ZBike Infinity Bluetooth protocol: integration guide for developers

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How to read from and control the Zycle ZBike Infinity indoor bike over Bluetooth Low Energy: Fitness Machine, Cycling Power, Device Information and the Zycle service.

• Applies to: ZBike Infinity, firmware 1.39 and later
• Edition: 1.0 (web edition, October 2026), based on the Integration Guide V1.0 of 31 August 2026
• Published by: Zycle, for third-party integration
• Questions about this guide: https://help.zycle.eu/hc/requests/new

Key rules (read this first)

These are the rules an application must follow to work correctly with the machine. If you are using an AI assistant to write your application, give it this whole document and ask it to follow these rules.

1. Scan for the name ZInfinity, or for service 0x1826 or 0x1818. Never filter a scan on 0x180A or on the Zycle service UUID: they are not advertised.
2. Request an ATT MTU of at least 185 octets right after connecting, before subscribing to anything. Without it, the Zycle Power Diagram and Command notifications never arrive.
3. Do not pair or bond. Every characteristic in this guide is open.
4. Subscribe to indications on the Fitness Machine Control Point (0x2AD9) before writing to it, and write to it with Write Request (write with response).
5. Write Request Control (00) first, then the target-setting procedure you need. Only four op codes work: 0x00, 0x04, 0x05, 0x11.
6. FTMS and Cycling Power values are little-endian. The Zycle service is big-endian, except the ERG target inside Zycle Information.
7. The resistance scale is a plain integer from 0 to 200, everywhere. Do not apply the 0.1 resolution that the FTMS specification defines for resistance.
8. To send a gradient, use Set Indoor Bike Simulation Parameters (0x11) and always send all four parameters. Set Target Inclination is not supported.
9. Expect 80 xx 05 (Control not permitted) when another app or an ANT+ device holds control. Keep reading data and retry later; do not disconnect and reconnect to force it.
10. Never read, write or subscribe to anything not described in this guide. The reserved characteristics and commands start calibration or drivetrain configuration procedures, and a machine left part-way through one can require Zycle support to recover.

Contents

1 About this guide

1.1 Purpose

This guide describes how an application, a game, a simulator or a head unit made by anyone other than Zycle can talk to the ZBike Infinity over Bluetooth Low Energy: how to find it, how to connect to it, how to read what the rider is doing, and how to control the resistance it applies.

It is not a restatement of the Bluetooth specifications. Where the machine follows a standard service (Fitness Machine, Cycling Power), this guide documents what that service actually contains on this machine, which optional fields are present, and what the values mean in practice, so that a developer can build a correct integration without inferring the machine's behaviour from the standards.

Everything in this guide applies to the ZBike Infinity only. Earlier Zycle and Bkool trainers expose different services and are outside its scope. The machine identifies itself over Bluetooth as ZInfinity.

1.2 Scope

This guide covers the Fitness Machine Service, the Cycling Power Service, the Device Information Service, and the part of the Zycle proprietary service that is published for third-party use.

It does not cover:

• other vendors' proprietary services that are also present on the machine;
• the first-party parts of the Zycle service (factory procedures), which are reserved to Zycle applications;
• communication with the machine over ANT+.

A client will encounter some of the reserved items on the machine. Section 2.1 says how to recognise them; the rule is simply to leave them alone. They are not published because they are not safe to explore: several of them start hardware calibration or drivetrain configuration. Read and write only what this guide describes.

1.3 Firmware versions

This guide describes firmware 1.39 of the ZBike Infinity. Later versions will also be compliant; if for any reason they are not, a new version of this guide will be released.

The running firmware version can be read in two ways: as a text string from the Device Information Service (section 6), and as a pair of octets from the Zycle service (section 7.4).

