How to determine load for a suspension component

So my Google fu sucks and I can't seem to find anything on the web about this. I'm building a hypothetical suspension for a largish multi purpose truck that would tow a race car and trailer and / or handle some pretty good trail obstacles with minimal adjustments required. The part that gets me stuck is how suspension designers determine how much load a piece will experience so the components will be strong enouh to survive expected conditions, whether that is stopping 8,000 lbs of truck and trailer or dropping the front wheel into a deep hole and back. Does anyone have advice on where to look?

Unless you're a mechanical engineer, you're gonna need to read up on your statics and dynamics.

"Load", "piece", and "components" are vague words. I would assume you are thinking over vertical loading, except for the mention of braking and trail obstacles.

SPECIFICS, MAN, WE NEED SPECIFICS!

This is the stuff they actually teach you in engineering school. Most people just overbuild it and hope.

I should tell you right now that towing and offroading are two very competing interests in suspension design...for towing you want low and hard, for offroad you want tall and fairly soft.

So as a more specific example. I am thinking that a triangulated 4 link in the rear might be better than leaf springs in that it would alow for more range of movement for off-road but be able to be "leashed" by a strong roll bar when towing. It would allow for the use of adjustable coil overs which would make ride height adjustment much easier and supplemental load assist air bags for large payload and trailer weights. To build it I would assume heim joints on tubular bars (yes bushings ride better but lt tires soak up a lot of harshness on their own) but there are a zillion differnt ones with load ranges from a cous thousand pounds to 300000 + pounds. Tubing sizes also vary a lot and if a 35 pound link bar will do the job it seems that would be better than a 50 pounder that ws overkill. I'd think that gravity would be the easy part as I know the weight of the vehicle and can calculate the bending and shear moment from a given "beam liar condition" but what about impacts and directional forces?

Does anyone know of a source for this kind of engineering?

In reply to GameboyRMH:

Yes. I know. That is why I am going through this thought process. Adjustability is key. I would assume that disconnects at sway bars and ride height adjustment are built in and I also assume 2 sets of tires and rims are also part of the design. Who wants to hear mud tire howl on a three hour trip anyway!

Kenny_McCormic wrote: This is the stuff they actually teach you in engineering school. Most people just overbuild it and hope.

I learned all of that stuff, once upon a time...And I'm still using the overbuild it and hope method in my Locost, where weight is also rather important. The 4x4 community has found material and component sizing through trial and error that has been proven to work. I would start there.

The loads for basic on-road conditions, are relatively easy to calculate. Most of the variables are known, or can be reasonably approximated, and it's just force balance equations from there. It's the off-road and unexpected on-road conditions that greatly complicate matters, and are often discovered through trial and error...Which goes right back to overbuild it and hope.

Yeah, for all the years of math and physics classes I took in college, all I did as a real-world engineer was to figure out how big it needed to be to handle such-and-such load, then make it four times bigger so the dumbass end-user could royally Berkeley it up and not break anything too badly.

I don't know where to tell you to start. There are a million suspension and chassis engineering/design books out there. Maybe try to find one at the library, see if you can get a good enough grasp to then over-engineer something for your application?

Static conditions are easy to predict and there are plenty of material strength/dimension calculators available. A little bit of vector math to arrive at the real xyz loads and you are golden. Great if you are doing a building where the loads can be predicted. Not so easy for a dynamic situation like a vehicle where the loads and rate of loads are all over the place.

Even most engineers use the "Metric System"...ie double and add 32 just to be safe.

The "proper" way to do what you want is to draw the suspension up in SolidWorks and do some "Finite Element Analysis" on the design. It will tell you if you are good to go. Engineers used to do these kind of calculations with a slide rule and a lot of Vector math.

My suggestion is to head over to Pirate 4x4 and peek over their shoulders to see what they are doing.

I could care less about the harshness of heims versus bushings, but would not be happy with the wear rate of heims as they are exposed to grit and grime.

Yeah. I had concerns about grime and burns too but looking at the hardcore jk and trail keep suspensions out there most of them run at least 1 heim per link. Your idea to model it in solid works is interesting and I will look into it. Is it freeware?

And pirate is an interesting place. There are some neat rigs, but no matter what question you ask about designing an IFS front your answer wi be "do an SAS swap or go away"

In reply to itsarebuild:

Solidworks is big money, but you can probably get a cheap/free student license(watermarked). I know Autodesk gives out the student license of their Solidworks competitor (Inventor) for free.

itsarebuild wrote: Yeah. I had concerns about grime and burns too but looking at the hardcore jk and trail keep suspensions out there most of them run at least 1 heim per link. Your idea to model it in solid works is interesting and I will look into it. Is it freeware?

Solidworks is the opposite of freeware, however if you know a student you can get a copy for cheap and it is fully loaded with all the expensive modules. Certainly a learning curve but if you are so predisposed, you could become addicted to this stuff. YOUTUBE is your go-to tutor for learning solidworks basics.

I don't think they ship the cosmos pack with the student edition. So your simulation's will be a little limited.

This is not F1, find someone who has done it in the past and survived and add strength till it gets to your weight.

In reply to NOHOME:

Yeah. Found it and broke my jaw on the desktop. Off to find inventor!

http://www.autodesk.com/education/free-software/students-university/popular

They might demand a .edu email address, but that's easily faked, googling ".edu email" gives tons of places to get a free one.

