[Question] Disc dimensions that don't add up

[jonkimbertx]

I've noticed that sometimes the PDGA measurements of discs don't fit with descriptions from manufacturers, and I'd love it someone could explain why. Specific cases:

A- DD has a Wraith bottom with anyhyzer top, but the rim width is 2.2 instead of Wraith's 2.1
B- Judge has a slimmer profile while the Dagger is crazy deep, yet their heights are 2.0 and 2.1 cm
C- MVP Volt and Shock are in the same 18.5 mm rim class, but are actually both larger not the same at 19.0 and 20.0 mm

I know all of these come down to a millimeter of difference, but it seems like measurements would line up across the board. The first one especially bothers me because both discs are in my bag. How can a disc share the same bottom as another and not measure the same?

 

[PMantle]

That's why I bought calipers and ignore PDGA numbers.

 

[VictorB]

I've noticed that sometimes the PDGA measurements of discs don't fit with descriptions from manufacturers, and I'd love it someone could explain why. Specific cases:

A- DD has a Wraith bottom with anyhyzer top, but the rim width is 2.2 instead of Wraith's 2.1
B- Judge has a slimmer profile while the Dagger is crazy deep, yet their heights are 2.0 and 2.1 cm
C- MVP Volt and Shock are in the same 18.5 mm rim class, but are actually both larger not the same at 19.0 and 20.0 mm

I know all of these come down to a millimeter of difference, but it seems like measurements would line up across the board. The first one especially bothers me because both discs are in my bag. How can a disc share the same bottom as another and not measure the same?

I'd venture to say the differences are probably in part due to how the different pieces of the mold come together - they probably feel more drastically different because of the shape of the disc. The dagger is way more blunt profiled while the judge is pretty slim and almost ballistic - but the overall height probably isn't that different after all.

...of course if someone with good calipers wants to show what the measurements really are, I'm all for that

 

[bradharris]

The first one especially bothers me because both discs are in my bag.

And if the measurements aren't what you think they are, that's going to somehow change how you decide to use them??

 

[garublador]

Injection molded plastics tend to expand when they cool, and how they expand depends on many different variables. The assumption is that it isn't cost effective to control it to the point where all runs would fly the same.

 

[Marmoset]

Injection molded plastics tend to expand when they cool, and how they expand depends on many different variables. The assumption is that it isn't cost effective to control it to the point where all runs would fly the same.

Good to hear from you again Garu, it's been a while!
I agree with everything above except for the word "expand."
To my knowledge, every plastic shrinks as it cools, not expand.
Plastic pellets come with shrinkage rate values on their technical data sheets. It's usually labeled like 0.5%- 0.8%. These values vary slightly among formulations in the same family and can greatly from family to family, I.e. DX vs Champ.
I've never seen a pellet that comes with an expansion rate.

 

[Marmoset]

I've noticed that sometimes the PDGA measurements of discs don't fit with descriptions from manufacturers, and I'd love it someone could explain why.

I know all of these come down to a millimeter of difference, but it seems like measurements would line up across the board. The first one especially bothers me because both discs are in my bag. How can a disc share the same bottom as another and not measure the same?

I have 2 answers for you:
1- measuring inconsistencies. You would think measuring would be A highly repeatable process but that isn't always the case. There are many variables that could cause two different people to come up with two slightly different measurements on the part that has curvature. There's curvature where the inside of the wing meets the bottom of the flight plate. Did both people Measure to the same part of the radius? Also, was there the same amount of flashing on each disc, or did one disc have less flashing than the other? Those are just two examples off the top of my head. I'm sure there's more.

2- plastic shrinkage. As I mentioned in my post above different plastic string at different rates, sometimes by a lot. If one disc was measured in DX and the other in champion, the measurements might be vastly different... easily in the realm of a mm. Because shrinkage rates are a percentage, you're more likely to see differences on wide winged drivers rather than narrow winged putters/ mids.

 

[JOwens]

Having worked in plastics, the only way to insure 100% consistency is to have a climate, and pressure controlled facility. We would have to trash several hours worth of work during the middle of the summer or dead of winter because parts stayed warm too long or cooled too quickly. It was really aggravating if QC had the night off and we found out an entire shifts run of part x was too small. Luckily we ran so many different parts that one being out of spec did not kill us.