1.4 Conventions

• Byte order is not the same in every service. The three Bluetooth SIG services are little-endian, as their specifications require. The Zycle proprietary service is big-endian.
• Octet offsets are zero-based and count from the first octet of the characteristic value.
• Hexadecimal is written 0x2AD2 for scalars and as space-separated octets, 74 08 A8 0C, for frames.
• 16-bit UUIDs are shown in their short form. The full 128-bit form is the usual Bluetooth Base UUID expansion, 0000xxxx-0000-1000-8000-00805F9B34FB.
• Shall and must mark a requirement a client has to meet for the integration to work; should marks strong advice; may marks something optional.

1.5 Support

Zycle publishes this document so that third-party products can communicate appropriately with the machine. For questions about this guide, and requests for behaviour it does not describe, submit a request at https://help.zycle.eu/hc/requests/new.

Zycle does not provide support for third-party applications, nor for code written with AI tools from this guide.

2 The machine at a glance

2.1 Services exposed

The ZBike Infinity presents several GATT services. Three of them are standard Bluetooth SIG services that any client may use, one is the Zycle proprietary service, and the rest are other vendors' proprietary services which a third-party client should discover and ignore.

2.2 Which service to use for what

2.3 A minimum viable integration

An application that wants to show power and cadence and control the machine needs nothing beyond the Fitness Machine Service:

1. Scan for a peripheral advertising the complete local name ZInfinity, or advertising the Fitness Machine service UUID 0x1826.
2. Connect and request an ATT MTU of 185 octets.
3. Discover services and characteristics.
4. Subscribe to indications on the Fitness Machine Control Point before writing to it, and to notifications on Indoor Bike Data.
5. Write Request Control, then whichever target-setting procedure your application needs.
6. Read Indoor Bike Data notifications, which arrive once per second.

Sections 3 and 4 give the detail. Everything else in this guide is either a different way to get the same data, or data that the standard services cannot express.

3 Connecting

3.1 Advertising and discovery

The machine advertises continuously, as a connectable and scannable undirected advertiser on the 1M PHY, at an interval of 250 ms and a transmit power of +8 dBm. Advertising does not time out. It stops only when the machine is holding its maximum number of connections (section 3.5), and resumes as soon as a connection is dropped.

Filter on the device name or on 0x1826 / 0x1818. Do not filter a scan on the Device Information UUID or on the Zycle service UUID: neither appears in the advertising data, so a scan filtered on them will never find the machine. The appearance value is in the scan response, so a client that filters on appearance must perform an active scan.

The name is fixed in firmware and is the same on every unit, so it identifies the model but not the individual machine. Where a rider owns more than one, distinguish them by the identifier your platform gives the device, or read the serial number from the Device Information Service after connecting (section 6). Note that iOS and Web Bluetooth do not expose the Bluetooth address (section 9).

3.2 Connection establishment and parameters

The machine is a peripheral and never initiates a connection. Its preferred connection parameters are:

A client may connect with different parameters. If it does, the machine asks for its own: five seconds after connecting it sends a connection-parameter update request, and repeats it every thirty seconds while the request goes unaccepted, up to three times.

The machine produces one notification per second on each subscribed characteristic (section 3.6), so a connection interval much longer than 100 ms combined with a supervision timeout shorter than four seconds will start dropping them.

3.3 ATT MTU

ATT carries every notification in a single packet whose payload is the negotiated MTU minus three octets of ATT header. The default MTU is 23 octets, which leaves 20 octets of payload. A notification longer than that is not fragmented and not truncated: the machine simply cannot send it, and the client sees nothing at all on that characteristic while everything else keeps working. The symptom is silence on the long characteristics only, which looks like a subscription failure rather than a size problem.

The MTU is negotiated once per connection, by an ATT Exchange MTU Request that the client sends; the peripheral cannot start it. The result is the smaller of the two sides' maxima. This machine supports 185 octets, giving 182 octets of payload, which is enough for every frame in this guide.

Request the largest MTU your stack allows, and do it before writing to any Client Characteristic Configuration descriptor. Requesting 185 or more is enough; asking for more than the machine supports is harmless. A client that only uses the standard services works at the default MTU, but cannot receive the two long Zycle frames. On some platforms the operating system negotiates the MTU itself (section 9).

