Mid-motor Losi XX

[stickboy007]

Figured I'd make a formal post about it here while also doing my usual YouTube content...

The "idea" came to me on a whim a few weeks ago and I figured I'd search around to see if anyone else has tried this. At least as far as I've searched, I did not find anything. Ok, then <cracks fingers>. The concept is pretty simple, as the bulkhead and gearbox mounting holes are well-aligned to just flip everything around. There is a bit of cutting you have to do to the chassis to make it fit, but otherwise it slots right in.

Some things I haven't covered yet in the videos are the need for longer screws going into the motor plate from the chassis, to keep the chassis from flexing either forward or backward. I did that already, but just haven't made a video for it yet. The aluminum brace which attaches to the top of the gearbox and runs against the bulkhead won't quite work the same in a mid motor configuration. If you think about it, in rear motor config, if the car is going forward and flips over, the tower gets pushed back, which pushes the bulkhead back and can snap the top of the gearbox. That aluminum brace resists that force; however, in mid-motor config it is flipped around and so the brace won't protect the gearbox as much. It may still help a little, as it is spreading out some of the stress applied to that region, but only time (and some horrible driving which I'm not prone to) can tell if it still helps at all with this configuration.

I also added a front brace supplement and modified rear tower, both from Factory Works. The rear tower fits a B6 wing mount, although I find that the wing looks like it's leaning backwards, even with the wing mount spacer that's supposed to lean it forward. I lightly modified the way the wing mount mounts to the tower, to give it more forward lean (will show up in the next video). Body is also cut and wasn't too bad to make it fit correctly, although I would say it helps to screw an extension onto the rear tower to help brace the top of the body. Otherwise there's just not enough support to keep the body from sloshing around if it flips over or something hits it from above.

General thoughts so far on this project:
1.) I do like the idea of a mid-motor XX and, at least for now until others start copying it, it is unique. The Atomic Carbon XXX mid-motor conversion was pretty cool, but custom jobs are always more fun.

2.) As far as vintage mid-motor buggies go, I have this, a Tekin RC10 mid conversion, and a Kyosho Rampage Pro electric conversion. The Losi is probably the lightest, but also possibly more fragile in certain areas (front arms come to mind) and certainly harder to work on and tune. The Kyosho is much easier to tune, as it was designed as a nitro buggy and so naturally has a mid-motor configuration. Just makes it easier to reach ball studs, tune gear ratios, etc. That Kyosho has also been my benchmark to-date for vintage 2wd buggies, as nothing else I have comes close to it on the track. We will see how the "XXM" compares when it's ready. My original plan was to run my TRX3 (maiden run) this year at VONATS and bring a Pro-X as backup, but now this XXM has me thinking twice. Spoiled for choice...

3.) Worth noting that this is a XX and not a XX-CR, for which the rear pivot block assembly is a little different and so a mid-motor conversion for that will be largely the same story but may require some minor tweaks.

[GoMachV]

That's awesome, I'm helping a friend with one currently. Well, it's in the mail. He did the basic work and wants me to design the braces. It will be interesting to see the different takes on how to go about it. I'll post up some pics when it arrives, all I have for now is this

[stickboy007]

Yeah that's D'Wayne's. He's converting a CR. He hadn't started that project until I showed him what I had and how I did it, as previously he had a completely different approach in mind which would have increased the wheelbase significantly and was convinced that was the only way to do it. I'm a little peeved that he gave me zero credit when he posted his project on FB, but whatever. I'll kick his butt on the track

[GoMachV]

Doh! Good to know. Maybe I'll engrave your username into the back side of the parts for him

[stickboy007]

That would be hilarious!

[stickboy007]

Front brace reinforcement from Factory Works. You just have to drill one hole into your front brace, which is a small price to pay for the extra strength. Serviceability of the XX makes itself known here, because you need to take out the hinge pin to remove the brace. However, taking that out means removing one of the inner ball studs AND the shock on that side, so the hinge pin can slide all the way out. It just means more disassembly than necessary to do some simple thing. Typical Losi

This is the not-at-all-phallic extension I mounted to the body mount portion of the rear tower (It's just a Kyosho long ball stud). In mid-motor configuration, the tower is all the way in the back and the body just isn't long enough to really reach it. So I will mount the body to the chassis sidewalls with velcro and use this rear extension to help brace the top rear of the body.

I cut some lexan out and attached to the rear bulkhead in order to capture the rear inner hinge pins. Seems to work okay, but a shiny, pretty G10 or carbon part would of course look sexier...

The Factory Works rear tower is cut to allow the use of a B6 wing mount. Fits nicely, but there is not enough forward lean in the wing. So I put a spacer between the tower and the bottom mounting holes, which puts the wing mount at an angle. You do have to eyeball it a bit with each of the four mounting screws, so that the wing sits symmetrically left-to-right, both in terms of forward lean as well as being level left-to-right.

