Yep, just stiffens the riser. Bow designerhave to go through a lot of math to get these things right. People aren't aware of it but the torsional pressure on a riser is tremendous. The last time I saw an actual mathemtical figure was about 20 years ago. At that time somebody came up with a figure of about 70,000 pounds per square inch of torsional (bending) pressure on a riser. I'd venture a guess that with today's modern technology (limbs and aggressive cams) it would have to be closer to 100,000 psi.
There are videos on You Tube showing in super slo mothion some of the things that happen when a bow is shot. It can be scary. So is a bridge riser neccessary? Basing on the amount of riser failures I say NO, but I'm not an engineer either.
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