I'm wondering what the deflection is with stock bushings Definitely good info in this and I can't wait to see how a reinforced trailing arm compares.

This is a cool thread. I'm just thinking out loud and sorry about the rough graphics.

The "focus" of the stress would move out board and not be a static force?
Uhhm...
The twisting force would change relitive to the trailing arm as the approach changed. Not just the Ft. lbs.
Do I make any sense?

QUOTE (r_towle @ Dec 19 2005, 09:08 PM)

so, is carbon fiber an option here.. Can it be effectively bonded to steel... Rich

I've got Boeing's best composite engineers (three of them actually) working on this very question. I posed the question to Jim, a senior engineer at Boeing (and a motor-head at heart), about which would be the most structurally sound way to stiffen my 914's inner longs with the 900' roll of uni-CF I got recently.....lay-up and bag the material directly to the longs, or make molds (or use a parting agent between the longs and composite) and bond the composite long stiffeners to the longs using a high-strength structural adhesive. He loves this kind of shit (anything car related) and said he was going to get with a couple of other guys on the subject and "crunch some numbers" . Personally, I think the HSSA method would be stronger than laminating directly to the steel, but either method would virtually eliminate corrosion from forming between the long (or swing-arm in this case) and stiffener. I may just make a couple of test billets while I'm off for Xmas break utilizing both methods, and abuse them until something cries "Uncle".....

Kevin,

I think the reason that people don't glue to steel is corrosion. A rust bubble can place a lot of stress on a glue joint and would probably enlarge until it caused delamination- right when you don't want it.


I have done a lotta aircraft structural type work with structural epoxy and rivets but aluminum doesn't form rust bubbles as readily as steel. The rivets are used both as reinforcement and as a means of squeezing the glue as thin as possible (max strength). Glue joint strength is primarily dependent on adhesive strength and glue area.

Ken

Hey Chris,

Any new data on the trailing arm flex experiment?

-Josh2

Yes, just a small amount of new info.
I was measuring the flex with the digital level mounted on my lever arm. After further examination it appears that there is measurable flex in the mounting end of the lever, so I moved the level to the brake mounting ears on the trailing arm. This change subtracts 0.3 degrees from the previous measurements.
Also my latest reinforcement, as described in the last picture, did not reduce the flex any more than a single bulkhead in the middle of the trailing arm. My next test will be two bulkheads, parallel to each other, several inches apart.

Chris,

did the measurement changes (the .3 degrees) subtract from both the before and after?

so .7 before
and .5 after


brant

How's about filling the arm with some hi-density foam.
The problem would cleaning the inside of the arm well enuff to have the stuff adhere.....plus selection the right foam. I don't know what is available.

Molten aluminum poured in would be gud too.......then start drilling holes.

QUOTE

How's about filling the arm with some hi-density foam.

Not as crazy as it sounds....we once filled all the cavities in a bugeye racecar with foam and got a noticible increase in chassis stiffness. We abandoned the idea later though...when we had to do some welding on it Another possible filler would be a lightweight epoxy mix with lots of micro-baloons.

QUOTE (goose2 @ Dec 29 2005, 09:52 AM)

Not as crazy as it sounds....we once filled all the cavities in a bugeye racecar with foam and got a noticible increase in chassis stiffness.

I may be crazy, but I ain't stupid.

If a guy could come up with sumthing to pour in and not add more than 4-5 lbs of weight you'd be in fat city.
Epoxy: maybe half full & rotate along the long axis till dry.
Maybe add an inch of wall thickness.

Thinking outside the box (NPI)
What else do I have to do while sanding off orange peel?

There are several companies that sell a foam injection kit for cars. Sport and Compact did a project Miata with one of the kits. I know all of the new Ford SUVs have a two part foam injected into the hollow areas on the chassis. There is a substaintial difference in the harmonic resonance levels as well as chassis stiffness.

Many of our chassis are fairly clean on the inside of the longs and other areas. You could do this and probably pick up some serious reductions in NHV. The problems are when you go and put this in rusty areas it will not bond, nor will it stop the rust. Also if you do this to a chassis it will cause problems if you try and weld over it. Poof, your car burns to the ground.

Otherwise it would be a good upgrade. You could acid dip the arms and then inject them with foam. Won't five substantial improvements in just the arms but it will help out a little.

