I have an RER recurve that I can shoot 500 or 400 spine carbons and get good flight.I have a Bear Super Kodiak same poundage and draw length that the 400 spine is stiff.Why would that be ,educate me please.

My first guess would be how the shelf is cut and the true draw weight. There are a lot of other little intricate things going on that make up how different bows react to an arrow but those are usually the big ones IMO.

The weight is within one pound ,the shelf on the RER is crested compared to the Bear which is flat.The arrow has a bigger contact area side and bottom on the Bear how does this influence the arrow

The farther away from center cut you get the more the arrow has to bend around the riser on release. Usually leading to a softer spined arrow flying true. If a bow is center cut, very close or past center usually a stiffer arrow is needed and or it tolerates different spines better.

Smarter guys than me will chime in I am sure, but from my limited experience I'd say bows perform at different level of efficiency, center cut depth, string material, ability to draw fully, cleaner release, and nock fit can all effect spine options. I went down this road last year. Additionally, sometimes arrows SEEM to be flying well, but either bareshafting or broadheads can reveal a different story. Not saying thats the case hear, nut the answer lies somewhere in all of that.

Some bows are more efficient than others I am told. Limb design, limb materials, riser shelf all can effect performance.
I had a similar experience with my newer SK and my Bear TD. The TD limbs were 10 years older than the SK. Both 55# at 28in. Both using same string and both shot off of a Para rest. I just figured it was the materials in the limbs or age of TD.

Even though both spines look like they fly well off of that one bow, one will certainly be better than the other. Fletched arrows certainly hide imperfections of tuning. I'm sure the RER is cut past center too. That doesn't make it better, just different...it is a nice bow though.

BigJim

Maybe, for the RER, a 450 spine would be perfect. Being cut slightly past center, it's very tolerant of spine variances and does in fact shoot 400's and 500's equally well since they are on each side, but close to a "sweet spot" for arrow spine.

The Super Kodiak may like a 500 spine and the 400 is on the stiff side for it. It might even tolerate something slightly weaker than the 500, where the RER would not.

I don't have an SK, but do have a couple of RERs and they seem to be VERY efficient bows for their draw weight.

I'm pretty sure if I shot aluminum's, where there are really small steps between spine ratings, I could find an ideal spine weight for each of my bows at 10GPP and it would likely be different for each bow. Carbons recover faster and the difference is not so readily apparent. I can generally shoot GT 3555 or 1535s from nearly all of my bows; the 3555s with 180 total point; the 1535s with 135 point weight and get good arrow flight from both to the same point of impact.

Now if I could keep all my shots in a tea cup at 20 yards, I'd worry more about having the perfect setup. Given my lunch plate size groups, I can pretty well mix up the arrows and not increase the groups size. Darn it.

The static tip on the RER makes a difference in performance vs the Bear Super Kodiak and therefore, will be able to shoot a stiffer spine.

Bill

I bare shaft tuned my RER XR recently with  400 and 500 Easton Axis and had great results. But arrows where not the same at all.
I did it just to test which I liked best. My bow is 62" and 50@29".

400 ended up needing cut down just a little,  full 75 grain insert, 150 grain head.

500 ended up around 1.5 inches shorter with 50 grain insert and 150 grain head.

Careful though because bare shaft tuning can get you lots of false results is form or other things are off.  Im not expert and it took long time but I think results are great and bare shaft groups with fletched perfectly.

Jack Hoyt has it right. I think most archers will find that the amount the bow is cut past center is the main reason bows of the same poundage require different spines.

Carbon reacts differently than wood - I can get 15/35s, 35/55 and 55/75s all to fly good out of most of my bows.... the impact point varies but I can change that with rest/side plate material.

There are two schools of thought - manipulate your arrow to fit your bow or manipulate your bow/rest material to make your arrow hit where you look.  

If you really want to get technical use a chronograph and differnet point weights and them use a formula to figure KE - that will tell you the best arrow set up for that bow.  Then you can manipulate your shelf/side plate material and nock/brace height to make that arrow hit where you look.

I use to spend hours tuning my arrows (bare shaft, paper tune, broadhead/field pt impacts, etc.) to fit a variety of bows... now I stick to 55/75s cut to 28 inches and I make them work out of ALL my bows.  I learned this from Rick Welch when I went to his class.  Proper helical fletching (at leasdt three 4 inch feathers will straighten carbon pretty quick - then it is just a matter of manipulating rest/shelf, nok and brace to make them fly where you naturally look.

Archery is suppose to be simple and man we sure fin ways to complicate things.