Loads/Forces acting on the upright

[Tudor Miron]

Hello All,
I have an urgent problem to solve - making initial upright design. I just don't have time to make it the right way - calculate the loads myself. This is not about lazyness - trust me http://fsae.com/groupee_common/emoticons/icon_smile.gif

So my question is what loads/forces (magnitude and direction) act on upright (boll joints locations, toe location, caliper, bearing)during dynamic events?

Thank you
Ted

[Tudor Miron]

Hello All,
I have an urgent problem to solve - making initial upright design. I just don't have time to make it the right way - calculate the loads myself. This is not about lazyness - trust me http://fsae.com/groupee_common/emoticons/icon_smile.gif

So my question is what loads/forces (magnitude and direction) act on upright (boll joints locations, toe location, caliper, bearing)during dynamic events?

Thank you
Ted

[Mark Bacchetti]

Determining accurate loads from dynamic events is a fairly complicated process that requires lots of testing. Most teams attempt to envelope all events into 3 loadcases in the X, Y, and Z directions (applied at the spindle). For example, 3g vertical, 2g lateral, 1g longitudinal. I'm sure you can find out what loadcases teams use by running a search on these forums. After you have the three loadcases run the statics to determine loads on the locations you are interested in. Another way to get loads is to join the tire consortium and use the data to find out actual tire loads. You may want to knock-up those loads to make sure you envelope any unexpected dynamic events.

[Tudor Miron]

Thanks Mark,
Do you mean that those loads should indicate "worst case"? Also do you mean applying all thos X/Y/Z at the same time? How about aligning torque?
Unfortunately my tires are different (not FSAE) so I can not take advantage of this great tire consortium

Thanks
Ted

[Jersey Tom]

MAKE time to do it the right way. There's a whole 24 hours in a day. This is a critical part of the wheel assembly, that sees all the forces the car generates. You and your buddies are gonna be riding on it.

Why has this been put off? Do you have anything from last year's design report regarding forces?

The forces at your ball joints depends heavily on the weight of your car, how much weight transfer you get (dependent on car CG and overall dimensions), and your suspension points.

But I'd go with Mark, find the forces acting on the spindle. Don't forget not only do you have linear forces, but moments as well since your wheel is cantelevered off your upright. Then its a statics problem...

[Mark Bacchetti]

Yeah, those loadcases will represent worst case with conservatism. The conservatism in this method is there because you really don't know and don't have time to determine the actual dynamic loadcases.

Applying all three loadcases at the same time will definitely be too conservative (hence too heavy) since the tire cannot experience 100% of the lateral and longitudinal loads at the same time. I've seen people running vertical+lateral and vertical+longitudinal loadcases and I've also seen all three cases run seperately. This is really where the art of engineering comes in.

Like Jersey Tom implied, apply these at the spindle bearing's center plane. This location will add moments into your static equations. Make sure your static boundary conditions are right too (eg kingpin bearings don't react moments).

Has your team built a car before? If so, you can make a judgement on the best X/Y/Z loadcases to use and compare those loads to the strength of your old car. This will give you an indication if your loadcases are within a ballpark (however this may not be a 'light' ballpark).

Theres a SAE Technical Paper by Cornell on Chassis Design that has some pushrod strain gage data that may be helpful with your assumptions. Even though you arent running FSAE tires, Tire Consortium data will give you a better idea on where to start with tire loads.

Long story short, unless you do extensive testing (which is rarely done in FSAE) then the loads you choose is really more of an art. Read up on the subject (books, forums, papers, etc) and make some engineering assumptions based off of the small data you have.

Hope this helps.

[Denny Trimble]

(Tudor isn't building an FSAE car).

An easy way to do your analysis, if you're comfortable with FEA, is to model your upright and spindle, then connect the spindle to the contact patch of the tire with either a "connector" restraint, or a dummy structure. Then, apply loads at the contact patch, restrain the balljoints minimally (on a pushrod car, the lower balljoint would be fixed in XYZ displacement but not rotations; the upper balljoint would be restrained in the horizontal plane only; the toe link would be restrained laterally only).

