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Seat tube angle? Whadaya mean?

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Seat tube angle? Whadaya mean?

Old 08-26-13, 11:25 AM
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Seat tube angle? Whadaya mean?

I could post this in the the builders' forum and maybe get a good answer. Or post it in the roadies' and get a flame war . Instead I'll post it here where you are my friends and really know what you are talking about.

I've read over and over about seat tube angles, 72 vs. 73 vs 74 vs 75, how it affects the position of the saddle w.r.t. the BB, etc. So let's work out some geometry.

For a 73deg ST angle, if I slide the saddle forward or backwards 1cm in the seatpost on a 59cm frame, it affects the position of the saddle w.r.t. the BB by the equivalent of about 1 deg in angle. Now, IIRC without having a bike here to measure, most saddles can be moved by at least 3cm, maybe more. That means I can change the effective ST angle by several degrees.

For most of my bikes the dominant fit feature is the distance from saddle to bar or to brake levers, once I've got the saddle height correct. Can I reach it or them without feeling too stretched out? So I've swapped stems and positioned the saddles to give me the reach that felt just right, and I've set the stem height too for what feels right. Okay, different bikes have different TT lengths, and I have to accomodate that. But what this means is my effective seat tube angle is not what the builder nominally thought it would be. It is whatever the TT and stem reach combination requires me to get. A shorter TT means I can, and may have to, set the saddle farther back. A longer TT (usually the case with me) may require to steepen my ST angle by moving the saddle forward.

In the end, the relative position of BB, saddle, and handlebar, i.e. the three contact points, are what they are. The only thing a nominally different ST angle gets me is the chainstay length. Now that affects the F/R weight balance and possible tires sizes, and maybe the frame's bump response. But it sure isn't a fixed number, forever frozen to what the builder said it was.

Do any of you feel the same way? Does this make sense?
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Old 08-26-13, 11:37 AM
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A longer or shorter stem thrown into that mix will effect just where that comfy zone lies in regards to the positioning to the BB AND the feel of the steering...This crazy stuff gets complicated! I am always feeling a little scrunched up, usually slam the saddle all the way BACK. I have a "Square" 58x58 I am looking to build up and see how THAT one fits me....Stem/seat fitment will be the deciding factor.
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Old 08-26-13, 12:59 PM
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As stated frames are designed so that the seat tube angle coincides with the effective seat tube angle for the a typically proportioned cyclist for that particular size of frame. However, I disagree with the philosphy of changing the saddle fore/aft position to correct for reach to the handlebars. The saddle fore/aft is agusted based on femur length, to establish the proper position of the knee over the pedal spindle. One of the reasons for the growth is seat tube steepness was to adjust for trend to longer crankarms. As the angle increased the stays were shortened, not because thet could be, but primarily to maintain the weight distribution. Reach should be adjusted only via the stem and bars.
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Old 08-26-13, 01:11 PM
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what T-mar said. I ride long cranks and big frames. I need a zero offset post to position my saddle angle correctly. I don't think I have a frame with a seat tube steeper than 73 degrees. On my road bike the saddle is clamped rear of center on a zero offset post! it looks ugly, so I built my first bike for me with a 74 degree seat tube so I could clamp my saddle in a more aesthetically pleasing position. Once the saddle position is dialed in, it is time to dial in reach and bar height.

Like you have noticed you can change the effective seat tube angle by sliding the saddle fore or aft. This is the reason bike fitters have been championing stack and reach for years and most manufactures of road bikes now report stack and reach in the geometry. The reason is that if you report the position of the center of the top of the head tube from the bottom bracket you can actually compare one frame to another for fit without working out how frame angles affect the fit. If you see a professional fitter they will dial in your saddle position first and then your handlebars.

