Author Topic: Weight and Balance  (Read 11981 times)

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Offline Sumner

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Re: Weight and Balance
« Reply #15 on: June 21, 2014, 12:08:31 PM »
Note the article on CP CG in the tech & FAQ section.

Thanks, that is a good article ....

http://www.landracing.com/docs/CG-CP_Hakansson-Dube_4-2014.pdf

and is a little more technically orientated finding the CG vs. placing two jacks under the car on each side to find where it balances and where the CG should be above that point.  If I had weight scales available I'd try that and compare it to the 'jack method'.

=======================================================

SSS if you can you might want to fix a small mistake though. There was a mistake in the part on figuring the CG.  It should be....

Rear Left     117.25 ...... 1130 ....... 132492.5  (not 112492.5)

Total ......................... 4674 ....... 292446.0 

292446.0 [inch-lbs.]/4674 [lbs.] = 62.56 [inches]  

I also turned the numerator and denominator around in the line above as that is the way I'm use to seeing them, but maybe I'm wrong,

Sum
« Last Edit: June 21, 2014, 12:11:44 PM by Sumner »

Offline jl222

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Re: Weight and Balance
« Reply #16 on: June 21, 2014, 02:29:56 PM »
You CANNOT get the CP of a complicated 3D object with a 2D side view! FYI: Sparky's CP is right on top of the CG!  I will have more to say on this topic later - just really slammed right now! Meantime take a look at this: http://www.youtube.com/watch?v=WLm4wkTBbuc
Some of you will know this guy has real life experience with CP/CG location!  :-o

  That test shows good results for a road car and ignores why spoilers were invented.

  Any hi hp Bville car set up like that would spin from spinning the tires;

  I know the Lindsley's and Liggett Camaro had a big setback for the engine and a bunch of weight on rear tires, small spill plates and short spoiler and had an average time of 330+ mph. How fast was it going at the end of the mile? They didn't run the last mile.

  Set up as in the video they would have melted the tires or spun.

             JL222


 It seems that air flowing over a down sloped hood would add weight to front and change the cp. But it also adds down force flowing over rear spoilers.

  Lindsley Liggett had trouble with front end rubbing on tires.

        JL222 

Offline SPARKY

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Re: Weight and Balance
« Reply #17 on: June 21, 2014, 02:49:34 PM »
As have others  George Fields comes to mind
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Offline tortoise

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Re: Weight and Balance
« Reply #18 on: June 21, 2014, 03:13:39 PM »
The increase in CP can let you add more weight forward of the rear axle for added traction

I don't think there's anything magical about the rear axle that makes adding weight behind it bad.

The same total weight and center of gravity location can be achieved with weight concentrated near the CG or far from it.  More weight at the ends gives a higher polar moment of inertia, having a good effect; being slow to get out of shape when wheels lose traction, and a bad one; a tendency to keep spinning once you've started.

Road racers like the lowest possible polar moment of inertia. Does anyone have a well-founded opinion as to whether ballast for a land speed racer should be placed to increase or decrease polar moment of inertia?
« Last Edit: June 21, 2014, 03:24:23 PM by tortoise »

Offline Sumner

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Re: Weight and Balance
« Reply #19 on: June 21, 2014, 04:24:00 PM »
....  More weight at the ends gives a higher polar moment of inertia, having a good effect; being slow to get out of shape when wheels lose traction, and a bad one; a tendency to keep spinning once you've started....

I don't have a scientific answer but I don't want that weight at extremes.  It might help initially to keep it from coming around but as you mentioned once it does it will increase the chances that it will go around and sooner or later any car with HP is going to spin the tires and start around.

I once made a pump trailer to pump deep wheels with and it had a lot of weight at the ends of the trailer and was a handful to tow as once it started fishtailing it didn't want to stop.  The next one I designed totally different to keep the weight central and it was a dream to tow.

Other than the run where the throttle stuck on the other runs when I experimented some with WOT and the tires spun and the car started around as soon as I lifted enough to stop the spinning the car immediately straightened out with the new vertical stabilizers.  We have a lot of weight just ahead of the rear axle on both sides and more forward of that and more in the nose.  The car was very easy to drive,

Sum

Offline tortoise

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Re: Weight and Balance
« Reply #20 on: June 21, 2014, 04:43:49 PM »
I don't have a scientific answer but I don't want that weight at extremes.  It might help initially to keep it from coming around but as you mentioned once it does it will increase the chances that it will go around and sooner or later any car with HP is going to spin the tires and start around.

That makes sense. I'd suspect that many builders, deciding that they need x pounds more on the rear, and y pounds more on the front, add those amounts in two slugs, each around one axle. They might do better, if there's room, to put it all in one place.

Offline Interested Observer

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Re: Weight and Balance
« Reply #21 on: June 21, 2014, 06:21:23 PM »
Quote
and the tires spun and the car started around as soon as I lifted enough to stop the spinning the car immediately straightened out with the new vertical stabilizers.

So---was it the re-established traction or the vertical stabilizers that regained directional control?  If the stabilizers were effective in providing aero stability it shouldn’t have gone very far around and should have corrected the yaw even with the rear tires still spinning.

Offline John Burk

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Re: Weight and Balance
« Reply #22 on: June 21, 2014, 06:46:46 PM »
Sprint cars have the engine and driver close to the center for a low polar moment so it can be snapped sideways going into a corner so rear tire thrust adds to cornering Gs . The price of low polar moment is less time for the driver to react when things start going wrong . For land speed you'd think you'd want spins to start more slowly .

