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My New Custom Printed Chassis

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  • My New Custom Printed Chassis

    NOTE: If you are not logged in the photos in this post may not appear. I'm not sure why that is. Don't know how to fix it. Sorry.

    *****************

    Heck, I'm just not the guy to keep his secret sauce under wraps. I have spent years -- yes years! -- trying to develop my own custom slotcar chassis to the point where they can be competitive in the IHSR racing series. But I think I have finally achieved that and I am here to brag all about it.

    I first started building my custom chassis from brass, about 3 years ago. They were too heavy and underpowered. I was new to 1/32nd. I was on a learning curve. Almost all my experience had been in HO, where cars have huge motors and are typically way overpowered. I had created some highly competitive HO gravity cars with brass chassis. But what worked in HO wasn't working in 1/32nd.

    Almost exactly two years ago I bought a Creality Ender 3 Pro filament printing machine. That started me on printing 1/32nd chassis components in plastic instead of brass. First in PLA (PolyLactic Acid), but soon I graduated to PETG (PolyEthylene Terephthalate - Glycol modified), which is much tougher while still being easy to print. I have also used TPU ( ThermoPlastic Urethane) for parts that need some flex, particularly front wheel/tires.

    I was determined to make my car as light as possible to make the best use of the limited power of the motor. My designs were using the body as a structural component -- what is known as a monocoque design. And I went to considerable trouble to make the whole chassis adjustable in length . So the chassis was actually divided in two -- a front 'sub-chassis' that included the guide and the front wheels attached to a rigid frame that could pivot in the 'roll' direction, and a rear sub-chassis that was essentially a motor-pod, with the motor, gearing, rear axle and rear wheels all in one rigid assembly that was bolted directly to the body.

    It was soon apparent that, while I had no issues keeping the guide in the slot, weight was needed to improve the handling of the car. I added the weight as a third independent element, designed to float so it could provide shock absorption.

    Well, all this experimentation ended up getting me nowhere. Too often my car was finishing dead last, or at best mid-field. A few months ago I decided it was time for a major rethink.

    I decided to scrap the monocoque design as well as much of the adjustability of the chassis. Both features made the whole car too fussy to work on. What weight savings the monocoque design allowed were low on the car, where weight was needed. So I ended up with a more conventional frame. Still an assembly, but of a fixed length. I'll explain all its features in a bit, but here is a look at the main structure. All printed in one piece, with flexibility designed in at three critical locations -- the rear cage that captures the motor, the front which supports the guide and front wheels, and is designed to pivot in the 'roll' axis, and the side 'pans' that support the brass weights that lower the center of gravity and provide shock absorption.

    Also note that the mounting posts are inboard rather than at the very front and back of the chassis. They are a faint echo of my monocoque experiments. The front mounting post receives an undersize screw which allows a small amount of roll, but otherwise limits the motion of the front of the chassis.



    So much for history. Now a bit about the rules that impacted the design of my new chassis.

    There are two classes in IHSR competition that allow custom chassis to race: the Division 1 Vintage Sports Cars (1962 and earlier) and the Division 3 Classic Sports Cars (1963-1974). My latest development is designed to compete in the Vintage class. The body of my car is a Monogram model of the Cooper Monaco sports car.



    The Vintage class restricts the overall width of the car to a maximum of 2.25 inches (57.2mm). It also restricts tires to rubber or urethane and a maximum width of 0.315 inches (8mm). The motor is specified to be an H & K Racing "Jack Rabbit" motor -- 14.7K no load RPM at 12 volts.

    The width limit makes it difficult to fit that motor in as a sidewinder, but I was determined to do it. I was able to manage that by fitting hubless rear wheels inside out, so that the hub of the spur gear fits inside the adjacent rear wheel.



    What I call the 'Axle Carrier' is a separate piece from the custom-printed frame. It captures the ball bearings for the rear axle and is attached to the frame by 4 each 0-80 UNC stainless steel flat-head screws. This is where some very necessary adjustability comes in. By making the axle carrier a separate bolt-on piece I am able to fit shims between it and the rest of the frame. By using different thicknesses of shim I can fit different sets of gears, for different gear ratios. I can custom-print the shims I need to thousandth-on-an-inch accuracy so I can fine-tune the mesh of the gears pretty much as finicky as I want. As it is there is just the smallest bit of backlash between the gears, about as perfect a mesh as I could want.

    The front of the chassis is actually an assembly of three structural pieces. A frame that supports the front wheels and a bracket that supports the guide are bolted to the front of the main chassis with a pair of 2-56 UNC stainless steel flat-head screws and nuts. That whole assembly is rigid in itself, but is designed to pivot a few degrees in the 'roll' direction, so all four wheels can stay in contact with the track despite any twist in the track due to banking. And, more importantly, the guide can stay square versus the slot walls despite any track twist.



