# COMMUNITY : FORUMS : KNEX

## Zipdart help

I'm struggling to figure out why range is so low for this thing. The barrel ends right after the bands stop pushing the projectile, the effective length is over a foot for the barrel, and even after adding 15+ lbs of band power the projectiles are going maybe 40 ft no matter what projectile I use. Any ideas?

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Downunder35m4 months ago

In my young years we had no knex, but we had wood and knifes ;)
When using rubber bands to launch something you need two thing to be right:
a) A smooth surface for both the projectile and the rail with as little gap as possible.
b) Suitable rubber ;)

Normal rubber bands are used to hold things together and offer quite a lot of stretch but most are not really suited to go back into shape really fast.
Rubber bands of the standard type are good for paper clips but above that weight they just suck.
If you want real power on your dart you have to things you can try:
Surgical latex tubing or better still the bands for spear guns as used by many divers.
Problem on a toy is to get the massive power down to something the assembly can handle.
As a kid my gnadfather was so nice to cut surcical tubing for me.
Two razor blades formed an "arrow head" that was mounted in reverse on some steel rod.
He simply pushed the tube over the rod and out came two halfs of the tubing with no serrated areas at all.
To further lower the power you can always cut again or use 4 blades to directly get quarter strips.

Ff = μN

where:

• Ff is the resistive force of friction
• μ is the coefficient of friction for the two surfaces (Greek letter "mu")
• N is the normal or perpendicular force pushing the two objects together
• μN is μ times N

Ff and N are measured in units of force, which are pounds or newtons

+ c for whatever else is in the way. Good luck.

I've tried to minimize how much pressure is exerted on the sides of the projectile as much as possible, but even despite that changing how much force is exerted should at least minimally change how far the projectile goes.

4 months ago

The energy is robbed by the bumpy surface it hits between each block. That's why the guide tube or channel needs to be continuous and smooth all the way through.

Dreamwave (author) 4 months ago

Photos: https://goo.gl/photos/Xafs8yHM6DzeAedP7

Dreamwave (author) 4 months ago

photos coming as soon as my ISP chooses to be stable for 5 minutes