# To measure the draw weight of a catapult at various stretch factors . . .



## mike160304 (Aug 10, 2018)

. . . would this little spring scale make sense? And can I equally use it horizontally, not just vertically?

https://www.ourweigh.co.uk/suspended-weighing/salter-brecknell-little-samson.html

I was thinking of the 6kg x 100g one, because a catapult of mine with single tapered 20mm to 15mm Theraband Gold bands seems to have a draw weight of around 3kg at 4x stretch factor, so I am unlikely to want to measure draw weights over 6kg?

The purpose of this would be to check the weakening of the bands as time went on, and cut the guesswork out of "When should I replace these bands?"

Any help would be appreciated.

Mike


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## mattwalt (Jan 5, 2017)

You know... I wonder if draw weight is the best approach... Not sure different elastics have the same action - so draw weights may be wildly inconsistent. Also pseudo vs flats behave very differently.

20-15 TBG with 3/8 is a good starting place I''d think.

I'd probably suggest a chrony - its all down to speed. Also you may be surprised by the results.

For replacing bands - either when they break - or when you can see visible damage that you're sure will lead to a failure.


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## mike160304 (Aug 10, 2018)

mattwalt - many thanks.

So draw weight may be one of several indicators.

I have a Chrony F1 (? the F1, it's in a box on the top shelf). I used it for my old .22 Webley Service Mk 2 air rifle and my .22 Crossman 2240 CO2 air pistol.

I rigged 2 LED torches under the "arches" and used it indoors in muted light. This gave me consistent results, whereas outside under the changing sky it was all over the place.

I'll have fun trying that. I shall have to be far enough away so that the pouch does not interfere, but as long as I always use the same shooting distance, it should give me answers.

Your 3/8" steel ball is around 9.5mm - I have some 8mm steel balls here, which will suffice. I was surprised when I weighed these and found that they only weigh 2.1 grammes. It will be fun putting my 27mm (1.06"), 10.6 gramme ice balls through the Chrony too.

And if my neighbour Dave strolls across to see what I am doing, it will be fun watching his face as I explain.

I'll probably go for the 6 KG Salter spring scale anyway - the unbranded cheapo I have goes to 12 kg and is not readable enough for the catapult application. And Salter is a great brand, and I like new toys . . .

I would get the 3 KG one, but I am over that limit already with this single Theraband Gold.

Re wear, yes, as you say, this will be visible on the flat bands and on my band-to-post bindings, and it should be visible before it gets dangerous.

Mike


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## mattwalt (Jan 5, 2017)

That is one seriously cool rifle to own.

Keep your chrony protected - you don't want to hit that accidentally.

3/8 and 9.5 same thing really. Whatever you're using just match the bands accordingly. You'll be surprised how little rubber you need to move things fast.

Regarding draw weight - I'm not sure its as much of a factor as you'd think. Elastic dynamics can be a little weird. But seen to hear your results (experiment data is always cool)

Pity - I used to live up the road - would have been cool to see use balls being hurled.


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## mike160304 (Aug 10, 2018)

mattwalt -

Yes, the ice balls are "cool" - around -18 Celsius. 

You lived near here - whereabouts?

I played with the Chrony F1 yesterday. I shot indoors using the 2 torches installed - see pic.

I got a lot of ERR1s and ERR2s meaning 1st or 2nd sensor did not see the 8mm ball.

Best I could get at around 4x stretch was 142 and 153 fps.

I am not professional enough to get consistent results on the Chrony F1, it's a bit out of my comfort zone. It's easier with a CO2 pistol or air rifle I think. The Webley Service Mk 2 by the way was my Dad's, pre-WW2.

I do think that, for checking the falling strength of the same set of rubber bands - not for comparing performance of different catapults - the little 6 kg spring scale used at 3x or 4x stretch factor will be a good indicator that is very easy to use, but we shall see.

Regards,

Mike


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## mike160304 (Aug 10, 2018)

I have the Salter 6kg spring balance now, I'll do some tests and let you know who wins. 

Mike


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## ForkLess (Aug 15, 2018)

Fishing scale laying around works great up to 40 lbs


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## jazz (May 15, 2012)

Draw weight is one of couple parameters which is of crucial importance when determining exit velocity of the projectile and the common sense tells you that without draw weight/power there is no movement of the projectile. Also, more draw power should bring more velocity, although not in a linear way, in the sense that two times more power will bring two times more velocity, but somewhat less.

The other parameter is elongation, that is the difference between the active length of the rubber (the length of the rubber in its non stretched state) and the length of your total draw.

The third, fourth and fifth parameters are the weight of the projectile, the weight of the pouch and the weight of the rubbers themselves.

