ok, it's taken me three weeks of reading and re-reading everything I have at home and on the internet about rake, trail, caster and steering angles. This thread has kept me awake at night dammit! Just as Dan says on his
Steering geometry page, "the sum of facts I know is greater than the degree of understanding I possess..." This may explain why Dan and I disagree on 'ideal stem length for road'.
Stem length does not affect handling of a bike.
Nowhere in the study of bicycle dynamics or frame design is the stem length discussed.
Stem length affects the level of comfort for the rider. Given the same frame, changing from a 70mm stem to a 120mm stem only changes the riders' reach, some weight distribution and SOME relationship on how that rider behaves moving thru space suspended between the two hubs. The steering axis of the frame, rake and trail remain the same. Unless you are doing loaded touring, and must consider 15-20lbs of load on the front end. But that is a secret password-protected section of Slowtwitch. Cyclenutz' chart is cool, yes, but his shaded band is showing how negligible the change is when playing with spacers, pitch and stem length. Which is a better solution, the 100mm stem at -17/20 spacers at 305 or or 100mm stem at -6/20mm spacers with 308 score? Determining which solutions 'handles better' there madness lays. But a shorter or longer stem for the rider may reduce neck/shoulder tension, reduce hand fatigue or cause other tangible (and squishy) results. Plus we're not even talking bar reach either.
Solving the x/y position for hood fit to the hands is pretty much 'it' in regards to 'ideal stem length'. Dans' conventional wisdom of Rider Height/stem length works here: if your frame size calculated with Stack/Reach is requiring a 150mm or 40mm stem something is wrong in the fit or the frame choice.
Anecdotally, whenever I have fit a lady, and she walks in with a Lemond frame, I instantly know the frame is too big. Lemonds' steepest seat angle is 72deg (61cm!) and his average seat angle is 72.5. The smallest frame size (45cm) is a 75deg SA. So ya, I put a lot of stumpy small stems on ladies' bikes over the years, didn't affect their handling at all. The fit was certainly better, but you can only move the saddle forward so much.
There is some Ego involved in stem length too, I admit. Can't imagine myself riding anything shorter than a 90. :P
for Tri bike: I'm still struggling, but instead of hand position we're looking at elbow position as the weight bearing point of contact. As we have more experiences with pedestal-style tri bikes, we need to see what sort of handling issues come out of rider position.
Anne Barnes
ABBikefit, Ltd
FIST/SICI/FIST DOWN DEEP
X/Y Coordinator
abbikefit@gmail.com