Weight placement question
- Thread starter John-newbie11
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Also, question for another car; what do you use to adhere cubes to the bottom of a hollowed-out car?
Heavy duty Double stick tape. I have used 2 kinds, the one included in window shrink kits and thicker gorilla or 3m double stick tape. The thinner heavy duty tape is good for very thin bodies and the thicker one works well if the body is a bit thicker. You can finish it off with aluminum tape on the bottom of the car.
As there were a few responses about where my cog should be, I'm not exactly sure what I should be shooting for.
I can move the dome back a bit and the cog moves back from between 3/4 and 1", to closer to 5/8 or less. I assume I don't want to move the dome any farther back than that as then my cog would get too small. Is 5/8 pushing it as well?
I could also move the dome back a bit more (moving the cog to less than 5/8) but then move a small cylinder forward to keep the cog at 5/8 or a bit more.
Aside from figuring out what my cog should be, I'm getting circular references in my head as to whether it makes a difference if I move the dome too far back and then compensate with a small cylinder in front of rear axle to keep the cog where it should be, or if that's the same thing as just not moving the dome too far back... I know I want as much weight as I can in the very back, but then the cog is too far back, so I have to move some weight forward. But if I don't want weight too far forward, am I better of putting less weight way in the back, and more around the rear axle.
I apologize, I know I'm going in circles and asking the same question in different ways...
I can move the dome back a bit and the cog moves back from between 3/4 and 1", to closer to 5/8 or less. I assume I don't want to move the dome any farther back than that as then my cog would get too small. Is 5/8 pushing it as well?
I could also move the dome back a bit more (moving the cog to less than 5/8) but then move a small cylinder forward to keep the cog at 5/8 or a bit more.
Aside from figuring out what my cog should be, I'm getting circular references in my head as to whether it makes a difference if I move the dome too far back and then compensate with a small cylinder in front of rear axle to keep the cog where it should be, or if that's the same thing as just not moving the dome too far back... I know I want as much weight as I can in the very back, but then the cog is too far back, so I have to move some weight forward. But if I don't want weight too far forward, am I better of putting less weight way in the back, and more around the rear axle.
I apologize, I know I'm going in circles and asking the same question in different ways...
Can the cubes stick out below the car just a little bit? 1/8" or less? I'm using standard axle slots so I think there's plenty of clearance, but am not certain. Thank you.
As a clearance issue, definitely. Ive put two tungsten plates stacked on top of one another under the car and it cleared fine. In fact I've wondered if placing the weights below the car might be a worthwhile choice in scout races where you can't extend the wheel base to help improve stability. My real life car has a preposterously heavy battery underneath it and I couldn't get it to wiggle in the rain if I tried.
I would try to avoid having them stick out, but as long as you clear the track you are ok. Anything increasing the profile/height of the car will increase wind resistance but am sure alot of others will have weights glued to the bottom. Most rules state what your minimum clearance needs to be and should be based on the height of the center rail.
I wonder if a little added wind resistance is a small price to pay for some added stability and lower weight placement which should improve speed (albeit by a tiny bit)
Everything else being equal, it’s better to have your weight as tightly grouped as possible. The physics involved here is Mass Moment of Inertia, which governs how much torque is needed for a given amount of angular momentum. The more dispersed the mass, the more torque you need. Kind of like lifting a jug of milk at arm’s length vs tight to your body.
For Derby cars, the tighter your weight, the easier it is for your rail rider to recover following a bump that knocks you off center.
For Derby cars, the tighter your weight, the easier it is for your rail rider to recover following a bump that knocks you off center.
Well, I'm curious if its specifically advantageous when your rules require the kit wheelbase, which I assumed no pros use.
In the overall aspect of gaining speed the wheel and axle prep are key. The wheel base affects your tuning. With those in consideration you should find speed. The weight best serves those purposes being compact and focused accordingly but car by car and builders skill set.
Currently, I plan to utilize the 2 holes in the very back of my car to put two .5 ounce cylinders (about half will be countersunk the other half will stick up), and will have the 2.5 oz dome as far back as I can (slightly in front of the rear axle), and will have a 3/4" cog.
