Release of 3d printed suspension treads from frc team 3737

Technical CAD
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In conjunction with out just-released 3D printed swerve drive, team 3737 is pleased to release the CAD * for our fully 3D printed, TPU suspension tread wheel. This wheel is fully interchangeable with the ‘standard’ 4” diameter x 1.5” wide wheel.

  • stp and 3mf files.

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Another team (88) has developed a great suspension tread but it is only suitable for printing on an SLS printer. SLS printers are far less common than FDM printers. Our goal was to not only print on an FDM printer (we use PRUSA MK3S or newer printers), but also produce a tread with variable suspension characteristics. The clamshell hub is also 3D printed (in PETG) and forms an integral part of the wheel with six interlocking lobes to fully capture the tread and prevent any slippage or side movement.

Our complete wheel costs approximately $12 ($10 tread, $2 hub) compared with a typical Aluminum wheel costing about $45 each with shipping (saving $132 per robot). The final design (with printed hub) has been tested in several off-season competitions. The tread appears to last up to 2x the conventional Nitrile treads.

The tread pattern is a closed cell, double helix design providing long life and excellent grip, especially on smooth surfaces (the pockets tend to act like suction cups).

Significantly, the infill percentage can be adjusted to suit the robots’ weight so that roughly 1” of flat is contacting the carpet. Compare this with the effective 1.5” line contact of a hard rubber tread. This large increase in contact area provides the following advantages:

  • Better grip – faster acceleration and deceleration.
  • Greater ability to defend (we were able to push a 6 wheel robot – sideways!)
  • Absorption of shocks (eg: landing after air-time) – lower stress on whole robot
  • Smooth running over cable tracks
  • Higher speeds possible over obstacles
  • Self-leveling – chassis are never perfectly flat. Swerve does not track well when only three wheels touch the floor. Suspension treads effectively level the playing field ensuring four-wheel contact at ‘all’ times.
  • Improved negotiation while climbing certain field elements (ramps, steps etc.).

The breakthrough in the tread design came from a technique using merged models in the slicer. This provided a constant thickness infill while properly printing all the internal lobes. We have provided a 3mf file to permit you to make infill percentage adjustments to suit your robot weight. Our 89lb robot required a 40% infill. The print uses a ‘gyroid’ infill pattern which provides a constant resistance in all directions.

We have experimented with various TPU filaments over the years for different types of treads but for this suspension style, we strongly recommend Fiberlogy Fiberflex 30D filament for its combination of flexibility and wear resistance.

We use Narrow path 3D for this material and they provide excellent customer service.

Full disclosure: we have no affiliation with or benefit from Narrow path 3D or Fiberlogy.

After three competitions the tread looked almost like new.

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Comments and questions are welcome.

If you use our design, we’d love to hear and be credited

https://drive.google.com/drive/folders/1CjWdC4LNWXU0OlBI9rZEyhhYrZ0VYS8N?usp=sharing

3D printed Swerve Drive link on CD:

26 Likes
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Have you tested the coefficient of friction? How long does it last?

Also, quick correction, they used a SLS printer, not a resin printer.

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No idea on CoF but real world results are very good in both grip and life.

Thank you for your correction. I wrote from (my clearly failing) memory! I have now updated the text.

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Neat. Interesting to see someone come to the same conclusion I did back in July when I was tooling around with how to print these faster with more adjustability on stiffness. Gyroid was the best cross-section I tested out of several.

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Would definitely be good to see a simple CoF test. A hard plastic wheel would have great lifetime, but not be the best for competition.

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I absolutely agree that a CoF figure would be interesting for comparison. However, varying surface finishes and robot weights have a considerable effect on actual grippyness. Our treads are Shore 30D, roughly equivalent to 80A. This is a fair bit softer than the 95A TPU, common on Amazon for example. Our real world experience is that they last at least three competitions (with no perceptible loss in grip), enabled us to climb the charge station with ease and can push a six-wheel robot sideways. I’d say, for $12 each, give them a try and see how they work for you.

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Another advantage to this tread type is that it does not use button head screws that eventually tear up the carpet when the tread is worn down. (and if you have ever changed such tread, you know how easy it is to ruin a nice Aluminum billet wheel by cross threading the screws!).

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Although we have a set of 3" Colson wheels we are testing on our Rev MAXSwerve, I would really like to try out this suspension wheel design.
Any chance these can be modified for use on these swerve modules?

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While none of these are that design, here are some tpu 3d printable maxswerve wheels.

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I realize this is a basic question but curious how you store and handle your TPU for this? I’ve considered TPU but have not gone for it yet. It’s hygroscopic so I assume a drybox is necessary for the spool?

I was encouraged to see your PETG hubs!

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It may be possible to make a 3" version but our 4" wheel (tread and hub) is fully integrated so it would not be possible to just make a 3" tread. Clearly, to design a full 3" wheel with suspension tread would require some work and testing. I’d be glad to offer some advice on how we merged the models in slicer to achieve our 4" design.

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We have found that keeping the filament in a sealed bag with the desiccant pack during storage has been sufficient. Also, the filament spool is open to the air while printing and has not caused any issues. We do have a dryer but have only used it for Nylon.

