Rodeo
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| posted on 29/12/11 at 07:19 PM |
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Centre of Gravity Q's
Hi All,Im a newb to the site ,cant see the newb intro area ,so thought I would dive straight in here....Apologies if this isnt the site etiquet!
Ive just completed and MSVA'd a new build Reliant engined trike with tube frame etc,but am now thinking I would like to build a 2 seater
reverse trike .Im only at the toe dipping stage at the moment. I read with interest a few of the posts on here regarding CofG . I believe below G.L.
is best according to the info on here. How do I work this out please?
Thanks
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designer
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| posted on 29/12/11 at 07:29 PM |
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quote:
I believe below G.L. is best
You better tell us what G.L. is!
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Rodeo
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| posted on 29/12/11 at 07:32 PM |
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lol! Ground Level
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JoelP
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| posted on 29/12/11 at 07:40 PM |
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think you mean roll centre re below ground level, i cant see how centre of mass can be below ground when all the mass is above ground!
Basically, CoG wants to be low, and in a reverse trike, id imagine quite forwards for stability.
Beware! Bourettes is binfectious.
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Rodeo
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| posted on 29/12/11 at 07:43 PM |
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Yes,sorry,I got the two things mixed up!
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Chippy
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| posted on 29/12/11 at 11:36 PM |
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With regard to roll centre calculation, this diagram may be of assistance. HTH Ray
[img]
 
Roll centre diag
[/img]
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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daviep
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| posted on 29/12/11 at 11:42 PM |
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quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray
[img]
Roll centre diag
[/img]
Can you explain for me please 
“A truly great library contains something in it to offend everyone.”
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Chippy
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| posted on 29/12/11 at 11:50 PM |
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Ermmm! whats to explain, thought the diagram was self explanatory. Cheers Ray
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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speedstar
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| posted on 30/12/11 at 01:16 AM |
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quote:
Originally posted by daviep
quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray
[img]
Roll centre diag
[/img]
Can you explain for me please
Take a line from each of the wishbones straight out to the side of the car. They SHOULDN'T be parallel (thats bad) and this means they will
intersect.
From the point they intersect, draw another line to the contact patch of the tyre.
Repeat for the opposite side. Where the two contact patch lines cross is your roll centre.
And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set
up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.
Regarding CofG, an IDEAL situation would be to have it at exactly that same point as your roll centre. This means you get zero roll. This has other
repercussions which can be detrimental (its never easy, is it?). If your CofG is i]below your roll centre, the car will actually roll the opposite
way you would expect (weight transfer stays the same though).
Happy planning 
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Hector.Brocklebank
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| posted on 30/12/11 at 03:41 AM |
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quote:
Originally posted by Chippy
Ermmm! whats to explain, thought the diagram was self explanatory. Cheers Ray
+1
Some people can never handle the truth and always try to shoot the messenger instead of taking an honest look in the mirror (its always easier to
blame another than to face reality), but secretly they wish they could grow a pair and be the messenger !!!
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snapper
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| posted on 30/12/11 at 08:32 AM |
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Just to add a bit of info, on a 4 wheel vehicle if the roll centre is below GL ground level then the car is more inclined to slip sideways.
It was always considered best to have the roll center a little above ground even at full suspension compression.
Well that's what I was told.
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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Rodeo
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| posted on 30/12/11 at 08:43 AM |
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Chippy and speedstar,thanks for the info
[Edited on 30/12/11 by Rodeo]
[Edited on 30/12/11 by Rodeo]
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Halemini
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| posted on 30/12/11 at 08:45 AM |
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quote:
Originally posted by speedstar
quote:
Originally posted by daviep
quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray
[img]
Roll centre diag
[/img]
Can you explain for me please
Take a line from each of the wishbones straight out to the side of the car. They SHOULDN'T be parallel (thats bad) and this means they will
intersect.
From the point they intersect, draw another line to the contact patch of the tyre.
Repeat for the opposite side. Where the two contact patch lines cross is your roll centre.
And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set
up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.
Regarding CofG, an IDEAL situation would be to have it at exactly that same point as your roll centre. This means you get zero roll. This has other
repercussions which can be detrimental (its never easy, is it?). If your CofG is i]below your roll centre, the car will actually roll the opposite
way you would expect (weight transfer stays the same though).
Happy planning
Ah 3 wheelers! Sorry Speedstar, This is very close to my heart, so I must disagree here! The reason Morgans were so successful was because of their
stability and speed! This comes from parallel cross tubes and sliding pillar front suspension!
