so i have to correct myself here. i just went through and derived the equation you put in your first post from scratch. turns out it is already taking the bending of both lever arms into account already.

so the correct way to use it is to use B as the full length, just like you said, but then double it to keep it consistent with calculating roll stiffness in lb/in wheel rate. ...also just like you were doing.

but short story is the stock front bar is the equivalent of roughly 525 lb/in of spring rate (aka 165 lb/in of wheel rate with the stock front bar motion ratio included).

 

Ok good, I was pretty sure it was that way but didn't want to argue . So our numbers match up and taking into account stock front rates, the bar is 50% of the roll resistance. 165lbs from bar and 166 from spring.

Considering most of us run 700+ front springs, that's about 650lb/in at the wheel, if we were to run the whiteline 26mm bar which is 285lb/in at the wheels with a cusco adjustable bracket set max we would be around 330lb/in. That's moving the ratio down to about 35/65 bar to overall rate.

Is this a good balance? Does it really matter if you have enough rear bar to work it?

Going through the number my 900lbs springs with the stock bar will have the same roll resistance as 700lbs springs with the whiteline 26mm and cusco bracket set full stiff.

A lot of this is just an exercise in understanding the system better, but I cant say for sure what it will all mean for track times until I get to try it at the next event.

 

I think the bar/spring ratio is of far less importance than having the correct roll stiffness.

it is interesting to test out different front roll stiffness as a single change and see how that effects the balance.

 

I just re-read this, Whiteline claims their bar is 38% stiffer than stock... if you take your stock bar measurements and add 2mm of diameter to it, for the "Whiteline equivalent," it works out to be just about that... 268.63lb/in -vs- 369.99lb/in. So yeah, it looks like Whiteline didn't take into account the bar's geometry, at all ...their little chart is kind of useless then ain't it.

 

Heh, you noticed that too?

Yeah the 38% looks exactly only at bar diameter but the arm torsion and perpendicular distances are both different. It all amounts to quite a bit different stiffness than claimed.

 

dallas,

we need to work up the rates on my bar lol

 

Dude, yours should be easy. That bar setup is pretty awesome but definitely need to know what its doing. Send me all the measurements and we can get your roll rates balanced

 

so.... like this you say?



I'm going to carefully measure the Hotchkis rear bar and reverse math it to see if/what measurements line up with their published lb/in rates....

 

Because I like to share what I find, here's so more interesting stat from the stock suspension.

Given F/R spring rates, spring and bar motion ratios front and rear, and bar rates, it looks like the Evo is as textbook as you can get.

Stock springs give around 1.3hz front and 1.5hz rear natural frequencies and swaybars are sized to be as close to 50% of overall roll rate as you can get. To me very interesting, both front and rear have the same percentage of the overall roll rate and that they just happen to land a 50%.

The setup I'm gonna try puts me at 31% front 33% rear until the Whitline 26mm bar shows up. Then Ill be able to up that a few percent up front and moreso in the rear since I run less rear spring than most.

 

have a rzr race this weekend so im prepping for that. ill get you the info next week

 

good analysis/good idea. I found a post here where a guy physically measured the OE IX spring rates. His numbers throw the NF's off a bit, but yeah, w/ the stock setup it looks like the bars are doing a LOT of the work!

 

Thanks . It really makes you think about what throwing a big rear bar on an otherwise stock car is doing. Huge relative percentage of roll stiffness coming from a bar and a very unbalanced setup in terms of bar to spring ratios. If that even matters that is.

I may go as far as trying softer rear springs and full stiff on the WL 26mm. Roll rates will all be the same as my standard setup but more weight transfer to the inside front mid corner.

 

I am going to re-read this after some sleep, but wanted to thank everyone for sharing their findings to this point.

 

I measured my collection of rear swaybars last night and thought I would share here. I can't find/verify any of these numbers except to say that Hotchkis published their rates as 170/190/215 lbs/in, but they don't/won't publish the wall thickness of their hollow bar. Without that number I'm a bit dead in the water as far as that bar goes...

Stock Rear Bar (where "L" is the total width between endlink holes)
A (using "L" and Pythagorean) = 9.888"
B = 32.05"
C = 11.457"
D = 0.866"
L = 43.625"

24mm Hotchkis Rear Bar
A'soft=9.491" - A'medium=8.961" - A'hard=8.449"
B = 32.05"
C'soft=11.1" - C'medium=10.55" - C'hard=10"
D = 0.945"
Wall Thickness = 0.127"??? (I fudged this to line up the results)
L'soft=43.563" - L'medium=43.188" - L'hard=42.75"

24mm Whiteline Rear Bar
A'soft=9.968" - A'medium=9.171" - A'hard=8.53"
B = 32.05"
C'soft=11.496" - C'medium=10.71" - C'hard=10.04"
D = 0.945"
L'soft=43.504" - L'medium=43.11" - L'hard=42.638"

so, the stock rear bar looks to be about 168 lb/in, anyone care to verify that with their own measurements?

If you take Hotchkis's published bar rates and what they say their percentages are over stock, my calculations/measurements have the stock bar about 12% over what Hotchkis claims it is..

The 24mm Whiteline rear bar looks to be about 235/280/327 lb/in
or 140%/166%/194% over my measured version of stock, about 17% more than what Whiteline claims it should be, again based on my measurements of stock.

Not knowing what Whiteline's measured version of "stock" is, my percentages are 21%/24%/27% higher than what they claim their bar is (119%/142%/167%).

Also of note, the Whiteline bar is a lot more similarly shaped to the stock bar than the Hotchkis bar is. If you take what I measured for the Whiteline bar on the soft setting and only remove the 2mm of extra diameter, it validates the rate I measured for the stock bar (mostly).

 

sure got quiet around here....