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Nope, didn't say that . I said that it would not in fact, but it's torsional constant per unit mass would be greater. Net stiffness is different. The same OD hollow would have less stiffness/spring rate but on a more than proportional basis would have less mass. To put it another way, a hollow bar of the same stiffness as a solid bar will be significantly lighter....but the larger OD of the hollow bar might be a hindrance to mounting unless your class allows the modification of mount points.

But you could get a hollow bar of the same OD as a solid made with the same stiffness and of the same material by changing the hollow bar's geometry compared to the solid ie: shorter moment arms or shorter length torsional length (or both). Both of these would require changes in endlinks (and they'd have to be stiff stiff stiff endlinks) or bar mount points that make them less than ideal means of increasing ARB stiffness (particularly on a stock class car).

My conversation brought in the weight factor which is probably irrelevant in autox. The extra 10 lbs on a .3 mile run probably represents a couple joules of extra energy to expend per run . I guess in a track setting where you have less constraints but yet the drive to make the car as light as possible, the J/m (torsional constant per unit mass) becomes a much more significant metric. As in if a team feels that they don't want to add more than 35lbs worth of ARB and the second goal is XX lb/in worth of wheel rate, they will maximize J/m to reach their mass requirement and then start playing with geometry and mounts to get their wheel rate requirement.

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2011/2012 Autox VP
2013/2014.5 President
2013 Top Gun

2015 Fit

22R-EC => 4G63 => D16Y7 + D16Y8 => EJ255 + K24Z2 => K20Z3 + K24Z2 => K24Z2 + M54 => L15B

James,

Thanks for the tutorial I actually know (or knew . . . memories fade) this stuff but it is good for newer folks who haven't read, and reread, all the books, etc. over the decades.

FYI for my FF, the bars are solid but they are very small and light. The "twist" portions are about 8 inches long. Diameters range from 1/2" up to 5/8" (mine are 9/16 and 1/2 currently) and the arms are roughly 6 to 8 inches also. The car has rocker arm suspension (i.e. inboard coilovers) and the bars connect to the same point as the coilovers. This means the bar and spring motion ratios are the same . . . makes calcuations easier. I don't recall the weights but probably a pound or 2 each. Van Diemen did a neat thing to make bar changes easy. The mounts for the twist portion are all roughly 1/4 inch diameter and simply insert into easily removeable bearings similar to a transmission/clutch pilot bearing. The arms are all 1/2 inch (round) and are clamped by split blocks which only need one small bolt each loosened to slide on the arms. (no cockpit adjustment). The links have heimjoints to compensate for adjustment angle changes. Race cars from experience manufacturers are NEAT.

Clarification on the bar mounts: Both ends of the bars have the 1/4 protrusions which insert into the bearings which are mounted in aluminum blocks which bolt to the chassis. The arms are welded just "inside" the ends.

FYI for anyone who is still reading at this point: Bar stiffness at the wheels is a factor of diameter, bar length, and arm length. My thin bars are very short. I don't recall the equation but I think diameter is to the 4th power, arm length is squared. I forget length (maybe just the first power) but short length does make a difference. With a light weight non aero race car a lot of the roll stiffness is done with the springs (my calculated roll angle at 1.4 g cornering is about 1 degree.

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Dick Rasmussen

FS 50 2018 Mustang GT

I'll throw out some of the equations:

T(orque) = JG*theta/l

where
theta = angle of twist for a given amount of body roll

J = pi/2*r^4 (solid)
J= p/2*(r_o^4 - r_i^4) (hollow)

G is modulus of elasticity (dependent on the kind of steel used)

l is the torsional length (twist length)

So you have a resistive torque and for a given vertical displacement of the suspension from 'rest'/equilibrium, if you know the moment arm (lets just assume the moment arm is perpendicular to the torsional arm) you can convert that displacement into an angle pretty easily with good ole' tan.

Also, you can convert the torque to an effective force at the endlink F=T/l_moment_arm.

Using that you have k = F/disp ie spring rate at the link.

And you mentioned the moment arm squared and that's true but 1/l^2 as follows:

F=T/l_moment_arm

displacement of bar end = l_moment_arm*tan(theta)

k=F/displacemnt_bar = T/(l_moment_arm^2*tan(theta))

Anyway, the important equation is:

k_bar_end*MR^2 = wheel rate

This is the equation important to the STI (getting back on topic ) since it has front strut suspension (say .99 MR) and rear double a arm (say .6 MR). So to get good balance of wheel rates, the rear needs more spring rate to get there. That is the nice thing about the suspension updates on Aarons STI SE since they went with a bigger RSB AND stiffer rear spring (compared to the front).

