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Author Topic: Weight and Balance  (Read 11983 times)

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Offline jbryant200

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Weight and Balance
« on: June 20, 2014, 11:50:26 AM »
Hi, my name is Jeff Bryant. I live in Northern California and am a second generation land speed racer. My father started coming to Bonneville in 1957 (Tom Bryant). I have been coming since the mid ‘60’s.

I have been a private pilot since 1991. I hold (have held) 200 mph records at Bonneville, El Mirage, and Muroc. On the way to the Bonneville 200 mph record I drove our D Competition Coupe. I crashed it at over 200 mph. Fortunately, I was only bruised in the ordeal. We made many changes over the years and learned about concepts called “Weight and Balance” and “Center of Pressure”. These are physical truths that apply to landspeed racing and flying (Private pilots are required to know their “numbers” related to centers of pressure and balance). If you don’t pay attention to them, you can load your plane in such a way that you can take off but not safely land. This information is a matter of life and death and not theory.

Over the years we have found that these physical “truths” apply to landspeed racing in the same “life and death” way they do with private pilots and their planes. This became painfully clear when we lost my youngest brother, Barry, in August of 2009 at Bonneville.

Since that day, my “therapy” has been to immerse myself into safety at Bonneville. I have been a tech inspector for several years now and am reaching out through this forum to tell other racers about the important calculations called “weight and balance” and “center of pressure”.

This information is not official and SCTA does not require that racing teams include this information in the design and modification of their vehicles. But I believe it is imperative that we know about these things. Here’s why:

Every landracing team should assume that someday their vehicle might leave the ground. Denial of this possibility puts your life at risk. A properly configured vehicle will keep its nose relatively straight when off the ground, much like a dart. That way the chute remains effective and helps stop the car. There are exceptions to this, but the odds of safely coming to a stop increase with a properly configured car. How your vehicle handles in crosswinds and the effectiveness of your steering are also affected by these calculations.

So, if you are building a new vehicle, have a vehicle that progressively gets harder to handle the faster you go, have a vehicle that seems to spin easily, and especially if you plan to exceed 200 mph, you should know about and apply this information. KNOW YOUR NUMBERS. We can discuss your question here or I can be reached at jbryant200@hotmail.com.

P.S. We knew our numbers on our competition coupe and still got in trouble because we did not RECALCULATE after making modifications to the car. Don’t you make the same mistake. Always recalculate whenever you change the weight distribution or shape of the vehicle. You always want the center of gravity to be four to six inches in front of your center of pressure.

I’ve heard many in landspeed racing say they have trouble with traction and handling. The common thought is to add weight to the rear of the vehicle when tires spin. This can be seriously dangerous in some cases. ALWAYS make changes to your vehicle with the proper configuration of "The Numbers" in mind.


Offline Seldom Seen Slim

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Re: Weight and Balance
« Reply #1 on: June 20, 2014, 12:10:05 PM »
"My father started coming to Bonneville in 1957 (Tom Bryant)."

Jeff, I think that at least one or two of us on this Forum know you and your family. :roll:  Okay, maybe a bunch of us.  Hey - dang near everyone knows Tom Bryant and his family and the very successful racing career - that has turned into legend.  Thanks for taking time to broach this subject.

Oh, yeah -- and say hi to your dad for Nancy and me, too.  Thanks.
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Offline SPARKY

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Re: Weight and Balance
« Reply #2 on: June 20, 2014, 01:56:15 PM »
Jeff,  thank you SO much for taking the lead on this  Center of Pressure in relation to Center of Gravity is so critical if our cars step out on us  ---Scrub Radius with to much CASTER has a lot to do with them getting out of shape in the first place.  On my new car I am in the process of working out the CP/CG before I go to T&T in July   I would like to have more weight in the back but before I can put more in the back I need more in the front---I am looking at a SOILD BAR frt axel  ---I have few other easy  :? options
Miss LIBERTY,  changing T.K.I.  to noise, dust and RUST!!!

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Offline manta22

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Re: Weight and Balance
« Reply #3 on: June 20, 2014, 02:51:41 PM »
Jeff;

I think your admonition about adding weight to the rear of a car changing its CG in relation to the CP speaks for using a wing to add rear downforce. True, the additional drag is a penalty but if you have excess HP creating wheelspin, it may be a good tradeoff.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline dw230

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Re: Weight and Balance
« Reply #4 on: June 20, 2014, 03:26:12 PM »
Neil,

You still have to within the rules for your class.

DW
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Offline toclub

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Re: Weight and Balance
« Reply #5 on: June 20, 2014, 03:31:26 PM »
How do you calculate the center of pressure?

Offline Sumner

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Re: Weight and Balance
« Reply #6 on: June 20, 2014, 03:34:11 PM »
Jeff;

I think your admonition about adding weight to the rear of a car changing its CG in relation to the CP speaks for using a wing to add rear downforce. True, the additional drag is a penalty but if you have excess HP creating wheelspin, it may be a good tradeoff.