Dc
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MikeRJ
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| posted on 30/12/11 at 10:19 AM |
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quote:
Originally posted by Halemini
Ah 3 wheelers! Sorry Speedstar, This is very close to my heart, so I must disagree here! The reason Morgans were so successful was because of their
stability and speed! This comes from parallel cross tubes and sliding pillar front suspension!
Dc
I strongly suspect any positive handling attributes they had was in spite of, rather than because of the sliding pillar system. How many modern cars
use the sliding pillar design?
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randombloke
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| posted on 30/12/11 at 11:22 AM |
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quote:
Originally posted by speedstar
quote:
Originally posted by daviep
quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray
[img]
Roll centre diag
[/img]
Can you explain for me please
Take a line from each of the wishbones straight out to the side of the car. They SHOULDN'T be parallel (thats bad) and this means they will
intersect.
From the point they intersect, draw another line to the contact patch of the tyre.
Repeat for the opposite side. Where the two contact patch lines cross is your roll centre.
And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set
up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.
Regarding CofG, an IDEAL situation would be to have it at exactly that same point as your roll centre. This means you get zero roll. This has other
repercussions which can be detrimental (its never easy, is it?). If your CofG is i]below your roll centre, the car will actually roll the opposite
way you would expect (weight transfer stays the same though).
Happy planning
The CofG is difficult to calculate but can be estimated to be somewhere in the middle of the car. This is dictated by how the vehicle is built, where
the components are mounted on the chassis and how high the chassis sits.
The vertical location of the roll centre dictates what the car does under load. With a roll centre above ground the car will squat under load whereas
a roll centre below ground level will jack the car under load (Useful for drifting). Theoretically a roll centre at ground level would be ideal.
However because the CofG will be above the ground, the car will want to roll, the further away the RC from the CofG the more it will want to roll.
Having the RC at the same point as the CofG means no roll but will have other unwanted effects on suspension geometry so a compromise is required that
puts the RC somewhere between the ground and the CofG.
What is also important is the distance from the instantaneous roll centre (Marked B on Chippy's diagram) to the wheel contact patch (A). This is
called the Swing Axel Length (SAL), short SAL gives very good geometry in corners but creates too much camber in bump and droop, long SAL is the
oppersite so you need to compromise based on what you will be using the vehicle for.
It is quite common practice to have the lower wishbone paralel to the ground. You can then decide your own compromise of SAL and RC height. Then you
just need to choose your upright and everything else falls into place. Hopefully?
Suspension is all about compromise which is why Lotus developed active suspension that just hydraulically adjusted the geometry to suit every
situation. And like everything that works well, it was promptly banned from F1 
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minitici
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| posted on 30/12/11 at 11:54 AM |
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Found this interesting article which may be of assistance?
3 wheel stability article
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speedstar
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| posted on 30/12/11 at 12:54 PM |
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quote:
Originally posted by Halemini
Ah 3 wheelers! Sorry Speedstar, This is very close to my heart, so I must disagree here! The reason Morgans were so successful was because of their
stability and speed! This comes from parallel cross tubes and sliding pillar front suspension!
Dc
What exactly do you disagree with?
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Chippy
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| posted on 30/12/11 at 11:48 PM |
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quote:
Originally posted by speedstar
And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set
up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.
Happy planning
Whilst i agree it is possable to design a suspension system with the roll centre below ground, it is generaly considered to be bad practise,
due in part to the very poor handling that this can give. It is generaly considered that the roll centre should be below the C of G and above ground
level, as this configeration will give feel as to what the car is about to do, "ie" loose traction or break away. The further that the
roll centre is from the C of G the more the car will lean in corners, adversly the closer to the C of G the less it will lean. To give an example the
SS100 Jag had its roll centre almost exactly where the C of G occured, this car had no roll in corners at all, BUT, would swop ends without the
slightest sign that it would do so, very unnerving for the driver, :-). Cheers Ray
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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Dusty
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| posted on 1/1/12 at 03:08 AM |
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Er theoreticaly if the roll centre is above the CofG the car will lift an outside wheel. Is that possible?
I do have a serious question Chippy. The diagram says choose the height of your roll centre. (By using your diagram and adjusting the geo of the
wishbones I can see how you do this) But How do you choose a roll centre. Whats the ideal height for the front roll centre in a sevenesque car?
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Chippy
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| posted on 1/1/12 at 02:05 PM |
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quote:
Originally posted by Dusty
Er theoreticaly if the roll centre is above the CofG the car will lift an outside wheel. Is that possible?
