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nowinaminute

Thunder Dragon Post Beach Run Autopsy!

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So I've been having a great time with my TD at the beach for the past week or so but on Thursday, the pinion and spur gave up and stripped out so it's time to see what went wrong.

The most obvious guess is sand ingress, and I know I didn't take my usual precaution of running a bead of grease around the gearbox cases or checking for any obvious openings, so that's definitely a strong suspect.

But I also know I had a questionable alloy pinion in there combined with brushless power and sand paddle tyres so there was a lot of power and a lot of drag on the drive train, so that could also be at least part of the issue.

I now have my new gear bag and a decent RW steel pinion so it's time to get stuck in!

General view of the work area:

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Rear end removed:

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This is a good example of why I like to run on nice dry dunes rather than the damp or wet sand. My first 3 runs were on dry sand and the car was spotless after a shake off. The last run was on the slightly damper sand on the beach and it just gets everywhere and stays there! Not as bad as running on the really wet stuff, but it certainly illustrates how a beach run can be pretty much zero cleanup or a fair amount of effort depending on the type of sand! I've also noticed quite windy days in the summer are good as parts of the main beach will tend to dry off completely and little ripples of dry sand will form. Anyways, I still like to run on the damp stuff now and then but you have to be willing to put the time in afterwards to clean it all off and maybe rub some oil or wd40 into the parts that can rust.

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First off was the prop joint cover and there's a lot of sand and crap in here! Interestingly, there was more sand here than in the main gearbox, so possible a small breach in the cover going on here. I'll check it on reassembly plus add the usual bead of grease. The gears are a little roughed up but I think I will reuse them as they have plenty of meat left and maybe some of that awful flashing from the factory has been ground down 🤣

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Hey! I too took my TD out for runs this week, football field runs instead of beach though. 🙂

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Next up, the motor and mount area. This is one of the places I don't trust entirely, just because the mount sort of has an open side and relies on the gear case moulding to make a tight seal, I think this could have been a potential source  of ingress, especially bearing in mind I didn't add any grease as a precaution the first time around.

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And then the gearbox halves themselves, definitely some sand made it partway into the seam. Whether or not it got all the way through, I don't know, but this is the kind of place I usually put grease, can't remember why I didn't at the time. Maybe I naively thought this was going to be a shelf queen 😅

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57 minutes ago, alvinlwh said:

Hey! I too took my TD out for runs this week, football field runs instead of beach though. 🙂

Nice!

I really enjoyed my beach runs till the little incident but I think it was mostly down to that alloy pinion and a lack of my usual precautions. Got some sweet video, though, so it's worth the wrenching time.

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Here's everything that fell out of the main gearbox! Interestingly, it seems like there isn't all that much sand, more a combination of shredded alloy pinion and some spur gear material...

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1 hour ago, nowinaminute said:

The gears are a little roughed up but I think I will reuse them as they have plenty of meat left and maybe some of that awful flashing from the factory has been ground down 🤣

Precisely what I was thinking honestly. After trimming the flashing, I packed the area with toothpaste and polishing compound (using bushings for this exercise) and ran the gearbox on low power for awhile before tearing it back down and cleaning/greasing and installing bearings. I could have skipped all that and just enjoyed a beach run instead, lol. (although my gears are now minty fresh!)

 

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The spur gear was shredded but the diff gear isn't too bad. But I think I will change this as it will no doubt have some dirt embedded in the teeth and will be in contact with the new spur.

I'm less concerned with the bevels as they are sort of separate so can only affect each other plus they seem to turn a lot smoother now 😂

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As you may have noticed, I put diff putty in these. I can't say for sure if it helped with beach running, but the thing sure does drive nice on sand and dirt etc. 

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8 minutes ago, Saito2 said:

Precisely what I was thinking honestly. After trimming the flashing, I packed the area with toothpaste and polishing compound (using bushings for this exercise) and ran the gearbox on low power for awhile before tearing it back down and cleaning/greasing and installing bearings. I could have skipped all that and just enjoyed a beach run instead, lol. (although my gears are now minty fresh!)

 

I was tempted to do the toothpaste thing too! Or try some lapping compound, maybe. 

The gears definitely seem quieter now although they are a bit chewed up looking, I guess because they have just had  some random exposure to sand and not a slow, controlled breaking in session. 

I wonder what it would be like after a few hours of sand mixed into toothpaste 🤔

Better still would be if Tamiya sorted it out!

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By the way, if anyone knows of any obvious weaknesses in terms of dirt proofing on this particular model, let me know!

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The pinion is utterly wrecked, I do wonder how much of this would have happened even if sand wasn't a factor. There was a tiny bit if misalignment, too but i don't think that would have saved the day.

