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Discussion Starter #1 (Edited)
I think I might be on to something. But be warned, this post is going to involve some math talk.

Transmissibility of a damped spring system looks like this (also pictured below):
http://personal.cityu.edu.hk/~bsapplec/Fire/Image486.gif

But note that the X axis of that graph is the ratio of F to Fn. When dealing with a bouncing seat, those two frequencies are the frequency of your car's suspension (determined by your springs) and the seat at which your butt actually oscillates on the seat. Yes, your seat has springs in it just like your chassis, even though they look different. As you can see, the ratio of these two spring rates is VERY important.

Well, we know that a stock STI has front and rear spring frequency of 1.6hz and 1.8hz, respectively. But what about the seat? The "bobblehead" as we've come to call it is a pretty fast oscillation, probably in that vicinity or just a little higher. We also know that the seat is poorly damped because your butt will bounce on the seat several times before settling. That means that the transmissibility of bumps through the seat will follow somewhere between the blue and green lines in the image below:

(source)

Notice that- especially at low damping- transmissibility varies greatly according to this frequency ratio. The X-axis is a ratio of the car's spring frequency divided by the seat's spring frequency. Transmissibility peaks when they are the same frequency, indicated by a "1.0" on the X axis. So, the question I have is: what is the spring frequency of a person sitting on the stock STI seat? In other words, during "bobblehead", how many times per second is your head bobbling?

My theory is that your body bounces on the seat around 1.6-2hz, roughly the same frequency as the chassis. This is the worst case scenario. According to this chart, when this happens your seat is magnifying the bumps sent to the chassis and making your ride uncomfortable. The seat won't begin to settle until the chassis settles, after which the bobblehead will decay according to how well damped the seat is (not much).

If this theory is true, then moving to much softer or much stiffer spring on either the car or the seat would alleviate the problem to a varying degree. So would gaining or losing a drastic amount of weight, since the weight of the person sitting in the seat will change its natural frequency.

So, there are several things worth experimenting with to try and fix bobblehead:

1) Simply remove the spring from the seat. Fullerton did this in a roundabout way (discussion) and liked the results. This would result in a transmissibility of 1:1 since there is no flex (except for the seat's foam). At least some racing seats are like this, but racing seats probably do this to save weight.

2) Drastically raise the vehicle spring rate. Many do this and report decreased bobblehead, but putting firmer springs in one's car will create a different kind of ride harshness. I've run firmer springs and don't really think it does enough to eliminate bobblehead, so I don't really like this solution. I think to make a difference you'd have to increase your spring frequency by at least sqrt(2) times to push us past that 1:1 transmissibility point, which would be really stiff. You're talking about 600lb/in or higher main springs. At that point the seat might behave, but your chassis is going to feel like a rock.

3) Install a damper on the seat springs. Yes, this is very possible- some trucker seats have 'em. I've actually looked into building one and would encourage a vendor with the resources to try it! If I were to do this, I'd use the Ace Controls adjustable hydrolic shock absorber part # HB-22-50. It's 5in long with 2in of stroke and seems about the right strength. Doing this would put us on the pink or green line in the graph above.

4) Lower the spring frequency of the seat. Attach multiple bungee cords beneath the seat to pull the seat springs downward. The idea isn't to pull the main spring (although many will like the feeling of sitting further in the seat), but to effectively lower the seat spring frequency since we now have a load pulling in the opposite direction. This would shift where we are on that graph to the right towards low transmissibility.

Since option #4 is so cheap and easy to try, I'm going to do it.
 

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this makes a lot of sense to me.

options 1 and 4 seem like the most logical to me. 3 sounds good but for the cost of the damper... it might be worth it to replace the seat anyway.

in the northeast this bobblehead issue is really freaking annoying... i go over bumps every day that launch me out of the seat.

keep us updated.
 

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I have been talking about how huge the role the OEM 04-07 WRX and STi seats plays in bobble head since May of 2003 when I bought my first STi !!
Yeah some have removed seat springs and or purchased aftermarket seats and commented on how much of an improvment it is, but MOST dont realize how much of their ride quality and or lack of it is caused by the OEM seat.

So to me options 1 and 4 would be my choice( had I not bought Recaro SPeeds!!).

Good thread.

Myles
 

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Ha..yeah, stretch, this has been a known issue for a while but haven't actually seen any numbers to verify it. I used the "hand sandwich" test, not exactly sound science.

Anyway, I'm liking #4 as well but am also considered that altering the seat springs might lead to "funny" (for lack of a better term) fitting seats.
 

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Dont forget to factor in size of RUMP and weight. My size 38 (250lbs)fits just barely "between" the main supports of the seat frame and allows for rumpage to use seat springs, where as when I weighed 309 a year ago( yep) my size 44 plus just sat ON TOP of the seat and it almost didnt matter. When I was 309 I probably was on the theoretical seat "BUMP STOPS" !!:lol:

Myles(in better shape now)
 

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Discussion Starter #8
A heavier person would have the same effect as #4, it'd lower the frequency of the seat. Are the seats more comfortable for heavier folks or ligher folks (IGNORING lateral support, a different issue)? That's actually a pretty important question.

