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Dromida diff flaw?

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Old 06-15-2017, 08:01 AM
  #1
celticsun33
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Default Dromida diff flaw?

I have a Dromida sc4.18bl. While disassembling to clean, inspect, and relube I noticed the outer bearing of the diff shaft was not spinning on the inner race but the whole bearing was spinning. I inspected a little closer and found that there are 4 tabs on the diff. the bearing is going inside these tabs and they are keeping the outer race locked in place. thus causing the whole thing to spin. Because of the width tolerance I couldn't put a spacer in or it wouldn't fit into the case. I therefore cut the tabs off allowing the bearing to spin correctly. both differentials were like this so I did them both. Has anyone else noticed this?
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Old 07-05-2017, 07:10 AM
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EXT2Rob
 
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Wow, that's interesting. I had not heard of this issue. I will have to inspect the ones we bought for the boys. The cars haven't had that much run time, so I haven't gotten into the diffs yet. Thanks for bringing it up.
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Old 07-05-2017, 12:21 PM
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Ok, so I just got back from having a looksee.....and you're right! The way they designed the diff doesn't allow the outer bearing race to spin! That seems really odd. The ONLY reason I can come up with is, it adds strength to the diff and the diff's output collars. The diff isn't resting entirely on the outputs when you put it in the housing. But it seems pretty clear that the outer bearing races are somehow spinning in the bearing seats in the housing. And that isn't a good thing. We're talking drag, motor strain/heat, friction, and possible melting of those seats, if you have a brushless model. I melted bearing seats in my Hyper TT, when I hadn't noticed one of the diff bearings had seized. Had to replace the whole housing.

However, the cars have been running just fine with that arrangement. So go figure. ?? Still, in trying to just turn the bearings by hand, it's impossible. I could turn the one on the opposite side, a little, but it just bugged me. So, I took an X-acto knife to it and whittled down the four tabs on top and the ring on the back to flat. The diff sits in the housing as normal. So........what the hey, Dromida? I only did one for now, to see what happens. But failing any unforeseen issues, this might just be the mod that needs doing on all. Try it yourself and see what happens. I'd think we'd get less rolling friction > lower motor temps.
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Old 07-05-2017, 04:47 PM
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trimming the tabs off is a common mod for these cars. i hard of people doing it and did it to all 3 of my cars. that was over a year ago, and even on brushless/lipo power durability hasn't been an issue. it just makes the cars roll a lot free'er.
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Old 07-05-2017, 08:49 PM
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Hey Hooligan! Haven't seen you 'round much! Cheers!
That's good to know. I did both diffs on one car and test drove it pretty hard, seems fine! Motor temp was only 137F.
I;ll start on the other one tomorrow.
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Old 07-06-2017, 05:59 AM
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celticsun33
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I've so far done the mod to all 3 of my cars. (2 SC and 1 touring car) I've never checked the temps but it has to be better. I'm a machinist and I know friction = bad.
I'm in the process of making titanium drive shafts with closer tolerences to reduce thr slop between the shaft and gears to reduce slop. I'm also making a slew of 2mm pins out of titanium as well.
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Old 07-06-2017, 07:36 AM
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Ah, a machinist! Titanium drive shafts and pins? Gonna make any to sell? 😃
Cool 😎
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Old 07-06-2017, 08:22 AM
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Quote:
Originally Posted by EXT2Rob View Post
Ah, a machinist! Titanium drive shafts and pins? Gonna make any to sell? 😃
Cool 😎
may be a possibility you never know. 😉😉
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Old 07-06-2017, 04:42 PM
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Quote:
Originally Posted by EXT2Rob View Post
Hey Hooligan! Haven't seen you 'round much! Cheers!
iv mostly been trolling the URC forum the last couple years
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Old 09-04-2017, 06:07 AM
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I actually contacted the company about this issue. The response was that they outsourced the design. The person I talked to seemed to think cutting the tabs was fine. I am leaving mine though. I am of the opinion that if people want to cut drag, there are better spots to do it. After having a couple of domidas, model doesn't really matter, there are other areas which introduce must more drag than those little tabs.
1) clean out the diffs of that stuff they call lube. Now, some run dry, I still use lube. I put a very small amount in, just enough to put a thin layer on the gears. This has dropped drag but at least half in the diffs.
2), if the shocks are allowed to push all the way out, the dogbones are at too much of an angle and that causes a lot of drag. Get a small piece of silicone fuel tubing and cut it about 1/16 inch thick and put it on the shock shaft inside the shock. This shortens the shocks just enough to eliminate that overextention drag.
3) Pull those bearings out and give them a good cleaning/lube. They are not quality bearings in the first place and they are gritty as hell out of the box. A good soak with shots of compressed air will clean them out. The lube them good and blot off the excess oil.
4) check spur/pinion mesh. There should be a very small amount of play between the teeth. Tight gear mesh saps tons of power and wears the gears and motor quickly. And these motors are not power monsters.

