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CoG Calculation gone wrong?

Suspension Tuning & Optimization

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Discussion and questions related to the course Suspension Tuning & Optimization

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I have what I think is a fault in my calculation. My car in low, and everything is low inside. Turbo is lowmount. Transmission is low and heavy and so on... I have done like the example in the course. I used Cornerweight scales and lifted 14,5 degrees. Solid chassie, no springs. But I get CoG at 700mm. This sounds too high.

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It seems strange, but actually makes sense if you think about it.

While the mean height of the engine, transmission, and other large masses are low, they're still going to be quite high from the ground, and so their average masses might easily be 400mm, or more, high. That would be a 300mm offset in your case, from the measured CoM.

On the other hand, there is a lot of the vehicles mass that's higher than the 700mm figure you have, and one of the big things is the glass and roof structure, that has a much larger offset above the measured value. You can thing of it as "leverage" around a pivot.

I don't know what the vehicle is, but a couple of things to remember - did you take the heights of the scales into account, was the "solid" chassis at correct ride height and what was done to represent the mass of the springs, etc? Were you using the vehicle with the wheels, etc, in place or was it a bare chassis?

There are many factors that can easily throw your figures a long way out, and I suspect this is what has happened, although many vehicle would be close to that, maybe even higher.

Even with, and without, the driver can have a massive difference - if a rally vehicle with a co-driver as well...

The car is a Z4 E86 BMW. Roof is carbonfibre. well... everthing is carbonfibre except the hood, thats oem aluminium. Rollcage ofcorse. I have whight in the drivers seat. I have the car in rideheight but have the springs switched for pipes in the right length. Race ready. Wheels are on and I lift the car in the wheels. Mabe it is right, but it seems wrong. eg people say that a normal car has CoG of aprox 400-500mm. This car is a all-in racecar. A replica of a GT3 car, but different engine and transmission.

Pic of the car here

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Sorry, took another look, it does look to be strange for a "race ready" car like that. Driver mass may need to be reconsidered for the position, but it would be better if the actual driver was sitting in the vehicle, in their race gear, as the actual mass position can't be too accurately placed.

Wheelbase figure I have is 2494mm, slightly shorter than yours, but that may be geometry/suspension changes. But as 2500mm is in the middle it makes the sums easier.

You have an angle entered of 14.8 degrees - that's rather steep and means the front was lifted on blocks by an additional 640mm, or so? Would that seem to be about right? Did you reference the "flat" angle first and that is the change, or did you just go straight off a chassis section - sorry about the "granny-eggs" thing, but working through it.

Being 'old school', I'd do the calculation with pen and paper and calculator (moved on from slide rule and "log tables"). Just got up and on my first cuppa of the day - I'll be back later with the long-hand way.

What happens if you get the tire radius correct? Pretty sure your tires are not 1.2m in diameter.

Next, if you created the maths to do this, make sure you don't really need radians as an input to a trig function. 14.8 deg = 0.258 radians.

Well spotted, David - 680mm is a common race tyre diameter!

While my maths is rusty, my (literal) back of envelope calc's suggest a CoG height from those figures of 194.5mm, which seems kinda low? Been a while so may be mis-remembering the process. Maybe a bigger sheet of paper will help?

I's suggest you go back over the weights you got, with the elevated fronts' values as well, and retry the program. I'd also reverse it with the rear elevated to allow for fuel movenent - especially if using half full tank(s) and oil/water reservoirs - and to get a double check that should be close to the first position, then average them for a more accurate real value.

I ran my calculations for those numbers (with correct wheel radius), and I get a CG of 376.68mm. If I enter a wheel radius of 680mm, then I get 715.7, pretty close to your calculation.

The reason the wheel radius makes such a difference, is the car pivots about this point, so you calculate the CG shift and add the wheel radius.

Thanks guys! Thats the thing. Diameter and radius. I was certain I had done something wrong. 376mm sounds more right. I have lifted the car quite high to get the 14 degrees. I did as in the course. I used my carlift and raised the front of the car as high as the lift could do it. I do NOT want to sit in the car as I do that. This was scary as h*ll anyways.

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