Suspension frequencies

[The_Man]

Noooo, no no no. Funny enough I ran into Doug Milliken today.

The term "suspension frequency" is kind of vague. The thing you're talking about is the sprung mass natural frequency. It is one natural frequency of a vehicle. Really just a measure of how heavily sprung a car is. High downforce open wheelers tend to migrate to very stiff setups to control to movement of the chassis (and aero elements), which in turn results in a higher natural rate.

The numbers in RCVD are representative of a broad range of vehicles and where things tend to end up, but by no means do you HAVE to be in that range.

Funny enough again this was one of the questions I forgot to ask amongst many that I asked 'Doug' when I met him personally last year.

Yes, i was talking about Sprung mass natural frequencies.

Now if the natural frequency a measure of how heavy your car(to be precise sprung mass) is then i can understand why lighter car would have higher frequencies than heavier one(the root m factor). Also for race car I'd want a higher wheel rate so higher frequency. However, the question then is why is it so important? When I do quick spreadsheet calculations for a suspension design for a FS car I design it so that I get the desired wheel travel in roll and a reasonable roll gradient, I always end up with a 3.3Hz+ frequency which makes me feel I've gotten something wrong somewhere. So not I guess the light FS cars and me liking stiffer suspensions explains these natural frequencies.

[Jersey Tom]

Like I said, there's nothing wrong with having sprung mass freq's higher than the "guidelines."

The last FS suspension I designed, when we tuned the car the natural frequency wound up being around 3 Hz, but it was a little over sprung. You really don't need THAT much stiffness in the car.

With FS... a big deciding factor in your springing is preventing the chassis from bottoming out under braking, since the wheelbase is so short. That's what drove us to such a high wheel rate.

There's nothing magical about the frequency in and of itself though. That's just where setups TEND to migrate as a balance of mechanical and aero grip.

(That is, until you get into ride dynamics and play with your transmissibility over a given frequency range, and how masses, spring and damper rates all play into that)

[fastback33]

One more question. Yaw is lateral movement correct? Does it also include front dive and rear squat?

[The_Man]

One more question. Yaw is lateral movement correct? Does it also include front dive and rear squat?

Yaw is the movement about the vertical axis(normal to the road surface)..
No it will not include dive or squat. That is pitch movement.

[RH1300S]

One more question. Yaw is lateral movement correct? Does it also include front dive and rear squat?

Not lateral movement - it's a rotation

Imagine sticking a 3 skewers through a car to define its rotation

A skewer pushed through the side and the car would rotate around the skewer nose up & down - that is Pitch

A skewer pushed down its length and the car would rotate (viewed from front or rear) in a side to side rocking motion - that is Roll

A skewer from the top down and the car would rotate around that point - that is Yaw

These axis converge at the centre of gravity - which is the point the car will move around in Pitch, Roll & Yaw

[Conceptual]

One more question. Yaw is lateral movement correct? Does it also include front dive and rear squat?

Not lateral movement - it's a rotation

Imagine sticking a 3 skewers through a car to define its rotation

A skewer pushed through the side and the car would rotate around the skewer nose up & down - that is Pitch

A skewer pushed down its length and the car would rotate (viewed from front or rear) in a side to side rocking motion - that is Roll

A skewer from the top down and the car would rotate around that point - that is Yaw

These axis converge at the centre of gravity - which is the point the car will move around in Pitch, Roll & Yaw

Wow, I have read some of the books mentioned in this thread, but this single post explains these things better than any book that I have read so far.

Thanks, this was probably the most simple to understand post that I have seen on here!

[riff_raff]

Suspension frequencies are only important from the standpoint that you don't want a condition where your front and rear suspension natural frequency modes may coincide and cause a coupled critical condition.

Race suspensions all have things called "dampeners", whose primary purpose is to absorb the kinetic energy and thus dampen the oscillations of the unsprung masses. The suspension energy is absorbed by forcing a hydraulic fluid through a restricting orifice, converting the kinetic energy to heat(thermal energy)in the hydraulic fluid.

[Shredcheddar]

Good point there by Riff Raff, that the idea of a front and rear suspension frequency is used to make sure the pitching mode is settled down quickly (this is typically done with a rear wheel frequency that is about 20% higher than the front wheel frequency).

It's also important, as has been mentioned, for ride comfort in passenger cars. If you *REALLY* want to understand suspension frequencies, pick up The Shock Absorber Handbook and read the first three chapters. I'm reading through it now and it is very nicely covered with plenty of supporting maths if you like that stuff.