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How aggressive would you go for weighting on a Scout car?

Discussion in 'Building tips' started by KyleBCostco, Jan 1, 2018.

  1. KyleBCostco

    KyleBCostco Pinewood Ninja

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    My son and I are building a Scout car, stock wheelbase, 5.0 oz rail rider running on 6 lane 42' Best track aluminum track. Here is a picture of us roughing the weights on the build and it on the scales.

    Is .6 oz too light up front? The track seems to be smooth. We have the fastest scout cars running roughly 3.0x with graphite only allowed.

    I'm concerned because this is the most aggressive we've ever gone with weighting.. Help me ease my fears!

    Thanks for your help! Kyle 20180101_134331.jpg
     
  2. TRE

    TRE PWD Royalty Pro Racer Pro Race Winner

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    You should be good..test the car if you can to see how much steer you need..If you can't test i would put 4 to 4.5 inches of steer in it
     
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  3. Ballistic Racing

    Ballistic Racing Workshop Leader #1 Ranked in the USA Pro Racer Pro Race Winner

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    I would slide those three cubes over to the dfw side of the car & bring the single cube back to make four. Weigh the car again & move the single cube back if needed. My guess is the front of the car will still be around .5 oz with the four side by side. When your done with the weights, cover all the holes.
     
  4. KyleBCostco

    KyleBCostco Pinewood Ninja

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    Thanks guys. I'll repost once we glue our weights in. We just finished rough sanding and removing 1/16" on the DFW. Looks like we will have another .1 oz of weight to play with depending on weight of paint and vinyl to cover the bottom...

    Is there a reason for adding weight to the DFW side? Does it make the car more stable? I'm curious because we have always tried to center the weight left to right.
     
  5. Thinkin'Bout Racin

    Thinkin'Bout Racin League Racer

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    Short answer ...yes. The reason is that if you move the cubes to the DRW side, then the load on the rear wheels will even out some. Right now if you were to weigh the two rears, you would probably see 2.45 - 2.55 ounces on the NDRW. The added load on that NDRW will increase friction and therefore drag on that wheel/axle.
     
    Last edited: Jan 5, 2018
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  6. CTDerbyDad

    CTDerbyDad Pinewood Ninja

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    We run on a wooden track and run just under 4 oz. right around the rear axle, but use a dome so the bulk of the weight is more right on top of the axle.

    What do you use to create the weight pockets? We use a series of forstner bits and then clean it up with a Dremel. I know guys use routers, but I can't see letting my kid use even my trim router and I can't see the pine that comes in a kit surviving without tear out.
     
  7. KyleBCostco

    KyleBCostco Pinewood Ninja

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    We have a horrible freight drill press. We use a routing bit 1/4" from horrible freight as well. It keeps my son from putting his fingers near sharp tools and allows him to play with power tools, win win!
     
  8. CTDerbyDad

    CTDerbyDad Pinewood Ninja

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    Looks good.

    How much weight are you using?
     
  9. KyleBCostco

    KyleBCostco Pinewood Ninja

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    Block is roughly 2oz cubes are roughly 1.7 oz. Car weighed out after rough sanding a hair under 4.9...
     
  10. KyleBCostco

    KyleBCostco Pinewood Ninja

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    Still very scared that this cars weighting is too aggressive... thoughts? We still have a bit of weight to add with putty. Where do we place it? By the cubes right in front of rear axle or in the center?
    20180106_153609.jpg 20180106_153321.jpg
     
  11. Thinkin'Bout Racin

    Thinkin'Bout Racin League Racer

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    First and most important, is that I am not a pro, so please consider the information below with that in mind. Also what is your wheelbase? Is it 4 3/8" ? Is the rear 7/8" from the rear? Based on this, right now you have a COG that is about 0.520" in front of the rear axle.

    Is this aggressive weighting... yes and no. Let me explain.

    You still have a few things to take into consideration if this is too aggressive of weighting or not.

