Ridley is one of the few brands that separates time trial from triathlon and sizes the two differently. The Dean Fast Disc is the UCI legal frame, sold as a frameset in S, M and L. The Triton Fast is the triathlon frame, in XS, S, M and L. It is also the one for which Ridley publishes an armpad fit window rather than frame numbers alone. Browse all Ridley bikes at Bikeroom.
Please note. The height rows in this guide are indicative only. Ridley publishes no rider height chart for either bike. Its own support guidance is to compare the actual top tube length of the bike you ride now rather than the reach figure, then see a dealer for a fit. Treat the table below as a starting point.
Quick size finder
Find your height, cross check your leg length, then read across to each model. The Triton Fast has four sizes against the Dean Fast's three, so the two columns do not line up.
| Rider height | Leg length (inseam) | Dean Fast Disc | Triton Fast |
|---|---|---|---|
| under 165 cm | under 74 cm | S | XS |
| 165 to 170 cm | 74 to 77 cm | S | XS |
| 170 to 176 cm | 77 to 82 cm | S | S |
| 176 to 182 cm | 82 to 86 cm | M | S or M |
| 182 to 188 cm | 86 to 90 cm | M or L | M |
| 188 to 194 cm | 90 to 94 cm | L | L |
| 194 cm and up | 94 cm and up | L | L |
Racers often sit a size above these rows. BikeRadar recorded Alec Segaert, listed at 181 cm, on a large Dean Fast in 2023.
How to measure yourself
Height. Stand barefoot against a wall, heels together, and measure to the top of your head.
Leg length (inseam). Barefoot, feet about 15 cm apart, press a book up between your legs as if it were a saddle. Hold it level against the wall and measure from the floor to the top edge.
The inseam formula does not apply. Ridley sizes these frames by letter, so the 0.66 multiplier used on centimetre sized road frames is no help. Bring a pad stack and pad reach from a fit session instead.
Dean Fast Disc (UCI time trial)
The Dean Fast was built around the relaxed UCI tube rules. The length to width ratio moved from 3:1 to 8:1, and compensation triangles allowed the head tube to be extended. Ridley ran the shapes through CFD targeting drag at 55 km/h, then validated them in the Bike Valley wind tunnel in Beringen. Claimed weights are 1,250 g for the medium frame, 460 g for the fork and 341 g for the base bar. It is sold as a frameset only.
| Size | Stack (mm) | Reach (mm) | Seat tube angle | Head tube (mm) | Wheelbase (mm) | Rider height |
|---|---|---|---|---|---|---|
| S | 495 | 373 | 74.5° | 90 | 964 | not published |
| M | 499 | 394 | 74.5° | 90 | 977 | not published |
| L | 518 | 411 | 74.5° | 110 | 1001 | not published |
Fit character: the S and the M differ by 4 mm of stack and 21 mm of reach. Here, size is a reach decision.
The 74.5 degree seat tube angle is worth a second look. It sits well below the 76 to 80 degrees usually quoted for time trial frames. The forward position comes from the post, not the seat tube. BikeRadar's gallery of Segaert's bike shows the saddle slammed almost fully forward on the adjustable aero seatpost.
Triton Fast (triathlon)
The Triton Fast is the non UCI frame. Ridley describes it as sitting the rider closer to the bottom bracket with a more open hip angle. It also gives it a longer reach and a taller head tube than the Dean Fast. Flip chips in the fork change the rake, so handling can be tuned towards stability or agility.
| Size | Stack (mm) | Reach (mm) | Seat tube angle | Head tube (mm) | Wheelbase (mm) | Rider height |
|---|---|---|---|---|---|---|
| XS | 476 | 406 | 80° | 70 | 1004 | not published |
| S | 508 | 420 | 80° | 100 | 1020 | not published |
| M | 538 | 450 | 81° | 130 | 1054 | not published |
| L | 568 | 480 | 82° | 160 | 1089 | not published |
Fit character: the seat angle steepens with size, from 80 degrees at XS to 82 degrees at L. A taller rider is rotated further forward, not just stretched out.
