Bino’s trainer comeback, part 2: Resurrecting the Minoura LR340 trainer

For indoor trainer use, I shod my folding bike Bino with Schwalbe Kojak slicks. The next step is to introduce a few modifications to the old Minoura LR340 magnetic turbo trainer.

While I’ve screwed up the LR340 with running it one too many times with a dirty rear tire (rookie mistake), the resistance unit is still operable despite the gouged roller the rear tire will drive. The real reason why the LR340 is out of commission is the entire mechanism required to press this resistance roller against the rear tire in the first place is broken.

The T-shaped nut above is key to the operation of many of Minoura’s turbo trainers. Photo credit: Steve Tan of the Hands On Bike blog.

That mechanism is made up simply of the resistance knob, which is attached to a threaded rod, which then threads into a nut with a special flange that make it T-shaped. Over time, this nut’s threads stripped out completely, and it is very hard to look for a direct replacement from Minoura unless you order one from, say, Amazon Japan.

Fortunately I came across this review on Amazon which pointed me in the right direction towards a possible fix. (DISCLAIMER AND WARNING: This post is purely for demonstration purposes only. If you decide to follow its instructions to modify your Minoura turbo trainer, proceed at your own risk. I will not be liable for any injury or warranty loss that may arise from you following these instructions.)

If it meant salvaging the LR340 trainer I had and having a shot at making it usable again for minimal cost…why not?

I ended up ordering the brad hole type T-nut and fluted knob with male stud, in both 5/16″ threading, from Amazon. Try as I might, I could not find any suitable equivalents on Lazada or Shopee. Amazon sells these in bulk, so I ended up with 12 of these T-nuts and 10 of these fluted knobs with male studs – way more than I’d need, strictly speaking, but they may have some other use.

The fluted knobs seem like a good replacement for the Minoura resistance knob. The knob itself is slightly smaller in diameter compared to the original, but the points are much more prominent and easier to grip and turn.

These brad hole type T-nuts are pretty beefy things, each made of steel with a base about 2.5 mm thick. As they are, though, they will not fit on the LR340. They will need a bit of work to replicate Minoura’s BF-11 T-nut, and that involves turning that round base plate into something more rectangular. As mentioned in the review, the reshaped T-nut will resist rotating while the knob itself is being turned.

After placing them in a vise and taking a metal hacksaw to them, two of the T-nuts now have a more rectangular shape to their base flanges.

The final bit of modification is to the LR340 resistance unit itself.

The threaded portion of the T-nut measures 11.8 mm in diameter, which is quite a bit bigger than the hole the original BF-11 T-nut used to inhabit, at around 10.2 mm. Using a drill and a stepped bit, I will need to enlarge the hole to an even 12 mm so that the new T-nut fits.

I had to remove the two pins that pivot the resistance unit on the frame so I could drill the hole out evenly from both sides. After reaming out the hole to 12.1 mm with my drill, I tried the new T-nut.

The new T-nut went pretty cleanly into its new home. Now I had to reassemble the pivot pins of the resistance unit, and reintroduce the small wheel adapter while I’m at it, as Bino is going to be using the LR340 almost exclusively now.

The final step is to drive the matching fluted knob into the new T-nut and the original conical spring. Before that, though, I measured the original metric-thread Minoura resistance knob and compered it with the new knob. The original Minoura unit was slightly too short to sufficiently push the resistance roller against Bino’s 20″ x 1.75″ (47-406 mm) rear tire without the aid of a cap nut jammed at the end. I wanted to see if this was still going to be a problem with the new setup.

The new knob’s threaded rod is longer than the old one by almost exactly 10 mm (39.4 vs 49.5 mm). If this is still too short to contact Bino’s 20″ x 1.35″ (35-406 mm) Schwalbe Kojak slicks, there may be much less of a gap to bridge. With all the modification work completed on the LR340, I decided to mount Bino and see.

As it turns out, the new resistance knob and T-nut combo is capable of pushing the resistance roller against the tire to a deflection of about 3 mm just before it runs out of thread. This is a decent result, but could be better. The pictures don’t show it, but I ended up removing the conical spring and adding a cap nut at the end to extend the threaded rod’s reach further. This way, there is just under 5 mm of rear tire deflection at the very end of the knob’s travel. More would have been good, but this is okay for now.

Cat Eye ISC-12 speed/cadence sensor. The cadence magnet is stuck magnetically to the pedal spindle.

To round out the changes, I transferred the Cat Eye Padrone Digital and its ISC-12 speed/cadence sensor over to Bino’s handlebars so that he gains cadence monitoring. The Bluetooth functionality also means support for a heart rate monitor strap is easy to add for real-time pulse data, although perhaps not the most reliable in connection.

My favorite bit about this hack is that I literally have a box full of spare T-nuts and resistance knobs in case these give out or strip their threads. Sure, the T-nuts and fluted knobs had to be bought online, and the T-nuts’ bases have to be cut to be usable on the trainer. Otherwise, if you’ve got the tools, this is a simple and very cost-effective way of bringing an out-of-commission turbo trainer back to life. I will gladly stand by this hack until Minoura make their small parts easier to procure for its users outside Japan.

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