    First is paint - I am assuming you are going to paint based on the body shape? (I hate to assume) Based on the amount of surface area in the front, you will gain some weight in the nose because of the distance from the fulcrum point. How much? It depends on number of coats, multiple colors, clear coat, multiple coats? So, you may want to base part of your weighting decision on once the car is painted.

    Second - More importantly, is the quality of your alignment. Did you use the slots? Did you drill above them? Is this a rail runner or 4 touching? If you drilled, did you use a jig? If you are sure that your alignment (drill) is perfect or near perfect, then the weighting can be more aggressive.

    Third - What type of track will you be racing on and what kind of condition is it in? The better the quality (aluminum vs. wood) and condition, the more aggressive the weighting can be.

    Fourth - Will you have a chance to test the car before racing? If so, the weighting while important, can be somewhat mitigated by adding additional steer to the car.

    Fifth - How stiff is the competition and what risk are you willing to take and have the car wiggle in order to win? If you can test before racing, then this element is less of an issue.

    If you have a perfect drill and the alignment is spot on, you have a good quality track that has been well cared for, you weigh the car after paint, you have the ability to test before racing, then I would probably move one more cube aft and then trim weight with putty until the car is stable and runs the best times. Remember, you may be running in a lane next to someone that does not have a stable car and that can cause your car to start to wiggle. Lots of thing to consider...having said all of that, it looks like you have a very nice start to your car!

    Only you can answer the above questions and then make a decision based on all of the information.

    Good luck, and let us know how it turns out!
     
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  12. KyleBCostco

    KyleBCostco Pinewood Ninja

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    Thank you for the detailed post.

    We can not run prior to the race and I don't have access to a track.

    Competition is tough. Last year we thought we had done every trick and my son didn't get 1st in his den. Sooooo

    My thought is put 4" steer over 4ft. It is a railrider. I think we will keep the 2 cubes up (since they are glued) and just add the remainder of weight by the rear axle hopefully it will even out the effect of the paint. *crossing fingers*
     
  13. Thinkin'Bout Racin

    Thinkin'Bout Racin League Racer

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    Did you do any testing or verification on the alignment? (Lightning Boy verification or at least a speed square test) If you did and it is perfect or near perfect, then 4"-5" is probably good. If not, I would lean toward 6 1/2" - 7 1/2" over 4 feet. The difference in that much steer might cost you 1 or 2 thousands. If the car wiggles, each wiggle cost you about a hundredth. Food for thought - If the other competitors are really good and you believe that nobody will shake going down the track ....otherwise, if you are on the edge of being stabile, the other car can cause you to wiggle also.

    Do some research on wheel gaps as there is some stability to be found with that.

    Good luck!
     
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  14. B_Regal Racing

    B_Regal Racing PWD Royalty Pro Racer

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    it looks to me as if BSA wheels are being used straight out of the box without being trued, the basic steel nails, and slots (i can see part of the nail in the slot from the pictures posted). The car is upside down on the scales. If it is a three-wheeled car, the weight shown for the front is from the non-dominant wheel. I do not measure COM, but weight on wheels. .589 is ~ 16 grams, which is nearing a pro weight on the FDW. Given this information alone, I would go a bit heavier on the FDW, just to be safe (I think it is heavier than shown given the weighting in the picture, as it is biased correctly towards the FDW). TBR is spot on that some extra steer or 2 or 3 grams extra on the FDW will cost a few thousandths (and gave you a lot of good information); the wiggles will cost you hundredths. Play it safe in pack to see what the car does in traffic; you can always get more aggressive in districts and council. BUT, I'm conservative, for sure...
     
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  15. KyleBCostco

    KyleBCostco Pinewood Ninja

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    All we are allowed to do to the wheels is smooth them and polish, remove 2 step.

    Stock polished nails.

    So for steering you are thinking more like 6 inches over 4'? I will add the difference of weight towards the FDW side in front of the back cubes which from what I was measuring will make the FDW weight 0.608 oz or so.

