Curb rein applies 4 x the rein tension to the mouth piece
A study was conducted to investigate the rein forces applied to double bridles vrs Snaffle - you can find the abstract here. This study is not yet released/published so I am going off abstract data only.(* Edit: it is now published)
The Results
The results found:
1- Over all rein tension was lower for double bridle than Snaffle ( Snaffle: 53.7 ± 17.6 N, Double Bridle: 44.9 ± 18.8 N).
2 - Total force was lower in the curb rein of the double bridle than the Snaffle rein in:
Walk (bridoon rein (snaffle of the double bridle): 20.7 ± 0.9 N, curb rein: 9.6 ± 4.5 N)
Trot (bridoon rein: 26.4 ± 15.2 N, curb rein: 19.1 ± 9.6 N)
Canter (bridoon rein: 39.8 ± 19.5 N, curb rein: 20.5 ± 7.9 N).
In all gaits, total force for the bridoon was lower than the snaffle bit.
Cristina Wilkins Presentation
Cristina explains the mechanical function of the curb bit and how the lever action works (you can find the presentation here). What we took from the presentation is that curb rein force is 4 x the rein tension applied.
So the above results for walk on the curb said the rein tension was 9.6kg, trot was 19.1N and canter 20.5N
Calculating the force applied by the curb in the results
When calculating the force applied to the mouth piece (curb rein tension x 4) it is as follows: Walk 9.6 x 4 = 38.4N Trot 19.1 x 4 = 76.4N Canter 20.5 x 4 = 82N
Now looking back at the Snaffle /bridoon forces, the applied force by the curb is up to double the force applied than the Snaffle.
Cristina also mentions this in her presentation that the reins will be lighter in the hands of the rider, even when higher forces are being applied than the Snaffle rein.
Be aware of misinterpretation
The results from the unpublished study only show what force was applied to the curb rein, this does not equate to force applied by the curb bit to the horses mouth. In the case of the bridoon as far as we know it is a 1:1 force, 2 N on the rein = 2 N on the mouth, as it's not a lever action like the curb.
There has been a misconception that the results demonstrates the light action of the curb and that "horses go better as the contact is so light", what is applied to the curb rein will be that times 4 in the horses mouth.
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