And you can see the spur is literally showered in alloy dust but there's not actually all that much sand present.

 

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That's the cases all cleaned.

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I h

2 hours ago, nowinaminute said:

Next up, the motor and mount area. This is one of the places I don't trust entirely, just because the mount sort of has an open side and relies on the gear case moulding to make a tight seal, I think this could have been a potential source  of ingress, especially bearing in mind I didn't add any grease as a precaution the first time around.

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I have a new in the box fire dragon that is going to be used most off his time on the beach, now I see this,  I going to use a lot of silicone sealant on this and the most on the motor mount, there is a lot of sand in there.

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27 minutes ago, dannymulder said:

I h

I have a new in the box fire dragon that is going to be used most off his time on the beach, now I see this,  I going to use a lot of silicone sealant on this and the most on the motor mount, there is a lot of sand in there.

It definitely needs intervention. I'm thinking silicone around the mount area at least, looks like that might have been the main issue.

Silicone usually does a great job. All my DT-02 and 03s have had the same weak point where the front of the gearbox meets the chassis, there's no lip there and if you hold it up, the gap is wide enough to see light through. The shock tower is supposed to seal that area but it's not even close to a good seal so a line of silicone alone there and underneath works wonders.

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Well, this hasn't quite gone as smoothly as planned! I thought I would be up and running in no time when the parts showed up yesterday, but still not there, yet!

I thought I had discovered some human error that may have contributed to the sand getting in but it turned out to mostly be a red herring. For some reason I put washers on the motor mount screws, not sure why I made that decision during the initial build, but I realised the washer on one side was perilously close to the lip on the gearbox casing. I wondered if perhaps it had prevented the mount from sitting totally flat against the case....

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But in reality, it seems like it had clashed very slightly, but had scraped down the inside of the lip, thus not causing the mount to be proud.

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So, with no obvious weak spot, I just have to assume it's just the usual case of small gaps here and there, possibly combined with some flexing now and then creating tiny gaps. Once a grain or two gets in, the gap is then held open for more to get in and so on.

I can clearly see several areas where sand has gotten where it shouldn't, I've used the flash on the camera to make it easier to see several areas of abrasion I've found around the "lip" area of the casing halves.

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Interestingly, I suspected the prop joint compartment may have been a weak spot as it had the most distinguishable sand inside, but there's virtually no sign of sand entering through the seam between the two parts. 

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So, with no obvious "smoking gun" it seems its just a case of doing the usual and putting a bead of grease around the seams to trap any grease that tries to get in. To be fair, this usually does the job nicely for most models. I may put a bead of silicone around the lip that goes against the motor mount, just to be sure, too. I was tempted to put silicone along the gearbox halves, too, but the diff gear gets precariously close to the seam there, and I'm worried that even with a tiny line of silicone, it might migrate onto the diff gear teeth when I close up the two halves and the excess gets pressed out of the seam.



 

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So, that brought me up to tonight, when I set aside some time to re-assemble the transmission, or so I thought!

First fail was that modelsport sent me an 18T RW pinion and not the 16T I ordered! I wouldn't mind so much, but 18T isn't even an option on this buggy! Guess I will have to email them in the morning.

Putting that aside temporarily, I continued with putting the gearbox back together. Next issue! With just the spur and diff gear installed, there's a noticeable tight spot when you turn the transmission. Now, I know too well that the bevel gears on the sides of the transmission are prone to tight spots due to poor moulding, but they haven't even been installed yet! It's literally just the spur and diff. I can't even remember if I checked the first time around, does anyone know if this is common thing with this transmission? I know that it can happen to any model, on a broader scale, certainly not unusual for there to be a tight spot somewhere on a gear but I'm not sure if these are known for it? I guess my choices are the usual, let it be and see what happens, try and work out the bad spot and gently file the teeth down or get another gear set and see if next lot are any good!

I naively thought I would be taking it out for a shake down test tomorrow after work!

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I decided to just live with the slight tight spot between spur and diff and hopefully it will bed in. So the gearbox is all reassembled now with a bead of grease all around the the contact area of the two halves.

For the lip on the motor mount, I put a (very untidy) bead of silicone sealant. Who knew it was so difficult to get a nice consistent layer with a little tube of poundland silicone sealant 😅

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Still, once its dry, it should do the job fine and make nice pliable and soft seal that can compensate for any flexing between the mount and the gearbox

Had to go with a 15t pinion for now while I wait for modelsport to send the correct 16T one I ordered (turned out RW racing put an 18t in a packet marked 16t)

I may be able to test it this evening but not sure yet. Fingers crossed!

