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Automotive theory question: What makes for a good chassis?

I read a lot of online automotive publications (,,, various forums, etc.) and they all talk about how important the chassis is. The chassis is afterall the body right? It’s unibody, right?

What makes one unibody chassis better than another? Is it basically just RIGIDITY?

You want it to be as strong and rigid as possible while being as light as possible. Is that basically it?

How can you tell if a chassis is good or not? IF you’re enjoying the handling of a car, how can you tell if its the suspension or chassis?
Unless the chassis is deficient in some sense (does it bend?!) ----how can one evaluate the chassis of a car???

They usually describe the things they like and I read and forget, read and forget. It matters to test drivers more than it does to most drivers anyway. Sports cars probably want stiffer chassis characteristics so that the car can handle the same way every time it corners at higher speeds. Other than that, the tires and suspension are more important. Read and forget, read and forget.

Structural engineers can write books on the topic. In general terms, divide the stiffness of an object by its mass, take the square root, and you get the natural frequency of the object. That is to say how often the body oscillate after you go over pot hole. More is generally better.

There are so many factors. One for sure is rigidity. If you go over a bumpy, uneven road, or take a sharp corner, the body should not flex at all. You may never have noticed when driving a cheap car or truck (and sometimes a not so cheap one), that sometimes you hear the dash creak or something in the cabin pop or crack when turning a corner or hitting a severe bump. That is often from the whole body flexing a bit, and it contributes to the handling as well as how the car is perceived, and can contribute to the safety in a crash.

Rigidity is one of the reasons that if you look at the GVW of a convertible vs. a coupe or sedan on the same model of car, that the convertible will usually weigh more. Without the roof to contribute to the structure, additional reinforcing is needed to eliminate flex.

I think in go-kart racing, flex is the suspension and the chassis is designed to flex. Kart racing rules forbid suspension systems.

Weight by itself seems to have little to do with good handling, some good handling cars are surprisingly heavy and some lightweight subcompacts have mediocre handling.

Everything flexes when a load is applied. Less is usually better, but that costs money and adds weight. So, like everything else on a car, it’s a compromise. Computer aided design allows major improvements by modeling the behavior without making any parts.

Having been a suspension engineer and vehicle dynamicist, I can shed a little light on this. The chassis is the overall term for the undercar bits and what they attach to. The suspension parts are the ones that move. The frame or unit body provides the connection between the front and rear suspensions and the mounting for the engine. All these things combine to provide the car’s handling.

Engineers want the structure that the suspension attaches to as rigid as possible so that the suspension can isolate bumps, provide directional control and carry loads. The go-kart is a good example of what happens when a stiff suspension (basically, none) is attached to a flexible frame, the frame BECOMES the suspension and can’t be controlled as well with shocks (none) springs (the frame itself) and stabilizer bars (again the frame). You can never get as much structure as you want because it is heavy, expensive and the car needs to carry people and stuff. The compromise we make is different from a luxury sedan to a sports car.

The magazine writers are driving the car and evaluating how smooth it rides, how well it responds to steering, braking and acceleration (the handling) as they drive the car normally to tire slidin’ fun. They evaluate how well it changes direction, comes back to and stays straight, the limits of traction, cornering and stopping all with the idea of what the buyer expects. Car companies have experts (much better ones!) who do this day in and out.

This is also an issue with racing motorcycles - there are constant interactions between the tires, suspension, and frame (the bike’s ‘chassis’). Make one too stiff or too flexible and the overall handling suffers.

A good chassis in one that’s stiff and corrosion resistant. Light weight would be nice, but not at the expense of the qualities mentioned.

Frame rigidity or lack of it is especially noticeable with a motorcycle. I have owned and ridden motorcycles with flexible and also with more rigid frames. The first Harley Sportster that I rode was from the late 1960s; had a flexible frame and felt like a rolling scrapyard full various pieces of metal, each moving in a different direction. Now Harley Sportsters have much more rigid frames; have that “All of a Piece” solid quality feeling as though you are riding one single chunk of metal, is the best way I know to describe it. BMW had rigid frames for years before the Japanese finally got it right around 1980. Before that, Japanese bikes had a loose feeling that was not a danger to riding safety for the average street rider in my view but a rigid frame gives a bike rider a certain feeling of higher confidence and quality.

Possibly frame rigidity is not as profoundly felt with cars as it is with bikes.

How can you tell if a chassis is good or not? IF you’re enjoying the handling of a car, how can you tell if its the suspension or chassis?

Unless you’re planning to harvest the chassis from a donor car, it doesn’t matter. The car is a system and all components work in concert to deliver the end product.

Mustangman said it well. Just one point to add about the tradeoff- the more rigid the chassis, the more perfect your suspension set up has to be for optimum performance.

I have vivid memories of the late '90s when engineers started to discover torsional rigidity in automotive chassis. The most publicized case was the '97 (C5) Corvette. The C4 Corvette that went from the early 80s thru 1996 handled pretty well, but the ride was crude at best, and the convertible was like a shoebox without a lid. The C5 introduced in 1997 was built on a much improved chassis that was originally designed to be a convertible. The ride and handling were both improved, especially the ride. The '97 Corvette quickly established itself as being a huge improvement over previous generations. It is now fairly well established that a comfortable ride and competent handling can co-exist much better in a car with a good, rigid chassis.

Low mass (weight) and limited flexing (to the purpose for which the car is designed) are what the designer aims to achieve for the frame I suppose. Add to that robustness. The frame should not bend or break or a crack a weld during the normal lifespan of the car. Likewise, it should not fail in a way that injures the passengers in a collision.

I guess the discussion has gone to handling as a “good” chassis goal. IMO, it is ( best chassis) one that most closely helps the engineer get what he wants form the vehicle he is trying to build. If all you want is handling, you sacrifice ride, load handling, perhaps ground clearance and even safety. Would you use as a base,the excellent handling qualities in a Corvette for a mid size sedan ? Of courses not. There are instances when you want a fair about of flex. There are instances when safety, space efficiency and durability trump handling. All automobile frames are designed to have some flexibility and ridgity in the right places depending on the what the engineer wants. Also, rust resistance is only relatively important. Any car can easily be made comparably rust free for very little added expense relative to the total cost of the car. That’s just an ancillary consideration in chassis design while reaching a goal that encompasses a lot of other good qualities… These are the real factors that make for a “good” chassis. In that respect, I agree with @twinturbo.

Dagosa, while I wouldn’t use the Corvette’s dynamics as a benchmark for a midsize sedan, I would say that the chassis developed since the mid '90s with improved torsional rigidity allow sedans to be created which have a comfortable ride, but at the same time have very sure-footed handling. The Corvette vastly exceeds the handling requirements for everyday driving, but some degree of competent handling is desirable in a family sedan. When you have to brake and steer quickly to avoid an unexpected dangerous situation, that dog-gone car better go where you point it, Corvette or not!

I hear and agree. I just don’ t fell putting quantifies like good, better and best, like this disscussion is based upon, is worthwhile IMO. A valid argument can be made that an Accord has a better made chassis for it’s intended use then a Corvette. Rigidity is an over used criteria that may have some validity in handling but in absolute terms, may have no place by itself alone in ride comfort and even safety. The development of a chassis for a fwd car that makes it a decent handler along with better economy, a safe secure ride, may actually be superior in execution then that of a Corvette given it’s inherrent, more limited expectations. The idea of just making a frame more rigid as an end all to overall better car performance just doesn’t hold water to me. I’m sure that was not anyone’s intent but I just want to go on record as saying, it can help or can be a detriment depending on the intended use of the auto.