PeterGT4
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| posted on 12/6/04 at 04:08 AM |
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Centre of Gravity height...
I've searched the archives here under different combinations of the above statement and have found lots of good info, but one thing I'm
not clear with, is where exactly the CG height is and where its measured from. I've seen numerous and widely varied measurements mentioned (
9" to 16" ) but to what point is that based on? From the ground up to? From the axle centre-line up to? Is it split exactly inbetween the
wheelbase, or the distance arrived at from the %front/rear weight distribution along the wheelbase (I'm thinking its that)? Is there an average
height that it's at (common among a few Locost's [the ones that use saloon-based, tweeked 4-cyl, Type 9 tranny, etc...])?
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Cita
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| posted on 12/6/04 at 06:36 AM |
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Good question Peter.
IMHO there can only be one point wich is the CG and this is the point where the object would be in perfect balance in space.
So in a car i would assume that the reference line heigth of the CG would be measured from the ground but nobody can prevent you from using the
wheelcenters or propshaft as the reference line.
So as i said...a good question!
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Hellfire
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| posted on 12/6/04 at 09:21 AM |
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I understand that it's generated from the axle base - i.e. distance from fornt to back axle's. The axle loads are generated by weighing.
This gives triangulation with which you can calculate how much weight is applied to each axle therefore the centre of gravity. Our's calculated
out 350mm from the ground almost inline with the seats.
As a BEC the weight distribution is nearer 50/50 than a CEC as I understand it.
But I'm not a Physics Graduate so I could be proved wrong. It could probably be better explained with the aid of diagrams.
Hope it helped.
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stephen_gusterson
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| posted on 12/6/04 at 09:35 AM |
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why would you need to know this?
A locost is a fairly 'simple' car to build, to a tried and tested formula.....
avoid complications.
atb
steve
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MikeP
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| posted on 12/6/04 at 02:29 PM |
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Hi Peter - yes, the CoG will be at a point along the wheelbase and between the track using the weight distribution taken from the corner weights just
as you mention.
The height of that point from the ground is a little trickier. Basically you raise one end of the car, find the difference in the weights at the
other end and apply some trig to sort it (there's a spreadsheet on the web somewhere, google should find it).
Puhn in his book suggests using the wheel center height as a good estimate for sports cars. Another way is to estimate the CoG of each part of the
car, and calculate it with a big table that uses the position of each of them.
Most locost weights that I've found on the web (including mine) come in at 50/50 or even 49/51 (F/R) when you include the driver.
[Edited on 12/6/04 by MikeP]
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Bob C
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| posted on 12/6/04 at 04:42 PM |
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I asked same question a while ago, got no satisfactory answer. So I made a table of major components their heights and weights (note I was the biggest
of these[BEC]) & let XL do the donkey work. This is where at least one 16" figure came from.
Bob C
PS you need to know for brakes rear proportioning valve calculations.
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Hellfire
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| posted on 12/6/04 at 09:40 PM |
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quote:
Originally posted by Bob C
PS you need to know for brakes rear proportioning valve calculations.
The SVA used our COG calculation together with axle brake forces to calculate whether the rears would lock before the fronts. This is why they need
the calculation.
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stephen_gusterson
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| posted on 12/6/04 at 10:45 PM |
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i was just gonna adjust the rear limiting valve to suit at mot time, pre sva...........
but then, my cars got abs, so might not be needed.....
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JoelP
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| posted on 12/6/04 at 11:52 PM |
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my CoG is about 2 to 4 inches infront of the gearstick, and probably about 14 inches off the ground. and maybe an inch to the left or right depending
on whether i am in the car or not.
at a guess, anyway.
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Peteff
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| posted on 13/6/04 at 12:00 AM |
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The SVA machine does it all for them, they don't need your calculations. I don't have any brake proportioning, just Fords own formula and
the tester said the balance was ideal for the car. The pedal force calculations at various pressures were checked front and rear with the foot
attachment to determine the lock up point. According to the tester they take the crankshaft as the COG.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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PeterGT4
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| posted on 13/6/04 at 07:23 PM |
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Thanks for the replies...
