Aaron Pfadt

Hey all! Just thought I would share this. I was doing a once over on our C6Z after two intensive days of track testing a bunch of existing and new suspension products at Reno Fernley last week (watch for results and video soon)and found some bushing carnage.

We were testing the Z in a stock configuration and adding suspension components to determine some lap time deltas. With that we retained the stock control arm bushings for all the testing because of the difficulty of changing bushings at the track.

Both of the front upper control arms had severe slippage of the bushings. The interesting thing is that is must be from rotor torque. The upper control arms are both pulled forward on the chassis. I would have expected them to be pushed back (toward the rear of the car) from holding the car back under braking. Instead the torque on the upright assembly under braking rotates the upper control arm forward.

Here is a little video of the spindle under braking also. Look at the rotor glow! I'm in the market for some new rotors







Looks like a perfect time to put in a set of our Pfadt C6Z bushings. Also note our Pfadt camber kit installed in the pictures. We put some tape on the shims to keep them from falling out while we were swapping components.

I'll post a full write-up on our testing very soon. It has lots of interesting data.

later,

wallyman424

gotta get some ducting on that rotor STAT.

and yeah, thats pretty much standard with stock bushings. it sucks.

Z06Fix

Very cool.

sleeper02Z06

Shoot, those bushings have at least a SEASON left in them on a grassroot car like mine!! As long as I can still align it........

Joe

WNeal

Pretty cool vid

jhester

Let me preface this post with 2 points;
1. I am a strict believer in the Pareto Principle (80/20) rule and realize I am not tackling the entire problem or system with this post, just most of it.
2. I mean my post with all due respect to a forum vendor

That said, Aaron, it seems like there are some fundamental misunderstandings of the physics involved in the system from your post.
The effect shown in the upper control arm bushings (this photo shows the driver's side of the car as indicated by the brake ducting) is a counter-clockwise rotational moment (torque) placed on the caliper (and therefore the spindle) due to reactionary force of the frictional force of the pads to the rotor (newton's 3rd law; for every action there is an equal and opposite reaction). The rotor is rotating counter-clockwise for forward motion, and the pad is exerting a frictional moment to slow the rotation of the rotor therefore the pad's frictional force is in the clockwise rotation. The equal and opposite force of this frictional force is then transmitted through the caliper, which acts as a torque arm (magnifying the CCW moment) through the spindle, which is flexing the bushing. Physically speaking, that is what is happening. You can deduce that simply by envisioning the forces at work, regardless of video evidence or not.

Another point to make is the fact that the results seen are almost entirely due to rotational moments, not lateral forces. The lateral forces in the system are 100% due to tire frictional forces during a turning event. The results of the bushing shown in the thread are due to braking rotational moments, not the lateral forces though in my opinion, as the vast majority (if not all) of the lateral force is transmitted directly into the chassis via the bushing mounts (the bolts are almost directly in parallel with the lateral force vectors).

All of this said, I believe there are some larger issues to be learned from the video evidence provided.

Several instances during the video the car is on the bump stops of the shock. This is a concern as the bump stop alters the spring constant of the entire system. This is no small matter. A spring system is just a simple harmonic oscillator, with the governing equation F = -kx. Where F is force, x is displacement, and k is spring constant. So you can see that the relationship between system (in this case, suspension) force and spring constant is multiplicative, so any modification of the spring constant due to bump stops has a vast effect in disturbing the tuned response of the overall suspension setup. A shock is tuned to the spring constant of the spring, not the spring + bump stop, so the fact that the car is on the stops will only throw the balance of the entire suspension system off; not a small matter. Now the system is complicated by adding the damper (shock) but regardless, if you can boil the device down to the component (spring) level you can see the issue with hitting the bump stop as frequently as you seem to be doing so.

Another point of note is the extreme lateral movement of the swaybar end link in the video. In the ideal case, the entire force of the sway bar is in the vertical plane as the bar rotationally flexes along the long axis to offset the motion in the suspension itself. Any lateral force in the bar is introducing added tension (tensile stress) into the bar which is in no way adding to the intended effect of the bar to the suspension setup.
For any specific bar geometry and material a specific tensile strength, which is consumed by both lateral and rotational (intended) stresses, as governed by sigma = F/A; sigma equals tensile stress, force is the force applied to the rod, and A is the cross-sectional area. This is why the larger the sway bar, the stiffer it feels, and the more tensile support it gives given identical force applied. Point being, by adding in a lateral load you are compromising the available rotational load-bearing capability of the bar, sacrificing intended performance.

I don't know what your engineering background is (I am an electrical, go figure) but it seems that generally speaking you were misunderstanding some of the forces at play in the system with your statement above.

