Chronographing Crossbows

I have a Skan Chronograph and thought it would be interesting to measure the velocity of the couple of bows I have. The first is I was told a 90lb draw weight, it's the small tournament bow thats my avatar >
It is shooting at around 140 ft/sec with 270 grain bolts
I have another 17th C Hunting style bow with around 200lb draw weight which chronographed at 170 ft/sec with 300 grain bolts.

I weighed the bolts on a kitchen scale so the weights are approx but the speeds are right.
Has anyone else chronographed their bows? Any idea what speed/strength historical weapons shot at?

Hi Scott, yes I just chronographed a crossbow I just finished.
It has a 157 pound draw weight and shoot a 428 grain arrow at 243 fps.
I use a gun powder scale to weigh my arrows which is very accurate to a 1/10 grain.

As a related item I have made a Yew english longbow that draws 70# at 27" draw and shoots a 750 grain arrow at 150 fps.

Also curious as to how other crossbow perform particulary a heavy draw weight medieval.
Todd

Hi Todd, thats fast/powerful! Thanks for the reply :)What is your bow made from? Are Steel bows generally fairly slow?
I will have a look for a powder scale, sounds useful.

Thanks Scott, I got the prod from Elk Ridge archery. It is laminated with fiberglass and wood and has antler overlays on the tips.

Yeah, I believe steel to be slower because of it's heaver mass.

You should be able to get a scale from places that sell reloading supplies (for gun cartridges)

Todd

All this talk of speed is making me think I should just invest in my own chronograph. Its not like I don't have enougth hobbies that I could use it with. Between making longbows and crossbows and reloading it should be in use a least once a month

Basilisk120 wrote:All this talk of speed is making me think I should just invest in my own chronograph. Its not like I don't have enougth hobbies that I could use it with. Between making longbows and crossbows and reloading it should be in use a least once a month

You do realize that if you do that, you have no turning back?

"Maybe by doing this minor modification I can squeeze 10fps more out of my crossbow..."

Yours truly,

Been there, done that

basileus wrote:

"Maybe by doing this minor modification I can squeeze 10fps more out of my crossbow..."

LOL yeah that will be me. I'll be the guy out there trying different fletchings just to see the different between 4 and 5 inch fletching and this will of course lead to trying wood or leather fletching.

I was amazed at how cheap chrony's are. Is there anything wrong with getting a $70 chronograph?

I don't think there is anything wrong with the budget chronographs. It would depend on which one your looking at.

Last time I looked at chrony's it seemed that the company I was checking out had two different models. Everything else was just addons and extras but that the actual measuring unit didn't change.

Reason why I was inquiring about building a ballistic pendulum earlier on is because I was at a range once and nearly shot their chrony...got a bit distracted by a couple of kids fighting in the background and didn't double check the angle at which the shooting vice/bench was...ended up with fletching skimming one of the chrony arms and looked around *oh man, I hope no one saw that* only to hear the voice over the intercom say - "I seeeee you #12" and the guys in the booth laughing.*I wasn't #12, but I got it that they were talking to me*

Overall chrony's are sweet and manageable on a moderate budget, I might get one in the future too, but building a ballistic pendulum is my choice for now.
...since it gives you the fun of actually shooting your measuring devise.

Have fun & Be safe.

Ivo

Hello,
Since I started being interested in making a crossbow I have been interested in this kind of thing. I was wondering what I would need to do to about equal the speed of my longbows. right now I have a 75 pound longbow that will shoot 700 grain arros about 170 feet per second. How would this compare to crossbows you guys have built?
Jamie

vabowyer wrote:Hello,
Since I started being interested in making a crossbow I have been interested in this kind of thing. I was wondering what I would need to do to about equal the speed of my longbows. right now I have a 75 pound longbow that will shoot 700 grain arros about 170 feet per second. How would this compare to crossbows you guys have built?
Jamie

It shouldn't be too difficult to achieve 170 fps (50m/s) with 700 grain (45,5g) bolts, I've made two crossbows that roughly equal your longbow and one that's about twice as powerful. A good candidate (for a steel bow) is 150 pounds with 40cm (16") draw. This is not especially surprising, as "pounds times draw length" is roughly the same as in your longbow . Anyways, a steel bow would have to be ~80cm (31,5") long, 0,5-0,6cm (~1/5-1/4") thick and 4-5cm (~2") wide at the middle, with width tapering to as near zero as possible (see here why) to achieve this performance.

