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  1. #1
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    Using Paterek method to find seat tube length: How to choose a BB height?

    I am designing a fixed road bike to be my first self-built custom. It will be a traditional horizontal top tube steel frame, lugged or fillet brazed. I downloaded the old paterek manual, and I'm in the process of hashing out the frame geometry right now. The manual has me starting with the seat tube length. His method for determining ST is to find inseam length and subtract from that the BB height and TT-crotch clearance, then do the whole pythagorean thing with the ST angle to find out the actual tube length. Pretty self explanatory. I know my inseam is 85.7 cm, and I want a ST angle of 74 degrees. Paterek recommends a crotch clearance of 6 cm for racing. The only thing that Paterek doesn't explain well is how to choose a BB height.

    I know that I want my BB placed so that it is as low as possible, without ever getting pedal scrape on tight corners. Currently I ride a Cannondale geared road bike that almost fits me perfectly, so i've been using that as my reference. The Cannondale has 265.25 mm of BB height with my current 700x23C tires (planning on using the same tire size on the new bike), and I've only scraped pedals once so far. I like that level of BB height, and it's probably a little bit more clearance than I really need.

    I AM planning on increasing from a 170mm crank (cannondale) to a 175mm (new fixie) crank length. I'm assuming I can take that original BB height (265.25), add 5 mm onto it (270.25) and that will accommodate for the longer crank. Since I probably could go lower I'm gonna turn that into a nice round 270 mm, and use that for my BB height.

    Any suggestions, critiques, etc...?

    Any better ways of picking out geometry? I know the old paterek manual is a little outdated (still recommends FOPS, cubit-based TT length, and a couple other relics), but I haven't found any better way of creating geometries from the ground up.

  2. #2
    Randomhead
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    what is driving that seat tube angle? Also, are you sure you want to increase the crank length? Fixed does make you pedal through corners so that's a consideration, do you really pedal through all corners on your geared bike?

  3. #3
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    Quote Originally Posted by unterhausen View Post
    what is driving that seat tube angle? Also, are you sure you want to increase the crank length? Fixed does make you pedal through corners so that's a consideration, do you really pedal through all corners on your geared bike?
    In order of asking:

    Paterek (and others) say that the normal range of ST/TT angles is 72-74 degrees, and that the steeper the angle the more responsive/aggressive/suited for racing the bike is. Since he stresses that anywhere within that range is acceptable, I figured I'd just go with the upper end of the range, since those are the characteristics i'm looking for.

    I've been reading about proportional crank lengths. While i'm not convinced the inseam x 2.16 = crank length equation is perfect (i think that comes out to a 180-something crank length for me), I do think that I would be more comfortable on a longer crank arm.

    I was considering the fact that I would be pedaling through all corners on the fixie, but I hadn't thought about the fact that I DON'T do that on my current bike. I went out yesterday and purposefully took some sharp turns while pedaling, and I definitely want more clearance (on the fixie) than I have now.

    The math I worked out earlier, 270 mm of BB height, would put me at about 8cm of BB drop, which i believe is on the higher end of the scale. I need to redo all of my math and double check it's accuracy, but I think if I just raise the BB one more centimeter I'll have plenty of clearance, but still have a nice low bottom bracket at around 7cm of BB drop.

    Here's another question: I might conceivably want to put treaded tires on my bike for rainy/muddy excursions. If I'm riding 700c rims, that's 622 mm of diameter in the rim itself. If I make room for 700 mm of diameter (an extra inch and a half past the rim on each side) will that be enough leeway for any tire I might want?

  4. #4
    Randomhead
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    seat tube angle is set by your body, not by responsiveness. Paterek was just parroting some ignorant "conventional wisdom" from back in the day. I suspect he has changed that in more recent versions. If you need 180 cranks, your femur is likely long enough to drive you to a slacker seat tube angle, not a steeper one. The handling of a bike is mostly controlled by the head tube angle, fork offset, and the weight balance. The seat goes where your fit tells it to go. Obviously you can get a long setback seat post to compensate for a steep seat tube angle, but that flies in the face of any reason to build a custom frame. You really want to put the seat tube angle where it makes your saddle of choice look good on your seat post of choice. In other words, it really doesn't matter what the angle is.

    BB drop of 7cm is pretty normal. I don't build fixies so I'm not sure if that is typical or not.

    As far as tires go, I would look at the online version of bikeCad, there is a view that shows tire clearances. You also might try Rattlecad, but I'm not sure how that works for tire clearances.

  5. #5
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    There are a few frame design "do's", like lower bb's for stability, that don't really apply for fixed gears. On a fixie, you'll never descend or corner as fast as you can on a geared bike so it's more important to have pedal clearance than a theoretical high speed stability that you'll never use.
    Road-going fixies also benefit from shorter trail.
    also, shorter cranks are better for riding fixed....they make it easier to spin faster.

  6. #6
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    femur length influences what seat post angle feels right.

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    Thanks for the answers everybody, they've been quite enlightening. I have no attachment to the 74 degree seat angle, but the placement of the saddle on my current bike feels too far back, and I've pushed the saddle up on the rails as far as it will go. I really need to get a nice angle finder so that I can figure out what my current ST angle is.

    In regards to "femur length influences what seat post angle feels right" I assume the longer the femur the more relaxed the angle. Though it's really only about the fore/aft position of the saddle right?

    All of my friends who ride fixed are trying to convince me not to increase crank length, which i guess I understand. It makes sense that you would want to be able to spin faster, and clip your pedals less. I'm probably going to stay with a nice 170mm crank arm, but some people have been telling me that I want 165. Is there any real reason or is that just some knee-jerk fixie dogma that I can ignore? I know some tracks have limits on crank length, is 170 acceptable?

