Fat Bob Instruments

Why Fat Bob speedometer? Where does the name come from?

The type of speedometer has been around since 1936, and since then it has adorned the tanks of Knucklehead, Side Valve Big Twins, Hydra Glide, Duo and Electra Glide. Was it already called that back then? We don't know for sure. When this name for the big 4-1/4" speedometers that sit so "fat" in the middle of the tanks was more common in the scene, there was indeed a Harley-Davidson model called that, the FXEF Fat Bob. On their tank sat only no Fat Bob speedometer, but two standard FX instruments with metric 80 mm, which was built into a small housing.

Most likely, the name simply comes from the Fat Bob tanks over which these speedometers are mounted with a base plate fixed to the frame. These tanks were available in a five-gallon version starting in 1967, and they looked pretty fat compared to the older 3-1/2 gallon version. From the "fat" tanks, the term was then probably simply transferred to the speedometers.

Fat Bob speedometers

are available in electronic versions for the Softail and Road King models with direct connection to the speed sensor.

Mechanical versions, on the other hand, are available with a wide variety of dials and the three calibrations commonly used on Harleys: 2:1, 1:1 and 2240:60. Yes, Harley-Davidson itself and the accessories industry have been very active, especially when it comes to speedometers. If the calibration is right for your bike you can exchange the parts just with a few tools thereby changing the style of the motorcycle.

2:1, 1:1, 2240:60 - what the h...ck?

These specs have already confused many of us. Sorry for that, but it wasn't us who made them up. What is meant is simply the calibration of the speedometer mechanism. Calibration makes sure that the pointer does not indicate just anything, but that the rotation of the speedometer cable corresponds to the correct deflection of the pointer. Or in other words, calibration is speedo set-up.

So, how many revolutions of the speedometer cable correspond to which speed read on the display?

A 2:1 speedometer shows a speed of 96 km/h at 2000 revolutions of the cable (per minute). Why such a crooked value? Well, because the parts are actually calibrated in miles, and 96 km/h = 60 mph.

A 1:1 speedometer shows 60 mph already at half that cable speed, i.e. @ 1000 revolutions of the cable.

And 2240:60? 2240 cable revolutions correspond to 60 mph = 96 km. This calibration type applied originally only to Fat Bob speedometers, whose speedo cable was driven by the front wheel, from 1983 onwards. At that time, Harley-Davidson began to buy instruments and front forks from Japanese suppliers, and these use "metric" values for calibration: 1400 rpm of the speedo cable → 60 km/h = 2240 cable rpm → 96 km/h = 2240 cable rpm → 60 mph. As you see, a little maths can be useful.