43 replies to this topic

[Larry Horner]

After Martin and I finished the MDC Bat car chassis, we were thinking it would be fun to create a 3D printed version of a vintage chassis design that was a bit more performant. The Bat car is fun in that it really invokes the spirit of the original design but being a hard body car with a high center of gravity, it’s never going to be winning the local Saturday night races. This is when Martin introduced me to Tony P’s drop dead gorgeous Starship. For those not familiar with this chassis, here is a build by Rick Thigpen in which he makes his own Starship (thanks Rick):  https://slotblog.net...ip/?hl=starship. Now for a big caveat: Martin thought it would be cool if we could make our’s a sidewinder. And from my tuning fork sidewinder (https://slotblog.net...ork-sidewinder/), I knew a sidewinder with a modern minican motor could have tires as small as 7/8”.  With this in mind, we laid out the basic parameters for our Starship:

 

- Minican sidewinder.

- 3 1/8” chassis width.

- 3 7/8” wheelbase.

- 7/8” guide lead.

- 7/8” rear tires. This mandates a 3/32” axle for clearance.

- .050 clearance under the rear.

- 3/4” front tires.

- .015 clearance under the front.

 

Another factor that influenced our design is that I knew we would be using SendCutSend for the custom cut brass pieces and they won’t cut anything thinner than .040”. And curiously, when 3D printing in PA12 nylon reinforced plastic, the minimum recommended thickness is also .040”. So Martin suggested a design in which our plumber rail would ride on top of a .040” printed lip that would blend perfectly with the brass pans along the seams. You can probably see where this is going but while PA12 plastic can take temps over 400 degrees Fahrenheit, it definitely can’t take the heat of soldering. So our Starship required a design in which the plumber rails, hinges and pans could be fabricated separately and then dropped into the plastic main section … what could possibly go wrong? Once the basic concept had been formulated, Martin started working on design sketches and this is where his talents far outshine my own … I’m a fabricator but not an artist!

 

After several design iterations and printed prototypes, we thought we were close to the goal line when I accidentally dropped a prototype with installed drivetrain on my hardwood floor only to have one of the leading wingtips break off … nicht sehr gut. So it was back to the drawing board and lots of additional gusseting was added for additional frontend support. I might add that the original Starship had a 24 degree included angle at the back while this one is only 20 degrees. We could have used 24 degrees but when we tried that, the motor ended up skewed off to one side losing the symmetry that I think is one of the strongpoints of this design. The raw chassis is 11 grams and something not obvious here is that the guide tongue extends forward at a 2 degree angle as is commonplace in today’s vintage racing. So without further ado, here is a CAD drawing and 3D print of the final design and I think you can see why I’m calling it the Burning Man Starship!

 

 

Here is a shot of the printed main chassis with a JK Hawk 6 motor mounted. We designed around this motor mostly because its tapered end-bell allows the motor to be “angled” in between the sidewalls of the motor-box. But additionally, there are lots of aftermarket arms if someone wants more poke. And since we opted for a predefined 12/48 gearing, the motor is solidly locked into position with flat head 2mm screws. Not only does this provide very secure and accurate positioning of the motor but the flush heads allow the spur gear to run inboard.

 

 

Here is a shot from the end-bell side. One late addition is that Martin wanted a cap to lock the end-bell from above. Neither of us was convinced this was really necessary but it does fill the void left by the slot needed to drop in the motor. This chassis is vapor blasted which cleans up the surface and also makes the color a bit darker and more uniform. But this process is only available for larger parts hence wasn’t available for the end-bell cap so it has the lighter salt-n-pepper finish of raw printed PA12.

 

 

And here is a shot with fully installed drivetrain. The spur gear here is a 48T 64P which I believe is made by Sonic (PCH sells them). And for those wondering about such matters (i.e. track owners), the bottom of the gear is ever so slightly above the bottom of the chassis. This can be used with either a 12T pinion or there is room for a 13T seen here if higher gearing is desired. More to come so stay tuned!

 

 

[JHMerriman]

This is just awesome! I love 3d printing but it has it short comings. I hope it works out well for your guys! Can't wait to see the final product.

[Alchemist]

Hi Larry, Martin,

 

What an stunning design!

 

Great job gentlemen!

 

Thank you for sharing and looking forward to your progress!

 

Ernie

[Larry Horner]

James, I too love both the accuracy and possibilities afforded by 3D printing. But in terms of short comings, the big issue for slot cars is that the stuff is just too light and as I mentioned above, the printed section of our chassis is only 11 grams. So we're hoping that combining it with classic wire and brass construction allows us to work around that hurdle.

 

Ernie, so nice to hear from you! Someday I want you to make me one of your amazing fiberglass board chassis ... maybe we can do a swap.

