Load calculation for control arm design? 1

[spdracer22]

I'm wanting to design some control arms for a track day car, but I'm unsure of what to use for the loads for the FEA.

The advice I got was to use 4-3-2 as maximum load multipliers, and to assume the car corners at 1.5G's (higher than the actual).

My problem is that I don't know how to calculate the maximum loads. I know my spring rates, motion ratios, unsprung mass, and static corner weights. But, I don't have the CG height, so I can't calculate steady-state load transfer using any method I know of, unless I make a guestimate as to what it is.

So, what should I do?

 

[GregLocock]

cg height=500mm

Cheers

Greg Locock

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

 

[spdracer22]

What about impact loads? The car is street-driven to and from the track, so I need to make sure it will handle the lovely Michigan roads without breaking. Should the 4-3-2 take care of it?

And, I'm assuming that the 4-3-2 is an actual multiplier...so if, for example, my lateral load is 1000lbs, I'll use 3000lbs in my FEA. This is correct, right?

 

[GregLocock]

Well, not quite.

If the corner weight is say 1000 lb, and I was using 5-4-3 then we'd apply 5000 upwards at the CP, 4000 longitudinally, and 3000 laterally.

Then work out the free body diagrams for each arm, for each of the three cases. That will give you the loads for the FEA

In my opinion the wisest thing to do is to add the sum of the squares of the stresses for all 3 directions, and compare the squarerrot of that with the yield stress.

I know some people say you just look at the most stressful case for each arm.

Cheers

Greg Locock

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

 

[izzmus]

Assuming "track day car" means the car will be using racing type tires, you may wish to up your cornering estimates.

I have seen 1.2g in a then 20 year old Mazda using nothing but the proper "attire". Handling was not so hot with the outside corner on the bumpstop, but this was acceptable for the course (smooth, wide, no major transitions) and traffic conditions. A friend, on better tires and a suspension that could handle it, could pull 1.5g easily. Still "street legal" race tires, not slicks.

I do understand that recent changes to the certification process will kill off these competition-special tires, so you may be right after all...

 

[spdracer22]

"I do understand that recent changes to the certification process will kill off these competition-special tires"


Could you explain?


Also, it's really mainly an autocross car. I run on both street tires (Falken Azenis) and DOT slicks (Kumho V710). I may go ahead and up the cornering force, though with the calculation I'm doing, it places nearly all of the weight of the car on the outside edge. I'm using the CG height as 500mm and the cornering acceleration as 1.5G's.

I don't have the roll center heights or proper roll stiffness calculations as of yet, so I can't figure out front to rear distribution during steady-state cornering. Since I'm just doing front control arms, my plan was to assume a f-r distribution of 75-25. My total weight transfer is around 1200lbs (spreadsheet's at home), so that would add 900lbs to the front outside tire, plus the roughly 750lbs static corner weight. So, I'm left with 1650lbs (car weighs 2250) on the outside front tire.

So, that would be my vertical load at the CP. Using 4-3-2, I would end up with 6600, 4950, 3300 as my loads (vertical, long., lateral). How does that sound?

 

[GregLocock]

steady state cornering loads are directly proportional to the axle weights.

Cheers

Greg Locock

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

 

[spdracer22]

Greg- Could you explain?

If you mean that if the static distribution is 60F/40R, then during steady-state cornering the distribution will still be 60/40, I don't necessarily agree...

 

[GregLocock]

Ignoring throttle (etc), I think you'll find that is the case.

Otherwise it won't be steady-state. Take moments about the CG.

Cheers

Greg Locock

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