Setting up the 935 for a 24hr race

Had you see Garmin's advice? https://www8.garmin.com/manuals/webh...DBF4A9759.html

I would add that you should disable all types of "auto-scroll" and use a watch-face that doesn't display seconds/sweep-second-hand - screen-refreshes DO take energy.

I don't have any real-world feedback but I'm curious why you want to use OHR? I use a chest-strap HRM even when I'm out for a regular run or walk, for accuracy and battery life maximization.

The thought of using a chest strap for 30 hours sounds like torture to me. I can see why the OP prefers OHR, especially as an ultra of this duration is mostly done at modest heart rates anyway.

Will there be any planned stops during this race? If you stop the activity during any pit stops and select 'resume later' you could top up the watch with a small USB power pack. With GPS off the charge rate is about 1% per minute. If you start with a fully charged watch you get about 20 hours use using GPS only and some sensible other settings. If you top up say 6 times 15 minutes during the event you can extend this to close to 40 hours.

Ultratrac should work as well but anecdotal evidence on these forums is that it only works well on straight routes without any switchbacks etc. This makes sense as ultratrac works by waking up the GPS periodically to take a fix and then shutting it down again. This method is best suited for routes with long (almost) straight sections.

I don't have any real-world feedback but I'm curious why you want to use OHR? I use a chest-strap HRM even when I'm out for a regular run or walk, for accuracy and battery life maximization.

Hi,

what is difference: OHR vs HRM in energy consumption? Near the same?

I want to use use OHR because, exactly as CloggyAbroad points out, i cannot imagine wearing a strap for 30 hrs given it does constrict breathing a bit and does carry chafing risk for such a duration (plus OHR is simply very good for me on the 935).

CloggyAbroad, i am still debating whether to use UltraTrac or normal GPS - it’s too bad that I haven’t yet been able to find by how much this one thing in isolation influences battery life. Indeed as you suggest, i am now planning to take a 2000mAh battery pack with me (weighs 60g) and recharge the watch during short stops, or even during movement (i have tried to charge while activity is in progress and it seems to work, minus OHR of course :) ).

But this does bring up a question which i hadn’t thought of before - can the watch’s memory even take 30 hrs of normal GPS data (i will have smart recording on for this race, even if normally i use 1s recording)?

...can the watch’s memory even take 30 hrs of normal GPS data (i will have smart recording on for this race, even if normally i use 1s recording)?

ABSOLUTELY YES! More like thousands of hours.

Garmin's specs say up to 60 hours ultratrac with no OHRM. Say about 40 hours with OHRM.

Backlight is the other big variable. Turn the brightness down to 10%. Keys off, alerts off, gesture after sunset, 8 second timeout.

I think there is room for about 300+ hours of activity on the watch even with 1s recording (depending on how many sensors are connected).

ran 2 x 100km del Passatore 2016 & 2017 with my old 920xt, HRM Run, pushed the old faithful to 16h10m recording in 2016: no light at all, only one 4-fields page, no Glonass, smart recording, no alerts, no sounds&vibration. ran a 2018 70km ultratrail with 935 borrowed from a friend so i made no modification at all but it easily recorded 11hours (no HRM, only OHR) even with any special setup.
set backlight to an absolute minimum (better off all the time, screen can be easily read with your headlight), only GPS, smart record, no external sensor, no sound & vibration and with some fast 5 minutes "refill" maybe 30 hours could be reached.

can the watch’s memory even take 30 hrs of normal GPS data (i will have smart recording on for this race, even if normally i use 1s recording)?

yes, no memory space problems. do a full sync then clean ACTIVITY folder via USB before start to avoid every space problem.

As you've said - charge on the go. I assume you'll be wearing a race pack. If so, just attach the watch to the shoulder strap and keep going. You might appreciate the clear wrist for a little while anyway. I'd rather this than cripple features or use ultratrac and have no reliable distance or pacing data.

Thanks all, very helpful discussion. One more thing for those of you with footpod experience: I normally use stryd for pacing and distance (much better than GPS for both), and was going to use it from the start until it dies (i expect 15ish hours). Can you foresee any issue with this, ie stryd dying mid-run, and the watch hopefully picking up gps for both pace and distance after it does?