How it works
Stopping distance has two parts: the distance covered while you react, and the distance the brakes need to bring the rig to a stop.
Reaction distance = speed × reaction time
Braking distance = speed² ÷ (2 × deceleration)
Deceleration with a trailer = road grip × g × (vehicle + e × trailer) ÷ (vehicle + trailer)
Road grip is the typical best-case deceleration on that surface, in g. The factor e is how much of its own weight the trailer brakes handle: 1 for working brakes, about 0.5 for weak ones, 0 with no brakes. Without trailer brakes, the vehicle’s brakes must stop the extra mass on their own, so deceleration drops.
Worked example
60 mph on dry pavement, 1.5 s reaction time, a 6,500 lb truck and a 7,000 lb trailer.
- Reaction: 88 ft/s × 1.5 s = 132 ft
- Braking solo at 0.7 g: 88² ÷ (2 × 22.5) ≈ 172 ft; total ≈ 304 ft
- No trailer brakes: deceleration falls to 0.7 × 6,500 ÷ 13,500 = 0.34 g, braking ≈ 357 ft, total ≈ 489 ft, 61% longer.
Stopping distance by speed
Dry pavement, 1.5 s reaction, 6,000 lb tow vehicle and 6,000 lb trailer.
| Speed | Solo | Trailer brakes working | No trailer brakes |
|---|---|---|---|
| 30 mph | 109 ft | 109 ft | 152 ft |
| 45 mph | 196 ft | 196 ft | 292 ft |
| 55 mph | 265 ft | 265 ft | 410 ft |
| 65 mph | 345 ft | 345 ft | 547 ft |
| 75 mph | 434 ft | 434 ft | 702 ft |
Braking distance grows with the square of speed: going from 55 to 75 mph nearly doubles it.
Brakes, controllers and weight
Electric trailer brakes are only as good as the brake controller setting. Set the gain with the trailer loaded, on a dry road, so the trailer brakes firmly without locking its wheels. Surge brakes on boat trailers work automatically but need regular service. Heavier loads stretch braking distance on every rig, so know what you are towing: try theGCWR calculator and the towing capacity calculator.
Common mistakes
- Following too closely. Leave far more space than you would solo.
- Never adjusting the controller. Gain set for an empty trailer is too weak for a loaded one.
- Riding the brakes downhill. Use a lower gear; overheated brakes fade badly.
- Ignoring tires. Worn or under-inflated tires reduce grip on both vehicle and trailer.