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I have tried to learn about bike steering traits and differences between brands and models, since I have two bikes, a Felt and a Giant, and I prefer to climb standing. My Felt bike (2012 F2, 58cm) feels great because it holds a straight line in a steep climb standing and hauling on the bars. While my Giant TCR (2016, 56cm) requires more attention to keep it going straight up. The Felt has a head angle of 74 degrees with 43mm fork rake, and the Giant has a head angle of 73 degrees with unquoted rake.
Recently, you came across as against steep head angles, but I can tell you that, in spite of the quick steering, I can confidently ride the Felt no-hands at any speed and through ruts and slight inclines in a moderate breeze, while I have a hard time doing the same on the Giant. What does this tell you?
It’s no surprise that your Felt holds a straight line better while your Giant weaves back and forth more when standing and pulling on the bars on a steep climb. The Felt’s fork trail is 52mm and wheel flop is 14mm (due to its head angle of 74 degrees, 43mm fork rake, and assuming 676mm tire diameter), while the Giant TCR has 56mm of fork trail and 16mm of wheel flop, due to its head angle of 73 degrees, 45mm fork rake, and assuming 676mm tire diameter. The stock TCR fork has 45mm of rake; if it were 43mm, your bike’s trail would become 58mm. At slow speed, the lower the fork trail — and lower the wheel flop — the less the bike weaves back and forth. I explain the fork trail and wheel flop with diagrams and photos.
As for riding no hands at all speeds and under various conditions, there are many variables to consider, the most significant one being the rider. As you point out, the Felt would be quicker steering at speed. This is due to the lower stability created by the lower fork trail and wheel flop.
Rather than you riding them without hands, if you were to instead load up, say, 150 pounds on the saddle of each bike and give it the same magnitude push on a flat surface, like a parking lot, the Giant would stay up longer before falling over than the Felt. If you were to roll the tires through the water at the beginning of pushing the bikes, the wet track would show the Giant weaving back and forth more. The Giant’s higher fork trail and wheel flop causes the front wheel to turn more sharply into the lean and get the front tire contact patch under the center of gravity of the loaded bike more quickly, thus righting it and allowing it to straighten up and then lean the other way and repeat the process.
The bike with greater fork trail (the Giant) will weave drunkenly back and forth longer and with greater amplitude, leaning and flopping one way and the other, as it goes slower and slower, before ultimately flopping over, while having gotten further across the parking lot before collapsing into a heap. This is what is meant by greater stability, although that doesn’t necessarily translate to feeling more stable to a rider atop it. Without a rider, who can, even no hands, keep a wide range of bikes upright, the characteristics of the bike can be more clearly seen.
At lower speeds and in dips, which magnify the steering responses of the bikes I describe above, the Giant will feel less stable due to leaning and weaving back and forth more than the Felt. At high speeds, I would expect both bikes to have enough stability that they would not give the rider a scare when riding no-hands; they both have sufficient fork trail to guarantee that. If that is not the case for you, I would look for other causes.
Interchange the wheels/tires on the two bikes, for instance, and then re-try riding them no hands at speed. Fore-aft position of the saddle relative to the bottom bracket, since the rider is not holding the handlebar, will play a key role.
Even the saddle shape is important. When sitting up no hands, a flat, wide saddle will make the bike feel less stable than would a more rounded, narrower saddle, because the weight is concentrated on one sit bone or the other as the bike leans back and forth, as opposed to a smoother weight shift with a rounded, narrower saddle. I recommend you consider these and other variables when comparing no-hands riding on the two bikes and get back to me with what you find out.
Having plenty of time on my hands this weekend during the COVID-19 shutdown, I got to thinking about bicycle frame geometries. My current one-bike solution is a 2016 Litespeed T5 gravel I built up from a raw frame — the best ride I’ve ever had — and I’ve noticed that over the past few years they’ve modified their geometries a bit, perhaps to reflect the trend for more performance-oriented gravel rides? Anyway, I wondered: Which frame would I buy to replace the T5g if something ever happened to it? Whose geometry comes closest and can be compensated for via seat post and/or stem/spacer adjustments?
I wrote up a little set of methods in Python to plot various geometries, and as I was coding, it occurred to me that not all of the frame specs were actually required to specify the frame geometry. For example, the 2016 T5g listed the following data:
- seat tube*
- top tube (eff)
- head tube angle*
- seat tube angle*
- bb drop*
- head tube*
- bb height
- front center
Where the * indicates my understanding of the fundamental geometric constraints on the frame’s geometry (i.e., the un-starred items can be derived from the starred ones.)
This reminded me of holonomic constraints and generalized coordinates from my graduate mechanics class. It was at that point that I found another frame builder that didn’t specify the wheelbase — a bit of geometry later and I derived it from the rake. As a sanity check, since Litespeed specified both rake and wheelbase, I applied the derivation in both directions… not a perfect fit, but a ~5.5 mm error in a 1039 mm specified wheelbase, or ~0.5 percent.
My question is, since you’re frame builder and I most certainly am not, does this sound about right to you? I figure there is some real-world play in these figures owing to measurement uncertainties and the finite diameter of actual tubes.
I’m surprised that you see the top tube (eff) (effective horizontal top-tube length) as something that can be derived from other measurements if you’re not given the actual top tube length and the top tube angle. Without knowing the offset distance of the top of the head tube from the center-center intersect of the top tube and head tube, you can’t get that from just the frame stack and reach and the frame angles and seat tube length, which must be center-to-center for being useful to derive other measurements.
Similarly, I don’t see how you can derive the standover height without knowing the top tube angle or the offset of the top tube from the top of the head tube and the head tube length, lower headset stack height, and fork length.
To derive rake, you need to know a lot of frame variables: wheelbase, bottom-bracket drop, seat-tube length, effective top-tube length, head-tube angle (head angle relative to horizontal) and seat-tube angle (seat angle relative to horizontal).
Bottom-bracket height is of course trivial to derive from wheel diameter and bottom-bracket drop, but that’s only as long as you’re given the wheel with an inflated tire diameter, and you haven’t listed that, so I assume you’ve not been given it.
Calculating to within 5.5mm on the wheelbase is not too bad. If you still have time on your hands and want to see if you can get closer on that wheelbase calculation, here is the formula for wheelbase:
Wheelbase = ((((SQRT((chainstay length)2-(bottom-bracket drop)2))-(center-center seat tube length)*COS((seat-tube angle)*π()/180))+(center-center actual length of top tube)*COS((top-tube angle)*π()/180))+(((((rear-wheel diameter)/2)-(bottom-bracket drop))+(center-center seat tube length)*SIN((seat-tube angle)*π()/180))+(center-center actual length of top tube)*SIN((top-tube angle)*π()/180))/TAN((head angle)*π()/180))-(front wheel diameter)/(2*TAN((head angle)*π()/180))+(fork offset)/SIN((head angle)*π()/180)
Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.