johnc

All in good fun... Scottie, I think it comes to your "third derivative of position" or jerk-factor: It is well known that the first derivative of position (symbol x) with respect to time is velocity (symbol v) and the second is acceleration (symbol a). It is a little less well known that the third derivative, i.e. the rate of change of acceleration, is technically known as jerk (symbol j). Jerk is a vector but may also be used loosely as a scalar quantity because there is not a separate term for the magnitude of jerk analogous to speed for magnitude of velocity. Many other terms have appeared in individual cases for the third derivative, including pulse, impulse, bounce, surge, shock and super acceleration. These are generally less appropriate than jerk, either because they are used in engineering to mean other things or because the common English use of the word does not fit the meaning so well. For example impulse is more commonly used in physics to mean a change of momentum imparted by a force of limited duration [belanger 1847] and surge is used by electricians to mean something like rate of change of current or voltage. The terms jerk and jolt are therefore preferred for rate of change of acceleration. Jerk appears to be the more common of the two. It is also recognised in international standards: In ISO 2041 (1990), Vibration and shock - Vocabulary, page 2: "1.5 jerk: A vector that specifies the time-derivative of acceleration." Note that the symbol j for jerk is not in the standard and is probably only one of many symbols used. In the aerospace industry they even have such a thing as a jerkmeter; an instrument for measuring jerk. So, Scottie, I think you need to hook up one-a-dem jerkmeters. Sorry list, I couldn't resist posting this 'cuz I found it yesterday when researching a physics thing. Gotta find me a jerk 'o meter somewhere (most people suggest that I peg the meter...) ------------------ John Coffey johnc@betamotorsports.com

What Pete says... Has anyone seen an "A" or a "C" spacer? I've dug around in a lot of Z rear strut housings and I've never seen anything but a "B" spacer. ------------------ John Coffey johnc@betamotorsports.com

I assume by precision you mean repeatability, correct? If that's true, I feel that a lack of repeatability in testing indicates a lack of accuracy - either as a result of equipment or operation. Let's take my Colt Gold Cup as an example: if I can't repeatedly hit the 10 ring at 25 yards can you say the gun is accurate just based on that sample? Regarding the lightweight flywheel and the underdrive pulleys - neither of those items add to the horsepower of an engine because they have zero effect on the volume of air moving through the engine or the amount of fuel being used. Yet an intertia dyno will report the addition of either of those parts as an increase in horsepower. Engine horsepower is torque over time (engine RPM). ALl the above being said, I think I am guilty of "picking nits." But its important that people be aware of the limitations of dynos. ------------------ John Coffey johnc@betamotorsports.com

> Won't the front hinge be less likely to > flex with the rocker box tube connected at > the bottom of 1 Yes, but the more leverage you can get via the door bar (and a resulting connection to and through the firewall) the better. At some point, if the door bar gets too low, there's a diminishing return - where you're just adding weight to the car. Where that point is, I don't know. FYI... in most roll cage construction the primary reason for the door bar is side impact protection. They do add a lot of stiffness but a petty bar is an even better choice. BTW... is this going to be a street driven car? If so, its illegal in most states to drive a roll cage equipped car on the street. Why? Unprotected heads hitting the cage in an accident. ------------------ John Coffey johnc@betamotorsports.com

