How to simulate a bump load? 2

[TG_eng]

How do you simulate a bump load? For example a wheel going over a bump. What type of analysis and what type of load? Would you do a dynamic analysis and if so what type of load? Or would you simplify it to a static analysis with just an equivalent max force? I know this is somewhat vague but I'm just trying to get some ideas here.

Thanks in advance.

 

[rb1957]

it sounds like it's a time varying load.

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

There are programs that can simulate vehicle motion vs. speed and road profile, including things like Belgian blocks and speed bumps. The challenge is determining what is the canonical speed bump, which doesn't really exist IRL. Equivalent max force doesn't quite cut it, since as with falling off a building, it's the impact that kills you, or the product. This is where your canonical speed bump can make it relatively smooth, or bone-jarring.

If you don't have something like

https://www.mscsoftware.com/product/adams,

you could possibly create a model of a single DOF sprung mass system that gets stimulated by your speed bump and use the result for your product simulation

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[GregLocock]

Some jurisdictions have a standard speed hump profile. Quite whether the road builders know or care is a separate issue. In parts of China they use prefabricated steel ones, so they'd be pretty consistent. In the past we've scanned particularly problematic speed humps and built copies.

So the usual process is to measure the forces going into the wheel with a wheel force transducer as it is driven over the hump, which measures 6 dof of force at the wheel/hub interface.

Attempting to model durability type events in ADAMS is the Holy Grail of the road loads community, the difficulty is that the tire has a very complex structure. Various complex 3d high frequency tire models are available, but I don't think anybody really claims to have cracked it as a general purpose method. I have simulated speedhump, square edge pothole and curbstrike. We aren't allowed to use the WFTs for the latter two because they might break, so we use strain gages in the arms, and load cells embedded in the road surface. Then I screw around with a tire 'model' until it matches both the force pulse and the measured motion of the wheel and body (done with a crash camera) and the forces in the arms. Then interrogate the model to get forces at each interface for the FEA boys to play with.

Typically the tire model looks like a mass with a nonlinear spring.

Loads you'll see in a full on event like SEPH are of the order of 6g loading, static, but that is not an upper limit. It is designed to rip the entire suspension out of the car.

Cheers

Greg Locock


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[IRstuff]

The speed bumps in old neighborhood, are very smooth, so long as you stay below 30 mph; anything above will likely bottom out the suspension. Those are about 2 1/2 to 3 ft, end to end, so it works out that at 25 mph your front suspension will have recovered its initial excursion before the back suspension gets going.

Other speed bumps have much steeper aspect ratios, and they like hitting the rising edge of a one-sided pothole, i.e, a Heaviside step function, and require you to slow down to 5 mph to protect your spine and your teeth.

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[Mohanlal0488]

Not sure how in-depth you would like to go with the analysis, but it would be easy to formulate a 1 degree of freedom problem for a bump in the road. Almost every dynamics text that I have seen deals with that problem

 

[IceBreakerSours]

If the previous suggestion is valuable to you, then here is one easy to follow resource.

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[TG_eng]

Thanks guys for all the comments. Part of it is that I am curious about how people develop a "bump" load. Are they primarily doing theoretical values using simple 1D dynamics equations? Or are they sticking an accelerometer somewhere and driving/riding around to see what the possible values are? I'm guess this is a mix of both.

 

[GregLocock]

"Are they primarily doing theoretical values using simple 1D dynamics equations?"

Doesn't work, tires are a critical part of the process

" Or are they sticking an accelerometer somewhere and driving/riding around to see what the possible values are? I'm guess this is a mix of both."

We use a mixture of load cells in the road surface, wheel force transducers, strain gages, high speed cameras, accelerometers and so on.

Then I correlate my model to those measurements, and then interrogate the model to find the loads at each point in the model for the FEA guys.

Cheers

Greg Locock


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