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Getting the right amount of room in the saddle for the rider can be tough if all you have to go on is the seat length measurement. That is because the seat length measurement doesn't actually tell you how far apart the fork and cantle are on the bars, and that is where the rider is. (Our Seat Length and Thigh Length Relationships page explains more about this.) The thigh length measurement can be very helpful in getting the right amount of room for the rider, and here is a quick explanation of how we use it and how we determine it.
So, what is thigh length? A lot of tree and saddle makers use this concept, though they may not actually measure it. We measure on the tree from the point of the cantle horizontally forward to the back of the fork where the nails are on a rawhided tree. This is where the rider's thigh actually sits and therefore it is important that there is enough, but not too much, room there for the rider's leg.
How to we figure out what we want? We ask the rider to measure the circumference of their upper thigh in inches and round up to the nearest 1/2 inch. It is an easy enough measurement to get.
Rule of Thumb for Starting Thigh Length
Take circumference of thigh
Divide by three
Add one inch
Then divide that number by 3, which is close enough to Pi for our purposes. This gives you the diameter of the thigh. Then add one inch, and you have a good ball park figure for the thigh length that rider will need. (By the way, we weren't brilliant enough to come up with this way of figuring it out. Dennis Lane, of the Dennis Lane Equine Back Profiling System, figured this correlation out too, and it has worked for us.)
Using this guideline, most people fit between 8 inches and 10 inches of thigh room. This is the starting point for figuring out the thigh length needed for a rider. It is often the finishing point too, as we find most of our customers like the finished length of the saddle when using this method. But there are some things that may cause you to adjust it.
This ballpark figure works well for people who like to ride with the traditional longer stirrup most commonly used in western riding. People who like to ride with shorter stirrups need more room because their thighs are closer to the horizontal and take up more room than a more vertical leg. So this needs to be taken into consideration when figuring out thigh length.
Another thing that changes this rule of thumb is specialty saddles. People who order cutting saddles want loooooong seat lengths due to the nature of their sport. Arena ropers want short seat lengths because they want to be right close to the fork so that when they stand up so they don't have to move or lean forward before they have the fork supporting them.
Then some people like to be tight in their saddles, while other prefer more room to move around. For some riders, whether they have bucking rolls on their slick fork or not may make a difference. These are questions the saddle maker may want to ask to figure out if they want to change the calculated thigh length or not.
The saddle is different
It is important to remember that this is the length in the tree. The similar measurement in a finished saddle will always measure longer. By the time the rigging is in and the rest of the leather is on, you will be measuring a fair bit higher on the cantle rim than you did on the tree. And since the cantle and fork angle away from each other, the higher up you measure, the longer the measurement will be. But since every saddle maker does things differently, how much longer it is will not be a consistent amount between saddles. You just need to recognize that you can't get a customer to measure their current saddle and use that as the thigh measurement for their next tree. It will end up too long for them.
Using thigh length instead of seat length means that it doesn't matter if the cantle is 3" tall or 6" tall, or if the fork is stood up or leaned ahead. Let the seat length measure what it may, by using the concept of thigh length you will end up with the amount of room in the saddle that the rider needs. And that is the whole point of these measurements, isn't it?