I undoubtedly believe the front center and rear center balance is the most important metric in bike design and am very uncompromising with getting this balance right when it comes to picking a bike.
If the front wheel is very close and/or rear wheel is far away, a rider will position themselves more rearward when riding tech. This is why it’s a custom to get the ass back on old short travel bikes. If the front wheel is further away from the rider, the rider feels that they need to adopt a more forward position. Sometimes this amount is just right and feels “centered”.
IME, 20mm of front center difference is very noticeable. I’m sure it’s like this for others, as many can feel the performance and fit difference between two identical bikes in different frame sizes. Generally, I found that a number of long travel bikes feel better and get better results in a smaller size, while short travel bikes almost always feel better in a larger size.
Slack HTA contributes not only in pushing the front wheel further away, but also affects steering and affects the angle of bump absorption (axle path) for telescopic forks.
The “auto-centering” steering effect is stronger with slacker HTA. It feels twitchy/floppy at low speeds until the rider gets used to it. It ramps up resistance to direct steering, forcing the rider to lean to turn, but keeps steering stable and straight at high speeds. This is more accurately determined by the metric measuring how much the front tire’s contact patch trails behind the steering axis. Fork offset directly affects this trail.
Over a decade of test riding bikes, I compiled a list of bikes that felt “centered” to me and found a pattern. It doesn’t match up to any simple math “ratio”, besides the FC and RC maintaining a ratio of 3:1 once you find your sweet spot.
My geo sweet spot at the moment is 1230mm wheelbase and 435mm chainstay on a typical 150mm (rear) FS 29er. The rear center (RC) is technically a virtual measurement, being a horizontal CS length, rather than the line drawn from BB to rear axle (BB drop factoring in). The front center (FC) is basically the WB minus the RC. To maintain the weight distribution balance, I found that extrapolating the 3:1 FC:RC ratio works.
1230mm WB 435mm CS
1250 WB 440 CS
… note: I’d subtract 5mm CS from 27.5mm bikes and would need to do testing for high pivot bikes.
Maybe this might be confirmation bias, but I made note of how many reviewers mentioned balance issues, such as the front wheel lacking traction. See enduromtb mag mentioning that the size med Commencal Meta AM 29 and Yeti SB150 performed better than the size L. See Loam Wolf criticizing the Fuji Auric LT and the new Marin Rift Zone. See many outlets complaining about the dangerous traction on the Arc8 Extra (their long travel model). All of these cases were ones where I’d predict that the CS was too short for the WB length in sizes L and XL.
I also made note of the bike of the year contenders and winners (in size L). There seems to have only been a few rare exceptions, like the Norco Range winning a bike-of-the-year, maybe since all the others felt too balanced/dialed (samesies?), and the Range commanded an “outstanding” feeling.
I also appreciated reviews from different height reviewers, like bikerumor’s Jessie May. Their opinion on bikes in smaller sizes provided contrast to the words of taller reviewers. Canfield got a lot of praise from the shorties, while it seemed like the tall folk reserved their criticisms out of respect for the shorty’s confidence, who named it their editor’s pick.
I’m open-minded enough to believe in a future with size-specific HTA and fork offset, where shorties get slack HTA with long fork offset and tall folk get steep HTA and short fork offset. Tall people typically downsize long-travel bikes, suffering from things like cramped cockpit and low grip height relative to their seat, making it uncomfortable to pedal on flat ground. Does a steeper HTA not address that? For shorties, the slacker HTA addresses the issues regarding the “doggie beg” position (see down-sloped stems on XC bikes) and the stability issues in the chunk due to a shorter WB (taller riders on longer WB bikes make the chunk seem more effortless ). If only people didn’t basically choose a HTA and treat it as sacred…