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Sway bar spring rates

Old 10-25-13, 11:02 PM
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Sway bar spring rates

I've been curious as to how much spring rate my sway bar has been adding on my FC, so I went out to my garage and made some measurements and put them into
Fred Phun's formula Fred Phun's formula
:

Code:
                           500,000 D^4
K (lbs/in) =  -------------------------------------
               (0.4244 x A^2 x B) + (0.2264 x C^3)


               B
       _________________
A|    /                 \  C
 |   /                   \

A - Length of end perpendicular to B (torque arm - inches)
B - Length of center section (inches)
C - Length of end (inches)
D - Diameter bar (inches)
My (rough) measurements of my 27mm Whiteline sway bar:

A= 7.75"
B = 36"
C = 8.5"
D = 1.06" (27mm)

which gave me ~600 lbs/in

motion ratio = 7.5"/12" = 0.625 (measurement to sway bar and measurement to ball joint)

so when you multiply 600 lbs/in by 0.625^2 to get the wheel rate you only get ~234 lbs/in which isn't as much as I was expecting....

but I thought it was interesting. just thought I'd share.
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Old 10-25-13, 11:08 PM
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You need to get the wheel movement for accurate motion ratio, not the ball joint. Different offset wheels will even affect the calculation.
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Old 10-25-13, 11:17 PM
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Originally Posted by jgrewe View Post
You need to get the wheel movement for accurate motion ratio, not the ball joint. Different offset wheels will even affect the calculation.
Yeah... but if anything that'll make it even softer.
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Old 10-25-13, 11:50 PM
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...and this is why spring rates are the main ingredient, and bar rates are just seasoning.

Absolute numbers for bars don't really matter because as long as you bars with the same material and construction, you can just go by diameter or the length of the ends.

Unless you're designing a car from scratch, that is.
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Old 10-25-13, 11:53 PM
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The main reason I did the calculations are because I've been toying with the idea of building a speedway style bar, or getting rid of the bar all together...
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Old 10-26-13, 12:04 AM
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Some cars will be faster without one of the bars, and just using higher spring rates.

My race car builder friend swears that AE86s are faster without the rear sway bar. Of course, he's using a rear spring rate of like, 14 kilos.

You are better off playing the the spring rates before you start going nuts building adjustable bars, unless you've already dialed your springs and shocks in. In which case, do it!

Also, unhook the links and see how it drives. It'll probably wallow like a pig.
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Old 10-26-13, 10:33 AM
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One thing a lot of people don't think about is that a sway bar is another spring to figure in the shock damping equation.

The smaller the better in my book. If you need a huge bar at one end of the car there is probably something wrong at the other end.

Very cool figuring out the rate though, most people don't have a clue what the parts they hang on their cars are doing.
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Old 10-26-13, 11:54 AM
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Originally Posted by Valkyrie View Post
Some cars will be faster without one of the bars, and just using higher spring rates.
it depends on the track too. when we did spec E30, the drivers liked no rear bar on the more open track, and they wanted the bar back on at the tighter track. IMHO, rear bar vs no rear bar is a drivers preference thing, and that depends on the track too.

i also encourage people try try both, as its an easy thing to do.

Originally Posted by jgrewe View Post
Very cool figuring out the rate though, most people don't have a clue what the parts they hang on their cars are doing.
true! it took us years to plot the wheel rate on the rear of the FWD car as it picks up the inside rear tire. we want it to pick up the tire, and B the interaction between the bar and the spring as the tire picks up turns out to be pretty important.
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Old 10-26-13, 09:43 PM
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Originally Posted by Valkyrie View Post
Also, unhook the links and see how it drives. It'll probably wallow like a pig.
that's not really comparable... If I unhook the front sway bar I'll add 250 lbs to the spring rate and see how I like it
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Old 10-27-13, 09:53 AM
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Originally Posted by eage8 View Post
that's not really comparable... If I unhook the front sway bar I'll add 250 lbs to the spring rate and see how I like it
so there is an SAE paper on the FD body, and one of the things they did was take an FC and put it on some kinds of stands, then they removed parts and measured the % each add to rigidity.

things like the strut bar and windshield are in the 3% area.

the sway bar is like -15%. if you look at it from the top, its basically so far forward, it makes a great lever to bend the body with.
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Old 10-27-13, 12:53 PM
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Originally Posted by j9fd3s View Post

so there is an SAE paper on the FD body, and one of the things they did was take an FC and put it on some kinds of stands, then they removed parts and measured the % each add to rigidity.

things like the strut bar and windshield are in the 3% area.

the sway bar is like -15%. if you look at it from the top, its basically so far forward, it makes a great lever to bend the body with.
That's pretty crazy... I was worried I'd loose rigidity since it ties the frame rails together.

