Symmetric v. asymmetric canopies

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FatSebastian
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Symmetric v. asymmetric canopies

Post by FatSebastian » Wed Jun 01, 2022 3:02 pm

This topic is intended to be about (dis)advantages of symmetric versus asymmetric canopies, not about the (dis)advantages of canopies versus some other weighting scheme.

I have noticed two styles of 2.5 oz tungsten canopies being offered. One has a symmetric dome and oval shaped, and can be mounted in two different orientations (either parallel or transverse to the direction of travel). Some offerings may have a shorter "stump" than others.

The other style has an asymmetric dome and is more egg shaped. I've seen the asymmetric shape on Amazon under the brand name "Gejoy" and described as "more aerodynamic". Although I doubt its aerodynamics present much of a performance advantage, the offset of the "stump" relative the canopy's center might offer some benefit in terms of placement in the vicinity of the rear axles?

I am curious if anyone has any experience with these and can comment on the advantages of one shape / design versus the other. For example, having to shorten axles to accommodate the "stump" of one shape versus the other would IMO defeat the value of the "easy-build" offered by a canopy.

I see these as being especially practical ballast for first-time builders, but I'd like to know the pitfalls before recommending either style to anyone.



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whodathunkit
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Re: Symmetric v. asymmetric canopies

Post by whodathunkit » Thu Jun 02, 2022 7:59 pm

Hi FatSebastian
Good to see you back here on DT.. From the Gejoy weight type your talking about .
I would have to say your thought's are spot on.. having never seen or used this weight type my self.
Had to look this one up..https://www.amazon.com/Pinewood-Weights ... B0819M19NY


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Re: Symmetric v. asymmetric canopies

Post by Eagle » Fri Jun 03, 2022 12:18 am

Wish I could help.
I haven't used canopy weights since our first two cars.
I bought a similar lead and a diamond shaped tungsten (for a Batmobile).
Honestly I don't think you are going to see any difference on the clock with styles you are asking about.
If you go with one Whodathunkit posted - wouldn't it make more sense (aerodynamically) to run it opposite of how it is shown (larger up front-tapered rear)



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Re: Symmetric v. asymmetric canopies

Post by Speedster » Sat Jun 04, 2022 6:53 am

Mr. FS made it very clear in his first sentence what he wanted to learn. He does not want anyone telling him how to build a fast Pinewood Derby car. He already knows how to do that. The cost of any dome plus shipping will probably stop most people. I think the vendors are missing something by not offering a dome without that 3/8" hunk sticking out the bottom. It could then be mounted any place and put on a 1/4" high block.

FS, I hope you get your answer.



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Re: Symmetric v. asymmetric canopies

Post by Eagle » Sat Jun 04, 2022 10:39 am

Agreed on the posts/holes. Best/fastest way to crack/break a block is to drill a hole in it.

Tungsten rounds are sold that have a slot for rear axle spar - thin car. Better aero and you can use a Forstner bit (less likely to have breakage of body).

Biggest problem I've found with tungsten canopies is not the aero, but getting a car to exact weight. It is far easier to go light body and build back up with weight then it is to have a heavy weight and shave car down to meet rules.

As to cost - there is a guy who uses spoons for his canopies/weight. He has posted a lot of pics on the FB page. I met him at Mid America this year.
Last edited by Eagle on Mon Jun 06, 2022 11:45 pm, edited 1 time in total.



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Re: Symmetric v. asymmetric canopies

Post by whodathunkit » Sat Jun 04, 2022 11:48 am

FatSebastian wrote:
Wed Jun 01, 2022 3:02 pm
This topic is intended to be about (dis)advantages of symmetric versus asymmetric canopies, not about the (dis)advantages of canopies versus some other weighting scheme.

Symmetric:
Made up of exactly similar parts facing other around an axis: symmetrical.

Asymmetric:
Having parts that fail to correspond to one another in shape, size, or arrangement, lacking symmetry, by having parts or aspects that are equal or equivalent; unequal.

The difference between (dis) advantage & advantage
Is that (dis) advantage is a weakness or undesirable characteristic; a con while the advantage is any condition, circumstance, opportunity, or means, particularly favorable to success, or any desired end.