Morgan, Your dyno chart example indicates where our disagreement is. You are talking about Power from the perspective of accelerating the vehicle. My original post and discussion is about Work (torque) produced by the engine. Horsepower is torque over time and is generally measured by engine rpm, not spinning drums. I agree the Dyno Jet can be accurate based on what it is measuring, but they are completely inaccurate when referring to engine torque and horsepower. The engine in your chart examples is not producing three different levels of work. The original premise of this thread was that engine of "x" car wasn't making enough horsepower and some absolute horsepower numbers generated from an intertia dyno were posted as an example. My contention is that the absolute numbers posted are inaccurate as a reflection of the horsepower producted by the engine. My whole point is to get people to think about the numbers they are quoting. And its not a "cop out" to say that lightweight flywheels and underdrive pulleys do no add horsepower to an engine. They don't! A vehicle may be able to accelerate faster, but that power increase is reflection of mass reduction, not a torque increase in the engine. > You just contradicted yourself - you say a > lighter flywheel doesn't add power. Then > you say power is work per unit time(which > it is). Correct, and the "time" measurement is engine RPM (as I stated), not 1/4 mile time. > Obviously a lighter flywheel lets the car > accelerate faster, which reduces the time > period over which the same amount of work > is done. Viola, your flywheel makes more > power. I agree, but not in the engine. The car can accelerate faster because the same amount of work (remember, no torque increase) has less mass to accelerate. The resulting measurement device (in this case, an inertia dyno) sees that as more power (work over time measure - in this cas turning drums), but there is no increase in work (energy). > You are picking nits, espicially for > someone who doesn't the difference between > accuracy and precision OK, I give, what's the difference? >And I still say dynojets are more than > precise enough for tuning and > informational purposes. We agree completely and what you've said above is exactly what I said earlier: "They are good for comparing one vehicle vs. another or one mod vs. another and coming up with relative numbers (most often a percentage improvement)." "Saying the engine "a" produces 10% more horsepower than engine "b" as tested on the same inertia dyno can be a very accurate statement." ------------------ John Coffey johnc@betamotorsports.com

Let's go back to my numbered items... Bad diagram: ..................1..........2 Front....__ _|_______| __ ...............O............O The Z chassis hinges upward at points 1 and 2 above. You need bracing to keep those hinges from moving and you need to control movement in these areas BEFORE you start worrying about torsional flexing. *** 1. Where the front firewall meets the floor pan/trans tunnel. Cracks at the front of the transmission tunnel and where the TC rods are attached to the frame are stress indicators. *** OK, in the original discussions I assumed we were talking about running low door bars to the front hoop of a 6 point cage or tubing in the rocker boxes. In those cases, unless you have something connecting the front of the 6 point hoop to the firewall it really doesn't help with #1 above. Now, the bracing/gussets that Peter mentions DO help connect the front of the cage to the firewall so they obviously reduce that front hinge on the 240Z chassis. Ideally, you would want that bracing higher and you also want something near the center of the car, but that's not part of the context of our discussion. And, I suppose, adding tubing anywhere in a Z chassis will add stiffness somewhere, but the two places I mention are where you NEED to add stiffness! Running tubes in the rocker boxes or subframe connectors under the car are bascially useless unless they connect to the chassis areas beyond points 1 and 2 above. *** 2. At the rear bulkhead/floor pan. Cracks at the rear of the driveshaft tunnel and at the C pillar body seams are stress indicators. *** Again, almost any 4 point rollbar helps with this problem. ------------------ John Coffey johnc@betamotorsports.com [This message has been edited by johnc (edited February 09, 2001).] [This message has been edited by johnc (edited February 09, 2001).]

Intertia dynos like the Dyno Jet are almost always optimistic in the absolute horsepower numbers they report (a lightened flywheel, underdrive pulleys, etc. will show a horsepower increase on a Dyno Jet dyno). Load dynos are more accurate at reporting the actual horsepower an engine is producting. http://www.fordmuscle.com/archives/2000/03/project1199/index.shtml http://www.supras.co.uk/resources/dynos.htm http://www.shotimes.com/dyno.html http://www.foxvalleykart.com/foxvalle/dyno1.htm Saying that an engine produces "n" horsepower based on a run on an intertia dyno is probably an inaccurate statement. Saying the engine "a" produces 10% more horsepower than engine "b" as tested on the same inertia dyno can be a very accurate statement. ------------------ John Coffey johnc@betamotorsports.com [This message has been edited by johnc (edited February 09, 2001).] [This message has been edited by johnc (edited February 09, 2001).]

I don't think any kind of "low" door bar or a tube in the rocker box will significantly help chassis stiffness. The 240Z chassis flexes like a door hinge in two places: 1. Where the front firewall meets the floor pan/trans tunnel. Cracks at the front of the transmission tunnel and where the TC rods are attached to the frame are stress indicators. 2. At the rear bulkhead/floor pan. Cracks at the rear of the driveshaft tunnel and at the C pillar body seams are stress indicators. A 4 point welded in roll bar will significantly reduce number 2, especially if you make if a 6 point welded in bar by attaching it to the roof right at the hatch hinges (illegal in ALL sanctioned racing classes). A 6 point welded in roll cage helps with number 1 but to really make the cage work you need an 8 point cage with a knee bar, door bars, firewall reinforcement behind the hood latch striker, longitudinal bars connecting the knee bar to the firewall reinforcements, and addition braces going from the firewall reinforcement to the front strut towers. As stated earlier, low mount door bars and rocker box inserts do very little to help with side impact. So, if they don't add to chassis stiffness or safety, all they are really adding to your car is weight. Sorry for the bad news... ------------------ John Coffey johnc@betamotorsports.com

