A Trace Spring is a coiled or laminated spring fitted between the horse's trace and the vehicle it pulls, absorbing the shock of each step so the load on the harness stays steady. The English carriage builders Holmes & Co. of Derby formalised the design in patents through the 1860s. The spring stretches under the horse's pulling effort, then releases that energy smoothly as the load eases — preventing jerking on the collar. The outcome is a calmer horse, a longer-lasting harness, and a smoother ride for the passenger.
Trace Spring Interactive Calculator
Vary the pulsing horse stride force and working travel band to see the smoothed pull and required normalized spring-rate range.
Equation Used
The worked example is represented as a normalized force waveform: 100 percent mean pull, 150 percent stride peak, 50 percent trough, and 25-40 mm working travel. The calculator uses Hooke law to estimate the spring-rate band needed for the spring to absorb the half-amplitude of the pulsing force over that travel.
- Forces are normalized to the worked example mean pull of 100 percent.
- The trace spring is treated as a linear Hooke-law spring.
- The spring absorbs the half-amplitude of the stride force over the selected travel band.
- Damping losses, friction, and bottoming impacts are not included.
Operating Principle of the Application of Trace Springs
A horse does not pull a carriage with constant force. Each stride spikes the draught load by 30 to 60 percent over the mean, then drops it almost to zero between steps. Without a damper between the trace and the vehicle, every one of those spikes hammers the collar into the horse's shoulders and snaps the carriage forward in a series of small jerks. The Trace Spring sits in line with the trace — the leather or chain strap running from the harness to the swingletree — and stores energy on the rising edge of each stride, then returns it on the falling edge. The result is a near-constant pull on the horse and a near-constant push on the vehicle.
Most trace springs are either a wound helical type or a stack of laminated leaf elements, sized so the spring's natural extension under mean draught load sits at roughly the middle of its travel. If you preload the spring too stiff, the horse feels every stride and you have gained nothing. Too soft, and the spring bottoms out on heavy pulls — the moment a wheel hits a rut, the coils close and you transmit the impact directly through to the collar. The sweet spot is a spring rate that gives 25 to 40 mm of working travel under the horse's nominal draught load, with another 20 mm of headroom before bottoming.
Failure modes are usually obvious. A cracked leaf in a laminated stack will let the trace lengthen suddenly under load — you would notice the horse lurch forward, then catch. Coil springs fail at the end loops where stress concentrates, and a corroded loop will snap on a cold morning under no warning. The bore of the swingletree hook the spring attaches to must match the spring's eye within 0.5 mm of clearance, otherwise the eye works against the hook surface and wears flat.
Key Components
- Spring body: The energy-storage element itself, either a wound helical coil or a stack of tapered steel leaves. Spring rate typically sits between 8 and 25 N/mm depending on the size of the horse and weight of the vehicle. Material is usually 1095 or EN45 spring steel, heat treated to 45-50 HRC.
- End eyes or loops: The attachment points at each end of the spring. These take the full draught load in tension and are the most common failure point — stress concentration at the bend root drives crack initiation. Eye inside diameter must match the swingletree hook within 0.5 mm.
- Trace strap: The leather or chain section connecting the spring to the harness collar or hames. Length is adjustable so the spring sits at its design preload when the horse stands square in the shafts.
- Swingletree (whippletree): The pivoting wooden bar at the front of the carriage that the spring's rear eye attaches to. It distributes pull between two horses and lets each horse stride independently. Bore for the spring hook is typically 12-16 mm depending on vehicle class.
- Retaining clip or split pin: Holds the spring eye captive on the swingletree hook. A loose clip is the single most common cause of a trace separating mid-drive — check for 1-2 mm of axial play, no more.
Where the Application of Trace Springs Is Used
Trace springs show up wherever a living animal pulls a wheeled or sliding load, and the engineering principle has carried over into a few mechanical analogues where pulsing tractive effort needs damping. The mechanism is still in active use today on working draught vehicles and competition driving rigs.
