Serk A21

NON-integrated front end: standard round handlebar and stem (the A21M successor on https://www.serk.cc/a21m/ adds an optional integrated bar/stem with internal front-end routing; the archive A21 page has no such option). Frameset includes Serk Semi-integrated headset, seatpost clamp, thru-axles, expander and Serk topcap. Headset ZS44 upper / EC44 lower (owners manual https://www.serk.cc/ownersmanual/: "The headsets on A30, A21, A10 is ZS44/EC44"), straight 44 mm head tube, tapered carbon steerer 28.6 to 39.8 mm. The owners manual A21 quick specs list "Additional top headset stack 10" mm on every size, i.e. the headset top cover adds 10 mm above the published frame stack (the A21M page quotes 28.5 mm for its integrated front end). Complete builds listed on the page: RIVAL AXS build = Uno Gravel handlebar, Uno stem, Uno 27.2 mm seatpost; EKAR and FORCE AXS builds = Serk Gravel Carbon handlebar, Zipp Service Course SL stem, Serk Carbon 27.2 mm seatpost; saddle Prologo Scratch M5 on all three. Stem lengths, stem angle, bar widths, bar reach/drop, spacer heights and maximum spacer stack are NOT published, and there is no per-size cockpit spec, so stock_stem_length and stock_bar_width are null. Seatpost: 27.2 mm round, offset not published; dropper post compatible with internal routing (A21 page: "Dropper post compatible (1x only)"). Fork: Serk Carbon Adventure, 12x100 mm thru-axle, adjustable rake via flip chip 47 or 52 mm (A21M page: "Party mode (47mm) provides a higher trail for more stability and Business (52mm) lower trail for more reactive handling"), 3-pack mounts on the legs, internal dynamo wiring, front light mount, carbon steerer.

Please refer to details and pricing of updated A21[M] here" (link to https://www.serk.cc/a21m/). The geometry table is plain HTML (sizes S / M / L / XL as columns, rows Reach, Stack and A to K) keyed to the diagram https://www.serk.cc/wp-content/uploads/2019/07/geo.png; units are unlabelled on the page and are mm and degrees. STACK DATUM: the diagram draws Stack vertically from the BB centre to the horizontal through the top of the head tube (frame only, headset cover not included) and Reach horizontally from the BB to the same point. B "Top Tube Length" is drawn horizontal (effective) and stored as top_tube_effective. A "Seat Tube Length" is drawn from the BB centre to the top of the seat tube. WHEELBASE is published explicitly at the 52 mm fork setting ("Wheelbase (52 Offset)"). FORK OFFSET is published as "47/52" (flip chip); fork_offset is stored as 52 because the wheelbase is quoted at 52 and the published trail of 73.1 reproduces exactly with 52 mm offset, 70 deg head angle and a 700x42 tyre (353 mm radius); the A21M page labels the identical trail row "Trail (700x42)". At the 47 mm setting the trail would be about 78 mm (computed, not published, so not stored). FRONT CENTRE (K) is recorded as published; it runs about 6 mm longer than sqrt((wheelbase - chainstay)^2 + bb_drop^2) would give, so the maker's datum for K is unclear. Standover, stem length and bar width are not published. MODEL YEAR: undated on the page; the page was published 2020-05-22, last modified 2026-01-19 and the diagram was uploaded July 2019, while the A21M successor page was published 2025-06-05, so this table describes the A21 as sold roughly 2020 to 2025. CROSS-REFERENCE (not used for the numbers here): the A21M page publishes stack 547/574/596/618 and top tube 521/547/567/592 with "Additional top headset stack 28.5", everything else identical to this table; the owners manual "A21 | The Dissident" quick specs repeat the A21M stack and top tube with additional top headset stack 10, wheelbase 1009/1032/1054/1075 and "Trail (28C) 73.7", which look like the 47 mm setting or a different tyre and are not the archive A21 figures. Panda Podium was not consulted (Serk sells direct). Frame facts from the page: 3Al/2.5V titanium, T47 cast one-piece BB shell (68 mm per the manual), flat mount, 12x142 rear, Di2 compatible, full internal routing, three bottle mounts, mudguard and rack mounts; tyre clearance 700c x 45 / 650b x 55 (measured); the A21M page says the geometry is "optimized for 700x45mm or 650x47mm tyres".