MRC BB Gun Bottom Bracket


Steve McMahon of McMahon Racing Cycles introduction of the B/B Gun bottom bracket brings new light weight concepts without compromise in spindle flex. The spindle is made of machined Titanium Ti 6Al-4V rod, with tapered ends for easy arm installation. This is not a hollow spindle, just the ends are drilled, then tapped for the crank bolts. Steve points out that a hollow Titanium spindle is subject to more flex than a solid one and that the saved weight doesn't justify the lost energy in spindle flex, the expense of cutting tool replacement, or the additional machining to drill the spindle through.

The spindle, in the middle, has a tiny 1mm wide cut made to a 1mm depth around the center of it. In this cut, MRC positions a cir-clip to act as a centering point for equal length aluminum tubing spacers that slide tightly around the spindle, moving from the center out to the bearing cup. The bearing cups are made of machined aluminum rod, bored from the back for the sealed bearing cartridge to be pressed in, revealing just the bearings' circular inner race from the outside.

Each bearing cup also acts as a water sleeve surrounding the spindle and spacers, when they are tightened in the shell, a tight joint in the center is formed. This sleeve is also an integral part of the mechanical design, and is one of two mechanisms used to set the spacing between the cups to conform to the 68mm B/B shell width exactly. The other is the tubing spacers that start at the cir-clip in the center. The small and large aluminum tubes butt against the inner steel bearing race and the bearing cup to hold the spindle in place, without the use of lockrings or locking collars, as the bearing cups and the entire assembly are turned to position the spindle and then tightened against each other.

The outer face of each cup has four equally spaced holes for final adjustments with a pin spanner. A Japanese SMT, model #6903Rk, precision sealed bearing cartridge is pressed into each of the machined bearing cups. The style of the spindle allows for up to 4 bearing cartridges to be used for increased rigidity and durability.

The B/B Gun comes with two machined from Titanium rod crank arm bolts, with two Titanium bolt washers, the pair weighs only 17 grams. (An earlier model of this B/B used machined aluminum crank bolts and washers, with the set having a total weight of 10 grams.) It is available with spindle lengths of 105mm, 113mm, 119mm, 125mm, or 131mm. The weight of a B/B Gun in the 113mm spindle length, with crank bolts & washers, is 180 1/2 grams.

Spindle Length-05-13-19-25-31 $Price in Catalog

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