3.4 Security, pairing and bonding

Every characteristic described in this guide is open: no authentication, no authorisation, no encryption requirement. The machine actively declines pairing (a pairing request is answered with Pairing Not Supported) and it stores no bonds. A client shall not initiate pairing or bonding.

Because there is no bonding, nothing ties a machine to a particular client, and any client in radio range can connect and take control. Section 3.7 describes how the machine arbitrates between clients.

3.5 Concurrent connections

The machine accepts two simultaneous connections and advertises whenever it holds fewer than two. This is deliberate: a rider may run a game on one connection and the Zycle app on the other, or a training application and a recording head unit.

• Both connections see the same machine state, and both receive notifications independently.
• Neither connection is told that the other exists. A client subscribed to Fitness Machine Status is told when the other changes a target (section 4.6); nothing else the other connection does is announced.
• Only one of them can control the machine at a time. Which one is decided by the rule in section 3.7, and a client that loses is told so in the control point's response.
• A client shall therefore treat every value it reads as the machine's state rather than as an echo of what it last commanded, and shall not assume a target it set is still in force.

3.6 Notification timing

Almost every notification on the machine happens once per second. Each subscribed characteristic is sent once per second, whether or not its contents have changed, on each connection that subscribed to it. There is no way to ask for a faster or slower rate, and none of these characteristics notifies on change.

There are three exceptions:

• The Fitness Machine Control Point is an indication sent in answer to a write, once per write and never repeated.
• The Fitness Machine Status characteristic is a notification sent at the moment the machine's state changes, once and never repeated (section 4.6).
• The Zycle Power Diagram is driven by crank rotation, not by the timer: it arrives roughly once per pedal revolution while the rider is pedalling, and not at all when they stop.

3.7 Control arbitration between clients

The machine can be commanded from more than one place: either of its two Bluetooth connections, and ANT+. Allowing two of them to adjust the resistance at the same time would be troublesome, so the machine arbitrates. The rule is first come, first served, held until release:

• When no one holds control, the first command received takes it.
• While a source holds control, its own commands are accepted and everyone else's are refused with 0x05 — Control not permitted.
• A Bluetooth client holds control until it disconnects. It does not time out, because a simulator may legitimately go quiet for a long time.
• Control is per source, not per channel. An application that commands the machine over more than one protocol on the same connection counts as one source and does not compete with itself.

A client that is refused should keep reading data, which is never gated by arbitration, and may keep retrying: as soon as the other client disconnects, the next command succeeds.

A client shall not try to force control by disconnecting and reconnecting. It does not help: reconnecting does not displace a controller that is still connected.

4 Fitness Machine Service

The Fitness Machine Service, UUID 0x1826, is the primary interface to the machine. It carries almost every quantity a training application needs and it is the only way to control the resistance. A client that implements nothing else can still be a complete training application.

4.1 Characteristics

Every value in this service is little-endian.

4.2 Fitness Machine Feature

Two 32-bit fields, read once. The values are fixed in firmware and do not change at run time. A client may hard-code its behaviour instead of reading them, but reading them is the better habit, because it keeps the client correct across firmware versions.

Fitness Machine Features   86 50 00 00   =  0x00005086
Target Setting Features    0C 20 00 00   =  0x0000200C

Every other bit is zero, and every zero bit means what it says. In particular the machine does not declare Average Speed, Inclination, Elevation Gain, Expended Energy, Heart Rate, Remaining Time, Spin Down Control or Wheel Circumference Configuration, and it implements none of them. A client shall not attempt a procedure whose feature bit is clear.

4.3 Indoor Bike Data

This is the characteristic a client spends its life reading. It notifies once per second, always fifteen octets, always with the same flags and therefore always with the same fields in the same places.