A fair amount of material must be removed from the rear of the chassis, especially on the motor side. This requires you to use a shorter front right screw for the rear pivot mounting, so it doesn't interfere with the motor. There is enough space to capture the left part of the rear pivot with some lock nuts, but no room for that on the right. The right rear screw does clear the motor, but there is just barely not enough space to put a lock nut there. Maybe if I took the motor out, then put the lock nut in, and re-mounted the motor, it would fit, but it's very tight. Even the ones on the left hand side are a pain. The left front screw, with a lock nut, basically puts the pinion gear into a box. Replacing the pinion means first loosening the pinion grub screw, then removing the motor mount screws and pulling the motor out, then pulling the pinion gear up (and vice versa for mounting it). You can't just pull the pinion "out."

Gear cover is also, naturally, cut to fit. I could have left a bit more material on the bottom near the spur gear, but overall not too bad.

I used a Kyosho UM733 and cut some of the rear flat section, to pick up the curved gap between the motor plate and the chassis. Needed to drill and countersink two new holes into the chassis. Then, with one screw mounting the piece, use a 2mm bit to drill into the motor plate, then 2.8mm to widen the hole (if you start at 2.8, the movement in the drill as you're holding it down will end up mucking up the hole you just drilled through the chassis, so start small to do the bulk of the work and then a quick blip with a larger bit to widen the hole). Then repeat on the other side. Then tap the holes in the motor plate and use a 3x12 screw and crank it down to tighten the motor plate against the brace. This will improve fore/aft stiffness and prevent excessive flex. Since we don't have an actual waterfall here, we need a trick like this to mimic the effect of a waterfall.

Static motor timing should be set to zero or negative. This is because the motor must spin in reverse orientation (you need a ESC to support this) in order for the car to go forward, since the motor is on the right side of the chassis with a 3-gear transmission. This means your timing advance is in the negative direction on the endbell. I still have to map out the negative locations (15, 30, 45, etc.) on this particular motor. Right now I just eyeballed it at somewhere between zero and -10deg.

[GoMachV]

so, here is how slow I am. I just now figured out who you are Getting old sucks. 2+2= squirrel!

[stickboy007]

so, here is how slow I am. I just now figured out who you are Getting old sucks. 2+2= squirrel!

hahaha.....

International Man of Mystery

[scr8p]

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[stickboy007]

So I'd say it's about 99% complete here. Need to do little things like set camber, ride height, etc. I added a few things like a battery brace (not shown here, but it's in the videos) and JC Racing Products repro XX front arms. Those arms are noticeably beefier than the stock XX arms, but you may have to drill some of the hinge pin holes to make sure everything mounts smoothly. Also, because there's more material around the shock mounting holes, the holes themselves are placed a little further apart. The inside hole is in the same position as stock, but as you move to the outside hole, they are a bit more to the outside than on the stock arm. You can lengthen the shock (unscrew the eyelet) to compensate for droop if you want.

General thoughts so far on this project:

1.) I maintain the Kyosho Rampage Pro-E is a better mid-motor vintage car. For starters, it is more tunable, more easily tunable, and more easily serviceable. Rear anti-squat can be tuned with shims whereas on the XX you are fixed to whatever the rear pivot support has. As long as you don't need to reduce anti-squat, then it's not a big deal, but on a bumpy track you may wish you had that option. With the XX-CR, you can probably tune anti-squat with shims under the pivot blocks as those are separate parts which attach to the T-plate.

2.) Original front rims seem a bit narrower than modern rims. Putting modern tires on is a tight fit without trimming the foams/inserts, but if you trim them then the tire profile is even more rounded off. I like the look of the 5 spoke rims, though, so I don't intend on putting modern rims on this car. Maybe the JC Racing Products repro XX rims are a bit wider at the front?

3.) Looking back, it seems you really have to pay close attention to how you drill the holes into the motor plate to hold it down to the chassis. If you're a little off one way or another, then that will flex the rear pivot support either forward or backward slightly, which affects anti-squat and wing angle (wing mounts to tower which mounts to bulkhead which mounts to pivot support). It's a "hand job," though, and as well all know, hand jobs are sometimes messy.

4.) I played a bit with negative timing due to the rotation reversal I mentioned earlier. Interestingly, going to negative timing increases cogging at low speeds, despite the reverse rotation. So I'm going to lay off of the negative timing for now. Maybe I'm missing something and positive timing is still the way to go for some odd reason (I doubt it but I could be wrong), but for now I have it at 0deg and will fiddle around with boost instead of static timing.

5.) Some basic testing in my basement and driveway suggest the car will drive quite well. Jumps neutral and handles nicely, as you'd expect from a mid-motor car. Looking forward to racing it, whether it's at VONATS this year or next year. Some people seem to think a vintage 2wd buggy win is only valid with a rear-motor chassis, so I either have to do that at least once to prove a point (with my Pro-X or TRX-3), or run this car as a mockery of vintage purists. Generally, I bring multiple cars and gravitate toward what works best with whatever I have already on the car, to avoid excessive time in the pits playing with setup. So we'll see if the XX-Mid wins the downselect game this year...

