Bike Frame sizing calculator

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Given that it’s pretty hard to walk into most bike shops and try out a HongFu bike frame (ignoring the many bike shops selling them as their own brand for the moment) it seemed useful to try and help out with sizing the frames as much as possible.

One recent innovation in sizing frames has been using Stack and Reach measurements because these can easily be compared across frames and sizes.

Most geometry charts still don’t come with Stack and Reach measurements yet, so I’ve created a calculator, preloaded with the FM015 & FM028 geometry.

Try it out now and let me know what you think: Frame Size Calculator

Discuss this at the new Bike Sizing Forum.

  • Graham

    Awesome! Thx for putting this together. Check the FM028 56CM dimensions though. I think it had the 58's dimensions.

    • CarbonNick

      Thanks Graham. You were right – there was a problem with the 56cm FM028. I've fixed that up so it should be ok now.

  • Adam_mac84

    I found that the FM028 S/R is VERY close to the Trek Madone H2 geometry. The 60cm FM028 is comprable to the 60 and 62cm frames… one is spot on for stack, another for reach. I chose to ride the one with the same stack, i can adjust reach a bit more with stem/bars

    • cynan

      Yes, the geometry between the FM028 and Trek H2 are almost identical. However, a major difference is the seat tube, which is longer on the FM028 (in the neighborhood of a couple of cm). This seems to indicate that the H2 is a slightly more compact geometry: Therefore the FM028 of the same size will have a greater stack height and shorter reach. This to me, would make the FM028 a better fitting frame for those with longer legs (larger inseam) relative to arm length.

      For example, for me, while I would probably fit closer to a 56cm frame with the H2, I would be closer to a 58 cm FM028. (But as I have a large-ish inseam – about 90 cm – this would result in my having to use a seat post length of at least 23cm, which is getting rather long. So for people with larger legs relative to arms, it would seem that the FM028 is a better fit than the H2)

      And as I just noticed, this is corroborated by the fact that the H2 geometries have longer reaches and shorter stack heights than the equivalently sized FMO28.

  • Harold

    I have been compiling a data base of stack and reach numbers to try to find a new frame. I took the numbers from the Cervelo RS frame geometry and added them to your calculator. The numbers do not match to what Cervelo has established. What are you using for calculations.
    I guess if I were to use your calculator and add all the relevant numbers I should be comparing each frame the same?.

    • CarbonNick

      Yes.

      Cervelo seem to do something different with their Stack & Reach calculations. I've spent quite a long time
      checking my calculations, and they are correct. I don't know how they do theirs.

      The formula I use is explained here: http://forums.roadbikereview.com/showpost.php?p=2907645&postcount=421

      • Brook

        Your REACH formula is correct. But, the value is computed using the STACK value, which isn't correct.

        Your stack values are high by about 12-13mm. Here's why: According to your definition and very nice CAD diagram, the 368mm fork length is measured along the head tube axis. The point on this line 368mm from the bottom of the head tube is not where the front axle is. Because of the rake, the axle is higher and further forward than this point. How much higher is given by (rake) * cos (head tube angle). For a 43mm rake and 73 degree HTA, the vertical distance is 12.6mm.

        STACK = BB_drop + (Axle_crown_ht + Head_tube_ht) * sin(Head_tube_angle) – (rake) * cos(Head_tube_angle)

        • Harold

          That formula does not work either. I have two examples from the Merckx bikes site. The site claims there EMX-1 bike has a Stack of 589 and a reach of 383. Using the first formula without the rake I get a stack of 601 and a reach of 379. the second formula with the rake gives me a stack of 570 and a reach of 389. Neither of them come close to what the manufacture is claiming.

          It would be nice to find out what formula there using.

  • cynan

    I went back to check the technical drawings and it seems that you may be correct. However, the results are a bit strange. The diagram I have for the 58cm FM028 shows the hypotenuse of the fork rake to be 43mm (the F015 drawings I have do not show this measurement). From this, you can calculate that the distance from the bottom of the top tube to the axle shown in the drawing exceeds the Axle crown height by 12mm. So, this would mean that the actual Axle crown height is 367.8mm – 12mm = 355.8mm.

    If you proceed to use 355.8mm as the axle crown height for the 58cm FM028, you now get a stack height of 595.6mm (instead of 607mm) and a reach of 390.9mm (instead of 387.4mm).

    And for the 58cm FM015, assuming that the same hypotenuse of the fork rake is also 43mm as on the FM028, you now get a stack height of 572.4mm (instead of 584.5mm) and a reach of 398.2mm (instead of 394.7mm)

    So while the reach numbers given by the calculator is close, the stack numbers do seem to be off by quite a bit.

    It would be helpful if someone with one of these frames could take a level and check if their stack height is really off by over a cm from what the calculator indicates…

    • cynan

      Was looking at the Cannondale CAAD8 geometry and noticed that the CAAD 8 size 56 geo is almost identical to 58cm FM015. Head and seat tube angles and head tube length are the same. Top tube length seems to be slightly shorter on the CAAD8 (only a few mm by the looks of it).

      Anyway, stack for the CAAD8 is 574mm 0 which is pretty darn close to the 572.4mm (see above comment) for the FM015. Interestingly, reach is about 10mm shorter (though again, this could have to do with how the top tube was measured and perhaps the CAAD8 has a slightly greater fork rake)

      Stack and Reach asside, if anyone is considering a 58cm FM015 and is not sure about fit, might be a good idea to go sit on a CAAD8.

  • otter68

    Can't you do reach as:   effective top tube length – (sin(90-seat angle) * seat tube length?

    Basically the ETT – (center of bb to center of top of seat tube(horizontally)).  As long as ETT uses the center of the top of the seat tube and ST is measured from center of bb to top of tube, I think that should work.

  • CarbonNick

    This thread is now closed. Please move discussion to the forum: http://cheapcarbonframes.com/forum/categories/bike-sizing