4 ms·

>A moving train generates a standing wave that is a function of the train's weight and its speed. No, it's really not. It's a function of the unsprung mass, th

>A moving train generates a standing wave that is a function of the train's weight and its speed. No, it's really not. It's a function of the unsprung mass, the roughness of the wheel, the roughness of the rails and other factors (fastener type, faster spacing, etc). Train weight never enters in to the equation (literally), only the combined weights of the train components south of the primary suspension. >Feeling the ground move down as the wheel trucks pass is dynamic, yes, but it is not vibration ??? That is exactly what ground-borne vibration is! >its "frequency" would be the rate at which the wheels passed by, Among other factors (wavelength of roughness, spacing of supports, etc), yes, and you can see that in train vibration spectra. You can also see the primary suspension resonance and other components. I'll refer you to the report "Vibration Characteristics of the Transrapid TR08 Maglev System" - (yes, I was a coauthor) were we discussed these issues in the section about the TR08 force density. http://www.fra.dot.gov/downloads/rrdev/fra0206.pdf http://www.fra.dot.gov/downloads/rrdev/fra0206.pdf section 3.3. It's not a perfect analog with steel-wheeled trains but a lot of the same principles apply in terms of the different factors that go into the input force. You mentioned "the early days of trains" - what era are you talking about? Specifically are you talking about the days before trains had suspensions, or had very stiff suspensions (which was common as late as the 1950's-1970's in transit vehicles until it's effect on vibration was well known - this was a huge issue with BART)? In that case I could very will see that vibration was proportional to gross weight since a stiff suspension is basically a short circuit to ground. That generally isn't true any more, the only exception being some types of freight cars. edit: a quick Google search provided a paper that supports my thesis: http://library.witpress.com/pages/PaperInfo.asp?PaperID=19575 http://library.witpress.com/pages/PaperInfo.asp?PaperID=1957...

WalterBright 15y ago
I'd really like an explanation of how, if you push down on the track at a point, you are not bending the track down at that point, sprung or not. Move that point along the track, and you create a standing wave, sprung or not. The amplitude of that wave will be proportional to the total weight.
- Anechoic 15y ago
  It's not that the train isn't pushing down on the track, it's that the force created by the unsprung mass is several orders of magnitude higher than the static force. The wheelset force is higher than the static force because the primary and secondary suspensions allow the wheels to move independently of the vehicle body. Imagine you have a 2x4 wood beam suspended across two blocks, with a 200-lb man standing in the middle of the beam - the beam will clearly sag in the middle. But if you place a 50 lb girl jumping up and down on the beam next to the man, the beam will sag significantly more. If you are looking at the effects of forces on that beam so see how it bends or breaks, it's the dynamic forces from the jumping girl that will control things, not from the heavier guy. At this point I don't know how to explain things without jumping into two degree-of-freedom mechanical models.

WalterBright 15y ago

I'd really like an explanation of how, if you push down on the track at a point, you are not bending the track down at that point, sprung or not. Move that point along the track, and you create a standing wave, sprung or not. The amplitude of that wave will be proportional to the total weight.

Anechoic 15y ago
It's not that the train isn't pushing down on the track, it's that the force created by the unsprung mass is several orders of magnitude higher than the static force. The wheelset force is higher than the static force because the primary and secondary suspensions allow the wheels to move independently of the vehicle body. Imagine you have a 2x4 wood beam suspended across two blocks, with a 200-lb man standing in the middle of the beam - the beam will clearly sag in the middle. But if you place a 50 lb girl jumping up and down on the beam next to the man, the beam will sag significantly more. If you are looking at the effects of forces on that beam so see how it bends or breaks, it's the dynamic forces from the jumping girl that will control things, not from the heavier guy. At this point I don't know how to explain things without jumping into two degree-of-freedom mechanical models.