Christina Krajewski [email protected] Series of articles published in Reiterjournal (Baden-Württemberg) 2015, Part 3:
Lever Bits – Myths and Facts What exactly is a Lever Bit? The second article of the series centred on the snaffle bit. Thus, what has yet been left unexplained is the performance of the so‐called lever bits – the category, which comprises most other bits, which are allowed under FEI rules. There exist even more variations of lever bits than of snaffles, and the creative names some manufacturers have invented for their specialities have resulted in a hopeless confusion among riders. The aim of this article is a scientifically supported analysis and explanation of the performance of lever bits, and we will start by classifying them according to the underlying physical principles: A closer look reveals that some of the so‐called lever bits do not resemble a lever according to the physical definition at all. The snaffle being the most simple bit, we use it to introduce the forces acting through (and on) the bridle. Fitting the bit to the cheek and neck pieces of the bridle causes a constant force Fc exerted by the ring on the poll of the horse, respectively the counter‐force Fp (equal magnitude but opposite direction) by the poll on the ring. When the rein is taken, the rein tension FR is applied to the ring in direction B (angle of appr. 20° upward backwards). The arrows representing FR and FC originate at the mouthpiece, which exerts the counter‐force. No lever action is possible. This photo shows a Weymouth or Pelham bit. Basically, they are fixed cheek bits with shanks, and comply with the criteria, which define a lever: There is a longer and a shorter lever arm, and a fulcrum (the mouthpiece). The rein force FR is applied to the longer lever arm (the lower shank) and induces a counter‐force on the shorter lever (indicated by the thick blue arrow), which is theoretically equal to FR multiplied by the mechanical advantage MA (the ratio of the lengths of both shanks). The counter‐
1 Lever Bits – Myths and Facts force is divided into component FP, directed to the poll, and a horizontal component, which transmits rein force to the curb chain, thus relieving the corners of the mouth. The Pelham with a bar mouthpiece is a “true” lever bit, too. A different mechanical principle is employed by gag bits: They represent a class 1 pulley redirecting FR to the poll. This force then acts directly on the poll (named Fp now). An amplification of the rein tension FR is not possible. The cheek type on the right is known to provide effective communication and is quite popular in cross‐country and show jumping. There is no lever construction present. The mouthpiece (fulcrum) can slide freely on a segment of a three‐ring cheek, thus acting as a class 1 pulley, transferring the rein force to the poll in addition to the constant force FC . The cheek piece ring is smaller than the rein ring, thus the radius of the circle it describes around the fulcrum is smaller than that of the rein ring. When the stop point is reached, the rein force induces an increase of the torque (turning moment), which can be compared to a lever action. It is only present until the rein is slacked again and the ring slides back, thus it has impulse qualities. Though we can conclude that most of the so‐called lever bits are technically speaking not a lever, we will go on to use this name because it is established. In which situations are lever bits used? Lever bits are quite popular in show jumping and cross‐country, they are also used by pleasure riders to give more confidence when hacking. In advanced dressage competitions, double bridles are mandatory, and they are the basis of Academic, Baroque or Spanish riding style. On the first glance, these are rather different situations, but they have all common that they require an “advanced level of communication”. 2 Lever Bits – Myths and Facts In the first article, it has been explained how bit and bridle utilize the acupressure points at the horse’s skull to establish a short connection to the brain. Snaffle bits mainly employ the tongue, lever bits make use of further communication points: The poll and the chin groove (through the curb chain). In communication theory, this principle would be referred to as “employing several communication channels”. This allows a faster perception of signals, the effect can be compared to the simultaneous warning by a light and sound signal in your car when the fuel gets low. How much „lever action“ is actually acting on the poll? Using more than one perception channel for a more effective communication sounds like a brilliant idea, but many riders are sceptic. In a web forum, someone claims that a Weymouth amplifies the rein force by a factor 10. Others warn that a lever bit is able to crush the horse’s jaw, and squeeze their own shin with the bar mouthpiece to prove it. Recently extensive research has been carried out to enlighten the truth about these myths: For different cheek types, the magnitude of rein tension and the additional force transmitted to the poll have been monitored. The results are presented in detail in the Technical Information below, and can be summarized as follows: When a rein tension of up to 2.5kg is applied, (walk and slow trot), snaffle bits cause a relief of the poll. Especially notable is this effect with the Baucher bit. Bits with shanks have a theoretical mechanical advantage of 1.1 to 1.7, depending on the ratio of lower/upper shank length, but in reality, not more than ¼ of the rein tension is transmitted to the poll. The most direct transfer of rein force to the poll can be realised with gag bits, but not more than half of the original rein tension could be monitored to reach the poll. Higher rein tensions in trot work (up to 4kg with snaffle, 5kg with lever bits) cause no additional