A Study in Handshock - Please Help

[simk]

What I believe to see is that most of the bend comes out of the fades. Top limb is pretty stiff from mid to outers, bottom almost only bending in the 1st 1/3rd. Tiller looks like a d/r imho. I would therefore suggest moving the bend more towards the outers. jm2c 

[John Scifres]

Tips are just shy of 1/2".

[John Scifres]

SIMK,

Agreed.  Thanks, I'll work on that.

[Mad Max]

1/2" is not bad but I always go less

[Kirkll]

The reflex helps, and if the nocks are at least an inch ahead of bow back is my pref. This stops the string because of more preload, so there is more tension when it slams home and less limb movement.

Kirk has some pretty good ideas on this, maybe he will be along to comment.

Well.... I'm not going to be much help on self bow tillering except for agreeing with these guys that each bow is going to be different, and should be balanced to the pressure point put on the riser to minimize hand shock.

What i will say is that string tension at brace height has a huge effect on hand shock even when the limbs are balanced well.....Your slow motion video is not really slow enough to see what those limbs are really doing.  You need a high speed camera, a LOT of light, and a white back ground to really get clear high speed video of how much movement a limb has after the shot, and how clean the limbs stop.....



I've got one clip here on You tube you can look at of a recurve limb, and have another video  i'm ediring right now with some different types of limbs for comparison that i'll post later with an explanation of what you are looking at.....

 

[John Scifres]

This is after several rounds with the scraper trying to get the limbs to bend farther out and also to time better.  I also reduced more of the width at the tips to control the mass.  It's better but still some work to do.

[John Scifres]

Thanks Kirk. That's helpful.

[Kirkll]

Ok John..... Here is a few bows that i compared that I will not mention manufactures names. But if you watch these limbs you can see the ones that stop the limb tips clean and and the ones that do not. I have a recurve bow in there that stops the tips well, but has limb bulge. That bow is a perfect example of a wee bit too much preload and forward taper. I believe that one had an .002 FT.   

There is another recurve in this batch that has very low preload, and was filmed before the limbs were balanced... You can see this limb flopping around after the string hits brace again....

Then i have a couple clips with my own hybrid long bow with 90+ pounds of string tension at brace.... There is a noticeable difference in these bows performance levels as well as hand shock.... The last bow that i filmed here was very fast, and i know the bowyer well.    I believe the two go hand in hand hand shock and performance... The more hand shock you have, the less energy is being transferred to the arrow shaft.

Once you have your limbs balanced out where they are stopping at the same time, the only way to tame the hand shock down from there is using heavier weight shafts. The momentum build up in the power stroke will often time launch a heavier shaft as well as a lighter one in low preload conditions with less hand shock.... It's hard to explain...But more stored energy gets transferred into the heavier shafts. The lighter weight shafts require more string tension at brace to stop the limbs dead and transfer the energy.

[John Scifres]

Thanks a lot Kirk. This is very helpful!  Can you help me out with a couple terms you use?

Limb Bulge
Preload
Forward Taper
String tension at brace

Also, what is your setup for shooting the video?  What camera and settings do you use?

Thanks for all the help.  I feel like the first go around in my bowmaking career was my Bachelors.  I may have partied too much on campus to truly understand what I was doing.  I made nice bows and had a decent career but I'm ready for my Masters now.  Thanks

[Kirkll]

Limb Bulge:

This is the condition when the limb tips are stoping clean but the center portion of the the working limb bulges forward after the limb tips stop. Most commonly seen in RC limbs with too much forward taper and borderline vertical stability issues.
It has no effect on performance, or energy transfer. but can cause vibration to be felt in the grip.

Preload:

The preload on a limb is the amount of tension put on the limbs when stringing the bow. The more reflex in the unbraced limb, the higher the string tension is at brace. The higher string tension,  or higher preload is what stops the forward limb travel dead, and transfers more of the stored energy into the shaft.

 But high preload alone isn’t the whole secret to better performance.  The shape of the limbs, and where the reflex is located on the limbs make a huge difference. You can just flip the tips keeping the limbs straight and gain some tension. Or you can do a gradual reflex along the whole length of the limb…. Depending on taper rates, you can manipulate where the limbs bend on a laminated glass bow. The same theory applies to self bows, but it’s done by scraping the limbs.  The bottom line is a lighter mass weight limp tip that bends further out from the fades will give you better performance with lighter weight shafts, better string angle at full draw, and less hand shock if the preload is adjusted correctly.

Forward Taper:

This is the taper rate from you riser fades to limb tips… On glass limbs they vary from .002 - .004 per inch on long bows. For RC. Limbs they run from  parallel to ..002 per inch…. The use of tip wedges is very common on glass limb bows to force the limbs to bend and store energy in a shorter section of the limbs without loosing string angle at full draw.


String tension at brace:  equals preload. This can be measured accurately using an in-line digital scale, or just doing a DFC chart.  Higher string tension or preload will give you a higher pounds per inch early in the draw cycle and will let off as the bow is drawn…. Where the limb is bending determines string angle at full draw.

Also, what is your setup for shooting the video?  What camera and settings do you use?

I used a camera with a 1200 FPS ( frames per second) on these clips, with a photo slide screen for a back stop. I set the bows up on a shooting machine too. I needed two double 500 watt halogen lights to get a clear photo. But I suppose they have special lighting for this available…. It’s expensive, and very time consuming to edit and trim these high speed video clips. It requires editing software and getting familiar with it… it’s a rather large learning curve depending on the software.

Kirk

[Kirkll]

Btw… the second bow in my video is the one with a limb bulge issue…. There are 5 different clips in this video, so be patient and wait for the next one…. Trying to trim these film clips frame by frame is tough to do.

[Stagmitis]

Hey john just watched your video and it looks like a string alignment issue to me.

[Mo_coon-catcher]

Whats the original side profile like (before any set) and what’s the front profile? It looks like you have too much bend in the inner half compared to the rest of the limbs. I had an Osage long bow I tillered circular to hit weight since it was a small stave without much meat to work with. It felt like it tried to jump out of my hand even though it took no set. I tillered it elliptically and the shock went away.

Kyle

[John Scifres]

It had maybe 1" of reflex to start and is almost straight now right off the string.  Maybe 1-1/2" total set from the start.

[Mo_coon-catcher]

Thats not bad. What’s the profiles like from the front and side?

Kyle

[John Scifres]

She's 1-3/8" for about 1/3 of the limb, tapering to 1/2" at the nocks.

[Mo_coon-catcher]

I would say it’s Beni g a bit too much in the inner 1/3 compared to the rest of the limb. I put a couple marks on the bow from a screenshot of the last video. I don’t think it’ll take much, but I feel that you’ll notice less hand shock with a bit more movement between the lines.

Kyle