That's about right, except I am using 1.1 for the original stock measurement and I get 0.8 with the level attached to the brake caliper mount (ie. vertical).

Today I added a second bulkhead to the most recently modified trailing arm. There was no further improvement - still a deflection of 0.5 deg.

I think I'll try adding some Chrome-Moly sheet in selected areas next.

QUOTE (J P Stein @ Dec 29 2005, 04:32 PM)

If a guy could come up with sumthing to pour in and not add more than 4-5 lbs of weight you'd be in fat city.

4-5 lbs???
JP, I'm barely willing to add one pound.

QUOTE (Racer Chris @ Dec 29 2005, 05:34 PM)

QUOTE (J P Stein @ Dec 29 2005, 04:32 PM) If a guy could come up with sumthing to pour in and not add more than 4-5 lbs of weight you'd be in fat city. 4-5 lbs??? JP, I'm barely willing to add one pound.

I hear ya, I ground down my trailing arms castings, cuf off the ears for the backing plates and all them little support ears to get 2-3 lbs off them.....even painted them yellow (YPAF, ya know ). But I'd put some back on to stiffen em'.....but not them fishing lures "they" sell for the job.

I had time for one more trial before 2005 ran out.
This was a continuation of the modifications to the first trailing arm I upgraded. This one already had the bulkhead added to the center of the box section and a fancy gusset between the box and the pivot shaft tube.
My observations indicated that the pivot shaft tube was flexing significantly, after the box section was stiffened, and the gusset didn't stop that. I decided to add a reinforcing layer over the exposed front side of the pivot shaft tube, as shown in the picture. The overlay is .065wall mild steel, a half section of 2 1/2" od tube about 7" long.
The original trailing arm flexes about 0.8 deg. under stress.
With the bulkhead added the flex drops to about 0.5 deg.
The gusset didn't stiffen the trailing arm noticeably - at least not in torsion.
The tubular overlay resulted in a reduction of the twisting to 0.4 degrees.
The trailing arm "feels" stiffer when I put my weight on the lever and is 50% stiffer than stock by the numbers.
I will probably use these three reinforcements (that weigh less than 16oz.) on my race car's trailing arms, but there is one more area where I want to focus my attention.
The part of the trailing arm that is pocketed to make clearance for the brake caliper bothers me. I want to fill that in so the pocket no longer exists. I think there is still room for a front caliper after filling in that area.

Looks great as usual Chris, good job!

This is where my arm cracked multiple times. After boxing the arm it is the weakest point it seems. The fix was to press in a piece of mild steel tubing (same size as my roll cage, I think). The arm was drilled in multiple spots and then welded to fix the pressed in tube to the arm. This seems to be a good fix - I do stress my suspension severely andso far no cracks. The additional advantage is that the weight that you add is mostly sprung weight and not unsprung as it would be if it were far out on the moment arm of the suspension. I am sure that this also minimizes flex in this area.

Chris great stuff! thanks. My $.02 . Have you thought about using a lazer pointer witha magnetic base on the part you are monitering. Then put a piece of sheetrock on the ceiling to put the bean on. This way you would see deflection in two axis at the same time. Comments? Al

So Chris,

now that you about have it nailed down.. one thought still sticks in my head.

how much negative camber can you easily dial in?
would it be basically limit-less?
I don't know that I need any more, but it makes me think......

here is what I think: "too bad I have monoballs at this point, its just not worth new wheel bearings and re-doing the monoballs."

two more questions sir.
since you have the arm with the old style kit handy.
how much does that one weigh, and how much total does your lovely new one weigh?

do they both have similar bearings and pivot shafts (weight wise)

and finally,
I lost track a tiny bit.
with the 2nd style of measurement, what is your measurement for an old style reinforced arm. Is it also at .5 deflection now? (I know at one point it was .8, but I'm not clear if it was remeasured when you switch measurement modes)

brant

QUOTE (Al Meredith @ Dec 31 2005, 07:03 PM)

Chris great stuff! thanks. My $.02 . Have you thought about using a lazer pointer witha magnetic base on the part you are monitering. Then put a piece of sheetrock on the ceiling to put the bean on. This way you would see deflection in two axis at the same time. Comments? Al

Good idea, Al.

Paul

QUOTE (Al Meredith @ Dec 31 2005, 10:03 PM)

Have you thought about using a lazer pointer

Thats a great idea Al.
I have a laser pointer so I will try it.