Maybe adjust reach by moving the saddle. This is just wrong. However, some people are so insensitive to position I am jealous. My body is plauged with old injuries and asymmetries and it very sensitive to fit over long distances.
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Old 08-26-13, 03:58 PM
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Post too long, ha! Recently build an MX Leader 60cm. Found that this Frame had a shorter bb to stem dimension, horizontally, compared to my 57cm Pinarello. Zero offset stem was needed, obviously. Stem .5cm shorter. The overall length of the frame is longer, which leaves the rear wheel(which is tight to the seat tube) set back in relation to my saddle. The frame handles beautifully. However, riding w/o hands requires more speed, as I am more forward in relation to the rear axle.
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Old 08-26-13, 04:47 PM
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Seat tube angle? Whadaya mean?

I'm all over the place with my bikes. I guess I need to get some measurements to make my point, but I'm pretty tolerant of short stems, long stems; wide bars, narrow bars; lots of setback, no setback; etc. I've even been known to ride metric centuries and longer rides with platform pedals.

Am I abnormal?
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Old 08-26-13, 04:58 PM
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Adjusting the seat fore/ aft to arrive at a comfortable reach is just plain wrong. The fore/aft adjustment is for proper pedaling position. Once the pedaling position is correct, install a stem that delivers a comfortable reach.

Regarding st angles, I've had to use setback seatposts on frames greater than 74 degrees. I have size 12 feet and long femurs.

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Old 08-26-13, 05:04 PM
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Originally Posted by photogravity View Post
I'm all over the place with my bikes. I guess I need to get some measurements to make my point, but I'm pretty tolerant of short stems, long stems; wide bars, narrow bars; lots of setback, no setback; etc. I've even been known to ride metric centuries and longer rides with platform pedals.

Am I abnormal?
Maybe just "lucky."

I'm the same way. I can tolerate just about anything but a short toptube/short stem combo. (or a soft saddle, but that's a different thread)

All my keeper bikes are technically "too big" for me (6'0" and riding 63cm frames w/slack angles) but the seat goes all the way back on the rails and I've never met a top tube that felt too long for me. OTOH, I can still be fairly comfortable on a 56cm square bike if it has a crazy-ass long stem and nice long seatpost, but I much prefer a huge frame.
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Old 08-26-13, 05:11 PM
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Originally Posted by T-Mar View Post
The saddle fore/aft is agusted based on femur length, to establish the proper position of the knee over the pedal spindle. Reach should be adjusted only via the stem and bars.
Yep, exactly correct.

Here's the voice of experience on the relevance of seat tube angles:

"aggressive seat tube angles -seventy four degrees- often make it impossible to move the saddle back far enough to achieve the right pedaling position....with a seat tube angle of approximately seventy two or seventy three degrees you should be fine."
-Greg LeMond
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Old 08-26-13, 05:39 PM
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Originally Posted by Bandera View Post
Yep, exactly correct.

Here's the voice of experience on the relevance of seat tube angles:

"aggressive seat tube angles -seventy four degrees- often make it impossible to move the saddle back far enough to achieve the right pedaling position....with a seat tube angle of approximately seventy two or seventy three degrees you should be fine."
-Greg LeMond
Greg's comment was biased to his own body proportions. a 72 degree seat tube on my road bike would not allow me to get the saddle fore enough with my desired crank length of 180mm.
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Old 08-26-13, 05:49 PM
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There are many "right" answers on bike fit. Many races/tours have been won on varying theory's of fit. If you're just throwing your seat around cause you feel like it, & you ride a lot of miles, you are going to end up with injuries. Regardless of theory, you should properly be fit your bike.
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Old 08-26-13, 06:12 PM
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I'm still learning about fit, and I still have a long ways to go. I'm finally beginning to admit, that the Bottecchia is actually just a bit too big for me. I think my "ideal" frame will be either a 58 or 60 cm seat tube, but with a much longer top tube than normal. I've had a couple nice Cannondales in those sizes, that fit pretty good, but too short a top tube. And why is it, that most bikes have such short top tubes, compared to seat tubes?
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Old 08-26-13, 06:23 PM
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Okay, I have to bow down to T-Mar and admit that the connection between seat tube angle and crank arm length has heretofore escaped me. And --please note, I do not scoff-- it still does. I must ponder this. The idea is just barely within my grasp. Gimme couple days and mebbe I'll come to terms with it.

