KEWL!

(jealous)

 

Sure looks alot like my Caltracs ::
Atleast from the back.

 

Considering what a beautiful job you did, I hope a bunch of VMF'ers will pile on me when I tell you I think you will have geometry problems with this setup. This comes from my vast experience working with ladder bars and the four link on the race car.

I hope I'm dead wrong but my instincts give me a stomach ache on this one....

 

Beautiful job. Sent you a PM.

 

Geometry problems?!?!? I thought that's what I was fixing with the old setup?!?! So what bad things will happen with this setup?

 

Show me your engineering drawings and perhaps I can comment in more detail. There may be some parallax error in your photographs (This means my eyesight ain't what it used to be *G*)....

I'm particularly interested in your travel arc studies, which examine the arcs the spring/housing travel in vs the arc the traction bar travels in. This is the area where I hope, for your sake, the other racers here pile on me.

I do concur with your use of the aircraft quality heim ends; they are the only kind I've used in my suspension systems. I have eight of them on the rear and 6 on the front of the race car. We both know how expensive that is, yes? *G*

 

I don't see anything different with his set up from the shelby under ride bars?

 

Great job! Nice looking parts...Where did you get the radius arms?

I've tried to figure out why worn underrides would cause bump steer, what is your explanation?

 

Pat, I'm not a racer but I believe I see what you said as soon as I looked at the pictures without reading your post. The bar is too long. It should pivot right below the spring bolt. As it is now, as the suspension moves up and down, the verticle travel isn't going to be on the same plane as before. It's going to alter the travel and put things into a bind and also make the rear axle do more steering. Am I correct Pat???

 

So, now I suppose your next project is a 5/8" rear Anti-roll bar? ::

 

Bingo. The big drawback to underridders. The solid arm even with the pivot in exactly the same point does not exactly follow the arch of the spring. No sure about the impact on the drag strip, but in tight, fast accelerating road course turns this set will bind and noticely shutter the rear end. I run a spring arm setup (Afco). I ocassionally will still incounter some bind.

Bob.

 

Am I correct Pat???

I don't know about correct Tom but we are certainly in aggreement, based on my experience. I'm predicting suspension bind if what I'm seeing in the pictures is in fact reality. Matrix notwithstanding...*G*

 

I don't see anything different with his set up from the shelby under ride bars?

I don't have access to any good pictures or drawings of the Shelby setup other than the inconclusive picture in my old Mustang Does It book. Perhaps you could direct me. Certainly, both the override and underride Shelby traction devices were engineered to work with the stock suspension in a manner conducive to smooth suspension travel. This freedom of movement is key to achieving maximum torque and weight transfer in the chassis.

There is a whole science of suspension tuning which I researched when implementing and tuning the 4 link on the race car and it's way too involved for a forum post. There are likely good books on the subject though. I've found tuning the rear suspension to be critical to both consistency and lower ET's, at least with my car.

 

The solid arm even with the pivot in exactly the same point does not exactly follow the arch of the spring.

This has been my experience as well. I came up with some really creative ways to change the instant center on the race car suspension (through manipulation of the travel arcs of the 4 link) when utilizing leaf springs (this was before the advent of affordable coil-over shocks). The modifications have withstood the test of time, scrutiny and performance parameters for over 2 decades now with nary a complaint. I believe a VMF'er got the extra leafs from my springs a couple of years ago, as I only use the main one and one other, with each side being different. IIRC, I got some Corona. Very fair trade, IMO *G*

Chassis science is a complete other discipline, IME, totally unrelated to almost everything we routinely discuss on the VMF. The road racers know what I'm talking about, I'll bet *G*

 

What I see is that he replaced the rubber/poly bushings, that can bind, with heim joints, which move freely. If there are any geometry problems, they would've been problems already inherent to the original under-ride design.

http://www.americanpony.com/store/graphics/00000001/5/0291.jpg

 

I cant tell from the pics if the attachment points are exactly the same as the aftermarket Shelby bars. Are they? It almost looks as if the from attchment point is further forward than the Shelbys, but it might be the camera angle.

 

IME, polyurethane is much more flexural than steel, and I work with both materials every day in my shop. In fact, polyurethane can be a PITA to machine precisely, because of this trait.

It remains to be seen whether Dave had geometry problems all along and this is just the lastest chapter in the story. Such systems, IME, will function, but not optimally, as related by one of the road racers in this thread. Most of my fellow racers, especially during my street days, knew little to nothing of the nuances of a freely travelling suspension and its impact on weight transfer and traction (in drag racing). Many had bound up suspensions, including myself at the beginning, and suffered traction loss and a harsh ride as a result. I figured there was a better way (that's why I do what I do for a living) and spent lots of hours scribbling and experimenting to come up with the chassis parameters for the race car.