I do have a serious question Chippy. The diagram says choose the height of your roll centre. (By using your diagram and adjusting the geo of the
wishbones I can see how you do this) But How do you choose a roll centre. Whats the ideal height for the front roll centre in a sevenesque car?
I know that if the RC is above the CofG then the car will lean into the bend, not sure about if it would lift an outside wheel though, :-).
I believe that this however does not give the same indication that you are aproching the limits of grip in the way that having the RC below the CofG
does, not sure on that but apears logical to me.
With regards the RC heights for a seven type car, its pretty much up to the individual. Mine for instance is 3" at front and 4" at rear,
but then with the weight of my engine etc. the CofG is going to be in the region of 15" or 16", (by my very rough calculations). I think
though that generaly the sort of height used is 1" or 2" front and 3" or 4" rear, (rear should always be slightly higher than
the front). HTH Ray
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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randombloke
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| posted on 1/1/12 at 03:14 PM |
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quote:
Originally posted by Dusty
Er theoreticaly if the roll centre is above the CofG the car will lift an outside wheel. Is that possible?
I do have a serious question Chippy. The diagram says choose the height of your roll centre. (By using your diagram and adjusting the geo of the
wishbones I can see how you do this) But How do you choose a roll centre. Whats the ideal height for the front roll centre in a sevenesque car?
There is no simple answer to roll centre height and in fact RCH is just a consequence of SAL and ride height. With the assumption that you have chosen
your uprights and that their geometry is fixed the only way to change the RCH is changing the SAL and RH. Ride height changes the CofG, lower being
better but lower effects ground clearance, SAL changes how dynamic geometry handles cornering versus bump and droop (See my post above). So it comes
down to which compromise suits what you want from the car.
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randombloke
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| posted on 1/1/12 at 03:22 PM |
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quote:
Originally posted by Chippy I think though that generaly the sort of height used is 1" or 2" front and 3" or 4"
rear, (rear should always be slightly higher than the front). HTH Ray
A general rule of thumb the rear should be higher than the front. Having different heights allows weight transfer from the front to the back and vice
versa, which effects whether the car oversteers or understeers. The same can be achieved by using different spring rates.
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Chippy
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| posted on 1/1/12 at 10:25 PM |
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quote:
Originally posted by randombloke
There is no simple answer to roll centre height and in fact RCH is just a consequence of SAL and ride height. With the assumption that you have chosen
your uprights and that their geometry is fixed the only way to change the RCH is changing the SAL and RH. Ride height changes the CofG, lower being
better but lower effects ground clearance, SAL changes how dynamic geometry handles cornering versus bump and droop (See my post above). So it comes
down to which compromise suits what you want from the car.
Whilst I agree with you in part, this only applies if you are using a chassis that somebody else has designed, ("ie" Locost, MK, MNR etc.
etc.). Starting with a blank sheet of paper then you can make your own mind up as to where things will be, and where your pickup points will be
located. This also gives you the opertunity to adjust the length of your wishbones to give the geommetry that you require. As my chassis was home
designed I used the stick and pin principal to see where my designed RC and length of wishbones would place the wheels in regard to body roll, thereby
getting the wheels to stay somewhere in the ball park that I calculated they would perform best. This has appeared to be well founded as the car is
directionally stable, and only has a little overstear, (but that could be from a heavy right foot and 238 bhp). Cheers Ray
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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Dusty
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| posted on 2/1/12 at 02:55 PM |
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Ok, that's clearer on roll centre but chucking in swing arm lengths is a bit mean. No definitive answers to any of these questions? I take it
short medium and long all have their + and - points so I'm not going to ask that question.
Instead it looks as if the roll centre leaps up and down on suspension movement. What would you do to stop this? Having chosen where I want the little
bleeder I would like it to stay there. At least I think I would unless there is some advantage in having it move.
Would it be more helpful to put together a list of things you definitely don't want to do with suspension?
[Edited on 2/1/12 by Dusty]
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Chippy
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| posted on 2/1/12 at 04:28 PM |
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Hi Dusty, well it's pretty much impossable to not have any movement of the RC, the trick is to try to get as little as possable. Using a long
SAL is the easiest way to attain this, (reason for the length stated in my diagram), but you still need to play around with wishbone, WB, lengths to
try to get the least movement. With mine I started with a lower WB of 18" and upper at 11", and after some experimentation ended up with
16" and 10", which gave me the least movement of the RC. Really if you want to get the full SP on suspension there are some very good
books that go into it in detail, though some are very maths orientated, (not my strong suite), I found "Andre Jutes" book, "Building
Special Cars", very informative, not only for suspension but the complete design of a car. Regards Ray
To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy
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