You can then read off the reaction forces at the balljoints, and look at strength (stress) and stiffness in your upright and spindle.

A simple and conservative set of load cases for strength is:

1) Cornering: vertical load = mass of car, lateral load = mass of car * tire coefficient of friction

2) Braking: vertical load = mass of car, rearward load = mass of car * tire coefficient of friction

3) Bump: vertical load = 2X mass of car.

You should use appropriate factors of safety based on your engineering judgement. The more you know about your loads, materials, and manufacturing processes, the lower you can go.

[D J Yates]

Hi Ted,

Calculating the loads on the upright is just like any other engineering problem. Start with a free body diagram. Don't be put off or intimidated by the fact that it's a 3D situation, it's exactly the same method of solution and no harder than a 2D FBD.

Finding the forces on that you need to apply at the tyre contact patch of the free body diagram is a little harder, but still is much simpler than the previous posts suggest. Just think about what is happening.

Static loads can be calculated by simple weight transfer. You don't need to include suspension properties, just CG height, mass, wheelbase and track - that'll be close enough for now.

This is where i disagree with everyone before me...

If you want accurate force calculations you cannot assume that a dynamic system is static. If you want to know the dynamic loads on the wheel, analyse it as a dynamic system. Do a simple quater car spring mass model and work out the amplitude of displacement for a given road surface bump/displacement.


As a general note to all. Finding the correct forces to use in an FBD is much more important than the method of analysis you use. You could spend hours doing FEA, but if your initial conditions are not acurate enough then the answer will be no better than a simple hand calculation.


Hope that made sense as it's kind of late...

And by the way. It doesn't take long to get through such calculations. Took me a couple of hours at the most.

[Z]

Ted,

Draw force vector pointing from approx centre of tyreprint (ie. contact patch) to car CG. This is direction of maximum force (think about it http://fsae.com/groupee_common/emoticons/icon_wink.gif). Force magnitude, including safety/stupidity factor, is say 5 x car-weight. So the car is doing a 5G one-armed handstand, perhaps from hitting a kerb/pothole during braking/cornering.

Then a simple statics problem to find forces in wishbones, toe-link, etc.

Z

[RacingManiac]

loads through the caliper mount are also important....

[Frank]

using 7075 I...

design at about 300-350 Nm (from cornering) = 75Mpa at "stress concentration" according to a fairly fine FEA mesh in COSMOS

webs are 4mm
CNC'd it myself (cut 4 uprights from 1 billet)
hard anodised

CAD of front upright
http://www.uq.edu.au/fsae/front_upright.jpg

Mechcam of rear upright
http://www.uq.edu.au/fsae/rear_upright.jpg

[Denny Trimble]

Nice work Frank!

[Tudor Miron]

Nice work Frank! What is the angle at halfshaft?

Z - your idea sounds very interesting. I'd like to ask (because I find it difficult to visualise it in my head) is the same 5 car weights pointing from contact patch to CG in case of rear wheel under that hard cornering/hard braking/hitting curb/pothole scenario?
Thank You
Ted

[kwancho]

That's one gorgeous upright!

[Z]

Ted,

IMO front wheels usually see higher maximum loadings than rear wheels, even on rear heavy cars, because the front wheels are more likely to hit things "head on". Although rear wheels (bearings, etc.) will have higher fatigue loadings on a rear heavy car ('cos higher average load).

Maximum load on a rear wheel is when the car spins and hits kerb/pothole when travelling backwards - so force direction is again from tyreprint to CG.

(Edit: Actually, if the car is spinning the maximum impact force has a direction that both stops/reverses the translational motion of the CG, and also stops/reverses the spinning motion. So the force direction is from the rear wheelprint to somewhere behind the CG (further behind for a faster spin). So worst case might be if the car spins ~120 degrees and hits kerb with outer-rear-wheel, giving an approx. lateral force direction from wheelprint which stops the car dead - it momentarily "standing" on that single wheel.)

