Sunny SF-B1851 Pro Indoor Cycling Bike | Innovative Flywheel

Start a new in-home fitness routine with the Sunny SF-B1851 Pro Indoor Cycling Bike. The Synergy Pro bike is among the most advanced Sunny indoor exercise bikes. Ergonomically designed, this bike has a rounded multi-grip handlebar and a 4-way adjustable seat to help maintain your comfort and stability while exercising.

The highlight of the SF-B1851 Synergy Pro bike is its exclusive innovative patented flywheel design based on Sunny’s years of experience and research. The 40 lbs (18.1 kg) flywheel adds more momentum and smoothness than standard flywheels.

The Sunny SF-B1851 Pro indoor cycling bike uses a belt drive magnetic resistance system that works perfectly with Sunny’s flywheel, making the ride remarkably smooth and quiet. Twist the resistance knob to increase or decrease the resistance to keep your riding workout challenging and effective.

Monitor your workout efficiency by tracking your average and max speed and cadence on the SF-B1851 fitness monitor. Then, use the speed and cadence data to assess and analyze your general fitness progress or cycling performance. Cycling on an indoor bike is a low-impact exercise that can strengthen cardiovascular performance and engage muscles in your arms, core, and legs.

Learn more in my complete review below!

Sunny SF-B1851 Pro Indoor Cycling Bike
Sunny SF-B1851 Pro Indoor Cycling Bike

Product:  Sunny SF-B1851 Pro Indoor Cycling Bike

Price: For a great deal on this bike, merely go here to Sunny Health &

Warranty: Frame 3 years / Parts 180 days

My rating: 9.25/10

Affiliate Disclosure: As a Sunny Health & Fitness Affiliate Program participant, I earn commissions on purchases made through links in the post. Full disclosure

Details 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

SF-B1851 Frame Construction

Sunny SF-B1851 Pro Indoor Cycling Bike

The Sunny SF-B1851 Pro indoor cycling bike features a pretty standard triangular frame. It combines oval and rectangular tubing, precision-welded for improved sturdiness. The mainframe piece comes double-coated with a chip-resistance and corrosion-resistant gray paint finish. 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 materials.

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 advisable to allow at least 24″ (61 cm) on each side of it for easy 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 and the low center of gravity resulting in 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, two people should transport the boxed unit. However, once assembled, one person can quickly relocate the bike. The front base stabilizer comes equipped with two large silicone wheels.

Seat and handlebars

The Sunny SF-B1851 Synergy Pro features a four-way adjustable seat. Its vertical post has multiple adjustment holes, while its horizontal slider adds five front-to-back adjustment positions.

Therefore, the applicable user inseam range for this bike is 29″ – 39.5″ (73 – 100 cm). The bike can comfortably accommodate users between 5’1″ and 6’5″ (155 – 196 cm). It also has a maximum user weight limit of 300 lbs (136 kg).

Sunny SF-B1851 Pro Indoor Cycling Bike
Sunny SF-B1851 Pro Indoor Cycling Bike

The included saddle features a racing design. Its internal shell is a durable nylon material, while its cushioning section consists of a thick urethane layer. It has a relatively wide median cutout for improved breathability and reduced soreness. It also has a standard collar-type attachment system, making compatible options 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 comes with a layer of high-density foam material to improve grip. Its side grips incorporate metallic pulse sensors. It is only two-way adjustable and 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).

SF-B1851 Resistance system

The Sunny SF-B1851 Pro indoor cycling bike has a manual magnetic resistance system. Therefore, you don’t need a power adapter for it. But unfortunately, 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 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.

The main advantage of the magnetic brake is that it eliminates the friction between the pad and flywheel, which leads to a very smooth pedaling motion. Also, the bike usage 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

Sunny SF-B1851 Pro Indoor Cycling Bike

The Sunny SF-B1851 Synergy Pro bike has a heavy 40 lbs (18.1 kg) flywheel and a perimeter-weighted design, leading to consistent momentum and more incredible pedaling difficulty.

The flywheel is bi-directional. However, 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 to the flywheel.

The bike comes 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 that you can use spin shoes. Their threads are standard 9/16″, so compatible options should be available in any bike shop if you ever want to replace them.

SF-B1851 Fitness monitor

The Sunny SF-B1851 Pro indoor cycling bike has a fitness monitor with a 4-window LCD that indicates speed, average speed, maximum speed, cadence (RPM), average cadence, maximum cadence, distance, calories, time, and pulse.

The distance and speed are in English units, but you can change them to metric units. Its pulse reading function only works with integrated sensors in the handlebars. The monitor cannot read your pulse from a wireless transmitter.

The fitness monitor offers time, distance, calories, and cadence targets. After choosing a time, distance, or calorie target, the display will count down from the selected target until it reaches zero.

For example, when selecting a target for cadence, the monitor will prompt you to increase your pedaling rhythm if your cadence falls below the chosen target and slow down if your cadence exceeds the RPM.

Sunny SF-B1851 Pro indoor cycling bike
Sunny SF-B1851 Pro Indoor Cycling Bike

The monitor 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 specified distance by keeping a higher cadence than it does.

