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axle paths seem to be the rage at the moment, but how much does that actually affect forward momentum? My gut feeling says that it helps. But, after looking into it, things got fuzzy...
https://www.santacruzbicycles.com/en-US/news/344
Specialized has added height to the main pivot above the BB to add some rearward motion, but nothing dramatic. Is there more to the concept?
Maybe the extra stability people rave about is simply due to the longer rear center?
https://www.vitalmtb.com/videos/member/Mixed-Wheel-Sizes-Longer-Chainstays-and-Adjustable-Reach-Neko-Mulally-Does-More-Off-Season-Testing,38740/BHowell,45918
Or, Could it be due to the fact that the chain is stabilized better?
[url=https://www.youtube.com/watch?v=grNUgu0H9YA]
https://www.santacruzbicycles.com/en-US/news/344
Specialized has added height to the main pivot above the BB to add some rearward motion, but nothing dramatic. Is there more to the concept?
Maybe the extra stability people rave about is simply due to the longer rear center?
https://www.vitalmtb.com/videos/member/Mixed-Wheel-Sizes-Longer-Chainstays-and-Adjustable-Reach-Neko-Mulally-Does-More-Off-Season-Testing,38740/BHowell,45918
Or, Could it be due to the fact that the chain is stabilized better?
[url=https://www.youtube.com/watch?v=grNUgu0H9YA]
There is two ways about high pivot: either raising a pivot with a conventional drivetrain, that is compromising other things elswhere. Or rising the pivot so much you now need an idler pulley to adjust chain tension and other characteristics. This is another world and you can really benefit from a high pivot bike.
Yes, the added stability comes from higher rear center. You have a short rear center when suspension top out with it's advantage on maneuverability, and you have the long rear center when the suspension bottom out (longer than the allready long rear center on 'conventional' bikes). So you can more or less tune how and when you have stability, or maneuverability.
But the main advantage of that route is the bump compliance.
There is a third way now. There is 29" bikes that needs an idler pulley to have an acceptable height of the pivot (not too much "forward travel"), but they aren't "true" high pivot bikes.
Edit: Or the 2022 Trek Session, same deal. It's rearward through only the first ~50% of travel.
(Rear) wheel diameter is part of the issue when it comes to pivot height. With 26" bikes and older geometries (higher BBs) a standard suspension layout was still somewhat 'rearward'. That is because the rear axle was below the BB and the pivot a bit higher than the BB. The pivot was nevertheless close to the BB, so there wasn't much pedal kickback due to chainstay length increasing through the travel.
Going to 27" wheels showed a problem with that, which was just intensified by 29" wheels - if you kept the relative pivot locations the same in regards to the rear axles on larger wheels, you'd essentially get a semi-high pivot. A pivot height that would case additional antisquat (due to chainstay length increase) and pedal kickback. And you would keep the same axle path. When you move the pivot closer to the BB to remove the pedal kickback, you get a mostly forwards axle path on 29ers, as your BB is mostly below the rear axle height - caused both by a higher axle placement and lower BB geometries.
That change additionally causes a more forward movement of the rear wheel, that is also much larger than before, which makes packaging of everything harder - that is how we got bent and offset seat tubes.
Want a 26"-era axle path, no packaging issues, 29" wheels and modern BB height geometries? You need an idler pivot to not have 150+ % of antisquat and insane amounts of pedal kickback. That or the output chainring that is not concentric to the BB (Effigear). And once you're on the idler bandwagon, why not go all in and make it a bump eating cookie monster?
When we look at leverage ratio, they are always plotted for vertical force at the contact patch but in reality leverage ratio is not fixed and depends on the orientation of the force or bump height.
For a given axle path, leverage ratio is highest when a force is aligned with axle path and go all the way to 0 when they are perpendicular.
Let's take 2 bikes with identical leverage ratios, one with a vertical axle path and the other with a 30 degrees rearward axle path.
If you set both to have a similar vertical compliance based on rider preferences for sag, natural frequency or bottom out resistance, the latter bike will always be more compliant than the former under any bump.
Correct me if I'm wrong but I see very few edge cases where I would like my bike to be less and less compliant as bump size increase as this leads to deceleration and harshness.
Now there's many other factors to consider like has been said but overall I think rearward axle path are great when riding rough grounds.
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