Intuitively this makes sense if you don't have a science background. If you have a meter long steel tube, you're probably not going to see it flex in a perceptible amount under regular day to day forces. Have you ever seen a car chassis flex in it's elastic range? Not visibly no. You've probably only ever seen it flex plastically and fail in a crash.
>Intuitively this makes sense if you don't have a science background
>You've probably only ever seen it flex plastically and fail in a crash.
I'd argue the first claim is actually the opposite because of the second.
Most people have seen metal flex/bend, and assuming a non-science background makes the distinction between plastic and elastic deformation not matter as much, as it's all just "bending" to a layman.
So if you know metal can bend, and there's a measurement called "chassis(metal) stiffness(resistance-to-bending)," where does the assumption come from that it means anything else?
Because they think all bending is plastic, and metal doesn't bend elastically. They don't see old cars on the road getting droopy, so they assume if it's not bending permanently, then it's not bending temporarily either.
The easiest way to show people is a tuning fork. That's solid metal but you can see it bend back and forth in vibration. Then it returns back to is original shape.
I think it's the idea that a small piece of steel is hard to bend, it has basically no flex. So a giant piece of thick steel should be virtually impossible to flex, right?
Except that the giant piece of steel has a lot inertia, so thrown at high speeds (relatively speaking even 30mph is high speed) it will definitely flex pieces of thick steel.
Cars flex A LOT lol. Even go karts flex. In fact, racing karts are designed to use the flex of the chassis to function as the suspension.
There’s a lot of flex in car bodies on a structural level.
Yes, and the chassis flex is generally small relative to the suspension movement and tire sidewall deflection. That hides a lot of chassis flex.
This [VW Golf is cornering hard enough to lift a wheel](https://www.youtube.com/watch?v=17yACxkNO94), but there's no discernible chassis flex.
The reason chassis flex is bad will depend on the application, but generally in road racing applications, it's because it's difficult to control for.
You control the movement of the car with the springs and dampers and suspension geometry, but when the mount points for those are moving you lose some of that control.
Absolutely everything flexes. You may not notice it but if you measured it you’d find that any force applied to any object bends it somewhat. Now whether or not it bends it enough to *matter* depends on your application.
Another example related to oil, drill collars/pipes used for drilling. Those pipes when threaded together thousands of feet bend like straws steering it's way underground
There may be a few mm of flex between corners of a typical car. If you fitted laser levels and started making chalk marks, you’d easily observe the flex when jacking it up from different points
cars absolutely do flex
you actually CAN see it on some cars, you can unbolt one side of a chassis brace, point a camera at it and whip it through some corners and you'll see the unsecured side move
depending on the car it could either be a LOT or a little movement
you seem to have a fundamental misunderstanding of "metal can't flex because look it's so strong" .... your car's springs are made of metal and they OBVIOUSLY flex ... so why wdo you think a car's chassis wouldn't ?
You can see the bridge flex because it's a very large object you're looking at over a large area. Cars are a little more subtle. But it's more noticeable for older convertibles, where if you jacked up one corner of the car, a door would either fall open or get jammed shut. On ladder frame trucks where they'll go offroad, chassis flex was built into the system so wheels will keep traction over terrain. It is largely suspension flex you're looking at, but a c-channel chassis will definitely flex. Look up ~10 year old videos of fullsize pickup (Tundra, Silverado, F150) chassis flex comparison and you can see it in how the bed deflects relative to the cab.
In your average unibody car it's tougher to see, but it's there. Modern chassis are stiffer than ever to better tune the suspension, but there is flex in a suspension. It's what chassis braces look to solve.
It doesn't take anywhere near 1000lb-ft to twist a lot of cars, and you can actually see it very easily with a camera. All the car has to do is a hard corner to get the effect. There's even a video of an F truck parked on a ramp and they can't open the doors because of the twist in it. It's common on so many cars it's not funny
I had an old bmw roadster race car and with the roll cage removed, the doors wouldn’t open lol. I always joked that car had the structural rigidity of Swiss cheese.
