Off-road Long Travel Suspension 1

[GEspo]

Hello eng-tips experts. For starters: I have a complete, fairly high fidelity, RBD simulation model of an off-road race vehicle, in Maplesim. Currently the model sits with IFS and a rear 4-link, triple bypass and dual rate coilovers on ea corner, real Modelon fluid flows. After a month or so working with the completed model, it's apparent the usual(pavement) race car dynamics analysis will only go so far due to the low coefficient of friction, ie dirt and tire, looks to be about .6, so lateral g's are restricted, and lots of sliding happens, just watch any race etc... SO to my question: Turning my focus to BUMP characteristics and analysis, are there any good resources that layout the study of long travel suspensions? Seems like studying a slinky would be more helpful than opening a dynamics book(I've opened a few btw)..

(As a guide, I'll generalize and define "long travel suspension" as a VERY Soft system with anywhere from 20" to 30" of travel..)

 

[SwinnyGG]

What information are you looking for exactly that you don't already have?

The kinematics aren't any different- you just have very different goals.

 

[GEspo]

I suppose the txt I've read have very limited info on the kinematics of BUMP, and much more on dynamics, slip etc, probably because they're dynamics books. Looking for recommendations on good kinematics books or resources, and of course any general studies of long travel suspensions if available.. I should say kinematics is new to me.. thx

 

[GregLocock]

I have not found anything authoritative that is freely available, but as usual 70% of the unknowns are the tire. The Baja FSAE boys are your best bet.

Cheers

Greg Locock


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[GEspo]

I'm ok starting from scratch, tend to learn better that way. A basic question if you wouldn't mind: with the model described above the dof of the system would be 10 w/o the wheels: 3 coord and 3 rotation for sprung, 1 for ea front suspension DW "arms", and 2 for the rear axle(slight side to side).. am I on the right track here? thx

 

[GregLocock]

Yes, in the absence of steering and compliances, that fully defines your vehicle's geometrical state.

Cheers

Greg Locock


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[GEspo]

Another question:

I did find a resource and going through it there's a section on "torque steer effect", described by the author here:

Torque steer effects, i.e. changes in longitudinal forces during cornering, are an
important criterion for the definition of transient handling characteristics. The
torque steer effects depend on the size of the change in the longitudinal force,
the adherence potential between the tyres and the road, the tyres and the kinematic
and elastokinematic chassis design.

To summarize the rest: when braking or reducing accel in a turn, the front will load and rear unload given an unchanged slip angle, thus allowing the vehicle to turn into the corner, creates a yawing moment.. but too much brake or decel can cause undesirable over/understeer

This can be designed for a bit since the suspension is loading and unloading, ie toe and camber change etc...

Looking online "torque steer" looks to be defined and discussed differently:
Torque steer is the unintended influence of engine torque on the steering, especially in front-wheel-drive vehicles

Which is correct or what is the industry standard terminology here?

Thanks!

 

[BrianPetersen]

"Torque steer" is the unintended influence of powertrain torque on the steering, and yes, it is of particular interest on front-drive vehicles, although all-wheel-drive vehicles are subject to it, too.

The other things you are describing are more often called "understeer" (front losing traction first) or "oversteer" (rear losing traction first) and in engineering terms can be described as "slip angles", i.e. the angle between the direction of travel and the direction that the wheel in question is nominally pointing towards ... front and rear (in fact, all four wheels) have different slip angles.

 

[GregLocock]

Yes, the second is torque steer, the other terminology is something he made up. The tests we do for that are called Brake in Turn, and Throttle on in turn, but with ESC as standard the electronic side is far more important than the basic vehicle response.

Cheers

Greg Locock


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[GEspo]

Thanks for the comments. It seems with such large travel and soft suspension("long-travel") the yawing moment due to braking or accel in turn may be important to look at from a design perspective, ie unintended changes in bump/droop.. the book mentions heavily camber and track change, both things I've been able to optimize(minimize) together, so from a kinematics standpoint I'm checking the right boxes so far..

 

[GregLocock]

The importance of camber change is very tire dependent. Which book are you using?

Cheers

Greg Locock


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[GEspo]

The Automotive Chassis 2nd Ed by Reimpell has some good discussion on Kinematics

Vehicle Dynamics by Jazar for general dynamics topics

From my limited study in vehicle Kinematics and dynamics, roll camber, as well as camber change is what I’ve targeted at the top of the list for the DW, along with track change (besides the general knuckle parameters like scrub, sai etc)..

 

[GregLocock]

Reimpell's good, but rather focused on production cars. Jazar is a new one on me.

