Real tech question - suspension mount load approximation

2.8 · Post by **2.8** » Thu Nov 18, 2004 2:33 am

If this forum is going to be more than a bunch of people jerking off in public because it's the first chance they've had (not that there's anything wrong with that) then it needs some real tech stuff. So here it is.

Let's say I'm designing a suspension mount. This will be the upper mount (fixed to the chassis) for a coil over on a double a-arm suspension. Standard stuff, no linkages with inboard shocks or anything fancy like that.

I want to build this mount as strong as it has to be but no stronger. First I need to figure out the maximum load it can take. How do I do that? Here are my thoughts.

Let's say the largest bump I will hit will be equivalent to driving up a 3" tall square edge ledge and that this will bottom out the suspension. Let's say the car tops out at 120mph so that's the fastest I'll ever hit this bump.

Knowing the speed of the car and the distance from where the the leading edge of the tire hits this bump, I can calculate the amount of time that it takes the wheel to move up 3". I can calculate the speed of the wheel when the suspension bottoming out.

I think that I can make a change in perspective and say that instead of the wheel moving up, the car is moving down. I know the weight of that corner of the car so I can now calculate the load that this will put on the suspension mount.

To make calculations easier I'm ignoring things like spring rate and shock damping because I'm guessing they will be insignificant in this case. For a safety factor, let's double the weight of that corner to say I might hit this bump in a corner.

Any problems with this methodology?

2.8

THE PUNISHER · Post by **THE PUNISHER** » Thu Nov 18, 2004 12:24 pm

There is no problem with your methodology , it's your motives I question. SInce a setup like this would be illegal in just about every form of racing with the exception of SCCA's EM class. What is the use of the car?

Most people who race their fiero's have found that the stock 84-87 suspension with an 88 rear cradle will yield the best resulats. George Ryans Old AP autocrossed with the manapart Quad 4 worked best with the 88 rear and earlier front.

Give us some insight as to why you want to undertake such a project and perhaps we can offer some alternatives or help with your current project.

Nice to see a serious post..keep it up!

JM

Post by **eHoward** » Fri Nov 19, 2004 4:41 pm

I did read this and will post a response for you.

I think you can set this up as a simple statics problem. I don't trust myself to give you actual equations. I will ask my friend that did modeling of forces for tubmounts on the formula car, what he did for those and also what he'd do for a shock tower.

Gixxer · Post by **Gixxer** » Sat Nov 20, 2004 8:04 pm

If I remember right (and I do...),
read a book on chassis design, the author said that people were designing spring mounts and other chassis bits to approx. 6 Gs. But if the spring and shock were coaxial, the loads would double. The shocks really put a huge load into the frame.
Steering component loads were 10 Gs. (Don't know how THAT was figured!)
I'd make the chassis mounts really friggin strong, so if you are gonna bend something, let it be the upper and lower links.
Cheers,
Brian

2.8 · Post by **2.8** » Tue Nov 23, 2004 3:10 am

THE PUNISHER wrote:What is the use of the car?

eHoward wrote:I will ask my friend that did modeling of forces for tubmounts on the formula car

Thanks Howard. I figured you'd have ideas on this.

Gixxer wrote:people were designing spring mounts and other chassis bits to approx. 6 Gs. But if the spring and shock were coaxial, the loads would double.

Great info. Maybe I better dust off Tune To Win.

Thanks everybody.

2.8

Fred Savage · Post by **Fred Savage** » Tue Nov 23, 2004 6:28 am

I know if you hit a curb at 90 you bend a control arm

your rims crack and your rearview mirror busts off, the rest is peachy. If that helps in any way, shape or form.

Gixxer · Post by **Gixxer** » Wed Nov 24, 2004 7:04 pm

A bit more detail for the "G" number stuff-- from "Race Car Chassis- Design and Construction" by Forbes Aird.
Assume 3G bump loadings in combination with 1G cornering load, or 3G bump in combi' with 1G braking, etc.
Multiply it by 2 for a safety factor, and you get about 6Gs.
Cheers.

The Dark Side of Will · Thu Nov 25, 2004 12:41 am

I think you're going for overkill... kitting a 3" step at 120? That's offroad racing shit.

what spring rates will you use? figure out how much it takes to compress your spring to coil bind. Add in how much force it takes to compress your bump stop to the point of spring coil bind. Add an appropriate safety factor and build to that. If you coil bind your spring, you'll probably bend a control arm as well as the spring mount, so they both might as well bend at the same time.

I'm interested in how you're going to build to a certain strength? Are you going to test the mount outside the car or just make a WAG about what material thickness, etc to use? FEA softare?

2.8 · Post by **2.8** » Thu Nov 25, 2004 2:23 am

The Dark Side of Will wrote:hitting a 3" step at 120? That's offroad racing shit.

Yep.

The Dark Side of Will wrote:I'm interested in how you're going to build to a certain strength? Are you going to test the mount outside the car or just make a WAG about what material thickness, etc to use? FEA softare?

I haven't exactly figured that out yet. Probably just use the SWAG (S is for Scientific WAG) method. No use using 1/2" steel if 1/4" steel will work, right?

2.8

Gixxer · Post by **Gixxer** » Thu Nov 25, 2004 2:45 pm

At work we have a separate stress anaylist group to give our designs a good looking over. They look at the design from a variety of viewpoints--uploads, downloads, fore and aft loads, CRASH loads, loads from multiple angles, vibration loads, fatigue, abuse, etc.
the important thing is that they have gone to school to learn how to calculate static and dynamic stress. But that's not good enough. Then they spend about 3 years learning the ropes about how to actually apply book learnin' to airplane (oops, now I said it) design.
Here's Gixxers short course on how --"I"-- would do it--
1)In your case- first thing you want to do is get the expected anticipated loads from somewhere.
I've given you that.
I got my numbers out of a book whose author spent some time interviewing race car designers for that info.
2) Find an EXPERIENCED automotive stress engineer. in his absense, find a student, or ex-student that has an engineering degree. then the two of you can collaborate on step 3, 4 and 5.
3) Make a 3 view drawing (or 3D model) of the cars supension showing all the suspension pickup points, tire(s), shocks, etc.
4)Then using this info divvy up how all the loads that are fed thru the suspension links and into the frame (see where and how much of the loads go to what).
5) Assuming that you've picked a decent point for your suspension and have figured out a stiff (triangulated) structure to go to it, you can now determine the SIZE and strength necessary for all of the parts--including brackets, bolts and the structural tubes that you will use.
OR--
6) You can make everthing hell-for-stout, and forget about (1) thru (5)!!!
Nobody says it's easy, that's why not everybody does it.

FYI the numbers I gave you were for ROAD RACING. Not for off-road racing. Nobody cares about off-pavement racing. We leave that for the people who were wet-nursed for 6 years too long.

2.8 · Post by **2.8** » Fri Nov 26, 2004 12:51 am

Gixxer wrote:We leave that for the people who were wet-nursed for 6 years too long.

I believe that's the cause of a number of my problems.

2.8