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A Flexible Chassis is Slow

A Flexible Chassis is Slow

(OP)
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”.


RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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.

RE: A Flexible Chassis is Slow

(OP)
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!

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

(OP)
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.


RE: A Flexible Chassis is Slow

"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.

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

(OP)
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(?)

RE: A Flexible Chassis is Slow

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.

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

Quote (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.

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.

RE: A Flexible Chassis is Slow

(OP)
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.

RE: A Flexible Chassis is Slow

Quote (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.

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.



RE: A Flexible Chassis is Slow

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.

RE: A Flexible Chassis is Slow

(OP)
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.

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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.

RE: A Flexible Chassis is Slow

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.

RE: A Flexible Chassis is Slow

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?

RE: A Flexible Chassis is Slow

"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.

RE: A Flexible Chassis is Slow

"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.

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

"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

RE: A Flexible Chassis is Slow

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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

Increased compliance will improve NVH, especial impact H.

RE: A Flexible Chassis is Slow

Don't ignore compliance steer effects . . .


Norm

RE: A Flexible Chassis is Slow

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: A Flexible Chassis is Slow

Quote (NoahLKatz)

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.

RE: A Flexible Chassis is Slow

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.

RE: A Flexible Chassis is Slow

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

RE: A Flexible Chassis is Slow

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


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: A Flexible Chassis is Slow

The difficulty in sensing via seat of the pants is understood, and I make no claims as to the resolution capabilities of my own butt-meter beyond picking up the coarsest differences. That said, I do have some datalogged information around a rather unofficial test loop where the only difference was in wheel widths and tire sizes (same tire make and model), plus (still) some memory of the differences in 'feel' out toward the maximum lateral g's experienced (0.7 - 0.8g or a bit more for the wider set).

On edit, this is with a car having a claimed 21,000 NM/deg torsional stiffness per the list linked to in another thread.


Norm

RE: A Flexible Chassis is Slow

Quote (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.

I'm not sure what you're arguing, I don't think anyone is disagreeing with you.

RE: A Flexible Chassis is Slow

I believe you're right.

I misinterpreted the below statement to say that because the horizontal force component is low, that it's not an important contributor top NVH.

"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."

BTW, how do quote someone else's post?

RE: A Flexible Chassis is Slow

Click the quote button, type the name of the poster you want to quote, copy and paste their text.

It's pretty clunky on this board compared to most others that use VBulletin or whatever and allow automatic quoting.

RE: A Flexible Chassis is Slow

Yes, it sure is; I'll just stick to c/p.

thanks

RE: A Flexible Chassis is Slow

It's just as easy to just type the HTML for quoting straight out.

{quote name}<paste your text>{/quote}

Replace the above brace characters { and } with the respective brackets [ and ]. There is a space between "quote" and the name you want displayed as your source.


Norm

RE: A Flexible Chassis is Slow

Thanks, Norm, but I'm lazy; that's too much extra typing and/or mousing around for me

RE: A Flexible Chassis is Slow

(OP)
"21,000 NM/deg torsional stiffness"

Highlight their text, ctrl c, then ctrl v it into a new post.

Having accomplished that, this is over 15,000 ft lb per degree?

RE: A Flexible Chassis is Slow

Quote (Buggar)

Having accomplished that, this is over 15,000 ft lb per degree?

Guess so. A little units conversion utility called 'Convert' came up with the same result. Probably 3 to 5 times what the 1960's originals were good for.


Norm

RE: A Flexible Chassis is Slow

(OP)
I use the same Convert. I guess if everybody's watch says the same, then that's the actual time.

RE: A Flexible Chassis is Slow

Is there any data on this at all? I want to see how a controlled test on chassis rigidity's effect on speed when the vehicle travels over a given set of bumps.

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

RE: A Flexible Chassis is Slow

http://www.pulpmx.com/stories/look-back-old-moto-m...