[RogueIV]

So I'd say it's about 99% complete here. Need to do little things like set camber, ride height, etc. I added a few things like a battery brace (not shown here, but it's in the videos) and JC Racing Products repro XX front arms. Those arms are noticeably beefier than the stock XX arms, but you may have to drill some of the hinge pin holes to make sure everything mounts smoothly. Also, because there's more material around the shock mounting holes, the holes themselves are placed a little further apart. The inside hole is in the same position as stock, but as you move to the outside hole, they are a bit more to the outside than on the stock arm. You can lengthen the shock (unscrew the eyelet) to compensate for droop if you want.

General thoughts so far on this project:

1.) I maintain the Kyosho Rampage Pro-E is a better mid-motor vintage car. For starters, it is more tunable, more easily tunable, and more easily serviceable. Rear anti-squat can be tuned with shims whereas on the XX you are fixed to whatever the rear pivot support has. As long as you don't need to reduce anti-squat, then it's not a big deal, but on a bumpy track you may wish you had that option. With the XX-CR, you can probably tune anti-squat with shims under the pivot blocks as those are separate parts which attach to the T-plate.

2.) Original front rims seem a bit narrower than modern rims. Putting modern tires on is a tight fit without trimming the foams/inserts, but if you trim them then the tire profile is even more rounded off. I like the look of the 5 spoke rims, though, so I don't intend on putting modern rims on this car. Maybe the JC Racing Products repro XX rims are a bit wider at the front?

3.) Looking back, it seems you really have to pay close attention to how you drill the holes into the motor plate to hold it down to the chassis. If you're a little off one way or another, then that will flex the rear pivot support either forward or backward slightly, which affects anti-squat and wing angle (wing mounts to tower which mounts to bulkhead which mounts to pivot support). It's a "hand job," though, and as well all know, hand jobs are sometimes messy.

4.) I played a bit with negative timing due to the rotation reversal I mentioned earlier. Interestingly, going to negative timing increases cogging at low speeds, despite the reverse rotation. So I'm going to lay off of the negative timing for now. Maybe I'm missing something and positive timing is still the way to go for some odd reason (I doubt it but I could be wrong), but for now I have it at 0deg and will fiddle around with boost instead of static timing.

5.) Some basic testing in my basement and driveway suggest the car will drive quite well. Jumps neutral and handles nicely, as you'd expect from a mid-motor car. Looking forward to racing it, whether it's at VONATS this year or next year. Some people seem to think a vintage 2wd buggy win is only valid with a rear-motor chassis, so I either have to do that at least once to prove a point (with my Pro-X or TRX-3), or run this car as a mockery of vintage purists. Generally, I bring multiple cars and gravitate toward what works best with whatever I have already on the car, to avoid excessive time in the pits playing with setup. So we'll see if the XX-Mid wins the downselect game this year...






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I've never owned a XX but I wonder if a new Tplate could be fashioned to ultilize separate pivot blocks like the XX-4 and later XXX series used. Like milling a tplate out of Carbon or G10 would likely make that part of the car a little stronger.

Also as far as rear motor support, you could always run a turnbuckle from the top of the motor plate down to the front area of the chassis.

p.s. Love this build!

[stickboy007]

Thanks, and yes, the turnbuckle is a good idea. Would need to figure out how to make it not interfere with battery ingress/egress.

[RogueIV]

Thanks, and yes, the turnbuckle is a good idea. Would need to figure out how to make it not interfere with battery ingress/egress.

could make it part of the battery hold down, that way it just levers up out of the way. just and idea anyway.

[stickboy007]

I got to the bottom of the bending of the rear T-plate, which was affecting the rear tower and wing angles. Basically, the Kyosho UM733 part that I had mounted, I pushed it too far back. That was lifting the motor mount slightly and causing some flex. So I removed that part and tried a different approach. Ultimately a good thing, because the threading into the bottom of the motor plate really wasn't enough to handle long-term stresses of racing. I filled the holes I drilled into the chassis for that with epoxy and drilled one new hole to accommodate what you see in the pictures below. Basically a 3x20 screw with spacers going into a rod end, which itself is attached to the front of the motor plate. This secures the motor plate to the chassis and eliminates fore/aft flexing like a waterfall would. It does take up some space in front of the motor mount, which limits battery position options. This means if you want the effect of the battery further back, then you just have to add some weights back there around the screw. Overall a much more elegant solution and far less messy than what I had earlier.

I also played around a bit with negative timing on the motor endbell. Because of the reverse rotation, more negative timing will increase top speed; however, what I've noticed, at least for this specific motor/esc combination (Novak Ballistic 17.5 w/ Trackstar 120A V2 ESC), is any amount of negative timing increases cogging at low speeds and it gets worse with more negative timing. Full throttle from a standing start usually does nothing, especially above ~ 15deg negative timing. So that's a limitation here, which I'm getting around by setting the fixed timing to 0 (or slightly positive to be on the safe side) and setting the boost to 40deg kicking in at 6,000 rpm. Seems to be plenty fast enough going up and down my street. It is possible that other motor/esc combos won't have this cogging problem with reverse rotation and negative timing, but I'm not about to blow my wallet running that experiment