914 control arm stiffness test
 

I have possibly come up with a way to improve the stiffness of the 914 control arm without the weight gain of boxing it. Does anyone have any idea as to how to test an arm to measure the improvement (or lack thereof) in a meaningful way i.e. increased stiffness only matters if its in the direction really counts (what direction is that in this case?). My goal is to test all three permutations (stock, boxed and ???) to see which works best. I promise I will share this info with one and all whether it works or not! SmileWavy

Seems that I remember someone determining that most of the 'flex' in the trailing arm is not from the arm itself. Bushings and other items can be changed to remove some of the flex.

Also, Andy's red, white, and blue race car that was on the cover of Pano earlier this year has a unique trailing arm configuration. Their mechanic made custom trailing arms along with a tubed chasis. They were able to maintain a constant camber angle through out the range of trailing arm movement. Anyway, their system works. While I was turning about 155 Seconds at CMP, they were in the 135 range. (I am sure some of that is driver, ie. I need more track time)

Good thoughts. My goal is to use the stock arms. Undoubtedly a custom fabricated arm could be done, but at what cost/effort? I agree there are any number of other sources for flex and minimizing those is a given. One could even ask the question as to how effective it is it stiffen the control arms by boxing them in the first place- does it really help? Why do people rush out to do it for competition oriented cars- is it habit and hearsay or does it really work? Someone out there must know. :confused:

Jay,

Theres an old saying that applies quite well here - "you can never do just one thing."
I have looked into doing the boxing bit myself on my current Ravenna 914's control arms (having bent one years ago when I had a 76 2.0L), but have decided against this for my everyday driver/autox car. The reason for this is that in the event of a undesired collision with a curb/wall, the stiffened control arm will quite effectively transfer all the impact to the frame which will be more expensive to repair than replacing a bent control arm.

In fact, I have just had this frame repair done, as one of the POs did hit a curb, and although the control arm may have been replaced, they failed to notice or repair the frame at the inner mounting point of the control arm. Fortunately for me I was able to take the car to Don Jackson's in Phoenix and he was able to get it straightened out for me. In addition they welded custom-made support plates on both frame inner mounting points to re-inforce them.

So although I'm not opposed to stiffening the control arms (which I plan to do for the racer), I strongly suggest you consider the need to stiffen the frame at the inner mounting points as well.

My $0.02

I guess thats my question- why will you stiffen the arm on the race car? To make it stiffer of course. Okay then my question to you is, "How much does it bend and in which direction?" Is boxing the best way to do it? Does anyone 'know' the answer to this question? Where's the data (even if its a before and after SOP comparison)? Why do we buy control arm kits if nobody can prove they work? You say you are not opposed to stiffening it- great. Now please tell me why you are in favor of it. What does it do and how can you tell if its doing it? (all of this is in the hope of coming up with a better design OR proving the old way is best)

cheap test
 

here ya go

get a stock arm and bolt it to a solid bench ( i mean solid )

make a bar that goes though the bearings tightly

from that bar weld another bar to it that is the same differnce from center as the wheel and tire size you run

then start hanging weight from it

when you start getting the arm to flex record the weight

if it flexes 1 degree with 500 lbs figure that at 1 g turn force you will lose about 1 degree of camber, i know this assumes no weight shift and all 4 tires gripping the same but it is a good start to test the stiffness because it is also very repoducable

next take the same arm and box it, them install the same set of bars and same amount of weight and check the flex, now you know if it helps

btw if you set it up right with the bar through the bearing level it gives you an easy way to read the amount of flex with a cheap digatal level ( 30 bucks )

It would be tough for me to say that after I boxed my control arms I was faster by "X" number of seconds on a given track or could somehow feel the increase in control through the wheel or the seat of my pants. But it is a piece of the overall stiffing and tighting up of the car when I started making modifications. The added weight of boxing the arms is not a big deal and to go to the trouble of testing different methods of stiffing is more work than the benefit. You could weld steel angles and gussets all over the arms, but who's to say it would be be better than boxing them. People are quick to box the arms because it is simple and it works. I think you are looking too closely at a single piece of the puzzle. Maybe if you have a GT2 car with a 2.8 running 17" slicks you would need something more than boxing the arms, but for the majority of autocross/track 914s boxing the arms gives you the added stiffness you need by a simple/cheap means.

Remember when you start upgrading parts you have to look at the "weakest link". If you weld a full cage in the car with NASCAR side bars and tie the suspension points together you better stiffen the control arm mounting points as well as the control arms and change to solid bushings. Everything needs to be upgraded relative to one another to see the greatest benefit. Stiffing one piece will give you some increase performance but do it together with other pieces gives you much more.