    Here is a look at the full chassis assembly. I'm going to post again on this topic, but I'm going to give myself a break right now. 'Sides which, there is a limit to how many photos I can include in any one post.

    Attached Files
    HO RacePro
    HRW Forum Veteran
    Last edited by HO RacePro; January 10, 2022, 06:48 AM.
    Ed Bianchi
    York Pennsylvania USA

  • #2
    I’m assuming it looks fantastic, but I think you forgot to attach the pics.

    Comment


    • #3
      Nothing here, either.

      Comment


      • #4
        Strange. The photos show up on my desktop iMac but not on my laptop MacBook Pro. I'll try reloading everything.

        This is not the first time this has happened to me. Dunno what's causing it.
        Ed Bianchi
        York Pennsylvania USA

        Comment


        • #5
          I get them on my Dell G7. Can I buy one of them from you ? I have that body but no chassis for it .
          Austin
          Merrimack, NH

          Comment


          • #6
            I think I've fixed the photo problem. Is anybody still not seeing them?

            Austin, I could possibly sell you a chassis. Do you want a complete rolling chassis with motor and body mount? I'd need to price that out for you.

            Ed Bianchi
            Ed Bianchi
            York Pennsylvania USA

            Comment


            • #7
              I see it now and it’s pretty neat. I like the bolt on rear member holding in the bearings. With the bolt on front axel carrier could you print one in different lengths for a longer chassis?

              Comment


              • HO RacePro
                HO RacePro
                HRW Forum Veteran
                HO RacePro commented
                Editing a comment
                Yes. The length of the chassis is not adjustable in hardware, but easily adjusted in the computer file that runs the printer. Giving up the hardware adjustment makes the whole design simpler and more robust.

                Ed Bianchi

              • Michael Squier
                Michael Squier
                HRW Forum Veteran
                Michael Squier commented
                Editing a comment
                It looks pretty good, how does it handle?

            • #8
              The handling is wonderful. More on that in a bit.
              Ed Bianchi
              York Pennsylvania USA

              Comment


              • #9
                What software did you use to create the file? FreeCad?

                I'm curious about the "roll" that you get; is that center piece of the chassis relatively flexible? I'm assuming the small rod through the center that extends to the brass "pan" weights is there to limit the roll?

                A fascinating chassis.
                Stan S.
                Newberg, OR
                Autodromo Rossa Colline (Red Hills Raceway)
                Member NASTE (Northwest Association of Slot Track Enthusiasts)

                Comment


                • HO RacePro
                  HO RacePro
                  HRW Forum Veteran
                  HO RacePro commented
                  Editing a comment
                  Yes, the center of the chassis is designed to flex, and the piano wire both helps limit the roll of the front and supports the pan weights. The CAD file was created in VectorWorks 2022. But any software that can create an STL file could have been used. The gCode file that runs the printer was created in the latest version of Cura, 4.12.1.
                  Ed Bianchi

              • #10
                So the motor "pod", "pans," central spine and base for the guide and front axle are all one piece, yes? Front and rear axle holders are separate, as is the center mounting plate that takes the body attachment bolts. The other pin and spacer nuts just behind the front axle are meant to control side-to-side movement of the "pans", right? There is that much flex in the central spine? Some ingenious planning in this one...
                Stan S.
                Newberg, OR
                Autodromo Rossa Colline (Red Hills Raceway)
                Member NASTE (Northwest Association of Slot Track Enthusiasts)

                Comment


                • HO RacePro
                  HO RacePro
                  HRW Forum Veteran
                  HO RacePro commented
                  Editing a comment
                  Yes, Stumbley, you've got it right. And the flex in the central spine is limited by the front body-mounting screw, which is not quite threaded down tight. The front assembly only needs to pivot a few degrees. That is more than enough to compensate for any twist in the track.
                  Ed Bianchi

              • #11
                " Every so often God lets you win one, so the government has something to tax." Ed Bianchi

                In my 40+ years of engineering I learned that dumb luck always plays a crucial role in any successful project. Yes, dumb luck is the savior of us all. But a ton of hard work usually is needed before dumb luck happens, and still more is needed to take advantage of it.

                So I'll acknowledge that the wonderful handling of this car happened as much by chance as by engineering. A lot of features were designed in to help make good handling happen. But wonderful handling the first time out on the track was a fluke.

                And that wonderful handling allowed me to run up some very good numbers on that first outing. I started running, and saw that I was turning really good laps. Once I reached 50 laps I decided to stop and see how I did.

