Bob C
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| posted on 12/8/03 at 11:00 AM |
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Height of centre of gravity
Hi guys,
trying to do some calcs for brake compensator valve & to estimate weight transfer under barking I need to know the hieght of the centre of
gravity. Now I know that a lot of you have built 3D solid models of the vehicle (without driver...) and have attempted nifty suspension calculations
so I'm hoping the information is "out there". I'm looking at a bike engined car with sierra based IRS.
For those who will (undoubtedly) say get a bias bar arrangement instead - that's fair enough but you have to tweak it for the road conditions -
a well sussed compensator setup does it automatically for you!
Or better still - only use it when it's dry & sunny!
Cheers
Bob C
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pbura
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| posted on 12/8/03 at 02:05 PM |
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Just the other day I read about a guy who had his measured at 9.75" as part of his SVA exam, but blast if I can find it today so you could
compare cars. 
If you've got a rolling chassis, you could weigh it for yourself. I'm waiting until the car's built up to do that for springs.
Pete B.
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Rorty
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| posted on 13/8/03 at 01:46 AM |
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Bob, have you got a couple of bathroom scales, tape measure and a jack? You can measure your CoG, rather than relying on someone else's,
possibly different/unreliable data.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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Bob C
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| posted on 13/8/03 at 09:05 AM |
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CoG
Hmm - fraid measuring/weighing won't be an option for a year or more....... 10 inches sounds lower than I'd expect - I have a plan now
though - one or two guys have logged the weight of all therir bits - if I perm that against expected installation height I should get a result - job
for lunchtime....
Cheers
PS original post, meant braking! barking does apply to us all though dunnit....
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Peteff
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| posted on 13/8/03 at 09:44 AM |
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For SVA purposes I'm sure they take the crankshaft level as the C of G in a car engined locost, but I don't think that would work in a
bike engined format as they are usually mounted higher in a cradle.
yours, Pete.
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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pbura
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| posted on 13/8/03 at 11:38 AM |
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quote:
10 inches sounds lower than I'd expect
It IS very low, probably why I remembered it. Maybe with 165/50-13 tires, a 4" ride height, no driver and an empty fuel tank. 
COGs do tend to be a little lower than the eyeball method would indicate, though.
I'm going to display my ignorance about the compensator mechanism now. This is not the same thing as a proportioning valve? Is it
adjustable?
Pete B.
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Bob C
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| posted on 13/8/03 at 12:14 PM |
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CofG
Well I whacked heights and weights of the biggest lumps into excel & got the answer 41cm. Course the biggest lump of all was me at 105kg..
Yeah rear proportioning valve thing - you can get adjustable ones off the main brake vendors - I'm still looking at lightweight willwood 2pot
calipers all round with a handbrake on the transmission (doubling as big cog for electric reverse and speedo sender) so I'll have to do some
sums to get my brakes right! Means brakes are same front and back - convenient.
Bob C
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pbura
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| posted on 13/8/03 at 03:22 PM |
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Just for a laugh:
[Edited on 13/8/03 by pbura]
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Bob C
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| posted on 14/8/03 at 11:45 AM |
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anti -dive suspension geometry
nice pic - made me think about anti- dive suspension (OK so the pic is illustrating cornering behaviour) Has anyone bothered with anti squat &
anti dive suspension? (front inner wishbone pivots pointing up a bit at the back)(back opposite).
Just wondered what it was like (I reckon the 'dive' is part of the feedback & actually quite handy...)
cheers
Bob C
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MK9R
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| posted on 14/8/03 at 12:14 PM |
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i designed one for F27 when i was at uni, a prototype was built but i don't know how it behaved . To achieve 100% anti squat and dive requires
the pivot points of the wisbones to point at the C of G, these angles really make the suspension design very complicated
Cheers Austen
RGB car number 9
www.austengreenway.co.uk
www.automatedtechnologygroup.co.uk
www.trackace.co.uk
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JoelP
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| posted on 14/8/03 at 12:22 PM |
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I was wondering just yesterday how anti squat and dive would work, seems obvious now! that would feel odd wouldn't it though, the nose not
dropping under braking? Makes for consistent steering i guess...
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MK9R
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| posted on 14/8/03 at 12:30 PM |
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lots of talk about it here
clicky click click
Cheers Austen
RGB car number 9
www.austengreenway.co.uk
www.automatedtechnologygroup.co.uk
www.trackace.co.uk
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pbura
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| posted on 14/8/03 at 01:27 PM |
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The reason I put the pic up was because the CoG appears so incredibly low at maybe 13" for that big V8 sled, and here's poor Bob with a
16" CoG in a Locost! 
I don't think the picture's very lifelike with regard to the CoG or the 50/50 apparent weight distribution. However, it came from a
very interesting site with much good info about vehicle suspension and dynamics, even though geared to Trans Am-type cars:
http://members.aol.com/sccacuda/cars/1SmthTa.html
In either anti-dive or anti-squat, you're forcing the wheels into the droop position to cancel the downward motion. There's a couple of
methods, and you can have less than 100% "anti". It's best to read as much as you can for the pros and cons.
In Tune to Win (and I'm paraphrasing here), Carroll Smith doesn't like anti-dive in light cars because it causes the front
suspension to bind, but he thinks some anti-squat is OK in a high power-to-weight situation, but no more than 20% in most cases.
Superperformance says they use both, so it's worth thinking about. The front wishbones on the Superperformance are angled very slightly for
just a partial amount of anti-dive (10% is my guess). It's hard to tell what's going on at the rear.
Pete B.
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JoelP
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| posted on 14/8/03 at 03:39 PM |
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As you brake, the weight feels like it falls onto the front wheels, but it really turns around the CoG. By aiming the wishbone pivots at the CoG the
braking force cant press down on them, so there is less or no dive. Usual WB pivots aim below the CoG so the weight is above their plane and can press
down.
Obviously you cant point the WB pivots to far otherwise their primary function would be compromised.
I think it is only suitable to cars with a low CoG, like F1 cars, which need to avoid pitch change cos they run very low.
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JoelP
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| posted on 14/8/03 at 03:43 PM |
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and if you imagine a car with huge wheels, with the entire car suspended below the wishbones, its nose would rise under braking.
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pbura
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| posted on 14/8/03 at 04:29 PM |
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quote:
and if you imagine a car with huge wheels, with the entire car suspended below the wishbones, its nose would rise under braking.
Exactly what happens at the rear end of dragsters. 
[Edited on 15/8/03 by pbura]
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pbura
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| posted on 15/8/03 at 12:05 PM |
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I deleted most of my last post because it was way, way off the mark!
We had a massive power outage last night, so I spent a portion of my slack time staring at anti-dive diagrams until I finally got the point.
With full anti-dive, with the wishbones converging on the CoG, the shifting weight from the CoG pushes against the suspension but cannot get on top of
it, to compress the springs.
This is very similar to the situation of having the roll center at the same height as the CoG. The weight transfer stays lateral to the springs, and
is expressed as a thrust, binding the suspension.
Ta dah!
Pete B.
P.S. Geez, Joel, isn't this what you said?? Please excuse, I'm doing this as relief from a crushing work deadline that's driving
me nuts!
[Edited on 15/8/03 by pbura]
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