I know this may sound unreasonable to some, but just because a company sells a product doesn't mean it works right. Trust me, I've fixed plenty of them, including a few of my own, over the years...*G*

 

What did those cost you? Were you aware of TractionMaster underrides for less than 100 shells?
I've got a set of the tractionmasters and they eliminated the hopping problems, not added to them. Thier website explains...
However, I'm probably wrong somehow, as I've recently discovered that I'm a lowlife autoxer toad (on the pyramid of speed) that rarely gets above 35 MPH...

 

Tell me if this makes sense.

First, the leaf spring deflects in a straight vertical line, since it has two pivot points. It doesn't have an arc.

The traction bar would tend to pivot in an arc, since it only has one mounting point.

The ideal situation would be to get both pieces moving in the same direction. The longer the traction bar, the less curvature the arc will have.

So it seems to me that the traction bar should be as long as physically possible.

 

What'll really bake your cookies is that our Dave has created two different devices, with different properties, and is expecting them to work in unison.

The spring, when deflected by body loading, not only moves up and down in relation to the body, it moves forward and back. This is due to the design and arch of the spring, if even so slight.

I'll assert that the fixed axle housing plate (which the rear traction bar mount is welded to) indeed travels in an arc, with the front spring mount being the pivot point. Possibly, the difficulty with understanding this is that the rear point is fixed, rather than being free to rotate, as the traction bar rear mounting pivot does.

I personally think this a great discussion to add to a FAQ, since most VMF'ers experiment with traction devices at some point, but further technical input from me will have to wait, as I have a business to run and a mother to care for. I hope we can pick this up at some future date when I have the time to prepare the technical drawings to support my assertions.

In the meantime, I welcome any further input!

 

I respectfully disagree that the traction bar's pivot point is the front spring mount.

It's impossible to get the leaf spring and the traction bar to travel in the exact same path, since one wants to travel in an arc, and the other wants to travel in a vertical line. The goal is to flatten the arc and minimize the bind that the travel causes. The way you flatten an arc is to make the radius as large as possible. Placing the front traction bar mount directly under the front spring mount would shorten the radius, increase the arc, and cause more binding.

 

I'm with Pat on this one. I agree with some of what you said that the spring will flatten out...to a point. But it still will want to pivot in an arch off the front spring bolt. Now, what Dave has formed between the 4 points consisting of the front spring bolt, U bolts on the axle, the front heim joint which is foward of the spring bolt and the rear heim joint just below the axle, is a trapazoid. A 4 sided box wich 4 differnet length sides and 4 different angles. At some point, things are going to bind. Now, if the front heim joint was just below the front spring bolt and the center distances where equal between heim joint and spring points, you'd form a parallelagram. With 2 sets of equal angles and 2 sets of equal sides. In this case, it would pivot freely through out it's travel.

 

The leaf spring is NOT going to move in an arch, unless there's no traction bar. That's the function of the traction bar; make sure the leaf spring moves only up and down.

Here's my best quick Powerpoint Engineering to explain this. Simply, the bigger the radius, the straighter the travel, and best candidate to match a vertical axle travel.

http://www.knology.net/~dorner/vmf/springs.jpg

 

I more of a hot shoe than a setup mechanic, and certainly no engineer. With the traction bar, you want the
bar to control the movement (identical each time). However, if you plot both movements, won't you arrive
at the point the spring starts to "fight" for control of the movement earlier, effectively limiting upward
movement. As the suspension is limited earlier (bind) and abruptly, shutters.

The length of the arm has to be much larger to accomplish what your describing. That's why the placement
of a single "torque arm" ( like TCP is now selling) is much farther forward. Because of the narrowness of our tranny tunnels,
my understanding is the arm isn't longer enought to avoid some amount of bind.

I haven't used my Carroll Smith and Herb Adams setup books (absolute must reading for beginning racer's) in
so long their stored in the attic. The underridders are really a suboptimal solution. Shebly switched to the setup for easy
of installation, cost and interior noise. Overridders were used on factory prepped cars and inculed in the Boss 302
chassis prep book.

If this was a easy science, race car setup would be boring (and all setups would look the same).

 

The shorter the arm, the earlier it binds. Like I said, this setup will never result in identical paths, so you try to minimize it.