An issue I've been trying to hunt down and general questions

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9
Hello friends, brand new here but I've definitely lurked these forums for a while.

I think I've vacuumed up about all the information I possibly could online about this truck and if I am missing some key places for information (SBR2 or general RC teching/maintenance of this type/scale), there just isn't a lot of info about its inner workings that I can find. Losi has a laughable amount of support officially on their site besides the meager manual included. Like this is a no joke truck of price and size, and I think much more through documentation with some more advanced possibilities for tuning and longer term maintenance planning would have been nice to have right out of the manual, but binding the receiver is in there so thats good.

I do not come from an automotive background but I can definitely see a lot of parts generally function like their 1/1 counterparts and I think thats where my struggles the most are as I gain more interest and experience in this hobby. Are there any sites or youtube channels that kinda go into some detail about RC differentials and transmission stuff? I have the most questions in the drivetrain areas of the truck. Like an example of something more advanced being like shimming all your gears for a more perfect mesh and also why you don't shim them! Because if I think I might need to because I notice play, its nice to know the play is there for a reason too. I dunno thats a lot to ask and pretty ambiguous but maybe I'm missing a great writeup somewhere.

So I'll try and be brief about my actual trucks problem I'm currently triaging. I crashed pretty hard a few weeks ago, end over end on concrete and it was not my favorite day all-in-all. I'm guessing that crash was the cause of the issue I'm having, although it didnt start maybe 2 weeks later. My truck won't drive a straight line. Trim tunings played with hundreds of times, AVC off and full blast and the issue seems worse with AVC on telling me that something is physical is out of wack that the gryro can't correct for.

The truck drives slowly left then right kinda like a drunk driver that can't stay in their lane. Like wheel wobble but over a much larger cycle of the drivetrain. The wheels spin fine no binding and no wobbling not normal for the truck. I tore the truck down and my center driveshaft is ever so slightly bent. My center differential and the surrounding case plastic don't fit super cleanly together. The rear diff and axle seem fine and I'm about to tear down the front end. My question:

For a vehicle that has this long multi-section drivetrain, if my center shaft is bent, what kind of things happen to the truck while its being operated? Like if anyone has experienced this before. Because to me it looks like the bent shaft and continued driving on it after has bent other parts down the line as a result of its new angles its rotating. Can I expect to see diff spider gears stripping and non-symetrical wear eventually causing one side of the truck to start failing? Its a big question but kind of what I wish I had more knowledge about. I could very well have caused wear and damage to too many moving parts down the powertrain to actually fix this and that would suck super bad.

Yeah if anyone does read this thank you. Appreciate any insights or tips for info regarding tuning and theory.
 
Messages
9
Sorry I haven't added any pictures of the problem, although I really don't know what I'd take pictures of just yet because there are 100 things that could be causing the issue.

I replaced the center front drive shaft and I don't think that did much.

The chassis plate could be bent where it holds the bulk heads. I would try to see if it lay flat on the table.

I am starting to think that might be what is going on here. I can't find anything that seems damaged, but all the while everything seems to be kinda off. Nothing spins true until the parts are removed from the vehicle then they all seem to be spinning fine.

This is the hardest problem for someone like me to solve because just when I think maybe I've made some progress on it, it comes back sometimes worse. Or I never made progress at all and it isn't getting worse, and I'm just in my own head at this point chasing red herrings. All I know is that the more I tear down everything and get it back together, the higher the chance I'm going to wear out or mess up something else, especially hardware/threads.

One thing I am learning more about is the anti-roll/sway bars and how they affect things. I don't know much about tuning them, but there are grub screws that you can tighten down on the bars. Are those to stop the bar from sliding back and forth or are they affecting the function of them?

Thanks for suggestions so far as I continue deeper into this mess I've created.
 
Messages
9
Looks like you just don't have a consistent return to zero on your servo. Has the steering servo been upgraded?
The servo is the stock Spektrum S905. The entire truck is actually completely stock all I've added to it is braces in the rear to counter the flex issue.

The servo does have a tough time returning to zero. I cannot remember if it has been that way all along.
with the wobble in reverse i'd say have a close look at the steering/ servo saver,
see thats either broken/cracked or just way too loose (y)
Yeah I actually just tore down and cleaned/rebuilt the entire steering system on the truck this weekend. The servo horn is looking pretty much untouched and stays nice and firm to the servo output. The servo saver is implemented in with the bellcrank (not the most knowledgable auto guy here :p) and the spring/nut compressing it seemed pretty good. But I tightened it down more anyways. The tie rods look good as do the axles to the wheels. The spindles and upper A arms seem kinda soft and I dunno....too giving. But I'm also way reluctant to start swapping out plastic parts for aluminum/steel because who knows what else I'll break down the drive train as a result.

I'm beginning to fear that I have a frame issue on my hands and everything is just out of wack because the frame is. Sounds expensive!
 
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that s905 servo is absolute weak :poop: ive had like 3 go bad in a couple months, also 1 went bad in my uncles kraton recently. Before you do anything replace it with a real servo. Remember that it is 1/6 sized not quite 1/5 sized. I believe both hitec and savox make a direct fit, or if you can find either a 1/5 mount or an adapter to fit a really good 1/8 servo that will be much better than the spektrum one.
AND TURN OFF AVC IF ITS ON
 
Messages
9
that s905 servo is absolute weak :poop: ive had like 3 go bad in a couple months, also 1 went bad in my uncles kraton recently. Before you do anything replace it with a real servo. Remember that it is 1/6 sized not quite 1/5 sized. I believe both hitec and savox make a direct fit, or if you can find either a 1/5 mount or an adapter to fit a really good 1/8 servo that will be much better than the spektrum one.
AND TURN OFF AVC IF ITS ON
Oh wow you've got quite a bit of experience with this servo. Ok thank you for the push to replace it. At least now I have some kind of direction on what to replace instead of blindly buying parts :).

And AVC is off most the time I drive it. I use it occasionally but otherwise it is off. For testing and troubleshooting it has been off unless I purposefully turn it on for comparison.
 
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Got the servo and spent the majority of yesterday afternoon/evening getting it mounted using the stock 1/6 servo mount. I knew the 1/5 servo would be a little bigger and didn't think it would be too hard to shave down the mount to get it fit, but it was a task thats for sure. The Power HD servo is insanely powerful and would probably have no issues ripping the entire truck apart if endpoints weren't set.

Even with my ghettomounting the steering is noticeably better and the vehicle tracks much straighter than before. I think I'll have to cough up the cash for the 1/5 mount Losi makes for it (Aluminum and looks great but $62! shhheeesh) too make sure everything is straight and true to the intended geometry.

Thank you guys for the help! Seriously the only people that actively advised me on this maze of both red herring hell and self-isolated rabbit holes I enjoyed for a few weeks :) You all are a bunch of sweethearts.
 
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