Wheelbase filtering effect on the frequency response of a vehicle
Wheelbase filtering effect on the frequency response of a vehicle
(OP)
Hello all,
I am carrying out some measurements on a vehicle using accelerometers placed on each of the wheel hubs, each of the suspension towers (body side) and on each seat rail.
My investigation involves driving down a defined stretch of road at a fixed speed with a known frequency input characteristic and determining the frequency response of the vehicle by producing FFTs and transmissibility plots using the measurement data collected.
Attached is the typical frequency response measured at a wheel hub, suspension tower and at the seat rail.
I understand the first two plots for hub and body response but not for the seat response. I am lead to believe that the multiple large peaks are due to the harmonics associated with the pitch natural frequency of the vehicle. But why is a similar response not measured at the suspension tower? Why are the harmonics associated with the heave and wheel hop modes not as evident at the suspension towers? Is it due to a lack of damping in the pitch mode?
I apologies for the long explaination!
I am carrying out some measurements on a vehicle using accelerometers placed on each of the wheel hubs, each of the suspension towers (body side) and on each seat rail.
My investigation involves driving down a defined stretch of road at a fixed speed with a known frequency input characteristic and determining the frequency response of the vehicle by producing FFTs and transmissibility plots using the measurement data collected.
Attached is the typical frequency response measured at a wheel hub, suspension tower and at the seat rail.
I understand the first two plots for hub and body response but not for the seat response. I am lead to believe that the multiple large peaks are due to the harmonics associated with the pitch natural frequency of the vehicle. But why is a similar response not measured at the suspension tower? Why are the harmonics associated with the heave and wheel hop modes not as evident at the suspension towers? Is it due to a lack of damping in the pitch mode?
I apologies for the long explaination!





RE: Wheelbase filtering effect on the frequency response of a vehicle
Your explanation for the seat rail plot is dubious, but since the data looks wrong then I'm not going to invest much thought in it.
Wheelbase filtering, as mentioned in your thread title, is easy enough to work out, what is your wheelbase and vehicle speed?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Wheelbase filtering effect on the frequency response of a vehicle
I'm surprised with your comments on the hub response, seeing as this is measured on the unsprung mass, I thought it would be normal to expect a greater concentration of energy around the wheel hop frequency? (16/17 Hz from what I can see)
The wheelbase is 2.6m and the vehicle speed is 30mph.
RE: Wheelbase filtering effect on the frequency response of a vehicle
I don't much like the 'harmonics of pitch mode' explanation, FWIW.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Wheelbase filtering effect on the frequency response of a vehicle
RE: Wheelbase filtering effect on the frequency response of a vehicle
The peaks in the seat track response don't line up with the wheelbase filtering effect (5.16 Hz and integer multiples).
I never see zero at several frequencies on a body response. I haven't seen a road spectrum that looks like that- I'm also guessing this must be a very smooth road.
There doesn't seem to be much tire first order (fair enough) on the spindle.
wheelhop (15.7 hz?) on the spindle seems too high in frequency and too well damped, but isn't coming through into the body.
Sorry I haven't got anything very positive to add.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Wheelbase filtering effect on the frequency response of a vehicle
This is a highly damped 2 seater sports car running on a relatively smooth road.
Thanks for your help.
RE: Wheelbase filtering effect on the frequency response of a vehicle
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Wheelbase filtering effect on the frequency response of a vehicle
The reason I ask is to rule out that that the bumps in your FFT plots aren't a result of an FFT anomaly. I see you used a square top window, did you try a hanning window to see if the results looked different? What is the frequency resolution of your FFT?
RE: Wheelbase filtering effect on the frequency response of a vehicle
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Wheelbase filtering effect on the frequency response of a vehicle
The accelerometer is high pass filtered at 3 Hz. I think the vertical scaling is right but it isn't my data.
This shows the sort of behaviour I am used to seeing, jagged (this is 1000 seconds of data), one lump of response at primary ride (0-3 Hz), another at engine bounce (say 6-9 hz), and then wheelhop at 10-12 Hz.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?