Olitho

I just read all that and I could swear you said the same thing as Aaron. Rotor torque is displacing the bushings under heavy braking.

What did I miss?

jhester

In the end we did say the same thing. The issue is the job of an engineer is to predict performance, not react to data (Post hoc ergo propter hoc). His post did nothing of the sort. He even admitted that he was assuming the bushings would be pushed backward as he envisioned the arm "holding the car back under braking." This statement is completely incorrect.

My point is, from a very basic physical perspective this is fundamentally wrong and in-fact backwards.

For someone who is putting my life, this forum, and the rest of the lives of their customers in the design of their suspension parts, it is concerning if their understanding of the system physics are inaccurate.

Olitho

Well, I guess that is what research and study is for. It appears to me he did his research with the video to better understand the dynamics. I wish I could always be so clairvoyant and correct in all of my postulations. Those postulates need to be understood to be proven. Sometimes you learn something from trying to prove them. Sometimes your original conclusion is confirmed.

It seems to me you need to send your analysis to GM suspension engineers as they designed the system in question, not Pfadt. In case you think I am a shill for Pfadt, I will state flatly that I don't run Pfadt parts and I never will. Those parts are not allowed in the class that I race, but I have run with Aaron before. He is a great driver and I have seen the quality of his product and I think it is good. He continues to research, learn and refine those products in subequent revisions. I think that is what automotive engineering is all about. It would be nice if we could instantly go from the race cars of the Indy 500 fifty years ago to today by understanding everything perfectly, but that is not how it happens. In another fifty years our cars of today will look even more primitive compared to today's standards I am sure.

I don't feel any risk or deficiency in Pfadt products in comparison to others on the market from this post. In fact, he goes on to state a cure may be a better set of bushings. I think you guys are in vigorous agreement on that.

If you want to see crappy bushings getting puked out of their a-arms you should look at the bushings on my car. I would love to run different bushings but I cannot. I don't know that my car is any less safe because of it. It is just a more frequent routine maintenance component on the car.

Now... where is the closest track. I feel the urge to go drive...!

jhester

I agree with you on almost all points.

The issue is that research and development should occur prior to a product making it to market, not after it has been released to the general public.

By including the term "engineering" in his business title, AP is taking on a certain amount of responsibility about the products he produces, and as an accredited engineer, it is not a responsibility we take lightly.

In fact many states even limit who may use "engineer" in their title, see Ohio for an example [ohiopeps.org].

Point being, if AP was performing the same testing and found that he had incorrectly calculated safety factor on say the tensile strength of the sway bar, and the sway bar was fracturing, we would all say it was poor product design.

I am saying that R&D has a time and a place, and the public market is not it; plain and simple.

Now I need to go drive as well.

Olitho

Now I am really confused because now you are putting forth a hypothetical as a criticism for something that has not even happened. I am not aware of any Pfadt sway bars that have cracked due to tensile strength problems. To suggest so could be considered libel.

I don't see anywhere in his post that suggests he is expecting the public to be the guinea pigs for his products. He was testing someting on his own car and he pointed out the problem with the stock C6Z06 bushings and suggested that he needed to put in his bushings.

With that said, in real life no one and no firm wants to test out in the public market place, but real world failures do occur and responsible companies fix those as soon as they happen. But in no case is that happening here. I am not sure why you are going after Pfadt for scenarios that have never happened.

By looking at some of your other posts, you seem to have an affinity for LG. That is fine. I like LG, too. In fact, I had the fortune two years ago of paying a lot of money to be part of a race clinic he had at WSIR. I had the honor of having Lou seated in my passenger seat for 30 minutes around that track in addition to the four hours of class time. He is great and I would not trade it for anything. With that said, your criticisms of Pfadt seem out-of-place and unfounded.

Anyway, I just finished a kick-*** amount of work tonight on west coast time and it is time for me to turn-in now. I never did find a close-by track to go drive....

Fastest regards,

Oli

VetteDrmr

Where did you get that from the video? Watching the end link in comparison to the strut was the best I could do.

Since you don't have any idea of the specific orientation of the camera, visual left-right indications are virtually impossible to translate into what the bar is actually doing.

I don't mind critiquing a product, but I either qualify the critique or back it up with rationale.

In any case, have a good one,
Mike

mgarfias

The video shows clear signs of the arm flexing side to side under load. This is compromising the anti-roll bar system and adding in a variable that isn't accounted for and whose effect most likely can't be calculated.

See Carroll Smith's _Engineer to Win_ pg 211 for a discussion of a blade adjustable anti-roll bar and how its a bad idea. This is like that, but with a system not designed to be a spring. I'd discuss why its a bad idea, but Carroll describes it far better than I can.

