An enclosed glider trailer is the most expensive thing most pilots tow and the least often serviced. It sits at the field for months, gets loaded with a sailplane worth more than the tow vehicle, then covers two or three thousand miles in a summer. Most were built in Germany, and most still in service are 20 to 40 years old. That is why Cobra and Komet bearing questions keep coming back on the soaring boards.
Why an old European trailer is a parts problem
A Cobra or Komet was built around European running gear: usually an AL-KO or Knott axle with a mechanical overrun brake, a surge coupler with a handbrake lever, metric hubs, 13 inch European wheels, and the large European trailer connector.
Almost none of that crosses over to a US parts counter, where the stock items are inch-sized tapered roller bearings, electric drum brakes and the round 7-pin connector. Owners on rec.aviation.soaring report that many glider trailers still carry the original European plug and get rewired to a US plug, and that repair shops often fit new US trailer components rather than chase original parts. Find out what is actually on your axle before you order anything.
Step one: identify the hub, not the trailer
Model and year will not get you the right parts. Pull one hub and look at it.
If it is a serviceable tapered bearing hub, you will find a castle nut and a cotter pin, an outer and an inner tapered roller bearing, separate hardened cups pressed into the hub, and a lip seal at the back. That hub can be cleaned, repacked and reassembled with new seals.
If it is a sealed cartridge hub, there is no castle nut to back off and no separate cups. Owners on RAS_Prime put it bluntly: new sealed cartridge bearings cannot be serviced, and replacement usually requires a shop press. That is a shop job, and the part is metric.
While the hub is off, write down all of this. It is the list we need to match a kit, and the list any shop will ask for.
- Drum inside diameter across the friction surface. US drums are usually 7, 10 or 12 inches.
- Number of lugs and the bolt circle diameter.
- The numbers stamped on the old bearing cones and their cups.
- Spindle diameter at the inner and outer bearing seat, measured with a caliper, hub off.
- Seal outside diameter and the seal bore in the hub.
- Whether the brake is a mechanical shoe pulled by a cable from the coupler, or an electric shoe with a magnet and a pair of wires.
Photographs of the spindle, the backing plate and the coupler help more than a model number.
If the hub is serviceable: repack it properly
The most useful thing in the RAS_Prime bearing thread was not a part number but the failure pattern. One reply said the most common fault is lack of lubrication, or hubs and bearings with a mix of different greases. So clean the hub out completely, and never top up over old grease or mix two greases. Pack each bearing by hand until grease comes through the wide side, seat a new seal, set the preload with the castle nut, and fit a fresh cotter pin every time. Our full repack walkthrough covers the sequence in detail.
Age matters on its own. The thread behind this article was a 26 year old Cobra with about 35,000 road miles facing a 3,000 mile trip. The bearings felt smooth. The grease was the worry, and rightly.
If the numbers on your old bearings match a standard US set, we stock Timken kits in two sizes: small bearings for 10 inch drums and big bearings for 12 inch drums. Our kit-by-axle-rating guide explains how the ratings line up. Match the stamped numbers first. If they do not match, the hub is metric and we are not your source for it.
Be clear on this point: we do not stock metric European bearings, AL-KO or Knott surge brake parts, complete axles of any kind, or 13 inch trailer wheels and tires. If your hub is metric and serviceable, a European trailer specialist or the original builder is the right call. Owners report that Anschau, the Komet builder, still supplies parts.
If a US conversion is the answer
Owners reach this point for one of three reasons: a hub that will no longer hold a grease seal, cartridge bearings with no practical service path, or a wish to run electric brakes with a modern proportional controller.
That is exactly the 2007 thread on rec.aviation.soaring: an early 1980s Cobra with a hub chipped badly enough that it would not hold the grease seal. Replies pointed the owner at a Dexter 2,200 lb electric-brake axle at around $480 with US wheels at about $30 each. Those are 2007 prices and mean nothing today, but the shape of the job has not changed.
A conversion is not one part. It is all of this:
- A brake-flanged axle rated at or above the trailer's loaded weight, cut to the correct hub face width and spring center. That is a made-to-order item from an axle manufacturer. We do not stock axles, brake-flanged or otherwise.
- Electric brake assemblies bolted to the flanges. Our electric trailer brake pair is the assembly itself, magnet included.
- Drums and hubs to suit, with bearings, seals and caps. Our brake drum hub ships fully greased with two bearings, and wheel caps finish it.
- US wheels and tires in the new bolt pattern. A 13 inch European wheel will not transfer.
- A US 7-pin connector and a rewire, because the brake circuit did not exist before.
- A brake controller in the tow vehicle.
- A breakaway switch and battery, which US law requires on braked trailers in most states. The CURT 12V breakaway kit includes the battery and a test light.
Our electric brake conversion guide lists what a complete kit has to include, and the drum hub replacement guide covers fitting.
Two warnings from the same thread. Fenders rarely line up after a swap, because the new axle sits at a different height and the US wheel has a different offset. Owners recommended a high mount. More important, going electric or hydraulic means losing the surge brake's handbrake feature. That lever holds the trailer parked on a slope. Electric brakes do nothing when the trailer is unplugged, so plan on chocks or a separate mechanical parking brake.
Wiring: name the circuit, not the pin
On a US round 7-pin connector the circuits go by function and wire color. Ground is white. Tail and running lights are brown. Left turn and brake is yellow, right turn and brake is green. Electric brakes are blue. The auxiliary 12 volt feed is black, and reverse is purple.
European wiring uses a different connector and a different color convention, so do not assume the wires inside your trailer mean what a US chart says. Ring out every conductor with a meter before you crimp anything. Our 7-way plug cord with junction box makes that rewire tidy, and the 7-pin connector guide walks through the circuits one at a time.
Tires, briefly
Owners report that the original Komet tire is a P165/80 R13, and they warn against ST-rated trailer tires at the 75 mph speeds glider trailers actually see. Check the load rating against your loaded weight, and check the date code on the sidewall. We do not stock 13 inch trailer tires.
What to send us
Send the drum inside diameter, the lug count and bolt circle, the numbers off the old cones and cups, the spindle measurements, and a photo of the backing plate. We will tell you honestly whether it is a US-standard set we stock or a metric hub we do not. Everything ships from Upland, California, and the counter is open on the way to the field.