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”.
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
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
RE: A Flexible Chassis is Slow
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
Norm
RE: A Flexible Chassis is Slow
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
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
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
*exception: Ariel Atom which is fast and flexible and I can’t figure out why(?)
RE: A Flexible Chassis is Slow
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
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
"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'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
"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
Norm
RE: A Flexible Chassis is Slow
"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
Norm
RE: A Flexible Chassis is Slow
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
“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
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
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
RE: A Flexible Chassis is Slow
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
"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
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
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
RE: A Flexible Chassis is Slow
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
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
RE: A Flexible Chassis is Slow
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
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
Norm
RE: A Flexible Chassis is Slow
Increased compliance will improve NVH, especial impact H.
RE: A Flexible Chassis is Slow
Norm
RE: A Flexible Chassis is Slow
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
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
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
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
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
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
I'm not sure what you're arguing, I don't think anyone is disagreeing with you.
RE: A Flexible Chassis is Slow
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
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
thanks
RE: A Flexible Chassis is Slow
{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
RE: A Flexible Chassis is Slow
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
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
RE: A Flexible Chassis is Slow
"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
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
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.
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.
This is (with, perhaps, the exception of the top .01% of the sport) very likely to be true.
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
je suis charlie
RE: A Flexible Chassis is Slow
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
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
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
je suis charlie
RE: A Flexible Chassis is Slow
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
RE: A Flexible Chassis is Slow
Norm
RE: A Flexible Chassis is Slow
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
je suis charlie
RE: A Flexible Chassis is Slow
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
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
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
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
I might be more concerned about where (along the X-axis) that compromise down to 7x happened.
Norm
RE: A Flexible Chassis is Slow
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
Here is a discussion of F1 chassis torsional stiffness trends up to 2000 or so, allegedly gleaned from sources who might know.
http://www.f1technical.net/forum/viewtopic.php?t=1...
RE: A Flexible Chassis is Slow
RE: A Flexible Chassis is Slow
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
"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
RE: A Flexible Chassis is Slow
je suis charlie
RE: A Flexible Chassis is Slow
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
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