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TerraTrike offers a webpage concerning how to determine your X-seam. X-seam is a measurement similar to inseam which is used to determine how to adjust the boom properly to fit the rider of a tadpole trike. When seated properly on a tadpole trike the rider’s leg should be about 85 % fully extended when the pedal is rotated to its most forward position.

I have written about this subject previously. HERE is one of the articles.

HERE is another webpage on this offering a good illustration. And HERE is another.

I have noticed that the instructions given for this vary somewhat in the matter of how much distance the bottom of the board is out away from the wall. To my way of thinking the safest and best method would be to duplicate the seat back angle more so than a measurement off of the wall. Of course, one must be careful not to move the board while sitting down and going thru the process of obtaining the measurement.

A quick and easy way to adjust the boom to its proper position for the rider is simply to sit in the seat and place the “heel” of the foot (while wearing shoes) on the pedal in the furthest forward position. The boom can then be tightened down and when the balls of the rider’s feet are positioned on the pedals the legs should be about 85 % or so extended.

It is good to know our X-seam measurement as it can be of value for a mechanic to set up a trike for the rider without them being there to go thru this process. Having a trike set up properly helps us to …





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seam … what is it? I am sure many of you know the answer to this, but for those who don’t it is according to definition:

“A person’s X-seam is a measurement related to the person’s height. It is measured from a sitting position, and is the distance from the lower back to the soles of the feet. X-seam measurements are used for sizing (recumbent) bicycles and ultralight aircraft.”  source – Wikipedia

X seam measurement

Yeah, so what? What does this have to do with tadpole trikes? Well, it is like this. I am sure we have all seen bicyclists who ride diamond frame bikes with the seats down way too low where their legs do not extend to near straight as they pedal.

bicycle seat too low

This is a major no no when cycling yet so many do it. Riding a tadpole trike is much the same as our legs should be nearly fully extended as we pedal. Knowing our X seam is the means a mechanic has in setting up our trike to “fit us”. Of course, it can also be accomplished by our simply sitting on the trike while the boom and pedals are adjusted in or out to accommodate us.

boom adjustment for x seam

You might ask … why can’t the inseam measurement be used? That’s a sensible question. It sure seams like this would work. But wait … there is a problem. We are not all the same. Our bodies differ considerably from one another. We come in all different sizes and shapes. Two people may have the same inseam measurement but their bodies are not at all the same. One my be quite thin and the other might be quite “large”. One might be much larger in the butt compared to the other. You can readily see where this inseam measurement isn’t going to tell the story … won’t work. X seam comes to the rescue. Here is a video explaining how to determine the X seam.

Having the X seam adjustment set correctly is important. Having one’s legs nearly fully extended while pedaling provides maximum efficiency in propulsion as well as maximum benefit to your body while exercising.

Here is a video produced by Utah Trikes which shows and explains about moving the boom in and out and the resulting chain length/rear derailleur effect.

Although I have never made the adjustment according to the instructions below it makes sense … that it would work fine and come out correctly.

With the rider properly seated on the trike seat (with his or her back and butt all the way back against the seat) place

the rider’s heels on the fully extended pedal and move the boom out until the leg is straight. Tighten the boom

in this position. Now with the ball of the foot on the fully extended pedal the leg should be nearly straight

but not quite. There should be just a very slight bend left in the knee joint which proper.

Having the X-seam set correctly will help to …