Let's Talk Axle Path

Was re-reading through an article from the other site about the high pivot DW patent, and I thought it would be worth sharing Dave Weagle’s thoughts on axle path for the discussion:

So I the chance to ride a buddy's Dreadnought during the Christmas/New Year's Trip to Finale Ligure and I am impressed. So much better than the Kavenz for my style of riding! Corners where a bit different then on my Horst Link Bike but otherwise pretty good.

At least that confirmed that I am ok with HP Bikes but the short chainstays of the Kavenz were not my cup of tea.

My suspension tuner said that high pivots are a band-aid for crappy, position sensitive OEM shock dampers.

Suspension is then a band aid for crappy riding technique and stiff legs... 

A high pivot has a very specific set of advantages and disadvantages. If it's a "band aid" for anything it the lack of rearwardness in the axle path that comes from axle height on modern 29 and 27 inch bikes compared to older 26 inchers. 

What’s their opinion on aliens??

Jokes aside, if it’s a “bandaid” for bad shocks, does that make it superior compared to a non-high pivot bikes if both have a custom tuned shock? Or is the performance benefit they have only applicable to poor damping and drops off/stays the same as damping improves? Because how I interpret that comment is that the suspension performance of high pivots bikes aren’t inherently effected by the damping of the shock, which obviously isn’t true. 

Also, what shocks are still position sensitive these days?? They are all speed sensitive, no?

FYI, just listened to the Blister podcast about the Vorsprung shock and Steve mentioned that testing a shock on a HP bike will make for a bad shock on normal pivot bikes as shaft speeds on square edge hits are too low on HP bikes to develop the shock correctly.

It could be read as if it is a band aid, but damn that's an expensive band aid development wise and marketing wise...

I feel like this is exactly what I was saying, but said better/shorter/clearer

Tuning a shock on a high pivot bike is the same concept but you get different shaft speed results. Also depends on which high pivot bike you are taking about. Say a Commencal V4 or Dreadknot have axles that go reward the whole travel. This will give you lower shaft speeds on square edge than say a scott gambler.

But a high pivot like trek session, Commencal V5, Fury or Frameworks, is closer to low pivot shaft speeds because the axle goes reward half way then up and forwards.

So what Steve was saying makes sense, you can develop a stiffer tune for low pivot then back off the clickers on a high pivot vs being too soft if you developed the other way around.  

The compression tune for the low pivot bike would be less stiff than on the high pivot bike, wouldn’t it? Because the high pivot bike has lower shaft speeds at the shock and therefore requires more damping to get similar control of wheel/axle speeds? 

I understood Steve’s comment of using the low pivot bike (and one with moderate progression) for development as coming down to making sure none of the internal architecture caused harshness at higher shaft speeds. As well as having a test bike that was fairly “middle of the range” in a lot of ways so that they could confirm function of the main components, and then increase/decrease damping to suit a range of bikes on either side of this median range.

More related to this forum topic though, I forgot I snagged this picture while in Whistler in the summer. So he’s certainly tested the shock on a high pivot as well. 

I’ve never tried the same shock on low and high pivot. I was just thinking that if the shafts speeds were naturally higher on low pivot, you’d need a stiffer tune to contain that and lower the speeds to something acceptable. But I’ve never done my own shims, so not sure!

Bringing this thread back I am always pondering what is the best cornering feeling. I have played around with the Linkage program over the year and nerd out on old articles like Pinkbikes "By The Numbers".  For me in comes to predictability and what DW has stated in some of his comments that were shared earlier in the thread. DW says, keeping the rear center length consistent as possible thought the travel makes for a consistent weight balance on the rear axle. 

I have been jumping between my Slash HP and a Yeti SB160. The Slash rear end grows about 18mm from 434mm to 452mm at bottom out. What shocked me is how much the Yeti grows. Measuring my large SB160 it grows from 441mm to roughly 468mm, thats 27mm. I had no idea riding the bike the axle path would elongate that much. The model saved in Linkage shows a near perfect vertical axle path which is much different than what I measured.  

Who has measured there bikes axle path? All these high pivots are getting praise for there stability from the rearward axle path but how many other bikes use a reward path also? I know the high pivot has many benefits for kinematics such as tuning the antisquat, etc. 

Cheers! 

Are you measuring chainstay or rear center length? Measuring from the bottom bracket to the rear axle would give you chainstay length, which can be a useful approximation of rear center. It makes sense on the Yeti for that value to increase significantly with displacement given the constraint of a near vertical axle path thus relatively consistent rear center - think of chainstay as the hypotenuse of a right triangle with rear center as the adjacent leg and wheel travel as the opposite leg from the bb. With the Trek being essentially a single pivot with respect to axle path, the offset of the main pivot from the bottom bracket is what determines the change in chainstay and rear center. In the edge case of offset equal to zero, chainstay would remain constant throughout displacement but rear center would be strictly decreasing assuming the rear axle height is equal to or greater than the bottom bracket height at top out (not a valid assumption but it simplifies the model).

Rear center, which is the horizontal component of the chainstay length vector, is more difficult to measure as it is dependent on the orientation of the front triangle. Are you compressing the fork along with the shock when making the measurements, and if so is it normalized by relative or absolute proportion of the total or vertical displacement? One of the factors that makes comparing bike geometry and kinematics difficult is this ambiguity and lack of standardized measurement and analysis methods. For example, most (I believe Frameworks is the only one who does this) kinematic graphs are not accompanied by the location of the center of mass assumed to calculate the graphical data. And to best reflect the dynamics of riding should that location change throughout displacement and for different frame sizes? Probably, because unless you’re Loic Bruni (and even if you are) you aren’t going to be able to hold perfect technique in a full bottom out scenario and your com will drop. And is that increase in model accuracy worth the increase in calculation complexity? Maybe someone with more experience than myself can offer their opinions, methods, best practices, etc?

Center of mass height and suspension performance are mostly correlated when it comes to antisquat. You're not going to pedal the bike in full squish. 

As for rear centre, I would measure it with suspension proportionally compressed on both sides as I think that should be most relevant when balance comes into play (compression in berms and the like). Hitting obstacles will cause a greater shift in balance just by slowing you down than how far the axle moves fore aft. 

If I am wrong anywhere, please point out. This was though out while I was typing this reply.