Is the center of a car body ALWAYS the center of the chassis?

[Robert Scott]

Doing some more research here. I am finding through some cad work that the geometrical center of a car (aka the mid point between the longest points of overall length do not coincide with the geometrical center of the vehicle drive track rectangle. That seems odd in my brain but maybe that is normal?

First let me define what I mean by the drive track rectangle. Say you have a vehicle with a 95 niche wheel base and 55 inch track. If you made a rectangle on the ground with those points you would find a center of the rectangle.

Im finding in some of my cad models, in order for the wheels to be center in the wheel wells, the center of what would be the wheel base / track rectangle is not truly at the center of the body. Is that normal? Am I thinking to much into this? Seems to be off by about an inch in some cases.

 

[Michel]

Geometrical center of a car ???????????????????????????????????
There are many " center" in a car but never heard about "geometrical center "....... may be missing something !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
May I ask a question; for exactly what use in a CAD drawing would you use such point ?????
Weight distribution ?
True Akerman situation ?
King pin angle definition ?
Roll center cinematic ?

 

[Larry L.]

Am I thinking to much into this?

Yes. ;-)

 

[Robert Scott]

The short answer is I know the wheel base and track of a vehicle. And I can obviously find the center of those dimensions.
Further more I have a body shell. I’m trying to stick wheels under the body sheep but just noticed they two centers don’t line up. I am thinking to myself at this point perhaps my assumption that they would is incorrect. I just assumed the center of the wheel base would align with the center of the body. But I guess not????

the other assumption I’m making is the wheel base is symmetrical. If you had the center of a body and you knew the wheelbase that it was supposed to be is it truly supposed to be symmetrical front to back a.k.a. see you had 100 and wheelbase or 50 inches of the wheelbase supposed to be center of the body and 50 inches behind or can that be shifted back-and-forth maybe I’m not making too much sense

 

[Michel]

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[Robert Scott]

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Some elaboration might be needed here?

 

[Robert Scott]

Here is the take away! Maybe Hans can check my sanity here.
The key take away is the if you take a body and find the geometric mathematically central. For example a 100 inch long body the middle would be at 50 inches right?

The poor assumption I am making is that distance from the center defined above to the center of the front wheel wheel is not nesscerily the distance to the center of the Rear wheel well. These is nothing that says it has to be symmetric. That is the point here. And that is why with my model, I don't have to adjust the wheel base all I need to do is stop assuming that its symmetric.

Make more sense?
I will draw a picture and post back in a second

 

[Russell]

The centre of a car is exactly where the designer placed with regards to style, looks and practicalities. To confirm this theory just look and compare and old school Ford Galaxy 500 and a Fiat 500....or a mini.
You could damn near put the Fiat in the boot of the galaxy.
Another comparison on the race track would be the Mclaren`s and the Porsche`s
Time for another piece of Christmas cake.
Russell

 

[H.J.Bayer]

Robert, I completely understand your thoughts. if the two of us were redesigning a car like this, we would take that into consideration as well. But Ford built the cars within 1 year or so back then. Without CAD according to 2D drawing and a lot of manual work. When I look at my parts, there is nothing symmetrical at all. You are thinking too much here. In addition, you can also compensate for some things later when building. The GFK parts are quite flexible.

 

[Robert Scott]

Thanks Hans!

So check this out guys. You know what they say, a picture is always worth a thousand words!
So in these in picture you you can see what I call the chassis rectangle. Its just a geometric block representing the wheelbase

In this picture I made the assumption that the MK4 body center would share the same center as the chassis rectangle. I think this was just simply a poor assumption on my part. There is nothing that says those two have to align ( I think) !The numbers are NOT MK4 numbers but just something I made for a quick sketch.

So in picture one this is what I assumed to be the case.

Now for picture 2!

This is what im finding to be ACTUALLY the case of a MK4. Still fake numbers just for reference here.

As it turns out , the geometric chassis rectangle center in order for the wheel wells to align on a 95 inch wheel base that has the car at roughly 172.1 inches long has to actually be set back just a tad in order to get the wheels to align with the wheel wells. Well? Why do you ask? I think the answer is simple. The wheel wells on the body are not symmetric. They are off set a tad from the body center. As it turns out to be 1.03025 inches set back. In the below picture I use 15 inches of offset to make the point im trying to demonstrate. Please see picture two. The point is the wheel wells don't have to be symmetric with the body center. And that is what causes the slight offset. I wonder why this was done this way though.

The question I ask to myself is? Does this matter? I think not but I have learned something a bit with this experiment. This is what happens when you are divorced and have no kids. This is how you spend your Christmas.

 

[Robert Scott]

At any rate! Here is the corrected model. Im pretty happy with it. Now the next question to ask myself is???? Do I go build a virtual P 917 or do I break out CNC router

 

[H.J.Bayer]

Great work !!!
Start the CNC !

 

[BenL]

You might want to take a look at this article.
https://en.wikipedia.org/wiki/Automobile_handling

IANAengineer, but where the center of mass is located relative to the wheels is important. Also, where the engine and other heavy items are located relative to the center of mass and the wheels is also important.

 

[Tom]

As you've discovered, geometric center of the body does not necessarily coincide with center of the chassis. As above, designers contended with packaging mass items (engine, trans, driver) in the chassis, and try to establish good weight distribution. Driver position dictates where the visual mass lies in a closed coupe. The rest was designed to look good, and at least try to establish reasonable lift/down force distribution. These things are better understood now, 50+ years later, so designers look at aero in tandem with centers of mass, and work hard to keep those maximized and synchronized so the car doesn't pick up aero push or oversteer, while maintaining efficient cooling. Look at how much kerfuffle there was about team Racing Point using Mercedes front brake ducts in their F1 car, and that's a tiny (but important) piece of the aero puzzle.

 

[Terry Oxandale]

As an example, look at a McLaren M8 where there is a very short rear overhang, but almost 2 feet of front body overhang. Compare that to a Cobra, which as a lot of rear overhang, and very little front overhang. There is really no relationship, either by wheelbase, or overall body length.

 

[Robert Scott]

Yep! Totally agreed