[pvd]

I posted this stuff on PinkBike recently but it found no traction there. Maybe the folks here will find something to discuss.

The anti-squat problem. | Peter Verdone Designs

www.peterverdone.com

Huck to Flat | Peter Verdone Designs

www.peterverdone.com

 

[uncajohn]

In the second video (Canyon frame) it looks like the air shock is struggling to remain in line! Although looking at this video (rather low res) I cannot be sure, but it seems that aside from the combined action of the (real) primary suspension element of all bikes, the pneumatic tire & the shock absorb er receiving a “kick” (or additional load) from the tire’s compression, momentary overloading ti, the shock itself is receiving some distortion from the bike frame too!



No matter how controlled a frame’s distortion is, it still exists. I always thought that a plus element on suspension design is the isolation of the shock absorb er from “twisting” forces coming from the frame. After all, in the real world the bike has to deal with all shorts of loads coming form the trail!



Those small inserts between the shock eyes and suspension links are a good idea. Maybe a high quality spherical bushing would do the job (and NO, not those failed, ultralight ones, tried out from time to time!!! Here our primary goal is suspension performance, not lightweight…)



But.

The pneumatic tire was (and is) our primary suspension element. All other come second.

 

[uncajohn]

Also.



Somebody could try the following experiment:



On a cantilever arm, mounted on a single large diameter pivot (for reducing unwanted distortions) that will rest at 1/3 off center, thus the one side will be 3 times the length of the other, we may mount 2 shocks.

The first shock, mounted at the short part of the lever, will be our “frame’s” suspension absorb er.

The other shock, mounted on the long part of the lever will be our pneumatic tire.

Under the “tire” shock there will be a pressure piston.



So now with the aid of a high speed camera or other measuring apparatus we can observe how the & when the additional load from the tire’s suspension will affect the suspension’s shock absorbed.

AC

Just an idea (and IF all that actually means anything on bike handling).

 

[Ben.land101]

I will freely admit the anti-squat stuff is above my pay grade at the moment.
Full-squish bikes are a whole other endeavor that I'll get to soon enough.

But, this sort of came up in conversation the other day, talking about cushcore inserts. Or at least, a somewhat related discussion.
When you think about what the suspension is actually doing on the bike, it helps to categorize the 'inputs' ie, small bumps, big hits, that sort of thing.

So when people talk about the suspension's 'small bump compliance', It's really more of 'medium bump compliance', as the tire system takes up so much of the small bumps, vibration, and 'chatter' that we usually don't even realize.

Then beyond that, our bodies can take up the big hits (bending at the knees, elbows, etc)
But of course comfort is only a small part of what the suspension does. Maintaining traction is of course a big aspect as well.

I don't know if any of that is helpful to the discussion, but it was funny that it came up in conversation for myself recently.


Also, (in the huck-to-flat vid) did anyone else notice how some of the bikes seem to 'oscillate' between the front and rear suspension? Like, the phenomenon pvd brought up, but it looks like the front and rear seem to "trade places" of which one is the tire rebounding and shock compressing.
The Santa Cruz Nomad is a good example of what I'm talking about.
It could just be that some of the bikes did not land level, one tire or the other hitting first, or maybe they are interfering with one another in the system.
Again, not sure if that's useful, but it caught my eye.

 

[Ben.land101]

Also,
Imagine if the tire system had a relatively slow rebound. As in, the tire would deform to the bump or hit, then stay that way long enough to NOT give that oscillation into the suspension.
Then of course, since we're rolling forward, the tire would have the entire revolution's worth of time to get ready for the next hit.
I'm sure there's plenty of other problems with that though. Just a random thought.

 

[dr.welby]

Watching the videos in 0.25 playback speed I was also trying to figure out if some of the shock stall was from when your brain decides to let your legs absorb some of the shock. Too much was going on the process, but it would neat to grab the video and motion track a bunch of key points.

 

[Surestick Malone]

One thing I found interesting in those videos was the time delay between the fork compressing and the rear suspension.

With regards to the suspension stopping and restarting behaviour that PVD mentions in his huck-to-flat blog post, I think that's normal behaviour for a system with two weights suspended from two springs in a spring-weight-spring-weight configuration. When you start things oscillating there are points where one of the weights stops moving and transfers it's energy to the other making it move violently (that's probably really not clear).
I think tuned-mass dampers are designed to take advantage of this effect.

 

[Mudguard]

I don't think you can really take the huck-to-flat as some kind of suspension analysis. It's a very rare event riding, and when they zoom out and show what's happening it makes it look even worse. How often during a regular ride would you have a 1m drop at about 5kmh?

 

[uncajohn]

And (to add on that) some bikes were landed front wheel first & some rear wheel first. So if we wish to see things more seriously, we owe to have 3 different landings with precise repeatability:


-One landing with both wheels touching together.


-One landing with the front wheel touching first


and


-One landing with the rear wheel touching first.


But.


All these are just for getting more views on the specific page. In the trail the various loads are so many… If we add the habits of each rider, his/her preferences on bike tuning and the fact that even the same rider ridding the same trail with the same bike, won’t be able to keep the same line (like ridding on rails), it;s obvious that the the to flat videos are just for fun!

 

[Ben.land101]

Well of course the video is just for fun. As is this discussion.

Not expecting anything too profound to come from it, it's just interesting to discuss and think about what may or may not translate to trail riding.

And I don't know about you guys, but I definitely do some 1m drops at low speed quite frequently on my rides. So it may be more relevant for some riders than others.

 

[uncajohn]

This is true!
We tend to ignore this very importand factor. The rider himself!

 

[mickuk]

Interesting to see you using the scales and angle of tilt for measurement of bike and rider c of g. I've done it with trucks and buses but never thought of it for bikes (due to the issue of the trying to get the rider in the same position relative to the bike). Guess it is pretty much the only way and you had a good stab at it. What is the next project - a trifilar table?

On the huck to flat videos I'm struggling to see beyond watching two spring-mass systems, one (the tyre) without much damping. Would be interesting to watch the tyre in isolation with different inserts to see if there is much improvement in the damping (you can clearly see the decaying largely undamped sinewave if you just watch the tyre).

For context this is part of what I have to play with in the lab - most definitely the bounciest tyres I've dealt with....

 

[pvd]

It's going to a long time before I have a 4-post rig to play with.

 

[Erichimedes]

Both points are interesting. I've been pondering the COM problem with anti squat since the beginning, and the more you dive into it the more problems you find, as you noted, Peter. There are simply so many variables, it makes direct comparison to other bikes really difficult. I will say I've learned a lot by working on the problem in CAD, then building prototypes and seeing how they ride. One thing that surprised me was all the test riders on my proto paid less attention to the pedaling efficiency than I did. I'm still not sure if that's a good thing or a bad thing.

The thing I noted about that huck to flat video when I watched it was what Ben.land mentioned where there is oscillation between the front and rear taking in how and when they compress. That really surprised me, I expected them to compress together.

Though I agree with what mudguard said, the huck to flat isn't any type of suspension analysis. How it handles flat landings is the last thing I'm tuning my suspension to handle well. Though it is one more interesting facet to observe the suspension system at work, in slo-mo.