For once, I can't talk about bump stops Myles, haha.
 

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Thanks for the original post....very informative.

Aftermarket seats are my next purchase.
 

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Stretch- Someone (slidewaysteve) already tried a version that is between your idea and mine. While they did remove the springs from the equation (my idea), they lowered the seating height also (your idea...kinda).

http://www.iwsti.com/forums/1658432-post66.html


Maybe you could use slidewaysteve's bar idea and attach your "dampers" to it.
 

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A heavier person would have the same effect as #4, it'd lower the frequency of the seat. Are the seats more comfortable for heavier folks or ligher folks (IGNORING lateral support, a different issue)? That's actually a pretty important question.

For once, I can't talk about bump stops Myles, haha.
Trust me thats a good thing NOT to be able to talk about !!

I would say a very lite weight person would to some degree be sitting "on top" of the seat. Say like Mike ( my employee) who weighs like 146 lbs,.that the OEM seat is FIRM for him and while my lard ass on the other end of the spectrum( in comparison) pushes the OEM seat DOWN far more into its SOA intended " all Americans are fat" design range. So yeah depending on weight you are getting MORE of a bounce than say a drag bike tester( Mike) aka MR WKA Northeast karting champ!!

HEavier guys are prob pushing the envelope of the OEM seat springs so they "might" be getting no flex or movement...BUT I dont think so. I still remember the feeling of SEAT bounce in all these STI's I drive and track. As I get lighter though.....I will say I think i "feel" it get worse. So at some point this has to cross over. MEaning there has to be an "ideal" weight for the seat to NOT be bouncy( or at least the least bouncy).

Hope that made sense. I havent eaten yet today,....:rofl:

And since I CAN NOT eat this ..I will leave you guys with a very relative picture.



Myles
 

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i agree myles... i feel like the seat definitely still bounces... its like a trampoline.. when i go over a bump and the car raises up and the springs in the seat launch me at the ceiling, then when i come back down that effect decreases rapidly.
The place that this is WORSE is on the NJ turnpike from exit 5 to exit 7. There is a rimple in the road that makes these cars just bounce , bounce, bounce. CONSTANTLY. Its probably under the asphault from YEARS ago when it was concrete blocks. But as you said......as the car reacts , you react, the seat reacts THEN, it starts all over. STOCK shocks are the worse with stock springs on that stretch of road. I did my first JDMpinks comparison there and later others found that area and couldnt believe that the road was that bad.

Same thing in maryland on RT 97 going NORTH after you pass under RT 100. You want to test the seat theory someplace?..THATS the place for us local MD guys. Same thing as in NJ on the turnpike..constant bounce.

Myles
 

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That has nothing on 355 in the west suburbs of chicago. From 88 to 290, that road is bounce after bounce.

I can't imagine having boobs on that stretch.
 

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The place that this is WORSE is on the NJ turnpike from exit 5 to exit 7. There is a rimple in the road that makes these cars just bounce , bounce, bounce. CONSTANTLY. Its probably under the asphault from YEARS ago when it was concrete blocks. But as you said......as the car reacts , you react, the seat reacts THEN, it starts all over. STOCK shocks are the worse with stock springs on that stretch of road. I did my first JDMpinks comparison there and later others found that area and couldnt believe that the road was that bad.

Same thing in maryland on RT 97 going NORTH after you pass under RT 100. You want to test the seat theory someplace?..THATS the place for us local MD guys. Same thing as in NJ on the turnpike..constant bounce.

Myles

i know that section of 97 actually... and 202 between malvern and king of prussia up here is the same way as the turnpike discription. wavy lol
 

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I'm interested in #4. This would be my last stop before new seats. I have a sore left knee the day after track and autox because i'm bracing myself using the dead pedal so much.

That has nothing on 355 in the west suburbs of chicago. From 88 to 290, that road is bounce after bounce.

I can't imagine having boobs on that stretch.
It's fun to have a set in the passenger seat. It's hard to keep your eyes on the road though. Bobblehead is now officially a liablility!
 

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Discussion Starter #18
Stretch- Someone (slidewaysteve) already tried a version that is between your idea and mine. While they did remove the springs from the equation (my idea), they lowered the seating height also (your idea...kinda).
That's not really what I had in mind for #4. The goal is not to pull the springs down- it's just a byproduct of my idea. The real idea is to reduce the spring frequency by adding a load that pulls in the opposite direction. This will still allow the seat to have a spring motion, except the seat will think it has a 400lb man in it instead of a 200lb man. This lowers the spring frequency of the seat which I hope will make it more comfortable.
 
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