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    if you think about how your body is positioned on a bicycle, you can rotate the same position around the bottom bracket, and as long as the contact points stay the same, the only thing that will change is the angle of your neck. You are at a disadvantage here because you haven't ridden the style of bike you want to build, so you are really just speculating about what is a good position for you. Fixed gear will show the deficiencies in your position. What you say about your current position makes me wonder if the cockpit is in the right place for you. Just for your own info, you should try Peter White's system

    People tend to go to shorter cranks with fixed gear because that is marginally easier to spin with less chance of pedal strike. These are not inconsequential reasons. I'm sure people use 170mm, but I think on the track that is mostly used for pursuit where you stay in a straight line and ride alone.

  9. #9
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    The last concern I'd have is setting a ST length in the design process. Build the frame with the ST full length and cut to length when done. This assumes all the other fit parameters are correct, and cockpit sizing works for you. ST angle will for the most part be determined in the fitting and design process. It seems backward to me to work from a pre-determined ST angle, and try to build the frame that fits and rides nicely.

    I don't ride fixies, but if I were I'd go with as little BB drop as I could get away with while trying to make the fit and frame sizing correct. Crank length? I have built me a couple single speeds, and I determined the crank length I preferred by swapping out lengths and test riding. Having only one speed I find I like the shorter cranks (165s). I believe this would also apply to fixed gear. Braking power of longer cranks? Don't worry about it.

    It seems that all new builders, and especially first-timers have many concerns about design and fit. I was, and am, no exception. I'm also no authority on design and fit at only 13 frames, but I found something that works well. This site- http://strawberrybicycle.com/frames-custom.php has a fitting program that is based on personal measurements, and frame design and sizing is determined from those measurements.

  10. #10
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    Quote Originally Posted by unterhausen View Post
    if you think about how your body is positioned on a bicycle, you can rotate the same position around the bottom bracket, and as long as the contact points stay the same, the only thing that will change is the angle of your neck.
    I disagree. What will change is how your weight is supported -- more on the arms vs. butt the more forward you rotate that position.

    And even more important than femur length for comfy saddle fore/aft is the position of the center of gravity relative to the BB (in the fore/aft direction).
    DT
    http://www.bridersplace.com/

    In between the bright lights and the far unlit unkown...

  11. #11
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    I have not read the Paterek manual, so I cannot comment on any versions of it.

    What I do find interesting is that Ticktocktoe as felt it necessary to push his saddle all the way forward currently.
    Not saying that is wrong, but I get the feeling that other factors are at work to have caused him to go that way.

    Regarding BB drop, in general most framebuilders actually work with drop vs. BB height as this is how the geometry of building a frame gets locked in so you can actually build something. A fixed gear bike often has a slightly higher bottom bracket, before "fixes" became known the extra height was used to avoid scraping a pedal on a steeply banked track, there are a few in the USA where this is really necessary, but those conventions were when Campagnolo old Record pedals were supreme and the axles were quite long, with many current designs this is not a problem. So what pedals and cranks (width and length) would guide me as to how much drop I required, keeping in mind that scraping a pedal is no fun on a fixed gear.
    With all that said I would probably not have a BB drop of over 70 mm, and would probably be in the 60's for a road going fixed gear.

    Seat tube angle has everything to do with your femur length, pedaling style (at a high cadence) and shoe size.

    Way, way back I bought a track bike from a famous maker and assumed it was going to fit, it had a very slack seat tube angle, like 71.5, totally wrong for my dimensions and cadence needs as I was racing under a gear limit. I got the bike sorted out eventually, and was really happy with by next bike which was fit to me.

  12. #12
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by TickTockToe View Post
    Thanks for the answers everybody, they've been quite enlightening. I have no attachment to the 74 degree seat angle, but the placement of the saddle on my current bike feels too far back, and I've pushed the saddle up on the rails as far as it will go. I really need to get a nice angle finder so that I can figure out what my current ST angle is.

    In regards to "femur length influences what seat post angle feels right" I assume the longer the femur the more relaxed the angle. Though it's really only about the fore/aft position of the saddle right?

    All of my friends who ride fixed are trying to convince me not to increase crank length, which i guess I understand. It makes sense that you would want to be able to spin faster, and clip your pedals less. I'm probably going to stay with a nice 170mm crank arm, but some people have been telling me that I want 165. Is there any real reason or is that just some knee-jerk fixie dogma that I can ignore? I know some tracks have limits on crank length, is 170 acceptable?
    Measure the comfortable saddle position on your 'Dale, height from BB and position behind the BB. Plot that (scale drawing) in CAD or on graph paper, and that'w where your seat tube should go. Place the seat post rails in the middle of the saddle rails, and decide if you're going with a straight seat post like a standard Thomson or a setback seat-post like most of the 1-bolt designs. Then the saddle location and how it's to be installed will determine your seat tube angle. Assuming the 'Dale is really a good match to your femur length. I don't really know how to assess that - wish I did!

  13. #13
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    Real framebuilders are working to fit anyone, against a backdrop of everything. When you design a frame for yourself, you are working with one person, and presumably some known preferences and real world parts. Presumably you are serious enough about your riding to have your position worked out, and all you have to do is model that, or make some subtle changes if you think the structure can be improved, or are moving to some different parts. For instance a seat post with a change in offset.

    If in addition I was planing on a frame from a style that I was not familiar with, I would probably look at current models for general info, though if one is making a classic there are probably reliable rules of thumb in Paterek.

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