Regards, Neil  Tucson, AZ

Jeff good to see you posting here and bringing forth a topic that many more people should become conscious of.

Niel you make great points and we are moving in that direction but have been very conservative in the shape of and the use of the wing so far as a wing at speed can generate some really large numbers in regards to downforce.  The really big advantage to using....



..... a wing in my opinion is that now....



...you are allowed to have vertical stabilizers that can play a huge impact on increasing the center of pressure on the car immediately which as Jeff pointed out can make the car a lot safer.  The increase in CP can let you add more weight forward of the rear axle for added traction and as you become better acquainted with the use of the wing you could remove some of that ballast for better acceleration since weight hurts you there.

Not all classes allow the ....



...wing and verticals but if you are in a class that does you can add a lot of safety to the car.  On one of the license runs at about 180 the throttle stuck wide open and the car immediately started to go around with the tires spinning for over 3 seconds according to the data log.  I thought the car was too far sideways to pull the chute at first and didn't want it wrapping around the car so didn't.  Just then the vertical stabilizers caught the car and sent it the other way with the tires still spinning.  The verticals caught the car again and as it came back towards middle the chute went out and caught the car.  I'm a really big believer in them now.

There are two videos of the run here...

https://www.youtube.com/watch?v=xaGf2Dh7bPo

https://www.youtube.com/watch?v=mKGZrCyLSEI

It would be nice to have on here and possibly on the SCTA site an explanation of how to figure where your car's center of pressure is with relation to the center of gravity with some examples of how to roughly figure this for those people who 'kind of understand' what we are talking about here.  You can get a fair idea of all of this with a good side view of the car and a couple jacks,

Sum

 

Offline Sumner

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Re: Weight and Balance
« Reply #7 on: June 20, 2014, 03:36:45 PM »
How do you calculate the center of pressure?

I mentioned briefly one way in a post here...

http://www.landracing.com/forum/index.php/topic,9404.1710.html

Sum

Offline bubruins

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Re: Weight and Balance
« Reply #8 on: June 20, 2014, 04:16:57 PM »
Is Center of pressure calculated by side profile surface area the best indicator for how a car will react sideways in the air? I am just learning here, but I've got to think that the coefficient of drag over different parts of the car as you would look at it from the side has to influence it. For example - on the stabilizers of Hooley's car they are completely flat. Imagine trying to push a car with a flat front through the air forwards. I think that in the same way a big flat surface is difficult to push through the air sideways, it would be easier to push a round surface through the air. If the surface was round it would still count towards the center of pressure calculation, but may slip through the air with a much less drag. Does that sound crazy?

Offline Sumner

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Re: Weight and Balance
« Reply #9 on: June 20, 2014, 04:22:50 PM »
Is Center of pressure calculated by side profile surface area the best indicator for how a car will react sideways in the air? I am just learning here, but I've got to think that the coefficient of drag over different parts of the car as you would look at it from the side has to influence it. For example - on the stabilizers of Hooley's car they are completely flat. Imagine trying to push a car with a flat front through the air forwards. I think that in the same way a big flat surface is difficult to push through the air sideways, it would be easier to push a round surface through the air. If the surface was round it would still count towards the center of pressure calculation, but may slip through the air with a much less drag. Does that sound crazy?


What you are saying is all true but show me an easy way to figure that  :-).  Using the surface area will probably get you close and probably show a worst case situation and then take in the surface contours like you said to see if you are erring on the safe side or not.  Talked about that in Sparky's build where the link above went to,

Sum


Offline manta22

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Re: Weight and Balance
« Reply #10 on: June 20, 2014, 06:05:33 PM »
Neil,

You still have to within the rules for your class.

DW

That goes without saying, Dan. Right you are.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline 631

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Re: Weight and Balance
« Reply #11 on: June 20, 2014, 09:12:25 PM »
Note the article on CP CG in the tech & FAQ section.

Offline WOODY@DDLLC

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Re: Weight and Balance
« Reply #12 on: June 21, 2014, 10:44:38 AM »
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
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Offline Sumner

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Re: Weight and Balance
« Reply #13 on: June 21, 2014, 11:16:02 AM »
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! ...

While I agree that you aren't going to get a totally accurate CP with a 2D side view I think it is better than doing nothing.  With that in mind we need some method that we can use were we can at least see if we are close to having a problem.  The majority of us don't have the money for wind tunnel tests and/or CFD studies, yet we will still take our cars to the salt.  What do you suggest we do to fill that need?



On Sparky's car above what is going on that moves the CP up to the CG if the CG is where he said it was?  There is a lot more body behind the CG than in front of it and it is a lot flatter also for much of that length at the back of the car.

Thanks,

Sumner


Offline jl222

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Re: Weight and Balance
« Reply #14 on: June 21, 2014, 11:29:45 AM »
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
« Last Edit: June 21, 2014, 11:45:38 AM by jl222 »