I’ve experienced this problem many times.
When I ride into Zwift in simulation mode, the hill resistance comes in late, like 2 or 3 minutes after I passed the hill. It comes when I’m on a flat part.
This is very annoying and I don’t know why it happens.
I’ve also experimented some power stuck during a workout session on Erg mode. Or minutes delay, like before…
Zwift is running on an iMac on MacOS 10.15, a Tacx Flux Smart connect by Bluetooth.
@Bastien_Marragou have you put your log file from those rides where you experience delayed resistance into www.zwiftalizer.com ? It may show BLE or ANT+ interference on those rides that could be causing the issue.
I checked the logs. It seems that the iMac (or the Zwift app on MacOS) cannot send grade information to the HT controllable by BLE. I got this error :
[9:44:01] [FTMS] SIM (1.4)
[9:44:01] BLE : bledevice - SendBLEResponseToGame received the following error for device Tacx Flux 58818 characteristic 2AD9 ERROR: Unknown error. Error Domain: CBErrorDomain=0
[9:44:01] BLE Mobile Response error 0 : Unknown error.
If I connect the HT controllable part through my ANT+ dongle, Zwift sends correctly the grade information (and the resistance is well updated) :
[9:59:53] FE-C SIM GRADE 0.013
[9:59:53] ANT : Transfer Completed.
[9:59:53] ANT : FET grade set successfully
All the other paired devices (Power source, Cadence, Heart Rate) work perfectly by BLE.
@Bastien_Marragou I have the same setup as you (mac and flux) and I am also experiencing this issue A LOT.
@Alec_Smith_USMES The only solution I’ve found is to connect the controllable part through an ANT+ dongle, and all the other sensors through Bluetooth. It seems that the Mac Bluetooth driver does not control the Flux correctly. My ANT+ dongle does it well. Note that the powermeter sensor should be connected by Bluetooth, better reads than ANT+.
+1, same here, same errors, same symptoms. Delay gets progressively worse during the length of a ride. On a recent 2h10m ride, the latency was almost 8m by the end. Emailed Zwift and Tacx for guidance and submitted logs. Any resolutions other than changing to an ANT+ connection?
I´m having the same issues with my Mac and Tacx Flux Smart. When I connect the Trainer via Bluetooth with my Mac the resistance comes in way too late and the incline reading in Zwift and the trainer resistance doesn´t match at all.
I gave it a try with my iPad and it works absolutely brilliant and the resistance versus incline reading is totally accurate.
Apart of connection the controllable part of the Trainer by ANT+, has somebody found a fix for Bluetooth?
The iMac is from End 2015 and has MacOS 11.1 installed
I’m having the same issue with my OSX/Tacx Flux 2 setup and Log.txt filling up with the following error messages:
BLE : bledevice - didWriteValueForCharacteristic received the following error for device Tacx Flux-2 28937 characteristic 2AD9 ERROR: Unknown error. Error Domain: CBErrorDomain=0 userinfo= : NSLocalizedDescription Unknown error.
When using the Zwift companion app as the source, no error messages in the log anymore. But not happy with the RPM the Tacx is providing. Had a chat with Garmin and according to Garmin, the Tacx is calculating the RPM. I did some tests and it’s sometimes off by 10RPM when comparing it to my Garmin 530 which is still on the bike.
So just ordered an ANT+ dongle so I can use my old Garmin cadence sensor and use the ANT+ for the controllable part of my Tacx. Let’s wait and see.
Hi John,
I would be very interested in if it works with your ANT+ dongle. The odd thing is that its apparently just an issue with Macs. Tested it with my IPad and Apple TV and it worked absolutely fine.
That’s working much better. 
Did the TDZ Stage 4 this morning and no more strange bounces of the resistance. 
Bought a 17 Euro ANT+ receiver(Anself) with an extra 2m USB cable and connected it to my new Apple M1 Air.