[Larry Horner]

Unlike the original Starship, Martin and I decided ours would have a pan filling the triangular center section. And this was very easily accomplished since SendCutSend was doing the hard work of cutting the brass. The end result is just a piece of .040" brass with a couple of 3/32” tubes solder on. This drops into the center cavity and the main chassis has a couple of 1/16” holes in the beams thru which 1/16” locking pins are inserted.

 

 

There are several different usage scenarios available here. First, is that you can leave out the center section entirely to save about 8 grams of weight. Second is that you can use the center pan but only insert a locking pin at either the front or the back allowing the other end to float … sort of a cheap and easy rattle pan. Or lastly you can use both locking pins to attach it securely. Here are shots of the center pan installed using both locking pins.

 

 

And here is a shot of the underside. The gap between the chassis center and pan is again .040", same as the chassis base and pan thickness.

 

[pn6]

You guys are coming up with some great stuff!!

Where can I get that resin Larry??

Paul

[Larry Horner]

Glad you like it Paul! We're using MJF (multi jet fusion) printed PA12-GB (glass bead) plastic which is very accurate and very strong as it's nylon reenforced. Unfortunately MJF printers are also very expensive hence out of reach of the home hobbyist so I had the chassis above printed by Shapeways which is back in business.

 

That said, I know you have a resin printer and I noticed the other day that Elegoo is marketing what they call Tough Resin (https://us.elegoo.co...goo-tough-resin). My guess is that a chassis printed with this stuff would be just as good as PA12 but that's just a conjecture on my part. Maybe you can be the first on SlotBlog to try it out?

[Martin]

Hi Paul,

I just had a conversation with our local 3D printer. He is up for printing a test shot in the Tough Resin. Then we can do a back to back test.

Our  main concern is we do not want to give up on the accuracy. The PA-12 is so good. 

[The Sawdust Man]

Love this! Count me in for a couple of them when you guys are finished.

[pn6]

Larry I figured it was something high tech so I’m hoping Martin’s guy finds the Elegoo stuff is a decent replacement.

[Larry Horner]

Robert, glad you're enjoying the ride! But I must confess that I just like to fabricate and build stuff and wasn't really thinking in terms of merch. But I'm going to hand the STL (chassis) and DXF (brass) files off to Martin so I'll let you two talk.

 

Paul, I hope it pans out too as Shapeways is in the Nederlands and getting a print order thru them is taking about six weeks. Conversely all the US based MJF printers are double the price. So if the Tough Resin works, it's time for me to finally buy a printer!

[Larry Horner]

With the 3D printed main chassis and center pan sorted, next up was the plumber rail supported pans. And as is almost never the case, I managed to accurately bend the plumber rails correctly on the very first try. So feeling cocky, I began bending the front and back crossmembers and was instantly humbled … let’s just say these took a few failed attempts before I got it right. I figured out a technique of bending a basic loop at the appropriate width, clamping the tip of the loop in my workbench vice and then bending the rods extending out the top to the correct angle. Also the height of the front crossmember doesn’t really matter but the one in back functions as the plumber rail up-stop hence the height has to be just right.

 

Luckily once the cross members were soldered to the rails, everything firmed up and the rest of the work went pretty smoothly. But there was one small fly in the ointment. I wanted the pan pivot pins on the outside of the pivot tubes for maximum strength. But this means the forward pivot pins needed to be inserted before the pivot tubes were solder to the rail. I’d never encountered a situation like this before but Steve O’Keefe said a proven method for him is to simply coat the pins with a light film of motor oil before soldering. This worked perfectly so thanks for the tip Steve! Note the bite bars are soldered to the main rails with a slight upward tilt yielding .020” of clearance at the edges of the pans. Here is a shot of the finished plumber rail structure.

 

 

With the rail structure finished, I was ready to mount the side pans. And my first attempt at this was a miserable failure in which one of the pans apparently shifted position during soldering. Luckily I had ordered multiple sets of pans so just moved on to attempt number two which included much more careful checking of the position and using Rick’s method of soldering the pivot pins to the pans. This involves way more flux than I’m used to using and more patience to allow the heat to flow from the steel pins into the thick brass. But it worked out great so thank you Rick! And for the astute amongst you (that’s everyone on SlotBlog right), there is an ever so slight bend in the pan pivot pins to account for the .016” pivot tube wall thickness. I’ve also added 3/4” pin tubes atop the pivot pins. It still needs final detailing but here is the finished component.

 

 

Now it is time to attach this puppy and this is where the magic happens. The plumber rail and pan assembly is dropped down from above, the rear crossmember is slid back into the up-stop grove at the front of the motor box, the plumber rail pivots are pushed thru the 1/16” slots in Mr Burning Man’s wingtips and finally 3/32” tubes are slid in from the sides around the rail pivots to lock them into position and voila … an old skool plumber rail and hinge assembly!

 

 

And to quote Captain Rick, bottom’s up!

