Originally Posted by
palincss
Are you assuming fork offset will remain constant? There's no reason to do so, especially since we're in C&V where steel forks are the norm, rather than in the world of carbon, where forks only come in a very limited range of offsets. Put a 2 1/2" (63 mm) fork rake on that 73 degree head angle and the front center and wheelbase are going to be a lot longer (and trail a lot less) than if it had a 43mm.
To illustrate the point, yes of course the same rake was used. And many bikes do use the same rake, despite varying HTAs. But even a change in rake that is proportional to the change in angle will affect wheelbase by only a few mm per cm of HTA wheelbase change.
The example also assumes the same fork length, same head tube length and is only an approximation of the change in wheelbase per degree.
Originally Posted by
JohnDThompson
Head and seat angle and top tube length determine rider placement on the frame and weight distribution between saddle and handlebars.
Chain stay length determines wheel clearance, overall wheelbase, and also clearance for cargo panniers.
Chain stay length also determines weight distribution on the wheels. Long chain stays shift the weight toward the front wheel.
Head tube angle doesn't really change rider position because we measure HTA starting at the top of the head tube. But changing HTA (even with a refinement in rake to hold trail) will change wheel weight distribution if it changes the front center.
Originally Posted by
Spoonrobot
I think wheel-flop has a much larger effect on handling than trail and is felt much more than any other metric. A 3mm difference in wheel flop between two bikes is very obvious when riding, 3mm of trail, rake, wheelbase or really anything else does not create nearly the same feedback.
This is true, but there is no situation where you would get a 3mm wheel flop and 3mm trail change at the same time, so it is a misleading statement. If you change a particular road bike's fork offset (rake) by 5mm, the trail will change by 6mm, but the wheel flop will only change 1mm. And if you leave rake alone and change HTA by 1°. the trail will again change significantly - 7mm - while flop only changes by 2mm. The two measures are not proportional.
Originally Posted by
Spoonrobot
There are mathematical expressions, but the post from the guy from Schwinn seems to assume that neutral trail is not a universal number, but is proportional to wheel size. So of course his example of two different wheel sizes yielding two different rides despite the same trail - the trail was only "the same" as an integer, not the same in proportion to wheel size.
On road bikes we say that anything below 56mm is low trail and everything above is high trail. But that's only true with a 700c wheel and a common racing tire size like 25c. If you build a 650c bike and you also want "neutral trail", you aren't going to find it at 56mm. Trail will be centered elsewhere, and a comparison of high trail between two different size wheels will be in proportion of change, not a fixed number of mms.
This was especially illustrated to me when I rode a bike that substituted a long fork with a roller blade wheel at the end. An 8cm wheel can't possibly have a trail of 5.6cm - trail has to be in proportion to the wheel diameter. The bike rode fine and didn't display any obvious weirdness. I would imagine any offset or trail was less than 3mm.