A Flexible Chassis is Slow 3

[BUGGAR]

This is relative to my investigation of the Ariel Atom chassis:


A Flexible Chassis is Slow.

In my quest to understand the importance of chassis stiffness of the Ariel Atom, I was talking with some off-road guys. We have a pretty big off-road community here in the desert and they’re a pretty open group. I was talking to them about chassis rigidity and they were talking about how much it slowed them down. What were they talking about? So here is a transcript of what I remember them saying, as best as I can remember and with some editing.

When you hit a bump, it’s like the bump is hitting your wheel with one of those little arrows that engineers use to show a force. It always points towards the center of the wheel because of the same laws that also keep people parallel to each other no matter where they are standing on the globe (sic).

Those force arrows pointing at the wheel can be broken down into two directions, one vertical and one horizontal. This is because engineers think in X and Y directions. In addition, those are the only directions that things can move when hitting a bump (called degrees of freedom by engineers – kind of like all the places where you can’t go if you’re under 21).

As noted, there will be an up arrow and a back arrow. The back arrow is related to and proportional to the up arrow. This proportionality changes as the wheel rolls over the bump but the back arrow is always going to be some proportion of the up arrow and it’s always going to be there until that bump is finished with the wheel. You can forget about the bump pushing back after cresting the bump because your car is flying off that bump without looking back for any help.

This back arrow forces back on the car’s suspension, and the car’s reaction is to slow down slightly. To reduce this slow-down arrow, we must reduce the up arrow, or more accurately, the vehicle’s reaction to the up arrow. This reaction to the up arrow comes from inelastic and elastic force/energy absorption: inelastic from the shock absorbers, elastic from the tires, springs and chassis flexibility.

For a given energy input from a given bump, the reaction force from the up arrow and its evil back arrow is less for an inelastic reaction than for an elastic reaction. Thus, the more energy absorbed inelastically by shock absorbers, the less the evil back arrow restraining force. If the chassis is flexible and does not permit the shock absorbers to absorb as much energy as they can, the resisting forces will be increased. That’s one reason to run multiple or progressive rate springs; they leave more work to be done by the damper than by the springs, and the slow-down arrows are smaller.


I think he’s right. I think. At least it was a great conversation!

Added by me: There are, however, minimum amounts of elastic energy that are required to restore the chassis and wheels to their neutral position. Also, experience shows that less than critical damping is “best”.

 

[gruntguru]

Sounds counter -intuitive to me. Chassis flex is essentially an (un-damped) spring in series with the existing spring/damper.

I see two sources of energy loss in the simplified model - bump-damping and spring energy not returned (due to flight as you mentioned and/or rebound damping).

je suis charlie

 

[BrianPetersen]

I think it's an oversimplification. When the wheel hits a bump, something has to give, and the wheel has to be set down on the other side of the bump in a controlled fashion. Your "evil back arrow" is a function of total compliance, whatever the source. A stiff chassis with compliant springs and damping ought to be a whole lot easier to tune.

 

[BUGGAR]

I was thinking more of the off-road guys with their little arrows slowing my car down, so I did some graphics to help me understand what’s happening. I took a packet of bump energy of 600 in-lb, and distributed it over a bump distance of 3 inches. One graph shows elastic (spring) energy and the other inelastic (shock absorber) energy, and their resultant reaction force for each. Note the lower reaction force for the inelastic.

They teach this stuff in school but there’s nothing like hanging out in a desert bar to really learn how it affects the real world.

Of course, a vehicle is a complex combination of these two charts but I found this interesting as a clarification.

Uno mas cerveza, por favor!

 

[NormPeterson]

Assuming that the springs or dampers are not changed, why wouldn't a more flexible chassis reduce the magnitude of the upward force (and hence the rearward force as well)?


Norm

 

[BUGGAR]

The thought is that the chassis is participating as another spring in series with the suspension springs. We're looking at making the shocks do more of the work in absorbing the bumps, and the springs, including the chassis, do less of the work. The theory is that we want the arrows to come from the shocks and not the springs. Of course, if you have too much shock capacity in your vehicle (damping above critical), then a flexible chassis is not bad.

Besides a stiff chassis, off-road guys here use progressive (multiple) springs to help keep them "on the shocks". Maybe this theory is only good for off road vehicles. Maybe like the Ariel Atom, you can throw some rules out the window?

Interesting theory.

 

[BrianPetersen]

"Maybe this theory is only good for off-road vehicles"

The suspension tuning that will be required for operating on a relatively smooth paved pavement race track is very different from that required for driving on extremely bumpy surfaces off road. Even circle-track suspension tuning (always turning left and always in the same range of road speed) will be different from road course (random lefts and rights at random speeds and with random simultaneous acceleration and deceleration).

My thing is motorcycle roadracing, and we are not concerned about how much hitting a bump slows the bike down, but we are very concerned with ensuring that the chassis maintains stability through and after the bump and maintains the grip as consistent as possible throughout. There is something of a trade-off between "grip" and "feel". A very stiff chassis with firm damping and high spring rates will give good "feel" - the rider will be able to feel for traction limits more easily - but it will not have good "grip" on bumpy surfaces, because it will not have the compliance needed to allow the wheels to follow the pavement surface. In my world, progressive spring rates are a bad thing - because they are soft when the rider doesn't need it (straight up and down) and hard when the rider would prefer to have compliance (leaned way over). A good many front forks have long, soft top out springs which have the effect of raising the spring rate when riding straight (which minimizes the geometry effect of changing ride height) but allowing it to be softer when leaned over.

 

[Panther140]

Do any off road cars use rake in the same way that a dirtbike's front suspension does?

