Ivan
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| posted on 28/4/07 at 08:38 AM |
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Anti Dive and brakes
Hi
My brain is on holiday this weekend so rather than think it through I thought I would get someone else to think for me  so here goes:
If anti-squat improves traction does anti-dive similarly improve braking?
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blakep82
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| posted on 28/4/07 at 08:57 AM |
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as far as i know (which could be wrong)
anti dive is the same kind of thing as anti squat, but for for braking. it stops to front of the car dropping down under braking
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gottabedone
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| posted on 28/4/07 at 09:42 AM |
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Hi, Blakep82 is right, it is where suspension geometry is used to convert the forward dive motion of the body into a vertical movement. The anti
squat is the same on the rear - the suspension geometry converts the squatting of the rear under acceleration into vertical force that raises the
rear.
hope this helps
Steve
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Ivan
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| posted on 28/4/07 at 11:30 AM |
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So from the above if the body is forced to rise on braking the reaction will be to force the wheels down increasing braking ability.
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JoelP
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| posted on 28/4/07 at 03:59 PM |
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thats not the way i see it, you cant change the amount of weight on the wheels. All you can do is prevent (reduce) the nose rising or falling and thus
maintain better geometry and contact.
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tks
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| posted on 28/4/07 at 04:08 PM |
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thats not totally true...
if a car dives his centre of gravity moves forward..
and thus increases the weight on the front wheels...
while accelerating the weight trasfers to the rear... thats the reason that a FWD cant accelerate as fast as a RWD.....
Tks
The above comments are always meant to be from the above persons perspective.
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britishtrident
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| posted on 28/4/07 at 05:45 PM |
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Anti-dive is only to be used with caution -- it can cause the suspension to go solid under braking cause wheel patter. It also has major effects on
steering geometry --ie castor and bump steer and toe in
Anti squat is useful in small amounts ---
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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novacaine
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| posted on 28/4/07 at 05:57 PM |
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surely having the front end of the car dive a little must be benificial? as was said previously the weight transfers onto the front wheels, increasing
the grip, increasing braking ability and making the turning better because the front wheels have more grip?
anyone care to enlighten me?
Matt
And you run and you run to catch up with the sun but its sinking, Racing around to come up behind you again, the sun is the same in a relative way but
your older, shorter of breath and one day closer to death
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tks
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| posted on 28/4/07 at 06:09 PM |
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I didn't say its favourable i only say that it happens...
if the weight transfers it means you loose grip on the rear wheels...
Tks
The above comments are always meant to be from the above persons perspective.
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Ivan
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| posted on 28/4/07 at 08:06 PM |
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The question remains - do you stop quicker with anti-dive or without it.
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gottabedone
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| posted on 28/4/07 at 10:00 PM |
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when you brake and you car dives, to a point this is beneficial as you have good contact between the tyre and road (largely also affcted by camber)
but as braking forces cause the front to dive they also lessen the forces on the rear. This decrease in the forces pushing the rear tyres onto the
ground will effectively decrease the effectiveness of the rear brakes (rear wheel lockup). This is more controlable than front wheel lockup but not
ideal in any car.
Britishtrident is right in saying that it is to be used with caution but part of the reason for anti dive is so that the suspension is allowed to
continue to do it's work in absorbing bumps etc during braking rather than the shocks and springs compressing, going solid while the front end
dives and the tyres bounce from bump to bump causing a skid.
regards
Steve
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Lippoman
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| posted on 30/4/07 at 06:11 AM |
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Anti Dive
Anti-dive does not change the weight transfer, weight transfer is a function of the location of CoG (vertical and horisontal) in relation to the
wheels. Anti-dive only "carries" a portion of the weight during braking instead of deplacing the springs, similar to anti roll bars only
resisting body roll, not weight transfer to the outside wheel.
Both anti-dive and ARBs have unwanted side effects, so their use/dimensioning should be made with care.
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NS Dev
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| posted on 30/4/07 at 12:09 PM |
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I would personally avoid very much anti-dive.
I don't know all the techy wording for this, but there are two things:
1) Dive shifts weight forwards helping braking (as has already been said)
2) Retaining compliance in the front suspension when braking reduces the tendency to lock wheels. Anti-dive can under some circumstances cause wheel
locking on bumpy surfaces, when no anti-dive leaves wheels able to be properly controlled by dampers etc.
Having dampers that respond differently in two wheel bump to single wheel bump, and even roll, would be an interesting development, but out of our
budgets!!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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britishtrident
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| posted on 30/4/07 at 05:23 PM |
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As has been said already Anti dive dosen't change weight tranfer -- it can't re-write old Newtons laws.