What do you guys think of this? I don't think offroad truck guys actually know what stiff is to be honest. They might just know what "too soft" is. By the way, I have ridden the bike in that article at a competitive level in various conditions. If I had not ever done so, I would not be dissenting from the consensus in this thread. There are some things that are not readily apparent through the situations that you are currently capable of simulating in your head based on the experiences that you currently have/don't have. Do you have test labs where you work? Test labs exist for that very reason. If you disagree with that, it is because you miss understood what I just said, or you don't actually work in engineering.

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

RE: A Flexible Chassis is Slow

Quote (Panther 140)

Is there any data on this at all? I want to see how a controlled test on chassis rigidity's effect on speed when the vehicle travels over a given set of bumps.

Maybe someone has tried... In my opinion it would be very very difficult to conduct a test of this interaction where the actual effect of chassis stiffness is discernible from noise in the experiment.

Quote (Panther140)

What do you guys think of this?

I think that motorcycle chassis and full scale off road vehicle chassis have very, very little in common. I don't debate that for the particular Honda motorcycle described in the article, what the author says is true (the gist of which is, this bike sucked because the chassis was too stiff) but that has zero bearing on a trophy truck or sand rail or whatever.

Quote (Panther140)

I don't think offroad truck guys actually know what stiff is to be honest.

This is (with, perhaps, the exception of the top .01% of the sport) very likely to be true.

Quote (Panther140)

They might just know what "too soft" is.

I doubt this strongly. To know what 'too soft' is with regard to an off road chassis, you would have to build multiple off road chassis of known stiffness, with the stiffness of each varied by a statistically significant degree, outfit them with the same components, and then test extensively. I doubt VERY strongly than any off road racing team has ever done this, with the explicit goal of trying to determine the exact effect that chassis stiffness has on suspension tuning. In my experience, they (attempt to) build things so that they don't break, and then tune the suspension to suit their attempt at a bomb proof chassis. There's very little engineering-from-first-principles going on. That's not a knock against those teams- it's a tough environment to design in.

RE: A Flexible Chassis is Slow

There are rules of thumb for "too soft" or not. Basically the wheel rate attributable to chassis flex should be at least (10x ?) the wheel rate attributable to springing components (springs and ARBs).

je suis charlie

RE: A Flexible Chassis is Slow

As Milliken nearly said, if you are designing thumbs, use rules of thumb, if you are doing vehicle dynamics, perhaps not.

The problem with that one in particular is that correcting the springs for a soft chassis is easy enough, schoolboy physics. The hard part is figuring out how to modify the shocks.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: A Flexible Chassis is Slow

Quote (gruntguru)

There are rules of thumb for "too soft" or not. Basically the wheel rate attributable to chassis flex should be at least (10x ?) the wheel rate attributable to springing components (springs and ARBs).

Right.. but how many guys building a sand rail in their garage, or how many custom shops building a sand rail for a rich guy to park in his garage, know their design's true wheel rate due to chassis flex?

My guess would be close to 0.

RE: A Flexible Chassis is Slow

Greg. I think that particular rule-of-thumb is meant to be a standard for building a back yard chassis rather than a tuning tool for compensating a sub-standard design.

je suis charlie

RE: A Flexible Chassis is Slow

(OP)
ps: the 10 percent generally applies to torsional rigidity of the chassis vs. the roll stiffness of the spring/anti roll bar stiffness? Correct me if I'm wrong. Is it valid? See my posts on the Ariel Atom. But that's another subject.

RE: A Flexible Chassis is Slow

It's always sounded more like "ignore effects of less than 10% as a first cut approximation" to me.


Norm

RE: A Flexible Chassis is Slow

If I was building a racecar in my garage, the goal would be to make the chassis as stiff as possible within reasonable weight constraints. If it turns out to be less than 10x the roll rate I have in mind, I should worry about the tune-ability of the suspension. If not, I should not.

je suis charlie

RE: A Flexible Chassis is Slow

Quote (GregLockok)

I suspect winning cars are uilt as light as possible with a low cg as a secondary consideration, and stiffness third.

