Engineering Behind the EponaShoe
(and other ramblings from the engineering department)

John Craig, Phd
V.P. Engineering, EponaShoe

There have been plastic shoes in the horse world for over 20 years.

Proponents of flexible shoes, propose a list of potential benefits. I believe that most people (farriers, owners, trainers, and veterinarians) will generally agree with the list of benefits of flexible shoes. We'll discuss these benefits later in this article.

Now, if you talk to farriers and owners who have tried some of the previous plastic shoes on the market, the typical complaints are:

* They lose shape
* They wiggle on the nails and get loose
* Wiggling nails tear up the hoof wall
* Their wider web goes under the sole further and makes flat-footed horses sore.
* The back of the shoe flip-flops off of the heel of the hoof, and debris gets in there
* They are too expensive

In these complaints, we find a list of practical problems with previous designs for flexible shoes. Most people do not disagree with the beneficial goals of a flexible shoe, but reality forces them to focus on the practical problems with them.

To me, these all sounded like engineering problems that could be solved. And now, after about 4 years of work, I think we have solved them to a large degree, as I'll attempt to explain in this article.

After using other plastic shoes for a number of years, we set out to work on these "engineering problems", and to design a better flexible shoe. Our goal was something healthy for the horse, easy to use, practical, and not too expensive.

I believe that previous plastic shoe designs have just not had the right ingredients. In some ways, they have been too simple. It turns out to be quite difficult to create an ideal shoe from one material. This bears a strong relationship with human footwear -- ever take apart a tennis shoe? Human shoes are made of a variety of different materials bonded together in layers. Shoe companies do not go to this trouble and expense for no reason. Different materials have different mechanical properties, and it is generally only in combination that all design goals can be met. Our (past and present) competitors sell shoes that are too simple, and hence either too soft, too hard, too grippy, too difficult to use, or other problems arising from a design that just couldn't address all the things that must be considered.

On the other hand, we need to try to keep things simple. We need to keep cost down.
As Einstein said: "Things should be as simple as possible, but not simpler!".

The EponaShoe consists of 3 different types of special high-quality polyurethane bonded together, and also two small steel stiffeners inside the shoe (you can't see them, and most people are unaware they are there.) Like the manufacturers of human footwear, we are not going to the expense of using 4 separate materials just for the fun of it. We have come to believe that this sort of combination of materials is required to implement a flexible shoe that really works for the horse.

A principle idea behind the EponaShoe, and one that is covered in our patent, is that on the sole-side of the shoe, two materials must be used - one that is hard and stiff where the nails go, and one that is softer towards the interior of the sole. This simple idea allows the nails to hold, but at the same time avoids any pressure points on the sole or frog. Prior to EponaShoe, when we used old-fashioned plastic shoes on our own horses, we had to carefully bevel away plastic to avoid pressure on the sole. When we tried softer shoes, we found that they would not stay secure on the nails for 6 weeks in a stable fashion.

A third type of polymer is used on the ground-side of the shoe for it's wear resistance at the toe, and shock absorption at the heel.

The internal steel stiffeners (U.S. patent pending) do two important things: first, they again aid with making the shoe stable when nailed. The internal stiffeners also encircle the rearmost 4 nail-holes with metal. These are the nail-holes which take the most stress, and so they are internally reinforced. Secondly, these steel stiffeners solve a big problem with other plastic shoes -- they tend to 'flip-flop' away from the heel of the hoof. When this occurs, debris can get in between the shoe and the hoof, and this starts the shoe distorting, and results in a less-than-ideal situation. The goal, which we feel we have achieved, is to make an internal stiffener which keeps the shoes rather stiff from the last nail hole back to the heel, while at the same time, leaving the shoe flexible in all other dimensions. If you take a radiograph of the shoe, you can see the internal stiffeners!

We do not agree with the farriers that say that only a shoe with a rigid steel core will work -- there is a whole continuum of designs from solid steel to solid plastic. We have built a shoe that is close to 100% plastic, but with just a small amount of steel to stiffen and reinforce where needed. This is an important feature of our design.

Figure 1 shows one sign of the benefits of our design. This image shows a shoe that has been on for 6 weeks. This horse is free-run every day, ridden every 2nd or 3rd day, and turned-out in hard California summer pasture during the day. The clinches (in the red circles) have not wiggled in the hoof wall, nor have they begun to 'open up'. They are stable, and the shoe is stable on the foot. There is not another shoe on the market that can do this while at the same time being flexible.

Figure 1: Clinches have not moved nor 'opened' after 6 weeks. Further, this was attained with only 6 nails (not 8). We used Mustad MX-60 nails in this case.