    Thanks for your help!
     
    Last edited: Jan 9, 2018
  16. S@HD Racing

    S@HD Racing Pack Champion

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    Reading this post caused me to do some thinking (watch out) and have a question about weighting. The few times I checked weight on individual wheels the NDRW is around 2.5g and DWR around 2.0g. But according to an online CG calculator the Left to Right CG was pretty close to being right on center. Question is: would it be better to shift weight to dominant side to balance weight on rear wheels which would cause shift of L/R CG towards dominant side, or would car be more stable if the L/R CG was centered?
     
  17. B_Regal Racing

    B_Regal Racing PWD Royalty Pro Racer

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    SECRET: I balance the rear wheels weight-wise
     
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  18. KyleBCostco

    KyleBCostco Pinewood Ninja

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  19. Thinkin'Bout Racin

    Thinkin'Bout Racin League Racer

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    So this caused you to do some Thinkin' ... Careful that's my line. ;)

    I have some thoughts on this and they are from my simple mind, so take them for what they are worth.

    I think there is/was many of us that use the term COG (center of gravity) interchangeably with COM (center of mass). In the past I know I have. However, they are two separate things and should be considered as such. I know that I have read that many Pro's do not measure COG or COM. However, I believe they consider it in other ways including weighing each wheel. I also believe that the COG/COM is a metric that us beginner builders can relate to. I know that I like tangible data that I can compare from one car to another.

    COG is the measure of how much "engine" we have (the fore and aft balance) while COM measure both the COG and the left to right bias. Both of these have an effect on how a car will perform and are not mutually exclusive.

    The COM balance can/does have an affect on how much friction each of the rear wheels/axles has along with the "natural center" that the car wants to follow/track while going down the lane (remember Newton's laws of motion). Each of the rear wheels shares in the largest portion of the friction loss due to friction between the wheel bore and the axle/axle head. Because the NDRW normally has a greater load to carry, it suffers the greatest loss to this friction. I believe that if some of this weight is transferred to the DRW, then some of that friction is transferred to that DRW. I further believe that losses due to friction are based on a non-linier curve. If this is true, there becomes a point at which the load sharing becomes beneficial by reducing the total friction loss based on the non-linearity of the total friction loss. Clear as mud? o_O

    So, what does this mean ...my belief is that some of the weight needs to be shifted to the DRW side when considering the COM bias of a car. How much ...that is something that the best pros have figured out and are probably not willing to openly share. I also believe that the magical amount of weight bias is unique to a number different factors. It is most certainly different on different cars. Things that can/do effect this - wheel bases, DFW axle angle, amount of steer, weight on the DFW, wheel/bore prep, fenders/no fenders, each builders process, track surface, along with many others.

    Having figured this out, I have more questions than answers. I do know this with certainty ...Everything on a build has some effect on everything else. We just need to think about how each step during the build effects the other elements of the build. With all of this in mind, don't forget to have some fun! :)
     
    Last edited: Jan 10, 2018
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  20. FDM

    FDM District Champion

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    This is a very interesting thought process, which made me pull out my balance just to find out that on our 3 wheeler cars (extended wheel base ~5.5 inches) the balance ratio on the rears NDRW : DRW = ~3:2. So there is a significant increase in weight on the NDRW. I tried to even out the weight balance but it seem to require quite a bit of extra weight on the DRW side of the car. I am afraid that if I want to balance the rear wheels out weight wise I will be ending up putting more weight towards the front end of the car or putting weight higher up in the back in the car. So how important is it to even out the weight on the back wheels?

    However these cars with the 3:2 weight distribution have done well for us. They have about 3 inch steer over 4 feet and COG's around 3/4 inch. I am wondering if the increased friction on the NDRW you mentioned is actually giving an extra steering effect. The extra friction could cause the NDRW to rotate a bit slower relative to the DRW resulting in a steering towards the NDRW side of the car helping the car stay on the rail.
     
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