Some Losi CAD fun

[fredswain]

I've been modeling various things up in Inventor lately and thought I'd share this one. It's a JRX-PRO chassis in Carbon Fiber with a set of Bullet Racing trailing arms also in carbon fiber. I was on team Bullet back in the day and thought I'd remake some of the old parts again. I've been measuring some old parts and started to get things into CAD so I can have actual prints. Certain CAD files will also be necessary when I get a 3D printer up and running this summer. This chassis is drawn up exactly like the PRO chassis with the exception that I am using the Bullet arms mounting holes. I haven't added battery box or steering holes yet. Anyways, here's a teaser.

[slapshot1979]

Well isn't that way cool

[bearrickster]

that looks cool

[fredswain]

The real thing is kind of neat as well, minus the carbon fiber weave. I'll get these parts out on the pin router soon though so I'll have the real thing soon enough.

[bearrickster]

were these ever made for the Losi's

[fredswain]

No. Not directly at least. There was a kit called the RC-X2 which used the RC10 front end, the JRX rear end, and Bullet trailing arms. When it came out the Stealth tranny wasn't around yet. The JRX gearbox was better than the 6 gear and cheaper than the MIP. The Bullet Powerglide belt drive wasn't out yet either. You could buy the trailing arms and adapt them to a car of your choice though but there was no Bullet JRX specific chassis for them.

If you've got a car that uses telescoping driveshafts or at least has separate outdrives that can be bolted on then these are easy to run. There used to be adapters that bolted into the stock outdrives of the 6 gear that allowed shortened sliders to be used. I personally thought it was a bit ghetto though. The arms pictured are actually from my Bullet RC10 which has an MIP SP-1 on it.

[bearrickster]

I didnt think so Ive never seen them. and Even Losi dropped the 5 Link almost as fast as they kitted it. the H arm kit came out very soon after the release of the Jrx2.

[fredswain]

I love the trailing arms. These things are so smooth to drive. I wish I could tune back in the day as well as I can now. If I ran these on a hard packed clay track with really large jumps and landed wrong I'm afraid I'd break an arm. On the old dirt tracks with smaller jumps I never saw one break. Barry ran them back then while I was using H arms and he never broke one ever. I hope to redesign them and refine their strength a bit. They'll definitely have less play and flex.

[bearrickster]

. They'll definitely have less play and flex.

I never liked the Losi 5 link because it was so sloppy.

[Coelacanth]

I see some potential design issues. There would need to be at least 2 bolts to mount each of those vertical swing arm posts, because a single bolt would too easily allow the posts & arms to swivel around with shocks or stress. It also seems like a lot of stress focused on one mount point (per arm). I'm not a fan of swing arms, but that's just an opinion.

[fredswain]

I see some potential design issues. There would need to be at least 2 bolts to mount each of those vertical swing arm posts, because a single bolt would too easily allow the posts & arms to swivel around with shocks or stress. It also seems like a lot of stress focused on one mount point (per arm). I'm not a fan of swing arms, but that's just an opinion.

Keep in mind this isn't my design. This is from the late 80's and actually have a mounting design heavily borrowed from the MIP trailing arms. Here is a picture of the bottom of my Bullet RC10 clearly showing the mounting points. You are correct. You can't have only 1 screw on each upright, and they don't. That's not an area that has any problems though. The issues with these particular arms is the tolerances.

There is a swivel that mounts through the upright that the arm screws into. This swivel has a 15 degree face on it which explains why the 2 mounting holes for the arms are angled 15 degrees in this picture. Depending on how the arm is oriented in relation to the swivel, you change the camber amount. These arms like most of the standard trailing arms do give camber gain with suspension travel which the hot trick trailing arms do not. The tolerance between the swivel diameter and the hole diameter it goes into in the upright is a bit loose. There is no bearing here either so over time as this wears the tolerance only increases causing the arms to move more and more side to side. This design really needs to be improved to use a set of bearings with very close tolerances or perhaps even a slight interference which would require freezing certain components to assemble them.

The other big problem is the lone 4-40 screw holding the arm to the swivel. Once set this needs to stay very tight or else it could slip on the swivel and your camber change would be all screwed up. There actually isn't a whole lot of concern about the strength with only 1 bolt since it is loaded at it's strongest dimension. A way to use a second screw would be very nice to have though. The hubs have loose tolerances for the driveshaft bearings. These should be tightened up. The arms can also flex slightly.

Now that I've pointed out all of the existing flaws, they actually drive remarkably well. These arms are 3" long which is far more than any other trailing arms. They soak up bumps like no others. The uprights also have 2 different holes to mount the swivel to. The high hole is greater antisquat and the lower hole is less antisquat. Even in the top hole the geometry of the system allows for much less antisquat than a-arms. However since the mounting point of the arms is so far forwards you can't pop a wheelie. You don't have the leverage. The rear end will just sit down and in extreme cases depending on how everything is adjusted, the chassis could actually drag on the ground during acceleration. These allow you to accelerate quite hard. Due to this trait the front wheels don't unload as badly on hard acceleration and this is especially useful during on power corner exit since you have more traction left to steer.

For all of their flaws, they do some remarkable things. I want to improve them and install them on my favorite old car, the JRX. I'm not sure about these on clay since historically trailing arms haven't been as good on it but we do have an older dirt track here still and they work fantastic there.

This actually turned into a Bullet trailing arm thread rather than a CAD thread so I'll get this back on track again as I model more parts. I'll either add to an existing Bullet thread or start one specifically for the trailing arms. I still need to do the same thing for my Bullet belt drive.