Syncing intervals.icu workouts to a Hammerhead Karoo

What the integration does, and where you turn it on

Intervals.icu appears on Hammerhead's list of linkable third-party accounts [Hammerhead Account Linking], and the sync runs both ways: planned workouts reach the Karoo, and rides recorded on the Karoo land back in intervals.icu. The non-obvious part is where the connection gets made.

Intervals.icu's developer announced the integration on 24 January 2025, describing it as connecting to intervals.icu to sync your rides and planned workouts, and noting that although there is a Hammerhead box on the intervals.icu settings page, the connection has to be created from your Karoo [Intervals.icu Karoo Sync]. That is the step people miss. If you have been hunting for an intervals.icu button on the Hammerhead Dashboard and coming up empty, you were looking in the wrong place — start on the device, and make sure it has been updated first.

There is a second capability here that almost nobody uses, and it is arguably the more useful half. Hammerhead's third-party sync pulls training metrics — FTP, power zones, heart-rate zones, and rider weight — from a platform you nominate, and intervals.icu is on that list alongside Strava and TrainingPeaks [Hammerhead Training Metrics]. Set it there and the numbers your Karoo trains you by follow your intervals.icu profile instead of drifting out of date on the device. The sync partner can only be changed from the Dashboard, not from the Karoo.

Once a workout is on the Dashboard, delivery is automatic: the Karoo collects it the next time it connects to the internet [Hammerhead Workout Import]. On the device, a workout scheduled for today carries a Today tag in the workouts list and raises a notification in the Control Center [Hammerhead Karoo Workouts]. Nothing else needs pressing.

The format trap, and the steps the importer rejects

Intervals.icu will happily author a workout the Karoo refuses, and the failure is all-or-nothing rather than partial. Hammerhead publishes exactly what it accepts, and two of the rules catch intervals.icu users specifically [Hammerhead Workout Import].

The first is ramps. A .ZWO step must be a warmup, cooldown, free ride, repeating interval, or steady state — maximum-effort and ramp steps are explicitly unsupported [Hammerhead Workout Import]. Intervals.icu makes ramped efforts easy to write, so a single ramp inside an otherwise ordinary session is a common cause of a workout that simply never appears. The whole file is rejected, not the offending step, which is why the failure looks so much like a sync bug.

The second is targets. Every .ZWO step must carry a power or cadence target, and zone-based power targets are not supported at all [Hammerhead Workout Import]. A session written as ride zone 4 for ten minutes rather than as a percentage of FTP will not import. Where a step has both power and cadence, power wins. Distance-based workouts are converted to time-based ones on the way in.

The trap proper is heart rate, and it decides which export you should reach for. A .FIT workout's steps can target speed, heart rate, cadence, or power — heart rate and power as absolute values or percentages, speed and cadence as absolute values — and heart rate does not appear on the .ZWO list at all [Hammerhead Workout Import]. The two formats are not interchangeable. If you train to power, either export works. If you train to heart rate, only .FIT carries your targets, and exporting the same session as .ZWO silently strips the thing you were training by. Note too that RPE-based workouts are not supported on the Karoo in any format [Hammerhead Karoo Workouts].

Where this route stops

For getting a planned session onto a Karoo, this is a genuinely good answer and it costs nothing. What it does not do — what no calendar does — is revise the plan when the week that plan assumed stops happening.

Be clear about how good the free option is, because plenty of pages have an incentive to be vague about it. Intervals.icu's analysis is stronger than most paid tools', the sync is official rather than a workaround, and pairing it with the training-metrics sync gives you one consistent set of numbers across the site and the device. If that is what you came for, you are done, and you can ignore the rest of this page.

The limit is structural rather than a missing feature. A calendar holds the plan you made on Sunday. It does not know that Wednesday was erased by a work trip, that a chest infection took ten days out of the block, or that Saturday's unplanned four hours changed what Tuesday should be. Those are the events that actually end amateur training years, and they arrive after the plan is written.

That gap is the subject of our broader guide to training with a Hammerhead Karoo, and it is where AdaptCycling sits. We read your rides from the source you already use, estimate threshold from your own power curve rather than asking you to schedule a maximal test [Jones et al. 2010], and rebuild the coming fortnight around the week you actually had. Karoo-recorded rides help here more than most, because the device opens a new lap at every workout interval [Hammerhead Karoo Workouts], so the analysis reads what you held rep by rep instead of guessing where the reps were.

Quick answers

Sources cited in this guide

1. 01
   Intervals.icu Karoo Sync. Hammerhead Karoo now syncs with Intervals.icu. Intervals.icu.
2. 02
   Hammerhead Account Linking. Dashboard - Linking Third-Party Accounts. Hammerhead Support.
3. 03
   Hammerhead Workout Import. Dashboard - Importing ZWO & FIT based Workouts. Hammerhead Support.
4. 04
   Hammerhead Training Metrics. Dashboard - Training Metrics. Hammerhead Support.
5. 05
   Hammerhead Karoo Workouts. Karoo OS - Workouts. Hammerhead Support.
6. 06
   Jones et al. 2010. Critical power: implications for determination of VO2max and exercise tolerance. Medicine & Science in Sports & Exercise.

More inside Training with a Hammerhead Karoo

Other articles in this series

1. 01

   Karoo structured workouts without TrainingPeaks

   TrainingPeaks is not required, and Hammerhead's own docs are part of why people think it is. The free upload route and the providers that qualify.

   →
2. 02

   Training plan apps for the Hammerhead Karoo

   Hammerhead ships no training platform, so the app you connect is the plan. What TrainingPeaks, TrainerRoad, Intervals.icu and Xert each actually do.

   →
3. 03

   Karoo workouts not syncing: how to find the real fault

   Check the Dashboard first — that one look halves the problem. The provider rules that block a push, the device half, and the failure that is not one.

   →
4. 04

   ZWO file not importing to a Karoo: the rejection rules

   Hammerhead rejects the whole file, not the bad step. The step types it accepts, the target rule, and the difference between rejected and ignored.

   →
5. 05

   Karoo structured workouts without a power meter

   A heart-rate strap satisfies the Karoo's sensor requirement, but heart-rate targets exist only in FIT. What HR can target, and what it cannot.

   →