Sliding Dropouts

There are two unique challenge when designing single speed frames with rear disc brakes:

(1) Chain Tensioning: An eccentric bottom bracket shell can be used but they are heavy and become hard to adjust when dirty. Another disadvantage with an eccentric is that it's off-center approach changes frame geometry by lifting/lowering BB height (as well as head and seat angles) to get proper chain tension. Alternately a horizontal dropout with an axle adjustment screw can be used which is very common on BMX and track bikes.

(2) Disc Brake Caliper/Rotor Alignment: The ISO Disc Brake Standard requires that the (2) disc brake mounting holes are aligned to the rear axle's centerline. These mounting holes are thus 'fixed' into proper position on standard hardtails with simple disc tabs. With horizontal dropouts, adjusting the rear axle to tension the chain causes the caliper to de-align and ultimately lose contact area to the rotor...not good. One solution: add slots to the ISO disc tab; this strategy is undesirable since the fixing bolts can slip under heavy braking loads.

Optimal Solution: Horizontal Sliding Dropouts

Sliding dropouts provide flexibility to the single speed frame design by allowing different 'sliders' to be mounted to the frame. The chain is tensioned by using a single bolt to push the axle rearward like BMX and track bikes. For disc brake applications, the 'disc slider' and rear axle move rearward at the same time in order to maintain proper caliper and rotor alignment while satisfying the ISO Disc Brake Standard.

For steel frames, sliding dropouts are readily available from Paragon and they work nicely. For aluminum frames, there are no 'off the shelf'' dropouts that fit the bill. So I decided to design my own dropout body machined from 7005 billet aluminum. The exploded view below shows the 'disc slider' in yellow and the 'horizontal dropout body' in dark gray. The parts mate together using (2) M6 button head screws and there is a brass wear guide shown in orange which prevents the frame from getting damaged when tightening the assembly. The 'disc slider' and rear axle are positioned using (1) M5 socket head cap screw and jam nut....this motion is what ultimately tensions the chain.

For simplicity, the 'horizontal dropout body' is identical on both left and right sides of the frame. The round hoods assemble easily to round or square stays with a 3/4" radius cut. Due to their modular nature, sliding dropouts from Ahrens Bicycles save time during the frame building process and offer the rider a solid, lightweight solution when running disc brakes!

Weight per Pair = 220 grams*

*Includes ISO Disc Slider, No Hanger on Drive Side, Wear Guides and Mounting Bolts

Left Slider Options:
-ISO Disc Mount
-No Disc Mount
Right Slider Options:
-No Hanger > single speeds
-Hanger > geared bikes