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Lunch Box rear suspension... that old chestnut

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I've been staring at the Lunch Box's rear suspension again trying to get my head around some of the various fixes and their effectiveness. So far, my one Lunchie with the rear flipped around backwards and 4-linked has proven to be the most effective, but I'm not fond of its highly non-stock appearance. This led me to examine the other common fixes.

Two of the most common are the Ampro brace and the 3rd shock mod. The brace is a more rigid form of locating the axle and preventing it from "winding up" on acceleration. The 3rd shock can dampen the wind up effect while still allowing some of it to a degree. The issue I see with both of these is that regardless of how well they do their job, the rear of the axle is still pulled down by the forces of acceleration (once the tires get traction), also locking the rear shocks by pulling them to full extension. I'm guessing this is what Tamiya was aiming at eliminating by laying down the rear shocks on vehicles like the Madbull.

The next idea rattling around in my head was adding a collapsible top link to the top of the diff case, one the would compress as need be for suspension action but lock at extension to prevent axle wind up. This is a fancy way of implementing the fishing line mod pioneered by @shenlonco, which accomplishes the same thing. Would spreading the "link" points out to the top of the gearcase and lower portion of the gearcase at the pivot aid effectiveness? Part of me thinks it might but shock position still doesn't seem optimal.

A 4 link with the gearbox in its normal position seems the next step up, but I still don't see the shock position as optimized until the rear is spun around and the shocks are mounted ahead of the axle centerline.

Much of this comes from studying the effectiveness and evolution of mod-Clod setups over the years. Mod-Clods have great suspension for a MOA design. From the start the Clod is both 4-linked (in a way, with the ladder bars) and minimally has the shocks mounted at the axle centerline. As they evolved, the shocks moved more and more toward the chassis's center, great adding compliance. Any thoughts?

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Is the Parma / You-g Hornet sway bar something that might work?  It prevents the rear axle from "wrapping" whilst allowing the axle to pivot side-to-side.

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I have seen another brace alternative on thingiverse, which is just a solid block that would fit in the slot at the front, stopping the front of the gearbox moving up and down completely. this would mean that it turns the front in to a pivot point. The bad bit about this is that it stops any suspension movement, so if the left wheel went over a rock it will probably just tip the car over. With the ampro style brace it still has the ability to move up on one side, thus could absorb the bump.

Ultimately until you can fashion some sort of independent suspension (think typical buggy or even monster beetle) its never going to be great.

The next best thing I can think of is if you cut off the original front mounting and then from that mounting point on the chassis run bars backward and have them attach to the axles that protrude from the gearbox you'd have a 4bar suspe nsion, this would let the axle tip to some extent (so that left wheel could go over that rock), the whole assembly couldn't "wind up" as the bars would hold it steady. you'd likely need a transverse brace though, because the whole gearbox could move left to right under the chassis. So imagine yet another bar connected where the shock mounts and then going from left to right and connecting to a bracket on the chassis. This is often called a panhard bar and stops that left to right movement.

I stole someones pic and made a quick sketch. All five bars would be the usual threaded rod with balljoint at attachment points. You'd need some brackets designed and 3d printed, something similar to the shock mounts I guess. Haven't put the panhard bar on the pic

0wzsG4jl.jpg

What you'd end up with is a geabox that cannot move up and down at the front, but can still articulate and has suspension offered by the shocks. Hereis a 1:1 version of what I mean, no shocks on this yet, but they can just go straight up off the axle.

08dfe663aeffacec7d5149f09dbdce47.jpg

The bars need to be equal length and parallel. they should be in a straight line back to front to avoid binding.

The alternative is to use triangulated arms - this prevents the ened for the panhard bar but you'd have to think about how to mount them. Thin of how RC crawler stuff works:

100625d1241731055-2.jpg

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Mark C's post about 4 parallel links with Panhard rod is a good idea.    I'd just like to add, once you have a panhard rod to locate the axle laterally, 3 parallel links are enough.   Using a Panhard rod with the 3 link setup eliminates the possibility of binding so there is no requirement for the 3 links to be perfectly parallel.   In the case of the Lunchbox, you can have 2 lower links, but the upper link on the left side only.   That will avoid any issue with the clearance on the right side where the motor is fitted.   There is a youtube video somewhere of a 3 link grasshopper rear suspension where the upper link is located at the top of the gearbox (positioned like the screwdriver in the picture used by Mark C).