The interplay of these parameters determine the exit velocity, the first two positively (more of each of them, higher the velocity) and the third, forth and fifth negatively (more of each of them, lower the velocity).

The device for measuring draw weight/power that you point to is ok but you have digital ones, more precise and easier to read and I think that those devices are called dynamo meters.

The draw weight of any given rubber will not be he same at various stretch factors - higher the factor higher the power, however, also not directly in a linear way, but this function looks somewhat curvy, or, getting tired although growing.. here are some graphs that show this relation to a stretch factor of 2.5 times, the other ones you can test yourself. By the way, I think that pounds in the first table and a graph in this link show powers that relate to the full width (some 12.6 cm) of various colors of Thera-Band).

good luck with your experiments,

cheers,

jazz


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## mike160304 (Aug 10, 2018)

jazz - thank you for your helpful comments.

I started my experiments by having a handheld catapult trying to drive a stick through my 35lb capacity digital scale, normally used for weighing travel baggage and parts of sailing boats. The wild movement of the digital readout made it impossible to get a consistent reading at a given stretch factor. Obviously with baggage or boat parts the force is static and the readout steady.

My new Salter 6kg spring balance only cost about £9/$12 delivered but with a handheld catapult or rubber band/tube it gives me a good enough readout for my purpose. Obviously I have to base my tests e.g. not on the force of a 12.5cm wide Theraband but on cut bands up to about 20mm, which suits the catapult environment. Accurate values are not important, it is the comparison between new and old rubber bandset, or between different types of bands/tubes, at a given stretch factor, that interests me.

Much more expensive and versatile digital scales are beyond want I need for these amateur tests.

It should be fun.

Regards,

Mike


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## jazz (May 15, 2012)

Hi mike160304,

Digital ones are not expensive at all, quite contrary, and just as an example here is one, but there are more:

https://www.ebay.com/itm/Portable-Digital-Hanging-Scale-Fishing-Travel-Luggage-Weight-BackLight-50Kg-5g/162700242174?hash=item25e1b0acfe:m:mMtpLGTY8d085FgjiobD5jg:

enjoy!

cheers,

jazz


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## JPD-Madrid (Apr 2, 2013)

jazz said:


> Hi mike160304,
> 
> Digital ones are not expensive at all, quite contrary, and just as an example here is one, but there are more:
> 
> ...


i had same one. the price is even lower than i got in China.


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## mike160304 (Aug 10, 2018)

jazz said:


> Hi mike160304,
> 
> Digital ones are not expensive at all, quite contrary, and just as an example here is one, but there are more:
> 
> ...


Thanks, quite right, I was thinking of more expensive "laboratory" type equipment.

But if you place a catapult pouch on the hook and pull, does the digital readout not "hunt", because your pull is not steady?

Mike


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## mattwalt (Jan 5, 2017)

Mike - also not that friction within the elastic comes into play.


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## mike160304 (Aug 10, 2018)

JPD-Madrid said:


> jazz said:
> 
> 
> > Hi mike160304,
> ...


Hey, I just noticed your

*PRECISE/SUMEIKE Flat bands, tubes, band sets, Targets,Cutting Ruler & Template ......*

*You just need to search "GM&BW" at Amazon marketplace for your country.*

I'll have a look at that!

Thanks,

Mike


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## mike160304 (Aug 10, 2018)

mattwalt said:


> Mike - also not that friction within the elastic comes into play.


Thanks.

Friction over the top post, with my Curiously Strong, between the 2 layers of Theraband Gold, is probably also a wear factor. There is sometimes a "click" during the pull, as it sticks-and-slips. I am trying a little silicone grease there. But I need to re-design to a no-friction form, like the tabs on my 1860 Victorian, but without using tabs that waste prime power space.

Mike


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## mattwalt (Jan 5, 2017)

No - also within the elastic itself...


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## skropi (Mar 22, 2018)

mattwalt said:


> No - also within the elastic itself...


Exactly. And that's the reason we don't get too scientific about it, it's an endless journey, with uncertain results, that when it comes to an end, you really have no idea what you got ????
Just measure draw weight with the scale when the bands are new. It's a good comparison between two band sets, and spend your remaining energy shooting a lot


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## mattwalt (Jan 5, 2017)

Also... the rubber has to move its own weight...


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## mike160304 (Aug 10, 2018)

mattwalt said:


> No - also within the elastic itself...


Yes, I got what you meant, my further comment was an "unrelated also".

Mike


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## mike160304 (Aug 10, 2018)

skropi said:


> mattwalt said:
> 
> 
> > No - also within the elastic itself...
> ...


Yes, comparing 2 bandsets, but also identifying strength fall-off in the same bandset. This interests me too.

It's all so quick and easy that major time input is not needed.

Thanks,

Mike


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