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How small is too small?

New to triathlon, I'm planning to get a proper bicycle, and have had my eyes on the Stealth Pro Carbon for some time. Problem is the "small" frame is a bit on the large side -- 520 mm -- and the "extra small" one, at least by crude mathematics, somewhat too small: 490 mm. According to the competitivecyclist.com fit calculator I should aim for something in the range of 504 through 524 mm, but even assuming this is correctly calculated (I understand this is not an exact science), there is considerable room for faults in the measuring, that could probably (given worst case scenario) add up to more than 4 mm, which could make the 520 too big for me. And, as I understand it, a too large a frame is not a good idea at all. So the question is, then: could I go for a 490 frame, a longer stem and possibly move the saddle back a bit, or is it likely to simply be impossible for me to get into an efficient position (in terms of getting maximum power transfer from my legs to the pedals and being able to fill the lungs properly) on this one?
I should probably mention I have no intention whatsoever to do Ironmans (yes, I'm pretty sure that's the correct plural) on this thing, just sprints, the occasional time trial (there are some nice 10K races over here) and possibly stretch myself to an olympic distance tri at some point. (As a former track runner, I never got the common obsession with going ever longer distances -- going faster is more fun than goning farther.) In other words, I don't mind being uncomfortable as long as it means more speed.

I suppose there are a couple of obvious advices here: (1) try a bike of this size to see if it fits, and (2) ask Planet X. Well, I've tried both.
As for (1), the bike shops around here really don't know all too much about TT/tri bikes ("but if you were ever interested in a dirt bike..."), don't keep a whole lot of them in stock, and will just pick the size off a chart. And whenever the one guy who might know a little bit about these things is not in (he never is), they won't even put it in a trainer for me...
And (2) when I've tried to ask Planet X staff (two different people, including their "technical expert"), they'll just quote their own rough sizing guide ("you're over 5'6"? then 490 is too small for you!"). They don't respond to emails. (To be fair, I'm sure to some part this is a language problem, as I'm obviously not a native English speaker. Hope I get the message across here, though. Anyway, I'm beginning to feel silly calling in to ask the same question over and over again.)

As for my measurements, I'm two inches shorter than the guy on the top of this page: http://www.planet-x-bikes.co.uk/help/what-size-bike but have the same inseam measurement.

Oh yes, a few words on 650c vs 700c wheels would also help.


  • andybrookeandybrooke Posts: 2
    Hi Simon,

    As a rough guide, if your inseam is 81cm then your saddle height should be around 0.877 times your inside leg (when measured from the centre of the bottom bracket to the top of the saddle) and 0.213 times your inseam gives you your crank length. So, you should be riding 172.5mm cranks and your saddle should be around 71cm when measured from the centre of your BB to the top of your saddle. As such, you could get a saddle height of 71cm on any of those Planet-X bikes so the top tube length is what you need to focus on.

    When it comes to estimating top tube length, it's pretty difficult without measuring you up properly. The guy you use as your reference at the top of that page uses a bike with a top tube of 490 but there's a guy further down the page who's 5cm shorter and uses a frame with a top tube of 530mm, so it all depends on your torso length. If I were you, I'd go for the Small with a 510mm top tube. I'm 166cm and that's the size I'd go for. I can't really offer anything more technical than that without doing a proper bike fitting I'm afraid.

    As for 650c versus 700C wheels, apparently there is a slight aerodynamic advantage to smaller riders when using 650c wheels but it effects your gearing, ground clearance for cornering and greatly limits options in terms of replacing wheels or buying tubes / tyres - very few bike shops will stock high end wheels / tyres in 650c for example. I'm a little smaller than you and I've always ridden 700c.

    Hope that helps

  • simonWsimonW Posts: 2
    Andy: yes, that was helpful. Thanks!

    You're right: the small is a 510, not a 520. Can't believe I got that wrong. It certainly tips the argument over a bit.

    The interesting thing about the "What size bike" page on Planet X that I linked to is that it seems to support my hunch, that I could go for the 490. The riders there I might be able to compare with are #1 (5 cm taller, same leg length; XS), #4 (+2 cm, -5 cm; S), and #5 (+/-0, -5; M (but pointing out that's a bit odd)). Body height minus leg length should be relatively consistent with torso length, and this difference for these three vs me is +5, +7, and +5, respectively.