2 hours ago, Mark_C said:

The next best thing I can think of is if you cut off the original front mounting and then from that mounting point on the chassis run bars backward and have them attach to the axles that protrude from the gearbox you'd have a 4bar suspe nsion, this would let the axle tip to some extent (so that left wheel could go over that rock), the whole assembly couldn't "wind up" as the bars would hold it steady. you'd likely need a transverse brace though, because the whole gearbox could move left to right under the chassis. So imagine yet another bar connected where the shock mounts and then going from left to right and connecting to a bracket on the chassis. This is often called a panhard bar and stops that left to right movement.

I stole someones pic and made a quick sketch. All five bars would be the usual threaded rod with balljoint at attachment points. You'd need some brackets designed and 3d printed, something similar to the shock mounts I guess. Haven't put the panhard bar on the pic

0wzsG4jl.jpg

 

 

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Mine has reversed gearbox and 4 link but shocks still above the axle. It made a huge difference, the truck squats under acceleration now and side to side articulation works great even when on the throttle. I often watch videos and enjoy seeing how much more compliant the rear end is and how independently it moves from the chassis:

CAbZMMK.gif

BUT when both wheels meet an obstacle at the same time, the whole axle still takes off. It could be better still.

Having the shocks closer to the chassis would help for sure but room is limited there.

Sometimes I stare longingly at videos of the new gen Kyosho Outlaw and marvel at how well it's 4 link works but when you look at the chassis, you can see why. The lower links/arms are very long, the chassis overhangs the inboard lower joints significantly and the shocks are far, far away from the axle.

If you've ever watched slow motion footage of these trucks, you'll see how well the rear suspension works, not just side to side but up and down too. It shows that solid axle+motor on axle can work very well and be responsive and compliant. 

I'm just not sure there's enough room under a Lunchbox to achieve the same results.

Kyosho_34361T1B__4.thumb.jpg.2099f421b29cef5764706f2243624881.jpg

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In response to nowinaminute, I had a chance to drive my Outlaw Rampage this weekend again. The truck is very scale from a driving standpoint. Easily the hardest car I have to drive lol. Every time you corner it really wants to roll, so you have to tap the breaks before you enter. 

The suspension with the MOA does work really well though. It looks like a true trophy truck jumping. The main reason for this is the weight in the back above the axle. With the tires mounted to the cage you can drop the truck and it will soak it up like a truck with IRS. Without the added weight it bounces like my Hornet. It's really a world of difference. 

My point is the weight is great for the suspension, but bad for turning in my experience. Just something to consider.

It might be worthwhile if using a 4-link to make the links longer with the ends closer to the chassis center so the battery can add more of the required weight over the rear axle and links. 

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1 hour ago, nowinaminute said:

There was a guy on ebay making a parallel 4 link kit with pan hard bar by the time I found out about it, he had stopped making them. Smirk Racing has one on his 6s Lunchbox.

https://www.worthpoint.com/worthopedia/tamiya-lunchbox-midnight-pumpkin-link-1834919363

I have a second set from the original guy who made them, which I'd be willing to sell. Drop me a PM if interested. Be warned, it is ridiculously expensive.

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7 hours ago, nowinaminute said:

There was a guy on ebay making a parallel 4 link kit with pan hard bar by the time I found out about it, he had stopped making them. Smirk Racing has one on his 6s Lunchbox.

https://www.worthpoint.com/worthopedia/tamiya-lunchbox-midnight-pumpkin-link-1834919363

Yeah thats pretty much what I was thinking, he did a better job on the panhard bar than my suggestion.

I wonder how cheap it could be done now with 3d printing the brackets.

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9 hours ago, Wheel_Nut said:

Mark C's post about 4 parallel links with Panhard rod is a good idea.    I'd just like to add, once you have a panhard rod to locate the axle laterally, 3 parallel links are enough.   Using a Panhard rod with the 3 link setup eliminates the possibility of binding so there is no requirement for the 3 links to be perfectly parallel.   In the case of the Lunchbox, you can have 2 lower links, but the upper link on the left side only.   That will avoid any issue with the clearance on the right side where the motor is fitted.   There is a youtube video somewhere of a 3 link grasshopper rear suspension where the upper link is located at the top of the gearbox (positioned like the screwdriver in the picture used by Mark C).

