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View Full Version : Woah.. skidpad calculated out to 1.23Gs



halz
08-11-2002, 01:57 AM
After finishing the install of a B&M shifter in my '89 240SX, I took a drive out to the theaters to meet up with some friends and catch a crappy movie at 12:50am..

In the vicinity of my place of work (not the theaters http://forums.freshalloy.com/ubbthreads/images/icons/wink.gif ) at mid-night, I decided to go ahead with something I've always wanted to do.. a skidpad.

Seeing the familiar empty parking lot of my work a decent place, I when't at it..

I counted out 7 parking stalls adjacent to each other and kicked over a banana peel (it was sun-baked to a crisp). Got in the car, and took a few laps around my two points of reference (the end of the 7 stalls, and the banana peel). Once getting the steering wheel at an angle that allowed me to round both points without turning the wheel any more, I started to push it.

I took it up to 25-30mph without getting a sound from the tires.. I thought if I were to make a noise on the pavement, for some reason, a police officer would spawn al a Serious Sam and give me a ticket for 'show-boating', so I ended up not pressing to the point of having the tires break.

I measure the width of each parking stall with my shoes (heel to toe, exactly 12") out to 8.5'. I added one more parking stall to make up for my cars diameter (the diameter of the skid pad needs to include the width of the car, as the overall skidpad diameter should be taking from the center/middle of the chassis). The total diameter therefore came out to 68ft.

The formula used to calculate lateral acceleration is 1.227 * (radius / time^2)

Because I didn't have a stop watch, I'll have to find the 'time' based off of my speed..
pi(D) = 213.62ft as the circumference of the circle. Using the most conservative speed, 25mph is 33.6ft/sec. So, 213.62 / 33.6ft/sec is 5.82 seconds for a full lap. Plugging the numbers into the equation:

1.227 *( 34 / 5.82^2 ) = 1.23

I'm pretty stoked, considering the suspension isn't all of what I feel it could be. After installing all of the components thus far, it hasn't even had a proper alignment!

The parts installed so far:
-Drummond Motorsports 40mm front, 50mm rear struts. Spring rates of 450lbs front, 350lbs rear. Car is at stock ride height
-Tein front camber pillow ball mounts, rear pillow ball mounts. Camber set on the plate to '0' in the front
-Whiteline 27mm front and 20mm rear adjustable anti-roll bars (they're still on the 'softest' setting from when I first installed them)
-RSPS adjustable front tension control rods (rod length is still stock)
-Pilotmotorsports front strut tower bar (at the time of the skidpad, the rear strut tower bar was not installed)
-'93 Infinity J30 VLSD
-Tires installed on the stock 15x6.5 steelies- (this is almost funny)
FL: Falken 'FK-06U', 195/60R15 88H Treadwear: 360
FR: Falken Ziex 'ZE-502', 195/60R15 88H Treadwear: ???
RL: Dominator Sport SR4, 195/60R15 87T Treadwear: 440
RR: Dominator Sport SR4, 205/60R15 90T Treadwear: 440

Once I figure out the maximum wheel-well clearances, I'll be looking for a set of 16" wheels to fill the gaps (16x8.5 +22, 16x9.5 +22, maybe? fender flares needed, I think). I'll still have to stiffen up the anti-roll bars, lower the car 1.5", corner weight, and get an alignment.. pushing front caster to the max, -2 front camber, -.5 toe out, -1 rear camber, 0 toe, I think.

Mav1178
08-11-2002, 06:16 PM
But the question is, where is your calculation wrong?

I ask because Don Nimi pulled 1.2 lateral G's on RACE TIRES.

-alex

AZ_Dave
08-11-2002, 07:54 PM
ZzzzZzzz

Reading that post is worth like 2 credit hours...

ADAM HUTCHINSON
08-12-2002, 03:36 PM
don't think so on 440 treadwear rubber....i can pull a max of 1.18G's on BFG 00 treadwear race tires..and thats with proper camber for them as well

uiuc240
08-12-2002, 03:46 PM
main problem is that skidpad isn't a normal size. the "standard" is 100-ft diameter. this discrepancy alone will cause wierd numbers. kinda like comparing 70-0 braking and 60-0 braking. or a 200-ft slalom and a 300-ft slalom. also, keep in mind that even 1.2 g's isn't that high for transient maneuvers. alex, i bet you can hit almost 3 g's around some turns at willow. it's sustained roadholding that we're talking about here, though.

halz, go do the test again, but this time make the distance 11 car spaces (9' wide is standard...and they are 20' long usually...with 24' aisles).

Eric

halz
08-12-2002, 05:12 PM
uiuc240:

Right.. I was realizing that the smaller the 'skid-pad', the larger an error would be. Say, a mis-reading of .10 seconds (or however many mph) around a 68ft skid pad as opposed to .10 seconds around a 100ft or 200ft skid pad can make for a HUGE difference between the final calculations

uiuc240
08-12-2002, 05:58 PM
well, just to show the impact:

using your formulas, if you keep everything the same and simply change the skidpad size to 100' diameter, the G's go down to .702. Now, of course you would then be able to go FASTER around the skidpad, but I'm just trying to say that the diameter has a large effect. Plus, the idea is not to keep a steady speed. Or a steady steering angle. The idea is to keep the front inside tire as close as possible to the line of the circle (without going inside it) and then go around the circle as fast as possible. The speed is actually irrelevant. It's a time to distance thing. I'm glad that you were able to have some fun doing this, but really, all your math is basically useless. Without using verifiable test methods, you're really comparing nothing. it would be like me pulling a donut and measuring the outer diameter of the burnout and the time it took to complete it. Ya see? Doesn't compute.

But this was fun food for thought. Thanks!

Eric

halz
08-12-2002, 07:38 PM
</font><blockquote><font class="small[/img]In reply to:</font><hr />
Without using verifiable test methods, you're really comparing nothing.

[/QUOTE]

Aww.. come on.. I used a banana peel as a marker!

uiuc240
08-13-2002, 11:13 AM
heheh! a truely reliable measurement device! dig it.

btw, halz, i wasn't trying to be mean or anything...just trying to explain why you might have gotten HUGE g's.

Eric