QUOTE (brant @ Dec 31 2005, 10:12 PM)

how much negative camber can you easily dial in?

I can make outer pickup brackets that change the starting camber - more positive or more negative.

QUOTE

how much does that one weigh, and how much total does your lovely new one weigh?

I will provide that data once I weigh them carefully.

QUOTE

with the 2nd style of measurement, what is your measurement for an old style reinforced arm.

It was 0.5 deg. when I remeasured it.

Chris,

I think this is an awesome product/service from CFR!

-50% stronger than stock and 20% stronger than the other "kit"

-I'm guessing 4lbs lighter (mostly unsprung weight)

-and available with an extra -1degree of camber (or other amounts)

sounds pretty hot to me!
brant

Here's some weight data.
Stock trailing arm: 15lb, 5oz.
Boxed trailing arm: 18lb, 2oz.
My reinforced trailing arm: 16lb, 5oz.
For reference, a pivot shaft with bushings weighs 1lb, 8oz which is included in the above weights.

When was the last time I read a 7 page thread?

Awesome!

Chris, thanks a lot

So, if I'm reading this right, a factory style stiffening kit is just under 3lbs. heavier (per side) and is 30% stiffer than stock?

Yup. Actually more like 38%. (0.3/0.8)
More than half the weight of the boxed kit is unsprung.
No more than 4oz. of my design is unsprung, and is another 12% stiffer.

I got some info from a friend tonight. He modelled the trailing arm and did some FEA for me. I haven't had a chance to analyze the details but everything appears to agree with my empirical data.

QUOTE

More than half the weight of the boxed kit is unsprung. No more than 4oz. of my design is unsprung, and is another 12% stiffer.

No, no... don't get me wrong. You're thing is the cat's ass, especially for guys in your arena that want to shed all the oz.'s they can.

I was just curious because I installed one years ago and was told that it was worthless, etc. Now I'm OK with the decision... it could be lighter and stiffer but frankly I'm just glad it helps and doesn't hurt. For me 6lbs isn't that bad. I saved that with my S-Calipers.

Thanks again. Killer thread. This needs to be book marked for a "Classic" for sure

QUOTE

I got some info from a friend tonight. He modelled the trailing arm and did some FEA for me. I haven't had a chance to analyze the details but everything appears to agree with my empirical data.

hehehe

I have the same data... Im looking at it now... I dont see any hotspots which is a good thing when you do a wireframe FEA

One thing that I may offer is that there are mulitple forces happening at once on a trailing arm.. not just torsion .

In order to investigate this further we should agree on some nomenclature, call X the horizontal plane, Y the vertical plane. etc

I didn't think you were giving me a hard time. I just thought it was a good intro for me to restate the facts.

I know, but I decided to ignore the lateral forces. I am guessing the lateral is only 20% of the torsional but I don't have any numbers to back that up. Can you do that calculation?
I also realize that my lever arm applies the force at 90 degrees to what would be required to measure the lateral effect but it was convenient to do it this way. Also the FEA says it doesn't matter whether the rear of the trailing arm is supported at the bottom as I did or at the shock mount, for the torsional measurements.

New guy here, I LOVE this forum, you guys rock.

If you want to watch it flex put a cable on the end of the pipe that you are using to load the trailing arm and hang your 220lbs on there, set them on a floor jack and just let it down while you watch.

QUOTE (TimT @ Jan 4 2006, 09:54 PM)

hehehe

Tim, guess who did the FEA for me. You know him from the PP racing forum.
















john luetjen

QUOTE (Pinepig @ Jan 4 2006, 10:14 PM)

If you want to watch it flex put a cable on the end of the pipe that you are using to load the trailing arm and hang your 220lbs on there, set them on a floor jack and just let it down while you watch.

That would work great except my weight consists of 12 pieces of steel of varying shape and size (plates, sq bars, rd bars). If I was into freeweights I could do what you suggest.
What I really like doing is having a friend bounce up and down on the lever while I watch the trailing arm closely.

Chris,

Im getting more concerned with the trailing arm mounts.... than the trailing arm itself... You obviously have by intuition, good ole common sense, and some inate engineering prowess cured a problem with the trailing arms in high load situations..

Now we bolt these arms back to the 914 tub.. I dont know the GCR but how much are you allowed to reinforce the pickups...

kudos to you and your development of these cars...

