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  1. #1
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    Deviations from Specified Geometry on MFG Bike Frames?

    I've been entering the details of my Lemond Sarthe frame (hand built, TIG welding in the USA) into the on line version of BikeCAD and have been surprised at both the difficulty and frustration in reconciling the differences between the published geometry/specs and what I am able to measure.

    Many tubing lengths are 1 - 2 mm different and angles are out by at least 0.5 degrees, maybe 0.75 degrees in some cases. Basically, it seems that it is impossible to make it work in Bike CAD without accepting some variation (1 to 2 %). I don't think the problem is with BikeCAD.

    How much do the specs vary for various brands of frames, or even between the same models/sizes? How much variation is considered acceptable?

    When these bike frames are being assembled, are the last tubes "cut to fit" or is this slop just due to the nature of welding (heating/expansion, etc.)

    Does any one know where/who used to build the Lemond steel frames?
    2006 Lemond Sarthe
    2000 Trek 7500FX

  2. #2
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    I don't know anything about that particular bike, but I don't think the kind of thing you are finding is unusual.

    I would break it down into two catagories. 1) difference between specs and frame, which is normally a good thing. 2) QC in manufacture.

    Where "1" is concerned I just looked at the table, and it isn't my kind of bike so I might not spot anything, but generally the fixed parts like CS length over certain sizes do appear to be accomodated to some degree under the variable items like the HT angle. Quite often the measurements in the geometry trables are for show. But there is some plausibility here.

    Where 2 is concerend I would wonder how able you are to measure some of this stuff, just knowing how difficult it can be to get a level to sit on head tubes unless it is the perfect length and there isn't any paint build or head badge to get in the way. It is pretty hard to mis-saw a tube by 2mm in a production setting, so I would tend to put apparent erors down to geometry fuzz. If they are left/right problems, and there are also alignment problems then the frame may have problems. Factory frames aren't built like custom frames though.

  3. #3
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    Quote Originally Posted by Peterpan1 View Post
    I don't know anything about that particular bike, but I don't think the kind of thing you are finding is unusual.

    I would break it down into two catagories. 1) difference between specs and frame, which is normally a good thing. 2) QC in manufacture.

    Where "1" is concerned I just looked at the table, and it isn't my kind of bike so I might not spot anything, but generally the fixed parts like CS length over certain sizes do appear to be accomodated to some degree under the variable items like the HT angle. Quite often the measurements in the geometry trables are for show. But there is some plausibility here.
    Where 2 is concerend I would wonder how able you are to measure some of this stuff, just knowing how difficult it can be to get a level to sit on head tubes unless it is the perfect length and there isn't any paint build or head badge to get in the way. It is pretty hard to mis-saw a tube by 2mm in a production setting, so I would tend to put apparent erors down to geometry fuzz. If they are left/right problems, and there are also alignment problems then the frame may have problems. Factory frames aren't built like custom frames though.
    You're right about the difficulty in measuring a bike, but some measurements can be done pretty accurately by using a plumb bob, level and tape measure once the bike is secured in a vertical/square position. Other measurements, like the top tube and seat tube (c-c) cannot be measured accurately.

    When I enter all the measurements I am confident about into BikeCAD and use the published specs for the rest of the measurements (that I can't take myself), that's when it becomes apparent that there is a problem. Something isn't right and like you say, usually it's something like a tube angle that has to move to make it work.
    2006 Lemond Sarthe
    2000 Trek 7500FX

  4. #4
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by bccycleguy View Post

    I've been entering the details of my Lemond Sarthe frame (hand built, TIG welding in the USA) into the on line version of BikeCAD and have been surprised at both the difficulty and frustration in reconciling the differences between the published geometry/specs and what I am able to measure.
    I don't know anyone's background here, but just being logical about it, if you're comparing your measurements with published specs and they don't agree there several possible explanations:

    1. The published specs are disingenuous, i.e. not seriously representing the product, aka, just not valid.

    2. The frames are not built to match a valid spec, either due to loose QC, or it's just the way they do it.

    3. The measurement technique or instruments are not capable of adequate accuracy to be compared to the published specs.

    I've tried very hard to measure bike frames, and I don't think it's easy to be accurate and consistent. Can you measure the same bike several times and come up with exactly the same results (I usually can't, BTW)? If not, your technique is not consistent (after all, the frame didn't just shrink 2 mm). And if it's not consistent, how can you say its not your fault if your numbers do not match those of the bike company?

    If it is consistent, how did you test if it was accurate?

    I come at this from an engineering background, not a frame builder's background. I've found I don't measure consistently, so I know this problem can be real. For me, accuracy is not the issue, since I'm not even consistent.

    Road Fan

  5. #5
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    I believe all three issues are at work here.

    Certainly when it comes to my own measurements, accuracy is a problem. I can only measure to the closest millimeter, so the accuracy is 0.5mm. Precision is also a problem, given that repeated measurements often vary by about 1 mm.

    The MFG specs are only given to the closest mm so that also is an accuracy problem.

    This is all about my trying to accurately determine the X and Y position of my handlebar position relative to the bottom bracket on the Lemond, so that my newly designed bike would have the same HB position with different geometry. I have been able to do this to within an overall accuracy and precision of 1.0 mm which is close enough, but I think this is approaching the same standard to which these bikes are built in the first place.

    The only way you could really find out would be to take a trial sample, say 10 same-sized bikes and measure each one in some kind of special jig so that they were all measured the same.

    I also suspect that there isn't a lot of communication between the engineering & design people, the marketing people and the fabrication people once the bike is in production and in fact all of these groups of people may work for different companies. What is published for specs may have been what was contemplated before production, but not what it evolved into once the realities of production entered the equation.
    Last edited by bccycleguy; 06-16-08 at 07:34 AM.
    2006 Lemond Sarthe
    2000 Trek 7500FX

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