Any force not pointing direct to CG causes the (previously non-rotating) body to spin, and there is less inertial resistance to this spinning action than to a translational motion of the whole car. The upright/suspension structure is squashed between the road-to-tyre contact force and the car body's "inertial resistance" (d'Alembert) forces. Picture the damage to two cars hitting each other in a "glancing collision" or "head-on" (or picture two footballers...).

Also note that the horizontal road-to-tyre force is not limited by the friction coefficient. A tyre shoulder pushing against a kerb, or against the rising edge of a pothole or rut in the road (only has to be a 1" - 2" step), has a direct contact force that can be much greater than Cf x Fz. Also the road can "push" much harder on a tyre than it can "pull", so the worst forces are from outside the car acting inwards.

Z

[Mark Bacchetti]

DJ- I agree with you. Making a static assumption that envelopes all dynamic events can be more conservative (eg less accurate) if it's not backed up with analysis like yours. I'm curious though... have you ever back calculated an equivalent acceleration (g) load out of your dynamic analysis? I wonder how conservative an educated guess is compared to more accurate dynamic model.

-Mark
Cal Poly Pomona Alumni

[Frank]

i wouldn't try and "dynamiclly model" this stuff.

and "engineering assumptions" and "typical safety factors" from texts will give you a 650 lb car

strain guages, rainflow analysis, and making valid comparison between design seems more logical to me (for your second car)

after that, it's NDT and heaps of testing, (including a crash or 2 is great also)

our parts are developed using the variables:
"static load"
hours driven
hardness, to guess tensile strength (or tensile strength if known)
stress concentration factor
section properties
typical fatigue response of the material

[Nihal]

How does everyone estimate their bearing forces from the press fit? I have no idea what order of magnitude they should be. Should I assume that for an Al upright that the diameter difference between the upright bearing housing, and the bearing would be the strain, and calculate backwards to the force and stress (assuming all the deflection is in the upright not the bearing shell)?

Also is it a valid assumption in FEA to model the upright with beam elements acting on the bearing housing from the tire contact patch and loading that. I would be constraining the ball joints and tie rods.

[D J Yates]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Nihal:
How does everyone estimate their bearing forces from the press fit? I have no idea what order of magnitude they should be. Should I assume that for an Al upright that the diameter difference between the upright bearing housing, and the bearing would be the strain, and calculate backwards to the force and stress (assuming all the deflection is in the upright not the bearing shell)? </div></BLOCKQUOTE>

Yep, just about. You can find the hoop stresses and radial stresses for an interference fit using Lame (pronounced la-may) equations. It should be in most static mechanics text books and will probably be found wherever there is mention of thick walled cylinders.

Dynamic simulation doesn't mean get out the MSC. ADAMS but just apply some equations of motion to a spring mass model. It really is the quickest and cheapest way to get accurate numbers for forces on the wheel suspension assembly.

Personally, i don't understand why you wouldn't look at the dynamics. I'm sure that you'd all test a materials properties if you weren't sure of them, so why not do some calculations to provide you with accurate forces. Assuming x g's static load might be about right, but you shouldn't trust it without doing some calculations to back it up.

Besides, how do you do fatigue calculations without knowing the dynamic forces?

Frank. We tried using strain guages on the suspension links last year, but there were just to many variables to make sensible use of the results.

[Jersey Tom]

Ha, that bearing press fit...

Pressing into aluminum, I'm doing about a .0010-.0015" undersize bore diameter for our wheel bearing.

When you're doing your analysis and fabrication, remember that is a VERY touchy tolerance. You have .0005" to play with, and it has to be dead on down the whole depth of the bore. If its just a little off, you'll either just get a slip fit, or a heinous pressfit that will require 2-3 guys hanging off the long arm of the press...

I just add the whole press fit in our FEA model and let it do its thing.

[CMURacing - Prometheus]

press fits have always been tough for us. however, i'd think its better to risk being slightly over and use one of the many henkel products designed for bearing slip fits than overstress your press fit and cause problems there.

