Project Logs

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• Log 3.5

  cmoon5511 • 06/29/2024 at 19:21 • 0 comments

  I finally test drove the RC after having it in pieces for a few weeks and there has definitely been an improvement in crawling abilities. Right now I am on a suspension setup with a shock angle of 35 degrees which theoretically increases suspension travel by 3.3mm over stock, and -4mm of vertical offset to increase travel in extension. There is some rubbing from the front tires when in full compression and steering fully locked to one side. 

  One thing that helped greatly is adding a limiting strap between the front axle and chassis; this limits how far the front end will lift on an incline, but still lets the front axle flex fully. I also increased the preload in the rear to help the front tires grab traction earlier on inclines. It's a little hard to tell from the picture but I looped a piece of kevlar string under the center diff links and between the battery tray and front shock mount. 

  As of right now I only have the rear magnetic body mount. I am keeping the stock front body mount attached to the body, and removed the screws that fasten it to the chassis. This lets me slide the front of the body into the frame, and the rear magnet locks it in place. For now this works extremely well, but I am still trying to design a proper front mount. The only reason I won't be keeping this setup is because I want to try experimenting with lifting both the body and bumpers to hopefully get some better tire clearance. I already have an idea for an adjustable rear mount, but the front will be a little tricky.
  

  So far I have magnet mount for the body that slides under the hood area, but there isn't anything securing it in place. I am trying to incorporate some set screws to the underside and see how that does. The front bumper mount is a little more difficult to reverse engineer than the rear so I'm expecting some trial and error before everything fits together cleanly. 

• Log 3: Rear Magnetic Body Mount V1

  cmoon5511 • 06/27/2024 at 01:56 • 0 comments

  I had bought some 25x5x3mm neodymium bar magnets from Amazon when I bought the RC specifically for this purpose so I'm hoping they work.

  There is a lot of room in the rear so I started designing those mounts first. I am going to reuse the stock mount holes on the body to install a magnet on. This initial design will use a tongue and groove sort of assembly so it will hopefully prevent any lateral movement.

  The front has considerably less room to put anything compared to the front. The only room you have is between the front of the body and the battery tray, and some of that room is needed to make room for the server horn in compression. Initially I was going to include the front mounts in this log but I think that will need to be its own log.

  These are the initial prototype models. For the bumper mount, I drew up the stock mount and just slapped a big box for the magnet on top. The mount that installs into the body is a simple cross bar with a protrusion for the magnet that will seat into the bumper mount. 

• Log 2: Redesigning shock mounts

  cmoon5511 • 06/26/2024 at 15:59 • 0 comments

  To help position the upper shock mount location, I created a simplified geometric model of how the shock should travel based on its compressed and uncompressed lengths and its angle compared to vertical.

  In this case, we are looking at the rear drivers side shock. The lower mounting point is green, and the upper mounting point is blue. L1 and L2 represent the uncompressed and compressed lengths, respectively. For the stock shocks, I measured these to be 42.5mm and 32.5mm, for a shock travel length of 10mm. H and V are the horizontal and vertical displacements of the upper mount when compared to the lower mount. The θmax value is the angle at which your shock will be completely horizontal at full compression. Ultimately, we are looking for the blue line T, which is the total travel you will see from the wheels. In reality, the line of action will be a curve rather than a straight vertical line, but I figured this is a good enough approximation as a better model would be a bit more complex to derive.

       My 'V1' rear shocks had an angle of about 24 degrees, which gave me about 1.25mm more travel. At θmax, we more than double the stock travel length. I wanted to test a setup with around 3mm more travel on extension and 1mm more on compression than my V1 setup, and an angle of about 40 degrees will get me the total travel needed. The 'mount circle' I described in log 1 will be dropped by 3mm to give the extension I want, rather than just increasing compression travel by 4mm. With such a steep shock angle, the spring loses 'effectiveness' in the vertical direction so I don't expect ride height to change significantly.

  Pictured are the new mounts installed on just the driver's side. The white mounts are there to help visualize how they look a little better. They are only 2mm thick and have a little bit too much flex, but they work as a prototype for now. As currently designed there is not much modularity or adjustability. I am considering a system where there is a "base" that either screws into the frame or is part of the chassis brace, and different "blades" that screw into the base to make it easier to adjust suspension setups. I could do different blades for different heights at full extension, with each blade having several mounts of different angles. 