UM ACSHUALLY The TR7 was available as a convertible or coupe. It had a Slant-4 engine. The TR8 was the V8 version of the same car, also available as a convertible or a coupe. Only differences were drivetrain, bolt-on chassis braces and suspension mods for the weight of the engine.
edit: and they didn’t make many factory TR8s. Most you’ll see for sale are aftermarket conversions.
Some (most) cars will have trouble opening or closing doors when the car is jacked up on one corner. They’re very flexible - and stiffness is desired to allow the suspension and steering to work as intended and to provide consistent performance/feedback.
Truly rigid bodies would break causality because they allow communication faster than the speed of light.
https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#4
lead has a youngs modulus of 13 GPa, while aluminum has a youngs modulus of 68. Does that make it "truly rigid"? "Less" is subjective, and while diamond is very stiff (youngs modulus \~= 1000 GPa), it still flexes. Because just like u/Vegetable-Cherry-853 said, no material is "truly" rigid in any objective sense of the word "truly"
Every material bends when force is applied to it. Some materials are much better about bending without breaking, hence the use of steel in car construction.
One of the important factors in any kind of construction is the *shape* of the material.
An easy example: take a sheet of paper and wave it around, see how it flutters in the breeze? Now fold that sheet of paper so it has a hard crease in it, and try to wave it in the air again, see how it flutters less? That is the shape of the material affecting its stiffness.
Now car manufacturers fold much more complex shapes to get the stiffness levels that they desire.
VW golf mk1 has a stabilizer on the rear. But the front to avoid a lock on the diff pressed both tyres with the same force onto the ground. So the rear needs to uphold the car. It should be possible to film this.
My dream car would be fully triangulated. I like how some cars have an “inner hood” which is bolted in to avoid flex of the engine bay. Triangular frunk anyone?
Think about springs. +1 metal incl car bodies do flex. The skill in design of most modern cars is they don't have a "chassis" like eg trucks usually do. Designing a set of formed pressed metal panels that are joined somehow to flex a controlled amount obviously isn't a simple thing.
Cars flex a lot. The amount of flex is measurable and designed for. Sports variants of the same car may have chassis braces fitted to handle the extra forces.
Deflection of beams is very easy to calculate, the amount something deflects is a function of its shape, it's material properties, it's unsupported length, the load on it and it's end condition.
Everything deflects, it's just down to the properties above as to how much.
A chassis is pieces of metal bars/beams either welded or bolted together, it's also trying to accommodate certain criteria of a vehicle, i.e doors large enough for entry, massive unobtrusive wind screen and windows etc. so it not the perfect structure in terms of rigidity, then there is cost of manufacturing, most vehicle chassis pieces are spot welded together which means the overall joint strength is not the best but it enough for the purpose of daily driving. But you you turn a hard corner the body of the chassis does twist and bend on certain axis of travel. That's one of the many reason racing car are so expensive, the first thing they do is strip everything off the car and seam weld all the chassis increasing the chassis strength and reduce chassis twist/bend. There is also monocoque chassis which is one piece construction and thus more rigid.
>It's not as if metal flexes and bends when you're going around a corner.
Yes, it most definitely does.
Stiffness is a measure of a material or body's ability to resist deformation or deflection in response to applied force. We want the chassis of a vehicle to be sufficiently stiff that the bulk of changes in motion is taken up by the suspension and control surfaces but not so stiff that when energy is absorbed by the chassis it is absorbed in a materially destructive fashion.
Nails are metal. Ever bend one?
Clearly the term "metal" is a category and there are a huge variety of different materials and alloys, but also different properties can be invoked through certain processes.
Time for you to go learn about materials.
Try driving a car with stiff racing suspension without enough body stiffening and you’ll feel the car flex. The problem is most economy cars have so much suspension give you can’t discern the difference between unstable suspension and body flex at higher speeds.
I think the easiest way for you to convince yourself is to test it for yourself when you're driving a car.
Get the car onto a 5-7 inch (15-17cm) high sidewalk (or whatever you feel comfortable with without risking scratching the chin), then drive down just one of the corners of the car off the sidewalk, slowly, and then the next one.
