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Thread: Centre of Gravity Height

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    Centre of Gravity Height

    Has anyone ever measured their COG height with a view to doing a brake force distribution calculation?

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    Re: Centre of Gravity Height

    Hi Keith, an intriguing question! Is this something that you calculate theoretically or physically?
    Cheers Steve

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    Re: Centre of Gravity Height

    Hello Steve,
    I was thinking physically. It involves lifting one end of the car up and measuring how much weight transfers to the other axle. So not simple to do. Need weighpads and some equipment and strategy for lifting the car safely. And a finished car of course. I've never attempted it myself but the information would enable anyone to calculate the weight transfer due to braking and therefore accurately predict the likelihood of rear lock with a particular brake configuration.
    Cheers,
    Keith

  4. #4

    Re: Centre of Gravity Height

    Keith, I suspect that nothing like this has been done before, and with your particular background and skill set, I suspect that you are likely to be the one person who can enlighten us to what your question asks?

    I for one will be intrigued to know.

    Robin.
    "Why make it easy when you can struggle"

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    SEC Member Iowstratos's Avatar
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    Re: Centre of Gravity Height

    Hi
    going by the fact that the car is about 44” off the ground, half that is 22”
    the main mass is the engine, and then driver that is lower than the chassis Centre,
    the CoG should be at about 16-18” off the road.
    Ian

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    SEC Member Griff's Avatar
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    Re: Centre of Gravity Height


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    Re: Centre of Gravity Height

    I have looked online for some COG height values and found a quoted value for a Cayman (20", 508mm) and an Elise (470mm). I think the Elise would be lower than my Hawk (smaller, lower engine) so I have started with 20" and see how it goes with the calculation. I also found some information on kerb and kerb + driver weights on the Forum (courtesy StruanR, thank you) and I've used these with some adjustment to make a first attempt at a brake force distribution calculation. And if I've uploaded it correctly the results should appear next.

    Click image for larger version. 

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    Many of you will have seen this kind of thing before. I have used a simple spreadsheet program from an old friend and brake engineer but I'm sure something similar must be available on the internet.
    For those who are not so familiar with the brake force distribution diagram, a short explanation.
    The blue line is the ideal brake force distribution characteristic where the brake force on front and rear axles is exactly matched to the vertical load on the axles and the available tyre/road friction. It is derived by calculating the vertical load due to initial static loading plus the load transfer due to deceleration and factoring that with the road mu value. It is important because if the actual installed brake force distribution of the vehicle always remains below this line the front wheels will always lock first on any homogeneous road surface from polished ice to dry tarmac.
    The orange line represents the "optimum" vehicle installed brake force distribution to give 1g deceleration on a road mu of 1.0. In practice I wouldn't want to match this line because it allows no room for variation in brake friction (between the pad and the disc) which will occur and could result in rear lock but it gives a limit to the braking ratio between front and rear brakes I might want to consider.
    The purple line is the actual installed brake force distribution I would get if I were to carry over the concept of the donor car brakes (Brembo 4-pot 42/38 fronts and 38mm rear pin sliders). It is a long way from the optimum, based as it is on a completely different vehicle layout. Its legal but the maximum deceleration is not going to be great.
    So I want to have a look at rear calipers with bigger piston areas.

    I found another piece of interesting information on the Forum, this time from Ken Tomblin where he stated the AP Works Gp4 Brake spec (Thanks Ken). That data gives the following result.

    Click image for larger version. 

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    I do not know the actual weight distribution for a Gp4 car and i don't know the actuation spec (which would have incorporated adjustable brake balance) so the result has to be viewed with caution but nevertheless its interesting. The Gp4 spec represents a Braking ratio of 57.6% Front. My "Optimum" line is 51.8% Front and the "carry-over" spec is 68.9% Front.

    So I am still asking myself how close I can safely go to the Optimum spec. This analysis is for straight line braking only. Its not for corner braking. If I have too much rear brake will that upset the cornering stability under braking? I don't know. But I can't be the first to have faced this challenge and plenty of you must have already found a solution you're happy with. Are any of you able to share your brake specification with me in order to help me decide mine? I could even collect your responses together in a data file for reference by future builders, if that hasn't already been done.

    Cheers,
    Keith

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    SEC Member Iowstratos's Avatar
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    Re: Centre of Gravity Height

    Hi
    When I upgraded my brakes on my Hawk I went on the fact that the weight distribution front to rear is 40/60 just like a F1 car.
    This give 55/45 brake forces so I fitted equal size and power brakes. For the balancing I only needed to put in a adjustable limiting valve to the rear.
    This setup works very well and tested on the dirt and track, and easy to do on any Stratos.

    For and ideal setup the rears need to grab first to reduce weight transfer from the rear, but with equal force which is hard to do.
    Ian

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    Re: Centre of Gravity Height

    Quote Originally Posted by Iowstratos View Post
    Hi
    When I upgraded my brakes on my Hawk I went on the fact that the weight distribution front to rear is 40/60 just like a F1 car.
    This give 55/45 brake forces so I fitted equal size and power brakes. For the balancing I only needed to put in a adjustable limiting valve to the rear.
    This setup works very well and tested on the dirt and track, and easy to do on any Stratos.

    For and ideal setup the rears need to grab first to reduce weight transfer from the rear, but with equal force which is hard to do.
    Ian
    Thanks Ian,
    That's just the kind of info I was looking for. Can I ask what caliper piston size you ended up with and what master cylinder set-up you use?
    Cheers, Keith

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    SEC Member Iowstratos's Avatar
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    Re: Centre of Gravity Height

    Hi Keith
    I went for HiSpec 285 Billet 4 brakes with the standard Fiat 131 Hawk setup. The rear calliper has a built in handbrake.
    Ian

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