The Astro Spiral Jump build

[markbt73]

I decided to write a lengthier post here about this, since it's easier than trying to explain it all on the main site. This is the "Making Of" my entry into the "Show Off" photo competition.

It's something I've wanted to try for ages: a complicated, difficult stunt sure to break a car, but spectacular if you manage to pull it off. The "Astro Spiral Jump" was the brainchild of Raymond McHenry and Jay Milligan, and it was originally intended to showcase the accuracy of the new computer-modelling of traffic accidents and crash tests. The idea of actually doing it was really just to prove the computer got it right.

But it caught on in Milligan's thrill shows, where it was first performed at the Houston Astrodome (hence the "Astro" part of the name), and was so impressive that the producers of the James Bond movies, always eager for a new thrill to put up on the big screen, wanted to put it in a movie: The Man With The Golden Gun.

That slide whistle still nearly ruins it. Nearly.

Now, I'm no mathematician, and I don't have a way to computer model a jump like this. But I have a beat-up Grasshopper and a lot of scrap plastic to try making ramps out of. And lucky for me, the original jump design was patented, and the patent ran out in 1991, which makes the design now public domain, downloadable from Uncle Sam himself.

The Grasshopper was ideal for this, because it's lightweight and tough, and has a rigid rear axle that should make it easier to "kick" the right rear wheel into the air at the end of the ramp, to start the spin. To make it look the part, I got a Protoform AMC Javelin body (the original stunt was performed with a Javelin, and nobody makes an AMC Hornet body like in the movie) and painted it to match the movie car.

The chassis started out stock, except for an elongated skidplate/body mount at the front, to which I attached the original front bumper about an inch further forward than stock, and a scrap of plastic for the rear body mounts. After the first round of testing, it became apparent that the front suspension was too soft, so I changed out the front springs for some very stiff touring car springs and perches. this kept the front suspension from soaking up all the energy of the "drop" section of the ramp.

I didn't get any good photos of the development of the ramp. I honestly wasn't expecting it to work, so I just kept plugging away at it without documenting very much. It wasn't until the second round of tests that I started taking photos.

The ramp was originally too low to get any height out of, so I stuck progressively larger cardboard boxes under it until I got the altitude I needed. The brick is there to weigh it down.

Well, that didn't work...

 

And in fact, it continued not working for most of the afternoon. I tried going faster, I tried going slower (which REALLY didn't work), I replaced the RS380 motor with a Sport-Tuned 540 to gain a little velocity. And then I started getting somewhere. I broke one rear shock mounting boss completely off the gearbox on a bad landing, making the rear suspension soggy. And suddenly, the jump worked... twice in a row. But the battery was fading, as was the light, so I vowed to go back out the next day with a fresh pack, find a more photogenic backdrop, and have my wife take photos.

And proceeded to fail a bunch more times.

I broke the other side of the rear suspension, cracked one axle housing, zip-tied it all back together, and kept trying. But I invoked a favorite saying from the Mythbusters, for luck.

And we got one - ONE - good jump.

I then tried to repeat it, with my wife recording video, but it never worked again. Poor Grasshopper. It looked so tired...

The car is now sitting on my shelf, the ramp is in the garage, and the one good shot we got is in the competition. I don't expect to win, but I haven't had as much fun trying something stupid with an RC car in a very long time. I need to order a new B parts tree to fix the car, and maybe redesign the ramp a bit so it isn't so kludged together, and we'll try again in the summer to get a good video of it...

 

[svenb]

That's was a great read and good luck sir👍

[Aerobert]

I love this project. It's awesome to see how much effort you put in this jump. But I think it's worth for that one cool photo.

Are you planning a landing ramp for the next try?

And you're right, the whistle almost ruined the whole scene in the movie.

Though little Grasshopper, by the way.

[Grumpy pants]

Nice thread,  a really good read and a great idea.

I bet you felt like a kid again, designing and testing your ramp and driving skills to the max 👏

[Mad Ax]

Quote

A secret agent?  On whose side!?

Top work there - I can see lots of effort went into that!  Getting jumps right is really tricky - I tried to video my monster truck jumping over a ramp for a "lockdown special" work video last year, it must have taken 20 attempts just to hit the ramp straight and not roll over on landing.  There's an awful lot of physics going on when a wheel hits a ramp and tries to change the car's direction, our little RC car suspension can't handle it without a lot of tuning (as you've proved!) - I wonder how much they had to modify the full-size car to do that stunt?

I remember speaking to a stuntman driver who drives Minis around on two wheels, aside from a rollcage and a welded diff they have to do a lot of reinforcing and stiffening of the suspension to handle driving on one wheel, and the heavy landing afterwards.  That movie jump actually looks quite sedate in comparison.

[Kol__]

12 hours ago, markbt73 said:

That slide whistle still nearly ruins it. Nearly.

Awesome project, gutted there is no video of it in action! I'll wait patiently for the vid in the summer

[markbt73]

Thanks all! Glad you like it. It sure was fun. And yeah, the stunt in the movie makes it look too easy... I have a new appreciation for what's involved now.

I didn't build a landing ramp, because I didn't think I needed it - watching videos of the original jumps, it looked like the landing ramp was only there to soften the blow on the driver. I think I might build one for the next round, though, because it softens the blow on the car, as well.

Speaking of which... it held up better than you might think!

It's 100% driveable as it sits. But it did take a serious beating, and it does need some repairs before next time.

The paint wasn't fully cured before I went out; I was on a tight timeline, between having to go to work and avoiding the worst of the weather. Uncured acrylic paint doesn't like water.

The chassis, intact and functional... but with no more rear suspension, and missing a few screws.

How on Earth did I lose one of those screws?!?

The gearbox took the worst of it. But to its credit, it still runs smooth as silk, and never missed a beat. That zip tie is both holding the gearbox onto the car, and that axle tube onto the gearbox. Note that the right-side shock mounting boss is completely gone.

So... new B parts are in order, and I think I'll go with the Hornet/Lunchbox style with the additional reinforcement. The rest of the chassis can stay as-is, I think; I don't see any other damage. This is why, deep down, if I'm being honest, the good old Grasshopper is still my favorite Tamiya model... it's just game for anything.

[markbt73]

Tore down the gearbox... the case is broken in several places (and some very strange places), but the internals are in fine shape. I did, however, manage to bend one of the cross-pins on one axle. Didn't know those could be bent...

I have a brand-new Hornet/Lunchbox "B" parts tree (wth the beefed-up axle housings) for it. It will get reassembled, and then go on the shelf to wait for springtime, at which time I think I'll try to finally get the jump on video. (What the hey, parts for these are cheap, right?)