_________________
2011/2012 Autox VP
2013/2014.5 President
2013 Top Gun

2015 Fit

22R-EC => 4G63 => D16Y7 + D16Y8 => EJ255 + K24Z2 => K20Z3 + K24Z2 => K24Z2 + M54 => L15B

Thanks for the heads up, I'm not going to bother setting up a second set of lights to get a 60' time now.

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Has no responsibility whatsoever.

There goes my chance at being fastest at something.

Aaron, how was the first outing in the new car? Is the engine still in one piece?

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Karl S.
2014 Baby, 2014 House, 2013 Ford Focus ST, 2013 BMW 328i, 1994 Mercedes E320
(Insert passive aggressive signature line here)

The engine performed fine Amazingly, we got dry runs (the last heat had a little drizzle).

So far, so good. I mounted up some old 295's and headed out to Danville yesterday. The course itself was certainly not an STi friendly course, with a metric ton of transition, but the car did pretty well. I found it rotated nicely and once the tires got some heat, turned in pretty well. Actually better than I thought it would.

I know coned out, but I was pleased with my raw times for the first time in the car. I know Keith got to a 57.7 +1 and I got to a 57.9 +1. Jadrice had a 57.2 clean but had a dirty 56.1 with multiple cones (which is pretty insane). I'll post up some side of the car video later tonight.

After talking with Jadrice (who used to own a '08 ), I have a few things to try. I was running the 32mm bar (I have a fitment issue with the 34mm bar), but should be able to run the 34mm bar at Blytheville. I also was only able to get about -1 degrees of camber up front. With loosening a few more bolts and going to a better alignment shop, I'm sure I can get another few tenths or so.

With the shocks and air pressures, I just guessed, but it felt pretty good. The car certainly doesn't suck at putting down the power

Launching this beast will be interesting, for I don't want to send the rear diff bushing through the floorboard I need to perfect the technique, which will take some time. I am looking forward to Blytheville in a couple of weeks. - AB

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'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

Do STIs shred tires or are they pretty okay for that? I know it's not going to wear tires like a Vette or an S2000, but hopefully it won't be as bad as, say, a DS E46. Just curious.

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Karl S.
2014 Baby, 2014 House, 2013 Ford Focus ST, 2013 BMW 328i, 1994 Mercedes E320
(Insert passive aggressive signature line here)

Well, the tires I put on I expected to cord the shoulders on with the 5 runs we got yesterday. To my amazement, they wore pretty nicely. I think they had about 40 runs on them prior and had been flipped once before, so I hoped they would make it through the day.

I think a lot of that is attributable to the 32mm bar and higher front spring rate, along with running pretty high pressures. - AB

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'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

Rev almost to redline then start bouncing the throttle up and down trying to keep it bouncing between 6k-ish and redline. Bonus points for timing it well enough that you're going up to redline when it's time to sidestep the clutch and let her fly.

That keeps it on boost, where just holding right at redline won't. The more power you can put to it, the less likely you are to hurt anything since the tires will spin. If, however, you just put a LOT of power to it but not enough to spin the tires, well, that's when stuff breaks, I think.

Other than the motor. That part is just a grenade with the pin pulled.


--Donnie

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My Blog

That sounds like good news then. It's be fun having you around in a BS car, especially since you're skinning the cat differently than the rest of us, so to speak.

Next question. Is it fun to drive? I remember watching Chris drive Donnie's at the Toledo Pro back in 2008. No question it was a fast car - it just didn't look like much fun, at least, not from the vantage point I had working course. It looked, I dunno, kind of violent. Face melting acceleration and braking, punctuated with lots of understeer. That's just how it looked from that one vantage point. I haven't driven a Subaru of any sort in years.

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Karl S.
2014 Baby, 2014 House, 2013 Ford Focus ST, 2013 BMW 328i, 1994 Mercedes E320
(Insert passive aggressive signature line here)

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Not spectacular just decent.
I'm not sure what I'm driving.
Maybe an ITR in DS.
Or half-assed STX prepped 330.

When you launch using this method, does it alleviate the rear diff bushing compressing to the point of clunking the floor of the car?

I'm going to have to practice. Some say to take up some slack on the clutch and rev from 4500 to 5500 to keep the boost up, then let straight out while flooring it. Others say there is no need to take up the slack. Hmmm. - AB

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'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

It depends. Compared to the Crossfire it is, but it's not nearly as fun to drive as the C6, RX-8 or your S2000. I'm sure that will be alleviated if it can win The good thing is that it drives better than I expected and I knew it wasn't going to be exciting to drive. - AB

_________________
'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

So you see what I did there.

_________________
Karl S.
2014 Baby, 2014 House, 2013 Ford Focus ST, 2013 BMW 328i, 1994 Mercedes E320
(Insert passive aggressive signature line here)