Combine that with the fact that it's probably 20 lbs in front of the front wheels... I'm pretty sure it's getting removed for next season.
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Old 10-28-13, 06:54 AM
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20 pounds!?

More like 5-10... they are tubular you know.
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Old 10-28-13, 06:55 AM
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Originally Posted by j9fd3s View Post
so there is an SAE paper on the FD body, and one of the things they did was take an FC and put it on some kinds of stands, then they removed parts and measured the % each add to rigidity.

things like the strut bar and windshield are in the 3% area.

the sway bar is like -15%. if you look at it from the top, its basically so far forward, it makes a great lever to bend the body with.

Does that mean no sway bar = 15 more rigid?
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Old 10-28-13, 09:06 AM
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Originally Posted by Valkyrie View Post
20 pounds!?

More like 5-10... they are tubular you know.
No they aren't. All FC bars are solid...

20 lbs might be an overstatement. I'll weigh it when I take it off.
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Old 10-28-13, 10:17 AM
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Originally Posted by Valkyrie View Post
Does that mean no sway bar = 15 more rigid?
it does appear that way yes.
Attached Thumbnails Sway bar spring rates-fc-stiffness.png  
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Old 10-28-13, 11:52 AM
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Originally Posted by j9fd3s View Post
it does appear that way yes.
apparently no sway bar = 35% more rigid...
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Old 10-28-13, 01:25 PM
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Originally Posted by eage8 View Post
apparently no sway bar = 35% more rigid...
if you think about it you can see why, the bar has a lot of leverage
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Old 10-28-13, 01:31 PM
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That is interesting to say the least...anyone here ever tried running without a front sway bar?
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Old 10-28-13, 01:33 PM
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Originally Posted by rx7 SE View Post
That is interesting to say the least...anyone here ever tried running without a front sway bar?
My endlink came unhooked at an autocross last year and I didn't really notice . that being said in pictures the car does look like it rolls a lot more (and I run 600 lbs/in springs). You'll want to add more spring if you try it...

I'm going to take mine off next year and up the spring rate to probably 900 lbs/in. we'll see how well it works.
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Old 10-28-13, 01:40 PM
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With front bar:



without:



also lifting front wheel with the bar:

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Old 10-31-13, 11:21 AM
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Everything is a trade-off of course, but on an FC with a lowered suspension (assuming you haven't relocated the LCA mounting points) I would be really concerned with the amount of camber loss under heavy roll and the ensuing under-steer. That's why (AFAIK) the general rule has always been "big front bar, small (or none) in the rear".

-bill
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Old 10-31-13, 09:20 PM
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Originally Posted by wrankin View Post
Everything is a trade-off of course, but on an FC with a lowered suspension (assuming you haven't relocated the LCA mounting points) I would be really concerned with the amount of camber loss under heavy roll and the ensuing under-steer. That's why (AFAIK) the general rule has always been "big front bar, small (or none) in the rear".

-bill
I hat a set of AWR roll center spacers up front that helped with that... until my car broke one in half

I'm working on a better solution but until then I pretty much have stock geometry.

the new spring rates will pretty much be the classic "if the suspension doesn't move it can't do anything wrong" philosophy
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Old 11-01-13, 10:20 AM
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If the bar lifts the inside wheel, try increasing spring preload. It's supposed to help keep the spring on the ground after you've transferred a wheel's worth of weight off that corner. You might also not enough enough droop travel.
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Old 11-01-13, 01:20 PM
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Originally Posted by eage8 View Post
I hat a set of AWR roll center spacers up front that helped with that... until my car broke one in half
Can you elaborate what broke and why? Or is there a thread I'm missing? I'm currently designing my own, and there are already a few things I didn't like about the AWR kit.
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Old 11-01-13, 01:35 PM
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Originally Posted by HotRodMex View Post
Can you elaborate what broke and why? Or is there a thread I'm missing? I'm currently designing my own, and there are already a few things I didn't like about the AWR kit.
http://www.rx7club.com/build-threads...turbo-1031251/

It broke at the pinch bolt recess:


it needs more support... let me know what you come up with.

I've been looking at modifying Mk1-3 VW golf ball joints to work since they have the same pinch bolt design, but they're 15/17/19mm and I think they FC is a 16mm stud...
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