andya
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| posted on 19/10/02 at 07:47 AM |
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Front Wishbone position and caster angle
Hi All
I have 2 questions for you:-
1) In Rons book the wishbones are mounted in an horizontal plane which means that even though the front upright is leant back by 5 degrees the wheel
still moves up and down in a vertical plane.....BUT in the Avon book the wishbones are mounted lower at the back allowing the wheel to move rearward
as it rises.....
What is the point of the castor angle and which is correct if any?
2) Does it matter if the top wishbone is mounted higher on the chassis as long as the front upright is in the correct position?
My reason is purely aesthetic, I want then to be more parallel.
Cheers
Andy
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interestedparty
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| posted on 19/10/02 at 10:06 AM |
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Caster angle should be around about 4-5 degrees. In the RC book this is built into the top wishbone, can't speak for the Tiger book, haven't seen
it
Correct setup for front suspension is bottom wishbone parallel to the ground, top wishbone sloping higher at the wheel, lower at the chassis end. This
gives more desirable camber change as wheel goes up and down
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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Stu16v
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| posted on 19/10/02 at 11:59 AM |
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Both will work, the RC way is the 'conventional' way for twin wishbone arrangements, the Avon is probably easier for the home builder to control and
measure with accuracy. In fact, look under most road cars with Mcpherson strut suspension on the front, the strut leans back to give the castor
required. Yes, the wheel will move backwards slighty, but the change would be small enough as to be insignificant.
Dont just build it.....make it!
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andya
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| posted on 19/10/02 at 01:17 PM |
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Thanks for the replies but what is the point of having a caster angle if the wheel moves vertically anyway??
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interestedparty
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| posted on 19/10/02 at 02:43 PM |
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The castor angle is there mainly to provide a degree of self-centering to the steering, a car without castor would feel quite unstable and be inclined
to wander
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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stephen_gusterson
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| posted on 19/10/02 at 10:09 PM |
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you will fail sva if the cars steering does not self centre, and as 'interested' said, caster is the cause of the self centering action.
HOWEVER I have heard from other builders that you need a bit more than the book suggests on some setups to get the correct action. More like 7 deg or
so..
atb
steve
[Edited on 19/10/02 by stephen_gusterson]
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wayner
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| posted on 20/10/02 at 08:26 PM |
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Caster controls the stability and self centering rather like a shopping trolly wheel, Camber controls the footprint of the tyre, ie being slightly
negative will allow some body roll bringing the wheel more upright giving max loading of the tyre onto the tarmac. Now add in bumpsteer, king pin
inclination, brake dive and about 10 other parameters all gets a bit complicated.
Does anyone know on a 4 A arm indepedant rear what the optimum settings are for best traction and roadholding.
Also why don't we use anti roll bars rather than brick hard springs to control the front
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Stu16v
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| posted on 20/10/02 at 08:37 PM |
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On IRS, 0.5mm toe-in and 0.5mm neg camber should be in the ball park.
As for anti-roll bars, its very much a personal preference, but like you mentioned in your post its just something else to complicate things. With
just making and fitting the bars there are so many variables.....
Also beware of going too soft with the front springs, as they also have to control the dive when braking heavily, too soft and it will be sat on the
bump stops with the absence of any anti-dive in the suspension geometry.
Dont just build it.....make it!
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johnston
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| posted on 20/10/02 at 08:56 PM |
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read a few books i got off amazo at the beginnin of week if u angle the wishbones down slightly and the shock tilted bac as well then yes the whell
does go bac under bump but it wouldnt be noticable but it also gices a bit of anti dive geomoetry
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Rorty
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| posted on 21/10/02 at 12:45 AM |
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Suspension geometry
I'm not familliar with either the RC or Tiger set-up, but generally speaking, There are 2 reasons the wishbone pivot axis might be angled.
1/ As the upright rises, the top of it also moves rearwards. This has the effect of increasing the castor, which can be beneficial in reducing drag
steer (not to be confused with bump steer) when the wheel hits a large bump or pothole.
2/ As Johnston pointed out, it can also contribute to anti-dive under braking, but only if it's been acurately and properly calculated.
I would advise, under no circumstances, alter or vary the geometry in your book/plans. Moving pivot points, even by milimeters, can have disastrous
results!
Cheers, Bob.
http://www.rortydesign.com 
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Dunc
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| posted on 21/10/02 at 10:14 AM |
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Front suspension geometry is too complicated to write into one paragraph but I would be a bit worried if the front wishbones on the tiger were mounted
lower at the back. This will increase the antidive effects. Ideally in an antidive/ squat geometry all wishbones should be inclined towards the cars
centre of gravity.
As for the springs, these should be chosen for a natural frequency of 1.5Hz, something to do with what the human body is designed for and feels
comfortable with, similar to walking frequency. Going higher begins to feel uncomfortable as your teeth are rattled out. I would recommend going for
softer springs and an anti roll bar rather than just going for a stiffer spring.
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wayner
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| posted on 21/10/02 at 09:42 PM |
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Anyone recommend a good technical book on this subject.
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MK9R
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| posted on 22/10/02 at 09:13 AM |
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For my disertation at uni i designed an irs for the Formula 27. I worked with steve porter at F27 and they actually built a prototype and displayed it
at the Stoneleigh show (about 6 years ago). I designed it to incorperate 100% anti squat (car would not squat during acceleration), minimum camber
change on inside wheel and positive camber increase during cornering and fully adustable toe. The basics of the anti squat was that both wisbones
needed to point towards the centre of gravity, but this means that the upright has to be on rose joints to allow for the opposing movement of the
wheel during bump. A trailing arm is then needed to hold the wheel straight, but by making this adustable in length allows toe in/out adustments easy.