Offline Sumner

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Re: Weight and Balance
« Reply #23 on: June 21, 2014, 06:49:22 PM »
Quote
and the tires spun and the car started around as soon as I lifted enough to stop the spinning the car immediately straightened out with the new vertical stabilizers.

So---was it the re-established traction or the vertical stabilizers that regained directional control?  If the stabilizers were effective in providing aero stability it shouldn’t have gone very far around and should have corrected the yaw even with the rear tires still spinning.

On the brief WOT tests I think it was a combination of both that settled things right down.  The car never jumped around to the same extend that it did when the the throttle stuck WOT with the tires continuously spinning.  That time I think the fact that it did not go all the way around indicates that the verticals did stop it. 

I feel there is a big difference in if the CP is just a little behind the CG or a lot behind and the results you could expect to see on the aero stability.  At this point we don't know where the CG is as it was a thrash to just get the car to the salt even 3 days late.  We didn't have corner weights or where the CG was after all the changes to the car and still don't have a good side view to work with to try and figure the CP.  We need to get that done and if someone has a side view from a distance I'd sure like to have it.

With the 8 lb. springs in the wastegates the car was probably gaining around 250-300 HP in less than a second as the car went from no boost to 8 lbs. with the throttle going from 40% to 100% instantly.  We now have 2 lb. springs in the wastegates and that should help since we can then use the E-boost controller to increase the boost over the 2 lb. in more controllable steps.  So a more normal application of the throttle would of probably not resulted in the car jumping out at all.  I was just playing at the time with 'what if I do this'.

I'll share one other item here.  Our two turbos have an AR of 1.32 which most people will shy away from thinking they will spool too slowly.  A second to spool at the drags might be slow but for what we are doing you don't need them to spool faster.   Granted the 572 is big but I'd stay with the same AR even with a much smaller motor.  We are trying to slow the power coming on in our case, if you can hook all the power you can make with a turbo up right away then go with a smaller AR, but I doubt that is the case most of the time,

Sum


Offline Seldom Seen Slim

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Re: Weight and Balance
« Reply #24 on: June 21, 2014, 06:49:56 PM »
I understand your request -- but I'll let the authors of the essay make the changes.  I don't want to do it -- and get it wrong in my own way just from a typo or something.  Rex, if you're seeing this, how 'bout asking Eva or Bill in to check to be sure -- and make a change if necessary?  Thanks.
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Offline Sumner

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Re: Weight and Balance
« Reply #25 on: June 21, 2014, 06:56:22 PM »
Sprint cars have the engine and driver close to the center for a low polar moment so it can be snapped sideways going into a corner so rear tire thrust adds to cornering Gs . The price of low polar moment is less time for the driver to react when things start going wrong . For land speed you'd think you'd want spins to start more slowly .

John I can see what you are saying but in our case at least we have a lot of weight barely ahead of the rear axle and a lot more quite a bit further forward and a lot longer wheel base and weight to begin with than a sprint car. 

Everyone still has to do what "they believe in" but I don't want to see us with a lot of weight hung out back behind the rear axle.  We do have a large and a smaller battery in the trunk area, but that is all,

Sum

Offline Sumner

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Re: Weight and Balance
« Reply #26 on: June 21, 2014, 06:58:18 PM »
I understand your request -- but I'll let the authors of the essay make the changes.  I don't want to do it -- and get it wrong in my own way just from a typo or something.  Rex, if you're seeing this, how 'bout asking Eva or Bill in to check to be sure -- and make a change if necessary?  Thanks.

Jon that is fine, but it is just a multiplication error on the one line that then carries forward to the rest of the calculations.  Probably just a slip on the keyboard at the time,

Sum

Offline RidgeRunner

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Re: Weight and Balance
« Reply #27 on: June 21, 2014, 09:07:22 PM »
The increase in CP can let you add more weight forward of the rear axle for added traction

I don't think there's anything magical about the rear axle that makes adding weight behind it bad.




     Wouldn't the axle location on an unsprung car [and relatively close to it on a sprung car depending on the type of suspension] act like the pivot point/fulcrum on a playground teeter board?  Add weight behind and all would be on the rear end plus some transferred from the front being unloaded.  Add it ahead and most would be on the rear with some going to the front.

      A pound might always be a pound but the location can have a large effect on the end results.  My first drive in an early Tempest with the rear transaxle , hitting a bump while cornering on a slick road comes readily to mind almost 50 years later..........

             Ed

Offline tortoise

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Re: Weight and Balance
« Reply #28 on: June 21, 2014, 09:25:43 PM »
You always want the center of gravity to be four to six inches in front of your center of pressure.

Can the center of pressure to be too far behind the center of gravity?

Offline SPARKY

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Re: Weight and Balance
« Reply #29 on: June 21, 2014, 09:58:08 PM »
not for LSR  in my opinion---I spent a lot of time with Ms Liberty today  I think I have figured out a way to add   a bunch of weight to the nose---but will be a pain---would have to pull the frt body off and build a weight box in the nose--  or build a weight box that will fit the nose area---our it full of lead---slide it forward and bolt and weld it in---sure hope we do not have to do this.  I have one more opinion I am waiting on---I have an opinion, Sum, Doug & Woody have an opinion---they diverge---my job is to decide where and what---if we have a decent track and I do not need more rear weight for traction---ok NA

When we turbo we will need more frt weight and more rear surface area   
« Last Edit: June 22, 2014, 08:21:24 AM by SPARKY »
Miss LIBERTY,  changing T.K.I.  to noise, dust and RUST!!!

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