It seems to me bicycle design should start with feet spinning around a fixed point, so so far you only need to know the crank arm length. Next, get the seat in relation to that; but this is highly flexible since seats adjust fore to aft. Next, place the handlebar in relationship to the other two. And now you set the steerer angle and offset... And the size... and... test ride?

Like my friend photogravity I can ride any bike 30-40 miles without complaint. Only after 80 miles can i distinguish between the too long and the too steep. And even then we can not disregard simple questions like, did you hydrate? It may prove bikes with water bottle cages fit me better than those without. Add a banana hammock and... Yes, potassium matters!

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Old 08-26-13, 06:37 PM
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rhm, I finally sat on that Norex saddle today for a bit, on the Bertin. I believe I "could" get a feel for that saddle, on the right bike, but it's just too scrunched up, on that frame, and I end up on the very back, which won't work. But if I could tip that saddle forward a touch & slide it back a few inches, it might work.
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Old 08-26-13, 06:43 PM
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Originally Posted by T-Mar View Post
The saddle fore/aft is agusted based on femur length, to establish the proper position of the knee over the pedal spindle.
That's another big can of worms to open, "the proper position of the knee over the pedal spindle" that could very substantually depending on rider and what works. KOPS means knee pain for me.
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Old 08-26-13, 07:05 PM
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Originally Posted by T-Mar View Post
...However, I disagree with the philosophy of changing the saddle fore/aft position to correct for reach to the handlebars. The saddle fore/aft is adjusted based on femur length, to establish the proper position of the knee over the pedal spindle.
T-mar, I highly respect your opinion, and I freely admit that I am no racer, never have been. But I must argue (quietly) on these two points.

First, the relative position of the femur or knee w.r.t. the pedal varies greatly through the power stroke because the pedal moves around, actually fore and aft by some fraction of 170mm, while the knee must follow the small arc allowed by the hip. Unless I'm standing on the pedals which means gravity is pulling me vertically then my force on the pedals could be applied over a range of possible angles, and if I'm pulling upward on the handlebar then that vertical direction may not be so vertical after all. So I can't see how there would be one optimal position. If you make it "optimal", whatever that may be, for one place in the power stroke, you will make it sub-optimal somewhere else. Yeah, I've read some of the treatises on optimal fitting. I know enough of the math and physics to understand that there are many factors at work, and I've never seen a complete analysis that satisfies me. In actual riding someone may, or at least I do, shift my seating position enough anyway to make tweaks of a few millimeters pointless. In any case, I've never found fore-and-aft saddle position to make much difference, certainly not as much difference as having the height correct. In most demanding situations I'm limited more by aerobic capacity than by leg power. But then, as I said, I'm no racer and have never been interested in that last one percent. What I do know is that if my back is comfortable I can ride longer and faster. I've swapped stems on 4 of my 6 solo bikes to help achieve that comfort. The frames are what they are, and I have to work within those limits.

The second point is that if one positions the saddle for the reasons you and others stated, then you are repositioning the saddle w.r.t. the BB to whatever it "should be" for your particular reason. Okay, it's a different reason than the one I raised, and the final saddle position may be different, but in the end the same geometric effect happened. A steeper angle but more offset backwards at the seatpost will produce the same effective ST angle as a shallower angle with less offset. It's simple geometry. The only thing the builder did for you by selecting a particular angle is define the limits of the adjustments you can make.