4.3.1 Frame layout

4.3.2 Worked example

A rider turning 90 rpm at 250 W, half an hour into a session, having covered 16.533 km since the machine was powered on:

74 08 A8 0C B4 00 95 40 00 54 00 FA 00 2D 07

Note the byte order in every multi-octet field: A8 0C is 0x0CA8 = 3240, not 0xA80C.

4.3.3 What the fields mean on this machine

• Total Distance is a partial odometer. It starts from zero on every reboot and counts with every pedal stroke. It is not the lifetime odometer of the bike, which is in Zycle Information (section 7.2).
• Instantaneous Speed is a simulated speed. There is no real wheel on this machine. The value is the road speed the machine calculates from the current cadence, the engaged virtual gear and the configured wheel diameter. Outside simulation mode, where virtual gears do not apply, a standard fixed gear ratio is assumed.
• Resistance Level is what is applied. It is the physical position of the brake, scaled to 0–200. In power and simulation modes it moves continuously as the machine works to hold the target.
• Elapsed Time is the time spent actually pedalling. It starts from zero on every reboot, begins counting the first time pedalling is detected, and only counts time spent pedalling.

4.4 Supported ranges

Two read-only characteristics report the limits of the target-setting procedures. Each is three little-endian 16-bit values: minimum, maximum and minimum increment.

The resistance range, the Resistance Level field of Indoor Bike Data and the Set Target Resistance Level operand all use the same plain integer 0–200 scale, so a client can write a value, read it back from the notification and compare like with like.

Note for generic FTMS libraries: the FTMS specification defines a resolution of 0.1 for the resistance range and for the Set Target Resistance Level operand. On this machine the raw values are used as plain integers: the maximum is 200, and writing 150 sets 150 of 200. Do not multiply or divide by ten.

The power range is a declaration of the control range, not a promise about the rider. A target of 4000 W will be accepted; what the rider actually produces is another matter. At the other end, a target of 0 W is accepted and releases the brake as far as it goes.

4.5 Fitness Machine Control Point

All control goes through this one characteristic. A client writes a one-octet op code followed by that op code's operands, and the machine answers with a three-octet indication.

4.5.1 Using the control point

1. Subscribe to indications on the control point before writing to it. The machine answers every write with an indication. A client that has not enabled the characteristic's Client Characteristic Configuration descriptor gets no answer, and cannot tell a command that worked from one that was refused.
2. Use Write Request, not Write Without Response. Write Without Response is not among the control point's properties.

A write is one op code octet followed by that procedure's operands. Operands are little-endian. The machine reads exactly as many operand octets as the op code defines and ignores anything beyond them. Write the exact length the op code calls for: a shorter write is not padded with zeros, and what the machine reads in place of the missing octets is undefined.

The response is always three octets: 80 (Response Code), the op code of the request, and the result code (section 4.5.6).

4.5.2 Request Control — op code 0x00

The specification requires a client to take control before issuing any target-setting procedure.

write        :  00
indication   :  80 00 01        success

4.5.3 Set Target Resistance Level — op code 0x04

One operand octet, an integer on the 0–200 scale of section 4.4. Writing it puts the machine into basic resistance mode, in which the brake holds a fixed position and the power the rider produces is whatever that position and their cadence produce. Values above 200 are clamped to 200.

write        :  04 96           set resistance to 150 of 200
indication   :  80 04 01        success

This is the right mode for a manual resistance control, and the wrong mode for anything that requires a specific power or specific simulation conditions such as a gradient.

4.5.4 Set Target Power — op code 0x05

Two operand octets in watts, a little-endian signed 16-bit value. Negative values are clamped to zero. Writing it puts the machine into power mode (ERG mode), in which the machine continuously adjusts the brake to hold the requested power regardless of cadence.

write        :  05 2C 01        target power 300 W  (0x012C little-endian)
indication   :  80 05 01        success

A client may re-send the target it already set. There is no separate "ERG off" procedure: to leave power mode, write one of the other two target-setting procedures; each switches the machine into its own mode.

