Hammerhead Karoo Setup Guide

Use your Apple Watch as a heart rate monitor with your Hammerhead Karoo. No chest strap, no extra hardware. Pair it once and your Karoo reconnects automatically before every ride.

What You'll Need

  • •Apple Watch (watchOS 10 or later)
  • •iPhone (iOS 15 or later)
  • •HRM app installed on both Apple Watch and iPhone: download here
  • •Hammerhead Karoo 2 or Karoo 3

Hammerhead Karoo Compatibility

Can connect
  • Karoo 3
  • Karoo 2

Full compatibility list →

Initial Setup

  1. 1 Open HRM on your iPhone and Apple Watch, then press Start on the Watch.
  2. 2 On your Karoo: go to Sensors → + (add) → Add Bluetooth Sensor.
  3. 3 HRM App will appear in the list. Select it.
  4. 4 On your iPhone: a Bluetooth pairing request will appear. Press Cancel.
  5. 5 Your current heart rate now displays on the Karoo.
If you're also using a chest strap or another heart rate sensor with your Karoo, disable it in Sensors before connecting HRM. Karoo will try to use whichever sensor is active.

Setup for Every Ride

After initial pairing, Karoo reconnects automatically on every ride.

  1. 1 Open HRM on iPhone and Apple Watch, then press Start on the Watch.
  2. 2 Karoo connects automatically. No need to navigate any menus.
  3. 3 Ride.
  4. 4 After your ride: press Stop on the Watch and terminate HRM on your iPhone.

If the Connection Drops

  1. 1 On your Karoo: Sensors → delete the HRM App sensor.
  2. 2 In your iPhone's Bluetooth settings: find "Karoo 2/3" and "Karoo 2/3 BT Audio" and select Forget. (This step is usually optional but helps if re-pairing fails.)
  3. 3 Re-pair following the Initial Setup steps above.

HRM Shows a Pulsing Circle but No Connected Device

"No Bluetooth Device Connected" is the expected state until a receiver connects. The pulsing blue circle means the iPhone is broadcasting; the connected-device line only reports whether anything has picked that broadcast up yet, and it clears the moment your equipment connects. If it never clears, the problem is on the receiving side: pair again with HRM open and in the foreground on both iPhone and Watch.

See HRM in action

Watch my Apple Watch heart rate show up on Zwift and a Hammerhead Karoo — in under two minutes.

Ready to Ditch the Chest Strap?

Open HRM on your Watch and iPhone, press Start, and ride. Try it free for 7 days. No charge if you cancel before it ends.

7-day free trial
$7.99 / year

after your trial ends

Less than a 3-pack of batteries.

Download on the App Store

Cancel before the trial ends and you won't be charged.