Stack and reach
How high and how far forward the top of the head tube is, from the bottom bracket. They are the two numbers that decide whether a frame can be set up like yours.
BB, or bottom bracket
The axle your cranks turn on, in the middle of the frame. Every number on this site is measured from it, because it is the one point that sits in the same place on every bike.
Headset cover
The little cap on top of the head tube, under your spacers. Most makers do not publish it, so I assume 15 mm and add 6 mm to the band, which on its own is enough to hold an answer at "probably" rather than "fits".
Stem clamp, and its centre
The collar on the steerer that your stem bolts to. I need the middle of it. Where a maker gives only the height, I assume halfway up and say so on the answer.
Steerer
The tube coming up out of the fork. The cover, the spacers and the stem all sit on it, and it is the thing a mechanic cuts to length.
Bar reach
How far forward the hoods sit from the middle of the bar. Two bars on the same stem can put your hands 20 mm apart, which is why I ask.
Setback
How far your saddle sits behind the bottom bracket.
Drop
How much lower your bar is than your saddle.
Rails, and the rail centre
The two rods under your saddle, and the middle of the straight part of them. That midpoint is the point I use, because it is the one you can find with a tape.
The band
How far out I could be, in millimetres, and why. It is a straight sum of everything I had to assume rather than a clever average, so it errs wide on purpose.
wb, stand, trail
Wheelbase, standover and trail: how long the bike is, how high the top tube is where you straddle it, and how steady the steering feels. I show them against your bike, and I leave them out when the maker does not publish them.
est.
Estimated: a number I worked out rather than read off the maker's table. Anything marked this way can move if the maker publishes the real one.
Close Escape closes it. The measuring guide has the pictures. Open the measuring guide
Serk A21
Gravel · 4 sizes
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- Bike
- Serk A21 · Gravel · 4 sizes
- Geometry
- serk.cc, fetched 29 Aug 2026, re-checked, no differences
- Cockpit
- Generic stem and bar, assumed
- Solved with
- cover 15 mm (assumed), max spacers 40 mm (assumed), stem clamp centre 20 mm (assumed), so every verdict carries ±10 mm at the bar
- Sheet
- 6 September 2026 · 210 bikes, 43 makes, 1,262 sizes in the catalogue
Geometry, every size, from the maker's table
The tinted row is the size drawn beside it. Press a size to draw it.The table runs on to the right: 6 more columns after Reach. Scroll it sideways to read them.
The L drawn to scale from the table, with no build on it. Stack and reach run from the bottom bracket to the top of the head tube; every size is solved from those two. One square is 100 mm.
The maker's cockpit
No stock cockpit I can name, so every size is solved on a generic stem and bar unless you choose one on the sheet.Generic stem and barAssumedNo bar is listed for this frame.
In the maker's words
Kept whole, as I read it off serk.cc.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).
Cockpit and seatpost, in the maker's words · 257 words
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.
SOURCE: the maker's own page for the A21, which now carries the banner "This is an archive page for the A21. Source: html_table
How I read the maker's table
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".
