Hello Martin, thank you for the post.
An unknown vibration at 25 Hz or, x 60 = 1500 rpm.
Here we are looking for a component rotation at this speed, or a component that is resonating at this frequency.
It does look as though engine speed is not linked to the vibration as we have the same condition at 3541 rpm
Can I ask that the tyre size entry and indicated road speed of 124 km/h are correct as I want to confirm T1 frequency based on these speeds.
They calculate correctly but I want to be sure road speed and tyre size are valid.
If we are looking at 1500 rpm we need to look at the drive train and associated components. (Transmission/road speed side) to work out if any components are rotating at this speed at 124 km/h
My gut feeling is we have a component that is resonating at 25 Hz and this can be challenging to find.
For example, under shields or body liners that flutter due to wind speed can resonate and produce intense cabin vibration.
Can I ask Martin, where can the vibration be felt at its worst?
Was this by the driver?
I would try to locate the source by repositioning the accelerometer, or better still, add an additional accelerometer whilst keeping the existing accelerometer (Y axis) in your current location as your reference.
Remember resonance occurs when a “forced vibration frequency” matches the “natural frequency” of a component. The relevant component will then resonate and I think this is what we have here.
If road speed increases or decreases, does the vibration reduce?
Is there a sweet spot for road speed?
Locating vibrations using amplitude requires careful consideration of locating points (accelerometer mounting points) as amplitude will differ depending on the mounting surface.
To clarify, an accelerometer mounted on a thin section of chassis may well measure an increased amplitude of vibration to an accelerometer mounted to a seat frame bolt due to the inherent differences in the mass and rigidity of the mounting surfaces!
Try as much as possible to keep mounting locations as similar as possible
Be aware that a resonating component will demonstrate the highest amplitude of vibration but may not be the offending component, it is the responding component, forced into a state of resonance from another source of vibration.
The classic example is steering wheel shake (responder) due to an imbalance wheel/tire (source)
In this scenario we tackle the source
I hope this helps and please feedback for any clarification
The entered tire size is correct, and the road speed in the Pico file matched the speed reported from the cluster.
The vibration is worst around the measured speed, but it is gone when the engine is loaded more (When you drive toward the Wind)
The vibration is felt as to be coming from the Bottom of the car, not the steering Wheel.
This is a simple front Wheel driven car, so I cannot figure out what that could be rotated at 25HZ
The gear ratios are:
1st
3.769
2nd
2.045
3rd
1.370
4th
1.036
5th
0.893
6th
0.774
Final gear ratio
4.059
I Think you are right in that this is a component that is resonating
All under shields are intact and correct mounted.
I was thinking that it might be a mass dampener that was missing, or damaged, but this is not the case.
I have an exact identical car for reference, and this car does not have this vibration.
I will try some more experiments, I have this car for a couple of weeks, as there are other repairs not linked to vibration, that also needs to be carried out.