Different arrows, achieving same spine

[FrankM]

I want to check with you guys to see if my reasoning is correct.

I wanted to add 100 grains to my 5575's. At the bowshop, they recommended I only add 50 grains and see how they shoot. Then if needed, add 50 more grains.  It sounds reasonable, except for this thought.

I have 2018's with 145 grain points and about 60 grain weights up front.

I want my 5575's to spine like my 2018's so I don't have to deal with nock or rest changes when switching arrows. This would have been accomplished by adding 100 grains, not 50 grains with 145 grain points according to Stu's Calculator.

Do you think I should have just insisted on the 100 grains, or is their way better? I didn't mention about the 2018's to them. They probably just wanted to help tune the 5575's to the bow.
Is my reasoning valid?

[JRY309]

For me carbons and aluminum are two different animals,they may spine a weigh similar but the react differently.I have had carbon arrows where I increased point weight by 50 grs. and they still shot with the lighter ones.I think also the bow has a big effect,depending on how he riser is cut.I always prefer to bareshaft tune and try things out for myself,I would try the 50 gr inserts first and see how the fly.

[FrankM]

Interesting. They are different animals. I thought I could get them to act the same out of one bow. Possibly not. Guess the best thing to do is give them a try and go from there. Thanks!

[stringstretcher]

Being that the two shafts are different diameters, is that not going to change you center shot and nocking point?

[FrankM]

I'm still a noob but it makes sense. LOL

[Jesse Peltan]

The diameter of the shaft is accounted for in the calculator.  If you didn't mention the 2018's the 50grains at a time was the most logical piece of advice so you should go back and get your 100grains.  The big difference between aluminums and carbons is diameter.  Diameter is accounted for in the calculator.  Go ahead and get your 100grains and see how they fly.

[JimB]

2018's and 5575's are the same giameter.I have always heard about carbons and aluminums acting differently and don't dispute that but mine have reacted about the same,inch per inch,pound for pound.I don't think they gave you bad advice,necessarily.Kind of like the doctor telling you"try these pills,if they don't work,come back and see me"(and pay for another office visit)Frank,you have fooled around with this stuff enough,that it may be time for you to do some of this yourself.You are starting to learn what you need.It isn't that hard after that and if you hit any snags,just holler here.You will get all the help that you need.

[FrankM]

Thanks guys. I shot some tonight. I tried the 2018's with horizontal and vertical tape. I also moved my rest to center. They shot well. They're coming in about 590 grains on a 51# bow. I tried the 5575's and they seem to shoot well with a 200 or 250 gr. field point with the tape. 175 or less grains is iffy with my rest centered. They're about 535 with the 200 grain point and 585 with the 250 grain point. I want to shoot a little more, but I think the first inclination from the guys here was correct, add 100 grains instead of the 50 I went with.