cymtriks
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| posted on 19/12/03 at 12:51 PM |
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Antidive and antisquat
I have seen percentages quoted for the amount of antidive or squat in a suspension. Please could someone explain how a percentage can be calculated
from a given suspension geometry. I suppose this will be a diagram type explanation or possibly one of you knows of a book that explans this properly.
My copy of Staniforths books are very light on this issue, no mention of percentages at all.
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timf
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| posted on 19/12/03 at 01:00 PM |
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no idea
i understood to get anti dive the geometry of the suspension when extended back intersects the cog of the car if that is the case you either have 100%
as it intersects or none as it doesn't.
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Matthew_1
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| posted on 19/12/03 at 02:54 PM |
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As I understand it to get 100% anti-dive/squat you'd need to angle the chassis pickups but quite a fair amount - in the models I have the front
would need to be about 15deg. This would cause fairly severe camber/castor changes which negate the advantages of anti-dive.
So people reduce the angle, e.g. if you needed 20degrees to get true anti-dive, and you actually used 5 degree inclination, you'd have 25%
anti-dive.
On a lotus 7 type chassis, which has low centre of gravity and low roll centres you don't really need full anti-dive. My chassis has 1.5
degrees inclination on the lower pickups and 1.25 on the upper, which gives about 10% anti-dive - same on rear for anti-squat.
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pbura
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| posted on 19/12/03 at 03:49 PM |
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Just pulled out my copy of Tune to Win by Carroll Smith for a refresher...
In 100% anti-either, lines drawn through the suspension pickup points will intersect at a line drawn from the tire contact point through the CoG.
For a lesser percentage, say 10%, a line drawn from the contact patch to where the lines through the pickups intersect will itself intersect a
vertical line from the CoG to the ground, at 10% of the height of the CoG from the ground. (Wish I had a picture)
Too much "anti" can cause the suspension to bind and not handle bumps well. Smith says that no anti-dive is needed in light cars such as
F1 and sports racing cars, and that anti-squat is OK in a high power-to-weight situation, with about 20% being the maximum practical amount.
Superformance apparently has both anti-dive and anti-squat, but not much from the pictures
I've seen.
I like the idea of a little "anti" at both ends as I plan to use fairly soft springs, with a small anti-roll bar at the front. I will be
pretty conservative in this, because the CoG location will be just a guess.
Pete
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jcduroc
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| posted on 19/12/03 at 08:29 PM |
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Humblingly sharing my little knowledge...
quote:
Originally posted by pbura
... For a lesser percentage, say 10%, a line drawn from the contact patch to where the lines through the pickups intersect will itself intersect a
vertical line from the CoG to the ground, at 10% of the height of the CoG from the ground. (Wish I had a picture)
...
N.B.: This post reply is due to my consideration for the requester (Cymtrick) and the last replier (Pete) which I quote; for an explanation see
http://www.locostbuilders.co.uk/viewthread.php?tid=9398
To have anti-dive your front wishbones rear pivots must be closer than the front ones. Let's imagine the lower bone chassis axis is horizontal
and the top one leans backwards (for instance rear pivot is 5 mm lower than the front one.
If we draw lines (in side view) trough both pivots of each wishbone respectively they'll intersect somewhere, usually much further back than the
rear axle. Now draw a line from this Instant Centre to the tire contact point; the height of the IC divided by the length is (AWAK) the tangent of the
Side IC angle.
As this is long I'll go over the deductions and say that if you multiply this tangent by the wheelbase divided by the CoG height and multiply by
the Braking Front % you'll get what is called "Brake anti-dive %".
Some example:
Side view IC height = 143
Side view IC length = 9530
Side view IC angle = 0.86º
Tangent = 0.015
Wheelbase = 2286
CoG height = 305
Front braking force = 70%
Anti-dive = 7.9%
Cheers
João
[Edited on 19/12/03 by jcduroc]
JCM
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Bob C
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| posted on 20/12/03 at 02:52 PM |
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I tried to estimate the height of my projected CofG a while back to determine what my brake bias should be, I got 16" off the deck in a BECn
This is not far from the height of the top wishbones.
I'm not bothering with anti-dive/squat, reckon there won't be much & what there is gives useful "driver feedback"!
Cheers
Bob
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