12 replies to this topic

[Larry Horner]

Whenever I visit a track, I like to buy my tires there as a way to give them some additional support. And most tracks generally have a good selection of .790 and .812 tires so this usually works out just fine. Unfortunately, vintage Thingies have very odd tire sizes that aren’t readily available. So for a Dynamic Renegade that I am currently resurrecting, I wanted to try some silicone tires to get around this issue. And luckily H&R makes some lovely 27mm silicone wheels & tires that in theory would be great for a Renegade. So I ordered a pair and while the tires are indeed a good size match, the wheels don’t have threaded hubs and worse yet, the collar where the set screw resides is exactly where the threaded 43T spur gear needs to be. Simultaneously, I was able to procure a 42T threaded aluminum spur gear but all I could find in the needed 43T size was a vintage Cox spur which again had a collar with a set screw which means the wheels would be pushed out way to far to clear the collars for both the wheel and spur gear.

 

Enter plan B. The slots in the H&R wheels are very uniform, clearly CNC machined. So I 3D printed an adapter plate that basically is a plastic disc (1.5mm thick) with 5 small protruding rods that snap into the H&R wheel slots to center it. There are also 1.6mm holes drilled thru the rods and the underlying plate. Lastly the adapter plate has a hole in the middle that is the exact size as the collar on the cox spur gear. Below are some shots of the plate and then the plate slid onto the collar of the Cox spur gear.

 

 

 

 

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[Larry Horner]

Now using a drill press, I used the adapter as a template in drilling 5 matching holes thru the gear itself. After I drilled the first hole, I drop a 1.6mm bolt thru the adapter and gear to assure that it would remain locked in place as I drilled out the remaining 4 holes. I then used a step drill to drill out the gear’s collar entirely. Finally I used a countersink drill bit on the 5 holes so the 1.6mm bolts could snug up recessed into the gear. Below is a shot of the adapter/gear sub-assembly ready for mounting on the hub.

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[Larry Horner]

Now for the final assembly. I mounted the wheel on a 1/8” axel using a bit of Loctite as the set screw won’t be accessible once the gear is mounted. I then snapped the adapter/spur assembly onto the inside of the wheel and used 1.6mm washers and nuts to secure everything to the wheel. And curiously, the Cox spur, adapter plate and mounting hardware are under a gram while my threaded aluminum spur gear is around 4 so this ends up being much lighter. And as an added bonus, I think actually seeing the mounting studs and bolts on the inside of the wheel hub is kind of a cool look but I’ll let you be the judge!

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[Alchemist]

Nicely done and well thought out!

 

I'm looking forward to seeing this mounted on the car.

 

Thank you for sharing Larry!

 

Ernie

[NSwanberg]

I understand why the bolts and nuts are not centered in the wheel slots but keep them!

 

Printed with a what? Looks a lot more accurate then my home printer. I could probably pull it it of but it would take a lot of cleanup.

 

Nice work!

[Martin]

Larry, you know I am on your side and commend the thought you put into this elegant solution.

But wondering why the boss on the adapter is round and not the kidney been shape to match the wheel? Filling the hole in the wheel would look better IMO and prevent shift under braking and acceleration. I get will never happen with the low HP/grip but it just the race engineer in me.  

[Larry Horner]

Thanks guys, this design was actually just a long shot that happened to pay off but sometimes you get lucky.

 

Ernie, I'll post my Regenade project soon ... just a few more details to resolve first!

 

Nelson, I use Shapeways for my 3D printing as I personally don't own a printer plus their industrial printers provide both higher accuracy and material selection than the printers available to most hobbyist (i.e. they are expensive and huge). This adapter plate was printed in what they call fine detail plastic as I wanted high accuracy and strength really isn't much of an issue here (https://www.shapeway...-detail-plastic).

[Larry Horner]

Hi Martin,

 

and an excellent question! And the answer is I'm lazy. No seriously, I wasn't sure this would even work at all so I wanted to attempted it with something easy to model first. Now that I know it actually works (in fact it works much better than I anticipated), I just might take your suggestion and refine the design a bit further.

[Martin]

I would not call this lazy but expedient to see if it all works, and it does.

Question, is the printed material strong enough for the gear it self?

You know where I am going next.

[Dave Crevie]

If you can get wheels with 3/32nds axle holes, they can be threaded for 5-40. The 3/32nds hole is close enough to get good threads. Start the threading from the front side ( the side that faces outward on the car) to avoid the set screw hole causing the tap to run off center. If you have a lathe, chucking the wheel in the three-jaw chuck and using a drill chuck in the tailstock to align the tap is the best way. You might be able to start the tap in a drill press, turning the spindle by hand. I have done this with many Pro-Tracks wheels so I could run my old timers while I was searching down the proper wheels and tires. Remember to remove the set screws first. 

 

If you are dead set on using silicone tires, (or urethanes, for that matter) you can thread Slot-it wheels the same way, and use slip-on tires. Pro-Track makes silicone coated tires in some sizes, but I don't believe they have anything as small as you are looking for.  

[Brian Czeiner]

 

Question, is the printed material strong enough for the gear it self?

You know where I am going next.

 

I am playing with this very concept right now with mixed results. Testing takes time....... 

[Larry Horner]

Martin, the answer is "I think so". The high accuracy plastic that I used is definitely not strong enough and is a bit brittle to boot. But Shapeways also offers a couple of different types of nylon impregnated plastics that I'm pretty sure would be strong enough.

 

Dave, great suggestion but sadly the H&R wheels are not available in 3/32" and Pro-Track doesn't offer Silicone in this size. I could probably get a set of each, tap the Pro-Track wheels and then mount the H&R silicone tires on them.

 

Brian, I feel your pain ... perfecting these models takes a lot of time!

 

However my "Plan B" is basically a stop gap measure to get my car up and running. But I will go ahead and share "Plan A". I want to see if I can recreate the original bespoke Dynamic American Mag wheel used on the Renegade (printed in aluminum). This will have to be a two step process in which I print a close facsimile and then do a bit of machining (lathe/tap) to yield the finished result. If I'm able to pull this off, there are probably a lot of folks out there that would love to have a set of these now unobtainable wheels.

[LindsayB]

Back in the 70's we did stuff like that (in Australia) we just had one pin locked into the the wheel worked well.