Worth the Extra Effort
One of the most common pieces of hardware on model planes is the lowly wheel collar. Du-Bro has been supplying them to hobbyists for decades and they do their job very well. Certainly some pilots will curse them for failing at times, but in my experience they fail me only after I have failed them first. Deceptively simple in appearance, properly installing a wheel collar takes a bit more time than a quick twist of the hex key.
Simply tightening the set screw against the typical music wire landing gear axle may seem secure, but it is a proven recipe for disappointment. Music wire is quite hard and getting a good “bite” against it means overtightening the set screw. The danger here is that wheel collars are plated brass and stripping their threads is a common result of too much torque.
With just a little extra attention during installation, wheel collars can provide a secure retaining method for the wheels on your model. Once you know where the wheel collar wants to be, moderately tighten the set screw and then loosen and remove the collar. The set screw should leave a light mark on the wire. I sometimes put a dab of ink from a sharpie on the mark so I don’t lose track of it.
Now grab your Dremel® tool with a fiber-reinforced abrasive cut-off wheel and create a small flat on the wire where the set screw scratched the axle. You don’t have to go too deep, but do be careful to make the ground surface as smooth and even as you can to provide a solid bearing point.
Check the flat by tightening the set screw against it, then backing off just enough to be able to slide the collar on the axle a little bit. You want the screw to catch on the sides of the flat to prevent the collar from sliding off the end of the axle. Creating this physical interference will let you lock the collar in place without needing to risk stripping the threads for the set screw. Now remove the collar and reinstall the wheel.
Remove the set screw from the collar and give it a drop or two of blue thread locker so you will be able to remove it at some point down the road. Most hobby shops should stock the Pacer line and their Z-42 thread locker works very well. Reinstall the screw and tighten it securely against the axle flat. Wipe off any excess and let the thread locker dry. The wheel collar should now stay in place until you decide to remove it.
One trend that I have seen increasing over the years is that ARFs sometimes come with conventional machine screws with heads on them for the wheel collars instead of hex-drive set screws. I would rather not see a threaded screw sticking out of a wheel collar and replace them whenever I can.
If you opt to go this route be very careful when checking new set screws. More often than not, the original screws and wheel collars have a metric thread and finding the right replacement in your parts jar could be a challenge. If I can’t find a suitable replacement I simply install them facing down or aft, and pretend that I don’t see them all that often.
While wheel collars are handy and easy to use, they aren’t always appropriate. They can look out of scale on smaller models and add unnecessary weight. In these cases, and when I want a more scale look, I’ll often use a lightweight cotter pin instead. This technique is low profile, looks great and can be used on most models. It also takes advantage of the common tendency for stock wheel hubs to have a larger bore than the axle size.
If your wheel is loose on the axle, find a suitable piece of brass tubing that will sleeve over the axle and act as a shim to fill the gap. Usually just one piece is necessary, but I have seen cases where two pieces of tubing sleeved together are needed.
Clean the tubing and solder a brass washer onto one end. This will keep the wheel from binding on the angled landing gear strut. I find soldering to the tube requires less heat than soldering to the wire strut. While you are waiting for the joint to cool, slide the wheel onto the axle, mark the wire and cut it off flush to the outside edge of the wheel hub, or even just short of that. The Dremel tool works well here and has been the tool of choice for generations of modelers. If you don’t want to shower your workbench with sparks though, there is another option. I often reach for a pair of high quality wire cutters that I got a while back from by Stevens AeroModel. Do not use side cutting pliers or diagonal cutters. The cutting edge on these tools is simply not up to the stress of music wire and you will most likely damage them on your first attempted cut.
Now slide your tubing over the axle and reinstall the wheel. Slide another washer onto the tube against the hub and make a mark at the outer face of the washer. Remove the tube and cross drill it with a 1/32-inch hole then cut the tube off about 1/16 outside of the hole you just drilled. Now reinstall the tube on the axle. While you can solder it to the axle, I usually just use a drop or two of thin Zap super glue to hold the tubing in place. Finally, reinstall the wheel and washer.
While I do use actual cotter pins on larger models, I typically use a short piece of copper phone wire on smaller models instead. This is the same soft copper wire that I use when binding gear legs before soldering. The wire bends easily and the insulation can be color-keyed to your model, as long as black, green, red or yellow suits you. You can also strip off the insulation to reveal the copper. Insert the wire through the tube, bend one end around the axle then bend the other side and trim with a pair of diagonal cutters.
Make Your Own
On occasion I have found wheels that needed a larger shim than was easily accomplished with stock tubing sizes. In these cases it only takes a few moments to turn a custom flanged shim using my Sherline lathe and a length of 6061-T6 aluminum.
Most recently I used this technique to secure a pair of wheels on my Stevens AeroModel FREDe 1.5x. I had chosen a pair of 4-inch foam wheels from my spares box that looked suitably oversized for this cartoony airframe. They originally were for a much larger and heavier model and the hubs were bored for 5mm axles. The FREDe 1.5x comes with 1/8-inch music wire landing gear. Machining my own adapters instead of using tubing meant being able to get a smooth, wobble-free fit.
Making the adapters went quickly on the lathe and was a gratifying project. After creating the center bore and turning the axle diameter and inner flange to size, I parted the piece off from the bar stock and moved it to my drill press. I first made a small dimple with a center drill, then switched to my final 1/32-inch drill size to pop the holes through for the cotter pin. After thoroughly cleaning all mating surfaces, I pressed the new parts onto the wire axles until they started to bind at the legs and secured them with thin Zap. Given the nature of the plane, I couldn’t help but go with a length of the red lead to key the color from the fuselage.
This is a simple and lightweight solution to keeping the wheels on your plane. While it does take a bit longer than installing wheel collars, I feel the extra effort is worth it on many models. The method looks great, especially on vintage flyers, and is quite light. I have used this technique for over 20 years on everything from park flyers to quarter scale without any failures.
If It Is Worth Doing …
Regardless of how you choose to hold your wheels in place, there is no good excuse for having them come off your model unless you say so. Wheel collars and cotter pins are both effective means of accomplishing the task, as long as you take a few moments to do the job right. A little extra effort in the workshop before the first flight inevitably saves much more time and aggravation later. Knowing you gave it your all will also give you one less thing to worry about whenever you take to the air.