Hopefully it's now sand proof but even if not, as long as there's progress, at least I will know I'm going on the right direction. 

To be honest, even the first time around I expect it would have held out 5x as long with a decent steel pinion so even if I've somehow made no improvements, which I highly doubt, it would still probably last a lot longer.

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50 minutes ago, nowinaminute said:

little tube of poundland silicone sealant

Lucky you still have a Poundland. My local one is closing down soon and almost everything is already cleared out. The next nearest one is over 100 miles away! 😭

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2 hours ago, alvinlwh said:

Lucky you still have a Poundland. My local one is closing down soon and almost everything is already cleared out. The next nearest one is over 100 miles away! 😭

Aww that's a shame. Despite half the stuff now being way more than £1, you could still find some useful stuff in there amongst the rubbish. Definitely picked up more than a couple of bits that have helped with RC stuff!

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Looks like I'll make it to the beach for a run today. Hopefully I won't be ordering another gear bag tonight!

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By the way, in the process of mashing the gearbox the first time, I did get some nice video, at least. So I'll post it up when I finally get around to editing it.

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2 hours ago, nowinaminute said:

Aww that's a shame. Despite half the stuff now being way more than £1, you could still find some useful stuff in there amongst the rubbish. Definitely picked up more than a couple of bits that have helped with RC stuff!

Shoe glue, CA glue, primer and AA/AAA batteries are the most useful stuff from there. Luckily we still have a Lidl here and it don't look like it is going anywhere soon. 🤞

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40 minutes ago, alvinlwh said:

Shoe glue, CA glue, primer and AA/AAA batteries are the most useful stuff from there. Luckily we still have a Lidl here and it don't look like it is going anywhere soon. 🤞

Lol that's literally all the stuff I get. Those Kodak Alkaline batteries are ace and I've used the primer loads of times. Now and then, if you're lucky, they'll get a job lot of brand name car paints in or various other brand name stuff for cheap, that's when I fill the backpack up lol

So many things but hard to recall off the top of my head. Cable ties, mounting tape, little LED head torches refurbished as makeshift night time running lights, a couple of little selfie sticks I use for RC videos that are still holding up after years, microfiber clothes etc etc

Clear nail varnish I use for retaining bearings on shafts and in seats, too, so they don't start slipping. (Thank god for self service lol)

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Took her for a spin, great fun as always. No idea how it went in terms of sand ingress. Not like I can go by ear, it made a right racket from the second I finished building it 😂

Will take a look inside tomorrow probably, when the 16T pinion turns up.

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OK...so

I was kind of nervous to see what was going in in the gearbox, hoping my efforts had at least made some difference to the amount of sand getting in.

I decided to be a wimp and take off the prop join cover first, just to get a sneak peek at what was going on. As you would have seen, that entire compartment was covered in a blend of sand and powered pinion and spur gear so anything in this little section would give a clue to what's happening in the main box.

Well, I took the cover off, and was met with this scene! I think there's one grain of sand in there and I'm pretty sure that just fell off from the outside as I was taking the cover off. The gears were already looking roughed up like that because I resued the same ones.

This is a very good start! I still need to take it all apart but that fact that this compartment is effectively spotless compared to the first time is very encouraging!

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Good news, everyone! The silicone sealant appears to have done it's job! It doesn't look like any sand has made it's way through, here. And even the crummy 15T kit pinion I was forced to use because I was sent the wrong one has held up fine and I really gave the thing badword on the throttle.

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The only sand present is stuck to the outside of the silicone. I could have left it time to cure fully before driving but I figured it might make a better seal if I installed the motor mount when it wasn't completely dry and it seems to have paid off. It made a lovely "schluck" kind of sound when I removed the motor mount and required a little effort to pull away, too, so it seems like it made a really good seal.

This would be a good time to stress the importance of using that little card gasket, too, even though it's a "sealed can" motor and even though it seems counter intuitive to not use the motor mount as an additional heat sink. This applies to many other models too and depends on how the motor mount area is designed.

But as an example, on this gearbox, the "lip" that is designed to prevent sand ingress does not prevent sand and dirt from reaching the some of the unused motor holes on the motor which partially line up with unused holes on the motor mount. Without a cover in place, sand is free to get into the motor via the holes and on some models, it can even come back out of over holes and get into the gearbox. Another option could be to put grub screws into the unpopulated holes, that way you could still get the heatsink effect from the motor mount.

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Anyways, looks like it's a win! I'm going to strip it all again just to be thorough then apply a new bead of grease and silicone and then run it till something breaks. 

 

 

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