Ill play around and try and determine lateral forces

Chris I know John did the FEA since he cc'd me in an email

John sent me the FEA based on he is a salesman not an engineer.. well Im an engineer, and John continues to impress me with his insights, and grasp of problems that face us..

QUOTE (TimT @ Jan 4 2006, 10:20 PM)

some inate engineering prowess..

QUOTE

how much are you allowed to reinforce the pickups...

Suspension pickup location and reinforcements are totally free in Production. That's why mine are already substantially altered, ie. moved and stiffened. The outer pickup bracket is a major concern so I designed a stronger unit. I still have to test it with my fixture so I can advertise just how much better than stock.

I shoulda known.

One thing the FEA didn't pick up is the amount of improvement from adding the bulkhead. Is that due to the wire frame model being different from sheet material or some other shortcoming of the software?
One piece of information that I have is a damaged trailing arm from the wheel being hit by another car. It buckled not far from where I added the bulkhead. I am guessing that the box tube dimples in that area during torsional force application as well, and the bulkhead prevents the dimpling. Adding more bulkheads had no additional benefit.

OK. I'll stop lurking on this thread and post some pictures of the model -- without the bulkhead.

Click for bigger image...

And from the back side...

Click here for larger image...

And from the top... (Note that the displacement is exagerated)

Click for bigger image...

QUOTE (Racer Chris @ Jan 4 2006, 11:20 PM)

What I really like doing is having a friend bounce up and down on the lever while I watch the trailing arm closely.

Hey, I resemble that remark!

Chris, looks like you had brainstorm in another direction since I was there. Good stuff!!

QUOTE (Racer Chris @ Jan 4 2006, 08:05 PM)

One thing the FEA didn't pick up is the amount of improvement from adding the bulkhead. Is that due to the wire frame model being different from sheet material or some other shortcoming of the software?

When I added the bulkhead, I used the same 2mm thick steel as I used for mostof the rest of the trailing arm model. While a bulkhead helps some, since most of the forces travel across the exterior of the arm, it's not going to be the best solution. Keep in mind that when modelling torsion bars, generally it doesn't make a big difference from a torsional displacement perspective if they are hollow or not since most of the forces travel across the exterior. This trailing arm really isn't much more then a stubby T-bar*. As you picked up with your experiments, most of the surface stress is in the area where the trailing arm's cross-section transitions from being an upright rectangle to being a laying down rectangle. Increasing the cross-sectional area of the trailing arm -- specifically in this area should yield the biggest improvement.

* Basic rule: The angular rate of a tube (in in/lbs/degree) is = (19700 * (OD^4-ID^4))/tube length. So if you were to generalize and say that the trailing arm is a 16 inch long tube with an OD of 4 inches (average) and an ID of (4 inches - (4mm or 3.84 inches)), then the rate of the trailing arm is 19700 * (256-217)/16 = 47.5k in/lbs/degree. If you separate it into 2 - 4 inch OD bars, one hollow bar of (16 inches - 2 mm (.08 inches) = 15.92 inches, and the other a solid bar of 2 mm in length, you get the hollow bar has a rate of 48k in/lbs/degree, while the solid bar has a rate of 63,040k in/lbs/degree. Combined using the formula 1(1/rate1 + 1/rate2) = total rate, we come up with a total spring rate for tube with a bulkhead of essentially... (wait for it!)... 48k in/lbs/degree! But, if you were to increase the OD of the the tube to 4.5 inches (still with an ID 4 mm less), you would get a rate of 19700 * (410 - 355)/16 = 68k in/lbs/degree. So you'll get much better results by increasing the cross-sectional area of the arm rather then adding internal bulkheads. This is what you did by adding the gusseting where the fabricated assembly attaches to the smaller tube.

QUOTE (jluetjen @ Jan 5 2006, 04:54 PM)

OK. I'll stop lurking on this thread and post some pictures of the model -- without the bulkhead.

I'm glad to see you over here John. Thanks for doing this for me!
In a previous life I was going to be a mathematician and I could have had this in hand with little more than a snap of the fingers. Instead I embarked on developing the weak side of my personality and now too much math gives me a headache.

