Sep 25, 2003 11:11 AM
I read some of your postings regarding fit and specific measurments for "stand over height vs. inseam", "height of seat from top tube", etc.
I'm not disputing any of your recs. (as a matter of fact all your guidelines fit my frame set-up).
I'm just wondering where you got these guidelines and if there are any others that can help to fine tune fit?
Sep 25, 2003 2:50 PM
|I've been riding seriously for twenty years. I do all my own mechanical work. I'm also a mechanical engineer, so I find the geometry and trig. interesting.
One of the most important things I've learned is not to set my saddle too high. Many years ago I followed the bad advice (often written in the bike magazines) to raise your saddle until your hips rock when pedaling at a high cadence. Worst advice in the world. I advise, as a very rough guide, to be sure that you can drop your heel well below horizontal with your leg locked out at the bottom of the stroke. This just insures that you have some bend in your knee with your foot relatively horizontal at the bottom of the stroke. My pedaling is a lot smoother since I've paid attention to keeping the saddle low enough. I frequently get compliments on the smoothness and speed of my pedaling.
I don't consider bike fit that tough. If you get 4cm of standover clearance (to a horizontal top tube), you can't be far off on vertical size. The vertical size of the bike is defined by the top tube height (with conventional geometry) and the head tube length. If the frame has a sloping TT, then head tube length is the critical vertical dimension.
Horizontally, TT length and seat tube angle must be evaluated together. As I've posted many times, TT lengths on two frames can't be compared directly, unless the seat tube angles are the same. If they are different, then applying the simple formula: saddle height x (cosA-cosB) provides the value for "equalizing" the two angles.
After reading dozens of geometry charts, I find that only a few brands differ significantly in their stock TT length. Most brands stay in a very narrow range. At the extreme ends are long Trek top tubes and LOOK's short top tubes (due to their shallow STA). LOOK's standard geometry with the 72.5 STA across the entire size range is well suited for those interested in trying a further-back position. LOOK has revised this geometry on some of their newest models and now seems to be following the steeper STA crowd.
My stem length guideline is for the racing oriented crowd. As long as your knees and elbows don't overlap when riding in the drops with the upper back horizontal, the stem is long enough. Additional length only moves the arms forward and puts unnecessary stress on the shoulder. It does not put more weight on the front of the bike, contrary to popular belief.
Providing guidance on saddle to bar height difference is a bit tougher. I like the bars low enough that I can ride in the drops with the back horizontal and only a slight bend in the arms. If you have the bars too high, then you need a lot of bend in the elbow to get low. Knee to elbow interference can then be a problem. Some folks ride with their elbows flared out to cure this problem, but I don't buy into this approach. It's quite easy to get the bars too low these days, particularly with integrated headsets. Seems like most head tubes are too short. Manufacturers are not making the total length of the head tube with the headset as long as it would be with a conventional (threadless)headset. That's why I stress the importance of the head tube length, to prevent ugly setups. I strive to get a no-spacer setup if possible.
|stem length & weight distribution.....||Synchronicity|
Sep 25, 2003 4:30 PM
Additional length only moves the arms forward and puts unnecessary stress on the shoulder. It does not put more weight on the front of the bike, contrary to popular belief.
c-40, not to start a flame-war or anything, but if you admit a longer stem moves the arms forward, it must put more weight on the front. Don't believe me?
Look at it this way: if you put a 500mm stem (say) on the front of your bike, you'll REALLY be reaching over a lot. That will bring more weight forward.
Think of the force as acting down through the place where your hands touch the handlebar, not where the stem attaches to the steerer tube. The stem transfers the force through it, sure, but think of your old "moment diagrams".....
Or think of it this way: if you put a stem the other way round (ie extending BACK over the top of the top tube), you'll have a very upright position..... which will bring weight to the rear.
Sep 25, 2003 7:36 PM
|What I proabably should have said is a SIGNIFICANT change in weight distribution. One of the things you learn in engineering is to pay attention to the things that account for 99% of a situation and be smart enough to know when to ignore the rest. Some folks seem to think that changing from a 100mm stem to a 120mm will significant alter the weight distribution, either for the better or worse depending on the riders imagination.
If you want to get real picky, you make insignificant changes to weight distribution when you scratch your ass or reach for a water bottle. The weight distribution would also be different riding in the drops, on the hoods or on the top section of the bars. Most of us somehow manage to stay on the road despite these constant changes to weight distribution.
Put your bike on two scales some time and move your hands forard or back 1cm. Unless those scales have extremely fine resolution, you couldn't tell any difference. If you are riding properly with your weight balanced over the saddle you won't have over a few pounds of weight pressing on the bars in the first place.
|That's the trouble with engineers...............................||Synchronicity|
Sep 25, 2003 9:02 PM
|"One of the things you learn in engineering is to pay attention to the things that account for 99% of a situation and be smart enough to know when to ignore the rest."