NOHOME wrote: The "proper" way to do what you want is to draw the suspension up in SolidWorks and do some "Finite Element Analysis" on the design. It will tell you if you are good to go. Engineers used to do these kind of calculations with a slide rule and a lot of Vector math.

The problem with that approach is there's a HUGE "bullE36 M3 in, bullE36 M3 out" factor. FEA is only as good as the boundary conditions you give it, and more often than not, the results aren't worth much more than the marketing of having pretty multicolor plots.

Not saying OP doesn't know what he's doing, but for multi-part suspension FEA, I'd be looking for something better than Solidworks/Inventor or just sticking to pen and paper where you KNOW you understand your load conditions.

unevolved wrote:

NOHOME wrote: The "proper" way to do what you want is to draw the suspension up in SolidWorks and do some "Finite Element Analysis" on the design. It will tell you if you are good to go. Engineers used to do these kind of calculations with a slide rule and a lot of Vector math.

The problem with that approach is there's a HUGE "bullE36 M3 in, bullE36 M3 out" factor. FEA is only as good as the boundary conditions you give it, and more often than not, the results aren't worth much more than the marketing of having pretty multicolor plots. Not saying OP doesn't know what he's doing, but for multi-part suspension FEA, I'd be looking for something better than Solidworks/Inventor or just sticking to pen and paper where you KNOW you understand your load conditions.

I think that we can all agree that 糞 in equal 糞 out. That applies even if you are an engineer. You would be surprised how many actual engineers mis-use the software not having a full understanding of load paths.

In reply to unevolved:

Yeah, FEA is more of a qualitative than quantitative thing for the most part, lets you see the weak spots.

Absolutely. I really don't want to come across as condescending, but OP doesn't sound too familiar with CAD and FEA, and I see lots of people look at it like a golden gun for design. It's awesome, but unless you're doing something topological optimization, it's just saving you some paperwork. I'd hate for OP to waste time pursing a license and learning a program if it won't do what he(?) needs.

unevolved wrote: Absolutely. I really don't want to come across as condescending, but OP doesn't sound too familiar with CAD and FEA, and I see lots of people look at it like a golden gun for design. It's awesome, but unless you're doing something topological optimization, it's just saving you some paperwork. I'd hate for OP to waste time pursing a license and learning a program if it won't do what he(?) needs.

If the op figures out the basics of solidworks, I see no downside. The ability to design stuff in 3D is well worth the initial learning curve.

That said, SW has a watermark on paper prints. Fair enough. Anyone tried a student version with a 3D printer?

As the OP I appreciate the discussion above. It is pretty spot on in that I know just enough about structural design to be dangerous. I still see some validity to learning the solid works program as it appears that it would also be helpful for really seeing in three dimensions how the system would work and where any deficiencies might exist for binding or unwanted effects.

I also appreciate the value of adding in some fudge factor. While I don't want to add unsprung weight carelessly it is important to remember this isn't a trophy truck either.

Like I said this is a mental exercise at the moment so I have the time to learn some software and hopefully a lot about suspension design.

If anyone has a recommendation for any literature on the subjects ( via electrons or dead trees) please post it up!

NOHOME wrote: That said, SW has a watermark on paper prints. Fair enough. Anyone tried a student version with a 3D printer?

Yep, it'll export STLs just fine. I've printed stuff students have sent me without any issues. STP, IGS, etc. also don't have any psuedo-watermark.

Also, itsarebuild, I 1000% endorse learning Solidworks. It's a fantastically useful program, and I've been using it for so long I'm not sure how I'd design something without it anymore. It's just so damn useful for making stuff. All my comments about exercising caution are in regards to analysis, not design. Go for it, you can't hurt yourself with CAD.

As stated above- towing and off-roading have some pretty different design goals related to suspensions... Towing requires a pretty stiff suspension to handle the tongue weights while off-roading typically works best with lots of suspension travel and softer spring rates. I would also look into limiter straps to tie the suspension down instead of using the sway bar for this- the sway bar won't be very happy with having to limit the suspension travel.

I agree on getting a copy of solidworks, if possible to design parts and systems. It is very easy to learn and easier than Inventor or the other higher power system- Catia, Creo, etc. There are some others out there that are cheaper but not sure how much they would help. You could do some reading in the Locostusa forums for some ideas on different cad software packages.

As for determining the suspension loading, there are several methods but this generally involves driving an instrumented vehicle over the typical terrain or roads under varying loads. The suspension will see different loads depending on how it is being driven- hard cornering vs severe bumps vs hard braking events. Lots of data to wade through and then decide what is real and how to apply this to the FEA...

A bit of reading through the Pirate 4x4 forums would probably get you most of the info you are looking for. I would also do some investigation on vehicles that may be similar to what you are trying to build to get a feeling for how the specific parts are designed.

It does sound like a fun project!!

I was loosely involved in that type of work for an FSAE car.

Basically, people do go to college for 4 years of rather rough classes to do this stuff.

We took our expected max loading (in every case) and used a minimum Factor of Safety multiplying the loads. NOTE: this was for a delicate formula car built for smooth pavement.

To see some of this type of engineering to give you a bit of a gist. http://www.dpcars.net/

(try page 11 of the ATOM)