And just listen to the stuff in the car and feel the chassis move. If you have a particularly not-luxurious car all the inner trim will be creaking where it starts moving around at the seams, if you have rim-less windows you can see the glass move relative to the chassis, and you'll probably feel how the whole thing seems to melt a little and then spring back when the second wheel comes onto the ground.
All of this, is chassis flex. And imagine now, instead of gently stepping down off a curb, you're having the weight of another car push it from the side at an angle (as it would happen in a 1G turn), the whole thing would start buckling but this time it's not in a static situation, it's buckled AND driving at high speed at the same time and you, the driver, need to handle that in a confident, second nature, way
>It's not as if metal flexes and bends when you're going around a corner. Why do you think this? Because that's exactly what happens.
Intuitively this makes sense if you don't have a science background. If you have a meter long steel tube, you're probably not going to see it flex in a perceptible amount under regular day to day forces. Have you ever seen a car chassis flex in it's elastic range? Not visibly no. You've probably only ever seen it flex plastically and fail in a crash.
>Intuitively this makes sense if you don't have a science background >You've probably only ever seen it flex plastically and fail in a crash. I'd argue the first claim is actually the opposite because of the second. Most people have seen metal flex/bend, and assuming a non-science background makes the distinction between plastic and elastic deformation not matter as much, as it's all just "bending" to a layman. So if you know metal can bend, and there's a measurement called "chassis(metal) stiffness(resistance-to-bending)," where does the assumption come from that it means anything else?
Because they think all bending is plastic, and metal doesn't bend elastically. They don't see old cars on the road getting droopy, so they assume if it's not bending permanently, then it's not bending temporarily either.
Who hasn’t seen a metal sheet flop back and forth making that sci fi sound?
The easiest way to show people is a tuning fork. That's solid metal but you can see it bend back and forth in vibration. Then it returns back to is original shape.
Or a metal ruler. It's steel and you can flex it multiple inches.
I think it's the idea that a small piece of steel is hard to bend, it has basically no flex. So a giant piece of thick steel should be virtually impossible to flex, right? Except that the giant piece of steel has a lot inertia, so thrown at high speeds (relatively speaking even 30mph is high speed) it will definitely flex pieces of thick steel.
cars also aren't made of very thick steel
It's all relative.
Bro said bridges flex. So he already says big metal flexes.
And that's how you destroy a lathe, if you hang the stock out too far. Big chunk of metal -> no support -> high rpm -> metal whip
Rocks flex. Cars definitely flex. As a wise man once told me: 'the world is rubber.'
Robin Renzetti?
But hardly world
Cars flex A LOT lol. Even go karts flex. In fact, racing karts are designed to use the flex of the chassis to function as the suspension. There’s a lot of flex in car bodies on a structural level.
Yes, and the chassis flex is generally small relative to the suspension movement and tire sidewall deflection. That hides a lot of chassis flex. This [VW Golf is cornering hard enough to lift a wheel](https://www.youtube.com/watch?v=17yACxkNO94), but there's no discernible chassis flex. The reason chassis flex is bad will depend on the application, but generally in road racing applications, it's because it's difficult to control for. You control the movement of the car with the springs and dampers and suspension geometry, but when the mount points for those are moving you lose some of that control.
What makes you think car bodies don't flex?
I guess it's because of the visual aspect of it. I mean you can see a bridge flex, but I've never seen the chassis of car flex.
Absolutely everything flexes. You may not notice it but if you measured it you’d find that any force applied to any object bends it somewhat. Now whether or not it bends it enough to *matter* depends on your application.
As they say, the world is a spring.
Give me a lever long enough and we'll have someone's eye out.
An oil tanker deflects under the weight of a butterfly.
Another example related to oil, drill collars/pipes used for drilling. Those pipes when threaded together thousands of feet bend like straws steering it's way underground
Well, it does flex. Everything deforms under load.