How do you know /when/ to ignore the rest if you don't investigate that 1% possibility?????
In science we are trained not to ignore the 1%. If you are designing or analysing something precisely, that 1% is HUGE. We talk in parts per million. (0.000001%)
Physicists talk in numbers even less than that. From memory, 760 torr is atmospheric pressure. An ultra-high vacuum is one of about 0.0000000001 torr. But a zero torr vacuum is totally unachievable.
To give another example closer to home, the difference between a 0.2% carbon steel and a 1.0% carbon steel is significant. One is very ductile, the other is quite brittle.
What about a 1% elongation in a bridge due to thermal expansion? The coefficient itself is even less than that, but if you don't take it into account the bridge will buckle in extremely hot weather and probably collapse.
I know you /try/ and be smart enough to know when to ignore the rest, but sometimes it comes back and haunts you.
Yes, you should have used the word "insignificant"; and I am nit-picking. But if you come across as an all-knowing engineer, then someone is bound to nit-pick when you make an oversight.
(BTW one of my best friends is an engineer)
|No, actually this is possible THE best example:||Synchronicity|
Sep 25, 2003 9:37 PM
|If your clocks were "only" 1% out at the very start of new years day on January 1st, then by the end of the year, you'd be having christmas a full THREE DAYS, 13 HOURS, 55 MINUTES AND 12 SECONDS EARLY!! (or late).
But only if it wasn't a leap year!
Then you'd be having it 14 minutes and 24 seconds earlier (or later) still.
Seriously though, I know you didn't /really/ mean 1% literally.
Sep 26, 2003 4:56 AM
|not to start a flame war but....||C-40|
Sep 26, 2003 6:45 AM
|your comments are far off base. You've manage to take numbers and misconstrue them into statements that are completely irrelevant to the topic. Are you one of the folks who've stated this theory about stem length?
In the case of a steel bridge, it's simple to calculate the amount of elongation using (.00000633 per degree F, per unit length) as the coefficient of expansion. It would take a temperature rise of 1580 degrees to create a 1% thermal expansion by the way, so in real life, engineers do worry about very small percentages when applicable. In the case of the bridge, the 1% value that would be ignored would be in the width of the expansion gap. Properly designed, the gap would have a generous safety factor and a width tolerance where 1% of the gap width was totally unimportant.
|Just an analogy, nothing more. Only to emphasise my point.||Synchronicity|
Sep 28, 2003 4:40 PM
|Okay, so I never bothered to calculate the expansion of the bridge. That's plainly obvious. You didn't happen to check my clock calculation, did you?
But my point is you still shouldn't totally ignore something seemingly small and insignificant (like thermal expansion). And actually engineers haven't forgotten about it, as you said it is taken into account with the expansion gap.
So maybe you engineers don't ignore little things as much as you think you do. Maybe you just like to think you have 99% of everything in control, and pretend not to worry about seemingy insignificant things like the 1%.
Still, my point is this: WHEN do you know if these "small percentages" are applicable? I'll tell you when - ONLY /after/ you've calculated whether the effect will be significant or not. It might be a rough calculation, but you should NEVER just dismiss anything if it has been brought to your attention.
You're the engineer: part of your job is to work out whether the insignificant things are indeed insignificant. If they are, then you don't incorporate it into your design. But how do you know?
I think a "smart engineer" /would/ calculate for small things that he has thought of, or that someone else has brought to their attention. Conversly, I don't think they'd be a very smart engineer if they just ignored things that people in other fields were telling them all the time.
Sep 25, 2003 10:33 PM
|That's pretty much true regarding stem length and weight distribution. Having said that, on my MTB I noticed considerably less grip at the front when I tried a 100mm stem versus my standard 120 (angle, make, model, further identical). It just seems to track less securely and constantly gives the feeling of washing out in higher speed leaning turns...not sure what the true cause is.|
|Your CG influences weight distribution on the bike,||Spunout|
Sep 26, 2003 3:50 AM
|so moving your arms 20mm will not do it in itself. The fact that moving your arms makes your upper body (and pelvis) tilt forward moves the CG and helps your bike track better.
On a shorter MTB setup, one would bend elbows and lay the upper body flat to move CG forward and track up a climb.
|That sounds logical. Thanks for the explanation. nm||divve|
Sep 26, 2003 5:32 AM
Sep 26, 2003 5:34 AM
|You've got some strange ideas. I rotate my arms forward or backward at least a couple of inches at the shoulder and never move the remainder of my upper body up at all. All that HAS to change is the angle of the upper arm.
If you want a significant change in CG, you need to move your entire body forward by moving the saddle or get a frame built with different geometry, for the specific purpose of creating a specific CG.
Sep 26, 2003 11:51 AM
|that's the case with my MTB because I'm not resting in my saddle by folding down so to speak. It's a more upright position and tilted toward the handles bars with more weight on my hands.|| |