QUOTE

If you separate it into 2 - 4 inch OD bars, one hollow bar of (16 inches - 2 mm (.08 inches) = 15.92 inches, and the other a solid bar of 2 mm in length, you get the hollow bar has a rate of 48k in/lbs/degree, while the solid bar has a rate of 63,040k in/lbs/degree. Combined using the formula 1(1/rate1 + 1/rate2) = total rate, we come up with a total spring rate for tube with a bulkhead of essentially... (wait for it!)... 48k in/lbs/degree!

Don't you want that to be two bars of 8 inch length held together at the middle by a solid bar of 2mm length? That would drastically alter the calculation if I'm not mistaken.

Something that might be interesting would be to load up the trailing arm in your rig and use a mic or a good set of calipers and measure across the arm and see if it is bulging or puckering (cross section) under load (compared to the unloaded condition.) If it was it would explain why the simple bulkhead works so well - "pucker facter reduction technology!"

QUOTE (srbliss @ Jan 5 2006, 06:46 PM)

"pucker facter reduction technology!"

lol, I like that!
It would probably take a while to isolate the location of maximum pucker but I expect that would be possible to measure a change there.

QUOTE (Racer Chris @ Jan 5 2006, 03:38 PM)

QUOTE (jluetjen @ Jan 5 2006, 04:54 PM) OK.  I'll stop lurking on this thread and post some pictures of the model -- without the bulkhead. I'm glad to see you over here John. Thanks for doing this for me! In a previous life I was going to be a mathematician and I could have had this in hand with little more than a snap of the fingers. Instead I embarked on developing the weak side of my personality and now too much math gives me a headache. QUOTE If you separate it into 2 - 4 inch OD bars, one hollow bar of (16 inches - 2 mm (.08 inches) = 15.92 inches, and the other a solid bar of 2 mm in length, you get the hollow bar has a rate of 48k in/lbs/degree, while the solid bar has a rate of 63,040k in/lbs/degree. Combined using the formula 1(1/rate1 + 1/rate2) = total rate, we come up with a total spring rate for tube with a bulkhead of essentially... (wait for it!)... 48k in/lbs/degree! Don't you want that to be two bars of 8 inch length held together at the middle by a solid bar of 2mm length? That would drastically alter the calculation if I'm not mistaken.

OK -- I was never very good at arithmetic...

3 springs in series, 2 of them have a rate of 96,037 in/lb/degree, and one of them has a rate of 63,037,000 in/lb/degree. So the formula would be...

1/(1/96,037 + 1/96,037 + 1/63,037,000) = 47,982 in/lb/degree. So the essentially solid bulkhead contributes very little to the overall stiffness because it basically replaces .5% of the springs total length with a solid piece. At best I would expect it to reduce the overall spring rate by about .5%.

Or did I get my math wrong?

This weekend I'll try running a bulkhead diagonally through the arm (end to end) to see what it does. That may make the whole arm act like it is (more) solid.

Actually, I was thinking about this while I was making dinner, and I decided the reason was something else.
The formula for a torsion bar doesn't properly apply in this application. The wall thickness is so small compared to the od that non-linear deformation occurs from the amount of load applied. The sides of the box deform laterally. That''s where the "pucker factor" comes in. The bulkhead stops the puckering and actually makes the torsion bar formula more appropriate to predict the spring rate.

OK. For all of the engineers out there-- what sort of forces cause "pucker"?

* Axial Load?
* Shear Y Load?
* Shear Z Load?
* Torsion Load?
* Moment Y Load?
* Moment Z Load?

Or any of the above as a stress?

I can model all of those with my software.

Could it be because the arm is square as opposed to round?
That causes the corners to stretch more than the sides since they are farther from the twist axis.
On body panels puckers occur when a part of the panel is stretched, like when it gets dented.
The difference is the trailing arm isn't stretched beyond the elastic limit of the steel, unless it gets hit by another car.

John continues to impress...

I havent built the model put it through a FEA yet.. I have many things on my plate right now.. which are distracting me., I am working day to day, the hammer may drop tomorrow morning when I get to the office. And then its look for a new job

Im modernizing my 1918 home.. replacing windows etc..the furnace got flooded a few weeks ago....

The cross section of the trailing arm has everything to do with its ability to resist torsion and bending forces. Square sections and circular sections behave differently in regards to torsion and bending.

Square is better in bending and round is better in torsion.
For torsion, having all the material equidistant from the axis is better.
For bending, a square section puts more material in tension/compression at a greater distance from the centerline.