force on the poll (and no further relief effect either). Increasing rein tension in canter (FR max. 7.5 kg) counteracts the initial relief effects when a snaffle is used, until the initial poll pressure FC is reached again. With lever‐ or pulley bits, the observed transmission of rein tension is increased, but only in the dampened way already described. Thus, no cheek type results in even half of the rein tension being transmitted to the neck, and impact has always impulse qualities. What exactly happens to the rein force? To evaluate these results, it is helpful to keep in mind some facts from your physics classes: The “golden rule of mechanics” states that forces (or rather energy) can never be generated or amplified “out of the blue”, and there is always a counter‐force of equal magnitude present. (Otherwise, the perpetuum mobile would have invented). Only the magnitude of the force the rider applies to the rein is distributed to the bridle via the mouthpiece as a fulcrum. The fraction that cannot be observed in the cheek peace (acting on the poll) must have been compensated elsewhere. If a curb chain is present, it provides a part of the counter‐force (generating a signal on the acupressure point, which is located there). The remains are absorbed by compressing the tissue of tongue and lips (being soft and compressible, they do not provide a fixed fulcrum according to the lever definition, we call this a “floating fulcrum”). This accounts for the very small mechanical advantage, which has been observed in reality. 3 Lever Bits – Myths and Facts The tongue, which is cushioning the lower bar like a mattress, is taking the biggest part of the rein tension (if the correct head position with vertical nose line is assumed, see article 2). Due to technical reasons, it has not yet been possible to actually measure the resulting pressure, but it can easily be estimated: A mouthpiece, which is 14cm wide with a “usable” breadth of 1 cm, offers a contact area of 14cm². If a tension of 7 kg is applied to the rein, that results in a pressure of 500g per square centimetre. If the contact area is not even, single features of the bit make the contact (see Technical Information in part 1). Then the full 7kg act on these small points (and that will be equally uncomfortable for the horse as the “shin test” mentioned above). Our general conclusion is that lever bits provide a clever solution to improve the communication with the horse and are not more severe or uncomfortable than snaffle bits – no amplification of the rein tension could be verified! Useful criteria to choose the appropriate lever bit or Weymouth will be the topic of article no.4 Technical Information: Force impact on the poll Recently, an extensive research project to investigate the action of lever bits on the horse’s poll has been carried out at Durham University. Prof. Graham Cross and his team have developed a series of experiments to monitor the rein tension and the tension in the cheek piece of the bridle (which is exerting pressure to the poll) at the same time. In the first step, the observations have been carried out in static equilibrium using two spring scales (see fig. 2 and 3 in the text above). Following the development of an intricate Bluetooth equipment (fig. 1and 4), these forces have also been monitored in movement. In the diagrams below, the rein tension applied by the rider is marked on the x‐axis (horizontal), and on the y‐axis (vertical) the resulting tension between bit and poll observed in the cheek piece. The dashed line indicates the initial tension FCP, which is always present after correctly fixing the bit and cheek piece, before taking up the reins. Snaffle bit (loose rings): The force acting between bit and poll FCP decreases with increasing rein tension FR (poll relief effect) 4 Lever Bits – Myths and Facts Baucher Snaffle: As soon as the reins are taken, the relief effect is observed, FCP decreases further with increasing rein tension FR Universal (3‐Ring)‐Cheek: The pressure FCP, which is acting on the poll, stays constant with increasing FR , the relief effect is counteracted by the augmented turning momentum. Gag bit: This type of cheek can be described as a class 1 pulley, directly transferring a proportion of the rein tension to the poll (the factor is equal to 0.36) „True“ lever bits: In the diagram, the theoretical mechanical advantage MA is calculated for different shank lengths. It is determined by the ration of lower to upper shank length. 5 Lever Bits – Myths and Facts Pelham and Weymouth: The pressure exerted on the poll increases, when more rein tension FR is applied, but the transmission factor amounts to just 0.25. This is much less than the theoretical MA, an amplification of the rein tension could not be verified! The Bluetooth equipment records both tension values simultaneously every 5ms over a time of 30s. The following diagram shows the original data recorded for the Baucher Cheek in canter: The recorded rein tension is depicted in black, the corresponding tension in the cheek piece in red. It can be observed that the signals from the rein have an impulse character, the average force applied is 25N (corresponds to 2.5 kg). In this diagram type, it is not possible to find out which value of rein tension FR results in which poll pressure FCP The diagram below shows the average of all FCP‐values recorded for a certain rein tension FR . The resulting curves are comparable with the results of the static measurements. Universal (3Ring)‐Bit: During walk (blue) and trot (green), the pressure on the poll stays constant up to a rein tension of appr. 2.0 kg. When cantering (red), it increases continually up to about 0.8kg corresponding to a rein tension of 3.5kg, whereas more rein tension in trot seams to have a decreasing effect on poll pressure. 6 Lever Bits – Myths and Facts