    On the 510 frame, the stack is 497 mm. I suppose you'll have to add a couple of inches to that, so let's assume the lowest position for the pads would be at 550 mm as measured along the same vertical line. The saddle height should be 732 mm according to competitivecyclist.com and 710 mm according to you, so let's make it 720 mm. 720 mm at a 76 degree angle (720 x sin76) equals 699 mm along a vertical line. This should mean I have a maximum saddle-pad drop of 699 - 550 = 149 [mm]. Again, acknowledging there is a lot of room for miscalculations, this could probably end up being a few cm's less (or more, but that wouldn't be a problem).
    Would this prevent me from getting into a really aggressive position? (Obviously, I'm not going to try this immediately, but I want to keep all options open in terms of sacrificing comfort for speed.)

    Just in case anyone is interested in the rest of my measurements (and could give me even more detailed advice based on this), here they are, along with the recommendations from the aforementioned fit calculator:
    Inseam: 81.3
    Trunk: 60.5
    Forearm: 32
    Arm: 60
    Thigh: 57
    Lower Leg: 51.5
    Sternal Notch: 142
    Total Body Height: 173

    The Aero Fit
    Top Tube Range: 50.4 - 52.4
    Stem Length Range: 8.0 - 10.0
    Saddle-Bottom Bracket Position: 73.2
    Saddle-Pedal Position: 90.4
    Saddle-Ground Position: 98.2
    Cranklength: 172.5
    Aero Bar Size: S
    Saddle-Aero Bar Pad Drop Minimal: 4.8
    Saddle-Aero Bar Pad Drop Moderate: 9.9
    Saddle-Aero Bar Pad Drop Intense: 12.1
    Saddle-Aero Bar Pad Drop Maximal: 15.8
    Pad-Ground Position Minimal: 93.4
    Pad-Ground Position Moderate: 88.3
    Pad-Ground Position Intense: 86.1
    Pad-Ground Position Maximal: 82.4
  • bathtubbathtub Posts: 280
    Might be worth you contacting Bike Science

    http://www.bike-science.com/bristol-stu ... 4_313.html

    Andy from Bike Science measured me up for my Stealth at Rotherham a few years back, he is a very knowledgeable chap.
  • andybrookeandybrooke Posts: 2
    Lots more interesting reading for you...


    I don't think you can really estimate the difference in torso length between you and the other riders on that page because they might have long necks or big heads! haha. Flexibility is also a key factor as it determines whether your back will be straight or curved.
  • I can certainly help with sizing. If you're in between sizes, the only way to see which suits best is to try them for size, either on the bike itself or or on a fitting jig. Truth is with a bit of adjustment you could probably get either to work. From experience, fit calculators aren't all that accurate, especially when it comes to tri bike fit.

    Where are you based?
  • JellybabyJellybaby Posts: 180
    Hi Simon,

    Where are you based? I'd recommend going to a few bike shops and trying a few bikes in your price range of differnet sizes. All bikes are not the same, and the fit on one will be differnet to the fit on another. If you're based around birmingham then go and see Mike at Bridgtown Cycles in Cannock. He will not pressure you into buying anything from him and will give you a lot of advice about what makes of bike will fit you, etc. If you can't get there to see him then give him a call. I'm based in London, but go up there for stuff to do with my bike that I can't do myself.


    *I am in no way connected to Mike, he's just a legend, that is all.
  • garyrobertsgaryroberts Posts: 869
    2 inches
  • when she has to ask where it is.
  • shadowone1shadowone1 Posts: 1,408

    Coma has returned. Where you been dude!!
  • power lifting, judoing and new job-ing plus i lost You and Ali's numbers when my phone died
  • shadowone1shadowone1 Posts: 1,408
    Good to see you back.

    Its still a bit quiet on this forum, but its got better recently.

    I'll drop you a PM with my No.

    Whats your race plans this year?
  • Turbo-x and Scottish swim are for sure... I see the strathy standard is not on?

    Everything else is sidelined at the moment from judo and powerlifting, will still enter anything that comes up! You ever get that event organised?
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