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Old 11-25-2007, 10:39 PM   #1
CNC Scott
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Default Bigger FSB=more grip / Bigger RSB=less grip

On a WRX a bigger FSB = more grip by reducing the loss of negative camber, then why does a larger RSB reduce traction with a similar strut suspension?
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Old 11-26-2007, 12:47 AM   #2
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Old 11-26-2007, 12:48 AM   #3
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larger rear swaybar improved mine. With the difference in roll, the alignment may have to be readjusted to maximize the contact patch.

In what way are you losing traction? Is it an overall loss or just a change in balance that seems to be a decrease in traction?
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Old 11-26-2007, 12:55 AM   #4
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Well speaking from experience, with my stock WRX struts and springs, when I set my Cobb RSB to the stiffest setting, the back end tends to come out more easily under braking and under throttle lift-off. I guess this is because the WRX is very front heavy and when you stiffen the RSB it makes it harder for the rear tires to act independently of one another in a hard corner so there is potential for losing grip.
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Old 11-26-2007, 01:19 AM   #5
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Quote:
Originally Posted by CNC Scott View Post
On a WRX a bigger FSB = more grip by reducing the loss of negative camber, then why does a larger RSB reduce traction with a similar strut suspension?
Because it's not as simple as that.

I'm guessing you're thinking that the larger RSB increases oversteer and thus it is assumed that this is due to "less traction".

But that's not the case. The larger RSB increases oversteer bias because the stiffer bar increases the rate of weight transfer. When the rate of lateral weight transfer in the rear is faster than the rate of lateral weight transfer in the front, you get an oversteer bias. Remember, the RSB still provides the oversteer bias even BEFORE the tires lose their traction.

The bigger FSB does 2 things: increases understeer bias, and increases front traction due to reduction of camber loss. So, the big FSB will contribute to understeer bias even BEFORE the front tires lose traction.

Disclaimer: No one seems to differentiate it like this, so maybe I'm wrong.

I think the confusion is that there is plowing understeer where the front tires have lost traction and now the front plows straight, and understeer bias where the car simply doesn't like turning (requiring the front tires to do a lot of work to move the front end in a new direction). Both are called understeer but are different things.
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Old 11-26-2007, 01:24 AM   #6
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The basic theory with anti roll bars is that by controlling the relative roll stiffnesses of the front and rear of the car, you control where the lateral load transfer ends up. The end with the higher roll stiffness will see a greater portion of the total load transfer of the car (NOTE: sways DO NOT CHANGE the amount of lateral load transfer. They CAN be used to change WHERE the load ends up). Tires do not respond well to increased loads. Two tires that are loaded evenly will hold a greater total lateral force than two tires that are loaded unevenly. By stiffening one end of the car, you increase the load transfer at that end and reduce it at the other end (remember, the total lateral load transfer for the car remains constant). By changing the load transfer in that way, you INCREASE GRIP at the end with reduced load transfer (and therefore tires more equally loaded) and REDUCE GRIP at the end with increased load transfer (and therefore tires more unequally loaded).

The Impreza, being a MacPherson Strut suspension with a pretty crappy camber curve, throws a bit a of a kink into the classic analysis of the effect of sways. When you increase the rear roll stiffness, not only are you getting the changes in loading, and therefore grip, described above, you're ALSO getting increased front end grip due to having better dynamic camber on the front end from the reduced chassis roll. Similarly, if you install a front sway, not only are you seeing the reduction in front end grip due to uneven loading as described above, you're ALSO seeing a MUCH, MUCH larger INCREASE in front grip due to better dynamic camber from reduced chassis roll.

Take this far enough via really big bars, and you eventually get to the point where you've gotten all you're going to get in terms of grip from better dynamic camber, and the uneven loading becomes more important as the classical analysis would suggest.

EDIT: Looks like chim was typing faster than I was. I said basically the same thing, but with more capitalization.
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Old 11-26-2007, 02:15 AM   #7
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I just added a 27mm FSB (replaced a Whiteline 22 adj set at full stiff) and during Friday’s auto-x, as I expected it was harder to get my car to off throttle oversteer, (the RSB is a Whiteline 22 adj set to full stiff). So I assume that the next step would be to go to a larger RSB, but is there a certain point that I would actually get better rear end grip due to better dynamic camber from the reduced chassis roll from a stiffer RSB and that would outweigh the effect of the lateral load transfer?
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Old 11-26-2007, 07:27 PM   #8
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If you stiffened the front bar and got more understeer, you're already past the tipping point on increased dynamic camber. Not to mention the rear camber curve, while not great, is WAAAAAY better than the front. If you want it to rotate more on course, you're looking at increasing rear spring rate or damping rate now.
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Old 11-26-2007, 10:19 PM   #9
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Old 11-26-2007, 10:24 PM   #10
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Actually stiffing the front gave me less understeer, the front grips pretty well now but it seems like I also got more rear grip. It is just harder to get the back to step out. I know loose is slow but it is helpful to be able to get the back to rotate sometimes.
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Old 11-26-2007, 10:27 PM   #11
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Quote:
Originally Posted by CNC Scott View Post
Actually stiffing the front gave me less understeer, the front grips pretty well now but it seems like I also got more rear grip. It is just harder to get the back to step out. I know loose is slow but it is helpful to be able to get the back to rotate sometimes.
Ahhh, I mis-understood then. What you got is what I would have expected. In theory, you could further increase rear bar and eventually, you'd get loose. However, at some point, you'll start picking up your inside rear wheel, which is even slower than being tight.

