Sunny Health & Fitness Synergy Pro SF-B1851 Indoor Cycle

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Sunny SF-B1851

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300 lbs40 lbsManual

The Sunny SF-B1851 is one of the bikes from the Sunny Health & Fitness Synergy series similar to the Sunny SF-B1879. This is the superior (Pro) model, though, the one equipped with a heavier flywheel, dual-sided pedals, and a more inclusive fitness monitor. It’s an exercise bike designed for spin workouts; it’s not a commercial-grade model but does offer a sturdiness and range of features that are pretty close to at least a light-commercial model.

Key Features and Specifications

  • Assembled dimensions: 49L x 22W x 46.5H inches (124L x 56W x 118H cm)
  • Assembled weight: 111.3 lbs (50.4 kg)
  • Flywheel weight: 40 lbs (18.1 kg)
  • Weight capacity: 300 lbs (136 kg)
  • Seat: 4-way adjustable
  • Drive: Belt
  • Resistance: Manual magnetic
  • Fitness monitor: Time, speed, distance, cadence, calories, pulse, race, goals
  • Extras: Bottle holder, device holder, dumbbell holders
  • Warranty: Frame 3 years / Parts 180 days

Frame construction

The Sunny Health & Fitness Synergy Pro SF-B1851 indoor cycling bike features a pretty standard triangular frame. It combines oval and rectangular tubing, precision-welded for improved sturdiness. The main frame piece is double-coated with a chip-resistance and corrosion-resistant paint finish, gray in color. The seat post, the handlebar post, and the flywheel are chrome plated.

Of course, the bike also incorporates a few plastic parts. For example, the flywheel side covers, the drivetrain cover, and several other small covers are made of durable thermoplastic material.

The bike is not too large. With all the parts assembled, it is only 49″ (124 cm) long, and 22″ (56 cm) wide. It is recommended, though, to allow at least 24″ (61 cm) on each side of it, for ease of access and safe usage.

The fully assembled unit is pretty heavy as well. Its net weight is 111.3 lbs (50.4 kg). All this weight, plus the low center of gravity, results in a superior stability. So, even if you engage in the most demanding spin workouts, the bike shouldn’t rock or move. Plus, both base stabilizers have adjustable levelers.

Due to its weight, the boxed unit may require two people for transport. However, once the bike is assembled, it can easily be moved by only one person. The front base stabilizer is equipped with two large silicone wheels.

Seat and handlebars

The Sunny Health & Fitness SF-B1851 features a four-way adjustable seat. Its upright post has multiple adjustment holes while its horizontal slider adds 5 extra front-to-back adjustment positions. Therefore, the applicable user inseam range for this bike is 29″ – 39.5″ (73 – 100 cm). Differently put, the bike should be able to accommodate users between 5’1″ and 6’5″ (155 – 196 cm). It is also rated for a maximum user weight limit of 300 lbs (136 kg).

The included saddle features a racing design. Its internal shell is made of durable nylon material while its cushioning section consists of a thick urethane layer. It has a rather wide median cutout for improved breathability and reduced soreness. It also has a standard collar-type attachment system so compatible options are easy to find.

The bike’s handlebar features a multi-grip design. It offers the possibility to adopt a close grip, a climbing grip, a racing grip, a cruising grip, and an aero grip. It also integrates a device holder.

The handlebar is fully coated with a layer of high-density foam material for an improved grip. Its side grips incorporate metallic pulse sensors. It is only two-way adjustable, though. It does not have a horizontal slider like the seat. It offers an adjustment span of approx. 5.5″ (14 cm), from 41″ to 46.5″ (104 – 118 cm).

Resistance system

Similar to the Sunny SF-B1918, the Sunny Synergy Pro+ exercise bike is equipped with a manual magnetic resistance system. This means you don’t need a power adapter for it. The fitness monitor cannot adjust the resistance automatically and does not have pre-set programs.

The resistance system consists of a magnetic brake placed above the flywheel and connected to the tension knob that passes through the main tube of the frame. By turning this knob clockwise, the magnets move closer to the flywheel increasing the pedaling difficulty.

The tension knob does not have pre-determined resistance levels. So, you basically have to turn it right or left until you find the difficulty level that suits you best. It does have the function of an emergency brake, though. You can press it down to stop the flywheel instantly.