4.5.5 Set Indoor Bike Simulation Parameters — op code 0x11

Six operand octets describing the virtual road. This is the procedure a game or a route simulator uses: the machine computes the resistance from the gradient, the rider's power, the engaged virtual gear and the coefficients below.

write        :  11 00 00 2C 01 33 33
                op wind  grade Crr Cw

wind speed   :  0.000 m/s
grade        :  +3.00 %
Crr          :  0.0051
Cw           :  0.51 kg/m

indication   :  80 11 01        success

Grade is signed: a descent is a negative value in two's complement, so −3.00 % is −300, which is 0xFED4, written D4 FE.

All four operands are applied together, so a client that wants to change only the gradient must resend the coefficients with it. The machine keeps the last values it was given; it does not merge a partial update.

There is no separate "simulation off" procedure: to leave simulation mode, write one of the other two target-setting procedures.

4.5.6 Responses and result codes

A write that changed something is followed by a Fitness Machine Status notification (section 4.6):

write        :  05 2C 01        set target power
indication   :  80 05 01        applied
notification :  08 2C 01        target power changed

write        :  07              Start or Resume (not implemented)
indication   :  80 07 02        op code not supported

write        :  05 2C 01        set target power
indication   :  80 05 05        refused: another client is in control

4.5.7 Procedures the machine does not implement

Every other op code defined by the specification is answered 0x02 — Op Code not supported — and has no effect. That includes Reset, Set Target Speed, Set Target Inclination, Set Target Heart Rate, Start or Resume, Stop or Pause, the targeted energy, step and stride procedures, Set Wheel Circumference and Spin Down Control.

Two consequences are worth spelling out. First, there is no way to start, stop, pause or reset the machine over Bluetooth. Second, even though it may sound appropriate, Set Target Inclination is not the way to send a gradient: Set Indoor Bike Simulation Parameters is (section 4.5.5).

4.6 Fitness Machine Status

The Fitness Machine Status characteristic, UUID 0x2ADA, is how the machine announces a change to its own state, as opposed to answering a request. It is notify-only: subscribe to it, and expect nothing from a read.

• The notification reaches every subscribed connection, including the one whose write caused the change. It is the only way a client learns that the other connection (section 3.5) has moved the target.
• It is event-driven, not periodic: it is sent once, at the moment the change takes effect.
• The frame is one op code octet followed by that op code's parameter, little-endian, with no flags and no length prefix: one to seven octets in total.

The machine sends three of the op codes the specification defines:

The indication always comes first and the status notification second. A client may treat the indication as the answer to its own write, and the notification as a statement about the machine:

write        :  05 2C 01              set target power 300 W
indication   :  80 05 01              accepted
notification :  08 2C 01              target power changed

write        :  04 96                 set resistance level 150
indication   :  80 04 01              accepted
notification :  07 96                 target resistance level changed

write        :  11 00 00 2C 01 33 33
indication   :  80 11 01              accepted
notification :  12 00 00 2C 01 33 33

A procedure that was refused (0x05) or is not implemented (0x02) changed nothing, and produces no status notification.

5 Cycling Power Service

The Cycling Power Service, UUID 0x1818, presents the machine as a power meter. It is useful when the client is a head unit or recorder that speaks the power-meter profile and not FTMS, and when the client wants crank revolution counts rather than a cadence figure. For everything else, the Fitness Machine Service is the better source, and the two agree.

5.1 Characteristics

The Cycling Power Control Point is not present. The service is read and notify only.

5.2 Cycling Power Feature

The feature value is 0x0000000C:

Pedal Power Balance, Accumulated Torque, the extreme magnitude and angle fields, Accumulated Energy and offset compensation are not declared and not present. The pedal-stroke information is not here; the machine exposes an enhanced version of it through the Zycle service instead (section 7.3).

5.3 Cycling Power Measurement

Fourteen octets, notified once per second, with a fixed flags value and therefore a fixed layout.

