grease helps a LOT in reducing rebuilds thanks to how thick it is, and the ceramics (thrust and balls) help as well. I use Much More's black grease on the thrusts which has been excellent as well.

I'm actually not having problems with frequent rebuilds, though. My challenge is that I have to run the diffs so tight so they don't slip that I think it's affecting the performance/setup of the car. If I loosen them to where they'd be with a normal motor, they slip like crazy both on takeoff, and when exiting corners or applying small amounts of throttle. And it's not like I'm mashing the trigger, I'm talking about fairly minor changes in throttle when exiting turns. It's easy enough to control on takeoff.
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Old 12-20-2006, 01:17 PM   #12
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Adrian, do you think making the diff rings more gritty (by sanding them) and switching to carbide balls would reduce the slippage issue? Could it be that they're too slippery?
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Old 12-20-2006, 01:51 PM   #13
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Default ding ding ding

We may have a winner there... i run the "cheap" diff balls, rings not perfect, and I don't see much slip (as long as I use the all-metal diffs).

I'm curious to hear Adrian's response.
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Old 12-20-2006, 02:00 PM   #14
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Quote:
Originally Posted by syndr0me
Adrian, do you think making the diff rings more gritty (by sanding them) and switching to carbide balls would reduce the slippage issue? Could it be that they're too slippery?
I sand the rings with 600 grit and motor spray so they actually look polished when you are done. I have been doing this for almost 20 years in pan cars and have started doing it to sedans in the last couple years. Sanding the diff rings makes the diff feel smoother and slip less.

Tungsten balls have a higher coefficient of friction against steel than most types of ceramic balls. This means ceramic balls are more likely to slip under the same amount of diff pressure.
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Old 12-20-2006, 02:01 PM   #15
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Huh, if downgrading the balls in my diff and roughing up the rings fixes this, that's gonna be pretty funny. The quest for buttery smooth diffs might be working against me here.

Last edited by syndr0me; 12-20-2006 at 02:13 PM. Reason: Question already answered about sandpaper
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