[ImmortalZ]

A flexing chassis doesn't feel comfortable - it feels very wrong. Almost like the car is going to fall apart. Personally, it makes me feel nauseous. A stiff chassis on a comparatively forgiving set of suspension components will feel very good indeed. With stiff suspension, it will feel Godlike.

[mallumowgli]

When i said a flexing chassis, i didnt mean a chassis flexing like a rubber band. It is only a relative term. I understand that the chassis of a Merc is relatively more flexible compared to a BMW. For that matter all our normal saloons and hatches have comparitively more flexible chassis than sports cars

My question is whether the stiffness increases ride quality as stated by Autocar or does it improves handling.

[Sankar]

Originally Posted by mallumowgli

In the recent review of the new 5 series by Autocar, they said that the stiffness of the chassis has been increased by 50% and this has resulted in an amazing increase in ride quality. I was under the impression that stiffness improves handling and not ride quality. Stiffer chassis means better handling. Better ride is brought about by softer suspension and chassis set up. Or, rather, this was my intuitive reasoning

Can anyone reading this clarify?

Stiffer chassis does not neccessarily provide better ride quality (read comfort). What Autocar may have missed out in the equation is a revised suspension alongwith a stiffer chassis. When a car is re-engineered with a stiffer chassis they usually revise the suspension too, both are done together when a manufacturer revamps a car inside out. So the better ride quality is a result of both not just stiffer chassis.

Your reasoning is quite correct, if you take a stock car, Maruti Esteem for example and go about strenghtening the chassis with seam welding, adding strut braces etc you will end up with a Esteem that rides (read comfort) bad compared to a stock Esteem, even if you leave the suspension un-touched. This is beause the engineered flex which absorbed a bit of shock is now gone.

[Sutripta]

Originally Posted by Sankar

....
This is beause the engineered flex which absorbed a bit of shock is now gone.

I don't think designers engineer flex into their chassis. They learn to live with it, and design round what they have achieved. To say they have designed a certain amount of flex as desirable is the equivalent of turning a bug into a feature in software.

Sutripta

[Sankar]

Originally Posted by Sutripta

I don't think designers engineer flex into their chassis. They learn to live with it, and design round what they have achieved. To say they have designed a certain amount of flex as desirable is the equivalent of turning a bug into a feature in software.

Sutripta

Not actually. Flex is sometimes desirable and is engineered into the design based on the application. Not just automobiles but motorcycles too. Sometimes flex is a result of economics.

Its better to flex than crack :-)

[Sutripta]

Originally Posted by Sankar

...Flex is sometimes desirable and is engineered into the design based on the application.

Agreed, but not in a chassis. The compliance is built into the suspension system, and in the mountings of the various heavy subsystems. Not in the structure of the chassis.

Regards
Sutripta

[Sankar]

Its all based on application Road/Off-road/racing etc. Sometimes its good to flex and sometimes its good not to Flex. Thats why i said its engineered in. Truth is all chassis flex, stiff etc is all relative, except maybe the carbonfibre honeycomb/space frame design, but cars sporting these kind of chassis are in a different league and focussed.

When it comes to motorcycles, in some cases a little flex has improved laptimes. Can't dig up the article now i read it a long back. Google should help if anyone is interested.

Edit: Sometimes reduced flex has also improved laptimes... point is the whole thing is a package

[Psycho]

Originally Posted by mallumowgli

My question is whether the stiffness increases ride quality as stated by Autocar or does it improves handling.

A stiffer chassis helps in improving handling as the suspension can be fine tuned easily, however as the others have said the ride quality is dependent on many more factors (suspension, busing, spring rates, damping, seats too) other than just the chassis stiffness

[SS-Traveller]

Isn't chassis stiffness also about weight as well as cost?

The stiffer the chassis, the heavier it would be (which is where monocoque shells win - they're stiffer but lighter than chassis-on-body construction, kg for kg).

OTOH, the stiffer a chassis is, the better the material inputs (carbon fibre, for instance) required - which pushes up costs. The end-product and its pricing targets determine how far engineers will go to make a stiffer chassis.

How many of us have seen those Ambys / Fiats with their roof chopped off by the local garage trying to create a cabriolet, and the centre section sagging down?

And folks in Kolkata will vouch for the fact that even today, brand-new Ambys fresh out of the factory get their chassis extensively re-welded and stiffened before they are registered as taxis, to survive the roads for a substantial time.

[mallumowgli]

Originally Posted by Sutripta

I don't think designers engineer flex into their chassis. They learn to live with it, and design round what they have achieved. To say they have designed a certain amount of flex as desirable is the equivalent of turning a bug into a feature in software.