The reason I asked is I'd like to integrate some form of anti-dive & anti-squat (and believe me, I've searched through the archives
about those and I'm very aware of the pros and cons. I'd like to have this as I will race this car... Don't want to get the Ferrari
dirty with cone-rash!). I suppose my best bet is to guess where the CG is (based on many posts of locations) and just aim the suspension arms towards
a point below it, to keep some compliance.
I am aware that these cars are great pretty much right-out-of-the-box, I'd just like mine to be a wee bit better
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dozracing
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| posted on 15/6/04 at 04:52 PM |
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Just for a laugh you could try this method of finding the CofG height. It'll be more accurate than the balancing method, cos that gives you
different values for different heights that you lift an axle off the ground, but some might consider it a little harder to do.
Basically tie each wheel to a rope and then each rope together above the car, connect this to a single rope and suspend car from ceiling, say
12" off ground. Then remembering your school physics you rock the car such that it starts to swing. You measure the time period for a complete
swing, and using the formula for a pendulum you will get a distance from the pivot point at the top, the distance will relate to the CofG height.
Would love to have a workshop big enough to try it and see how well it works.
Darren
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Bob C
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| posted on 15/6/04 at 05:01 PM |
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Like the way you're thinking Darren...
You'd have to swing sideways as front/back would give slightly low reading due to length of car,
I'll get me coat...
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jcduroc
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| posted on 15/6/04 at 09:47 PM |
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quote:
Originally posted by dozracing
Just for a laugh you could try this method of finding the CofG height. It'll be more accurate than the balancing method, cos that gives you
different values for different heights that you lift an axle off the ground, but some might consider it a little harder to do...
Darren
Nice try Darren
Seriously though, have a look at the (horrible) attached pic.
A) Put the car at ride height, in level position, on 4 scales
1. and read all of them.
Wfl is left front weight
Wfr is right front weight
Wrl is left rear weight
Wrr is right rear weight
2. Wf is front axle weight = Wfl + Wfr
Wr is rear axle weight = Wrl + Wrr
Wt is car's total weight = Wf + Wr
L is the wheelbase
3. Longitudinal location of the CoG
a = distance from CoG to front axle
b = distance from CoG to rear axle
4. Moments about rear axle:
Wt * b = Wf * L -> b = Wf/Wt * L
About front axle
Wt * a = Wr * L -> a = Wr/Wt * L
Note 1: Weight distribution Front/Rear Wf/Wr is equivalent to b/a.
Note 2: We disregarded the fact that the CoG might not be on the centre plane of the car.
B) Now:
1. Replace coilovers with some rogid links
2. Put the in level position with the rear wheels on scales
3. Raise the front of the car with the wheels on block stands or with a hoist in a way that the front is supported on the wheels
Note: block the rear wheels so the car does not move.
Take the readings of the rear wheels on the scales.
Let DW = Difference of rear weight
X = height of the plane the front wheels are standing
CoGheight = RearWheelCrushedRadius + DW/Wt * L/X * squareroot(L^2-X^2)
Simple isn't it?
Joćo
[Edited on 21/6/04 by jcduroc]
 
Rescued attachment Img_0103.jpg
JCM
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Bob C
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| posted on 21/6/04 at 10:21 AM |
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Much much much easier....
put loads of pressure in the tyres then tip the car sideways till it balances. Note angle. Do the same on the other side. Draw picture, measure height
of COG. Brew up for the lads who helped...
Bob C
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Bob C
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| posted on 21/6/04 at 10:22 AM |
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PS put sacks of spuds in the drivers seat to pretend to be you.....
Bob C
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Mark Allanson
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| posted on 21/6/04 at 05:45 PM |
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...or 4 sacks of spuds in my case!
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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