I've worked on cars that were clean sheet space frames, and cars that are based on a OE BIW, and in both cases, you'd be right.

In the case of a BIW turned racer, in my experience, much of the added structure which increased chassis stiffness is put in place because of rules and regulations regarding crashworthiness. The additional chassis stiffness is a welcome, but secondary, byproduct.

Of course, at a high level, those rule-mandated structures are optimized to provide as much stiffness per weight as possible, but *most* people building cars don't have the right tools to do that type of engineering.

RE: A Flexible Chassis is Slow

Regardless, I wouldn't compromise a 15x torsional stiffness chassis down to 7x to save 10 kg on a 500 kg car.

Stiffness mat only have third priority but the reality is everything is a trade-off - you can't compromise any metric beyond a certain point, for the sake of improving a "primary" or "secondary" one.

je suis charlie

RE: A Flexible Chassis is Slow

True, but the point we are struggling with is that it is hard to directly quantify the performance benefit from a stiff body at a vehicle level. A lower cgz, or lower mass, directly translates into higher lateral g at the limit.

Cheers

Greg Locock


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RE: A Flexible Chassis is Slow

Quote (gg)

Regardless, I wouldn't compromise a 15x torsional stiffness chassis down to 7x to save 10 kg on a 500 kg car.

I might be more concerned about where (along the X-axis) that compromise down to 7x happened.


Norm

RE: A Flexible Chassis is Slow

(OP)
Another star to Greg because he summarized my post about the torsionaly flexible Ariel Atom - why is it so fast? Is there a track that needs torsional rigidity and on which this car would not be competitive? (There is some large money on this particular answer.)
I'm looking at F1 cars and their only need for any suspension at all seems to be to provide a mechanism to transfer wheel loads diagonally in the turns which can only be done with torsional stiffness (am I wrong on this?).

RE: A Flexible Chassis is Slow

(OP)
Somewhat unrelated but I have to pass it on. Some of the guys do measure their frames for torsional rigidity. Did I say they were competitive? No stone left unturned! To eliminate transients that would possibly measure anything other then pure torsion, they suspend their cars between chains hung from a chassis lift fully extended above and chains fastened to inserts in the floor slab below. They don't even have to disconnect the suspension. Yes, the suspended cars look like a mechanics idea of S&M. They use come-alongs with cable tension meters for loading and a Craftsman protractor to measure angles. It seems to work. It cost me a couple more Dos Equis to find out, but around 3,000 to 5,000 ft lb per degree seems common.

RE: A Flexible Chassis is Slow

> I might be more concerned about where (along the X-axis) that compromise down to 7x happened.

Why would it matter, and why would there be a particular where (as opposed to the integrated effect of all of the torsion between the front and rear wheels)?

RE: A Flexible Chassis is Slow

""I might be more concerned about where (along the X-axis) that compromise down to 7x happened."

"Why would it matter, and why would there be a particular where (as opposed to the integrated effect of all of the torsion between the front and rear wheels)?""


I would think it was a lower order effect. There would certainly be some effect on dynamic roll response. Imagine a vehicle where the chassis was torsionally stiff between one axle and the engine (massive) and much softer between the engine and the other axle.

je suis charlie

RE: A Flexible Chassis is Slow

OK, I suppose so, though it's hard to imagine that not being the case, given that the engine is usually very close to one of the axles and far from the other, with the lion's share of the torsional compliance occurring in the structure between.

RE: A Flexible Chassis is Slow

As long as torsional compliance is low, the effect is of no consequence.

je suis charlie

RE: A Flexible Chassis is Slow

How weak chassis driver can feel on transition inputs?
for example like a insufficient lateral compliance?
is difference perception front or rear chassis twist?
gives difference over all lateral g levels?

I hope makes sense

(I have only steady state cornering experience ...... understeer change )

Thanks Radek

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