An example frame from the same rider as section 4.3.2 (250 W, 90 rpm, half an hour in):

30 00 FA 00 DB 60 00 00 00 68 C3 0A 00 B4

5.4 Deriving cadence from crank revolutions

Cadence is not a field in this service. A client that has any use for the Fitness Machine Service should take cadence from Indoor Bike Data instead, where it is a plain field. If strictly needed, cadence can be computed from two notifications by dividing the change in crank revolutions by the change in crank event time:

cadence [rpm]  =  (revs2 - revs1) * 60 * 1024 / (time2 - time1)

notification 1 :  30 00 FA 00 DB 60 00 00 00 68 C3 0A 00 B4
notification 3 :  30 00 FA 00 F6 60 00 00 00 78 C6 0A 00 BC

d revs         :  3
d time         :  2048 counts = 2.000 s
cadence        :  90.0 rpm

Handle the wrap-around of both counters (use unsigned 16-bit subtraction). The two frames above are two notification intervals apart, because 90 rpm is 1.5 revolutions per second and a whole number of revolutions needs an even number of seconds. That is the general case: the counters advance in whole revolutions, so consecutive notifications give a coarse figure, and a client that wants a stable cadence should compute it over a short window of frames. The machine may repeat an identical frame, since it notifies on a timer rather than on a crank event; ignore frames whose crank event time has not changed.

5.5 The wheel revolution fields on this machine

There is no physical wheel on the machine. The rider's cranks drive a flywheel through a fixed pulley ratio, so Cumulative Wheel Revolutions reports flywheel revolutions. It is not a road-wheel count, it does not account for the engaged virtual gear, and no wheel circumference turns it into a road distance. For virtual speed, use Indoor Bike Data.

6 Device Information Service

The Device Information Service, UUID 0x180A, carries five read-only strings. It is not advertised; discover it after connecting. Its main use to a third-party client is reading the firmware version and the serial number, to tell one machine from another.

The Zycle service offers the firmware version a second way, as two numeric octets (section 7.4).

7 Zycle service

The Zycle proprietary service carries three things the standard services cannot express: which mode the machine is in, which virtual gear the rider has engaged, and the shape of their pedal stroke. A client that does not need any of those can ignore this service completely and lose nothing else.

This service provides no control. Every published characteristic in it is read-only or a diagnostic channel. Target power, resistance and simulation parameters all go through the Fitness Machine Control Point (section 4.5), on the same connection, at the same time.

Only part of the service is published. The rest are factory configurations and procedures, reserved to Zycle applications and not documented here. The published subset is complete in itself: nothing in this section depends on anything that is not in it.

7.1 Service and characteristics

The service and its characteristics use 128-bit vendor UUIDs sharing a common base. Each characteristic differs from the others only in the third and fourth octets, shown below as a 16-bit alias.

service          db6dba4b-a18d-44ae-90f1-38d01dec8f51
characteristics  db6dXXXX-a18d-44ae-90f1-38d01dec8f51
                     ^^^^  the 16-bit alias from the table below

This service is big-endian, the opposite of the three standard services. The one exception is noted in section 7.2.1.

The service UUID is not advertised, so a client discovers it after connecting. If it is absent, the firmware predates it; a client should treat the service, and each characteristic in it, as optional and fall back to the standard services.

7.2 Information

Fifteen octets, notified once per second while subscribed. Subscribe rather than read.

7.2.1 Bike Mode and the two parameters

Bike Mode says what the machine is currently doing, and decides what the two parameter octets mean. The three modes below are the ones a client will see on a machine being ridden.

The mode follows whichever target-setting procedure was last accepted: for example, a client that sets a target power will see the mode become 0x02 within a second.

Any other value means the machine is not in a state a rider can ride (starting up, in error, etc.). A client shall treat an unrecognised mode as "not ridable" and ignore the two parameter octets.

In power mode the target is a 16-bit value split across the two octets, and, uniquely in this service, it is little-endian: Parameter 1 carries the low octet and Parameter 2 the high octet.