 

[Larry Horner]

Next up are the front wheels and you’ve probably noticed already that the main chassis has supports for these guys. Both Tony P’s and Rick’s Starships had spring rods to keep the front tires down on the track but 3D printing provides pretty much unlimited flexibility. So our A-arms have connectors between them that effectively act as the spring. The connector diameter can be varied to control the amount of spring tension. And the angle at which the connectors attach to the A-arm pivots can also be changed to adjust camber. An interesting caveat is that if one could make a real car with such a design, the connectors would also function as anti-sway bars but I digress. With the pictured A-arms, the connectors are .060” thick with no camber but when the car is on the track, the spring rate is light enough that the car drops virtually all of its weight onto the guide and the A-arms end up in a negative camber position. Here is a literary bonus question: Why are the A-arms red?

 

 

And here is the completed roller sans wiring. I won’t be trimming and fully inserting the plumber pivot tubes until after wiring.

 

 

[Alchemist]

It's coming along in a excellent way!

 

Fantastic job Larry!

 

Again, thank you for sharing the project with us.

 

Ernie

[pn6]

That is one beautiful chassis Larry!!!!! Excellent work

[Larry Horner]

Thanks guys but just to reiterate, it was a team effort!

 

We're now on the final chapter and here Mr. Burning Man has one last trick up his sleeve ... well under his armpit actually. Martin and I actually did a couple of iterations of this detail and here is a design sketch where we realized we finally had a keeper.

 

 

And now for a sad tale. While soldering the lead wires onto the motor, I inadvertently dropped the iron which naturally landed on the plastic chassis right above the pinion gear ... ARGG!!!! Luckily with this design, the flat head screws snug everything up firmly but I'm not going to solder the leads onto to the guide flag until I get a replacement chassis. So this is the end of the road for now but I do hope everyone has a lovely Holiday!

 

 

 

[Alchemist]

Hi Larry, Martin,

 

Such an admirable engineering exercise for the both of you to share with us.

 

It is definitely an inspiration, watching how this project of yours is coming together and reading the detailed explanation is such a treat for me - Thank you Larry/Martin!

 

I learn something new everyday that I have the opportunity to stop by Slotblog.

 

Thank you "Slotblog/Cheater" for having the forum available to all of us members!

 

Thanks.

 

Ernie

[Pablo]

Good thing the iron didn't land on your foot !! 

[pn6]

Ok, holidays are over. Back to work. Did you get a replacement yet? Have you run the car yet?

[olescratch]

  I'm thinking that the use of bearings will determine the size of axle that could be used in the rear, correct?  

[Martin]

Paul,

I will be back to working on my 3D StarShip today.  I am making a fixture so I can do all the soldering away from the 3D printed chassis.

Tooling always takes time but if we ever want to make more that one or two this will make the build less of a chore, and of course I like making tooling.

 

John,

3/32" axles was the obvious choice, availability of axles, wheels and gears but most important to get that axle close to the can to keep tire diameter to a minimum.

The printed material we are using could be used as a bearing but it could/would wear out down the road and I happen to have a pot load of 3/32" ball bearings.

 

This is the .040" thick steel base for the soldering fixture.

 

 

Here is the bending jig to bend the main plumber rails, the same every time and in-plane too.

 

 

 

 

[Larry Horner]

Paul, you are quite the taskmaster!  But in answer to your question, my Shapeway's shipment is in route so hopefully I will have the new chassis in hand in the next couple of days. The real problem is that my closest track (since Eddie's closed) is now a couple of hours drive away although I do admit that I've never been to the Modesto track and am looking forward to seeing their hillclimb.

 

John, it's not conclusive but I was able to slide a piece of 1/8" tubing over the axle between the axle and can. But the clearance between the can and tubing ranges from non-existent to SUPER tight so I'm not really sure if this would be viable without grinding the center of the axle down a tad. Also the axle carrier tube is designed with 3/32" bearings in mind so I'm not sure if the recesses for the bearings could be milled out to accommodate 1/8" bearings. Basically all our design parameters were set to use 7/8" tires which mandated the 3/32" axles.

 

Martin, you've been busy over the holidays ... your new jig looks great!

[Martin]

Larry I took a week off and would have thought I could get more done, where does the time go? 

 

John, I just put a .025" feeler gauge between the motor and the 3/32" axle and that means a 1/8" axle would have .009" clearance to the motor.  Happy days.

 

The easiest way, If I was doing it would be to ream out a pair of 3/32" bushings. 

Unless you can find 1/8" i.d. and 3/16" o.d. flanged  ball-bearings. I could not.

[Bill from NH]

Champion & Slick 7 both sold 3/16" o.d. bushings for 1/8" axles. You could contact Roger Schmitt at Mid America to see if they are still available. These were popular to use in Champion & some JK flexi chassis.

[Jay Guard]

Martin:

Check page 10 of the Slick 7 catalog on their website, I believe they have what you are looking for. They are excellent high quality bronze bushings, only "trick" is that you have to keep them oiled with a good quality synthetic oil as they are not Oilites.  They also have a ball bearing adapter that may work for your application.