Adjustable droop

[hardingfv32-1]

The rear rear bar support is probably attached to frame members coming back from the shoulder height side frame rails. This is required to locate/support the spring and shocks. These frame members would not have to have any responsibility for supporting the trans/engine package.

The control arms are attached to the trans rear mounting holes. Structurally this guy is being very optimistic. You look input any force beyond the normal operating level and this is going to break. The transaxle can take all these force IF you have provide for a way to feed them into the transaxle case. This is the challenge with such a design.

Brian

[brian]

If memory serves me right, we're not allowed to use the engine or tranny as a stressed member. See 9.1.1.C.8.C page 200,

Regarding the air over spring perches, they're used on four corner cars to weight jack the car to adjust handling. By increasing the spring in one corner, the weight is jacked cross corner to the opposite tire. In a dirt application, if the car is loose, you cross jack weight from the left front to the right rear. It will tend to give the right rar a bit more bite. Many years ago, we had craftsmen rachets in the cockpit with extension and swivels to reach down and allow the driver to crank down on the left front torsion arm. On a zero roll car you can't cross jack. Our rear suspensions self equalize. One could raise and lower the car and subsequently change the camber accordingly. Not sure of what the benefit would be but you could adjust the camber. Moving our suspension up and down will impact droop and that may be an issue.

[hardingfv32-1]

Very good point on that section. Of coarse it was intended to prevent stressed skin type construction. I would say the precedent was set with the Citations many years ago.

Brian

[brian]

I'm having trouble grasping your point. Are you saying that the Citations use the engine or tranny as a stressed member? The over the engine braces, while strengthing the frame, don't really use the engines as stressed members since both components are bolted to the frame at some other point. Maybe I'll have to go back and look at one again.

[hardingfv32-1]

Maybe your are right, the Citation is not a good example. I was thinking the engine, using the front motor mount, stiffens the engine bay. But in the case of the Citation, maybe that is not completely necessary. The situation illustrated here, clearly the rear control arms are mounted without any frame members. This would seem to be a violation of our rules as you pointed out. A flimsy frame member connected to this rear mount would probably make it legal.

Brian

[FV80]

Maybe your are right, the Citation is not a good example. I was thinking the engine, using the front motor mount, stiffens the engine bay. But in the case of the Citation, maybe that is not completely necessary. The situation illustrated here, clearly the rear control arms are mounted without any frame members. This would seem to be a violation of our rules as you pointed out. A flimsy frame member connected to this rear mount would probably make it legal.

Brian

The rules say CHASSIS rigidity - they make no reference about SUSPENSION mounting points.

But I do agree with you that the rule was intended for 'stressed skin' type help for the chassis - using the engine/trannie wasn't the target, nor is it even considered. I'd say about 1/2 the chassis out there use one or more braces from the roll bar to the top of the engine/trannie mount - (as does the one in the picture, except it goes even further back to the rear of the trannie). MOST of those have no other chassis support at that point-only the much older models with the "square" at that same point.
Steve

[Matt King]

To me, the concept of using the engine or trans as a "stressed member" implies the presence of an inboard suspension control point attached directly to either unit. Otherwise, I can't think of a vehicle in which the engine and trans are not "stressed" in some way by being attached to the frame of the vehicle.

[hardingfv32-1]

What makes this complex is that the rear axle is actually a suspension member or type of control arm. Almost all the side loads and a very high percentage of the of the vertical loads travel through the rear axles to the transmission case and then to the chassis through the transmission mounts and in some cases a front engine mount.

Brian

[dric53]

Isn't the loading of the transaxle somewhat dependant on which type of spring accuation is used? I'm under the impression the pushrods put on more of a load on than pullrods in that the pushrod tries to pull the axle tube away from the transmission as it compresses the spring and the pullrod pushes the axle tube in. Other than the D13 are there many cars that are still using a front motor mount?

Dennis

[hardingfv32-1]

Assuming zero roll:

A) Almost all of the side loads travel thru the axle tube. A very small amount will go thru the control arms based on the amount of toe that is used and another small amount thru the zero roll mechanism (rockers) based on the amount of camber used.

B) This I'm less sure about, but the vertical loads are probably shared with the axle tube and the zero role mechanism (rockers). What raises questions in my mind is that there are Vees out there that have the COMPLETE zero roll mechanism attached to the axle tubes. In this case ALL vertical load travels through the axle tubes. I'm just not sure how to draw the force lines in the normal zero roll setup.

Possibly the Citation uses a front mount.

Brian

[brian]

My understanding of using the motor or tranny as a stressed member implies that if you remove either component, the car has an incomplete frame and won't be able to stand up. If you look at many formula cars, from Ford on up, the engine, tranny or both are used as frame members. Remove either and the car is on the ground. Back to the rules, on page 200, while the GCR specifically mentions skin and monocoques, it clearly states that rigidity or strength must be derived from frame rails. Many vees have additional stiffeners or braces attached to engines or trannys but if one removes either engine or tranny, the car will still be able to stand up.

Unless a car is subjected to extreme conditions, a wreck or short flight, it really doesn't matter whether the rear suspension is pull or push. Have a hard landing and it's likely an axle cover will be damaged and you'll have a leaker. Some folks believe if you use a pull rod system, this damage is less likely. I'm not really sure of that. The major problem I've seen with pull rod variations is that it''s very difficult to reduce effective spring rate. Usually there's not enough room for proper leverage and travel resulting in the use of very stiff springs. Nothing wrong with stiff springs if you like go karts and never race in limited grip conditions. (rain). MY preference is for a softer spring. In the absence of wings and downforce, mechanical grip is dependent on compliance of the chassis.

