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Old 17th October 2013, 14:21   #1
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Default 50:50 Weight Distribution: Marketing gimmick or the real deal?

BMW cars have always had the reputation of being the ultimate driving machine. One of the factors that enhance this image is the design & weight distribution of their cars - many models have close to 50:50 front/back ratio.

BMW is obviously very proud of this, and you can see it being mentioned in the press, in the brochures & on their website:

50:50 Weight Distribution: Marketing gimmick or the real deal?-bmw.jpg

Now I understand the physics behind the benefits of 50:50 weight distribution. On my drive back in my Civic, I think I had the perfect combination of luggage weight at the back, and passengers weight in the cabin. And I really could "feel" the difference in stability/handling, so to speak.

My somewhat frivolous question is this:

When BMW makes a new car, it might have 50:50 weight distribution.

But when its actually on the road, being used normal people, the weight distribution on the front and rear axles depend on:

1) Amount of fuel in the tank
2) Weight of the driver.
3) Number of passengers
4) Seating position of the passengers
5) Weight of the passengers
6) Amount, weight & positioning of the luggage in the boot.

All these are variables, and 99 out of 100 cars on the road will never have close to purported weight distribution mentioned in brochures. So what's the point? Isn't it all marketing gimmick or am I missing something here?
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Old 17th October 2013, 15:23   #2
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Default re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Quote:
Originally Posted by smartcat View Post

1) Amount of fuel in the tank
2) Weight of the driver.
3) Number of passengers
4) Seating position of the passengers
5) Weight of the passengers
6) Amount, weight & positioning of the luggage in the boot.
60 liters ( 0.8 kg /liter) = 48 Kgs
All of the rest are dependent on the driver or the passengers.

A car weighs so much overall, and that is distributed – let’s assume for the sake of argument, equally - between front and rear. When the driver gets on the brakes, the total remains the same, but the effective – or dynamic – distribution, alters. Weight is effectively shifted to the front wheels but since the total remains the same, the rear effectively weighs less. Since there is less to press the rear tyres to the road, they will slide more easily, but this is not the same thing as saying a lighter car would therefore slide more easily at both ends... The point is that the balance of the car has changed and it is the change that makes the difference in the car’s behaviour. There is also the point that the weight has been lifted from the rear wheels but they are still receiving a similar amount of braking effort (forget ABS just for the moment). This makes them more likely to lock, which is akin to yanking on the handbrake.*
*http://www.evo.co.uk/trackdays/theor...s_effects.html


Is it a question of the car being perfectly well balanced in the first place versus a car which does not have perfect weight distribution to begin with.The other car besides bmw which featured the 50:50 weight distribution is the Honda S2000. a lot of sports cars often have a more rearward weight distribution.I Wouldn't be qualified to say if it was a gimmick

Last edited by abishek2222 : 17th October 2013 at 15:31.
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Old 18th October 2013, 14:27   #3
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Interesting thread, and very good point here :

Quote:
Originally Posted by smartcat View Post
When BMW makes a new car, it might have 50:50 weight distribution. But when its actually on the road, being used normal people, the weight distribution on the front and rear axles depend on:
Personally, i think as long as the car is within a 45/55 split (either which way), things wont change much.

I think once you start pushing to a more skewed split, that's when certain 'characteristics' become more pronounced; whether they are good or bad could be a personal preference.

Here's a list i put together just for reference:
(In ascending order)
Lotus Exige : 38/62
Porsche 997 C2 : 38/62
Porsche 959 : 40/60
Mclaren F1 : 42/58
Porsche 991 C2 : 42/58
Lambo LP560-4 : 43/57
Ferrari F430 : 43/57
Porsche Cayman : 45/55
F12 Berlinetta : 46/54
Mazda MX5 : 51/49
I just compiled the list randomly thinking of good cars. You can see that we have what are probably the 2 most well renowned cars for handling (the MX5 and the Exige) on either ends of the "performance cars i thought of" list.