If you scroll back up to the top of this article and review the list of "practical problems" with plastic shoes, you may now be starting to see how the EponaShoe has fixed them. Low cost is one final goal, and we have been moderately successful: the common sizes of EponaShoe are $19 per pair. Yes, this is more than a simple steel shoe, but read further, and you'll see that if
your shoeing philosophy includes supporting the frog and sole, the EponaShoe may be the least expensive way to achieve this!

What are the benefits of using flexible shoes? Most of our plastic shoe competitors harp on "shock absorbing" and "lightweight". While these are indeed benefits, I don't think they are the top two -- in fact, I put them at positions 5 & 6 on our list of benefits. Some shock absorption is a good thing in some pursuits such as when horses are used on pavement (carriage, mounted police, etc.) and perhaps for endurance horses. But otherwise, I feel that horses in nature run barefoot on rather hard surfaces, and most of them can take some concussion without problems.

In our view, the benefits of the EponaShoe would be listed in priority order as:

* The hoof can flex much as an unshod hoof can flex
* The frog is supported - it is not dropping as in a rim-shoe
* Plastic wears more than steel and this allows the horse to wear a bevel at the toe as it needs to
* Because the EponaShoe can be glued, a wide variety of therapeutic situations can be dealt with, and nail damage can be avoided if desired
* Concussion is somewhat reduced through the internal dampening of the shoe
* Lightweight compared to metal shoes

A major benefit to the EponaShoe is the way in which the frog is supported. This is not done with a hard bar of material cutting across the center of the frog. Rather, on the sole-side of the shoe, the soft polyurethane forms a "built in pad" within the shoe which is shaped to support the frog. Its not rocket science: the horse was designed to stand with all parts of its foot in contact (more or less) with the ground. Think of a barefoot horse standing on a rubber stall mat. The single biggest problem with the traditional metal rim shoe is that it elevates the frog 1/4" off the ground. As a consequence, the frog generally "descends" down by this distance to "seek the ground" and get in contact with it in an attempt to bear some weight. Of course, all the internal structures "sink down" somewhat in this process, and that is the start of trouble. Of course, there is a lot of variety in the horse world, and so it is certainly true that some horses (especially young ones) can tolerate this situation just fine.

There is an enormous amount of money being spent now on liquid urethane fillers, hoof pads, and other contraptions in an attempt to fix the deficiencies of the standard steel shoe in this regard. In light of the cost of these add-ons, the EponaShoe at $19 a pair can become the cheapest way to properly support the frog. The pad and frog support is "built-in" -- you don't have to add it. Well, we do recommend using a small amount of special silicon-based putty around the frog to create the contact between the foot and our shoe (this may cost an extra $2 per foot).

The EponaShoe was also designed from the start to be able to be glued-on rather than nailed. This is just an option - not a requirement. There is additional cost and time associated with the use of glues, but there also appear to be therapeutic benefits. We have been able to deal with a wide variety of situations through the use of the EponaShoe as a glue-on shoe. We continue to use glue for most of our own horses, eventhough they are totally sound. Attachment by glue spreads the load evenly around the hoof, avoids stress concentration at the nails, and avoids nail damage to the hoof wall. It gets you one step closer to the ideal of just having a protective 'hoof-extension' rather than an artificial shoe.

Another thing about engineering shoes for horses, or any other product: it is extremely useful to implement a "continuous product improvement" strategy. This simply means that you probably won't get it quite right the first time, so you need to redesign. Then, you'll need to redesign a third time, and so on. In the end, the best products come from continuing redesign and building a company culture of continual product improvement. Our first shoes were called "Model A" and had an "A" on them. Then came model "B", and then "C", and now we are producing shoes marked "C+" or "D". Each new model has improved on the last, and each of these steps required a new set of molds for injection molding the shoes. We've gone through these 4 generations in our first 2 years on the market. If you look at our plastic-shoe competitors, their designs have not changed in several years! In some cases, we know for a fact that they have known problems which they are electing not to fix (because new molds are expensive).

Another engineering aspect of note is that we are a sister company to EponaTech ( see www.eponatech.com ) which produces software for veterinarians and farriers for measuring the hoof and keeping track of things. Because of that connection, when we laid out our shoe shapes and sizes, we did so with the help of a database of more than 3000 calibrated and measured images of hooves. This means our sizes fit horses pretty well, and the exact details of how large the frog support portion of the shoe is, where the nail holes are placed, and a dozen other details were designed in the context of a lot of real-world data. I'm pretty sure that sort of input has not been available to other horseshoe designers.

This is all to say that we are really trying to get it right and I hope we never stop at that pursuit. The bottom line is that horses are comfortable and performant in the EponaShoe!