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Author Topic: Seeking ballasting and alignment input  (Read 3253 times)
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QikNip
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« on: May 31, 2016, 10:10:23 PM »

With my 1979 Porsche 924 Classic Production build finally nearing its' end, I scaled the car today to see where all my work came out to get the car as close as possible to just over 50% rear weight and as near to 25% on each wheel. BTW, those goals were gleaned from talking to several LSR racers, so I hope they're valid! With my weight ballast in the drivers seat, a full fuel load, and all fluids and equipment in place the car totalled 2,584 pounds (that includes 40 pounds of ballast located in the center rear) with the varous ways to measure that came out as follows:

LF weight 661 pounds or 25.5%
RF weight 612 pounds or 23.6%
LR weight 651 pounds or 25.1%
RR weight 660 pounds or 25.5%

Total left side weight 1,312 pounds or 50.7%
Total right side weight 1,273 pounds or 49.2%
Total front weight 1,273 pounds or 49.2%
Total rear weight 1,311 pounds or 50.7%

The LF/RR cross weight is 1,321 pounds or 51.1%
The RF/LR weight is 1,263 pounds or 48.9%

With all this as background, do I stand pat or should I tinker with the ballast to get more rear weight, or perhaps try to get closer to a 50/50 cross weight (assuming that matters much at all in LSR)? Likewise, I plan to align the front end at near zero camber, a very small amount of toe in and lots of caster. In the rear I plan to go zero camber and zero toe. In the case of both the intended alignment and ballasting issue, I'd appreciate any input as this is my first foray into the LSR sport after many years of road racing.
Rick
« Last Edit: May 31, 2016, 10:12:07 PM by QikNip » Logged

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SPARKY
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« Reply #1 on: May 31, 2016, 11:05:42 PM »

This is a loaded question  rolleyes  grin

Wow---how much torque will you eng produce?  what is the rotation direction?  can you detect body roll on pave under hard acceleration?
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Miss LIBERTY,  changing TKI  to noise, dust and RUST!!!

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« Reply #2 on: May 31, 2016, 11:12:49 PM »

I would ask... how fast are you trying to go?
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Michael LeFevers
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« Reply #3 on: June 01, 2016, 07:24:56 AM »

In reading through the opening statement I thought “Gee, this guy is building a road racer”, then I got to the last lines.
As I recall, the 924 has a driveline torque tube, so engine torque and consequent weight shifting is not a consideration.
Where is the center of pressure?  If target speed is realtively low, may want rearward weight bias for traction.  If relatively high, weight forward of CP for aero stability but rearward for traction.  The everpresent rear-drive LSR conundrum.
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QikNip
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« Reply #4 on: June 01, 2016, 07:43:50 AM »

I would ask... how fast are you trying to go?

My absolutely fondest hope is a miserly (by LSR standards) 160. At least that's what all the charts I can find say it could do under perfect conditions given the car's weight, frontal area, drag and given the dyno results for the engine. :
Rick
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QikNip
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« Reply #5 on: June 01, 2016, 07:48:57 AM »

In reading through the opening statement I thought “Gee, this guy is building a road racer”, then I got to the last lines.
As I recall, the 924 has a driveline torque tube, so engine torque and consequent weight shifting is not a consideration.
Where is the center of pressure?  If target speed is realtively low, may want rearward weight bias for traction.  If relatively high, weight forward of CP for aero stability but rearward for traction.  The everpresent rear-drive LSR conundrum.


You are correct on all counts regarding the car's design. As I mentioned above, the speed should be in the 150-160 range. Assuming that range is considered "relatively low" how much bias to the rear would you recommend?
Rick
P.S. Any thoughts regarding the alignment I mentioned in the post?
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QikNip
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« Reply #6 on: June 01, 2016, 07:53:50 AM »

This is a loaded question  rolleyes  grin

Wow---how much torque will you eng produce?  what is the rotation direction?  can you detect body roll on pave under hard acceleration?

Sparky:
The torque is pretty low (210 lbs) and because the car uses a torque tube driveline, and because the suspension is pretty stiff, I'd assumed that body roll imposed by engine rotation shouldn't be much of a factor. Do you agree?
Rick
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Peter Jack
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« Reply #7 on: June 01, 2016, 08:00:53 AM »

Rick, at this point I'd get it as low as possible with a little rake in it and then go out and try it and let the car tell you what it wants. Your alignment sounds really good for what you're trying to do. If you can get the weights a little closer it might help, but I doubt it. Your road race experience is showing through and you're probably a lot closer than most of the production based the cars running out there.

