Re: Matt, get back in the box!
MaXJohnson said:
That's a pretty radical concept.
It changes so much that it'll take some serious pondering (=lot's of beer) to grasp the impact.
A couple of things that come to mind:
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Suspension helps absorb shock loads when torque is transmitted to the tire contact patch. Your rigid mount would put 100% of the shock load at the tire. This would increase the possibility of wheel spin and require "twinkle toes" on the throttle pedal.
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In essence, your design eliminates suspension on two-wheel events and fixes a permanent roll axis. It also eliminates anti-dive, anti-squat and anti-everythingelse. This isn't necessarily a good thing. I have yet to see a valid discussion of the impact of anti-xxxx in relation to climbing. Still waiting for Mr. Peabody, er I mean Ed to lay this out.
Rock crawling suspension design is in it's infancy. Your idea could be as valid as any.
Why kill brain cells to explain what you
already describe (the need for twinkle toe driver control when on the throttle with high anti-squat on hills)? Damn challenges ...
This light touch need is due to the weight's traction resistance load being directly placed on the links with no suspension movement (100% anti-squat), or the even more unstable situation with suspension jacking (over 100% anti-squat) with a growing anti-squat value as the throttle is applied in increasing vigor (forcing the AS% even higher with less throttle). The suspension design should complement the drivers ability to control the vehicle, not accelerate the vehicle into more difficult driver control territory (where the suspension is isolated to the tire flex).
A suspension allows the chassis to store and release energy (for good, or bad). The suspension is a combination of tire, and spring/linkage, and chassis flex. Traditional competition has evolved to eliminate chassis flex, and tire flex (pavement racing low profile doughnuts), so we are usually left to play with the spring/linkage arrangement.
Matt proposes we exploit the tire, and chassis flex, and simplify spring/linkage solutions. It's an approach worth exploring (with a few :cheers: ... later).
Control fails, and energy fails to be stored, when the tires traction adhesion cannot deliver the power to the driving surface: when the tire spins. Energy is stored when the chassis jacks or squats. This is sometimes desirable in a high weight car with a small tire contact patch. It is also sometimes desirable to design in chassis-lean when cornering with a heavy vehicle and a small tire contact patch. The suspension stores the impact shock load, a shock load that would normally force the tire contact adhesion into a less desirable dynamic friction coefficient, a transition that would be difficult for the driver to control (an impact force that would break the static coefficient of friction for the tire material).
We have all experienced the ease of spinning a tire, once the initial rolling friction adhesion limit was exceeded (WhooHoo
, neutral drops & side stepping the clutch, for a smokey throttled burnout once the tire starts spinning). How about the weekend practice on the go-kart slick track? What resulted in a better cornering speed, radical lock to lock steering (Ted Nugent) style, or the smooth transition (Dan Gurney) steering style between corners. The same change in available traction, from fairly good static rolling friction to poor dynamic sliding friction, happens even with less violent of a power threshold transfer (compared to abusive throttle on a poor traction surface). What is easier for driver control, the tire at or below the static traction limit, or crossing the transition between static (rolling) and dynamic (sliding)coefficients for tire contact patch friction?
What if you could dampen, or better yet store, the excess energy that forced the shock load past the tire static friction limit? What if you could keep the tire contact patch at the superior static (rolling) friction coefficient? Cornering's dampening goal is more control to allow the driver to drive the transitional chassis motion, and the acceleration goal is to allow the driver control to more easily throttle & brake, at the tire's superior static traction limit.
This is also the goal of suspension improvements in super stock drag racing, and similar limited tire classes (cornering and straight line competition): store the impact energy until it can be released later (when the tire can afford the extra load, or when the shock load can be applied over a longer time period).
When competing a vehicle with a superior traction to weight balance we can ignore storing energy with the suspension: go-karts & F1, Pro-Stock and rail dragsters. The tires traction (and driver skill) is so superior in these arenas that the driver does not need a suspension aid to smooth out the rough spots that can break the tire adhesion. This is the design perimeters documented in most "racing" chassis design manuals: transition loads and driver skill that do not tax the tires adhesion limit. These vehicles are designed to exploit the high traction available (due to the generous tire friction) and can basically run with minimial suspension. The fact that driver skill has placed competition at the ragged edge of the superior tire performance is remarkable.
Rockcrawling and dirt surface racing does not have the advantage of a controlled track surface, and the suspension design must exploit storing shock loads, and the slow release of the energy. Take away the chassis's ability to smooth out the shock loads, and you make the driver control more difficult. This is what happens when the anti-squat percent reaches 100%, or exceeds 100%.
We get to the question ... why does the AS% change when climbing?
Take an FBD for a vehicle linkage system and isolate the IC and CG for level ground. Use a 90 inch wheelbase with 36-inch tires and a 36-inch high CG on the wheelbase centerline (50/50 weight distribution). Draw a linkage system with 75% anti-squat using a level LCA.
Rotate the FBD up 45 degrees (the ground), and recalculate the AS% (and exercise the skills).
The Einsteins will twist their head 45-degrees and tell you the AS% is still 75%. The wheelbase is the same, relative to the ground, and the IC and CG are all the same distance apart with the same linkage angles, relative to the (now angled) ground. The IC is still 27 inches from the front tire contact patch on the ground, and the CG is still 36 inches off the ground measured at the front axle centerline.
What they fail to see is the GC does not care about the simplified convention of measuring the IC and AS% in relation to the ground (or at the front axle centerline perpendicular to the vehicle wheelbase). The convention of using this method for calculating the AS% is a simplification constrained to the level wheelbase condition, where the gravity force (and reaction) is always perpendicular to the wheelbase, and the ground: where we measure the AS% from the ground along the vertical line along the front axle centerline.
Draw the level condition (before rotating and recalculating). The ICL is 27 inches over and CGL of 36 inches = 75% Anti-Squat (check the math).
The non-level ground condition places the force of gravity at an angle to the ground, or the axle centerline height of front and rear axles (the wheelbase) at different heights. The IC is still located the same way, the extension of linkage intersections. The true wheelbase is the same, but the virtual wheelbase perpendicular to the gravity force has changed. The relationship of the linkage angles, relative to the gravity force (not the ground surface) have been altered. The external FBD only cares about the gravity direction (down) at the CG, and the resistance (up) at the tire contact patch.
Have the drawing rotated 45 degrees. Draw a vertical line through the front tire & axle centerline parallel to the direction of the gravity force (down, and now 45 degrees to the wheelbase). Maintain the IC location points and CG (they did not change). Draw a new level line (perpendicular to the gravity force) at the rear tire contact patch (it moves, slightly). Calculate the revised AS% from the new line that is level (at the rear tire contact patch) along the (now much taller) front axle centerline? I measure (rough scale) ICR at 100.5 inches over the CGR at 65 inches = 155% Anti-Squat.
Somewhere during the rotation of the vehicle (in relation to the direction of gravity) that was forced by the terrain change (the 45 degree climb), the Anti-Squat percentage changed from storing 25% of the torque load on the springs (75% AS), to fully carrying the torque load on the suspension links (100% AS, @~22.5 degrees) with no storage in the springs, to storing the torque load in extended springs (150% AS).
Next is where you need to ignore the ground and get a feel for the suspension movement under throttle with gravity pulling 45 degreees to the ground.
This has progressed deep into :cheers: time. It is near worthless without drawings (not possible to scan at work, and I need to run some drawings through the OCE machine).