Sutripta

Every material has a certain amount of flexibility. Remember steel is one of the most elastic materials available? Materials that doesnt have flexibility tends to be brittle and break.

Now designers can decide how much of this flexibility they need for the type of car being designed. For a sports car, the flexibility needs to be reduced (or stiffness needs to be increased) by changes in design(type of chassis), materials used etc

So, as you said, flex is not designed into the chassis, but is an inherent quality. But, it is also not bad, since it helps in absorbing the shocks. Or rather this is one of the factors that helps in absorbing shocks

Praveen

[ramzsys]

I am speaking from intuition, but of all the cars i have driven, the ikon is the one which feels to have a stiff chassis and shell- a feel of stiffness(not indestructible like the palio). Ikon's suspension is also stiff(tight) but ride quality is still better.

[Sutripta]

Originally Posted by mallumowgli

So, as you said, flex is not designed into the chassis, but is an inherent quality. But, it is also not bad, since it helps in absorbing the shocks. Or rather this is one of the factors that helps in absorbing shocks

Praveen

Beg to differ.
From the performance oriented drivers point of view, a nonrigid car is difficult to setup and does not handle well.
From the nonperformance oriented drivers point of view, a nonrigid car does not age well, with numerous squeaks and rattles showing up very early in its life.

The figure of merit, the chassis engineers holy grail is torsional rigidity. And I think (and can be totally wrong) the most important constraint (beating cost/ weight) is packaging. You cannot have struts and braces running through the passenger space! Where that is not a constraint, check out the space frame/ roll cage in the passenger compartment.

Regards
Sutripta

[FlatOut]

Originally Posted by king_julian

Hi,

A question that keeps popping into my mind - Does the chassis itself become less stiff as a car puts on more kilometers?

A car that has a lakh of kilometers under it's belt just doesn't seem as stiff as it used to be, even with regular suspension work, regular tightening of chassis nuts, etc. It could also be due to the tolerance of the fasteners becoming looser after so many bumps.

Thanks,
Amit.

Metal fatigue and weld deterioration can begin to reduce the strength of a chassis and body. Micro-cracks and popping of spot welds, for example. Which in turn may stress other areas of the chassis.

However, there is the unmentioned business of steel losing its elasticity. This may only occur in the most stressed areas and can create a 'deadness' of feel to a vehicle when it has worked hard over many miles.

When this occurs in the elements which contribute to cornering, it is most obvious with a very dull feeling as loads are applied through fast bends, especially if the vehicle is loaded.

Originally Posted by Sankar

Not actually. Flex is sometimes desirable and is engineered into the design based on the application. Not just automobiles but motorcycles too. Sometimes flex is a result of economics.

Its better to flex than crack :-)

Very true, just as a tree only survives high winds through the strength of its rooting system and the ability of the branches to bend. Modern monocoques use rubber everywhere to allow for movement where necessary, but it is very important to minimise the concentration of stress in steel. Many modern designs are more about how they work on a racetrack (the Top Gear effect) than in the real world but computer-aided-design does eliminate almost all weak spots before a design is put into production.

A good example of an over-stiff chassis is the original Land-Rover long wheelbase model, which routinely snaps its halfshafts when negotiating difficult terrain.

The strongest piece of automotive design I have ever used is the small, ancient Citroën twin cylinder car which some think of as very flimsy due to its lightweight design. However, since it was designed by top-class engineers for very poor farmers, they envisaged all types of abuse and neglect, the result being something which very few people could break even if driven off-road at high speed, unless badly corroded in the chassis. Some of the engineering on this car owes more to race-car technology - it is quite an unbelievable machine.

Originally Posted by SS-Traveller

Isn't chassis stiffness also about weight as well as cost?
The stiffer the chassis, the heavier it would be (which is where monocoque shells win - they're stiffer but lighter than chassis-on-body construction, kg for kg).

A clever design will be strong and light. As any aircraft or bridge engineer knows - if it isn't strong, it will collapse under load, if it isn't light enough it will collapse under its own weight/never get off the ground!

`

[ptaneja]

Jeeez !!
Keeping technical jargon aside, can someone please list out the points I should point out to MASS or my service center guy to take a look at and repair/reinforce or do something about in my ageing 1,25,000kms driven swift ?!

[Shreyas Hande]

Car roofs have hydroformed steel tubes spot welded on to the pillars to increase roof strength. Does anyone know how much do those hydroformed tubes cost ?