I guess the point of my post is this. People and books and articles and BF posters wax poetic about angles. They make pronouncements about how a 74deg bike means such and such and 73 means such and such, etc. But if I can negate that extra degree by moving the saddle, regardless of why, then that angle is nothing more than a starting point. As you said, the trend for steeper angles is to accommodate a trend to longer cranks. I've never seen that argument before (not that I've looked very hard), and it makes perfect sense. Just don't anyone argue that steep ST angles are meant to make a bike more "racing" or "sporting" or whatever. You and I and everyone else who bothers with it are going to settle on whatever effective position works, for whatever reason is right for our riding.

I feel better now.
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Old 08-26-13, 09:02 PM
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Originally Posted by rhm View Post
Okay, I have to bow down to T-Mar and admit that the connection between seat tube angle and crank arm length has heretofore escaped me. And --please note, I do not scoff-- it still does.
Here's how I read that riddle. (In fact I am reading a book which bring up this point obliquely, and this is why I started this thread.)

The supposed reason for choosing a ST angle is to place the knee correctly over the pedal. (Exactly what that means isn't obvious to me. Presumably it is directly vertical over the pedal because the maximum power point would be with the crank level and forward. I'd argue that the angles and muscle effectiveness vary so much that this isn't necessarily the most important crank angle, and that in fact the integral of torque over the entire rotation is what really matters. But that's another story.) T-mar's point is that if the crank is longer then the foot will be further forward when the pedal is in the forward-level position. To keep the knee above the pedal the saddle must be pushed forward by that extra length. If the saddle can't go any more forward on its rails then you accomplish the same thing by using a steeper angle.
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Old 08-27-13, 05:40 AM
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http://www.stevehoggbikefitting.com/...leat-position/
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Old 08-27-13, 09:50 AM
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It's interesting that this thread has been entirely about fit, not ride and handling characteristics. Unless I'm missing something.

I'm sure T-Mar knows more about this than I do, but I would ascribe the rise in SP angles to the ever greater influence of race bike geometry on consumer road bikes more than a change in crank length.
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Old 08-27-13, 10:19 AM
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All very interesting.

I guess this as good place to mention that the Brooks B17 saddle frame has not changed for over a half century. Photos of bikes from the 40's show the saddle clamp mounted to the front of the post. By the mid 50's the clamp had migrated to the back of the post, and when integrated posts came out, they maintained that same position. More recently, you can get seat posts with more and more setback. And people now complain that Brooks saddles don't have enough fore-aft adjustability.
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Old 08-27-13, 10:23 AM
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Originally Posted by Fred Smedley View Post
That's another big can of worms to open, "the proper position of the knee over the pedal spindle" that could very substantually depending on rider and what works. KOPS means knee pain for me.
I did not mean to infer directly over the pedal spindle, as many cyclists prefer slightly ahead or behind. Perhaps I should have stated to use the saddle fore/aft adjustment to achieve your traditional knee position relative to the pedal spindle. The basic point was to use the saddle to position yourself relative to the pedals, not the handlebars.

Until fairly recently, most of the theory on riding position was derived empirically based on what had been successful for the pro cyclists. KOPS remains one of the most popular and in my 20+ years as a certified coach I use it as my starting point. However, if the cyclist's traditional position is too far off KOPS, I also do not immediately position him per KOPS. Large sudden changes can lead to trauma related injury.

The human body can be remarkedly adaptive, particulalry when young, large. Studies have shown that, for most cyclists, power does tend to fall off as you deviate from KOPS, however the drop is not as much as most cyclists expect and deviation is highly dependent on how long you have been using that position.
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Old 08-27-13, 10:40 AM
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Originally Posted by Standalone View Post
It's interesting that this thread has been entirely about fit, not ride and handling characteristics. Unless I'm missing something.

I'm sure T-Mar knows more about this than I do, but I would ascribe the rise in SP angles to the ever greater influence of race bike geometry on consumer road bikes more than a change in crank length.
My thoughts exactly. As I read, Eddy Merckx had back issues after his big crash early in his career. He spent untold hours working out fit & as a builder, geometry to build a bike who's comfort was as important as its effectiveness in its speciality. A lot of what we see in current production probably has a lot if influence from Eddy & other builders like him.
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Old 08-27-13, 10:58 AM
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Originally Posted by jimmuller View Post
T-mar, I highly respect your opinion, and I freely admit that I am no racer, never have been. But I must argue (quietly) on these two points....

