Magnet pull needed to hold a 230g phone at 80rpm on a spin bike stem

Part of our guide on secure bike mounts for heavy phones

Find the exact magnet pull force needed to keep your phone secure on a spin bike stem at 80 rpm without slipping.

Calculating the centrifugal force on a 230g phone at 80 rpm on a spin bike stem

Centrifugal force is the main physical effect that can cause your phone to slip off a magnetic mount on a spinning bike stem. It depends on the phone's mass, the radius of rotation, and the rotational speed in rpm.

For a 230g phone (0.23 kg) mounted on a spin bike stem rotating at 80 rpm, the centrifugal force F can be estimated with the formula: F = m * r * ω², where ω is angular velocity in radians/second and r is radius in meters.

At 80 rpm, angular velocity ω = 2π * 80 / 60 ≈ 8.38 rad/s. The radius is roughly the stem radius plus half the phone width; typically stem radius is 15mm to 20mm (0.015-0.02m). Using r = 0.02m, the centrifugal force is about 0.23 * 0.02 * 8.38² ≈ 0.32 N.

This force is quite small, but the actual magnetic force needed to hold the phone must be higher to overcome vibrations, torque from handling, and case thickness reducing magnetic coupling. The other half of this decision is balancing pull force and adhesive.

What you'll need

Digital Force 50K Meter Gauge Pull & Push Magnitude Test Tester Newton Meter

Measures pull force up to 490 N, suitable for verifying magnetic hold force in newtons.

See the one to get

How magnet pull force relates to holding a phone with case thickness and stem diameter

Magnet pull force is always measured at zero gap, but your phone case typically adds a 2-5 mm gap between magnet and phone metal pieces. Magnetic pull force quickly drops off with increasing gap.

For example, a MagSafe puck rated at 7 N pull with no case can drop to 3-4 N through a 4 mm thick Otterbox-style case. Larger diameters of bike stems reduce magnetic flux concentration, requiring higher-rated magnets.

Therefore, to hold a 230 g phone through a 4 mm case on a 25 mm diameter treadmill bar or bike stem, you should look for magnets with pull forces around 8-12 N at zero gap to compensate for losses. Before you commit to anything, it is worth looking at glue strength for spin bike mount.

As an example, consider a 230g phone with a 3mm thick silicone case mounted on a 20mm diameter stem. The gap reduces the magnet's effective pull force by about 40%. If the magnet's rated pull is 7 N at zero gap, the actual pull force holding the phone may be closer to 4.2 N. If the stem diameter increases to 30 mm, the radius of rotation grows, increasing the centrifugal force proportionally, which demands even stronger magnets to maintain a secure hold. This illustrates the need to scale magnet pull ratings according to both case thickness and stem size.

What magnet pull (N) do I need to hold a 230 g phone at 80 rpm on a spin bike stem without slipping

The key figure you want is the minimum magnet pull force to resist the outward inertia at 80 rpm without the phone slipping or rotating on the stem.

Given the centrifugal force is about 0.3-0.5 N depending on stem radius, add a safety factor of 10x to overcome shocks, vibrations, and case material gaps. This yields a magnet pull force between 6-12 newtons. There is more on using magnetic plates in cases in a separate guide.

To avoid slipping, look for magnets rated at least 8 N pull at zero gap if your phone has a moderately thick case and you mount it on a stem between 25-30 mm diameter.

This ensures the magnetic mount holds securely even under dynamic gym conditions.

For instance, if the spin bike stem radius is 0.015 m rather than 0.02 m, the centrifugal force reduces to about 0.23 * 0.015 * 8.38² ≈ 0.24 N, lowering the minimum magnet pull force needed to around 5 N after applying the safety factor. Conversely, if the case thickness increases to 5 mm, the effective magnet pull can drop by over 60%, meaning a nominally 12 N magnet might only deliver about 4.8 N of holding force, which is insufficient. Checking by gently shaking the mounted phone at rest can reveal if the magnet holds securely before spinning the bike. If the phone slips or rotates under light force, a stronger magnet or thinner case is needed. We go through phone mount holding weighted phone securely step by step elsewhere on the site.

Which MagSafe puck pull force holds a 240g phone through 4mm case on 25mm treadmill bar

A 240 g phone adds just slightly more centrifugal force than a 230 g phone, roughly 0.32-0.45 N at 80 rpm on a 25 mm bar. The 4 mm case reduces actual pull force by about 50%.

A typical standard MagSafe puck rated around 7 N pull at zero gap may only provide about 3.5 N through the case. This is borderline but can work with careful alignment and minimal movement.

For reliable holding, consider a higher-rated stick-on MagSafe puck or a neodymium magnet mount rated closer to 10 N pull. These better compensate for gap losses and dynamic forces during exercise.

Which MagSafe puck holds iPhone 15 Pro Max in 4mm Otterbox on 40mm squat rack

The iPhone 15 Pro Max weighs about 240 g, similar to previous examples, but a 40 mm squat rack bar increases the mounting radius, thus centrifugal force is higher, near 0.5 N at 80 rpm.

Combined with a 4 mm Otterbox case, magnetic pull force is halved or worse at the increased gap and larger bar diameter.

You will need a MagSafe puck or mount rated at a minimum of 10-12 N pull at zero gap to hold securely. Standard Apple MagSafe pucks rated 7-8 N pull may be insufficient and lead to slipping under exercise conditions.

Which stick-on MagSafe puck holds a 230g phone on 28mm treadmill rail at 10 km/h

At 10 km/h treadmill speed and 28 mm rail diameter, the phone experiences similar centrifugal forces as at 80 rpm on a bike stem because the rotational velocity correlates.

The centrifugal force ranges around 0.35-0.45 N. With typical case thickness and a 28 mm rail, magnet pull force losses are significant.

A stick-on MagSafe puck rated at least 8 N pull with zero gap is advisable. Lower rated pucks risk slipping, especially under impacts or quick movements.

Note that treadmill vibrations and user movements increase the effective force needed, reinforcing the need for a strong magnet.

Questions people still ask

I've tested magnetic phone mounts on spin bikes and know the difference a proper pull force makes.