Interesting Youtube vid on vertical compliance

[Buzz Killington]

Check this out for a take on vertical compliance. Must be scientific because I don't understand half the things he's talking about...
https://youtu.be/_kW2cfCrDb8?si=kCOMKAVbYOJ5FWNZ

[unterhausen]

As someone that has worked in structural engineering and vibration, I'm afraid to watch it and yet drawn to it. Maybe when I'm working in the basement.

This is the kind of thing I try to avoid even reading threads about. It's not going to change anything if I set people straight.

[Carbonita]

The YouTuber is an engg prof at cal poly, mech engg IIRC. I'd like to see a frequency domain analysis next.

[benb]

It’s definitely really interesting, it is indeed a frequency domain analysis.

The gist of it is he’s got two frames, one of them feels harsh and the other doesn’t when built up exactly the same.

His data is basically validating that there’s basically zero compliance in the vertical dimension but there’s a significant difference in lateral stiffness and his hypothesis is that we might interpret bikes with more lateral flex as more comfortable.

[martl]

What, 0 vertical compliance from a classic diamond frame can be measured?

Now that is a shocker! if only someone had told us!!11!

[marciero]

This is very cool. One limitation I didnt hear mentioned is that fourier based frequency domain analysis assumes a "stationary" process. What this means in practical terms is that there are no trends or transients in the data, among other things. In fact the power spectral density that he talks about is not well defined unless you have stationarity. Most real world processes exhibit at least some non stationarity. Very short time bursts might be approx stationary but it sounds like he is plotting the power spectrum from the entire run of many seconds or minutes. So I am curious how he addressed or if he simply checked and found it was close enough to approximate as stationary. Electrical engineers, and also economists and financial quants, have a whole bag of tricks for this kind of analysis. There are a family of adaptive filtering techniques, for example. He is using Matlab software, which has libraries for all these methods so he may be applying filtering or other processing under the hood.

[StressStrain]

Great video! I'm a structural engineer and I work in the frequency domain every day. That presenter did a fantastic job collecting, analyzing and explaining that data.

Personally, I would have moved the 2nd accelerometer up to the seat post since it's closer to the rider interface, and then calculated the transfer function between the rear axle and seat post acceleration. The beauty of the transfer function (aka FRF) is that it captures the relationship between input and output, instead of just looking at each separately. But that's just me.

[NHAero]

Originally Posted by StressStrain

Great video! I'm a structural engineer and I work in the frequency domain every day. That presenter did a fantastic job collecting, analyzing and explaining that data.

Personally, I would have moved the 2nd accelerometer up to the seat post since it's closer to the rider interface, and then calculated the transfer function between the rear axle and seat post acceleration. The beauty of the transfer function (aka FRF) is that it captures the relationship between input and output, instead of just looking at each separately. But that's just me.

As the OP of the thread about a bike feeling smooth, I agree about the 2nd location.

Going back 50 years - during the MIT Aluminum Bike project, we jigged up a lot of frames and hung weights on them and measured deflection. It became clear that the rear triangle (with an axle between the dropouts) was a tetrahedron and extremely stiff. But it was pretty easy to get noticeable deflection when we twisted the frames.

[benb]

Originally Posted by martl

What, 0 vertical compliance from a classic diamond frame can be measured?

Now that is a shocker! if only someone had told us!!11!

Nah.. I think a lot of cyclists need to be constantly reminded to counteract the marketing.

We literally have a thread about "which bike will have the better vertical compliance" right now don't we?

[NHAero]

Originally Posted by benb

Nah.. I think a lot of cyclists need to be constantly reminded to counteract the marketing.

We literally have a thread about "which bike will have the better vertical compliance" right now don't we?

What's interesting about the Youtube experiment is that he controls for everything except the frame by transferring all the components from one frame to the other. He is clear that he feels the Neuhaus frame is more compliant, but that according to his measurements it's not due to vertical compliance.

Similarly, in my thread about 'smooth', I'm clear that what I'm feeling is subjective, and I haven't controlled nearly as much as the gent on Youtube. We're allowed to acknowledge that a bike feels different to us, and then seek the reasons, valid or not!

[benb]

Yah definitely.

But big marketing will tell us it's vertical compliance in the frame over and over. "Vertically compliant but laterally stiff, climbs like a goat, slices through the wind!"

At least with flexy seatmasts there is something you can actually see flex.

It would be interesting if he could do those.

If additional people did experiments and could prove what actually makes us feel like it's "smooth" it would be really interesting if it caused the marketing to shift towards something more honest, especially if it meant they actually decided there was a limit to lateral stiffness where it started making the bike less comfortable.

I think minus some trickery the last few metal bikes I've had did seem really smooth compared to the carbon ones, and the metal ones have had considerably more lateral flex. My Ti Serotta had the most lateral flex IIRC, it was exceedingly smooth. My All City is very smooth too, I think it has slightly less flex than the Serotta, but it's a little hard to say. It's a harsher frame but it's never really had tires smaller than 32c whereas the Serotta never wore anything bigger than 25c.

Minus the ISO Speed doo-hickey I bet my Domane would be incredibly harsh. The BB is so overbuilt. You feel it in your feet.

[Spoker]

Interresting. Dutch magazines did use similar technique long time ago in testing.
I wonder if he accounted for the seatposts in subjective feel, as in the same posts.
A lot of verical flex seemed to come from the headtube flexing the front triangle vertically in some problematic brake chattering frames I have.

[marciero]

Originally Posted by martl

What, 0 vertical compliance from a classic diamond frame can be measured?

In fact we know it can and has been easily measured with simple stress test on the frame. What we are always debating is whether we can feel those differences while riding.

The most you can say is that this one test did not see a significant difference in what it measured.

[Dude]

can we get a TL;DW for those too lazy to watch a video?

[sparky33]

The Bike Sauce is a great channel for bike nerdery. Go Nolan.