A Hack For Measuring 39mm Fork Tubes Without Disassembly

So you wanna lengthen your fork, and you assume your Sporty, FXR, or Dyna has stock tubes. They’re all pretty close, right? Except then you start hearing some bikes had longer tubes than others, and maybe they're not all the same. That’s true. 

To get the right tube, you need to know your tube length. We know you really don’t want to rip your fork apart before payday to get the measurement, order the stuff, and then wait to get the parts in and get it back together.

First, I’d be remiss if I did not mention that if we show your fork tubes on the website here at TC Bros., you’re gonna get them fast. They’re on the shelf. We don’t display parts we don’t have. Order before 2PM EST and the parts go on the truck that day. But enough of my sales pitch.

Most “stock length” tubes you’ll see offered for the 39mm front end are something close to the most common length OEM tube, which is 24 1/4”. Note that I’m not saying that’s what you have—but you might. So here’s a handy way to measure and be extra sure (and a bonus third way to be pretty close if you don't have a way to lift your bike). Since the amount of the tube that resides inside the fork lower is somewhat consistent, you can measure the tube distance between the lower part of the triple tree and the top of the dust boot.

I’d recommend doing this with the front wheel in the air, but here’s a measurement for the bike on the ground, too,  if “close enough” is close enough for you. Both is better! These figures are based on an OEM front end.

Taking them out and measuring is always the best way, but this will get you in the ballpark. Remember different springs and damper tubes also affect the measurement—a stiff spring doesn’t let as much tube enter the lower, making the effective length “longer” when the bike is loaded.

It's possible you'll fall above or below this measurement, so if you are, you know you have one of the "other" length models. If you buy a TC Bros, kit, our measurements are based on 25" as stock, so you can reliably calculate what you've got coming if you order a given length over—even if it isn't a true two or four.

So now you can ride your freedom machine right up until the day you make the swap. And if you haven't been in there before, don't forget the fork oil and the bushings. And if you're ordering a stock-length set because you're a go-fast guy, we have other goodies to help in that department, too.