Information   :  00 FA 00 5A 00 12 D6 87 02 2C 01 02 00 02 DE
                                            ^^ ^^  parameters 1 and 2

target power  =  parameter1 | (parameter2 << 8)
              =  0x2C | (0x01 << 8)  =  300 W

7.2.2 Balance and Smoothness

Both are pedalling-quality estimates computed on the machine, in units of 0.1 %.

• Balance is the share of the revolution's power contribution attributable to the left half of the pedal stroke, derived from the same per-revolution torque samples that produce the Power Diagram. 500 is even.
• Smoothness is a proprietary 0–1000 score in which higher is smoother.

Both are reset when the machine detects that the rider has stopped: Balance returns to 500 and Smoothness to 0.

7.2.3 Worked example

The same rider as section 4.3.2, in simulation mode with the first chainring and the eighth sprocket engaged:

00 FA 00 5A 00 12 D6 87 04 01 08 02 00 02 DE

7.2.4 Using the gear indices

The indices are positions in the machine's configured drivetrain, numbered from one: not tooth counts and not ratios. Index 1 always exists and index 0 never occurs.

The two indices count in opposite directions, as is common in cycling: chainrings are ordered from fewest teeth upwards, so chainring 1 is the smallest, whereas sprockets are ordered from most teeth downwards, so sprocket 1 is the largest. That way, the lower the number, the easier the gear.

The drivetrain itself (number of chainrings and sprockets, and teeth on each) can only be configured from the Zycle app. For reference, there is a maximum of 24 sprockets for single-chainring configurations and 12 sprockets for double-chainring configurations, and the bike may be configured with as few as 9 sprockets in any configuration.

7.3 Power Diagram

One hundred octets describing the rider's power through a single crank revolution: twenty-five pairs, each an angle followed by a power value, both uint16 big-endian.

octets 0-3    : angle 0,   power at 0°
octets 4-7    : angle 15,  power at 15°
...
octets 92-95  : angle 345, power at 345°
octets 96-99  : angle 0,   power repeated from the first point (padding)

The twenty-fifth pair is padding, not a data point: its angle is 0 and its power repeats the first point's value, so that a client which draws the pairs in order closes the curve.

When drawing the diagram as a polar plot, scale the radial axis so that it does not start at zero watts; otherwise the differences in power through the stroke are too small to see and the diagram looks like a circle.

This characteristic is driven by crank rotation, not by the one-second timer. While the rider is pedalling it updates roughly once per revolution; when they stop, notifications stop. At one hundred octets, it is also the frame most likely to be blocked by an unnegotiated ATT MTU (section 3.3).

7.4 Command

The Command characteristic is a request-and-response channel. A client writes a two-octet big-endian command value, optionally followed by parameters, using Write Without Response, and reads the response from a notification on the same characteristic. Subscribe to notifications before writing.

At the moment only one command is available to third-party clients. Every other command value is reserved: do not send it.

write        :  00 01

notification :  01 27 00 00 00 00 00 00 00 00 00 00 00 00
                00 00 00 00 00 00 00 00 00 00 00 00 00 00

firmware     :  0x01 . 0x27  =  version 1.39

The first octet is the major version and the second the minor, each as a plain integer. This is the same version the Device Information Service reports as a string (section 6). Responses are always twenty-eight octets, zero-padded after the meaningful part.

8 Worked integrations

This section is informative. It puts the preceding sections together for the three kinds of application that account for almost every integration.

8.1 A structured-workout application

An application that drives the rider through target powers (intervals, ramps, a training plan) needs the Fitness Machine Service and nothing else.

1. Scan for ZInfinity or for service 0x1826, and connect.
2. Request an ATT MTU of 185 octets.
3. Discover services and characteristics.
4. Subscribe to indications on the Control Point before writing to it, and to notifications on Indoor Bike Data.
5. Read Supported Power Range and clamp your targets to it (0–4000 W on present firmware).
6. Write Request Control (00) and check the indication.
7. Write Set Target Power (05 plus the target, little-endian) at the start of each interval.
8. Read power and cadence from the Indoor Bike Data notifications, once per second.