Yes, I'm going through this with the rear of my foxbody 5.0 Mustang at the moment. There's debate whether a panhard bar can be added to the stock triangulated 4 link rear or if one top link should be removed (a cheap way of doing what the newer S197 has). The Mustang might get away with it due to its large pillow-like control arm bushings. The rod ends we use would bind for sure as the two systems would have conflicting roll centers. With parallel links, like the kit @smirk-racing has, it seems to get away with having the 4 links and panhard bar.

8 hours ago, nowinaminute said:

BUT when both wheels meet an obstacle at the same time, the whole axle still takes off. It could be better still.

This is where I'm at now. Its far better, but no perfect.

8 hours ago, nowinaminute said:

Sometimes I stare longingly at videos of the new gen Kyosho Outlaw and marvel at how well it's 4 link works but when you look at the chassis, you can see why. The lower links/arms are very long, the chassis overhangs the inboard lower joints significantly and the shocks are far, far away from the axle.

This is what I had in mind ideally, but in our Lunchie's, as we all know, we'd never get that optimized shock location. In my mind, I still see the shocks being extended upon acceleration when they sit behind the axle centerline.  Flipping the rear was the only way I could sneak them in front of the motor with the limited space we have.

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This vid of a grasshopper with a flipped gearbox might be interesting:

 

 

 

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1 hour ago, smirk-racing said:

This vid of a grasshopper with a flipped gearbox might be interesting:

 

 

 

Looks interesting and worth investigating , but the damper top mounts look to be under stress

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6 hours ago, smirk-racing said:

This vid of a grasshopper with a flipped gearbox might be interesting:

 

 

 

I've been tempted to try this on my Grashopper and Nikko F10 buggies. Maybe even a miniaturised version on my Tyco hopper!

He's a nice guy, he sent me an STL file for  a19mm-25mm screw spacing adaptor so I can use 380 size inrunners and outrunners in place of 540 motors for less unsprung mass. You can see he's using an outrunner on his too.

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I never tried any link type system but I did find that the fishing string mod works best even over 5th shock mod.

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Thanks for posting that @Wheel_Nut. That's pretty much what a S197 Mustang runs in the rear, a three link with panhard bar. Looks very effective, even without flipping the gearbox. I like it.

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20200619_155316

This is what I've come up with. Of all the things I've tested this is the best performing short of going to a full 4 link or 3 link with a panhard bar. As I alluded to earlier, the Ampro brace and 5th shock are slightly compromised. The brace works splendidly, but because it "locks in" or fixes the the front pivot point, the gearbox can only pivot upwards on that axis now. It does allow side-to-side, independent movement. In essence, for suspension compression (when both wheels are compressed simultaneously), the whole gearbox assembly can only pivot upward off the fixed point now. Upon hard acceleration, I still see the rear shocks being pulled to full extension, being located farthest from this fixed pivot point.

The third shock does not hold the front pivot fixed but allows the gearbox to both pivot or move upwards as a whole unit. Unfortunately, the rotational forces involved still cause gearbox rotation, compressing the 5th shock and extending the rear shocks. The 5th shock works on the same principle as the stock "axle springs" but unlike the axle springs, it continues to function rather than breaking on about the 3rd run or so.

What I have pictured is a collapsible link made from an old Grasshopper rear shock (with no spring). Several things to note. The front eye must not be made overly tight or rigid. This one is bushed in rubber. The rear eye is secured to a ball stud. These are required to allow independent, side-to-side play for the gearbox assembly. Ideally they should be ball jointed at both ends. The "link" (Grasshopper shock) is set so it is fully extended with the front pivot at the bottom of its slots. As such the rear gearbox cannot rotate in anyway. All rotational forces are transferred into the whole chassis. The instant the suspension begins the compress. The link's geometry will begin to allow the pivot to rise in it slots if need be or the link can collapse altogether allowing the gearbox to pivot. In other words the gearbox has total freedom to move in any direction during suspension compression, except for allowing the front pivot to slam upward on acceleration. Resulting test show the rear is fairly planted and the usual sound of the motor loading and unloading as the standard issue Lunch Box hops and tramps around in the rear is greatly diminished.

BTW, the fishing line mod does all this too. I just felt the need to make it more "mechanical".