[Matt King]

My understanding of using the motor or tranny as a stressed member implies that if you remove either component, the car has an incomplete frame and won't be able to stand up. If you look at many formula cars, from Ford on up, the engine, tranny or both are used as frame members. Remove either and the car is on the ground. Back to the rules, on page 200, while the GCR specifically mentions skin and monocoques, it clearly states that rigidity or strength must be derived from frame rails. Many vees have additional stiffeners or braces attached to engines or trannys but if one removes either engine or tranny, the car will still be able to stand up.

What do you mean by "stand up"? I would say that Vees already use the t-axle as a stressed member. Obviously if you remove the transxle from any Vee, it will be one the ground, since much of the rear suspension is incorporated into the gearbox. But on most of them (the car in the photos above being an exception) at least part of the rear suspension typically attaches to the frame structure.

[hardingfv32-1]

I think it might be more accurate to state when the engine is removed. It would seem the transmission is always required to be in place to control the axles,etc.

IF...a stiff suspension is better in dry conditions (not an accepted theory), would it not be the correct compromise over a soft setup as we race much more in dry conditions?

Brian

[jpetillo]

The Citation, for example, does not use the engine/tranny as a stressed member. They do not replace any frame member. The trapezoid connects the bottom frame tube to the top ones from the roll hoop. The suspension forces are directed mostly into this, and not the engine/transmission.

I don't think Citations have front engine mounts. Perhaps some do.

Regarding what supports the vertical loads along the axle tubes, it depends on where the spring pushrods/pullrods connect to the axle tube. The D-13 puts the shocks down very close to the wheel. In this case, most of the vertical force is carried by the shock and little carried by the tranny at the other end of the axle. I like this design. The Citation has its pushrods a bit further from the tires, and as a result there's a bit more vertical force taken by the tranny as a result. The Caracal D puts the pushrod much closer to the centerline of the car, but it's mounted on the leading arm. There's a lot more vertical force taken by the tranny than the other cars mentioned, but it's shared with the vertical force on the leading arm.

At rest, the horizontal force on any of the pushrods/pullrods/shocks(D-13) that have an angle of about 45 degrees puts as much force horizontally as it does vertically on the axle tubes.

Brian H, I agree with you that almost all of the horizontal cornering force is directed straight through the axle tube - this is true for all the Vees. In the case where the zero roll is mounted completely on the axle tubes, then the vertical load is 100% on the tranny. In the case where it's not, then what I wrote above applies. The horizontal force (at rest) on the tranny is in the direction of the push (outward) or pull rods (inward). This counters or adds to and horizontal cornering force.

Dennis, that's a good point. The vertical force of a pullrod or pushrod only has to do with the location from the centerline from the car of where it connects to the axle tube. The only difference would be that one pulls down and one pushes up, as you mentioned. The amount of force along the axis of the push or pull rod has to do with the location along the axle tube and the angle is has with the axle tube, and not whether it pushes or pulls. The vertical force is least when the pushrod connects near the tire and most when it's closer to the tranny. The pullrods underneath are usually at a much shallower angle, so although the vertical force would be the same at a certain axle tube mount location (each would hold up the same weight), the horizontal force is much greater due to the shallower angle.

Does that seem about right?

John

[dric53]

John,

Good explanation! If you ever want to go a bit deeper into suspension theory and design check the Milliken's book. It is rather weighty and does seem to produce headaches in those who read it, at least it does to me. I should lend you my copy so you could explain it to me.

Dennis

[brian]

I'm afraid I over spimplified my explanation regarding standing up. It's a given that without the rear axles, a vee would be on the ground. My intent was to explain the role of the tranny in regards to acting as the frame. As to soft and hard suspensions my point was to describe preferences. Like the blond vs. brunette issue, it's a matter of personal choice. MY experience has taught me that a softer, more compliant suspension, is a bit easier to drive and more forgiving. I have converted several cars to a softer set up and the owners have been more consistent and eventually quicker.

[jpetillo]

John,

Good explanation! If you ever want to go a bit deeper into suspension theory and design check the Milliken's book. It is rather weighty and does seem to produce headaches in those who read it, at least it does to me. I should lend you my copy so you could explain it to me.

Dennis

Dennis - Sounds like fun. Perhaps a discussion over a Fresca at the track! John

[remmers]

to touch on this one a little bit, i actually thought long and hard about the role that such a damper would use on the car. I came to the conclusion that this actually optimizes the balance of the car. At least my car, if not all vees, will push in slow corners and be loose in fast corners. in this scenario, because the roll resistance is coming from a shock instead of a spring, the roll resistance will be speed based, as opposed to position based. i believe the car will probably have more tendency to oversteer in situations where the transfer of weight is fast (throwing the car into a sharp turn) and less tendency to oversteer when the transfer of weight is slower (rolling into a fast sweeper). the reason for two shocks for roll resistance on this car is to keep the compression and rebound damping from being different one side to the other, this could also likely be resolved by mounting the shock proper longitudinally and utilizing a rotary shock for roll resistance. someone please speak up if my logic is flawed

[brian]

In general terms, the cornering camber is determined by the chassis roll and geometry. As the car rolls, preset camber diminshes. That's why so many cars have large camber presets to offset the change. If I have a car that's tight or pushes in slow tight turns and tends to be loose or oversteer on fast turns, I will suspect excessive preset camber. On the slow turn the car rolls more and reduces camber. On a high speed turn, things are a bit more gentle, and the preset camber is not reduced enough to establish a good tire contact area.