Weight distribution

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10range
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Weight distribution

Post by 10range »

Looking for a bit of advice on weight distribution. This year was the first year that my son has been old enough to race/"help" build a car for our Awana race. Most of his help has been watching me as I use tools and me talking with him about how to be safe around them (he is only 2 1/2 but already knows what safety glasses are 8). He was the one that chose the design for my car for the adult race. He is a big lightning McQueen fan so that is what we built.

My question is about weight distribution. This is the first time I have ever built a car where the rear wheels are not as far back as they could go. The rear wheels are moved forward about 3/4 inch. I already know that I am not going to get a perfect distribution as this is my first "full bodied" car with fenders and all the additions. Do I need to be concerned about how far back I get the weight and possibly making the front end to light by having weight too far behind the rear wheels? I am estimating that the car with axles and wheels with no weight added is going to end up in the 3 1/4 to 3 1/2 oz range. Car is a 3 wheeler and plan to rail ride.

Any advice?
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FatSebastian
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Re: Weight distribution

Post by FatSebastian »

10range wrote:Any advice?
10range, we have no personal experience with AWANA cars yet, but I would presume, like 5.0 ounce BSA cars, that you would want between 1/2 to 1 ounce on the dominant front wheel (the former specification being relatively aggressive, and the latter being rather safe). (You can use this equation to convert between front-wheel weight and CoM relative the rear axle.) My feeling, based on your description, is that your likelihood of ending with less than 1/2 ounce on the front wheel seems low.
10range
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Re: Weight distribution

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Had a bit more time to play around with it tonight. Car without any lead weighs 3.2 oz. With this I have 1.35 oz on the front and 1.8 oz rear. With lead added as far back as possible to bring it to 5 oz I measure 1.25 oz on the front and 3.75 rear. This means that I am actually counter balancing the front some by putting the weight in the rear. My concern is that it will cause the front end to get light as it transitions from the slope to flat and cause the car to wiggle. The COM is about 3/4 inch in front of the rear axle. Should I be concerned about loosing traction on the front or will I be ok? Hope I explained this somewhat understandable.
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FatSebastian
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Re: Weight distribution

Post by FatSebastian »

10range wrote:The COM is about 3/4 inch in front of the rear axle.
:thumbup: (...assuming this is after Pb ballast is applied).
10range wrote:Should I be concerned about loosing traction on the front or will I be ok?
If loss of control remains a concern of yours, you could always run a little harder into the rail than you might ordinarily... but we would not be overly concerned about losing traction with 1.25 ounces (25% of the total weight) on the front wheel. Because you are running a shorter-than-standard wheelbase, alignment is likely to be a bit trickier than a standard wheelbase (I admit that shorter-than-standard wheelbases are outside our domain of experience).
10range wrote:This means that I am actually counter balancing the front some by putting the weight in the rear.
Yes, but you seem to be adding counterweight aft the rear wheel because the forward part is already so very heavy. The alternative is to add weight forward of the rear axle which is guaranteed to reduce your competitiveness.
Last edited by FatSebastian on Wed Jan 11, 2012 7:36 pm, edited 1 time in total.
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pwrd by tungsten
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Re: Weight distribution

Post by pwrd by tungsten »

Can you place the wheels further back?
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10range
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Re: Weight distribution

Post by 10range »

Yes the com I measured was after the lead was added.

Moving the wheels is not an option. I just measured and the back is moved just under 1/2 inch forward.
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Stan Pope
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Re: Weight distribution

Post by Stan Pope »

One of the reasons that rail guiding works is that the lightly loaded DFW creates little friction losses. How would your more heavily loaded front wheel affect the DFW friction losses due to rail guiding? Would you have to turn into the rail more strongly or less strongly to accomplish the same stabilizing effect as is accomplished with light DFW loading?
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10range
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Re: Weight distribution

Post by 10range »

All good thoughts. When I started weighing this thing I had visions of it popping a wheely at the transition :thinking:

I don't have too high of hopes for speed on this thing since it is more about the design, although I won't complain if it is too fast. I am not sure if I will have time to test and tune at the race but if I do, I will probably try to work on the drift and report back if I learn anything.
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