Needless to say, the main advantage of the magnetic brake is that it eliminates the friction between the pad and flywheel. This leads to a very smooth pedaling motion. Also, the usage of the bike is very quiet and you won’t have to worry about replacing the brake pad as is the case for bikes with a friction resistance system.

Drive system

As we’ve mentioned above, the Sunny Synergy Pro+ indoor cycling trainer comes with a heavier flywheel than the Sunny Synergy SF-B1879. It weighs 40 lbs (18.1 kg) and has a perimeter-weighted design. This leads to a consistent momentum and a higher pedaling difficulty.

The flywheel is bi-directional. However, just as for many other spin bikes, coasting is not possible. When pedaling in reverse, you will get the same resistance as when pedaling forward, given by the position of the magnets in relation to the flywheel.

The bike is equipped with a belt drive system. It uses a heavy-duty poly-v rubber belt. This type of drive system comes with a few advantages. For example, as opposed to a chain drive, you don’t have to lubricate it. Also, it contributes to the overall quietness and smoothness of the ride.

The crankset is pretty durable as well. It features oval steel arms and dual-sided pedals. They have toe cages on one side, and SPD clips on the other so you can use spin shoes if you want. Their threads are standard 9/16″ so if you will ever want to replace them, compatible options should be available in any bike shop.

Fitness monitor

The Sunny Health & Fitness Synergy Pro+ exercise bike is equipped with a fitness monitor similar to the one included with the SF-B1876 model. It features a 4-window LCD display that indicates speed, average speed, maximum speed, cadence (RPM), average cadence, maximum cadence, distance, calories, time, and pulse. Distance and speed are by default set to English units but you can also change them to metric units. Its pulse reading function only works with the integrated sensors. The unit cannot read your pulse from a wireless transmitter.

The monitor unit offers target functions for time, distance, calories, and cadence. If a target is selected for time, distance or calories, the display will count down from the selected target until it reaches zero. If a target is selected for cadence, the monitor will prompt you to increase your pedaling rhythm if your cadence falls below the selected target and slow down if your cadence exceeds the RPM.

The monitor unit does not have standard pre-set programs and cannot adjust the resistance automatically. However, it does have one Race Program. This workout mode allows you to select a target for time and distance. The cadence is variable. So, you will have to beat the computer to the selected distance by basically keeping a higher cadence than it does.

The monitor unit is battery-powered. It uses two AAA batteries which are included. It starts up when you begin pedaling, and also automatically turns off after a few minutes of inactivity on the bike.

Assembly and Maintenance

The bike’s main body comes pre-assembled; the flywheel, the drivetrain, the crankset, and the magnetic brake, and the seat are already attached. You only have to connect the two base stabilizers, the pedals, the handlebar, and the fitness monitor. This shouldn’t take longer than 30 minutes. Also, all the necessary tools are included.

For maintenance, all you have to do is periodically check for loose bolts and parts and keep the bike clean. The drivetrain does not require lubrication.

The Pros

  • Heavy-duty steel frame – 300 lbs weight capacity;
  • Aesthetic design;
  • Adjustable stabilizers on both bases;
  • Transport wheels included;
  • 4-way adjustable seat;
  • Race saddle with a median groove for breathability;
  • Multi-grip handlebar with pulse sensors;
  • Smooth and silent belt drive;
  • 40 lbs, perimeter-weighted flywheel for enhanced inertia;
  • Magnetic resistance, with zero friction;
  • Performance monitor included – it tracks time, speed, distance, cadence, calories, and pulse;
  • Goal-setting function;
  • Race program;
  • Bottle holder included;
  • Device holder included;
  • Dumbbell holder included;
  • Easy to assemble;
  • Little maintenance required.

The Cons

  • No backlight on the performance monitor display;
  • The pulse and calories readings aren’t too accurate.


The Sunny Health & Fitness Synergy Pro+ SF-B1851 is an indoor cycling bike made for home use. It is equipped with a manual magnetic resistance system, a 40-lb flywheel, a 4-way adjustable seat, and a fitness monitor that tracks the most important metrics and also has a Race program. It’s a heavy-duty bike designed to support all types of spin workouts. It’s great for keeping a healthy cardiovascular shape, improve strength and stamina, tone muscles, and lose weight.

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