weight advice---pic

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davet
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Re: weight advice---pic

Post by davet » Tue Jan 26, 2016 12:10 pm

I think I get it. When you say that the greater the CMz you mean the higher on the car right? So our extra .21 oz of putty added below the rear 12 cubes would lower the weight on the DFW on the grade. Does this mean that at the transition it would be more likely to "wheelie" than a similar .62 oz DFW car that simply has a lighter front body and only 12 cubes behind the axle?



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Scrollsawer
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Re: weight advice---pic

Post by Scrollsawer » Tue Jan 26, 2016 3:38 pm

You're not going to pop a 'wheelie' with that extended wheelbase. Wheelies are more a function of too much weight behind the rear axles, versus weight residing underneath the rear axles.

But I understand your posit, and will let Stan speak to that one.

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davet
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Re: weight advice---pic

Post by davet » Tue Jan 26, 2016 5:35 pm

I don't know how to get the CMz value. Is that how high above or below the axle line the weight is?



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davet
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Re: weight advice---pic

Post by davet » Tue Jan 26, 2016 5:38 pm

Scrollsawer wrote:You're not going to pop a 'wheelie' with that extended wheelbase. Wheelies are more a function of too much weight behind the rear axles, versus weight residing underneath the rear axles.

But I understand your posit, and will let Stan speak to that one.

Scrollsawer
That's what we have here--- .21 oz of putty behind the rear axle and stuck to the bottom of the 12 cubes. Seems to fit OK there. We'll tape the bottom of the car and run her. Who knows. Even if it's slow...it will catch some attention with the lights.

Thanks guys.



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Re: weight advice---pic

Post by Scrollsawer » Tue Jan 26, 2016 7:22 pm

Davet,

I may be wrong, but I don't think there's a specific measure or formula (at least not to my knowledge) when it comes to the optimal location of weight vertically at the axle line. I can say that most positive experiences that I've read about on this forum (with regard to vertical weight location along the axle line) is that folks tend to report better results and performance when the weight is either at or below the axle line.

That's why packing the tungsten cubes right around the axles is preferred, and also why your putty weight location is good right where you put it. At, or behind the rear axles is where you want weight to be. With the extended wheelbase, and those electronics embedded in the middle of the car, you will not pop a wheelie, and the weight location behind the axle will benefit you with regard to optimizing the potential energy of the car while keeping the car's run stable, including at transition.

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davet
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Re: weight advice---pic

Post by davet » Tue Jan 26, 2016 8:41 pm

Scrollsawer wrote:Davet,

I may be wrong, but I don't think there's a specific measure or formula (at least not to my knowledge) when it comes to the optimal location of weight vertically at the axle line. I can say that most positive experiences that I've read about on this forum (with regard to vertical weight location along the axle line) is that folks tend to report better results and performance when the weight is either at or below the axle line.

That's why packing the tungsten cubes right around the axles is preferred, and also why your putty weight location is good right where you put it. At, or behind the rear axles is where you want weight to be. With the extended wheelbase, and those electronics embedded in the middle of the car, you will not pop a wheelie, and the weight location behind the axle will benefit you with regard to optimizing the potential energy of the car while keeping the car's run stable, including at transition.

Scrollsawer
"Nuf said then. I'll quit trying to find a problem where there isn't one. Thanks guys.



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Stan Pope
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Re: weight advice---pic

Post by Stan Pope » Wed Jan 27, 2016 1:19 am

re lowering CMz ... I'd only have concern if you were already right "on the edge." I don't think you are there.

re reference point for measuring CMz ... It is usually measured relative to the axle, positive as it rises toward the top of the car. I can not justify saying which part of the axle though. I think it would be either the axle center or the axle contact surface (bottom of the axle.) If the axle is cambered, I think that the part of the axle would be lowest part along the axle. I'm having trouble visualizing the physics of this tonight! :(

Lowering CMz provides a slightly greater potential energy that is turned into a (very) small amount of extra speed at the bottom of the slope.


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davet
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Re: weight advice---pic

Post by davet » Wed Jan 27, 2016 1:42 pm

I've been wondering how guys get COM's of .50". I was playing with the COM calculator last night and see that just by moving the front axles back the COM goes down. Looking closer at your posts in this thread I think I've been trying to get short wheelbase numbers out of a long wheelbase car. The long wheelbase seems to be pretty conservative design feature that probably won't allow a hack like me to go too aggressive on the COM.

It's funny how it takes having the same point explained a couple different ways can make it finally make sense. That and a good nights sleep.



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Re: weight advice---pic

Post by Stan Pope » Sat Jan 30, 2016 3:48 pm

davet wrote:I've been wondering how guys get COM's of .50". I was playing with the COM calculator last night and see that just by moving the front axles back the COM goes down. Looking closer at your posts in this thread I think I've been trying to get short wheelbase numbers out of a long wheelbase car. The long wheelbase seems to be pretty conservative design feature that probably won't allow a hack like me to go too aggressive on the COM.

It's funny how it takes having the same point explained a couple different ways can make it finally make sense. That and a good nights sleep.
I consider my "pups" cars to be "long wheel base," but they are not as long as possible. Since we may drill axle holes and locate them so long as wheels do not extend beyond the body, we set the rears even with the back of the car and set the front so that about an inch of "nose" sticks out beyond them. Seems to be a good compromise for us. That makes a wheel base of about 5" and CMx of about 0.59" ... not as aggressive as we have done before, but still in the ball park of "good" for our short tracks, all in a 1/4" thick body.
.


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