 

[hugheshilton]

Marmoset pretty much summarized my thoughts. Additionally, for the OP's question B, there is a big difference between overall disc height the inside rim depth. The flight plate thickness affects it of course. Depending on where they measured inside the disc and how much curvature there is where the rim meets the flight plate, there could also be some loss there as well.

As a real world example of plastic shrinkage differences, take the Colt. I have maybe 10 XT Colts which all feel about the same. I've also felt some DX's in the store which felt about the same as my XT's. Recently I bought a Star and a GStar to test. The Star and GStar feel much deeper and larger in my hand. I haven't measured them with calipers yet to see what the difference actually is, but I'm quite sure that there is some difference. The premium plastic shrinks less than the baseline plastic so it makes sense that the Star/GStar discs are slightly larger than the XT (or DX).

 

[garublador]

Good to hear from you again Garu, it's been a while!
I agree with everything above except for the word "expand."
To my knowledge, every plastic shrinks as it cools, not expand.
Plastic pellets come with shrinkage rate values on their technical data sheets. It's usually labeled like 0.5%- 0.8%. These values vary slightly among formulations in the same family and can greatly from family to family, I.e. DX vs Champ.
I've never seen a pellet that comes with an expansion rate.

Thanks for the correction! Apparently I have a case of the Mondays.

 

[Marmoset]

who wants to get nerdy?

Destroyers rim thickness measurements
according to the PDGA: 2.2 cm
according to my measurements from inside the wing where it meets the flight plate to the furthest point on the wing.
Champ: 2.366 cm
DX: 2.362 cm
Pro: 2.293 cm
Star: 2.321 cm
there is only a minimal radius where the wing meets the flight plate on the Destroyer so this makes measurements more repeatable. If there was a larger radius then I would have had much larger variation in the measurements. still, none of these- except for the Pro- are in the 2.2s. So I measured a different way.

From the furthest tip of the nose of the wing to the point directly horizontal on the wall... roughly halfway between where the bead would be and where the wing meets the flight plate:
Champ: 2.286 cm
DX: 2.987 cm
Pro: 2.987 cm
Star: 2.316 cm
These were closer to the PDGA spec.
the way I Interpret the PDGA procedure (found here and relevant pull quote below) I believe my first batch of measurements were the correct method.

(4) Rim Thickness - This attribute is recorded using a Vernier caliper. The rim thickness is defined as
the distance between the outermost and innermost edges of the rim.

The approval form (found here) says the final measurement is supposed to be the median of the measurements. Median out of how many measurements? I have no idea, I couldn't find that part. So I took...
5 rim thickness measurements from various points of the perimeter on an unthrown Pro Destroyer:
2.278 cm
2.286 cm
2.286 cm
2.291 cm
2.299 cm
so the median value is 2.286 cm. The PDGA mus not be rounding, they must be dropping everything after the first decimal place.

All of this assumes perfect measuring techniques (which I don't have). Rim thickness measurements need to be perfectly radial in order to assure you are measuring the shortest distance between the two points. Otherwise you are artificially exaggerating the rim thickness. In other words, if you are no longer measuring along the radius but instead you measure along a chord then the rim thickness will be larger. I tried this and depending on how extreme you go then your measurements could literally fall anywhere in between 2.278 cm (according to my measurements on the Pro) to a dimension outside the PDGA limits of 2.6cm if you are waaaaayyyyy off.

And the same goes for the tilt of the calipers. I'm assuming the calipers should be perfectly horizontal at all times. This really becomes an issue on "Plus" configuration discs such as the Roc+ where the inside of the rim is noticeably tilted. If you rest the flat of the caliper jaws on the flat of the inside of the rim then the measurement will be different than if you keep the calipers horizontal.

Anyway.

Conclusion:
measuring seems like it should be easy but it isn't always. I measured 4 different discs from the same mold and used the exact same techniques on each. They were all different. If I measured four more then they would still be different from each other as well as from the first batch.
If I worked for the PDGA then I would build a measurement rig to assure all these little variables would be eliminated. I don't think they've done this so take their numbers as a rough guide.

 

[hugheshilton]

who wants to get nerdy?

[...]

Conclusion:
measuring seems like it should be easy but it isn't always. I measured 4 different discs from the same mold and used the exact same techniques on each. They were all different. If I measured four more then they would still be different from each other as well as from the first batch.
If I worked for the PDGA then I would build a measurement rig to assure all these little variables would be eliminated. I don't think they've done this so take their numbers as a rough guide.

