What affect does riser length have on bow performance and bow weight?
Longer riser same stack heavier draw weight. More or less working limb for different draw lengths.
It's totally dependent on total bow length as far as draw weight goes. Let's use std ILF riser lengths for example. 15, 17, 19, & 21". If you use medium length limbs on all of them the length and have 50 @ 28" on the 21" riser you will have a 64" bow length .
Using the same limbs your draw weight will increase between 2# & 2.5 # as you shorten the bow length depending on limb length.
19"= 62" length= 52.5#
17"= 60" length = 55# ,
15"= 58" length = 57.5 @ 28".
The most common ILF limb lengths are medium and long limbs, but they do make short limbs too.(they are not very common) The long limbs are 1" longer than the mediums. So a 19" riser with long limbs will be 64". The longer limbs have more draw length potential and are smoother drawing using the exact same design and taper rates and just lengthening the butt wedge length. Maintaining the same string nock location on the hook will insure comparable performance rates. The difference in draw weights with long limbs are closer to 2# for every step down in riser size too.
Btw... riser length has nothing to do with bow performance... period.
Hope that helps. Kirk