Maximum radial G forces experienced on a wheel hub?

[Engineer2Alamek]

Hi,
I need to install some electronic sensing equipment to the wheel hub of various vehicles from a small car to a BW truck using the existing hub nuts/lugs. To determine how strong to make the casing and its mount I need to know approximately what sort of radial acceleration or g force, ie in the plane of the wheel, can be expected not including that due to centrifugal force? The worst case scenario would be going over a pothole or gutter I imagine. Also of use would be to know the maximum oscillation frequency that could be expected. Unfortunately there's no information available on these parameters that I could find.

I guess both these parameters, acceleration and frequency, could be theoretically determined based on the unsprung weight, spring constant k, damping constant, etc. Would anyone be able to point me to the pertinent equation?

Any help most appreciated.

 

[GregLocock]

"Would anyone be able to point me to the pertinent equation?"

doesn't exist.


Are you really interested in forces or accelerations?


Alternatively reverse engineer the wheel studs.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BobM3]

Fifty g's max in the vertical direction is the most I've seen. That's with a 50-60 hertz low pass filter. Further low pass filtering at around 40 hertz reduces that to maybe 40 g's max. It does occur during pot hole strikes when the suspension hits the jounce bumper. I don't know what you mean by "highest oscillation frequency". Wheel hop vertical natural frequency is typically 10-12 hertz. Other higher natural frequencies occur in other axes.

 

[Engineer2Alamek]

>Fifty g's max in the vertical direction is the most I've seen.<

Thanks, that's what I suspected, is this based on your own testing? if so was it a car or truck please?

>That's with a 50-60 hertz low pass filter. Further low pass filtering at around 40 hertz reduces that to maybe 40 g's max.<

I'm interested in the maximum instantaneous g force the hub might experience.

>It does occur during pot hole strikes when the suspension hits the jounce bumper.<

I would have thought that when the suspension "bottoms out" ie hits the jounce bumper, the g force on the hub would suddenly decrease since the unsprung mass has suddenly increased. That is the upwards force on the wheel now has to lift not just the wheel/suspension assembly but also effectively a quarter of the vehicle mass.

Not so sure what happens to the hub g forces on the return stroke however as you've now got both the spring and rubber bumper fully compressed and acting in series. Is the overall spring constant in this case follow the standard inverse rule:

1 = 1 + 1
Ktot Kspring Krubber

>I don't know what you mean by "highest oscillation frequency".<

I mean when a wheel hits a bump its bounces a few times before stabilising since most suspensions are underdamped, what's this maximum bounce rate or frequency? This would be largely dictated by spring rate, damper rate and unsprung mass.

>Wheel hop vertical natural frequency is typically 10-12 hertz. Other higher natural frequencies occur in other axes.<

OK I think you've just answered my above question.

 

[Engineer2Alamek]

>Alternatively reverse engineer the wheel studs.<

To do this with any accuracy I'd need to know what shear force factor of safety they used. This is also complicated by the fact the studs are designed to resist axial tension when tightened which also has it's own FoS no doubt. So there's both shear and tensile stresses involved in a wheel stud design.

 

[GregLocock]

Yes, you would have to apply engineering judgement.

Since the data you are asking for uses wheel force transducers that cost upwards of $250000 each last time I checked, and can take ~ 1 thousand man hours to acquire and analyse, I doubt anyone is going to be able to give you a number that you can use directly.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Bribyk]

This was by far the most difficult wheel load to estimate back when I was doing an FSAE design. We had to base our bump loads off whether the previous design loads had broken the part or not.