Very nice post! Real data is always appreciated. I'd also be willing to bet that if you took a Star Destroyer from 2007 and a Star Destroyer from 2017 and measured each of them you'd get different results. The plastic blends have changed over time which probably affects the shrinkage rate to some extent (not to mention conditions in the factory when both were molded). As you say, take the PDGA measurements as a rough guide. Just because the PDGA says a disc has a 2.2 cm rim doesn't mean a particular disc you grab off the shelf will have a 2.2 cm rim.

 

[ThomasOrion]

I have 2 answers for you:

2- plastic shrinkage. As I mentioned in my post above different plastic string at different rates, sometimes by a lot. If one disc was measured in DX and the other in champion, the measurements might be vastly different... easily in the realm of a mm. Because shrinkage rates are a percentage, you're more likely to see differences on wide winged drivers rather than narrow winged putters/ mids.

Shrinkage is always a concern, especially in these cold winter temps

 

[Marmoset]

...I'd also be willing to bet that if you took a Star Destroyer from 2007 and a Star Destroyer from 2017 and measured each of them you'd get different results...

Yeah I thought about that too. But not in reference to the plastic blends you mentioned.
I was thinking about how the Destroyer mold is so popular it has to have been re-tooled (and accidentally or purposefully tweaked in the process) by now... or possibly there are 3-4 destroyer molds in order to keep up with the demand? Who knows.

None of these scenarios add up to consistent measurements.

 

[hugheshilton]

I was thinking about how the Destroyer mold is so popular it has to have been re-tooled (and accidentally or purposefully tweaked in the process) by now... or possibly there are 3-4 destroyer molds in order to keep up with the demand? Who knows.

None of these scenarios add up to consistent measurements.

Oh yeah both those seem entirely possible. I think most people have noticed that the McBeth Destroyers tend to be more overstable than the last round of Avery's. This could just be a change in the plastic blend, but I don't think so. If you do side-by-side comparisons, the wing on my McBeth Destroyers is significantly higher and the concave scoop out of the underside of the wing is much more pronounced than older runs. The mold might have been tweaked or else a piece of the mold replaced. Or maybe as you say, they just needed a new mold to keep up with production and they tooled it a little more overstable. Fwiw, the GStar Destroyers I've bought recently have all looked like the old Avery wing without the extra scoop. I'd hazard guess maybe they're using the older mold for Pro/DX/GStar production and the newer mold for Star.

 

[Aim For The Chains]

@marmoset... ::bunch of good info removed::
If I worked for the PDGA then I would build a measurement rig to assure all these little variables would be eliminated. I don't think they've done this so take their numbers as a rough guide.

Ive seen the high-tech setup and it is pretty impressive.







PDGA measurement tools:

 

[Snap-it]

lol AIM, u jokester

 

[allenhoop]

Shrink rates are to blame here. There are so many variables involved in thermoplastic injection molding, it's economically impossible for disc manufacturers to do anything about it... Unless you want to pay $30 for stock premium plastic because the manufacturers move towards having climate and atmospheric controlled production facilities.

This is why IMHO the PDGA should consider simplifying their technical standards. Just make sure the disc isn't too dense or stiff, specify some dimensional standards, and what else should they have to worry about?

 

[@iDiscGolf]

Having worked in plastics, the only way to insure 100% consistency is to have a climate, and pressure controlled facility. We would have to trash several hours worth of work during the middle of the summer or dead of winter because parts stayed warm too long or cooled too quickly. It was really aggravating if QC had the night off and we found out an entire shifts run of part x was too small. Luckily we ran so many different parts that one being out of spec did not kill us.

Hvac

This seems strange to me that this isn't the case. I mean innova for one has their manufacturing in California which is pretty well temperature controlled to begin with... but you go into any business like a Best Buy and it's the same temp all year long.

 

[@iDiscGolf]

Shrink rates are to blame here. There are so many variables involved in thermoplastic injection molding, it's economically impossible for disc manufacturers to do anything about it... Unless you want to pay $30 for stock premium plastic because the manufacturers move towards having climate and atmospheric controlled production facilities.

This is why IMHO the PDGA should consider simplifying their technical standards. Just make sure the disc isn't too dense or stiff, specify some dimensional standards, and what else should they have to worry about?

There's of course this side to it.

And I don't need a $30 disc. I'm completely fine with inconsistencies. I'll roll with it with nooo problem.

Sounds like discmania and millennium both have ways to create a market around varied runs or treat it like it's a feature or special run lol