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Nice! Is there an easy tutorial for the fishing line mod anywhere? It wasn't clear on the video how it's actually done.  I'm assuming it completely replaces the little springs.

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41 minutes ago, Saito2 said:

20200619_155316

This is what I've come up with. Of all the things I've tested this is the best performing short of going to a full 4 link or 3 link with a panhard bar. As I alluded to earlier, the Ampro brace and 5th shock are slightly compromised. The brace works splendidly, but because it "locks in" or fixes the the front pivot point, the gearbox can only pivot upwards on that axis now. It does allow side-to-side, independent movement. In essence, for suspension compression (when both wheels are compressed simultaneously), the whole gearbox assembly can only pivot upward off the fixed point now. Upon hard acceleration, I still see the rear shocks being pulled to full extension, being located farthest from this fixed pivot point.

The third shock does not hold the front pivot fixed but allows the gearbox to both pivot or move upwards as a whole unit. Unfortunately, the rotational forces involved still cause gearbox rotation, compressing the 5th shock and extending the rear shocks. The 5th shock works on the same principle as the stock "axle springs" but unlike the axle springs, it continues to function rather than breaking on about the 3rd run or so.

What I have pictured is a collapsible link made from an old Grasshopper rear shock (with no spring). Several things to note. The front eye must not be made overly tight or rigid. This one is bushed in rubber. The rear eye is secured to a ball stud. These are required to allow independent, side-to-side play for the gearbox assembly. Ideally they should be ball jointed at both ends. The "link" (Grasshopper shock) is set so it is fully extended with the front pivot at the bottom of its slots. As such the rear gearbox cannot rotate in anyway. All rotational forces are transferred into the whole chassis. The instant the suspension begins the compress. The link's geometry will begin to allow the pivot to rise in it slots if need be or the link can collapse altogether allowing the gearbox to pivot. In other words the gearbox has total freedom to move in any direction during suspension compression, except for allowing the front pivot to slam upward on acceleration. Resulting test show the rear is fairly planted and the usual sound of the motor loading and unloading as the standard issue Lunch Box hops and tramps around in the rear is greatly diminished.

BTW, the fishing line mod does all this too. I just felt the need to make it more "mechanical".

Great work . I've done the 5th shock mod , and the AmPro brace , FX10 , Ultima beam front mods ,  and I think even with this mod or fishing line mod it will still be a wacky Lunchbox and it always will be - thank goodness

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1 hour ago, jellon said:

Nice! Is there an easy tutorial for the fishing line mod anywhere? It wasn't clear on the video how it's actually done.  I'm assuming it completely replaces the little springs.

Looking at the video, @shenlonco has the front of the fishing line tied to the bracket of the fifth shock mount he's already installed. This is a mounted by drilling a hole in the chassis tub, roughly in the same spot you see my front link mount in the picture above. The rear appears to be tied around the screw head (possibly backed out slightly to allow for this) at the top, rear of the gearbox. The fishing line must be tied or drawn tight with the shocks fully extended and the front pivot at the bottom of its slot.

1 hour ago, KEV THE REV said:

Great work . I've done the 5th shock mod , and the AmPro brace , FX10 , Ultima beam front mods ,  and I think even with this mod or fishing line mod it will still be a wacky Lunchbox and it always will be - thank goodness

Yeah, that's the beauty of all these little mods we do. They may slightly tame or improve the Lunch Box, but in the end its still a Lunch Box and that spells F-U-N. I had a DF01 rear gearbox mocked up under the back at one time, but IRS would strip the little van of too much of it original flavor.

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5 hours ago, jellon said:

Nice! Is there an easy tutorial for the fishing line mod anywhere? It wasn't clear on the video how it's actually done.  I'm assuming it completely replaces the little springs.

Hi here is how it works as shown in my video.

And this shows all lunch box mods and the string mod on the lunch box... but the above video I explain how it works and how to do the mod better.

 

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7 hours ago, shenlonco said:

Hi here is how it works as shown in my video.

And this shows all lunch box mods and the string mod on the lunch box... but the above video I explain how it works and how to do the mod better.

 

Excellent 👌 thanks shenlonco. I'd only seen the second video. 

I'm assuming I don't need to replace the little springs wires? They've both snapped.

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2 hours ago, jellon said:

I'm assuming I don't need to replace the little springs wires? They've both snapped.

No, you don't need to replace the little springs.

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