Go ahead and box the arms and than work on the rest of the car.

I agree URY914 (I usually run GT-3), but my hope is to do something more effective that weighs less, but might be a little more work- and costs the same or less. Scott, any way you could send a drawing? Sounds right, I'm just want to make sure I've got the visual correct. I think the real payoff to making things stiffer is increased predictability and tunability (sp?). Its a small part of a very large puzzle without a doubt, but so many people do this modification why not do the test. Thanks for the input!!! Anyone else out there with a suggestion? Ahem, a good suggestion- not where I can place the control arm . :eek:

Jay,
As I have so frequently said, here and elsewhere, the 914 trailing arms are about as flexible as a brick. The real soft spot is the inner mount for the arm. If someone slams a curb real hard, the inner mount lets go, perhaps the trailing arm will bend too but, I doubt it. I have disassembled many 914 cars down to parade rest (more aptly, perhaps, parade rust) and I have never personally seen a bent trailing arm. I have seen many inner mount failures, one forced by me at an autox. I have a half dozen laying around waiting to be bent. Maybe I will try to bent one just for fun.
BTW, it follows that the box kits for the arms are money and time for nothing. Reinforcing the inner mount suspension console is where it's at.
Phil

Point well taken. Please let me know the results if and how you do try to bend an arm. How does one reinforce the inner suspension mount? BTW, I think I will still have a go at one of the arms with my new, untried technique. It may be awhile, but it should be fun. A lot of people do this modification and/or sell the parts to do it. I'd love to hear more thoughts on the subject.

jay,

In response to your question - "How does one reinforce the inner suspension mount?"

Contact Don Jackson Entersprise in Phoenix AZ (602) 997-2785.
They make a custom set of plates that weld onto the dog ear of the inner mounting point as well as the portion above the ear. I believe they can sell them as a kit.

This is the reinforcement I mentioned in previous reply. Sorry I missed answering your question. I was trying to make the point clarified by "philinjax", that boxing for all intents and purposes is overkill.

Before you go welding a jig for the control arm to test it, if you're in California, I was told in a previous post of mine, that there are a couple of shops (no names) that have such jigs. Checking with one of those shops may get you more accurate results sooner, and they may have done some of their own tests. All you have to do is find one of them.

Good luck.

Unfortunately, I'm on the East Coast. Does any one know of these shops or what they came up with??

If someone slams a curb real hard, the inner mount lets go, perhaps the trailing arm will bend too but, I doubt it. I have disassembled many 914 cars down to parade rest (more aptly, perhaps, parade rust) and I have never personally seen a bent trailing arm.

I wish I had pics of mine. I did just that, slammed into a curb. Took out the wheel, the hub, the bearing, the caliper, the rotor, the CV joint, sheared the mounting ears off the tranny. Twisted the aft end of the trailing arm a good 30 degrees or more. The inner mounting ear was fine.

This was a left-side trailing arm.

--DD

Dave.
It's no wonder that your trailing arm bent. A locomotive hit might do it too but I expect racing stresses will have little to zero affect on the stock arms.
Phil

Don't the 911 turbos have larger arms than the NA's? The standard 911 arm is much larger than the 914s - Why would they put such big arms on the 911? I don't know, but unsprung weight is the enemy and I would think that the lightest arm that holds up would be what you (and porsche) would want.

A real to life structural engineer (works for Genderal Dynamics) did a "stress test" or failure analysis. There is a write up about his findings on the 914 control arm. True, he didnt test it in the chassis, but what he found was very interesting. I personally think the stiff kit is a waste of time. Yes the factory used it in the early 70's, but have you seen some of the factory cars ? Did they have roller bearing suspensions ?? or even hard plastic ?? I think they where trying to fix a problem with a band-aid.

A used 914 control arm runs about 50$. I would rather bend one of these and replace it than have a stiffened arm transfer its load to the inside ear (I gusset mine and I suggest you do too).

I have fully fabbed 5 inch chrome moly control arms in one of our tube frame race cars that rides on roller bearings. The car doesnt feel any different than my other tube frame race car with stock 914 arms and roller bearings. Both cars have had Goodyear R430 23.5/12/16 slicks on them at one time or another. Both cars came from the same jig.

Let me see if I can find this guy's testing.

B

BTW, Phil--my point was that the mounting points were fine, while the control arm bent.

However, I agree with you that it is unlikely that the control arm is flexing much. The serious SCCA racers that I have seen appear to run non-boxed trailing arms, and discard them after a while. They tend to be very serious about weight, particularly unsprung weight. (Which part of the trailing arm is.) They also tend to be pretty serious about chassis and suspension flex--or rather, a lack thereof.

--DD