  It might be difficult to see from the pictures, but there is a bit more travel on both extension and compression, although maybe not as much on extension as I wanted. I appreciate FMS for making the frames symmetrical. I have been reading that it can be beneficial for crawling to have a stiffer rear and to limit extension travel in the front by means of a center limiting strap or something similar.

  The next thing I will work on is making some magnetic body mounts, getting the body on and off is currently a pain in the ass and deters me from taking apart/putting the truck together so I can actually use it!

• Log 1: Shock Mounts V1

  cmoon5511 • 06/05/2024 at 17:52 • 0 comments

  This is my first attempt at increasing articulation with the stock suspension linkages and shocks. There are existing solutions; JoRcs on Etsy sells some 3d printed "shock keys," which are modular shock mounts that can allow for increased suspension travel by moving the shock mounting points inwards along the longitudinal axis of the truck. Some youtubers have also documented their DIY solutions, and I believe Injora has begun selling some aluminum threaded shock towers. 

  One issue I want to solve is that many of the fasteners on the FCX18 are self tapping into plastic; realistically this isn't a huge concern if you aren't constantly removing and reinstalling parts. However, it can be easy to strip threads by over torquing, and the threads can loosen over time. I designed these shock mounts around M2 threaded inserts to avoid any problems with plastic threads. 
  

  One thing to consider with these inserts is that they take up more space than just a 2mm hole; The inserts themselves are 4mm in diameter, and also require at least another mm of plastic to set into.

  Many of the existing shock mount solutions are separate pieces for each corner that screw into the stock mount points. The rear of the chassis has alot of room, so I designed a whole new chassis brace / shock mount to replace the stock brace. I modified the brace design to have more clearance for the rear diff but I might have gone a bit overkill with it. I added some ribs for stiffness as the brace design looks a bit floppy, but I've heard that some chassis flex is desirable; I'll have to look more into this later. As designed, suspension travel at full extension will reach the same point as the stock configuration but offer more travel at full compression, with travel increasing as you mount the shock farther inwards. 

  Unlike the rear, the front of the chassis is very limited in space. In order to make a weekend meetup I threw this design together and had it printed out in about 15 minutes, so it definitely has room for improvement. Right now It only has a singular mount point and the tolerances are very poor, but still gives me a bit more travel. A single piece mount/brace like I did for the rear doesn't seem feasible for the front, as the ESC and battery take up a lot of space.

  I did a quick coarse print out of PLA and it seems to hold up well. I should have made the fastener holes deeper since the shocks don't fully seat into the mount points for the rear. I haven't done any real analysis or testing, but the rear definitely has more squat now. At full flex (e.g. left compressed and right extended fully) the tires barely rub the edge of the fenders. I had designed in multiple mount points in case the steepest settings had too much travel but the suspension isn't bottoming out yet, so the shallower mount points are basically useless. Looking at the new brace on the chassis, I have a bit more room than I accounted for in the initial design. For V2 I am hoping to have a similar or greater range in compression, and increase the full extension range about 1mm. At some point the links will bottom out on the chassis which could be fixed by buying some high clearance links that are available.

  The steering servo hitting the battery tray and existing front shock mount is the limiting factor for how much movement I can get out of the front suspension right now. One of my next tasks is to take apart the battery tray and ESC holder and see if I can somehow optimize the design for front suspension travel, or just open up...

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• Log 0: Buying Things, free mods

  cmoon5511 • 06/05/2024 at 17:25 • 0 comments

  I bought the FCX18 LC80 back in March of 2024, but lost access to my printer for about 2 months. This initial log will detail a bit of the FCX18 design and the parts I have installed.

  The first free "mod" I did was flipping the front diff housing screws to get a bit more steering angle, there are several guides on youtube that show how to do this.
  

  Thankfully the FCX18 shares a lot of parts with the FCX24 so parts aren't too hard to find or expensive. I installed some metal diff gears from Aliexpress, as I have heard these are one of the first things to break. I also bought some metal portal axle gears, but it turns out the stock gears are metal. 
  

  As designed, the FCX18 is a bit top heavy so I figured I'd get some brass wheels. However, I do like the look of the stock wheels, so I ended up buying these brass internal rings; these replace the plastic rings of the 3 piece wheels. HOWEVER they do not fit out of the box. If you want to go this route, you will need to sand down the inner and outer faces of the wheel until they fit into the brass ring. You will also end up with a slightly wider wheel, effectively increasing the amount of negative offset. I paired these new wheels with some Injora 58x24mm tires.

  With wheels and tires, the truck is a little less prone to tipping at high approach angles. The next log will go over my first iteration of new shock mounts that are designed to increase suspension travel.

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