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I should also mention that the hub is designed to accept the 45T bevel gear from SDS. The six, #10-32 flathead screws are used to retain the gear while clamping the two halves of the printed hub together.

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Update on tread wear. Having finished a year of on and off season competitions, we have concluded that the treads are capable of two competitions. We also recommend that you keep a spare set on hand should you encounter any significant failure during a match. Cheers.

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1640 has run 3d printed treads for 2 years now. Tried many tread patterns. It’s interesting that our tread patterns are kind of similar. These are recessed diamonds and ours are raised diamonds. We have both found good wear. We had good traction on the polycarbonate ramps. We printed our hubs out of polycarbonate and so far in testing they have held up. We found the pin treads to wear fast and bad on polycarb.

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Gdeaver, thanks for your comments. Our tread pattern was specifically designed to assist in grip on the balance in 2023. But the suspension part, I think, makes the most difference. We aim for about 1" flat on the carpet which is an enormous increase in contact area compared with the effectively straight line contact of Nitrile treads on an Aluminum wheel (and, as mentioned, the self leveling effect ensures all four corners contact the ground - except when our driver did some air time off the balance).

What hardness TPU do you use? We use a 30D which is amazingly grippy, roughly equivalent to 80A.

We actually started with PETG hubs and apart from a small redesign of the lip next to the bevel gear, they have been perfect. Apart from the treads, every printed part on our robot (including our 3D printed swerve modules) are made from PETG. We have avoided any CF bearing material due to serious health concerns (the CF rods are not rejected by the skin and end up in the blood stream and therefore heart, brain, kidneys etc. Also, any CF bearing filament requires approved breathing equipment and gloves when handled). Fortunately, PETG is very compliant and has served us well over the last few years.

I was reading about your CVT swerve. Very nice. I keep wondering how we can improve upon our design; perhaps integrating CVT is our next step! But I hate to make our module larger which I think would be unavoidable.

Thanks again for your feedback.

Cheers,

Derek

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Thanks for sharing this. I think we may end up using these. We don’t love having to mess around with the billet wheels and treads and the Colsons are just too slippery. The test wheel I printed with the filaments I had to hand looks really good. Now my new Fiberlogy Fiberflex 30D and PETG have arrive so I’m about to start trying the design for real.

I just have a few questions, if you have time to answer.

  • What print bed do you use for the Fiberlogy Fiberflex 30D? I had a failure with the textured sheet on a small test print. It worked with the smooth sheet, but was very hard to remove. I don’t really have much experience with more challenging filaments.

  • Looking at your Prusa project file, I think I understand your overlay method. It appears that you are doing this to get solid infill in the lugs that hold the tread to the hub. Is that necessary? What was the failure mode without this?

  • Did you experiment with layer height and/or nozzle diameter? I’m mainly interested in the possibility of reducing print time, without loosing strength.

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Nick, you are welcome. Answers as requested:

  1. We use smooth PEI sheets (PRUSA OEM supplied). A tip for print removal, heat the bed to 80 deg and then peel off around the diameter bit by bit. ie: push the top of the tread inward to break the bond to the heatbed. We occasionally get a trace left over. Before printing, just clean with IPA (90+%), no Windex.
  2. Exactly right. The problem was that when the robot travelled slowly, you could see the chassis bounce in time with the lugs. ie: 6 bounces per wheel revolution. So it’s important to ensure the infill is a regular annulus and not a ‘star’ shape.
  3. I’m a fan of 0.2 layer height and so stuck with that for the tread (and hub) It seems to be a good compromise between the lower heights (extending print time) and thicker layers (possibly less sound structurally). However, you may find that in the case of the treads, a greater layer height is perfectly ok. I’d be glad to get your feedback if you try an alternative.
    We have not tried a larger nozzle, that may also yield a sufficiently strong tread at a lower print time.

Allow me to add a few comments, one that Narrow Path gave me:

  1. If printing on a PRUSA MK3S/+, back out the extruder screw 1.25 turns when printing TPU. For all other filaments, I keep the head of the screw flush with the side of the extruder body. (this technique may translate to other printers but I have no personal experience).
  2. If after a few matches, you get any delamination of the tread (usually the outer layers which get stressed the most during aggressive match play), just remove any loose material with side cutters. You should not notice any performance loss.
  3. For infill percentage, we have used 40% for a 90 lb robot and 50% for a near max weight robot. I suggest you interpolate/extrapolate as necessary but I would expect most teams would run between 40% and 50%.
  4. When splitting the hub (to replace a tread), use three of the six flat head screws as ‘ejector pins’. Turn each screw just a little at a time to keep the hubs parallel as they separate (the screws cut their own thread on the first insertion so the force will be higher first time. Optionally, use a dot of red’n’tacky grease on the tip of the screw first time).

Hope these answers and tips help. As mentioned, I welcome any feedback to reduce printing time or improve performance. Also glad to answer any further questions.

Cheers,

Derek

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Also curious what brand of filament you guys ran; maybe you could provide a link?

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A link to the 30D filament we use is in our original post (in grey box with Narrow Path 3D). We use Overture PETG exclusively for everything else on our robot (Amazon) including the wheel hubs. Oh and we actually use Overture 95A TPU for our handles (they provide a soft cushion in the Lexan handle cutouts).
Cheers,
Derek