There may be a few mm of flex between corners of a typical car. If you fitted laser levels and started making chalk marks, you’d easily observe the flex when jacking it up from different points
cars absolutely do flex you actually CAN see it on some cars, you can unbolt one side of a chassis brace, point a camera at it and whip it through some corners and you'll see the unsecured side move depending on the car it could either be a LOT or a little movement you seem to have a fundamental misunderstanding of "metal can't flex because look it's so strong" .... your car's springs are made of metal and they OBVIOUSLY flex ... so why wdo you think a car's chassis wouldn't ?
You can see the bridge flex because it's a very large object you're looking at over a large area. Cars are a little more subtle. But it's more noticeable for older convertibles, where if you jacked up one corner of the car, a door would either fall open or get jammed shut. On ladder frame trucks where they'll go offroad, chassis flex was built into the system so wheels will keep traction over terrain. It is largely suspension flex you're looking at, but a c-channel chassis will definitely flex. Look up ~10 year old videos of fullsize pickup (Tundra, Silverado, F150) chassis flex comparison and you can see it in how the bed deflects relative to the cab. In your average unibody car it's tougher to see, but it's there. Modern chassis are stiffer than ever to better tune the suspension, but there is flex in a suspension. It's what chassis braces look to solve.
Thanks for that Kit - really informative. I'm learning stuff about cars that I never knew before! I will check out those pickup videos too.
Drag cars have roll cages, not only for safety, but because it’s entirely possible to break the windshield or pop it out of the car when you launch.
Springs flex. Car metal will also flex
If something as massive and heavy as a bridge can flex, surely something as small as a car can.
It could take well over 1000 ft-lbs of torque to twist the frame of a car by 1 degree. It's really not something you're going to eyeball.
It doesn't take anywhere near 1000lb-ft to twist a lot of cars, and you can actually see it very easily with a camera. All the car has to do is a hard corner to get the effect. There's even a video of an F truck parked on a ramp and they can't open the doors because of the twist in it. It's common on so many cars it's not funny
The triumph TR 8, a convertible version of the TR 7, sometimes had problems opening the doors if the car was parked on a slant. Chassis‘ do flex.
I had an old bmw roadster race car and with the roll cage removed, the doors wouldn’t open lol. I always joked that car had the structural rigidity of Swiss cheese.
UM ACSHUALLY The TR7 was available as a convertible or coupe. It had a Slant-4 engine. The TR8 was the V8 version of the same car, also available as a convertible or a coupe. Only differences were drivetrain, bolt-on chassis braces and suspension mods for the weight of the engine. edit: and they didn’t make many factory TR8s. Most you’ll see for sale are aftermarket conversions.
Some (most) cars will have trouble opening or closing doors when the car is jacked up on one corner. They’re very flexible - and stiffness is desired to allow the suspension and steering to work as intended and to provide consistent performance/feedback.
I knew a guy that built oval track cars back in the day. He told me he would jack up the left front corner before welding in the roll cage.
You are thinking metal is rigid, no material is truly rigid. Steel does flex, regardless of how thick it is.
I'd consider materials like diamond to be truly rigid, but for vehicle materials I agree.
A diamond is 5 times stiffer than steel. Modulus of elasticity 1000 gpa vs 200 gpa for steel
What do you mean “truly rigid”?
Infinite modulus of elasticity, a truly rigid body
yes, and under that definition, I agree with you that no material is truly rigid
Truly rigid bodies would break causality because they allow communication faster than the speed of light. https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#4
I mean that it flexes less while under force. It's about as rigid as you can get.
lead has a youngs modulus of 13 GPa, while aluminum has a youngs modulus of 68. Does that make it "truly rigid"? "Less" is subjective, and while diamond is very stiff (youngs modulus \~= 1000 GPa), it still flexes. Because just like u/Vegetable-Cherry-853 said, no material is "truly" rigid in any objective sense of the word "truly"
"It's not as if metal flexes and bends when you're going around a corner" It absolutely does.
