Fat Bob FXFB Shock Upgrade: Fitting a Monoshock for Control

Bought for corners, not couches. So the job is judged by how the bike holds a line, not how plush it feels.

Does this apply to your bike? 2018-up FXFB Fat Bob on the M8 Softail chassis. Not for the twin-shock Dyna FXDF Fat Bob, which has its own guide. Riders whose main goal is a softer or lower cruiser may find this page's priorities do not match theirs.

DifficultyIntermediate

Time2 to 4 hours, plus a proper road test

Fat Bob owners get told to buy a softer shock when they complain of a rough rear, and that is often the wrong answer on this bike. It is the sporty M8 Softail, ridden for control and handling, and a shock that wallows through a fast bend is a step backward. Be clear about what you want. It also is not the Dyna FXDF.

Tools

Rear stand or lift
SAE and Torx sockets
Torque wrench
Tape measure
Helper
Notebook
Correct preload tool for your adjuster
Service manual

Say what you want the bike to do

Write down, in plain terms, the misbehaviour you are trying to remove. Does the rear skip across rippled pavement in a turn? Does it keep bouncing after a dip? Does it scrape earlier than expected? Those are three separate faults, and only some of them are cured by a new shock.

Compression through a corner uses up clearance, and a rear that sits too deep from insufficient preload will drag sooner and feel vague. Fix sag with what you have, then ride your favourite bends once more. You may find a large part of your complaint has gone.

If the rear keeps pogoing after a bump with sag set right, the damping may be tired or too light for how you ride. That is a sensible reason for a new unit, and you should tell a seller exactly that. If the shock is simply overwhelmed by your weight, spring rate is what to discuss.

Shopping for control, not brochure copy

Listings for this class of shock tend to promise everything. Ask what spring range suits your weight, what adjustments the unit offers and whether it fits FXFB in your year. A shock with adjustments you never use is no better than one without them, so buy only what you will actually set.

Do not assume that a shock built for a different 2018-up Softail is the right match. The family shares a layout, but preload adjusters differ across models and years, and your big rear tyre leaves you its own clearance issue to verify.

A shock that needs adjusting often also needs a rider willing to do it. Think about how you actually spend a Saturday. If the honest answer is that you will set it once and leave it, then spend your money on the correct spring and on sound damping, and pass on the extra dials that would just sit there collecting road grime.

Strip-down sequence

With the bike on a stable lift and the rear held firmly, take off the seat. The manual decides everything after that. On many of these bikes, parts around the battery tray and on the sides must come away, so read that section before starting, not halfway.

Photograph your connectors. Bag fasteners by location. Note your old adjuster's type and setting, as a reference to the ride you started from.

Remove the mount bolts as described, with the rear still supported, then bring out the shock by its intended path. Keep its spring in compression and intact. Opening one calls for the correct compressor and nothing less.

Mount check for hard riders

Sporty riding works mounts hard. Examine eye bushings on the old unit for slop. Look at frame and swingarm mounting points for fretting, cracks and elongated holes. Examine your bolts on a flat surface for a bend, and look at the threads.

Any slack here is felt as a vague rear in fast turns, and riders then blame the new shock. Take the time. Fix anything doubtful before going further.

Fitting, then clearance

Ease in the new unit and start each bolt by hand, keeping wires and any remote adjuster line free. Your torque values live in the service manual for your year; use them, tightening progressively to the final figure.

Before final assembly, get a light on that wide tyre. Look at its shoulders as well as its crown, along the fender and around the belt. A different shock length moves this relationship, so where the maker gives length and travel, use them to think through what full compression will do.

Reinstall everything, then set sag for your weight in riding gear. Write down the settings you started from, so changes can be undone.

Testing what you paid for

Start with a low-speed lap, listening for knocks. Inspect the fasteners before going further. Then ride those same bends from your original complaint. Change only one setting between runs. If you turn several adjusters at once, you will have no idea which one helped.

Judge it against your first notes. Is the rear stable over ripples in a turn? Does it settle after one movement? Is scraping where it was? Stay within legal speeds and your own limits, and look over the tyre and fender for rub marks when you get home.

Mistakes that cost money

Choosing a softer shock on a bike that needs a more controlled one.
Buying before setting sag on the old unit.
Assuming another M8 Softail's shock is a perfect match.
Changing several adjustments between test runs.
Checking only the crown of a wide tyre for clearance.

When to stop

If the rear feels loose or unpredictable in a turn after the swap, stop riding hard and check every mount before testing again. A mount problem at lean is not one to discover twice.

Other Harley how-to guides

Low Rider FXLR Rear Shock: A Monoshock Picked for How Many Ride Low Rider S (FXLRS): When to Replace a Shock That Is Already Better Low Rider ST (FXLRST) Rear Shock: Bags First, Then Everything Else Rear Suspension Preload and Setting Sag

This guide covers procedure and judgement. Torque figures, fluid capacities, gaps and tension settings are specific to your model and year and must come from the factory service manual for your motorcycle — we deliberately do not print numbers we cannot stand behind for your bike. Independent aftermarket retailer, not affiliated with Harley-Davidson, Inc.