I’ve had a CycleOps (now Saris) Hammer smart trainer since late 2017 and it’s been working great, but lately has been making a slight rattling sound during post-ride spindown just before the flywheel stopped. It felt fine during use so I kept riding, but then when putting out a couple hard, short efforts during high resistance periods (in slope mode via Zwift) it made a loud clank/bang sound. I would also sometimes get a rumbling feeling as if the notches on the belt weren’t smoothly engaging with the notched pulleys. Various posts online attribute this to a worn belt, so I opened up the trainer to take a look.
What I found led me to believe the issue is with the belt tensioner coming loose, not a worn belt. I suspect for many the belt replacement fixes the problem because the replacement process includes re-tensioning the belt.
Inside the trainer there is an idler pully whose tension is adjusted via a threaded rod on a spring. This rod is turned via a 5mm hex head accessible from the bottom of the trainer (without opening it). The threads on this rod seemed a bit worn, it seemed loose, and shaking it made a similar metal-on-metal sound to the rattling and clank that I’ve been hearing.
As the belt is a motor-type timing belt, it’s pretty unlikely that human-level output in clean conditions will stretch it much. Removing and inspecting the belt and all pulleys showed that everything was clean and free of damage, so I reassembled it, tightened the tensioner to compress the spring a bit and tension the belt, and rode the trainer a few times. After this there was no more rattle and I was unable to reproduce the clank/bang during hard efforts.
It seems the cause of the noise was the lack of tension on the threaded rod, spring, and idler and it likely came loose over time. The rattle was from the tension rod rattling as the as the heavy flywheel and main fan wheel and belt came to rest. The bang noise was from the sudden heavy pedal load vs. the strong resistance unit removing all belt pressure on the idler and it all slamming back together when my pedal stroke dropped off.
So, if you’re hearing this same sort of noise and rumbling, try using a long 5mm wrench to tighten up the tensioner a couple turns. If this works for you it’s much quicker (and cheaper) than replacing a belt.
This trainer is an original CycleOps Hammer, sometimes referred to as the H1, and the whole Hammer/H2/H3 family of trainers is now sold under the Saris name. (The CycleOps brand has been owned by Saris for years, but now all products are sold under the Saris name.)
To open a Hammer (H1) trainer you need a 1/18″ hex tool for the small screws, a T-30 Torx for the large screws. The cassette must first be removed, and after removing the screws the snug-fitting side panel pulls away. Belt tension can be adjusted with a 5mm hex and the fastener is accessible without opening the trainer. The stock belt is an MBL 150S5M930 and available for under $10 online. (Saris sells the replacement belt with the three required tools for $59.99 + shipping.)