weight advice---pic

[davet]

12 cubes behind and 6 ahead of rear axle. I have a bunch of putty behind the rear axle to get a DFW of .62. Will the weight behind the axle cause a problem even though the DFW is good? COM of .68.

[Scrollsawer]

If I'm understanding your post correctly, I don't think you need to worry about the added putty underneath the car and behind the axle. So long as the putty clears the track guide, then you should be good to go.

So long as the car is drifting the way you want (toward the DFW) during your tuning and alignment checks, you're good. I would be tempted to just pack it into that void in your car in front of the rear axle, but I know you're trying to squeeze out better COM numbers by placing it underneath and behind the rear axle.

Scrollsawer

[davet]

If I'm understanding your post correctly, I don't think you need to worry about the added putty underneath the car and behind the axle. So long as the putty clears the track guide, then you should be good to go.

So long as the car is drifting the way you want (toward the DFW) during your tuning and alignment checks, you're good. I would be tempted to just pack it into that void in your car in front of the rear axle, but I know you're trying to squeeze out better COM numbers by placing it underneath and behind the rear axle.

Scrollsawer

Great. We've run well before with the DFW weight and COM I listed above and just wanted to hit that again. We tune on the kitchen table so we won't know until race day.

My boy was applying some of the covering and some paint and called me at work and said he messed it up and we'd have to reapply the covering. Since it was wired up I couldn't just sand the layer off. We applied another over the top and that along with epoxying the covering on the fenders really messed us up for weighting. If we make Districts we may remove the front fenders.

Now, any advice on how to attach the wires to the battery. Tape isn't cutting it. Thanks.

[Stan Pope]

spring clips, commercial or home made.

[davet]

spring clips, commercial or home made.

what type?

[Speedster]

I don't quite understand. With Positive camber and toe-in, will the car not drift toward the Non Dominant Front Wheel?

[davet]

I don't quite understand. With Positive camber and toe-in, will the car not drift toward the Non Dominant Front Wheel?

The wheels in the pic were just stuck on for pic purposes. They are not aligned yet.

My concern was if added weight behind the axle, besides just the 12 cubes, would cause odd handling characteristics. Do 2 cars with the same COM handle the same no matter if it's heavy in front like ours with extra weight behind the rear axle or if it's light in front and has only the 12 cubes behind the rear axle? The COM is the same but doesn't the front of the car with more than the weight of 12 cubes behind the axle tip up quicker and easier?

[Speedster]

I was confused by a previous post by scrollsawer. Perhaps I'm reading it improperly.

[Stan Pope]

Differences between cars with identical CM but different arrangements are "lost in the noise." I've heard of no one who has produced experimental results that "barbell" weighting is inferior to "concentrated" weighting. Nonetheless, I would concentrate ballast to the extent convenient.

[Scrollsawer]

Not that I've used them, or have any idea as to their effectiveness, but are wheel weights considered 'barbell weighting'? I am curious as to the proliferation of this type of weighting, over, say, concentrated (cube or molten lead) weighting.

[davet]

Differences between cars with identical CM but different arrangements are "lost in the noise." I've heard of no one who has produced experimental results that "barbell" weighting is inferior to "concentrated" weighting. Nonetheless, I would concentrate ballast to the extent convenient.

At Council last year I saw a couple cars get light front light off the transition. I asked a tech inspector (memeber here) if he knew anything about why. He said he believed they were weighted with the wheel weights.

So Stan, you're saying we'll be OK with the weight of roughly 13 cubes plus another .04 oz behind our rear axle and a COM of .62 when compared to last years car that had 12 cubes behind the rear axle and a COM of .62?

[davet]

[quote="davet"][quote="Stan Pope"]Differences between cars with identical CM but different arrangements are "lost in the noise." I've heard of no one who has produced experimental results that "barbell" weighting is inferior to "concentrated" weighting. Nonetheless, I would concentrate ballast to the extent convenient.