Another thing to note: A majority of cars in the list are rear biased (but that is also because most are mid/rear engined). That aside, i can't imagine a car with anything more than 60% at the front being a great handler... but then again, i haven't driven the Focus RS...
Ford Focus RS : 63/37
cya
R

Last edited by Rehaan : 18th October 2013 at 14:30.
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Old 18th October 2013, 14:58   #4
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Quote:
Originally Posted by Rehaan View Post

Personally, i think as long as the car is within a 45/55 split (either which way), things wont change much.
Guess so.

Quote:
I think once you start pushing to a more skewed split, that's when certain 'characteristics' become more pronounced.
Quote:
Another thing to note: A majority of cars in the list are rear biased (but that is also because most are mid/rear engined).
Is understeer/oversteer bias of a car one such characteristic? Could there be a link between front/rear axle weight distribution and understeer/oversteer bias of a car? The general perception though is -

FWD/AWD + Power = Understeer and
RWD + Power = Oversteer.

A powerful car that oversteers on the limit generally sounds more fun than a powerful car that understeers on the limit. So I can imagine why a sports car company would design a car that would oversteer when pushed. And that's why perhaps most of the cars in the list have a rearward weight bias (along with RWD)?
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Old 18th October 2013, 16:09   #5
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Quote:
Originally Posted by smartcat View Post
So I can imagine why a sports car company would design a car that would oversteer when pushed. And that's why perhaps most of the cars in the list have a rearward weight bias (along with RWD)?
Were interesting tread.

I think Over-steer would be associated with Gripper front and loose Back-end.
While Under-steer would be associated with loose Front and Gripper Back-end.

Gripper front - dose not depend only on the Weight Bias, and depends on many factors. (Aerodynamic, Load Transfer, Lateral forces while cornering or breaking etc.) Hence most sports car are Rearward Weight Biased.

Gripper Back-end - depends mostly on the Weight Bias and passenger or luggage weight.

Hence for over-steer, companies would have more balanced (44-55) Weight Bias. So that under breaking or while cornering the front become Gripper while the rare would be looser.

I am not qualified to say if the Weight Bias is a marketing gimmick or essential for a passenger car.

But definitely Over steer cars are fun to drive.

ps. i am not an Automobile Engineer, nor a track Racer. These are just my views.
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Old 18th October 2013, 20:59   #6
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Quote:
Originally Posted by smartcat View Post
So what's the point? Isn't it all marketing gimmick or am I missing something here?
This is a topic of discussion that's been going on for a very long time. Some folks call the 50:50 weight distribution feature as more hype than actual benefit. An analogy of hype in another area - alloys technically improve handling by reducing unsprung mass whereas in reality you do not feel the difference.

And then again if you look at Formula 1 cars they have between 55-65% on the rear wheel and they are arguably the best handling cars in the world.

Having said that, other conditions remaining normal, the 50:50 weight distribution will improve handling with just 1 up or 2 up with minimal luggage etc. Manufacturers do not usually expect owners to test handling while on a family holiday with full luggage etc at which time the 50:50 weight distribution would not hold good.

You will find handling varying according to load conditions in every car. This is why it is considered a good practice to distribute weight (luggage + persons) evenly as far as is possible - for instance if you have 3 people traveling, you will find an improvement in handling if you place some luggage in the seat next to the third person rather than in the boot.
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Old 18th October 2013, 22:07   #7
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Smile Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

I do not think it as a gimmick, cars with different weight distributions behave very differently!

The dynamic weight distribution would also depend on the suspension design than just the static axle load distribution.

The lower moment of inertia of the car will help agility (time to change direction) , and is probably why BMW's have their engines pushed right against the firewall / bulkhead.

For instance take almost any petrol vs diesel in the same chassis, they are remarkably different to drive.

The petrol Vento / Rapid is much more agile than the diesel! Weight difference is only 60 kg, but because it is entirely on the front axle it makes a big difference.


The difference of one/two passenger's in the middle of the car between two axles do not affect the car's balance much, typically (80/1200 KG = 6 %).

But a fully loaded car will drastically change the handling, as weight is also on and behind the rear axle (assuming boot is loaded).

I think when BMW claim 50-50 weight distribution it is with some fluids!