Going heavier will increase bolt velocities; just how much depends on the bow material being used. In steel bows reducing bolt weight reduces efficiency (energy in -> energy out) pretty quickly. Glassfiber should be better in this regard, as it's lighter. Anyways, here are some statistics from my heaviest steel crossbow:

• Leaf-spring cut to an enhanced pyramid front-view profile
  
• Draw weight: ~315 lbs
  
• Power stroke: 27cm (10,6")
  
• 81 gram (1246 grain) bolts: 50 m/s (167 fps)
  
• 47 gram (723 grain) bolts: 62 m/s (207 fps)
  
• 30 gram (450 grain) bolts: 71 m/s (237 fps)
  

There are a few small tweaks I can still do for this beast, giving it perhaps 5% performance boost, with velocities rising at most 2-3 m/s (6-9 fps).

Hope this did not went too much off-topic .

Basileus

Free chronograph software. Use your computer or laptop as a chrony - heres a link to a page of mine. At bottom of page is a how to video. Have fun and good luck.
W.
http://warhammer1.wordpress.com/warhammers-ballista-physics-links/

Admin Edit:

basileus wrote:Glassfiber should be better in this regard, as it's lighter

I think this is something I'm willing to try very soon.

Warhammer1 wrote:At bottom of page is a how to video

So that's where I first saw that video! Thanks for posting it, I was looking all over for it.

Ivo

PS: Warhammer1...You really got to start to posting(embedding) videos on the forum- you got good stuff man!

That was a cool video warhammer. Nice simple idea.
Played around w/ audacity before. For a sound editing program it is fairly simple to use.

I should add:

Basileus, thansk for the numbers. Its always useful to see real numbers and for different weight bolts.

Warhammer1 wrote:Free chronograph software. Use your computer or laptop as a chrony - heres a link to a page of mine. At bottom of page is a how to video. Have fun and good luck.
W.

Indeed, this could prove useful for measuring the velocity of sling bullets. It's damn difficult to shoot them through an optical chronograph without breaking it . Video+audio combo would be even better, as a sling bullet leaving the pouch probably does not make a clearly noticeable sound.

basileus wrote:Video+audio combo would be even better

Only if you do have a 1000 frame per second camera.. Sound from a first hit paper sheet have to be very clear.
basileus, send me dimensions, so I'll make a shield for your chrono.. I have a lot of crab plywood around workshop..

Regerald wrote:

basileus wrote:Video+audio combo would be even better

Only if you do have a 1000 frame per second camera.. Sound from a first hit paper sheet have to be very clear.
basileus, send me dimensions, so I'll make a shield for your chrono.. I have a lot of crab veneer around workshop..

Silly me. Slinging the bullet first through a paper sheet on to a thin steel plate should do the trick. The microphone could be placed close to the paper to reliably record the first sound. The second sound should be loud enough to be recorded even at a long distance. In this case there'd be no need to determine when the bullet has left the sling pouch. The same technique might be useful for seeing how much velocity of a bolt or and arrow is reduced when going through armor, shield or whatever.

Regerald: thanks for the offer! However, I got large, 2cm thick birch plywood pieces lying around in the attic which I can use those to build a chrono shield.

Basileus

There is some advantage in keeping the microphone in the middle between the two sheets of paper. It would cancel out the difference in time it takes for the sound to reach the microphone.

Just for reference at 20°C at sea level and standard atmospheric conditions it takes approx. .00146 seconds for for sound to travel .5 meter.
That is actully enought to slightly throw off the calculations if it is not accounted for.

microphone should be evenly spaced between two sheets of paper but a tin foil plate is louder. Also important to shoot a small distance away from first sheet of paper. If too close it is hard to separate sound of bow from paper for a proper calculation.
It is worse for my spring powered machines, so I make sure all shots are taken at least two meters from first sheet so there is a clear separation.

Basilisk120 wrote:There is some advantage in keeping the microphone in the middle between the two sheets of paper. It would cancel out the difference in time it takes for the sound to reach the microphone.

One could also measure the distance from the microphone to the papers / aluminum foils, which would allow making appropriate corrections to the results. Of course, placing the microphone in the middle makes this unnecessary.

Of course, placing the microphone in the middle makes this unnecessary.

A reminder from doctor Doppler -

Even if you place your mike right between the sheets, the time delay will still be there. And should be accounted for.

yeah the time delay is still there but the time it takes for the sound to reach the mike from the first hit is the same as the time it takes for the sound from the second hit to reach the mike.



This means the "Hit To Mike" sound times are the same for both sides so they cancel out. or looking at it anouther way the delta between the actual first hit and last hit and the delta between the recorded first hit and recorded last hit are the same.



I can come up with a better proof of this but the limitations of the text box make it difficult.



Unless your thinking of something else that I'm missing in which case please let me know.

Never mind . Basilisk , You are absolutelu right.
I just tumbled into my own cleverness, have a bad habit of trying to think too complicated sometimes.
Just ignore my previous message.