- Carriage restoration: Original Holmes & Co. spring-trace gigs and dog-carts restored for British Driving Society events still run their period trace springs, often re-tempered rather than replaced.
- Competition driving: FEI combined driving teams use modern sealed coil trace springs on marathon carriages from makers like Bennington and Kühnle to reduce horse fatigue over 18 km cross-country phases.
- Working agriculture: Amish freight wagons and Pennsylvania Dutch farm carts continue to use laminated leaf trace springs for field haulage, where the smoothing effect on the horse's shoulders extends working hours.
- Brewery dray service: Samuel Smith's Old Brewery in Tadcaster runs Shire-horse drays on traces fitted with helical draught springs to handle the 2-tonne barrel loads on cobbled streets.
- Logging: Belgian and Ardennes draught teams skidding timber out of soft ground use heavy-rate trace springs to absorb the snatch load when a log catches on a stump or root.
- Ceremonial transport: The Royal Mews in London maintains trace springs on state landaus and broughams used for royal carriage processions, where smoothness under public scrutiny matters more than absolute speed.
The Formula Behind the Application of Trace Springs
The job is to pick a spring rate that lets the horse pull through a working travel band — not bottom out, not float free. The spring rate k determines how far the spring extends under a given draught load. At the low end of typical operating loads, the spring barely moves and you get almost no benefit. At the high end, you risk coil-bind or leaf contact. The sweet spot is a working extension of roughly 30 mm at mean draught load, with reserve travel left for stride peaks.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| x | Spring extension under load | mm | in |
| Fdraught | Mean draught force from the horse | N | lbf |
| k | Spring rate | N/mm | lbf/in |
Worked Example: Application of Trace Springs in a restored Edwardian governess cart
You are fitting trace springs to a restored 1908 governess cart with a single 550 kg cob in the shafts, total vehicle-plus-passenger weight of 320 kg, on level macadam. Manufacturer's drawings call for a coil trace spring rated at 12 N/mm. You need to know whether the spring will work in its sweet spot or bottom out under stride peaks.
Given
- k = 12 N/mm
- Fdraught,mean = 380 N
- Stride peak factor = 1.5 —
- Stride trough factor = 0.4 —
Solution
Step 1 — at the nominal mean draught load, calculate spring extension:
That puts the spring sitting almost exactly in the middle of its working travel, which is what you want. The horse pulls steady, the spring breathes 31.7 mm at the mean of every stride.
Step 2 — at the low end of stride force, the trough between steps:
The spring is still under tension at trough — this is critical. If xlow went to zero the spring would go slack between steps and clatter against its end stops. 12.7 mm of residual extension means the trace stays taut through the full stride cycle.
Step 3 — at the high end of stride force, the peak:
47.5 mm under peak stride load. If the spring's free travel before coil-bind is 60 mm, you have 12.5 mm of reserve — enough to absorb a wheel dropping into a pothole without slamming the coils solid. If reserve travel were under 5 mm you would feel every road imperfection straight through to the horse's collar.
Result
Nominal extension is 31. 7 mm at mean draught, sitting right in the spring's working band. The horse pulls into a compliant cushion rather than a dead strap — the rider in the cart feels a smooth glide instead of a series of small jerks, and the cob's shoulders show no rub marks after a 90-minute drive. Across the operating range the spring breathes from 12.7 mm at stride trough to 47.5 mm at peak, which is the sweet spot — enough preload to stay taut, enough reserve to absorb a pothole. If your measured extension at mean draught is significantly less than 31.7 mm — say 20 mm — the most likely causes are: (1) a stiffer spring than spec because the supplier substituted a heavier wire diameter, (2) the trace strap adjusted too short, raising effective preload and stealing working travel, or (3) corroded coils binding partway through travel, common on springs left out over winter without oiling.