First, the relative position of the femur or knee w.r.t. the pedal varies greatly through the power stroke....

The second point is that if one positions the saddle....The only thing the builder did for you by selecting a particular angle is define the limits of the adjustments you can make.

...You and I and everyone else who bothers with it are going to settle on whatever effective position works, for whatever reason is right for our riding.

I feel better now.
I'm glad you feel better.

Yes, the position of the knee is going to vary thoughout the revoltion and therefore power will vary thoughout the stroke. For the typical competitve cyclist, the goal is position the knee relative to the pedal to achieve the highest total power poutput over a revolution. Most studies I've read corroborate the KOPs method, however as mentioned earlier the fall-off based on the deviation from KOPS is not nearly as high as expected and can often be mitigated by other factors, such as how long one has been rding in a non-KOPS position.

Agreed (and I believe I stated this earlier), the seat tube angle is chosen by the builder to coincide with the effective seat tube angle for a average portioned cyclist using that size of frame. This will give most owners approximately equal fore and aft saddle for fine tuning the position to personal taste. Of course, this can also be affected by saddle and post design.

My philiosophy has always been that you need to be comfortable on the bicycle, as you are most efficient when you are comfortable. All the rules-of-thumb are only starting points. They'll put most cyclists in the ballpark, but many will require fine tuning and some can require radical adjustments based on their physiology.
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Old 08-27-13, 11:32 AM
  #24  
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Originally Posted by Blue Belly View Post
My thoughts exactly. As I read, Eddy Merckx had back issues after his big crash early in his career. He spent untold hours working out fit & as a builder, geometry to build a bike who's comfort was as important as its effectiveness in its speciality. A lot of what we see in current production probably has a lot if influence from Eddy & other builders like him.
I have in mind the Atala that got me into road biking when I was 11. 70x70 geometry. Great for long rides on rolling hills-- no fun going up the bigger hills, but all in all a bike that asked to be ridden long distance. Steeper head tube angles and fork trail characteristics have major and complicated impacts on handling.

It's more than just fit and efficiency.

A fork rake and trail calculator: http://yojimg.net/bike/web_tools/trailcalc.php
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Old 08-27-13, 11:34 AM
  #25  
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Originally Posted by Standalone View Post
It's interesting that this thread has been entirely about fit, not ride and handling characteristics. Unless I'm missing something.

I'm sure T-Mar knows more about this than I do, but I would ascribe the rise in SP angles to the ever greater influence of race bike geometry on consumer road bikes more than a change in crank length.


Great thread here, much thinking going on! (!)

Cool that I both so much agree with so many comments here, and also so much disagree.

This stuff is like the Holy Grail of cycling wisdom.

As far as handling, this is what brought me to collecting bikes in the 90's.
I had bought a relatively large Epic Allez with 74-degree angles and wondered why the bike (actually me the rider) seemed to always take to the inside line too early in high-speed turns.

So I sold and re-bought the same bike, this time in a smaller size, using a longer stem.
The handling was transformed, now it was a neutral handler!
I learned right away that a shorter stem and higher handlebar each caused the steering to be more flighty, whereas the identical, smaller frame with slightly longer stem and lower handlebar was plenty stable.

I later built up and used frames with very relaxed angles, like 70 degrees for a Schwinn Varsity, 71 degrees for a Steyr Clubman, and 72 degrees for a Nishiki International. I used frames from 56cm up to 62cm and reversed the seatpost clamps where necessary, to move the saddle foreward.
The more-relaxed-angled frames needed to be larger sized, both for handlebar height and for not having to use a long stem above 90 or 100mm.
The relaxed angles didn't play nice with the steering feel when longer stems were used, making the steering floppy when pedaling while standing for sprinting and climbing (I needed a wider bar to control the steering as the stem length was increased).

