Originally Posted by
Rick_D
Maybe a variation on what you're experiencing, but I have a bike with a shock-absorbing stem that amplifies a paving irregularity as I descend a nearby bike-pedestrian bridge. First time I experienced this it nearly had me crashing into the bollard at the end of the ramp. Very sketchy.
I feel the undulation on other bikes but they don't amplify it. Learned to use back brake only when slowing my descent and that lets the stem travel but not transmit as much vibration to the entire bike. The whole thing is very weird, and site-specific.
Have experienced oddities on long descents with other bikes and just had to cope in real time, usually getting off the saddle and moving my weight back. Sometimes, rural roads will have periodic irregularities that a road bike will resonate to. Exhausting to ride. IDK how folks race cobbles.
Motorcycles with bad suspensions and whippy frames are a whole other level of "fun." Some were even given a shock to stabilize the steering, which to me was always a sign to "stay away."
This is known in the auto industry as "ride hop". Some heavy trucks with long frames have enough longitudinal bending flex (with no damping of that) for this to happen on some roads with a regular hop period at certain speeds, like about 50 mph. Some L.A. freeways had/have a nasty ride hop due to slab spacing and some curvature, enough that car makers, when doing final tuning on the suspension dampers, would spend a week in tuning in L.A. on that. I was there on business, they gave me for rental the first model Ford Escape, a combination of the short wheelbase, short overhangs, and suspension springing and damping, and the ride hop was *awful*. End of week I needed to drive to SF and back, I said give me anything else, they had a Taurus, which I happened to know was tuned for ride hop. Rode fine, easy drive.