Load on lower shock mount

[highboost]

Hi there,

Quick question, hoping to find some advice...

I am trying to perform an FEA on an end eye I want to use on my rear shocks, (lower mount)

How do I determine the load to apply? I have the vehicle weight and damper curves.

Here's what I was thinking: Use the weight at the given corner, and the force applied at 4in/sec (simulate load seen while going over a bump)

Then apply a factor of safety, is it really that simple?

Thanks guys

 

[JayMaechtlen]

Well, you could cheat and copy one or two standard mounts, see what kind of loads they can take.
Look at the shock mounting hardware and see what kind of loads it can possibly transmit.
As long as your mount can accept as bit a load as the shock can transmit through its hardware, you should have some factor of safety.
(actually, I just welded up some shock brackets for my bastard project car. So far, nothing has bent or fallen off.)

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[GregLocock]

4 in /sec is incredibly low as a maximum for a shock absorber.

It does depend whether you are circuit racing or on roads or Baja, and whether you are kerb hopping when circuit racing.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[wrightti]

Also dont forget to take into account the fatigue life. This brings in quite a few more variables such as material fatigue performance and surface finish.

Tim

 

[GregLocock]

I disagree. Shock absorber mountings are usually designed to a maximum load criterion, they are not fatigue limited.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[highboost]

Thanks for the responses guys. I agree, 4in/sec may be a litle too slow.

I would think the lower mount would be fatigue limimted as well, why is this not the case?

 

[GregLocock]

Because by the time they are designed to handle the forces they will see (4 in /sec is comically low, I assumed it was a typo with a missing zero or the wrong units, not a little too slow) fatigue is no longer an issue.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[gt6racer2]

If it's a pure shock, ( not a coil-over ) then vehicle weight is not important. For a pure shock the design limit of the lower mount is not damping - it's the load seen when the shock limits the extension of the suspension. ( Unless of course you have a provision such as axle straps to limit the travel which is separate from the shock ).
As Greg mentioned, this is a rare "overload" type event and hence is usually specified as a one time load rather than a fatigue validation. Load range 20-30kN is typical.

 

[wrightti]

Yea ok fair call, I'm prepared to accept that. Was just throwing out a suggestion based on my short time in the industry.

Tim

 

[JayMaechtlen]

Even if it's a pure shock (vs. coil-over) wouldn't vehicle weight be important?
Doesn't damping have to be proportional to spring rate?
Spring rate for given type of ride (luxury vs sport) would be proportional to vehicle weight?

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[wrightti]

I will try (again) and answer this one. Feel free to shoot me down, yes the vehicle weight is probably related to the damping forces but its an indirect relationship. The weight force of the chassis doesnt act directly on the shock.

The damper can only transmit so much force and this is directly related to the damping curves, so you are much better off looking at those to find your worst case load.

Tim

 

[gt6racer2]

Jay,
Yes, damping forces may be adjusted for the vehicle mass / spring rate, but as I noted above, damping is not the load you need to worry about.
In fact, the added damping that might accompany a spring rate increase actually helps the worst case shock loading as it will reduce the velocity of the suspension at the end of travel ( Vs no damper change). A secondary effect in changing the spring rate will be that it will also change the spring load at full extension - assuming no other changes the stiffer spring will have less force.