The ideal situation for the camber change is the same as on a motorbike (lean into corners) but to a leaser degree due to th profile of the tires.
That is one of the reasons why when you see the front wheels turn they lean into the corner. Its a very complex issue but with a bit of research
(can't remember names of books i used, i will look them up) you can improve things. If anyone is interested i could send them my thesis.
Does anyone rember seeing the suspension set up at stoneleigh? I was out of the scene for a while and didn't know if F27 put then design into
production. After looking at car recently i have seen they have gone for a more conventional, easier setup. It was complicated but looked very trick.
I wonder if Dax saw it?
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interestedparty
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| posted on 22/10/02 at 10:24 AM |
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quote:
Originally posted by MK9R
The basics of the anti squat was that both wisbones needed to point towards the centre of gravity, but this means that the upright has to be on rose
joints to allow for the opposing movement of the wheel during bump. A trailing arm is then needed to hold the wheel straight, but by making this
adustable in length allows toe in/out adustments easy.
A bit more explaination is required here. If a wheel is mounted on twin wishbones how can you also use a trailing arm? Surely the trailing arm, with
its pivot in a different plane to the inner wishbone pivots, is going to try to pull the wheel forward when the wheel is in its highest and lowest
positions?
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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Dunc
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| posted on 22/10/02 at 10:41 AM |
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John, I may be wrong but I'm sure the GT40 has a similar arrangement at the rear, can't remember if it was a lower wishbone or control arm and
trailing rod. As for the books, there are couple out there, A speed pro book with a picture of a 7's front suspension is quite good but the best on I
found had a picture of a Merc A class cornering. I'll need to look out the books to get the names but I'm pretty sure they have them in the CCC mag.
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Dunc
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| posted on 22/10/02 at 10:44 AM |
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MK9R, I wouldn't mind a look at your thesis. Wish I'd kept mine, Active Exhaust Noise Cancellation. A modded version would be quite useful for the
SVA noise check. Switch it on and off at will.
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MK9R
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| posted on 22/10/02 at 11:09 AM |
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It depends where you put the piviot and the length of it. You can actually position it so to give you 4 wheel steering.
There was/is a car with a similiar arrangement because i remember taking pictures of it during my research, but can't remember what it was.
Will search thesis out, hopefully i still have an electronic copy.
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interestedparty
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| posted on 22/10/02 at 12:20 PM |
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quote:
Originally posted by MK9R
It depends where you put the piviot and the length of it. You can actually position it so to give you 4 wheel steering.
Can't see it myself, not unless there is some compliance built into the suspension somewhere, cant do it with all rose joints, not unless something
is bending somewhere
Trailing arms work fine where the wishbone is replaced by a single arm, but not with wishbones. Don't need a book for that, simple geometry will
do
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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MK9R
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| posted on 22/10/02 at 01:16 PM |
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I know it works as it has been built. Have a look at this attachment showing the principles (not to scale) 
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interestedparty
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| posted on 22/10/02 at 02:55 PM |
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quote:
Originally posted by MK9R
I know it works as it has been built. Have a look at this attachment showing the principles (not to scale)
Downloaded it but don't have the software to open it, can you convert it into something else?
Will it address my point about there either being compliance in one or more of the joints (similar to the rubber mounts of the trailing arm on
Jaguars), or one or more of the rods, arms or brackets bending?
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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Dunc
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| posted on 22/10/02 at 03:17 PM |
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The dimension would have to be crucial for it to work correctly. Is the trailing arm there to allow higher powered engines while keeping the wishbones
small or primarily as a form of track rod while cornering?
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interestedparty
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| posted on 22/10/02 at 05:04 PM |
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quote:
Originally posted by Dunc
The dimension would have to be crucial for it to work correctly.
The only way it could work correctly, with nothing bending or stretching, is if the pivot for the trailing arm was in line with the two pivots of the
wishbone.
It doesn't matter how long the trailing arm is, it could be the length of the whole car, if its pivot is not in line with the wishbone pivots then
something has to bend or stretch to accommodate suspension movement. Jaguars have what is in effect wishbone suspension and they also have a trailing
arm. The trailing arm has big rubber joints to allow suspension movement.
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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MK9R
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| posted on 23/10/02 at 06:31 AM |
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quote:
Originally posted by interestedparty
Downloaded it but don't have the software to open it, can you convert it into something else?
Jpegs for you
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interestedparty
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| posted on 23/10/02 at 06:51 AM |
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Yes, it is as I thought. I don't know why it isn't obvious to others but I will try to explain-
Consider the traing arm. It is anchored to the chassis at on end, by a rose joint. This means that the movement available at the other end is
circular, with the anchored end the centre of the circle.
Add that movement to the movement of the upright and perhaps you can see that the suspension as shown simply doesn't work.
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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MK9R
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| posted on 23/10/02 at 07:05 AM |
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i'm starting to get annoyed now!!
The trialing arm is not parallel to the chassis and you get a 3d sperical motion, so the length of the trailing arm in side veiw would appear to
change in length as it moved from side to side as the wishbone tarvel up and down. This side to side movement obviously rotates the upright (toe
in/out).
If you still don't believe it works, make it from some pencils on you desk. I'm not claiming that the system i designed worked fantastically (as i
don't know, as steve porter never gave me any feed back!), but the principle does work.
[Edited on 10/23/02 by MK9R]
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