Garmin HRM weirdness - any ideas?

Hi all, hoping for some advice here.

I’ve been using a Garmin HRM-Dual for all of my cycling since January 2020, probably averaging around 5 hours/week, with absolutely no problems at all. I always track my SUF sessions on my Garmin Edge as well as in SUF.

A couple of rides ago I noticed that the HR showing in SUF was a lot different from what was showing on the Edge - at the start of each recovery the SUF HR would go UP while the Edge HR was going down, then they’d swap, and eventually converge.

Today I did Nine Hammers but just after the 8th Hammer the HR dropped steadily to 124 and then got stuck there for a few minutes. I eventually pulled off the HRM because it was distracting me and I wanted to push through Hammer 9 (!). When you look at the trace (picture below) you can see there are also a LOT of dropouts, but no dropouts during the steady alleged drop in HR.

After the session I tried reconnecting it, making sure the strap was tight and correctly positioned etc, and it reported a HR of 74 which was about 40 too low according to a manual check and also my Garmin Forerunner watch.

So… I have checked the Garmin FAQ which doesn’t seem too helpful (eg I definitely had enough holy water!)

Could this just be a battery issue? There hasn’t been any low battery warning. Or something more serious? Has anyone had a similar problem?

Yep! It could be a battery issue, but first I would also suggest making sure your chest strap is still adequately tight and not a bit loose due to wear or washing.

A “dirty” strap can cause signal problems. Body fat and salts affect the resistance. Try washing the strap (not the transmitter). Usually they can be washed with sports gear in the washing machine otherwise hand wash - check with Garmin what applies

Thanks @coach.jinger.g and @ut-och-cykla, I will try again tonight with a well rinsed and tightened strap. I wash it in the machine every now and then but admittedly not every 7 rides as recommended :shushing_face: so perhaps that’s contributed.

Hey there i used other HRM (polar oh1 and a whoop strap) and maybe not as bad as yours but notice some “lagging” on the sufferfest app, running the polar beat or whoop app on my phone there were some discrepancies between the readings from the phone and the suff app. I don’t know if some BT lagging or a “refresh rate” on the app itself. But well didn’t report as it was never really a huge gap, but still interesting the same sensor sending data and different apps displaying something different

So I had some issues with the Dual HRM. I switched to the Wahoo Tickr. The Garmin would drop out all the time and it drove me crazy. I had it connected to my iPhone via Bluetooth. I have 0 issues with the Wahoo Tickr. It sounds like you’re broadcasting your HR via both channels: ANT+ and Bluetooth. When I was having issues with the Garmin HRM I reached out to support and, I can’t recall the exact wording, but they said it struggles when you Bluetooth to more than one device so you have to go in and make sure the device you’re not using isn’t connected. I can’t find any reference in the FAQs but maybe it’s less accurate broadcasting via both channels simultaneously. Generally speaking though, I use the Wahoo with the ap and keep the Garmin strap in my car so I always have it when we go to the MTB park. I do notice it giving me too low of a HR occasionally and I HAVE to wear quick drying moisture wicking fabrics (I earn my downs). I noticed in the FAQs that the synthetic materials can make it difficult to get an accurate reading. The shirt I always wear is the OrNot trail shirt and it has some silver whatever jumbled in to make it not get stinky. As much as I hate sweating in cotton, have you tried swapping your synthetic jersey for a cotton t? It seems like my Garmin HR eventually reads properly but I have to keep it wet and warm.

That solution always bugs me. Why have multiple BT channels if you can’t use them? :frowning:

Thanks everyone for your input.

I have tried a few different things and concluded that the Bluetooth signal is lagging variably behind the ANT+ signal.

Here is a comparison of my HR recorded via the SUF app (Bluetooth connection) vs the Garmin Edge (ANT+) connection. This is from Revolver which I did last night. You can see that the Bluetooth HR is always lagging behind the ANT+ one, plus it has a few dropouts:

By comparison the cadence (transmitted from my power meter) and power (Edge from power meter, SUF from the Kickr Core) align perfectly in terms of timing, although clearly my power meter doesn’t have power smoothing!


So the problem is isolated to the heart rate monitor.

If you then look at the first few intervals you can see that the Bluetooth heart rate is going DOWN during each interval, then going UP during each rest (!):

However, the HR recorded on my Edge over ANT+ actually fits with the effort:

So, that’s weird…

I have replaced the battery today (even though the Edge reported that the battery status was okay) and will see how it goes the next few rides.