                Now I should mention that during those 50 laps I got a bit sideways, once, but had no offs at all. And this with no great effort on my part. The car just wanted to run.

                So after 50 laps my median lap time was 4.45 seconds. And my benchmark was the winning average lap time by 'Rick' from my first Vintage IHSR race I hosted on my new track -- 4.50 seconds.



                Now I am not so stupid as to claim my car could have beaten Rick's in that event. Too many factors and variables to make that claim. But what I do see is lap times and handling that look competitive. And yes, maybe, potentially, a winner. Stranger things have happened.

                A couple of factors worth noting. The rear tires are Slot.It N22's, which were recommended to me by Sherm, the acknowledged IHSR guru of car setup. Said tires were treated overnight with NSR Tire Oil. All four tires were trued on my Professor Motor truing machine to within a dial-indicated thousandth of an inch. The guide is a Slot.It SICH07, one of the smallest deep-slot guides available, placed WAY up front, again per Sherm's recommendation. And the brass weights suspended over the chassis pans on that blue painter's tape were spec'd out by eyeball and a finger in the wind. Therein lay much of the aforementioned luck.

                Other numbers of interest. The rolling chassis, sans body, weighs in at almost precisely 60 grams -- 21.4 grams of that is motor. The bare main chassis weighs in at -- get this -- 3.32 grams!

                And -- lookee here! -- the center of gravity of the rolling chassis is low enough that the car can be tilted this far without rolling over. Point of fact it could probably be tilted farther except at this angle the rear tires start to slip.

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                So, a good design informed by a lot of experimental messing around and useful lessons learned from abject failures, plus a generous serving of slop luck, all maybe resulting in a decent, competitive car.

                'Course that just ain't worth diddly until I get real race results.

                To be continued...
                Attached Files
                HO RacePro
                HRW Forum Veteran
                Last edited by HO RacePro; January 9, 2022, 03:46 PM.
                Ed Bianchi
                York Pennsylvania USA

                Comment


                • #12
                  Okay, one more question: how do you work the front stub axles? Do the wheels rotate independently of the axle?
                  Stan S.
                  Newberg, OR
                  Autodromo Rossa Colline (Red Hills Raceway)
                  Member NASTE (Northwest Association of Slot Track Enthusiasts)

                  Comment


                  • HO RacePro
                    HO RacePro
                    HRW Forum Veteran
                    HO RacePro commented
                    Editing a comment
                    Yes, the front wheels are independent. The stub axles are 3/32 inch diameter flat-head aluminum rivets, pressed into the front axle carrier and superglued in place. The front wheels are printed from NovaMaker TPU, with a short K&S 7/32 inch OD brass tube bushing press-fit and superglued inside. Those printed wheels have a conical taper so only the outside edge of the tire contacts the track.
                    Ed Bianchi

                • #13
                  I see little pics, let's see if we can do them justice...

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                  Kevan - Isle of Man
                  Print It, Build It, Race It, Improve It, Repeat...

                  Comment


                  • #14
                    Well there's enough to look at and ponder in those pics for quite some time.

                    I like the floating pan weights kept in place by the piano wire over the top.
                    I like the TPU front tyres, I must get some, I'm sure my Prusa can handle it.
                    I do like the sills on my bodyshells imparting load onto the pans, the body floating left and right is useful weight to utilise
                    That gearing is interesting...
                    There's a few bits could be chopped off to lose more weight, screw, motor shaft, front axles...and does that motor really need such thick wire?

                    I'd love to see a video of that going round the track.
                    Kevan - Isle of Man
                    Print It, Build It, Race It, Improve It, Repeat...

                    Comment


                    • HO RacePro
                      HO RacePro
                      HRW Forum Veteran
                      HO RacePro commented
                      Editing a comment
                      Kevan, that thick wire is also used in place of pickup braids. I stripped the ends of the wires, fed them down through the guide and spread them out beneath it, then inserted small set screws down through the front of the guide to hold the wires in place. So why? I got tired of fussing with separate braids and trying to keep them connected. Using 'Pickup Wire' is something I borrowed from my HO Slide Guides. Works just fine. It does mean I'll periodically need to replace the wires, but I'm good with that. What I should do is tie down the wires. I do that on my HO Rattler cars, and that lowers the cars center of gravity enough to make a difference.

                      As for a video, I can do that. Stand by.

                      Ed Bianchi

                  • #15
                    I like it. It looks pretty functional. Cool concept. However, the little plastic tab over the end of the motor requires some explanation. Is it a spreader? I am curious about the type of track you are running on. I have tried a number of aftermarket and custom chassis that don't work on my track with silicon tires. I wouldn't mind trying one that actually worked on a very demanding track.

                    Comment

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