Theory on bobblehead and fixing the bouncy seat

stretch · 01-17-2008, 04:35 AM

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.

chaos311clarity · 01-17-2008, 04:40 AM

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.

**RaceComp Engineering** · 01-17-2008, 04:52 AM

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

chaos311clarity · 01-17-2008, 04:53 AM

oh sure rub it in that you have nice seats lol

Boxer · 01-17-2008, 04:59 AM

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.

**RaceComp Engineering** · 01-17-2008, 05:02 AM

Quote:

Originally Posted by chaos311clarity

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.

Myles

**RaceComp Engineering** · 01-17-2008, 05:07 AM

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)

stretch · 01-17-2008, 05:17 AM

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.

chaos311clarity · 01-17-2008, 05:28 AM

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

05_GD7 · 01-17-2008, 05:37 AM

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

Aftermarket seats are my next purchase.

***fullerton*** · 01-17-2008, 05:44 AM

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).

How-to Cure Bobblehead for cheap

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

**RaceComp Engineering** · 01-17-2008, 05:49 AM

Quote:

Originally Posted by stretch

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,....

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

Myles

chaos311clarity · 01-17-2008, 05:51 AM

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.

**RaceComp Engineering** · 01-17-2008, 06:10 AM

Quote:

Originally Posted by chaos311clarity

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

***fullerton*** · 01-17-2008, 06:13 AM

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.

01-17-2008, 04:35 AM	#1
stretch Silver Member Car: Aspen 2005 STi Fav Mod: TurnInConcepts.com Coilovers Location: Silver Spring, MD Posts: 1,780 Join Date: Mar 2006 Trader Rating: (2)	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. This ad is not endorsed by this member. Please register or login to hide this ad. Last edited by stretch : 01-17-2008 at 09:47 AM.

01-17-2008, 04:40 AM	#2
chaos311clarity STI Guru Car: 05 CGM STi Fav Mod: grip. Location: Southeastern PA Posts: 5,544 Join Date: Aug 2006 Trader Rating: (7)	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.

01-17-2008, 04:52 AM	#3
RaceComp Engineering Authorized Vendor Car: 2008 Pinarello FP5 Fav Mod: New Underwear after a 147mph slide into turn 17 at Sebring in the rain....still got 3rd.. Location: Columbia, Maryland Posts: 2,996 Join Date: Apr 2005 Trader Rating: (10)	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 : 01-17-2008 at 08:40 AM.

01-17-2008, 04:59 AM	#5
Boxer Spec C Club Location: FL Posts: 1,467 Join Date: Jan 2004 Trader Rating: (0)	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.

01-17-2008, 05:07 AM	#7
RaceComp Engineering Authorized Vendor Car: 2008 Pinarello FP5 Fav Mod: New Underwear after a 147mph slide into turn 17 at Sebring in the rain....still got 3rd.. Location: Columbia, Maryland Posts: 2,996 Join Date: Apr 2005 Trader Rating: (10)	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)

01-17-2008, 04:53 AM	#4
chaos311clarity STI Guru Car: 05 CGM STi Fav Mod: grip. Location: Southeastern PA Posts: 5,544 Join Date: Aug 2006 Trader Rating: (7)	Re: Theory on bobblehead and fixing the bouncy seat oh sure rub it in that you have nice seats lol

01-17-2008, 05:17 AM	#8
stretch Silver Member Car: Aspen 2005 STi Fav Mod: TurnInConcepts.com Coilovers Location: Silver Spring, MD Posts: 1,780 Join Date: Mar 2006 Trader Rating: (2)	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.

01-17-2008, 05:28 AM	#9
chaos311clarity STI Guru Car: 05 CGM STi Fav Mod: grip. Location: Southeastern PA Posts: 5,544 Join Date: Aug 2006 Trader Rating: (7)	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

01-17-2008, 05:37 AM	#10
05_GD7 Spec C Club Car: 05 STi - Black Fav Mod: No Lien Location: Ontario, Canada Posts: 1,758 Join Date: Mar 2006 Trader Rating: (2)	Re: Theory on bobblehead and fixing the bouncy seat Thanks for the original post....very informative. Aftermarket seats are my next purchase.

01-17-2008, 05:44 AM	#11
*fullerton* abracious Car: AW GDF-D Fav Mod: Track time Location: Near a 100oct pump Posts: 16,207 Join Date: Oct 2005 Trader Rating: (10)	Re: Theory on bobblehead and fixing the bouncy seat 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). How-to Cure Bobblehead for cheap Maybe you could use slidewaysteve's bar idea and attach your "dampers" to it.

01-17-2008, 05:51 AM	#13
chaos311clarity STI Guru Car: 05 CGM STi Fav Mod: grip. Location: Southeastern PA Posts: 5,544 Join Date: Aug 2006 Trader Rating: (7)	Re: Theory on bobblehead and fixing the bouncy seat 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.

01-17-2008, 06:13 AM	#15
*fullerton* abracious Car: AW GDF-D Fav Mod: Track time Location: Near a 100oct pump Posts: 16,207 Join Date: Oct 2005 Trader Rating: (10)	Re: Theory on bobblehead and fixing the bouncy seat 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. This ad is not endorsed by this member. Please register or login to hide this ad.