Regarding shorter cranks, I prefer them for the same reason that I prefer a more-foreward saddle/bar position, that is that transitioning from seated to standing becomes less of a heave that can put an ache in your fatigued knees. I like being able to more effortlessly change position according to the pitch of the terrain.
A more-foreward rider position seems superior in that one can get comfortably aero with less severe of a bend at the rider's waist, which allows a higher continuous power output to be maintained.
Being more-foreward of the bb also means that a higher pedaling force is needed to prevent too much of the rider's weight from being transferred to the rider's arms, so shorter, higher-effort rides will benefit from a more-foreward position, while casual-paced century rides will not.
As for the go-fast type of riding, one adapts to the arm stress by achieving some muscle development in the arms and shoulders, and while this is initially uncomfortable, the rider's arms will probably be nowhere near as sore as the rider's legs.
I use auxiliary "safety" levers on the brakes so that my tired arms can apply the brakes from the top of the handlebars after a long ride has left my arms and shoulders too tired to comfortably use the main brake levers while descending.

So, in conclusion:

A twitchy steering frameset benefits from a longer stem and/or lower handlebar, so mustn't be too large for the rider.

A relaxed-steering frameset may benefit from a shorter stem and higher handlebar, so should be a larger/longer frame, and a longer stem at some point needs to be balanced out with a wider handlebar.

A more-foreward rider position relative to the bb eases high-effort and/or shorter rides by allowing a comfortably higher power output while maintaining a more-aggressive aero position.

Shorter cranks and especially a more-foreward seating position ease the transition from seated to standing.

The rider/builder needs to know the basic relationships between seat tube angle and top-tube and stem length in order to effectively select and get dialed into a proper frame fit.

A more-foreward handlebar gives the rider more of a direct push from behind with every pedal stroke, instead of the bike wasting the rider's energy by moving flexibly fore-aft under the rider. This really helps sprinting and helps attacking short, rolling hills!
The handlebar position should firstly be adjusted for optimum position when sprinting and climbing while out of the saddle, then and only then should the saddle be adjusted to meet the rider's seated position requirements.

The reach from the saddle to the bars needs to be what it needs to be, i.e. comfortable. Tipping the saddle down in front allows a somewhat longer reach to be employed if the rider is enough of a hard-pedaling type to suspend more weight on the pedals (instead of the saddle and handlebar), but having to reach too far from a rearward saddle just doesn't work and would of course be further aggravated by a leather saddle's hard nose.

Lastly, Greg LeMond's position is very unusual in that he achieved an aerodynamic, long foreward reach ahead of the bars while leaving his saddle further back than a typical racer's!
I concur with the larger frame-sizing spec of his LeMond-branded bikes, but could never stretch that far from saddle to handlebar, so my saddle comes foreward before riding my LeMond Chambery. This frame is labeled 57mm, but is identical to a similar Trek frame that is labeled as 60cm.

PS; My recent preference is toward a relatively large frame (for my 5'9" height) with more-foreward saddle/handlebar positioning.

I'm not doing any roadracing, so also prefer a relaxed head-tube angle.

The blue bike pictured below has such relaxed angles that I had to use a wider handlebar in order to control the steering with the longer 110mm stem.

The rider position on the silver bike is so far foreward that I have to limit my rides to 3 hours or so, but the aux brake levers allow me to more comfortably finish the longer rides. This old bike can achieve good aerodynamics and surprisingly high speed.

The faded 1952 purple bike has angles like a track bike's, i.e. north of 76 degrees head and seat tube angles.
The bike is almost too twitchy to reach down a grab the bottle while riding in less than perfect conditions.
The handling would benefit from a 2cm longer stem and foreward saddle, and would then be even more of a climbing rocket (whether seated or standing). The old bike is a super-quick climber as it is (and has to be with a 47 tooth small ring).



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