At Council last year I saw a couple cars get light front light off the transition. I asked a tech inspector (member here) if he knew anything about why. He said he believed they were weighted with the wheel weights. I'm guessing that those running wheel weights are using them to get lower COMs than would be possible with the cubes only. I don't think that it's the placement of the weight but, rather, the COM that made them light in front.

So Stan, you're saying we'll be OK with 12 cubes behind plus an additional .21 oz of putty spread on bottom behind our rear axle and a COM of .62 when compared to last years car that had 12 cubes behind the rear axle and a COM of .62?

[Stan Pope]

Differences between cars with identical CM but different arrangements are "lost in the noise." I've heard of no one who has produced experimental results that "barbell" weighting is inferior to "concentrated" weighting. Nonetheless, I would concentrate ballast to the extent convenient.

At Council last year I saw a couple cars get light front light off the transition. I asked a tech inspector (memeber here) if he knew anything about why. He said he believed they were weighted with the wheel weights.

So Stan, you're saying we'll be OK with the weight of roughly 13 cubes plus another .04 oz behind our rear axle and a COM of .62 when compared to last years car that had 12 cubes behind the rear axle and a COM of .62?

Yes, if, in fact, your CM's are/were 0.62". Measuring CM by balancing is difficult and more error prone than measuring by weighing and calculating.

I would consider wheel weights as "barbell" only in the side-to-side CM location (CMy). CMy has small role in the apparent lightness of the front end. For fore-aft location (CMx) the CM of the wheel weights is quite close to the desired overall CM location. The CMz (distance above or below the rear axles) may play a larger role in the "startup front end lightness" because if CMz is below the axle, the front end weighs less when the car is on the slope and may be more prone to pop a wheelie!

[davet]

At Council last year I saw a couple cars get light front light off the transition. I asked a tech inspector (memeber here) if he knew anything about why. He said he believed they were weighted with the wheel weights.

So Stan, you're saying we'll be OK with the weight of roughly 13 cubes plus another .04 oz behind our rear axle and a COM of .62 when compared to last years car that had 12 cubes behind the rear axle and a COM of .62?

If, in fact your CM's are/were 0.62". Measuring CM by balancing is difficult and more error prone than measuring by weighing and calculating.

I would consider wheel weights as "barbell" only in the side-to-side CM location (CMy). CMy has small role in the apparent lightness of the front end. For fore-aft location (CMx) the CM of the wheel weights is quite close to the desired overall CM location. The CMz (distance above or below the rear axles) may play a larger role in the "startup front end lightness" because if CMz is below the axle, the front end weighs less when the car is on the slope and may be more prone to pop a wheelie!

Oh man Stan. I wish I could be in your head when you come up with this because maybe I'd understand it better. We came to our COM by weighing DFW and by placing rear wheels on 2 identical scales both calibrated to 300 grams. We enter those numbers and other variables (left or right DFW and WB) into the fsfiddle COM calculator.

[Stan Pope]

If, in fact your CM's are/were 0.62". Measuring CM by balancing is difficult and more error prone than measuring by weighing and calculating.

I would consider wheel weights as "barbell" only in the side-to-side CM location (CMy). CMy has small role in the apparent lightness of the front end. For fore-aft location (CMx) the CM of the wheel weights is quite close to the desired overall CM location. The CMz (distance above or below the rear axles) may play a larger role in the "startup front end lightness" because if CMz is below the axle, the front end weighs less when the car is on the slope and may be more prone to pop a wheelie!

Oh man Stan. I wish I could be in your head when you come up with this because maybe I'd understand it better. We came to our COM by weighing DFW and by placing rear wheels on 2 identical scales both calibrated to 300 grams. We enter those numbers and other variables (left or right DFW and WB) into the fsfiddle COM calculator.

Good!

Now, apply that method with the cars angled nose-down at 30 degrees. The greater the CMz, the more the weight is shifted toward the DFW at the starting line. (This is why when measuring CM by weighing one must take care that all the scale pans are at the same level!)

Are the CMz values for both cars the same?