[kwancho]

Noob FEA question of the day:
How exactly do you guys apply, say, a bump load to the upright, if you've got a pushrod attached to the lower A-arm? Assume the lower balljoint is in single shear and is pulling down on the upright. Can you apply the load on the entire face, or just a circle the width of your bolt head or washer? If you apply a 3G bump (~1200lbs for a loaded wheel in our case) on just a 7/16" circle (bolt head diameter for 3/8" bolt), it just deforms massively at the circle unless the upright is 1/2" thick there. If you apply it over that whole face, you get a more meaningful result, but I'm not sure that's a realistic condition. I can post pics if no one gets what I'm talking about.

[Marshall Grice]

apply the load as a "shear" force acting on the threads of the bolt hole.

what are your constraints? You're pretty much assured an overstiff model unless you use a rather extensive FEM, or use a remote load with appropriate upright constraints. Just applying a force at the LBJ doesn't take into account all of the moments that are generated, unless of course if you are adding in the moments as well.

[Jersey Tom]

Remote loads are the way to go.

You sure 3G bump with 1200lbf is the way to go? I'd think it should be based on spring compression and shock velocity. And theres ways of getting that without data acquisition

[kwancho]

Oh, what if I don't thread the bolt hole and use a nut instead? Is that horribly bad? In theory, a vertical load shouldn't apply any serious moments, right?
As for the actual load, I've got a 400lbs loaded wheel and a 3g bump = 1200 lbs (not 1200*3 if that's what you're wondering). And it certainly sounds like a decent idea to do the m d2x + b dx + k x = F calculation.

[Conor]

What program are you using to do your modeling? If it's ProE (only one I'm familiar with so this may be possible with others), you might want to try building the upright assembly as a mechanism, and obtain your forces that way. Then load the part into Mechanica (FEA), and use the forces found from modeling the mechanism. I'm just throwing this out there and I've actually never done it myself. I asked an engineer at work about doing something similiar for our differential carrier and he gave me this solution. He'll be helping me with all the FEAs in a couple weeks so I can pass on what I learn if you're interested.

[Jersey Tom]

I'd find the loads from the dynamimc displacement of the wheel and the spring and damper. 1200lb load is going to lead to way overdesigned part.

[kwancho]

Conor- I'm using Solidworks. I have to settle for COSMOSXpress right now, but I'm going to get the disc for the full version some time soon, which can do assemblies.

Tom - What about running over a sizable rock? Won't that cause a load that's greater than what the resisting spring and damper provide?

[Jersey Tom]

Dont use Cosmosxpress! It is worthless for the analysis you should be doing.

Formula cars are not designed to go offroading. I've seen and tested at some pretty nasty parking lots (like Coors.. where the weather changed every 30 minutes, sand/rock storms would attack us, and we'd be rained on by thousands of empty beer cas) and have never seen 'sizeable' rocks or holes. If its really that bad, you shouldnt be testing there. The competition surfaces these days are very smooth.

Irregularities on the order of a centimeter or so the suspension should be able to manage. Pretty easy to model. I reccomed doing ANY system dynamics stuff in Simulink.

Btw hows the rest of the car shaping up?

[kwancho]

Gotcha. Haven't really learned Simulink yet. Hm..
Car's on its way. When I have something nice to show I will.

[Jersey Tom]

Simulink is AWESOME. Hadn't used it before, hadn't taken system d, and my comfort level with 2nd order differential equations was.. just about non existant.

Within a couple hours I had a cool suspension modeled that shows chassis displacement and suspension load as the wheel hits road surface irregularities.

[Devd]

I am working on front upright.i had searched the web,but failed to find correct info about it please guide me.i am talking about format for design calculation.

[wagemd]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Devd:
I am working on front upright.i had searched the web,but failed to find correct info about it please guide me.i am talking about format for design calculation. </div></BLOCKQUOTE>

Well, at least is appears you searched before posting since this thread is ancient...

However, no one here is going to give you a serious reply to your question. What exactly are you asking? You want someone to give you an upright design? What incorrect info did you find?

The format should be 12 pt Helvetica. Don't forget to shrink the margins so you can fit more text in.