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Theory on bobblehead and fixing the bouncy seat
Theory on bobblehead and fixing the bouncy seat
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Old 01-17-08, 04:35 AM   #1
stretch
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Default Theory on bobblehead and fixing the bouncy seat

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/~bsappl...e/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.

Last edited by stretch; 01-17-08 at 09:48 AM.
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Old 01-17-08, 04:40 AM   #2
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Default Re: Theory on bobblehead and fixing the bouncy seat

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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Old 01-17-08, 04:52 AM   #3
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Default Re: Theory on bobblehead and fixing the bouncy seat

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

Last edited by RaceComp Engineering; 01-17-08 at 08:41 AM.
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Old 01-17-08, 04:53 AM   #4
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Default Re: Theory on bobblehead and fixing the bouncy seat

oh sure rub it in that you have nice seats lol
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Old 01-17-08, 04:59 AM   #5
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Default Re: Theory on bobblehead and fixing the bouncy seat

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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Old 01-17-08, 05:02 AM   #6
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Default Re: Theory on bobblehead and fixing the bouncy seat

Quote:
Originally Posted by chaos311clarity View Post
oh sure rub it in that you have nice seats lol
HAD nice seats. Gone whe I sold it last Feb 14th( yep Valentines Day)......so now I am back to the crappy Legacy spec b seats.

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Old 01-17-08, 05:08 AM   #7
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Default Re: Theory on bobblehead and fixing the bouncy seat

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" !!

Myles(in better shape now)
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Old 01-17-08, 05:18 AM   #8
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Default Re: Theory on bobblehead and fixing the bouncy seat

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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Old 01-17-08, 05:28 AM   #9
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Default Re: Theory on bobblehead and fixing the bouncy seat

i find the stock seats to be semi comfortable, but they dont hold me for **** in the turns... i usually end up bracing myself against the door and such
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Old 01-17-08, 05:38 AM   #10
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Default Re: Theory on bobblehead and fixing the bouncy seat

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

Aftermarket seats are my next purchase.
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