Well Fat Sebastian.. maybe the pitfall's of two canopies boil down to which one is particularly favorable to the builder.


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Re: Symmetric v. asymmetric canopies

Post by FatSebastian » Mon Jun 06, 2022 2:17 pm

Eagle wrote:
Fri Jun 03, 2022 12:18 am
I haven't used canopy weights since our first two cars.
We have never built a car using a W canopy. I don't think they existed when we started out. (In fact, derby-specific W was essentially non-existent then, although Pb domes were in use as people sometimes poured melted lead into rounded molds.)
Eagle wrote:
Fri Jun 03, 2022 12:18 am
Honestly I don't think you are going to see any difference on the clock with styles you are asking about.
It is fair to ask how fast one can make a plank+canopy go, but for the moment, I am really more curious about having a recommended "emergency weight" or just the simplest possible weight. Although a bit off topic, here are some scenarios where this might be relevant:

a) A finished kid's car shows up on race day having no ballast, and needs some kind of simple ballast "dropped on" to be competitive (perhaps just to make it to the finish line)
b) Somebody wants to participate in a race that wasn't planning to, so we need to quickly build a simple car (e.g., plank or wedge, using slots) from spare parts

I foresee several advantages with the 2.5 ounce canopies:

1) Canopies install easily and almost instantly with a 3/8" bit.
2) Canopies can be removed after the race and reused if a mild adhesive or press-fit is used.

Canopies are relatively expensive but (2) above compensates a bit for that. In the past, I've recommended (and maintained a pit supply of) bullet-shaped sinker weights to be hot-glued on the top as far back as possible, and that approach has provided excellent performance improvements to underweight bodies in the past, but it takes multiple sinkers to equal the mass of one canopy, and they are much trickier to securely install, especially in the pit environs.

(There is also a 3.5 ounce canopy, but it is more expensive, and its use requires a car body with less than ~1 ounce of wood. Here I am assuming we are mostly dealing with car bodies that are probably already too bulky to accept 3.5 ounces of ballast, hence the question about 2.5 ounce ones.)
Eagle wrote:
Fri Jun 03, 2022 12:18 am
...wouldn't it make more sense (aerodynamically) to run it opposite of how it is shown (larger up front-tapered rear)
As you said, one is probably not "going to see any difference on the clock" between the two styles, so orientation aerodynamics are also likely to be an imperceptible effect. Because the one style is asymmetric, the stump is not in the geometric center, so installing it aerodynamically with aft taper puts the stump closer to the front. Might this interfere with rear axles? This is the heart of my question - does asymmetry of the stump location offer some advantage or disadvantage with standard and/or extended wheelbase depending on its orientation. Does one type have greater installation flexibility than the other? Does one style use a longer a stump to get to 2.5 ounces? Etc.

Ultimately I'd like to have a couple of simple canopies available in the pit. Tungsten canopies are efficient but expensive so I'd like to invest in the most practical style. Perhaps I may just need to play with each type, and eat the cost, as most active contributors to DT may be past experimentation with canopies. But our pit is already full of a lot of expensive items that will probably never be used (or, used again)...
Eagle wrote:
Sat Jun 04, 2022 10:39 am
Best/fastest way to crack/break a block is to drill a hole in it... you can use a Forstner bit (less likely to have breakage of body).
Yes, I personally use Forstner bits at low speed for block holes 1/4" or larger, and I cannot recall seeing a block crack with that style bit. I think these 2.5 ounce canopies use a 3/8" diameter stump and are 1/4" to 3/8" deep.
Eagle wrote:
Sat Jun 04, 2022 10:39 am
It is far easier to go light body and build back up with weight then it is to have a heavy weight and shave car down to meet rules.
Yes, this is the issue of mine to be addressed with the 2.5 canopy question, except there is less concern about "exact weight" (landing at 5.0 ounces) and more concern about just having the simplest possible ballast for a car design that ends up less than 2.5 ounces. And one might also mill out space for shallow trim weights underneath the canopy if necessary to push the 5 oz limit.
Eagle wrote:
Sat Jun 04, 2022 10:39 am
...there is a guy who uses spoons for his canopies/weight. He has posted a lot of pics on the FB page. I met him at Mid America this year.
Yes, I've seen a DT thread about that. Pretty cool!