The monitor is battery-powered, using two AAA batteries with the bike. It starts when you pedal and automatically turns off after a few minutes of inactivity on the bike.

SF-B1851 Assembly and Maintenance

The bike’s main body comes pre-assembled; the flywheel, the drivetrain, the crankset, the magnetic brake, and the seat are pre-attached. Then, you only have to connect the two base stabilizers, the pedals, the handlebar, and the fitness monitor. Therefore, it shouldn’t take longer than 30 minutes. Also, all the necessary tools for assembly come with the bike.

For maintenance, you must periodically check for loose bolts and parts and retighten or replace them as needed. Lastly, you keep the bike clean. The drivetrain does not require lubrication.


Sunny SF-B1851 Pro Indoor Cycling Bike
  • 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;
  • Fitness 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 is required.


  • The fitness monitor LCD does not have a backlight;
  • The pulse and calorie readings aren’t too accurate.

Sunny SF-B1851 Pro Indoor Cycling Bike Summary

The Sunny SF-B1851 Synergy Pro is a home-use indoor cycling bike. It has a manual magnetic resistance system, a 40-lb flywheel, a 4-way adjustable seat, a fitness monitor that tracks the most important metrics, and a Race program.

Moreover, it’s a heavy-duty bike that supports all types of spin workouts. As a result, it’s great for keeping a healthy cardiovascular shape, improving strength and stamina, toning muscles, and losing weight.

I hope you found this article helpful and informative. Would you please speak up if you have anything you’d like to share on the topic or any opinions about my website? I look forward to your comments, questions, and the sharing of ideas.

Please remember to consult your physician to determine if you have any medical or physical conditions that could put your health and safety at risk or prevent you from using the equipment properly. In addition, your physician’s advice is essential if you are taking medication that affects your heart rate, blood pressure, or cholesterol level.

10 thoughts on “Sunny SF-B1851 Pro Indoor Cycling Bike | Innovative Flywheel”

  1. Thank you, Robert, for your complete and pure information about Indoor Cycle Bick, mainly Sunny SF-B1851 Pro. It’s one of the best out there in my research because they built a perfect seat and handlebars for this model, and you will be so comfortable during train time. Also, the 40 lbs (18.1 kg) flywheel adds additional momentum and smoothness compared to standard flywheels.

    Nice work!

    • Hi Farzad, thanks for stopping by and taking the time to comment. Would you please let me know if I can ever assist you with exercise equipment? I’m glad to be of help.

      Kind regards, Robert

  2. Hi there Robert. The Sunny SF-B1851 Pro indoor cycling bike is just what I’ve been wanting. I have started 2022 with a bang, and one of my resolutions is to take my physical fitness very seriously. I was looking for an excellent indoor cycling bike with just the features I had seen on the Sunny SF-B1851 pro. I am going to get it right away and get to work. Thanks for the detailed review here. 🙂

    • Hi Dave, thanks for stopping by. Could you please come back and let us know what you think about the Sunny SF-B1851 Pro after you’ve had it for a while? I believe you’ll be thrilled with this bike.

      Kind regards, Robert

  3. Hi. Thanks for the detailed review! I’ve wanted to purchase home gym equipment that won’t make much noise. And based on my research, a cycling bike is a great piece of equipment for beginners like me. So I clicked on the pricing, and it’s not that bad! It’s actually not that expensive compared to other indoor bikes that cost around $600-700. 

    • Hi Martina, thanks for stopping by and sharing your thoughts with us. Would you please let me know if I can assist you with the Sunny SF-B1851 or any other exercise equipment? I’m always happy to help.

      P.S. There’s a great sale going on right now. Save $320.00 off the retail price.

      Kind regards, Robert

  4. Overall this Sunny SF-B1851 Pro Indoor Cycling Bike has everything we look for in an exercise bike, especially the seat looks pretty comfortable. I did a little bit of research, and it has excellent reviews. And the price is very affordable, and it is easy to assemble. 

    Do you have any indoor cycling bike that is foldable?

    Thank you for doing this great review about this indoor cycling bike.

    • Hi there, thanks for stopping by and sharing your thoughts with us. I do not know of a foldable bike, the Sunny SF-B1851 (spin bike). However, there are many foldable upright bikes, such as the Marcy Foldable Upright Bike.

      Would you please let me know if I may further assist you with exercise equipment? I’m always happy to help.

      Kind regards, Robert

  5. Sunny has been a trusted and reliable brand for exercise equipment for many years, and are always coming out with innovative new designs. So it is not surprising to find that they have a patented flywheel design. A flywheel weight of 40lbs is quite heavy, which would result in a heavy and sturdy exercise bike. 

    The Sunny SF-B1851 Synergy Pro will be a great addition to a home gym, but I would have liked online connectivity in this innovative new bike. 

    • Hi there, thanks for stopping by and sharing your thoughts with us. Sunny has released the all-new free SunnyFit app, where you can access Sunny’s expert fitness trainers. In addition, the app allows you to track your progress online, view workout videos, and ride on various virtual trails worldwide.

      Would you please let me know if I may further assist you with exercise equipment? I’m always happy to help.

      Kind regards, Robert


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