Pete
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QikNip
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« Reply #8 on: June 01, 2016, 08:33:28 AM »

Rick, at this point I'd get it as low as possible with a little rake in it and then go out and try it and let the car tell you what it wants. Your alignment sounds really good for what you're trying to do. If you can get the weights a little closer it might help, but I doubt it. Your road race experience is showing through and you're probably a lot closer than most of the production based the cars running out there.

Pete

Thanks Pete:
I've got about two inches of rake in the car now but (regrettably) the ride height is a lot higher than I would like. Since the car uses torsion bars in the rear suspension, lowering that end is a fairly major undertaking that I'm saving for the post LSR season.
Rick
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SPARKY
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« Reply #9 on: June 01, 2016, 08:36:32 AM »

After looking at the body shape of a 924--- I am with PJ, but I would also come up with a way to monitor the body lift as speeds increase.

 I have no idea how lift neutral that body is-- but it looks like in might not be.
« Last Edit: June 01, 2016, 08:39:21 AM by SPARKY » Logged

Miss LIBERTY,  changing TKI  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
   Center for Self Governance            tncsg.org     mrspowell.org

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« Reply #10 on: June 01, 2016, 08:48:36 AM »

Run it like it is.... or balance it closer, won't know till you run it  undecided
take a couple hundred pounds of ballast with you. (with proper facility to secure it, I would have it mounted for inspection or  take it back to inspection if you decide to run it)
Then follow the "old salt" advice....
if the tires slip add weight, if they don't add HP  cheers
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« Reply #11 on: June 01, 2016, 02:02:33 PM »

As a road racer you are probably familiar with bump steer.  While the typical LSR course is pretty smooth, there are still some bumps or heaves, and depending on your aero lift/downforce there may be suspension movement due to that.  About the last thing you want is to have some dynamic toe-out, front or rear.  As a starter, static toe-in should probably be sufficient to preclude toe-out under likely suspension travel.
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Rex Schimmer
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« Reply #12 on: June 01, 2016, 05:30:56 PM »

Using a coefficient of friction for the salt of .35 ( it can easily vary between .2 and .5) and multiplying your rear weight  by it give approx. 400 lbs of max forward thrust available. Using your target speed of 160 mph this thrust would be equal to around 170 HP, you probably have more, so I would suggest that you have some ballast available and a place to legally mount it. Preferably in the back biased to the left side of the centerline.

Rex
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QikNip
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« Reply #13 on: June 01, 2016, 09:27:08 PM »

Using a coefficient of friction for the salt of .35 ( it can easily vary between .2 and .5) and multiplying your rear weight  by it give approx. 400 lbs of max forward thrust available. Using your target speed of 160 mph this thrust would be equal to around 170 HP, you probably have more, so I would suggest that you have some ballast available and a place to legally mount it. Preferably in the back biased to the left side of the centerline.

Rex

Rex:
Thanks. You're correct on HP. It dynoed at 205, but that was at nominal sea level, so it'll probably loose 30 HP at Bonneville, leaving me pretty much at your referenced number. As for adding rear ballast, at what percent rear weight distribution will I see the car begin to have handling issues? I've been told anything over 55% isn't advised. As for biasing the rear ballast to the left side, is that biasing related to torque unloading the left rear wheel? BTW, the rear gear is locked.
Rick
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« Reply #14 on: June 01, 2016, 11:32:20 PM »

Replicating the process Rex undertook, I get a result that, given the static weight distribution and a 0.35 coefficient, the coefficient would support 195 Hp at 160 mph.  So, it would appear you would likely be power limited (to something less than 160 mph).  (Was the 205 hp resulting from an engine dyno or chassis dyno?)  Are tire size, gearing, and rev range compatible with this speed?
As far as biased ballast, I think Rex may have missed the part about the torque tube.  However, lead or steel plates bolted into the footwells is a good location, that is, low in the car, if it is seen to be needed.

There is no clear and fast rule about advisable weight distribution.  Each car will be different.  The 55% you were told is probably the result of not wanting to get too much in the rear which could lead to aero instability.  It depends on the car’s aerodynamics and how close to instability you are comfortable with.  Roadster drivers explore this frontier all the time.

Also bear in mind that due to aero drag of about 458 lb, there will be some front-to-rear weight transfer which would help load the rear, however, the rear is liable to be generating some lift, which would detract from the rear loading.
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