REF-1516 · Andre et al., 2013 · Journal of Strength and Conditioning Research · Paper · PubMed

Why does my gym's sled feel lighter than the race one?

One sled, one set-up, winch pull: friction 0.31-0.35 moving, falling as load rose. Friction changes with load; it compares no venues or surfaces and no people pushing.

Main caveat: Abstract only. The journal is closed and no open copy was found by identifier.
limited, mixed evidenceMeasured on a mixed group

Andre and colleagues wanted a reliable way to find the static and dynamic friction coefficients of a weighted sled, so that coaches and researchers can estimate horizontal force and work. A nylon tether joined a sled-mounted force transducer to a winch, which pulled at constant velocity for 20 seconds.

Each of three loads, 44.8 kg (the unloaded sled), 90.0 kg and 136.2 kg, was pulled 10 times. Both coefficients fell as load rose (p < 0.01), and each was very repeatable at a given load. Combined across loads, static was 0.43 ± 0.04 and dynamic 0.33 ± 0.02, with higher CVs of 8.3 and 5.6%, because the coefficient changes with load.

The authors suggest others use the method for their own equipment and surfaces. Our reading: a small, useful fact about one sled. Its main use for HYROX is a caution: the feel of a sled depends on more than the plates, and this study gives no number for a race floor or for any gym.

Protocol

Design: repeated mechanical trials; n = 30 winch trials (10 for each load); no human performers.
Set-up: a nylon tether attached to a sled-mounted force transducer; a winch pulled the tethered sled at constant velocity for 20 seconds.
Loads: 44.8 kg (unloaded sled), 90.0 kg (additional load 45.2 kg), 136.2 kg (additional load 91.4 kg).
Not in the abstract: sled model, surface type, velocity, days of testing, and force values in newtons.
Funding and interests: not given in the abstract.

Key Results

LoadStatic friction (mean ± SD; CV)Dynamic friction (mean ± SD; CV)
44.8 kg0.47 ± 0.01; 2.2%0.35 ± 0.01; 1.6%
90.0 kg0.42 ± 0.01; 3.0%0.33 ± 0.01; 3.7%
136.2 kg0.39 ± 0.01; 2.7%0.31 ± 0.00; 1.0%
All loads combined0.43 ± 0.04; 8.3%0.33 ± 0.02; 5.6%

Differences across loads: p < 0.01, as printed.

Mechanisms

Measured: the coefficient of friction at each load, on this set-up; static was higher than dynamic at every load.
Authors' interpretation: the method is reliable, and users can determine the coefficients for their own equipment and surfaces.
Inference: the coefficient falling with load means force rises less than in proportion. Using the dynamic values, the force to keep this sled moving would be about 154 N at 44.8 kg and about 414 N at 136.2 kg (our arithmetic, F = μ × m × 9.81): three times the load, about 2.7 times the force. Holds for this set-up only.
Inference: static friction above dynamic means that starting this sled takes more force than keeping it moving. That is physics on this set-up; no person was measured.

Practical Application

Why does my gym's sled feel lighter than the race one? One sled, one set-up, winch pull: friction 0.31-0.35 moving, falling as load rose. Friction changes with load; it compares no venues or surfaces and no people pushing.
To check your own sled: the authors say coaches can determine the coefficients for their own equipment and surfaces with this method, a force transducer and a winch. A cheaper gauge was not tested.
Do not use it for: a correction factor between a gym and a race; a claim that race carpet is harder or easier; or an overload rule for sled training. None was measured. Not from this paper; the base's own practice: practise the station weekly for technique (SYNTH-71).

Limitations

Abstract only. The journal is closed and no open copy was found by identifier; surface, sled and velocity are unknown to us, and nothing here should be quoted beyond the abstract.
One sled, one set-up, three loads. No generalisation across floors, carpet, moisture or wear can come from this.
A winch, not a person. Constant velocity, steady line of pull, 20 seconds; humans push and pull unevenly and start from rest, and force through the body is not measured.
Repeatable is not valid across places. The low CV is trial-to-trial on one set-up, and it does not show the coefficient holds elsewhere or over time.
Pull, not push. A pushed sled could load differently, and the abstract does not address it.
Conflicts and funding: not available from the abstract.
How this entry was checked and changed
Why this level

a test-method study is B; held down because only the abstract was read and its surface and sled are not described, the result comes from one set-up, and the outcome is a friction coefficient, not anything an athlete did