Guys, you also need to realize that single pull dynos like the Dyno Jet are not very accurate at producing "absolute" numbers. They are good for comparing one vehicle vs. another or one mod vs. another and coming up with relative numbers (most often a percentage improvement). I've seen a racing Viper V10 dyno'd on a Clayton engine dyno product 805hp numbers repeatedly over different days. That same engine in the GTS race car pulled between 605 and 746hp ON THE SAME DAY on a Dyno Jet dyno - with no changes to fuel maps, etc. Clayton recently released an electromagnetic chassis dyno that allows vehicles to do steady state power runs. These are more accurate and much better tuning devices than the single pull dynos. ------------------ John Coffey johnc@betamotorsports.com

Unfortunately I'm a frikin' expert on early Z stub axles! I've broken 3 in the last year. The most common failure mode is the separation of the wheel stud flange from the axle due to fatigue failure of the fusion weld joining the two. Caused by side loads (cornering, hitting curbs, etc.) Second most common is a stress fracture starting from the spline area. Caused by overload (horsepower, bad downshifting, excessive weight, etc.) Differences between the 240Z and 280Z stub axles: 1. 25 spline vs. 27 spline. 2. Thicker shaft. 3. Slightly different (harder) heat treating on the bearing surfaces (new stub axles only). 4. More consistent flange to axle fusion welds. 5. Better radius on the flange to axle fusion welds. New 280Z stub axles are still available. Companion flanges are NLA. Don Potter occasionally blueprints a matched set of stub axles and companion flanges (shot peened, micro-polished, cryo treated, Nissan Comp studs, AN/MS nuts/bolts) that are supposedly bullet proof. They cost $1,000 per set too! ------------------ John Coffey johnc@betamotorsports.com

I've been queried twice today on this topic, so I'll post it here for those that are interested: Nissan Motorsports recommends 70 ft. lbs. I normally ran mine at 85-90 ft. lbs. for autocross. It falls off from there after a few events. If the breakaway torque gets down to 35-40 ft. lbs. its essentially an open diff. If the breakaway is 110 ft. lbs. or over its essentially a locked diff. You will need to check it at least once per year and expect to replace the clutches every 12 to 18 months. Autocross is especially hard on clutch type diffs. ------------------ John Coffey johnc@betamotorsports.com

Regarding spring length (8" or 10")... 8" springs - lighter and faster response. 10" - more compliant. Careful with 10" springs though, you may not have enough threaded collar to lower the car where you want it. ------------------ John Coffey johnc@betamotorsports.com

Offest bushings (inners) are how you get the toe-in. Depending on the type of offest busing you use, you might have to oval the hole a little bit to reduce bind. Another way to achieve this is to put shims between the bushing and clamps. Now, you probably don't need to make this mod if its a street driven Z or you're running street tires. With race slicks or DOT R tires the rear of the car will "hunt" during transitions (corner turn in, corner exit) which will make it more prone to lift throttle oversteer on corner entry and on corner exit you'll get snap oversteer if too much throttle is aplied. > PS whats with that zcarnbranson dillhole > on zcar.com talkin smack about you? I think he was agreeing with my post on the "This link is why beating rice is so..." thread. He just had a funny way of saying it. ------------------ John Coffey johnc@betamotorsports.com [This message has been edited by johnc (edited February 02, 2001).]

A handling tip... If you run wheels wider than 7" on the back of your 240Z you need to dial-in 1/16 to 1/8" toe in. Why? You need to compensate for the increase track from the wider rims to get the stability back in the rear of the car. ------------------ John Coffey johnc@betamotorsports.com

Call the scca national office and ask for the road racing department. You can get the number from http://www.scca.org. I "think" with 1.75 diameter mild steel tubing you can run .095 wall. The .120 wall requirement is for 1.5 mild steel. FYI... the scca also stopped giving a wall thickness break for cro-moly tubing so you can't save any weight there. ------------------ John Coffey johnc@betamotorsports.com [This message has been edited by johnc (edited February 02, 2001).]