To end the session, stop writing targets. There is no "stop" procedure.

8.2 A route or gradient simulator

A game or a route replay wants the rider to feel the terrain, and to be able to shift gears.

1. Connect and subscribe exactly as in 8.1.
2. Write Set Indoor Bike Simulation Parameters (11) with the gradient at the rider's current position, together with the wind speed and the rolling and wind resistance coefficients you have chosen for the rider and bike. All four are sent together every time; the machine does not merge partial updates.
3. Re-send whenever the gradient changes.
4. Read power and cadence from the Indoor Bike Data notifications, once per second. If desired, also read speed, which takes the engaged gear into account and is therefore the most realistic source.
5. If the application shows the engaged gear, subscribe to Zycle Information and read the two mode parameters (section 7.2).

8.3 A recorder or head unit

An application that only records what the rider does, or a head unit that treats the machine as a power meter, needs no control at all.

1. Connect. Subscribe to Indoor Bike Data (recommended if the application records cadence) or to Cycling Power Measurement.
2. To show a pedal-stroke diagram, subscribe to notifications on the Zycle Power Diagram characteristic (this needs the larger MTU, section 3.3).
3. If desired, take distance from Zycle Information. It is a lifetime odometer; subtract the initial value for a session distance.
4. Do not write to the Control Point.

This integration can safely run alongside any other client, as it never attempts to control the bike.

9 Platform notes

This section is informative. It collects the platform details that most often stop a first integration from working.

9.1 Web Bluetooth (browser apps)

• Works in Chrome and Edge on Windows, macOS, ChromeOS and Android. It does not work in Safari, in Firefox, or in any browser on iPhone and iPad.
• The page must be served over HTTPS (or from localhost), and navigator.bluetooth.requestDevice() must be called from a user action such as a button click.
• List every service you will use in optionalServices, or the browser will refuse access to it after connecting:

const device = await navigator.bluetooth.requestDevice({
  filters: [{ name: 'ZInfinity' }],
  optionalServices: [0x1826, 0x1818, 0x180a, 'db6dba4b-a18d-44ae-90f1-38d01dec8f51']
});

• There is no API to request the MTU: the browser and operating system negotiate it. If the Zycle Power Diagram or Command notifications never arrive, the MTU negotiated on that platform is too small.
• Write to the FTMS Control Point with writeValueWithResponse(), and to the Zycle Command characteristic with writeValueWithoutResponse().
• Chrome blocks reading the Serial Number String (0x2A25). Use device.id to tell machines apart.

9.2 Android

• Request the runtime permissions BLUETOOTH_SCAN and BLUETOOTH_CONNECT (Android 12 and later), or location permission on older versions.
• Call requestMtu(185) (or higher) after connecting and wait for onMtuChanged before enabling notifications.
• Android performs one GATT operation at a time: queue reads, writes and descriptor writes, and wait for each callback before starting the next.

9.3 iOS and macOS (Core Bluetooth)

• The operating system negotiates the MTU automatically; maximumWriteValueLength(for:) shows the result.
• The Bluetooth address is not exposed. Use the peripheral identifier (stable on one device) or the serial number from the Device Information Service to tell machines apart.
• Declare NSBluetoothAlwaysUsageDescription in the app's Info.plist.

9.4 Desktop and scripts

On Windows, macOS and Linux, libraries such as Bleak (Python) let the operating system negotiate the MTU. Check that the Power Diagram arrives to confirm it is large enough.

10 Troubleshooting

Annex A — Test vectors

Every frame in this guide, collected for use as unit-test fixtures. The same vectors are available as a JSON file.

A.1 Fitness Machine Service

A.2 Cycling Power Service

A.3 Zycle service

Annex B — Quick reference

B.1 Identifiers

B.2 Limits and constants

Annex C — Revision history