# Inertia in HDV file

#### seamount0

Hi , may I ask if you can give me some info about "Inertia" in HDV file ? As an example, how to read those figures: HDV=Inertia=(649.7,681.5,127.3) ?
what the meaning of the 3 figures ?

#### muzarati

AFAIK.... Inertia for the vehicle as in the movement of the vehicle forces or the tendency to (rock up and down, spin left and right, roll over) in that order.... like I said...AFAIK

#### decpau

Muzarati is right, it's the primary weight transfer code for the car in the sim. The numbers are weight moment arms of force (I believe in Kg/m2) in the X axis-Front to back, Y axis-Side to side, and Z axis-Up and down. You can see that the Z axis is always pretty low because any force has to only overcome the weight of the car (there is no friction involved from the tires in contact with the road). This can be handy to look at when setting up the spring weights on your car.
So in your example, X axis is 649 Kg/m2. That is 1430 lbs/m2. SWAG math puts that at @ 1400 lbs/yd2 or 155 lbs/ft2 or 65 Kg/10cm. So under max acceleration or braking, you car needs let's say 155lbs (65-70kg) of spring weight over and above car weight spring settings to just soak up the inertia of that movement both front and back. You really need a bit more because in the Y axis the inertia numbers are higher at 681 in your example above. So this is pretty much the minimum spring weight, over and above the car weight at rest, (paying attention to the car weight distribution front and back) for your spring settings. Otherwise your car suspension will be hitting the travel stops before you loose frictional tire grip and you always want to find that limit with some suspension affect still remaining without bottoming the car.
Before anyone thinks I'm too smart, or just BS'ing; there are a lot of things that aren't clear in these .hdv files that I'm not clear on either. In this case for inertia, the transfer has to be tied to a specific force applied (acceleration, braking, lateral G or lateral lift like going over a hill). And I'm not sure how that force is coded. All I know is that when playing with the car setups in a car I've never played with before, I look at car weight, fuel weight, the car inertia numbers and the fuel tank inertia numbers and work out some basic spring settings. Sometimes the math works, sometimes it doesn't. When it doesn't it's usually because of the tire grip values. But it always seems to get me close. Then you get to start working on dampers, roll bars, ride height, aero, etc...LOL

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