Short riser longbow question

[Carpdaddy]

I currently build a one piece reflex/deflex longbow that is 62" total length, 60" NTN. The form looks similar to the one Kenny M. has, maybe a little more reflex and more forward riser, but close. I was thinking about going even shorter for use in my ground blind, maybe 1" off each end. And yea; not just for the blind but just cuz I want to do something different (for me that is). I already did this once as I formerly built it 64" total length, I had to make it a little thinner to compensate. I start the limb taper about an inch past the riser fades and usually end up sanding back onto the riser fades, and usually down to 5/8" tips. I currently use a 14" riser and was considering dropping it to 12". My question is this; if I drop the riser length by 2" along with the total length; will I need to go thinner, thicker, or keep my lams the same? Or do you feel that I would be better off leaving the riser length the same and dropping lam thickness resulting in less working limb? What are your experiences with shorter risers?    
 Thanks in advance.

[chackworth3]

I would keep them the same on the first one just to see what you are gonna end up with as far as weight goes and adjust from there. I built a 54" bow one with a 16" riser one with 14" and with same stack and there was about an 8# difference in the weight. So I would just try it with the same the first time. Just my opinion...

[LittleBen]

Yeah likewise. I think you'll end up almost exactly the same weight.

[Carpdaddy]

Thanks for your response guys. I know that the shorter riser will make for a few challenges but I think with my form it will be okay. I was hoping to hear more from others who use short risers with their words of caution or advice. I will stay the same on lam thickness them.

[canopyboy]

I think you'll drop slightly in draw weight, everything else (materials, widths, overall length) being equal.  I think to keep the same draw weight with longer working limbs, you will need to increase your thickness slightly.  My full explanation in a different thread this morning, although he was trying to do the opposite:

 http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=125;t=011160

In the real world, you may or may not see that difference as much in variance in wood properties, glue-up consistency, and accuracy in grinding your profile.  But for the sake of increasing your odds of successfully matching draw weight, I would increase your stack by 25/24 (4%).

[Carpdaddy]

Thanks for your response canopyboy; I was hoping you would give your input on this. I had read your response about five times already in the other post and think you are on target. But what you may have missed is that I am not just taking 2" from the riser, I am also shortening that same 2" from the tips. Each one will have 24" of working limb. Do you think with the same amount of working limb that it will change as a result of the shorter riser. Again; thank you.

[Carpdaddy]

More to consider on this; taking an inch from the tips will reduce very slightly the amount of recurve. Also to consider is the fact that having a usual .003 per inch taper, the end being shorter would be slightly thicker. I know that these changes would be small but just wondering if it would be enough to change draw weight?

[canopyboy]

You're right, I did miss that.  If your working limb length doesn't change, you're into the secondary effects of your string angle changing slightly and the reduced recurve.  There are no good ways I know of to account for these from a simple first principles calculation basis.  You'd be getting into more of an FEA-type analysis.  Easier just to build it and see.

I think how much you sand and any variabilities in your materials (not to mention phase of the moon during glue-up) will have about as much or more effect than these two changes to your design and now agree with Ben and Chris that you shouldn't change anything (dang those people who read carefully.)

But please remember that while I like to talk big about anything I can put in engineering terms, I'm just a greenhorn bowyer still learning as I go.  Take it all with a grain of salt, I've learned to do the same no matter how experienced the person teaching is.  Everyone experiences things a little differently.  Keep an open, but slightly cynical mind.

[Carpdaddy]

LOL canopyboy! And here I was thinking that a feller that can use such big words must know everything! Thanks to all of you for your input.
Carp