It would not make sense for them to design 50-50 weight distribution for an empty fuel tank, as they do crazy things like put the battery in the boot!
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Old 19th October 2013, 07:45   #8
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Manufacturers engineer cars using ballast weights to simulate intended loads during testing.
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Old 19th October 2013, 08:40   #9
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

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Originally Posted by Diesel_convert View Post
The lower moment of inertia of the car will help agility (time to change direction)
Lower moment of inertia will help acceleration not agility. It is the Polar moment of inertia that helps agility. A larger polar moment of inertia will make the car handle better.
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Old 19th October 2013, 09:40   #10
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Quote:
Originally Posted by VeyronSuperSprt View Post

Lower moment of inertia will help acceleration not agility. It is the Polar moment of inertia that helps agility. A larger polar moment of inertia will make the car handle better.
Sorry sir, but as far as I know they are not interchangeable.

A lower moment of inertia about the pivot axis will make the car more agile.


Polar moment is in units of m^4 vs N-m^2 for mass moment

Polar moment is for calculating torsion and mass moment for angular acceleration

or agility as we call it :-)
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Old 19th October 2013, 10:37   #11
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Quote:
Originally Posted by Diesel_convert View Post
Sorry sir, but as far as I know they are not interchangeable.

A lower moment of inertia about the pivot axis will make the car more agile.


Polar moment is in units of m^4 vs N-m^2 for mass moment

Polar moment is for calculating torsion and mass moment for angular acceleration

or agility as we call it :-)
Polar moment of inertia is for calculating resistance to torsion and not to calculate torsion. In other words, this will help the cat roll less while cornering thereby making it handle better. It is moment of inertia which is angular acceleration.
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Old 19th October 2013, 10:58   #12
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

50:50 weight distribution which BMW claims is without any payload in it, and I personally feel (After thorough Test Drive) it makes a noticeable difference, majority of cars in India are 60:40 due to their front wheel setup which causes some amount of Oversteer when driven enthusiastically.

And in cars with front engine and rear wheel drive this effect is even more exaggerated and handles like a dream.
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Old 19th October 2013, 11:43   #13
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Quote:
Originally Posted by VeyronSuperSprt View Post

Polar moment of inertia is for calculating resistance to torsion and not to calculate torsion. In other words, this will help the cat roll less while cornering thereby making it handle better. It is moment of inertia which is angular acceleration.
Car rolling is dominated by suspension stiffness and mass moment of inertia in roll axis.

Torsional stiffness of the chassis has a very small role to play. This is affected by the Polar moment of inertia.

The pivot axis I was talking about is the yaw axis.

For a given steering input/reaction from the tires, the angular acceleration is mostly a function of the mass moment of inertia of the car about the yaw axis.

I agree that rolling of the car affects this, mostly because it changes the weight distribution amongst the tires and the reaction from the tires will change as a result.
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Old 20th October 2013, 11:57   #14
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

Quote:
Originally Posted by smartcat View Post
...weight distribution of their cars - many models have close to 50:50 front/back ratio.
My somewhat frivolous question is this:
...on the road, being used normal people, the weight distribution on the front and rear axles...
All these are variables...
So what's the point? Isn't it all marketing gimmick or am I missing something here?
You didn't take into account the weight distribution between left and right sides, did you? Neither did BMW. And what about the gyroscopic effect of the direction of spin of the engine, and the effect that has on handling? I don't presume any manufacturer discusses that either.

I'll decline to participate in the maths contest.

Last edited by SS-Traveller : 20th October 2013 at 11:58.
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Old 20th October 2013, 12:16   #15
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Default Re: 50:50 Weight Distribution: Marketing gimmick or the real deal?

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You didn't take into account the weight distribution between left and right sides, did you?
I did. That's what I mentioned the number of passengers and where they are seated, and whether they look like Laurel or Hardy.

Some of the cars solve this issue to some extent with the help of electronics I think. Sending power to one wheel, braking another wheel during cornering etc. And you can bet that the marketing team has a cool sounding abbreviation for that technology (like PDK or ESC or something like that!)
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