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Re: Symmetric v. asymmetric canopies

Post by Eagle » Tue Jun 07, 2022 12:08 am

FatSebastian
Not sure why cars are arriving so under weight at your races.

Had you started with Best Race Day Add on Weight... things might have gone differently.

How/who convinces the kid who has built the car the way he/she wants that you someone is going to drill a hole in their car. But, not to worry, after the race his car will be returned without the weight. The hole, however, is forever.

Speedster has the right idea. That post and the needed hole is a problem. Unfortunately, lead has been deemed unsafe for use. If you agree then look at tungsten round discs. There are 3 pack options that are less expensive in different weights on eBay.

If not, consider pouring canopies. They will all end up different weights. Seal them. Use as needed. If for some reason the sealer is compromised in removing from car, reseal before next use.

Please also know the likely reason for the post is once in motion the weight wants to remain in motion. So, the glue job on a flat glued on weight will need to be significant. I won't bore you with the details. I will just say that I have attempted to attach weight for reuse and it has not gone well.

Lastly, if it is tungsten weight that you are looking to add last minute I may have some options that are less expensive then the PWD vendors offer. They come in different sizes and weights that you could glue on and take off. They are not domes. They are no rounds. They are not pretty. But, they would get a car up to weight in a concentrated area. If interested PM me.



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Re: Symmetric v. asymmetric canopies

Post by FatSebastian » Wed Jun 08, 2022 1:44 pm

Eagle wrote:
Tue Jun 07, 2022 12:08 am
Not sure why cars are arriving so under weight at your races.
It is because many people just aren't adding any ballast to their designs, even when told that added weight really is needed, and why it is needed (via an appendix to the rules). The mentality behind this would be the subject of another thread.
Eagle wrote:
Tue Jun 07, 2022 12:08 am
Had you started with Best Race Day Add on Weight... things might have gone differently.
:thinking: "Best Race Day Add-on Weight" as the topic of this thread? Indeed, but that title would not reflect my primary interest, which is, as noted at the outset:
FatSebastian wrote:
Wed Jun 01, 2022 3:02 pm
This topic is intended to be about (dis)advantages of symmetric versus asymmetric canopies, not about the (dis)advantages of canopies versus some other weighting scheme.
The 3-pack W discs mentioned seems to have potential as a "Race Day Add-on Weight" (keeping in mind that it is an alloy containing 90% W, and 10% iron or nickel). But, having a recommended "emergency weight" or "the simplest possible weight" were just illustrative of some potential applications of a 2.5 ounce dome, and certainly not the only usages I might envisage. As I said before, I foresee these as being especially practical for first-time builders, but I'd like to know the pitfalls before recommending one or the other. Although too expensive to freely hand out, having one or two canopies in our pit box helps us demonstrate them to others should that style ballast work best with their design or situation...
Eagle wrote:
Tue Jun 07, 2022 12:08 am
Please also know the likely reason for the post is once in motion the weight wants to remain in motion.
The stump exists to secure the dome to the body. It also adds substantial "hidden" or "internal" mass to lower the center of mass and decrease the exterior profile of the dome. (Consider this "canopy" which is mostly "stump"!) I believe the stump can be press fit if the hole is sufficiently tight (or if the stump is wrapped with something like tape). This again is another potential inquiry about these domes - maybe their stumps have slightly odd diameters that do not accommodate a press-fit using a standard drill bit size...

If the dome is placed in its most rearward possible location (such that it reaches the rear edge of the block), it would seem that the "aerodynamic" (aft-taper) orientation might interfere with the standard wheelbase locations, and the opposite orientation might interfere with an extended wheelbase, based on the online retailer's illustrated dimensions. I have since ordered an asymmetric canopy for experimentation. Should I discover something interesting with this canopy, I will report back. Thanks!



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