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”.

 

[LionelHutz]

I don't at all follow how the forces applied to the base of the wheel are reduced by absorbing the resulting energy in the shock. My gut says that is complete nonsense.

They use the stiff chassis so they can setup a properly tuned spring and shock package which ensures the wheel is controlled and maintains ground contact instead of bouncing off the ground where it's useless at providing forward propulsion.

 

[BUGGAR]

Yeah, the guy said something about "it's all in the knee" and I took that as my queue to leave before I got kneecapped or something.

 

[Panther140]

Any vector of force that is not collinear with the wheel's travel is partially absorbed through the chassis. Picture a wheel rolling toward a horizontal railroad tie laying on the ground.

A certain amount of the impact is going to be absorbed by the chassis regardless of how stiff the chassis is. Stiffening the chassis increases the impulse of the impact.

The forces I'm talking about are analogous to drag on an airplane's wing, except they are transient, which means they can be dampened. Not dampening them causes erratic behavior of the vehicle (deflecting/bouncing off of them)


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

 

[Panther140]

Not seeing the value of proper chassis stiffness is what caused the engineers at Honda to create the 1997 CR125/250 aluminum frame. It rides like a bronco no matter how soft you make the suspension.

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

 

[SwinnyGG]

I think you are confused about the many different ways in which 'stiffness' can apply to an automotive chassis.

When *most* people talk about 'chassis stiffness' they are talking about stiffness in roll- the rate at which the front and rear suspension groups are coupled.

If you really want to dig into how bump forces are dealt with along the axis of travel of a vehicle, you're talking about 'stiffness' in a very different sense- the rate at which any suspension member will deflect relative to the center of mass or centroid of the chassis due to an axial force (along the centerline of the vehicle). Also keep in mind that in order for the chassis pick up points to be loaded in this way, the suspension links involved all need to be loaded as well- and suspension links are not going to bear nearly the stiffness of the chassis they are attached to.

The vast majority of bumps that any vehicle- even an off road vehicle- encounters in its life are going to have applied force vectors that are much more vertical than horizontal- because bumps are usually small and wheels are usually big. In order for the vector to have a large horizontal component, the vehicle needs to be traveling VERY fast, or the bump needs to be tall enough that the point of contact is not far removed from the axis of travel of the wheel. Even out in the desert, there's not going to be a lot of 24" tall, non compliant bumps (i.e. rocks) that a smart driver is going to hit at high speed.

Point is, no matter how you try and rationalize it, the horizontal component of bumps forces is VERY small relative to the kinetic energy of the vehicle, and impulse applied to the chassis is controlled by the compliance of the suspension bits between the upright and the chassis, because those bits are going to be much, much less stiff than the chassis itself.

 

[NoahLKatz]

I think the offroaders' explanation misses the point.

Any energy imparted to the vehicle by the bump slows it down, because it's unlikely to be returned to it as forward moving kinetic energy.

So in that respect it doesn't matter how it's absorbed, though if it's by the shocks the vehicle is likely to be more controllable.

I'd agree with the initial premise on that basis, i.e. if the chassis is flexing all over and giving oscillating wheel loads and steering angles.

 

[140Airpower]

Buggar, If I interpret them correctly, the diagrams you presented show the static force from a spring in one instance and then a comparative force from a "shock absorber" and then it talks about damping. Is that right?

 

[NoahLKatz]

"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...The extremely tight undampened spring that is your frame will rebound violently and often before you are even clear of the bump."

The energy absorbed by a spring is inversely proportional to the square of its deflection, so that in a well designed chassis it will be insignificant.


"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."

Total compliance is the sum of all of the compliances in series, so the tire and suspension bushing compliances would make the chassis stiffness irrelevant.


"Point is, no matter how you try and rationalize it, the horizontal component of bumps forces is VERY small relative to the kinetic energy of the vehicle, and impulse applied to the chassis is controlled by the compliance of the suspension bits between the upright and the chassis, because those bits are going to be much, much less stiff than the chassis itself."

I think the second part of the above belies the first.