With anti-dive the only difference is that some of the weight transfer is taken by the wishbone pivots not the spring or damper.
Some dive has some advantages --- for example it generates aerodynamic down force
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Johan
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| posted on 1/5/07 at 05:08 PM |
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(my way of looking at it)
Lateral weight transfer:
The distance between CoG and rollcenter (RC) determines how much of the weight transfer is taken by the springs (causing roll), and how much is taken
"stiff" by the wishbones. (If the CoG is a distance 1 from the ground and the RC is 1/3 from the ground, 2/3 of the wieght transfer is
taken by the springs, the rest goes stiff trough the wishbones)
The same reasoning can be made for longitudal weight transfer: The % of "anti" determines how much is taken through the stiff suspension
components.
I guess that 100% anti is not good because sudden changes in weight transfer cause sudden load changes on the wheel that can break traction.
Springs/dampers will smooth things out.
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britishtrident
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| posted on 1/5/07 at 07:49 PM |
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Anti-squat first became fashionable in the late 1960s. Jaguar took the plunge introduced a little anti-squat on the on the original Mk1 XJ6 it worked
fairly well. Since them most softly sprung luxury RWD saloons have had anti-squat, and increasingly some anti-dive on this type of vehicle it works
well.
Always with an eye to developments Chapman dictated that the Lotus 72 design were to have generous ammounts of both anti-squat, both were removed
pretty quickly at the drivers request.
Look at modern F1 cars, they pull 3g but can you see any noticeable anti-dive inclination of the front wishbones ?
Anti-dive has a big hidden disadvantage that anti-squat dosen't suffer from ---- it plays havoc with the steering geometry particularly caster,
bump and roll steer.
[Edited on 1/5/07 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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dnmalc
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| posted on 1/5/07 at 09:19 PM |
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During braking the front tyre contact patch becomes the point through which the braking force is applied which can be represented as a moment about
the C of G plus an applied force that represents the general retardation. In normal suspensions the moment causes the front suspension to dip but as
the retardation forces is along the line of the wishbone pivots there is no other vertical loads applied to the front suspension as a result of the
retardation forces.
For anti dive suspensions the same load is applied to front supspension as a resut of the moment, but the retardation force is now a force that is
applied at an angle to the wishbone pivots and therefore can be resolved into a force along the pivot as well as a force perpendicular to it. This
perpendicular force resists the load that results from the moment. Therefore on an anti dive suspension the load on the front is less and hence at the
extreme allows better braking due to the systems ability to follow the bumps in the road.
Note however that as the frictional force between the tyre and the road is generally related to the applied load, anti dive systems will have less
grip in the same circumstance and therefore more likely to skid.
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Johan
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| posted on 2/5/07 at 09:01 PM |
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quote:
Originally posted by dnmalc
Therefore on an anti dive suspension the load on the front is less ...
Unfortunately this is not correct. (As has been said before) for a given retardation/acceleration the amount of weight transfer ONLY depends on the
wheelbase and the CoG height: weight tranfer = mass*cgh/wheelbase
What anti-dive can affect is how much of the weight transfer that compresses the springs and how much goes directly to the chassis via the stiff
suspension components.
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dnmalc
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| posted on 3/5/07 at 09:57 PM |
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Johan
what you have in your equation is the load transfer that results from the moment created by the moment about the C of G. If what you have indicated
was all that was concerned there would be no forces acting to prevent the front dipping.
The whole point about an anti dive system is that the wishbones are inclined and when you consider the applied force they produce a counter load .
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Ivan
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| posted on 4/5/07 at 07:04 AM |
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I said
quote:
Originally posted by Ivan
Hi
My brain is on holiday this weekend so rather than think it through I thought I would get someone else to think for me so here goes:
If anti-squat improves traction does anti-dive similarly improve braking?
After all the realy interesting debate it appears that I will have to switch on my brain again some-time and try and make sense of it all - the
interesting thing is that most agree that the result of anti dive and anti squat is (other than opposing body movement) the exact opposite in the
impact on traction at the affected wheels - anti squat improves traction - anti dive reduces it. (this is where my brain is tempted to switch off
again as it's all just too confusing)
Thanks all and see if you can convince me and each other as to why it happens so.
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Johan
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| posted on 4/5/07 at 02:31 PM |
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quote:
Originally posted by dnmalc
Johan
what you have in your equation is the load transfer that results from the moment created by the moment about the C of G. If what you have indicated
was all that was concerned there would be no forces acting to prevent the front dipping.