You could experiment with either stiffer rear springs, stiffer rear damping, reduced rear camber, or significant rear toe out.
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Old 11-26-2007, 11:30 PM   #12
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Quote:
You could experiment with either stiffer rear springs, stiffer rear damping, reduced rear camber, or significant rear toe out.
I am getting ready to install koni inserts so increased rear damping should be easy. I am running -1 deg rear camber, I am not sure if I can get closer to 0 deg with just camber bolts. I am running SPT Pink springs and they feel pretty good so I am not ready to change them and lots of toe out is too much trouble to set at the track and too scary to run on the street. Is there any advantage to go to the slightly stiffer 24mm adj RSB from the 22 adj RSB, I don't think it would be enough to cause the inside rear to lift?
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Old 11-26-2007, 11:39 PM   #13
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Quote:
Originally Posted by CNC Scott View Post
I am getting ready to install koni inserts so increased rear damping should be easy. I am running -1 deg rear camber, I am not sure if I can get closer to 0 deg with just camber bolts. I am running SPT Pink springs and they feel pretty good so I am not ready to change them and lots of toe out is too much trouble to set at the track and too scary to run on the street. Is there any advantage to go to the slightly stiffer 24mm adj RSB from the 22 adj RSB, I don't think it would be enough to cause the inside rear to lift?
A quick trip to the suspension spreadsheet says that the inside rear wheel will not lift at 24mm, but in a 1G corner, it'll only have 107lbs on it. Easy tire smoke territory.

The SPT Pink/USDM STi rates are quite good. But rates closer to even will rotate better, so will toe out. At a certain point, you have to optimize the suspension for either the track or the road. Looks like you're sneaking up on that point.
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Old 11-27-2007, 12:03 AM   #14
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Quote:
Originally Posted by CNC Scott View Post
Actually stiffing the front gave me less understeer, the front grips pretty well now but it seems like I also got more rear grip. It is just harder to get the back to step out. I know loose is slow but it is helpful to be able to get the back to rotate sometimes.
By understeer, are you referring to at limit driving at autox type of stuff? If so, that's probably because the limit of traction for the front was bumped up. In this sense, you have less of this plowing understeer.

Just as Williaty explained, your thicker front bar is helping the back stay planted, which is why it's harder to step the back out. This is understeer bias.

So... you got less of the former, and more of the latter.

I think the understeer confusion is (yet) another misnomer case of car guys who aren't engineers trying to be technical. Just like "apex".
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Old 11-27-2007, 03:07 AM   #15
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A quick trip to the suspension spreadsheet says that the inside rear wheel will not lift at 24mm, but in a 1G corner, it'll only have 107lbs on it. Easy tire smoke territory.
WOW! that is really cool that you are able to calculate that. It really does illistrate what a stiffer RSB will do.
Quote:
By understeer, are you referring to at limit driving at autox type of stuff?
Exactly.
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Old 11-27-2007, 11:31 AM   #16
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1G corner, it'll only have 107lbs on it.
I am trying to set-up my WRX for STU, assuming I get it well set-up, can it generate 1g of cornering force on street tires like the RS2 or RE01's ?
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Old 11-27-2007, 12:05 PM   #17
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That really depends on how well set up it is. Short answer would be yes, probably, or at least very close. You'd DEFINITELY hit it in transient loading, just not 100% sure at steady state.
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Old 11-27-2007, 03:51 PM   #18
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I am trying to set-up my WRX for STU, assuming I get it well set-up, can it generate 1g of cornering force on street tires like the RS2 or RE01's ?
On 22mm F/R adjustable WL sways (both on full stiff), front/rear strut tower braces, and 225/45-17 RE070's on 17x7.5 '04 STi BBS, on maxxed out stock alignment, I regularly hit 1.1ish G at the autocross in transient... interestingly enough, peak G usually occurs just past apex on corner exit. In steady state, I'd guesstimate about a 0.9g capability, but that's a little more difficult to determine from the data, and I've never done a flat-out skidpad test.

(I've actually seen LatG's approaching 1.3 (and not just one data point, a few consecutive ones AFTER the data is already filtered), but it's GPS and not accelerometer based, so...)

So yes... if you get springs/struts/sways/bracing/235-245 sticky RE01-R/camber plates, I think 1g is a reasonable expectation.
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