Cheers

Greg Locock


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[GEspo]

So I have a few basic questions as I start digging through all the literature I can find on "roll center". I've begun analyzing what looks to be a 9bar linkage(front suspension DW), 2 dof, that will have lateral force acting on one link, thus moving it around.. done to study a cornering vehicle. Intuition(and simulation) tells me the vehicle(sprung mass) has a pitch moment while in turn, so here go the questions:

1) Forces at tires-
Tires are force generators, and in turn the front tires by friction generate a centripetal force(say Fy) pointing inward toward the center of the turn.. intuition tells me the back tires are still generating a "forward" force(say Fx), so like tripping, the rear will pitch... (I haven't ignored inertia here, just trying to analyze the forces)

2) I see very little in the vehicle dynamics books regarding the mechanical advantage of the 4bar/DW... in my question 1 above there is a longitudinal lever produced by the chassis and front DW, ie when in pitch a longer wheel base will create a larger mechanical advantage than a shorter thus the moment on the front springs will be greater with longer wheel bases(when braking or turn).. thoughts?

 

[BrianPetersen]

You are heading towards force-based instant-center analysis (or, perhaps not quite as accurately, force-based roll-center analysis). The procedure that you are trying to do ... That's what it's called.

Your thinking about "mechanical advantage" is bass-ackwards. There is a certain vehicle deceleration that you are analysing. That requires a net force acting on the vehicle's center of gravity in order to achieve it. Since the force is not applied at the center of gravity but rather at ground level, there is a moment (torque) arising from the height of the center of gravity and the mass and the acceleration that has to be resisted. It is resisted by the normal-force at the tire contact patches increasing or decreasing (relative to static weight distribution). A longer wheelbase (when considering pitch) or wider track (when considering lat-acc) results in what is in common terminology "less weight transfer". As does, a lower center of gravity.

 

[BrianPetersen]

I should add that a by-hand pencil-and-paper analysis of static conditions (steady-state acceleration, braking, or lateral acceleration) is valuable for purposes of getting your head around the concepts (perhaps so that later on, you can better understand what a computer analysis is telling you), but real world situations involve transients, and bumps, and 3-D geometry, and non-linear damper valving, and non-linear tire characteristics. The complexity shoots off the scale REAL quick.

The most common independent rear suspension layout in production cars nowadays - flexible trailing arm, upper and lower lateral links ahead of the wheel centerline, toe link aft of the wheel centerline on which the spring is frequently acting - has toe and camber and compliance reactions that defy pencil-and-paper analysis because of their 3-D geometry. And good luck analysing a twist-beam rear axle by hand.

 

[GEspo]

Thanks for the response.. I like starting from the basics and asking lots of questions.. but.. if you(or anyone) has a resource for force-based instant-center analysis I would much appreciate it, no need to reinvent the wheel everytime. I've been digging around on force-based roll centers etc and found one book Tires, Suspension and Handling by Dixon but dont have it yet..

I agree pencil and paper every time for starters, then code up some math if needed(and possible), and lastly simulation(which is a mere guess if you dont know the math behind it as you mention)..

Compliance reactions... ugh.. baby steps for me..

 

[GregLocock]

FBRCH is the change of the vertical force when a horizontal force is applied at each cp, multiplied by the track/2

ie

dFz/dFy*track/2

The practical setup is with the contact patches are on slip plates on top of a load cell, and then pushed sideways, with the sprung body fixed in space. (I imagine, our K&C is a hexapod and doesn't work like that at all).

For a relatively simple setup like panhard rod live rear axle the hand calc would not be terrifying.

what's a DW 9 bar link when it is at home?




Cheers

Greg Locock


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[GEspo]

Vertical force on ea tire correct, which is just vertical force acting ON the com distributed to ea tire? or is it vertical force FROM the com? That force would be sloped downward(not totally vertical) toward the front(when viewed from side), and to the outside tire(when viewed from above), in turn.. or vertical meaning the vertical component Fz? I'd like a more precise picture of whats going on.. Force vector feels(seems) like its referred to as scalar in many context..

Help w/ force based roll basics(excluding drag):

Normal force on ea tire from/distributed by the sprung com, along w/ ea unsprung element com and normal force

Long/lat forces from ea tire at contact patch from friction and work done by motor

What forces are involved in tire "load"?

 

[GEspo]

Online paper states:

The steering link accounts for the differences between FBRC and KRC. Proper accounting for the forces on the steering link explains the differences.
The difference between FBRC and KRC would be more significant but the following analysis will show that minimizing bump steer reduces the difference. The practical desire to eliminate bump steer minimizes the theoretical difference between FBRC and KRC.

Any validity to this?

Side note: I developed a nice procedure to eliminate bump steer even with anti-dive

Another online paper is using “inertia force” in their force based analysis.. valid, or no?

Why are there so many interpretations on roll center and roll axis? I’m not accustomed to the ambiguity in such a scientific area.. usually you have a claim then prove it, case closed... SO many professionals and experts not agreeing..