The whole point about an anti dive system is that the wishbones are inclined and when you consider the applied force they produce a counter load .
What I want to point out is that if you break with a certain retardation this will result in a weight transfer (depending only on wheelbase and CoG
hight) that will have to be carried by front wheels contact patches. No antidive can make this weight transfer smaller.
EDIT: dnmalc: What you describe is correct for the "system" of the car from the contact patch and upwards. By angling the pivot points of
the wishbones the springs are unloaded because the pivot points take up some of the forces, but the sum at the contact patch is constant, anti-dive or
not
[Edited on 5/5/07 by Johan]
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sebastiaan
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| posted on 11/5/07 at 02:51 PM |
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quote:
Originally posted by Johan
What I want to point out is that if you break with a certain retardation this will result in a weight transfer (depending only on wheelbase and CoG
hight) that will have to be carried by front wheels contact patches. No antidive can make this weight transfer smaller.
[Edited on 5/5/07 by Johan]
This is not true. Weight transfer can be divided into two parts:
1) weight transfer due to the deceleration and thus the moment around the center of gravity (as pointed out earlier). Anti-dive does NOT influence
this.
2) weight transfer due to the different rake angle of the vehicle. This part of the weight transfer (often called geometric weight transfer) can be
reduced by anti-dive. Small experiment: take a piece of straight square tubing you just *might* have kicking about ;-) and lift it at one end. When
it's just off the ground, you're lifting half its weight. If you lift this one end higher of the ground, you're lifting LESS then
half the weight. This is called geometric weight transfer.
Will it reduce the distance you need to stop? In a perfect world, yes. But you'd need to have your brake balance set up really well to take
advantage from the increased braking possibilities of the rear wheels. (and yes, you can brake harder if the weight is more evenly distributed over
the tires. Traction is NOT directly proportional to the weigth on one tire)
personally, I wouldn't bother on a road car, our cars are often sprung hard enough to almost avoid geometric weight transfer. On a softly sprung
road car (Citroen, anyone?) it can be nice to have, and in most citroens it is incorporated by having trailing arms at the back that squat under
braking due to the moment of the rear brakes....
Hope this helps.
Sebastiaan
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kb58
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| posted on 11/5/07 at 03:24 PM |
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It depends on your camber curves. If you have a lot of camber compensation, as the nose drops, the tires camber in and you end up riding on the edge
of your tires. In that case, yes, in a straight line, anti-dive would help. BUT, if you're in a turn, the effectively stiffened suspension,
it'll cause more understeer. Sorry, there is no black-and-white answer to this.
And regarding some earlier comments, suspension design has nothing to do with weight transfer. The same force moves forwards under braking regardless
of anti-this or that. Oh sure, your CG cants forward maybe 1/8" but that's not going to measurably matter.
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NS Dev
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| posted on 11/5/07 at 04:42 PM |
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Heh heh all this brings me back to the lively anti-squat debates on here!!!
Whatever the science of the matter (all of which I understand and will not argue with! ) I could always get faster starts from a standing start in my
grasser using PRO-squat geometry, not anti-squat!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Anti-squat was great on a dry, flat track, but on anything with
bumps and water on it, pro-squat worked better...................................
Regarding anti-dive:
Am I being stupid (I don't think so!!! ) in my assumption that you are effectively jacking up the front of the car under braking, by
transferring a proportion of the longitudinal force generated by the contact patch and brakes, into a load acting along the control
arm/wishbone/tension strut/compression strut etc etc etc...................................
In which case, you are therefore effectively reducing the load seen by the spring and damper, hence the lack of dive for the same weight
transfer..............................
In which case, how is this good for braking???????????????? We know the overall weight applied to the contact patch does not change, that is dictated
by the weight of the vehicle, the c of g and the deceleration.
Which means that instead of applying all that load through a nice spring and damper combination, you are now applying a percentage of it through a
rigid metal strut with no compliance whatsoever, which means that any undulation in the road surface is MORE likely to cause lockup and skidding as
the load at the contact patch will vary more rapidly and in a less controlled fashion........................
which may well be why F1 don't use lots of anti-squat!!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Bob C
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| posted on 11/5/07 at 10:53 PM |
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Don't think so Nat - if you pivoted your wishbones on ballraces (perfect ones of course with no friction) you'd still get anti- dive but
nothing's locking up, it's not moving because the force is perpendicular to the available movement. So the suspension would still work for
bumps :^)
Bob
PS I never considered anti dive or squat for my car - the low CoG does the job nicely, even with very low rate springs!
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