I could drill another hole in the back to add another .5 oz weight (or even .25, or something), but I'd have to remove a lot of weight from the front/middle of my car in order to do so, and I don't know how much weight I can save by just drilling a bunch of holes (whether they go all the way through or not). I'd rather not try to sand down the car to make it any thinner as it's already painted, and I don't have the time/tools to neatly cut out a large area.
Do you think that just 1 ounce in the very rear, and 2.5 in front of rear axle, is not good enough? Is it really in my best interest to try and get a lot more weight in the back? I could move the dome further back (and put the .5 cylinders in front of it to balance the cog) but then I worry that I'll have too much in the back which might make the car unstable.
Another option (if I have enough spare cylinders) is to nix the dome and just drill a bunch more holes in the back. Then I can place multiple more cylinders in the very back, over the axle and in front of the axle as necessary, to get more weight in the back and a good cog.
Is my current setup of just 1 oz in the very back going to cost me tenths of seconds, hundredths...? I know that's hard to answer...
I am going to do my best to properly prep the wheels and axles, so perhaps the weight is fine as is, but if you think it would save substantial time then I may try to take some drastic measures at this point.
For reference, our top 3 finishers last year had cumulative time (top 3 of 4 races) of 9.3821, 9.4503 and 9.5387, which is an average race of 3.127, 3.15 and 3.18. Therefore, hundredths do matter even in our local pack race.
Thanks again.
Let me see if I understand what you are trying to do ....you are thinking of add two taller weights to the rear of the car that weight .5 oz each and sliding the canopy back as far as t you can. And this will give you a 3/4" COG correct?
I might consider drilling a third hole and placing .25 oz weights in all 3 holes for a total of 3/4 oz. I would also then take a lead sinker and pound/shape it into the size of the holes that you have drilled. I would add this to all three holes until they are about flush. This should give you very close to 1 oz of total added weight behind the rear axle, but you won't have the weight sticking up or the added wind resistance. I would still move the canopy as far rearward as possible.
Have you weighed the FDW? What does it weight? If I were to guess, I would think it is probably between 20-22 grams. If your alignment is decent, you won't have an issue with the car being unstable with a FDW weight of 16 grams or more. It can go less, but let's start there. If the car is unstable with a FDW weight of 16 grams or more the problem lies in the alignment, wheel prep, wheels, axles, and maybe even wheel gap.
On a separate note, do you have a Forster bit? How about a drill press? Those two items would make it relatively easy to cut out weight from the bottom front of the car. This might not be the time to try it, but keep that in mind for the next time.
You would probably need to move the COG to around 0.600" or less to gain another 0.0100-0.015. For example, if you set the car up as a rail rider and do not put in enough steer, and the car wiggles 1-2 times it will cost you 0.010-0.020. The proper set up along with a near perfect alignment will help way more than moving the COG 0.100" at this point. My sons first rail runner from 4 years ago has a 1 1/8" COG (lead car) and a 5 1/8" WB. It is a graphite car and runs consistent 3.06x times. But, it runs straight and true.
Almost but not exactly; if I put the two .5 in the rear, I can't move the canopy back too far or the cog gets really small (where it is now (maybe 1" in front of rear axle, I'm not exactly sure right now), I'm at 3/4") cog. I could move the canopy all the way back (or much farther back), and instead put the cylinders in front of it. That would of course put more weight in the back, but I'm not sure if this (2.5 oz) puts too much in the back though (or would I still be ok as long as cog is at 3/4)? While I of course want more weight in the back, I don't want to overweight it and pop a wheelie...
I could do the 3 holes in the back with .25 each (.75 total plus some tungsten putty, I wouldn't use lead), but I don't know how close to 1 ounce it would be (is the wind resistance a real factor or am I better just keeping the 1 ounce as is?).
I'm not sure what FDW is.
I do have a forstner bit but no drill press. I could indeed take some weight out from underneath, I'm just not sure how much it adds up (but I guess every fraction helps...).
Keeping the cog at 3/4" for example, and all else being equal, any idea how much benefit there might be to having 2 oz in rear and 1.5 oz in front of rear axle, vs. 1 ounce in rear and 2.5 in front (adjusting exact location to keep cog at the 3/4)?
Thanks again!
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