Mechanical derivation of Anti-dive?

[PlatinumZealot]

I don't have a book for suspension design and I never learned any of this car stuff at school. Some of you know that I am currently taking on a Race car design project in my spare time. Recently I was thinking about the locations of the control arm mounts then I decided to have a look on some Pictures of those on the Nissan GTR and I notice that the top control arm is tilted towards the rear. I read that it was for anti dive. I figured that since the arm is tilted towards to the rear and since it can only offer free movement in one axis.. Anti dive has something to do with the Control arms (which can not move along the plane of the axis) taking some Percent of the load under braking along that plane... Right?

Now I want to quantify it so I can apply it to my car.
So I went on some websites (NASCAR it seems) and they are giving all type a stuff about drawing lines and getting all sorts of things like Percentage anti dive.. That's all good and simple.. but those lines and that Percentage are not really intuitive to me

I am not used to those types of methods, I am one of those peole who have to see the derivation from scratch.. What is the proper way to approach this? (Something with numbers and forces and moments, dynamics etc) and the different implications??

[The_Man]

In the side view much like the front view we find the instant centers of the wheel motion by extending the line connecting the 2 control arm pickups from the chassis.

% anti-dive is basically the percentage of the longitudinal load transfer that is transmitted from rears to the fronts via the control arms. That is without causing the front spring to compress, very similar to the jacking forces in roll.

Why all the IC is fairly simple to imagine. The ratio of the slope of the line connecting (the wheel center and the IC) and the slope of the line connecting the CG and the contact patch determine the ratio of the load transfer of the front braking that is transmitted by the control arms. If this ratio is 1 then the pseudo force acting on the CG during braking can not cause a moment about the IC for the suspension to move or spring to compress so all load transfer happens via control arms. Similarly if the ratio is zero then there is no path for force to travel via the control arms all the force acts in to form of moment about the instant center. Anything in between these two extremes has the force braking into 2 components moment about the IC and direct force along the path of the control arms. Its now not hard to imagine negative anti dive I hope.

[RH1300S]

Honestly, if you don't have a book on suspension design and are planning on designing a car - it's a good time to get some books.

I can recommend Carroll Smith's - Engineer to Win & Tune to Win.

Also (not everyone would agree, but it's good simple and easy to understand) - Allan Staniforth - Race & Rally Car Source Book - Plus - Competition Car Suspension by the same author

Time to start reading......

[PlatinumZealot]

Ok.. Thanks.. I just figured it using some of the suggestions.

I used a slightly different reasoning though.

I just used the basics.. Most of the diagrams i saw online did not use all the forces present so that is what put me off.

[riff_raff]

n smikle,

The amount of anti-dive (or anti-squat in the rear) that you employ in your suspension, will depend upon the mass properties of your chassis. The NASCAR guys tend to use lots of anti-dive in their front A-arms due to the heavy, forward weight distribution their cars have. But a mid-engined F1 chassis, with even front-to-rear weight distribution, doesn't like much anti-dive or anti-squat in the suspension, since it tends to upset the pitch attitude and aero CofP of the car.

Good luck with the project.
Terry

[PlatinumZealot]

Yes, makes more sense now that i think about it, I just realised that the top front control arms on middle engined cars (like the new mclaren, ford GT, and mid engine Ferrairis) are parallel... and control arms on front engined cars like the Corvette and the Nissan GTR are tilted back..