are two unique challenge when designing single speed frames
with rear disc brakes:
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.
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.
Solution: Horizontal Sliding Dropouts
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.
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.
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!
per Pair = 220 grams*
ISO Disc Slider, No Hanger on Drive Side, Wear Guides and Mounting