IMHO... I'm not sure if the adjustable end links MSA sells will actually work. It appears that from the pictures that the link itself will pivot slightly if the offest hole is used, negating the benefit of the offset. You're probably better off with an adjustable anti-roll bar to begin with. ------------------ John Coffey johnc@betamotorsports.com

> Hmm I just talked to a guy here at work > who is a master welder and he said it > would be better to bolt it together than > weld it, because when you weld something > you weaken it, bolts dont weaken it. A good weld is as strong as the surrounding metal. A welded in roll cage is much stronger/stiffer/safer than a bolted in cage. BTW... what kind of welding does your master welder friend do? Hopefully not nuclear powerplants... ------------------ John Coffey johnc@betamotorsports.com

Simon, If you have a rear mount rear anti-roll bar you are better off putting spacer blocks on the mounts themselves instead of messing with the bar ends. Most rear mounted anti-roll bars on the Z have to be spaced back about 1" to keep the bar from binding under full rear bump. ------------------ John Coffey johnc@betamotorsports.com

What Pete said, only with a few more subtleties. A lowered 240Z front suspension will toe-in in bump slightly. Its not that much and if you have to have any toe change as a result of bumpsteer then toe-in is what you want. Toe-out in bump is dangerous. Bumpsteer spaces work very well to get rid of that toe-in in bump, but they take away the 1/4 of a degree in camber gain (more negative camber) in bump that is built into the 240Zs front suspension. Moving the lower control arm pivot point up reduces the toe-in in bump, but not as much as bumpsteer spacers. You will still have some toe-in in bump, but you will also not lose the 1/4 degree (or more depending on where the hole is) of camber gain. Now, as we can see, both of these changes are good things in one way or another. Which change is appropriate in which situation? For autocross, where your suspension is constantly moving, its a low speed course, short radius turns, etc. then the camber gain is very important and the bumpsteer issue is less important. In this applicaiton, moving the lower control arm position is the best choice. For road racing where you are traveling at high speeds, and speed maintenenace/smoothness is critical, then the bumpsteer spaces are the best choice. And to answer the obvious next question: no, I don't know if doing both is good or bad. I'll have to put that in my little computer program, but at first glance it looks like it might be a bad idea. FYI... when I am talking about bumpsteer spacers, I'm not talking about the ones sold by MSA. They are the wrong thickness and shape. ------------------ John Coffey johnc@betamotorsports.com [This message has been edited by johnc (edited January 19, 2001).]

If money is not a big issue, companies like Painless will custom make a harness for any vehicle. You just have to send them your old harness. ------------------ John Coffey johnc@betamotorsports.com

I think most "regular" anti-roll bars are made from the same kind of spring steel. The only real difference would be bar thickness and end link type (stamped or welded on). Again, this is only a guess on my part. The real differences in anti-roll bars comes when you start comparing regular with tubular and blade. There you see huge differences in quality and tuneability (and price). ------------------ John Coffey johnc@betamotorsports.com [This message has been edited by johnc (edited January 11, 2001).]

They make trailer hitches. ------------------ John Coffey johnc@betamotorsports.com

Monocoque Taylor/Litespeed Complete Custom Wheel Kiezer... Will all make custom wheels for the Z. I have a set of 15 x 8 Monocoques on my 240Z and they cost $280 each back in 1994. All the above are comparable in price and quality. ------------------ John Coffey johnc@betamotorsports.com

What problem are we trying to solve by choosing between Rabbit and MR2 (or any other) ready made strut inserts? We are trying to get the right shock valving for our particular 240Z application. In general, rebound damping is what we are most concerned with because that is what most folks use to tune with. Without going into a lot of discussion, as Mike has said, the MR2 inserts have the best rebound damping range for a BSP autocross and ITS road race 240Z. So, if your 240Z has close to a 50/50 weight distribution, weighs under 2,500 pounds, and your front spring rate is 300 lb. in. or under, I would go with the MR2 inserts in the front. ------------------ John Coffey johnc@betamotorsports.com

What kind of traction are you looking for? Do you drag race? Do you autocross? Do you road race? Do you rally? Do you drive on the street? - John ------------------ John Coffey johnc@betamotorsports.com