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I have Tacx Boost trainer and axle that came with the trainer is too short (cannot put the nut on the drive side as it's just too short). Axle on my current bike (Trek Emonda SL5) is seen in the first picture. I want to buy a longer axle for the trainer but I am not really sure what measurements I should look for.
An this is the axle that came with the trainer:
The second picture shows a quick release skewer. This is a totally different mechanism than a thru axle. Even if the skewer were longer, it would not secure the wheel in your bike's dropouts.
Most direct drive trainers come with one set of nuts for quick releases and a different set for thru axles. If you have a thru axle bike, you just use your bike's thru axle. The Tacx Boost is a lower cost wheel-on trainer that clamps the quick release lever nuts from the sides. Garmin (which bought Tacx's trainer division) seems to have forgone a thru axle adaptor for this trainer.
You need to buy a thru axle adaptor. You'd screw this into your dropouts, and it protrudes out each side. Shane Miller did a video on how to install the adaptor. If you have a drop bar bike, your rear is probably a 142mmx12mm thru axle (but confirm this). You'll also need to know the thread pitch of the axle, i.e. how far apart the threads are. They can be 1.0, 1.5, and 1.75. The Robert Axle Project has a guide on how to measure thread pitch here. Alternatively, the measurements might be printed on the thru axle. Or you can search RAP's site for your bike's model as if you were buying a thru axle, and the dimensions should be listed.
Your thru-axle is indeed 12mm with a 1.75 thread pitch based on the photo you provided.
I am familiar with the Kinetic Traxle adapter for their trainers, and for that it would take the Traxle Coarse T-2101 to match up with your bike/thru-axle.
A specific note regarding using this product on a non-Kinetic trainer is to ensure that your trainer can accommodate the width of the Traxle. In your case, this adapter is 205mm long, so that is the critical dimension for fitting it to your Tacx trainer.
Link to KurtKinetic Traxle info
Garmin does have a FAQ on which thru-axle will fit your bike. Based on information there and what your information offers, it appears that a Tacx/Garmin model T1708 E-Thru is the one you would need.
Which Tacx Thru-Axle Will Fit My Bike?
Here is a screen grab from that FAQ page:
These two should give you a couple of options (assuming the Tacx Boost trainer is wide enough for the Kinetic axle) in case you have trouble locating one or the other. I would probably veer towards the Tacx/Garmin model in case the profile of the axle ends are not consistent between brands.