If horizontal forces were so small, engineers wouldn't invest so much effort in suspension bushings and accept the loss of steering precision which they cause.

Besides the tires they're the only significant source of horizontal compliance.

 

[140Airpower]

"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."

To what Noah said above I just want to amplify: the stiffer the chassis, the less it deflects, the less energy it stores and the less it rebounds. A very stiff chassis is effectively taken out of the interactions of the compliant members.

 

[BUGGAR]

I think my chart posted on July 27 was mislabed: Resultant Force should be Reaction Forces, which are those "felt" by the chassis.

 

[gruntguru]

"If horizontal forces were so small, engineers wouldn't invest so much effort in suspension bushings and accept the loss of steering precision which they cause.

Besides the tires they're the only significant source of horizontal compliance. "

I don't think bushings are there to increase horizontal compliance, rather for NVH.

je suis charlie

 

[GregLocock]

They are used as a rather unhappy tool to change the understeer of the car. Most times stiffer is better for other aspects of steering and handling, but sometimes you just have to use compliance to get linear range understeer. One place they are used directly for steering is on rear twist beams, where they give lateral force steer-in, to help counteract the tendency for oversteer because the wheels are at the back. People who have compliantly mounted subframes often tune the subframe mounts to increase linear range understeer.



Cheers

Greg Locock


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[NormPeterson]

Question for Greg - about where would you define the upper end of "linear range" for a reasonably sporty car that wouldn't be a total bust for about 90% use in normal street driving?


Norm

 

[NoahLKatz]

> I don't think bushings are there to increase horizontal compliance, rather for NVH.

Increased compliance will improve NVH, especial impact H.

 

[NormPeterson]

Don't ignore compliance steer effects . . .


Norm

 

[GregLocock]

If you plot the front and rear slip angles vs latacc the curves are pretty straight up until 0.6g for production cars, or even small trucks. However if you calculate the understeer over successive 0.1g increments it stops looking linear. I suppose that is another way of saying the derivative is small but measurable. Obviously in a normal car with terminal neutral throttle understeer at some point the us curve has to start getting seriously curvy.

Mostly you are looking at your tire f&m data, when do these start curving.


Cheers

Greg Locock


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[SwinnyGG]

I think the second part of the above belies the first.

If horizontal forces were so small, engineers wouldn't invest so much effort in suspension bushings and accept the loss of steering precision which they cause.

Besides the tires they're the only significant source of horizontal compliance.

I think there is confusion here because I used the word horizontal.

Horizontal forces during cornering (i.e. forces that actually turn the car, normal to the axis of travel) can be very large.

Horizontal (in the sense that they are parallel to the ground) forces due to bumps, parallel to the axis of travel of the vehicle, are usually very small relative to the KE of the vehicle which is what I stated above.

 

[NoahLKatz]

Regardless of the magnitude of the longitudinal horizontal forces, they are a significant contributor to impact harshness in suspensions w/o compliance in that direction.

It's been decades since I've ridden in one, but I distinctly remember that in VW's with trailing link front suspension, in which the front wheels move back as well as up, it felt like the wheels rolled over the bumps instead of crashing into them.

 

[NormPeterson]

Thanks, Greg.

I think I know what you're describing feels like from the driver's seat, but I don't have nearly enough datalogging capability to measure everything that's involved. At low-ish lat-g, maybe 0.25 g, the car hardly feels any different than it does when going straight ahead, totally stuck down with no sensation of sliding or slip angles that I can discern. At double that, the car starts feeling a little livelier. Much past that and actual sliding starts becoming noticeable, which I assume is getting at least into transitional range.


Norm

 

[GregLocock]

Understeer is very hard to sense directly, on a road. On a skidpad it is more apparent, trundle round the circle at a constant speed, increase speed a bit, did you have to steer more (understeer) no change (neutral) steer out a bit (oversteer). Note that this test is more complex than it appears as the tire properties change with speed. For completeness, another test is called swept steer at constant speed, so that gets rid of the speed dependency, but basically you just wind lock on slowly and measure latacc. This is more complex than it seems as the Ackermann is also coming into play.




Cheers

Greg Locock


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