Application of Trace Springs vs Alternatives
Trace springs compete with two other approaches to smoothing draught load: a fully rigid trace with no compliance at all, and a modern elastomeric shock absorber that uses a polyurethane block instead of steel. Each has a place depending on speed, load, and how authentic the build needs to be.
| Property | Trace Spring (steel) | Rigid trace | Elastomeric shock block |
|---|---|---|---|
| Working travel | 25-50 mm | 0 mm | 10-20 mm |
| Spring rate consistency over temperature | Excellent, ±2% from -20°C to 40°C | N/A | Poor, stiffens 30-50% below 0°C |
| Service life under daily working load | 15-25 years | Indefinite | 3-5 years before polyurethane creep |
| Cost (single trace, 2024 UK) | £120-£280 | £15-£40 | £60-£140 |
| Period authenticity for pre-1940 restoration | Correct | Incorrect for sprung-trace vehicles | Anachronistic |
| Failure mode | End-eye fatigue crack, gradual | No failure, transmits all shock | Sudden polyurethane tear under cold snatch load |
| Maximum draught load | Up to 4000 N for heavy dray springs | Limited only by trace material | Typically capped at 1500 N |
Frequently Asked Questions About Application of Trace Springs
Trace springs only smooth the load if they sit in their working band. If you set the trace strap length wrong, the spring runs either slack or near-coil-bind, and the horse takes the full stride pulse. Measure the spring's extension while the horse stands square in the shafts pulling against light resistance — you should see roughly 30 mm of stretch. If you see 5 mm or 50 mm, adjust the trace buckle.
The other common cause is asymmetry between left and right traces. A 10 mm difference in working extension between the two springs makes one shoulder do more work than the other, and that shoulder gets the sore.
Leaf-type trace springs handle higher peak loads and tolerate side-loading better — if your dray operates on cobbles or rough ground where the trace gets twisted, leaf wins. They also fail gracefully: a cracked leaf reduces capacity but does not let go.
Coil springs give a more linear rate and a longer working travel for a given installed length. For smooth roads and lighter vehicles up to about 1500 kg gross, a coil is the better choice. Above that, or for any vehicle that regularly carries shock loads (timber skidding, brewery work), specify laminated leaves.
Probably not the spring itself. The most common cause of a 20-25% shortfall in measured extension is friction in the trace path — leather traces that have stiffened from neglect, or a swingletree pivot that has seized. The friction load eats into the force reaching the spring.
Diagnostic check: disconnect the spring and pull the trace by hand at the swingletree end. It should rotate freely with under 5 N of input. If you have to lean on it, service the swingletree pivot before blaming the spring rate.
The spring is going slack at the stride trough — extension is dropping to zero and the coils are unloading against their end fittings. Two causes: either the spring rate is too high for the horse's mean draught (the spring barely stretches at mean load, so any load reduction takes it slack), or the trace is adjusted too long, removing preload.
Shorten the trace by 10-15 mm and re-test. If that does not silence it, the spring is over-rated for the application — drop one rate class.
Mechanically yes, but it will affect the carriage's behaviour and any concours judging. Modern sealed coils run lighter for the same rate, which raises the natural frequency of the trace assembly — at trot, you may see a sympathetic bounce that the leaf design damped through inter-leaf friction.
If period correctness matters, have the original leaves re-tempered. A competent spring smith can restore 1880s leaves to within 5% of original rate. If the carriage is for working use only, a modern coil at the equivalent rate works fine and runs cheaper.
Mean draught on level ground sits at roughly 8-12% of the horse's body weight for a well-balanced vehicle. A 600 kg horse pulling a properly maintained carriage runs around 470-700 N of mean draught. Pick a spring rate that gives 25-35 mm of extension at that force, so for 600 N target rate is 600/30 = 20 N/mm.
Add 50% to your draught estimate if the vehicle works hills, soft ground, or carries heavy variable loads like a brewery dray. Under-sizing the spring and bottoming it out is far more damaging to the horse than over-sizing slightly.
References & Further Reading
- Wikipedia contributors. Horse harness. Wikipedia
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