The armpad fit window
For the Triton Fast, Ridley publishes the range of stack and reach measured to the armrest. That is the pair of numbers a fitter hands you, so it is the table to use if you have one.
| Size | Reach to armpad (mm) | Stack to armpad (mm) |
|---|---|---|
| XS | 456 to 496 | 566 to 661 |
| S | 470 to 510 | 598 to 693 |
| M | 500 to 540 | 628 to 723 |
| L | 530 to 570 | 658 to 753 |
Each size covers 40 mm of pad reach and 95 mm of pad stack, and the windows overlap. The tie breaker is spacer height.
The older Dean Fast framesets
Ridley's configurator still lists a rim brake Dean Fast in XXS, XS, S, M and L. That is five sizes rather than three, and Ridley no longer publishes a geometry chart for that generation. If you are looking at one second hand, ask the seller for measured stack and reach, or measure the frame yourself. Do not assume the current Dean Fast Disc figures carry over, because the size count alone tells you they do not.
Why a time trial size is not a road size
- Pad stack and pad reach replace stack and reach. Your position is set by the distance from the bottom bracket to the armrest, not to the top of the head tube. The Triton Fast table above is that idea made official.
- The seat angle gets steeper. Road frames sit near 73 degrees. Time trial and triathlon frames work in the 76 to 80 degree band, whether that comes from the seat tube or from the seatpost.
- Most riders end up one size below their road size. The frame only has to carry the armrests, so a shorter head tube leaves room to build the position back up.
- The rules differ. The Dean Fast is built inside UCI limits on tube shapes, saddle setback and extension length. The Triton Fast is not, which is why Ridley sells both.
What riders say about Ridley time trial sizing
Three consistent points come out of the coverage rather than from owner forums, because the Dean Fast is a frameset for racers and volumes are small.
Racers size for reach. BikeRadar's pro bike feature by Jack Luke, published on 8 August 2023, put Segaert at 181 cm on a large frame. The extensions sat on two tall stacks of aero spacers. The spacers do the height work, so the frame is chosen for length.
Ridley itself distrusts the reach figure. Its support note, written for the Noah road range, tells riders to compare the actual top tube length of their current bike instead, because of the steep seat angles the brand uses.
The two bikes are genuinely different animals. Ridley's own comparison, published on 11 March 2025, sets the Triton Fast's longer reach and taller head tube against the Dean Fast's lower front end. Expect to redo the fit when you move between them.
Between two sizes?
On the Dean Fast Disc, treat S against M as a pure reach question: 21 mm of it, at almost the same stack. M against L adds 19 mm of stack as well, so that step really does raise the front end.
On the Triton Fast, use the armpad window instead. Take the smaller frame if you want a lower front end, or if you expect your position to drop. Take the larger one if you are already near the top of the smaller size's pad stack range.
Our Ridley road bike size guide covers the road range. If you are still deciding which of the two frames suits your racing, time trial bicycles versus triathlon bicycles explains what each rulebook allows. For a second hand frameset, the practical guide to buying used triathlon bikes lists the checks worth making first.
Shop Ridley at Bikeroom
- All Ridley bikes for the full range.
- Ridley road bikes if a road frame is the better buy.
- Time trial and triathlon bikes for the whole category across brands.
- New triathlon bikes for frames that have not been ridden.
If your pad stack and pad reach point to a Triton Fast in S or in M and you cannot separate them, talk to our concierge. We will check them against the published windows.
Disclaimer: the size suggestions here are indicative only and do not guarantee a correct fit. Geometry and armpad figures are official Ridley data taken from the current Dean Fast Disc and Triton Fast product pages. Ridley publishes no rider height bands for either model, so the height rows are editorial guidance, and the weights are Ridley claims. Always confirm against the exact bike and model year you are buying.
Sources: Ridley, Dean Fast Disc geometry; Ridley, Triton Fast geometry; Ridley, Triton Fast versus Dean Fast, 11 March 2025; BikeRadar, Dean Fast launch; BikeRadar, Jack Luke, 8 August 2023. All accessed 24 September 2026.
