Suspension frequencies

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fastback33
fastback33
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Joined: Wed Aug 29, 2007 7:45 am

Suspension frequencies

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Do they matter? In the context as long as the front is less than the rear. I'll be honest i don't know that much about it, however Carrol Smith in "tune to win" says they don't matter. He is fairly well renowned engineer and i'd like to know what you guys think.

RH1300S
RH1300S
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Joined: Mon Jun 06, 2005 2:29 pm

Re: Suspension frequencies

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Of course they matter :D - getting the right frequency is very important, after all it's a function of the wheel rate against the weight it's carrying (which includes aero load).

I think what Carroll Smith was saying was not to get too hung up about frequencies - in particular, the idea that the "wrong" frequencies front/rear can set up an undesirable reaction over bumps. His reasons were that whatever the frequencies, the tuning of the front/rear dampers effectively confuses the final result.

Jersey Tom
Jersey Tom
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Re: Suspension frequencies

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Higher rear ride frequency will help settle the car in the pitch mode after hitting a bump. Given how stiffly sprung aero cars are however, I would imagine that pitching dampens out VERY quickly, and in the grand scheme of things probably isn't critical.

As with anything, it depends on the application.

Regarding "what people say," don't follow --- just because Smith or Milliken or Rouelle say so. There is no gospel to this stuff, and no one ever has the complete picture. Gotta prove it out to yourself, regardless of what anyone else says.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

fastback33
fastback33
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Re: Suspension frequencies

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Great! Now, do you guys have a website (besides this one) where i can read more about it? :D

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Carlos
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Location: Canada

Re: Suspension frequencies

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General considerations
http://www.rqriley.com/suspensn.htm
Bit more targeted to frequency
http://www.smithees-racetech.com.au/per ... ctors.html

Try that search thingy ...

Krispy
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Re: Suspension frequencies

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Jersey Tom wrote:
As with anything, it depends on the application.
Tom, Do you recall what general ballpark values you ran for frequencies back in your FSAE days?

1-2 hz?

15-20 hz?

I have found that I like low single digit frequencies , higher in the rear than in the front, however I am led to believe that this is a function of your wheelbase when considering bumps and timing of disturbances between the front and rear suspensions.
"In order to finish first, you must first finish"-Stirling Moss

Jersey Tom
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Re: Suspension frequencies

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15-20Hz is.. a lot.

There are ballpark values in RCVD for what to expect in anything from a sedan to a high downforce open-wheeler.

I think we were in the low single digits at most. Anything above that on a no-downforce car and the car is gonna ride like a brick.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

fastback33
fastback33
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Re: Suspension frequencies

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Okay, so i've done some reading. Now, when is the correct time to decide what i want my spring frequencies to be used? I understand the frequencies are related to wheel rate and suspension geometry, i just don't exactly know how these are all connect or how to put this all together with the rest of the car. Can someone care to explain?

Jersey Tom
Jersey Tom
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Re: Suspension frequencies

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It's an iterative process.

Might start by thinking you want a ride rate of 1 Hz (or 2, 3, whatever). Knowing the vehicle mass and F/R split, that instantly defines all your wheel rates (for a symmetric car anyway). With that, packaging constraints, and physical constraints of what springs and dampers are actually available you can simultaneously set your installation ratio and spring rate.

That gets you started.

Then you go back, add ARBs and modify springs to get your right roll stiffness distribution... check to make sure the chassis won't bottom out under braking... take the car to a 7-post maybe you go up or down in springrate based on that... take it to a track, go up or down based on how sharp or lazy the chassis is to react...

There's definitely gains to be had by playing with spring and tire rates, unsprung mass, etc to pull tricks with your transmissibility curves in the suspension. Track to track and corner to corner youll have varying frequency content you can aim to deaden out. Mass dampers and inerters probably work quite well to that end.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

RH1300S
RH1300S
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Re: Suspension frequencies

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A good read is also - Race and Rally Car Source Book: The Guide to Building or Modifying a Competition Car by Allan Staniforth. Also Competition Car Suspension by the same author. (try amazon)

This explains frequencies very well (he uses CPM - cycles per minute), but the idea is the same. Also he gives you real world comparisons (i.e. what is a typical saloon car, rally car, single seater without downforce, single seater with downforce etc.)

Read this and you WILL have an understanding of what it's all about.

Belatti
Belatti
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Re: Suspension frequencies

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Mass damper has a lot do to with this.
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Carlos
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Joined: Sat Sep 02, 2006 6:43 pm
Location: Canada

Re: Suspension frequencies

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This worksheet might help. Wheel Rate. Suspension Frequency. Motion Ratio & other calculations.From Eibach Spring.

http://eibach.com/eibach/img/ers-14susp ... ksheet.pdf

The_Man
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Re: Suspension frequencies

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Suspension frequencies is something that catches my eye many times. Ever since I've been reading about suspension design it is always said it is very important etc.
Most references including Milliken, Gellispie and Carroll Smith directly talk about frequency ranges. Race car they should be between 2-3Hz, lesser for sedans,etc. They quote numbers and say this is the range they come from usually so you must follow the same. I've never come across anything that explains these numbers. Where do these numbers really come from, are the completely based on experience or is there some math behind it?

MattF1
MattF1
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Re: Suspension frequencies

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Is suspension frequency the same as the resonant frequency of the system?

Jersey Tom
Jersey Tom
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Re: Suspension frequencies

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The_Man wrote:Most references including Milliken, Gellispie and Carroll Smith directly talk about frequency ranges. Race car they should be between 2-3Hz, lesser for sedans,etc. They quote numbers and say this is the range they come from usually so you must follow the same.
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.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.