Also the video does show clearly that the car is hitting the bump stops while in a corner (not across rough terrain). That indicates that the springs aren't stiff enough to support the car in roll.

VetteDrmr

This isn't a flame, I'm just not understanding. Are you saying that the video shouldn't see the arm moving at all in relation to the steering knuckle?

Aaron Pfadt

I'm not really sure where to start when some information I post gets turned into a viscous personal attack. I'll start by saying that I do not know jhester, at least to my knowledge.

The first point I would like to make, is that nothing in my post is incorrect.
This:
Is a long winded overly technical way of stating exactly what I said. The forces from braking torque on the upper control arm (forward forces) are in fact larger than the rearward forces (the component of the tire force pushing rearward on both control arms).

Secondly, the video was not shown as 'evidence' of anything. It is video that I took during testing and thought would be interesting for forum members to see. Certainly it invites a lot of armchair engineering and anyone with a bone to pick will certainly do as much. I have nothing to hide about the way our products function. I'll address some of the video comments in another post.

I will not continue to discuss anything with jhester because his comments to and about me are absurd and inflammatory.

To address the insuations about my qualifications, I have a B. S. in Mechanical Engineering from the Rochester Institute of Technology. Look it up. From there I spent 10 years in Detroit as an engineer for both GM and after that DaimlerChrysler. It's not designing cul-de-sacs in Ohio, but it pays the bills.

One last thought. As a vendor my goal and my focus is on developing testing and producing my products. I know that if I do these things well, my company will be strong and grow. The moment I lose that focus and start spending my time attacking other vendors and members of the Corvette community, not only have I let my own company down but I also let the Corvette community down. I simply will not go down that path.

-Aaron

Aaron Pfadt

Mike,

You are correct in that the orientation of the camera and the different arcs of the components makes it difficult to determine what might be deflection in the arms versus rotational travel. There is also the issue of control arm bushing deflection that moves the control arm in relation to the sway bar (which is mounted to the chassis with solid bushings) that is clearly in play.

The answer still is that there is some deflection. The sway bar is a system that is not rigid, if it were, the bars would lock out roll completely. Not desireable in this case.

The rate effects of side deflection of the arms is certainly difficult to calculate, but the deflection is present in both stock sway bars (the same loads are present and the factory bars are not completely rigid in that plane either) and our parts. Testing is where the rubber meets the road and our bars work with some of this deflection. It is part of the system and it is why we test.

If you have any doubts about the capability of these bars, I invite you to look at the record of accomplishments of them. They were on both the National winning (Danny Popp) and 3rd place (Pfadt) NASA ST-1 cars. They were also on a variety of other top NASA national TT cars. A couple of Grand Am cars also use the Pfadt sway bars.

Hope this clarifies things a little.

-Aaron

petefias

The springs are augmented by the bar in the turns, so I'm really surprised that the car is hitting the bump stops. Imagine with just a stock bar. I think an adjustment on that bar would eliminate hitting the bump stops. Cranking up the shocks would also settle that suspension setup.

I think it is great that Aaron posted these results so we can all learn from it.

P.S. Great username!

sperkins

Pretty amazing the direction this thread has taken. jhester, you could have presented your initial thoughts/comments in a much better way. After pointing out the flaw in his slaw, you could have just stopped there. There was no reason to try to educate Aaron in suspension dynamics by using fancy terms like 'rotational moments, lateral force vectors, harmonic oscillator, F = -kx, sigma = F/A' and so forth. It's almost like you took his original comments personal. Do you guys have a history or something that would make you bust out of the gate trying to make him out to be a crook or an idiot by posting a seven paragraph rebuttal? I'm not disagreeing with any of the points you made, but you sure came across as a bully and a know it all. Try to remember that this is a discussion forum about corvettes and not engineering 101. Some things are just best left unsaid.

95jersey

Regarding the suspension hitting the bump stops, while I am sure this is not good, wouldn't basically most stock Corvette suspensions do the same thing under the same conditions (or even worse)? I have personally never run stiffer springs than stock Z06 leafs and my car definately has bottomed out on many occasions. Is it optimal, definately not, but I never seen any detrimental effects from the situation other than less than optimal corner speed. I mean to 90% of the Corvette community who may be running stock or close to stock springs on R compounds, wouldn't that cause the soft stock suspension to bottom out as well? I guess my point is unless you had very stiff race springs specifically for the track, wouldn't this situation almost be "common" and not necessarily inherint to PDFAT or any other coil over designed for multiple use (street AND light track) duty? I have even heard that T1 is too soft for REAL track duty. I have heard guys running 1000lbs springs and above.

John Shiels

you can have some real bad affects by hitting the bumpstops like losing control of the car. My leafs are 1200+ and 900+.