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 Post subject: Rear sway bar thoughts?
PostPosted: May 21, 2018, 4:05 pm 
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I've been debating installing the rear sway bar on my project. Front axle weighs around 1200 pounds, rear axles around 800 pounds. The coilovers I have on are a 8k/6k spring rate (448/336 lb/in). This is a Subaru Forester/Impreza with body swap in aspect of everything. The drivetrain and suspension, although modified is essentially the same as an Impreza WRX. These came standard with 19mm fron bars and 13mm rear bars.

I have no issue installing the rear bar, i even have the mounts done. But given the ecessive amount of power it has, the light weight, and the stiff rear springs. I thought i could do without the bar and save the room for a real muffler. Theres not really room back there for both.

test drives have been up and down my street, nothing to test the handling until i can get it street legal.

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PostPosted: May 21, 2018, 4:24 pm 
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Long answer = needing to know a lot more specific details and doing a bunch of math, preferably with the assistance of a spreadsheet.

Short answer = build to accommodate one, and let on-track testing be your guide.

Just from that pic, it looks like there might at least be room for a transverse muffler underneath at the rear, assuming a functional 'pickup bed' is also important.

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PostPosted: May 22, 2018, 9:06 am 
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What Justin (Driven5) said.

I'm not clear on which location you had in mind for the rear sway bar. Are you talking about placing it behind the axle or in front of it?

Cheers,

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PostPosted: May 22, 2018, 1:26 pm 
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Lonnie-S wrote:
What Justin (Driven5) said.

I'm not clear on which location you had in mind for the rear sway bar. Are you talking about placing it behind the axle or in front of it?

Cheers,


behind the rear crossmember. Heres a shot with old springless struts, th bar would mount to the white pad on frame and tie into the lateral arms

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PostPosted: May 22, 2018, 1:45 pm 
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In my experience, a rear bar is needed at low (autocross) speeds to help the car rotate. At high speed events (trackdays) it'll try to kill you. We always disconnected the rear bar for track events.

Also, there's a saying regarding bars, "bars degrade traction in order to make the other end of the car appear to stick better."

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PostPosted: May 22, 2018, 1:49 pm 
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KB58 wrote:
Also, there's a saying regarding bars, "bars degrade traction in order to make the other end of the car appear to stick better."


Yeah, What Kurt said! :agree:

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PostPosted: May 22, 2018, 3:05 pm 
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You wouldn't have an extra 4 or 5 inches to spare below the sway bar?...What about running 'stacks' vertically up behind the main hoop?

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PostPosted: May 22, 2018, 3:43 pm 
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Ok thanks guys. I think im going to try without the bar. Worst case it seems i'll have some understeer which isnt always a bad thing.

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PostPosted: May 22, 2018, 5:00 pm 
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Sorry, didn't mean to push you into not running one - design for a bar mount, then leave it out during testing and see how it goes. Always easy to add one later if the mounts are in-place already.

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PostPosted: May 22, 2018, 6:36 pm 
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With struts on all 4 corners, you have a near 1:1 motion ratio all around. If we assume say 125 pounds per axle of unsprung weight with your 1200/800 weight distribution, that leaves 1075/675 sprung weight. Combined with your 448/336 spring (and wheel) rates, that leaves you at 2Hz front and 2.2Hz rear. That gets you nicely in the 'flat ride' ballpark, and should inherently need at least some front sway bar but not necessarily a rear bar.

Just using this rough approximation, and depending pretty heavily on your actual sway bar rate(s), you may actually come out pretty good without any rear bar. I'm not sure what the smallest front bar that would bolt in is, nor what the one in your possession (if you have one already) is, but I would suspect there is a good chance that even if after testing you wanted less understeer you could just go with a smaller front bar to get the handling balance where you want it, rather than adding a rear bar.

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PostPosted: May 23, 2018, 9:06 am 
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Driven5 wrote:
With struts on all 4 corners, you have a near 1:1 motion ratio all around. If we assume say 125 pounds per axle of unsprung weight with your 1200/800 weight distribution, that leaves 1075/675 sprung weight. Combined with your 448/336 spring (and wheel) rates, that leaves you at 2Hz front and 2.2Hz rear. That gets you nicely in the 'flat ride' ballpark, and should inherently need at least some front sway bar but not necessarily a rear bar.

Just using this rough approximation, and depending pretty heavily on your actual sway bar rate(s), you may actually come out pretty good without any rear bar. I'm not sure what the smallest front bar that would bolt in is, nor what the one in your possession (if you have one already) is, but I would suspect there is a good chance that even if after testing you wanted less understeer you could just go with a smaller front bar to get the handling balance where you want it, rather than adding a rear bar.


thanks for the help. i appreciate your efforts and input. Do you think dampening adjustment may also help. They are ISC N1 coilovers with 32way adjustability. My guts tellin me the stiffer rear springs will do fine without the bar. I can go down to a 224# spring in the rear on the same dampers as per ISC tech told me. The buggies got 525hp, and like i mentioned before, a lil understeer may not be bad

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PostPosted: May 23, 2018, 9:52 am 
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Damping only has an effect when the suspension is moving. So during transitions, but not steady state.

Some further calculation of the sway bar contribution to the front vs rear roll rates might help get you to a closer approximation on the first try. I'll try to dig up one of the spreadsheets, or a link to one, that might be able to help you out.

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PostPosted: May 24, 2018, 7:20 am 
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I'll try to dig up one of the spreadsheets, or a link to one, that might be able to help you out.
I used this one when I was trying to design a front bar for the Slotus. Good luck!

http://www.auto-ware.com/calcs/swaybar.htm

:cheers:
JDK

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PostPosted: May 24, 2018, 7:41 am 
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GonzoRacer wrote:
Quote:
I'll try to dig up one of the spreadsheets, or a link to one, that might be able to help you out.
I used this one when I was trying to design a front bar for the Slotus. Good luck!

http://www.auto-ware.com/calcs/swaybar.htm

:cheers:
JDK


Thanks, coming up with 255 pounds

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PostPosted: June 4, 2018, 6:34 am 
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I'm pretty sure I have posted this ChassisCalcsPublic spreadsheet as an attachment before, but for the life of me can't find the thread it's in. So I'll just post it here again too. I did not create it, but have referenced others to it a few times over the years. It should be a good way to roughly get into the right ballpark. The general claim is that the Lateral Load Transfer Distribution (LLTD) should be biased ~5% more to the front than the weight distribution, for a slight inherent understeer. Of course, this notion operates under a number of general (all else equal) assumptions, not the least of which would be the tire width stagger should also roughly match the weight distribution. So if you're running the same size tires all around with a front weight bias, you'll already have inherent understeer from that as well.


Attachments:
ChassisCalcsPublic - LocostUSA.zip [11.93 KiB]
Downloaded 19 times

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