Owners on the tractor and farm boards ask it more than any other trailer question: my trailer has no brakes, or has surge brakes, and I want electric brakes. What do I need?
The answer is five things on the trailer and two on the tow vehicle, and the reason people get it wrong is that they buy the brake assemblies first and discover afterwards that the axle has nowhere to bolt them.
First, does the axle have a brake flange?
An electric brake assembly bolts to a flange welded to the axle behind the hub. An axle built without brakes usually has no flange, and an axle built for surge brakes may have a flange of a different pattern.
Pull a wheel and look behind the hub. A flat plate with four or five bolt holes around the spindle is a brake flange. Nothing there means the axle cannot take electric brakes without either welding flanges on, which is a job for a shop that does it regularly, or replacing the axle with a brake-flanged one.
This is the first check, and it decides whether this is a parts job or an axle job. Do it before buying anything.
The five parts on the trailer
1. Brake assemblies
One per wheel, and they are handed: left and right are different. They are sized by drum diameter, ten inch for 3,500 lb axles and twelve inch for 5,200 to 7,000 lb, and by the flange bolt pattern. A matched pair of electric brake assemblies covers one axle.
2. Drums, or hub-and-drum assemblies
An unbraked axle has plain idler hubs with no drum. Converting means replacing each hub with a hub-and-drum assembly that the brake shoes act on. The brake drum hub comes with both bearings fitted and greased, which makes the swap a straight replacement of the idler hub.
The drum's bolt pattern must match your wheels and its bearings must match your spindle, which brings in the next part.
3. Bearings and seals
If you are fitting new hubs, they need bearings matched to the spindle. Read the numbers off the existing bearings, or measure: 3,500 lb spindles take the 10-inch Timken set, 5,200 to 7,000 lb take the 12-inch set. A greased hub assembly may come with bearings already fitted; confirm they suit your spindle rather than assuming. New cotter pins for every hub.
4. Wiring from the plug to the axles
The brake circuit needs a run from the trailer's connector to each brake, with a junction near the axles. Use wire sized for the current, protected where it passes near the suspension, and grounded properly. A poor ground is the most common reason a fresh conversion works intermittently.
5. A breakaway system
Required by law for braked trailers above a weight threshold in most jurisdictions, and sensible regardless. It is a small battery, a switch with a lanyard to the tow vehicle, and wiring to the brakes, so that if the trailer separates the brakes apply. Our guide to testing a breakaway system covers keeping it working once fitted.
The two parts on the tow vehicle
A seven-way connector. Surge and unbraked trailers often run a four-way flat plug carrying lights only. Electric brakes need the seven-way round connector with the blue brake wire and the black auxiliary 12 volt wire. If the truck has a four-way, it needs a seven-way fitted and wired.
A brake controller. The device in the cab that sends a modulated signal down the blue wire in proportion to braking. Without it the brakes do nothing. Our guide to brake controller troubleshooting from the tow-vehicle side covers what the controller has to deliver and how to test it.
Putting it together
- Confirm the flange, or arrange the axle work.
- Fit the brake assemblies to the flanges, left and right on the correct sides.
- Fit the hub-and-drum assemblies with correct bearings and fresh seals, and set preload properly. Our repack guide covers it.
- Run the wiring from the connector to a junction and out to each brake, and ground it cleanly.
- Fit the breakaway system and test it.
- Adjust the brakes to the manufacturer's clearance, then re-adjust after the first few hundred miles as the shoes bed in.
- Set the controller gain on a quiet road: increase until the trailer brakes just begin to lock on a firm stop, then back off slightly.
A conversion that skips step six or seven gives you weak or grabbing brakes, which are covered in our brake troubleshooting guide.
Protecting the circuit
Where you add a circuit for the breakaway battery charge or an accessory feed, a resettable DC breaker is easier to live with than a fuse you have to carry spares for, and it is waterproof for a location that sees spray.
What we stock, and what we do not
We carry the brake assemblies, the greased hub-and-drum assemblies and the bearing kits above. We do not currently stock brake-flanged axles, breakaway kits, seven-way connectors or brake controllers, so those come from elsewhere, and this guide says so rather than pretending otherwise.
Send us your axle rating, drum size and flange bolt pattern, or a photo behind the hub, and we will tell you which assemblies and hubs fit before you order. Everything we stock ships from Upland, California.