No drivers needed, automatically detected:
When starting Zwift, BLE, and ANT devices discovered:
==========================================================
Non-Selected: HR Strap 15961 (0x783E59) [ANT]
Non-Selected: Speed/Cadence 6787 (0x791A83) [ANT]
Non-Selected: USB ANT STICK (0x19946192) [BLE]
Controllable Trainer: ANT+ Trainer 28937 (0x117109) [ANT]
Heart Rate: Polar H9 6F3E5926 158 (0x1D341C9E) [BLE]
Cadence: Tacx Flux-2 28937 199 (0x16D234C7) [BLE]
Power: Tacx Flux-2 28937 199 (0x16D234C7) [BLE]
==========================================================
connected the Tacx “Controllable Trainer” via ANT and the rest via Bluetooth.
In the Log.txt you will see the ANT FE-C protocol being used to control the Tacx.
FE-C SIM GRADE 0.032
ANT : Transfer Completed.
ANT : FET grade set successfully
But also seeing some RX Fails every now and then.
ANT : Rx Fail on channel 1
There is a nice youtube video on how to lower the amount of Rx Fails on the zwiftalizer results page.
Checking to see if I can bring it below the 10% but the controllable part via ANT is working much better.
John, many thanks for your detailed review. Then its probably worth it to buy a ANT dongle and it´s not that expensive.
I´m suprised that even with the newest Macbook the Tacx Flux Bluetooth connection is struggling. I suspected already my older Mac but it seems to be a general problem of Tacx Flux and Macs.
Cheers, Marcel
Did some low-level Bluetooth packet tracing using PacketLogger but OSX and IOS are showing the same problem when trying to write a value in Bluetooth GATT Characteristic 2D9 (Fitness Machine Control Point) 
My Tacx app is using other Characteristic ID and not showing any write error:
Asked Tacx if they can explain what’s going on here.
Hi guys I’m new to Zwift and have a tacx flux 2 and suffering from the trainer losing connectivity with Zwift which I run on my MacBook Pro. I’ve just been looking in the logs as happened again today and it’s full of the same errors being discussed here just before the disconnect. Is this all part of the same issue then and pointing to Bluetooth instability between the Mac OS and the trainer? I notice some people say the delay issue being discussed is fine if using an iPad instead so maybe I should try that (I don’t notice resistance change delays)? If not then it’s an ANT+ dongle I need I guess but seems a bit crazy that an expensive MacBook is somehow poor at handling this connectivity as I get no issues with any other devices.
My log example before the disconnect:
[13:19:19] BLE Response error 0 : Unknown error.
[13:19:19] [FTMS] SIM (0.2)
[13:19:19] FPS 51, 28167, 12194, -55431
[13:19:19] BLE : bledevice - didWriteValueForCharacteristic received the following error for device Tacx Flux-2 02730 characteristic 2AD9 ERROR: Unknown error. Error Domain: CBErrorDomain=0 userinfo= : NSLocalizedDescription Unknown error.
[13:19:19] BLE Response error 0 : Unknown error.
[13:19:19] [FTMS] SIM (0.4)
[13:19:20] BLE : bledevice - didWriteValueForCharacteristic received the following error for device Tacx Flux-2 02730 characteristic 2AD9 ERROR: Unknown error. Error Domain: CBErrorDomain=0 userinfo= : NSLocalizedDescription Unknown error.
[13:19:20] BLE Response error 0 : Unknown error.
[13:19:20] [FTMS] SIM (0.4)
[13:19:20] BLE : bledevice - didWriteValueForCharacteristic received the following error for device Tacx Flux-2 02730 characteristic 2AD9 ERROR: Unknown error. Error Domain: CBErrorDomain=0 userinfo= : NSLocalizedDescription Unknown error.
[13:19:20] BLE Response error 0 : Unknown error.
[13:19:21] BLE : bledevice - didWriteValueForCharacteristic received the following error for device Tacx Flux-2 02730 characteristic 2AD9 ERROR: Unknown error. Error Domain: CBErrorDomain=0 userinfo= : NSLocalizedDescription Unknown error.
[13:19:21] BLE Response error 0 : Unknown error.
[13:19:21] BLE : bledevice - didWriteValueForCharacteristic received the following error for device Tacx Flux-2 02730 characteristic 2AD9 ERROR: Unknown error. Error Domain: CBErrorDomain=0 userinfo= : NSLocalizedDescription Unknown error.
