I had really hoped that the thread would not get hostile, but I suppose that the wonders of the internet have a hard time preventing that. I can see the reasoning behind the questions, I would want to know also. And in defense, there are not a whole lot of hard numbers. Many of the questions I will not answer due to proprietary reasons, but I will answer some of the specifics.
My credentials are BS Applied Physics, Optics, with a smattering of graduate studies in Optical Engineering, as well as graduate level Business. Katz is a BSME. We also consult with professional engineering firms for supplemental analysis that we do not have the capability to do, these firms (one main firm) have a tremendous amount of depth. My last position in the corporate world was Director of Core Module Engineering for a medical laser company, where I had a team of highly skilled engineers from multiple disciplines that designed an incredibly complex opto-mechanical/electro-optical laser based delivery system for use in vision correction surgery. In relative terms, the engineering complexity of designing a performance suspension system is childs play compared to the projects I have successfully led in the past (this is no way intended to belittle what we are doing, nor is it to too my own horn). I left that post to start this business. And in response to the software "pumping out numbers," it is true that it is a really usefull tool(s), it is clear that a engineering team can not be competitive without the use of today's programs. I hired two guys that use Solidworks, and all of the other stuff, because I don't, I hate computers, and I do all of my own calcualtions using the same HP calculator I used in undergrad school, using the classical equations from Olley, Milliken, and all of the rest of the literature (which is extensive, and good). This creates an excellent check and balance system for design details which I am proud of, anyone can buy WinGeo and whip up a design, but far fewer can tell what the numbers mean. We can.
The system is designed to accomodate a variety of ride heights. Katz aggressive design nature pulled us to have a very low ride height, my more conservative approach drug us the other direction, I deal with speed bumps ona daily basis. The most challenging geometry issues are with the car at a very low ride height. In this form, and again, it is sensitive to ride height, approximate numbers are as follows: SVSA > 70", A/S > 70%. Roll steer is dependent upon two factors in our design. One, the roll center height is defined by the Watt's bellcrank location. Two, because we incorporated skew into the lower control arm (plan view), the convergence point defines an additional lateral restraint point. The line running through the RRCH, and the forward restraint point, in Side View, defines the roll axis. There's a great picture of this in Milliken's book, page 653. For very low ride heights, the angle of the LCA's in side view become more parallel, so the roll axis becomes very neutral. In no case, in the extremes of high A/S, and the lowest RRCH from the Watt's, is the roll understeer percentage more than a few percent. Not being evasive here, it is just that the darned numbers are HIGHLY sensitive to the ride height and Watt's location, etc. CG used for all examples is ~20" (with a 47/53-ish rear weight bias), which is conservative, but the reason is that what is really important is the CG of the SPRUNG mass, the CG of the combined sprung and unsprung mass will be a few inches lower (a reasonable estimate for this approach is to find the height of the cam centerline). Using the "other" approach will result in a lower CG, and all of the A/S numbers look even better, which is basically what we have seen competitors do, so a marketing comparison will show them as being "better," or at least higher. That's okay.
Here's a note on this stuff. With a coupled trailing arm suspension, there is a constant battle between three things (independent of packaging). SVSA, A/S, and Roll Steer. For a given setup, you get to pick two. If you want very high A/S and neutral roll steer, the ONLY way to accomodate this is to have a very short SVSA, and experience has shown, this creates a nightmare in heavy braking (violent brake hop). Might be okay at the auto-cross where the speeds aren't that high, but heading into turn #1 at Willow Springs and not having brake control is not a happy moment. Okay, so how about high anti-squat, but with a longer SVSA? In order to do this, you need to raise the Instant Center, which means the LCA's need to incline steeply upwards, so what gives? Roll steer, and in the wrong way. In this general condition, you will get a big time roll oversteer condition. Again, this is well known too, to quote a friend's experience in his AI race car, "corner entry was pretty good, but midway through the turn, I had to correct my steering big time in order not to run "over" the apex, it was kind of strange feeling. Corner exit was a blast, I felt like a hero being able to get into the throttle a lot sooner, but I later realized that this was partially because I was going through the middle of the turn a lot slower." Yes, Virginia, there is a Santa Clause, and yes, roll steer is a real issue.
This is the root of our design. The balance of Roll Steer, Anti-Squat, and a suitably long SVSA (Adam's says 40", Milliken 60", experience says at least 60") is what we worked so hard to achieve. Trying to make it fit was a challenge, we did our best, and we are very happy with the results. For the tech hungry folks, sorry for all of the vague marketing talk earlier on this thread, there is a real balance for describing a product in terms that everyone can relate to. For those of you who know me, Katz, etc, I would think you would attest that we are far better on the specific tech than the fluffy stuff.
Some other quick answers: The Watt's location on the axle is beefy for a reason, there is a very well known company that suffers from failure of the Panhard bracket on their product in a race environment, and if nothing changes in the design, there will be another company that suffers from this with a newly released product (it won't be us). The Watt's brackets are placed below the axle for a couple reasons: our philosophy is to have low RRCH, placing the pickup points lower on the axle puts the bellcrank more in line with the pickups, and therefore reduces the bending moment on the links themselves (more of a compression/tensile load in this fashion). The passenger side is the lower one, there is more support structure there, so we put it there to help improve left/right weight bias (it's a terribly small amount, but....). There is nothing terribly magic about the link length/bellcrank offset for the Watt's, specific dimensions are proprietary, and as an MSME, I am sure you know how these things work, they aren't exclusive to cars by a long shot.
Forward crossmember, 14.5 lbs. Rear, 14.5 lbs. Watt's bracket, 16 lbs (if this sounds heavy, it is because of need, deflection under lateral loads on the bellcrank pivot is the biggest driver for the design, it is bulletproof). 1/2 the weight of the coil-overs, control arms, and Watt's linkage arms is of the order of ~12 lbs(?), for a total weight of about 60 pounds, give or take. All sprung. With a leaf car, half the weight of the leafs, shocks is considered sprung, so about 45 lbs or so. So, we increased the sprung mass by a few pounds (while, at the same time, adding a significant amount of additional structure, helping to improve the torsional rigidity of the flexible car somewhat). Now consider the fact that the unsprung mass of ours includes 1/2 the weight of the coil-overs/trailing arms, and the differential. As stated above, this weight (less the rear axle assembly, we'll get to that in a minute), is about 12 pounds, unsprung. Leaf? 45 pounds or so, unsprung, so we have eliminated about 30 lbs of unprung mass relative to what is coming off of the car.
I have to start a new thread with the remainder, it limitedme to 10k characters. That's okay, I can use this for a FAQ section on the website!
M
|
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
Hybrid Mode