By increasing stiffness of the chasis, are you reducing the amount of longitudinal forces that act against the momentum of the overall system? Or are you minimizing chasis distortion/lozanging from the impact?

You're making the chasis more resistant to deformation during the impact, the chasis still has to resist that same amount of force in that vector, it just now has the strength to resist it without bending/distorting. The only difference that I can see is that instead of distorting the frame, you would be putting that longitudinal force up directly against the momentum of the vehicle with no dampening.

Do they have data to back up their "its slower" claim? Or do they just go by what feels fast?



"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[BUGGAR]

Mostly I’m trying to verify what some local off roaders are saying. When I design something vehicular, I like to talk to them because they break stuff and they seem to have an innate engineering knowledge. When they tell me something, I go back and try to prove or disprove them (several Dos Equis are always on the line). The engineering theory I came up with seems to bear them out*. But there’s a Dos Equis for challengers, too.

*exception: Ariel Atom which is fast and flexible and I can’t figure out why(?)

 

[SwinnyGG]

All a soft chassis does is make spring and shock sizing/valving less accurate. The chassis is undamped- so all you're doing by decreasing chassis stiffness is removing your ability to control the system.

Stiff chassis or soft chassis- when the wheel hits a bump it has to move up and out of the way. If the chassis is stiff and you use soft springs and dampers, or if the chassis is soft and you use stiff springs and dampers, the end result is the same.

'Chassis stiffness is bad because bumps slow down a stiff car' is a nonsensical statement. Not to say that these off road dudes can't build a fast rig- but do they even know how stiff their chassis are? Doubtful.

 

[Panther140]

jgKRI, your theory requires you to be able to design suspension that can directly dampen impacts of every possible vector and never bottom out.

A stiff chasis is still a spring that takes the same amount of force as a more flexible spring. It is still a non-dampened spring no matter how stiff you make it. However, rebound tends to be more violent with stiff springs. The concept that I am trying to communicate to you can only be truly understood by people who have ever ridden a dirt bike over a downed tree in a woods race.

Your suspension only moves in one vector. Everything else is resisted by your chasis.


"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[Panther140]

Stiff frames give you a very abrupt impulse against your momentum when those horizontal/longitudinal vectors of force impact the wheels. The extremely tight undampened spring that is your frame will rebound violently and often before you are even clear of the bump.


"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[NormPeterson]

I have a feeling that anybody who hits a bump badly enough in an Atom to slow it down by any measurable amount has already made a driving error of far greater time significance. Either that or he's made a rather poor choice of venues to drive it at.

I'm also not seeing why a more flexible chassis wouldn't reduce the amount of suspension compression over any given bump, which would tend to reduce both the elastic energy temporarily stored in the spring and the energy inelastically dissipated through the dampers. I am assuming that the suspension does not bottom out on the bump stop (a whole different spring & damper that changes everything).


Norm

 

[Panther140]

Norm, the force would be put through the shock when the frame rebounds, wouldn't it?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[NormPeterson]

If the frame is rebounding, wouldn't the axle also be rebounding away from the frame (because the wheel would then be on the back side of the bump)? To dissipate energy from the frame from its rebounding, you'd need the frame to be moving toward the axle, otherwise it's the energy stored in the spring that's getting dissipated.


Norm

 

[Panther140]

If the load has decreased enough for the dampened coil spring to be rebounding, I'd imagine that the frame has rebounded to a point where there is minimal load on the rame.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[NormPeterson]

That's going to come back to suspension vs torsional chassis frequencies, and might not be true for a car with ~1000 ft*lb/deg chassis stiffness and 2.2 Hz or higher suspension frequencies.


Norm

 

[SwinnyGG]

jgKRI, your theory requires you to be able to design suspension that can directly dampen impacts of every possible vector and never bottom out.

A stiff chasis is still a spring that takes the same amount of force as a more flexible spring. It is still a non-dampened spring no matter how stiff you make it. However, rebound tends to be more violent with stiff springs. The concept that I am trying to communicate to you can only be truly understood by people who have ever ridden a dirt bike over a downed tree in a woods race.

Your suspension only moves in one vector. Everything else is resisted by your chasis.

No offense, but this is still nonsensical.

You're right in that (most) suspension systems only control and damp movement in one direction- the vertical, and that the force imparted on a tire and wheel is not always applied through that pure vector- but if you want to have control over contact between the tire and the ground (which, ultimately, is the goal of any suspension system- to accurately control tire/surface contact under all possible conditions) you have to minimize uncontrolled displacements in the system- this includes the unsprung end of the spring/damper arrangement.

When the goal is to arrive at a well-controlled, properly damped system, adding uncontrolled displacement is never going to be the right move.

 

[BUGGAR]

From my recent conversation at the bar in the desert:

“So if I have a rigid chassis with rigid suspension and I’m driving along and someone has
half-buried a spring and a shock absorber on either side of the road ahead. If I drive over the shock, it will give me less of a poke in the ass than the spring will.”

I think he’s right but I would call it a reaction.

 

[SwinnyGG]

From my recent conversation at the bar in the desert:

“So if I have a rigid chassis with rigid suspension and I’m driving along and someone has
half-buried a spring and a shock absorber on either side of the road ahead. If I drive over the shock, it will give me less of a poke in the ass than the spring will.”

I think he’s right but I would call it a reaction.

I guess I'm not even sure what point that statement (By the mysterious soft chassis advocate?) is trying to make.

The amount of force applied by a shock to a sprung assembly is entirely dependent on the valving, and has little or nothing to do with the spring rate. Just as the amount of deflection created by a given force in a spring is entirely dependent on spring rate.

A blanket statement of "springs apply more (or less) force than shocks do" is entirely nonsensical.