Mounting instability issues with BFD and M-One follow focus motors on 15mm rod systems

2010Follow Focus forum

16 years old. Gear and prices may have changed since.

Motor or rod slipping during operation can result from flex in the rod or lens, particularly when the motor is positioned far from the rod mount point.
Stiff lenses with tight external mechanisms can cause motors to skip out of gear during rapid focus pulls, even if the lens barrel rotates smoothly.
Shallow gear teeth on some lens types reduce motor engagement and can lead to skipping during quick throws.
Alignment of compression slots in the rod collet is necessary for secure mounting.
The mounting bracket orientation matters—positioning the tightening lever to press toward the lens rather than away from it improves holding force.
A dog bone attachment bracket can help position the motor at a mechanical advantage to prevent swiveling out of gear.
Using a zoom support bracket or matte box with rod support keeps both rods locked together and reduces slipping.
Tightening the single rod adjustment point rather than over-tightening multiple connection points creates a more stable setup.
When motor skipping occurs, returning the lens and motor to infinity focus and reattaching restores engagement without full recalibration.
A PAM standard 15mm rod mount can serve as an alternative attachment method with improved security.
Proper reducer sleeves matching the exact rod diameter specifications work better than generic adapters.
Clean and dry rods and collets are essential for reliable grip.

Solutions described are based on experience with specific setups and lens types, not comprehensive testing across all equipment combinations.
The effectiveness of modifications like scratching adapters for grip is anecdotal and untested.
Some suggestions apply primarily to older lenses with external focus mechanisms.
Experience is documented primarily with Alexa cameras and specific lens designs like Cooke S4.