Of course body geometry is a big determining factor in choosing a frame size, saddle setback, bar reach and drop, stem length and crank arm length...and a whole bunch of other position tuning details. NO ONE is arguing otherwise.
Hip rotation, general flexibility, chosen handlebars, stem spacer setup ALL enter in the equation. As I originally stated, when it comes to speed and aero, long AND low are king. Aero rules in the world of going fast. In order to get aero, trialing out a longer stem or deleting stem spacers is how it's done.
And I 'think' the OP mentioned his stem was already slammed or close to it (with today's tall'ish head tubes on many models, almost a necessity for even 'midget arms' to get more aero) and his steerer tube is already cut. Does that sound like a guy worried about neck pain or brain cancer caused by a stem length change?
However, the OP has many, many miles on his current rig, is riding at advanced speed and mileage (he is NOT a rookie fresh off his Schwinn LeTour or Sears Free Spirit). He IS seeking more speed and a more aggressive position on the bike in order to achieve that speed requirement. And to that I say, "Good for him!".
The most difficult thing to do with your training is...to CHANGE it!
He asked, plainly, if the increased stem length is going to adversely affect handling.
Frankly and truthfully...it is NOT. Tu capisci?
Frame sizing, in general, has been on a trend towards 'smaller' for well over a decade. Back in the day a 90 MM-100 MM stem was max. That usually yields a cramped fit to the 'average' body type (if there is such a thing) and typical fits today start at 100 MM and often exceed that by over an inch.
As far as pro's riding stock frames, many, if not most, of them do. Many do not. Just off the top of my head:
Tom Boonen: Longer than stock top tube. Experienced lower back problems with stock geometry.
Lance Pharmstrong rode a 58 CM TREK in order to get the top tube length he needed.
Little Damiano Cunego used a custom TT length.
Too pressed for time to Google from BikeRadar, Cycling News, etc.
I really don't get it. It's not like the OP is tossing his old stem in the trash and can't bolt it back on if a longer one doesn't work for him. Spending $30-$50 on an experiment in your bike position is chump change. And there's always eBay and Craigslist if it doesn't work out.
Here's a little quality information from someone other than Campybob on stem length:
How does stem length affect a bike’s steering and handling?
by Matt Wikstrom
March 30, 2015
TECH SUPPORTED BY
Today’s marketplace provides a wide variety of stem lengths and angles, allowing riders enormous flexibility for fine-tuning their reach to the handlebars. While comfort and an effective position trumps all other considerations, Matt Wikstrom takes a closer look at the influence of stem length on the steering and handling of a road bike.
Almost since the invention of the safety bicycle, the handlebar stem has served as a crucial point of adjustment. While the earliest designs offered negligible reach, longer, adjustable versions soon emerged.
Marshall “Major” Taylor is credited with the invention of the first adjustable bicycle stem at the close of the 19th century. The design employed a sliding clamp on a protruding length: the handlebars could thus be adjusted fore and aft to suit the size of the rider. Taylor used the stem to great effect, winning a multitude of races, first in the US and then throughout Europe.
Taylor’s “outrigger” eventually gave way to sturdier one-piece designs and ever since then riders have adjusted their handlebar reach by fitting stems of different lengths. Nowadays, there is a choice of stems as short as 50mm and as long 150mm with a variety of angles for fine-tuning the position of the handlebars.
Such adjustability allows a rider to consider a variety of frame sizes and geometries. But the question arises: can a stem be too long or too short for a bike? And is there an ideal length?
No simple formula
Custom framebuilders are perhaps the best source of information on the matter of stem length. After all, they are in the business of building bikes that provide an ideal fit for their clients, and part of that process involves determining the best stem length for each frame they build.
When I put the question of stem length to Steven Jaegher of
Jaegher in Belgium he said, “We try to keep stem length within certain margins in relation to the frame size to keep a nice balance handling wise. Compensating a wrong frame length by mounting a short or very long stem is never a good idea.”
Richard Craddock of
Craddock Cycles based in the UK expressed a similar view, “In general I find the best balance is a medium length stem. Of course, all this depends on your position and whether you have a frame with the right top tube length to allow you to achieve that position with a medium length stem.”
For Tom Kellogg of
Spectrum Cycles in the US, one of the most important considerations is the rider’s reach, which he refers to as ‘cockpit length’.
“It comes down to the cockpit length not rider height. Longer cockpit riders should have longer stem/bar reach combinations,” Kellogg said. “And
the more aggressively a bike will be ridden, the (somewhat) longer the stem. Full on race bikes should have longer stems than touring bikes, all other things being equal, in order to get more weight on the front wheel.”
The importance of the bike’s purpose is something that also stands out for Ryan Moody at
Baum Cycles. “Stem length will vary with the design of the bike. It must also address the rider’s comfort, biomechanics, performance needs, and the steering and handling of the bike.
Deciding on the best stem length depends on the rider’s priorities for each because a relaxed position compromises performance, and it’s not always possible to achieve ideal biomechanics.”
Gererally speaking, longer stems belong on racing bikes for tried and true reasons. As I have patiently explained above. If some touron wants to run a 70 MM stem or someone has a horrible back (as do I and my 'long stem' causes me no problems while cleaning a bathtub kills it), let them. No one is saying the world has to subscribe to 'go fast' advice, but if one wants speed there ARE accepted means to go about it.