After doing these things, I have a very free drive train. The tabs have been a non-issue. Maybe the first couple of runs they were, but they have loosened up and I can put a feeler gauge between the tabs and bearing. So, obviously, they are not going to account for hardly any drag. Now, I am not going to tell people what to do, it's their car, so it's their choice. Just that both my cars have been like this. I have brushed models.

On a side note, I also reamed the pivot points in the arms just a bit to allow better suspension action. Talk about binding there. It was amazing the arms moved at all really. Speaking of which, check your shocks regularly. The stock shocks are not very oil tight. Mine, out of the box, were either empty or partially empty. I used 30wt oil I think, maybe 40 at the highest. I stuck a piece of fuel tubing on the shaft on the outside, just thick enough to prevent the chassis from slapping the ground easily. The fuel tubing in shocks is an ooolllddddd trick from at least the 80s.

As for machining parts, that would be cool, but I'd start with the god-awful steering slop. I think hot racing makes a bell crank to replace the stock one and that is supposed to help a fair amount, but those tie rods are so sloppy. And so is the stock servo saver.
I got a mg90 servo, 4 for $10 on amazon. There are metal geared servos, so I tossed the saver. It cleaned things up some. And I have yet to break the servo. I didn't even have to screw it down. I still have the stock components and the servo sandwiches between the chassis and receiver pretty tightly. A thin double sided tape would do the trick if it does move.
I did get a a 2s fluereon pack, cant remember the mah right now, but they barely fit in the stock location. I had to remove the battery posts. I used strips of velcro to hold the battery in place, works great. Do that on my rc10 gold pans as well. Then I just added a low voltage detector. Car goes like hell but the motor would burn out long before the pack is dead I think. These little motors just make so much heat. Too little motor for too much weight. However, the motor did not get so hot on the stock nimh pack. I could run that thing till it died with no issue. So the additional power from the lipo is the primary culprit. I'll either end up brushless(though the brushed motor does well), or a heatsink/fan unit.
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Old 09-04-2017, 08:02 AM
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Great info there, vinman. Thanks! 😀
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Old 09-04-2017, 08:54 AM
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Drag was only one reason the tabs should be removed. If the bearing is not allowed to spin the way it was designed and is "frozen" in place on the shaft the outer race of the bearing is spinning in the case. It is supposed to be seated tightly so the inner race is the only part that spins. Without that, you might as well have a plastic bushing. The bearing is not doing it's designed purpose. pull the diff apart and look at how the bearing is set up with the tabs on. The tabs have trapped the bearing onto the shaft, so when the shaft turns, the whole bearing turns. Spin the diff. What happens? the whole bearing rotates with the shaft. What's the point of the bearing then? All it will do is wear the plastic seat that is designed to trap the outer race in place.
The steering is bad I'll give you that for sure, and so is the pivot movement on the A-arms. By custom making my drive shafts I am able to tighten the tolerances, and drastically reduce the slop. when you're racing on carpet every turn, brake, and acceleration that has even the smallest amount of slop loses you time. Take your shaft and move it by hand. there should be hardly any slop. Mine has probably a degree and a half maybe two. That's no good. first reason being you lose time on accelerate and brake granted it may be a mere split second but multiply that by 100 brakes and accelerates and that adds up. Second and greater reason, that slop increases wear. Wear on the gears and wear on the shaft itself. Now, that being said, I haven't gotten an aftermarket drive set from Dromida so I can't speak for the tolerances on them. bottom line is, Bearings should spin freely, and anything with the drive train should be tight.
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Old 09-04-2017, 06:29 PM
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I don't have that problem with my bearings and drive cups. Like I said, I can get a feeler gauge between my tabs and bearings. If I have the motor out, I can give my car a light shove and it will roll a good number of feet. If your bearing is not rotating, then a new bearing is likely in order, or something is not seating right once you put the diff in and cover on.

As for tolerances, well racing is a different matter. But still, there are so many areas that can be tightened up on these sloppy things. I would tackle steering long before drive shafts. You waste far more time on sloppy steering than drive shafts. But hey, if you got the time and materials, go for it. I bet there would be a fairly lucrative business making parts for these dromidas. And honestly, I would buy parts like this instead of the plastic parts. Graphite shock towers would be nice, but I suppose that flex is what allows the car to bounce back so well!
Actually, I'd like to see associated do a race quality 1/18th, again. Maybe the reflex is good? I haven't looked into it. But then, I wish I could find the MIP 4x4 conversion for my old rc10 gold pans.
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Old 09-05-2017, 01:22 PM
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Well I'm game to purchase any and all titanium parts if you plan on selling any!
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