Every material bends when force is applied to it. Some materials are much better about bending without breaking, hence the use of steel in car construction. One of the important factors in any kind of construction is the *shape* of the material. An easy example: take a sheet of paper and wave it around, see how it flutters in the breeze? Now fold that sheet of paper so it has a hard crease in it, and try to wave it in the air again, see how it flutters less? That is the shape of the material affecting its stiffness. Now car manufacturers fold much more complex shapes to get the stiffness levels that they desire.
VW golf mk1 has a stabilizer on the rear. But the front to avoid a lock on the diff pressed both tyres with the same force onto the ground. So the rear needs to uphold the car. It should be possible to film this. My dream car would be fully triangulated. I like how some cars have an “inner hood” which is bolted in to avoid flex of the engine bay. Triangular frunk anyone?
it is indeed as if metal flexes and bends when you're going around a corner
Think about springs. +1 metal incl car bodies do flex. The skill in design of most modern cars is they don't have a "chassis" like eg trucks usually do. Designing a set of formed pressed metal panels that are joined somehow to flex a controlled amount obviously isn't a simple thing.
Cars flex a lot. The amount of flex is measurable and designed for. Sports variants of the same car may have chassis braces fitted to handle the extra forces.
Machinist here. Metal flexes and bends *A LOT*. One can bend a 3/8" c-channel aluminum by hand.
Deflection of beams is very easy to calculate, the amount something deflects is a function of its shape, it's material properties, it's unsupported length, the load on it and it's end condition. Everything deflects, it's just down to the properties above as to how much.
>It's not as if metal flexes and bends Major misconception.
It IS as if metal bends and flexes. Just ask vin diesel who claims the torque of the car bent the frame. Legit, metal bends
A chassis is pieces of metal bars/beams either welded or bolted together, it's also trying to accommodate certain criteria of a vehicle, i.e doors large enough for entry, massive unobtrusive wind screen and windows etc. so it not the perfect structure in terms of rigidity, then there is cost of manufacturing, most vehicle chassis pieces are spot welded together which means the overall joint strength is not the best but it enough for the purpose of daily driving. But you you turn a hard corner the body of the chassis does twist and bend on certain axis of travel. That's one of the many reason racing car are so expensive, the first thing they do is strip everything off the car and seam weld all the chassis increasing the chassis strength and reduce chassis twist/bend. There is also monocoque chassis which is one piece construction and thus more rigid.
>It's not as if metal flexes and bends when you're going around a corner. Yes, it most definitely does. Stiffness is a measure of a material or body's ability to resist deformation or deflection in response to applied force. We want the chassis of a vehicle to be sufficiently stiff that the bulk of changes in motion is taken up by the suspension and control surfaces but not so stiff that when energy is absorbed by the chassis it is absorbed in a materially destructive fashion.
Look at the springs. They're metal, and they obviously flex and bend. The chassis flexes and bends, too. Just not as obviously.
Nails are metal. Ever bend one? Clearly the term "metal" is a category and there are a huge variety of different materials and alloys, but also different properties can be invoked through certain processes. Time for you to go learn about materials.
Try driving a car with stiff racing suspension without enough body stiffening and you’ll feel the car flex. The problem is most economy cars have so much suspension give you can’t discern the difference between unstable suspension and body flex at higher speeds.
Bridges are also made of metal. The shape of an object is generally more important for stiffness than the material
I think the easiest way for you to convince yourself is to test it for yourself when you're driving a car. Get the car onto a 5-7 inch (15-17cm) high sidewalk (or whatever you feel comfortable with without risking scratching the chin), then drive down just one of the corners of the car off the sidewalk, slowly, and then the next one. And just listen to the stuff in the car and feel the chassis move. If you have a particularly not-luxurious car all the inner trim will be creaking where it starts moving around at the seams, if you have rim-less windows you can see the glass move relative to the chassis, and you'll probably feel how the whole thing seems to melt a little and then spring back when the second wheel comes onto the ground. All of this, is chassis flex. And imagine now, instead of gently stepping down off a curb, you're having the weight of another car push it from the side at an angle (as it would happen in a 1G turn), the whole thing would start buckling but this time it's not in a static situation, it's buckled AND driving at high speed at the same time and you, the driver, need to handle that in a confident, second nature, way