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 Most makers do not publish it, so I assume 15 mm and add 6 mm to the band, which on its own is enough to hold an answer at "probably" rather than "fits".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 A21M The Dissident
Gravel · 4 sizes
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This is the bike I ride
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- Bike
- Serk A21M The Dissident · Gravel · 4 sizes
- Geometry
- serk.cc, fetched 28 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.Frameset (from RMB 16,800 / US$ 2599) ships frame + Serk Carbon Adventure fork + Serk semi-integrated headset + seatpost clamp + thru-axles + expander + Ti top cap; "Integrated handlebars are not included." Front end is a choice: "Integrated front end cabling or External" / "Choose Integrated or Non-Integrated version" (internal routing = all electronic groupsets; external routing = 1x/2x mechanical or electronic).
Cockpit and seatpost, in the maker's words · 421 words
Frameset (from RMB 16,800 / US$ 2599) ships frame + Serk Carbon Adventure fork + Serk semi-integrated headset + seatpost clamp + thru-axles + expander + Ti top cap; "Integrated handlebars are not included." Front end is a choice: "Integrated front end cabling or External" / "Choose Integrated or Non-Integrated version" (internal routing = all electronic groupsets; external routing = 1x/2x mechanical or electronic). Integrated bar/stem upgrade options listed on the page: Deda Super Box / Deda RS2 (+1000 RMB), Serk Carbon integrated (+1200 RMB), THE ONE PRO Carbon Integrated (+1500 RMB), EXS Aerover Carbon Integrated (+2500 RMB, on the Rival build). Complete builds: SRAM Rival D1 AXS 1x = Uno Gravel bar + Uno stem (external routing) + Uno aluminium 27.2 mm seatpost; Campagnolo Ekar = Serk Gravel Carbon bar + Zipp Service Course SL stem (external routing) + Serk Carbon 27.2 mm seatpost; SRAM Force E1 2x AXS = Serk Carbon integrated bar/stem + Serk Carbon 27.2 mm seatpost. NOT published anywhere on the maker site: stem lengths per size, stem angle, bar widths, bar reach/drop, clamp/stack height of the integrated bars, maximum spacer stack, seatpost length or offset. Seatpost: round 27.2 mm (owners manual A21 quick spec), dropper post compatible with internal mechanical dropper cable routing. Headset: ZS44 upper / EC44 lower on the A21 (owners manual HEADSETS section: straight 44 mm head tube, tapered steerer 28.6 to 39.8 mm; compatible with Chris King, Cane Creek, White Industries etc.); the A30M-style EC44/EC44 handlebar-specific headset is NOT stated for the A21. Headset top stack above frame stack: 28.5 mm on the A21M page table ("* Additional top headset stack"); the owners manual A21 table gives 10 mm, and the manual's A40 table labels the same two figures as 28.5 = internal routing headset top and 10.1 = external routing headset top, so read 28.5 as the internal/integrated front end and 10 as the external-routing top cover. Fork: 12x100 mm thru-axle, adjustable rake 47/52 mm via flip chip ("Party mode" 47 mm = more trail, more stable; "Business mode" 52 mm = less trail, more reactive), 3-pack side mounts, internal dynamo wiring, front light mount, 520 g with 300 mm steerer. Frame weights published: S 1750 g, M 1850 g, L 2000 g, XL 2090 g. Frame: titanium 3Al/2.5V, T47 cast one-piece BB shell (68 mm), flat mount 160 mm front and rear (max 160 mm rotor), 12x142 rear, 3 bottle cage mounts, mudguard and rack mounts, UDH not mentioned, system weight limit 115 kg (rider + bike, owners manual).
STACK DATUM: diagram (geo_2023_550.jpg) draws STACK from the BB centre vertically to a horizontal line at the TOP OF THE HEAD TUBE (frame only), and REACH horizontally from the BB centre to the same head tube top point. Source: html_table (maker product page GEOMETRY section: HTML table plus letter-keyed diagram image
How I read the maker's table
the headset top cover is excluded and published separately as "* Additional top headset stack" = 28.5 mm every size on the A21M page (10 mm in the owners manual A21 table, see cockpit_options). TOP TUBE: labelled "Top Tube Length" (B), drawn horizontally in the diagram from the head tube top to the seat tube/seatpost line, so recorded as top_tube_effective (assumption). SEAT TUBE LENGTH (A): drawn along the seat tube from the BB; centre-to-top assumed, not stated. SIZES: named S/M/L/XL only; page footnote "*measured size" refers to tyre clearance, not frame sizes. FORK OFFSET: published as "47/52" (flip chip) in one row; recorded as 52 because the wheelbase row is explicitly "Wheelbase (52 OffSET)" and the trail row "Trail (700x42)" = 73.1 mm agrees with a standard trail calculation at 52 mm offset and 700x42 (about 78.5 mm at 47 mm); 47 mm is the alternate position and wheelbase shrinks by roughly 6 mm there (owners manual A21 table: 1009/1032/1054/1075, unlabelled offset, consistent with 47 mm). TRAIL: 73.1 mm all sizes (HTA 70 deg constant). BB DROP: 75 mm S and M, 73 mm L and XL. HEAD TUBE ANGLE 70 deg all sizes. Standover and front centre are NOT published (nulls). No stem length or bar width per size is published (nulls). MODEL YEAR: no year on the page; the diagram file is named geo_2023 (uploaded Oct 2022), the page was published 2025-06-05 and last modified 2026-02-07 (article meta), and the text calls the A21 "our 3rd generation gravel bike"; owners manual A21 table (older copy, external-routing headset stack, mislabelled "Trail (28C)" = 73.7, wheelbase 1009/1032/1054/1075) carries identical frame numbers, so the frame geometry appears unchanged across the A21 / A21M naming; model_year left null. NAMING: the page title is "A21M", the range page (serk.cc/titanium/) calls it "A21 - The Dissident" ("Offroad Anywhere Bike") and the owners manual "A21 | The Dissident" ("Gravel Bike"); the M suffix matches Serk's A30M convention for the internally routed / integrated front end version, and the A21M page offers both integrated and external front ends. Tyre clearance: 700c x 45 / 650b x 55 measured (owners manual says 700c x 40 or 650b x 55 measured, older); geometry "optimized for 700x45mm or 650x47mm tyres". Max chainring 50/34 (2x) or 46 (1x).
Source: html_table (maker product page GEOMETRY section: HTML table plus letter-keyed diagram image https://www.serk.cc/wp-content/uploads/2022/10/geo_2023_550.jpg)
