How Can We Help?
< All Topics

I hear about the “mechanical advantage” of certain broadheads and broadhead shapes. Is this important?

A broadhead’s mechanical advantage is something to consider as it can alter hunting effectiveness.

With something like a traditional bow, selecting an efficient broadhead is useful as there is a lack of force due to the equipment, so there’s a need to maximize the use of what force is available. Same thing for most women and kids shooting compounds.

Wise equipment choices explains how guys can manage to consistently pass through some of the largest game in the world using traditional equipment, and yet every year we deal with guys shooting compounds and crossbows that are failing to penetrate whitetail deer.

With higher poundage compounds, the “required” mechanical advantage is not as much of a concern, and the same is true of crossbows, but it’s better not to reduce base effectiveness and eliminating the advantage of using more powerful equipment.

Mechanical Advantage of Broadhead = Length / Height / Blades

3 inch long, 1 inch wide, 2 blade head gives us 2 blades that are each 3 inches long and 0.5 inches tall
3(L)/0.5(H)/2(BL) = Mechanical Advantage
3=Mechanical Advantage

Work = Force x Distance

The reference provided is looking at a broadhead as a number of ramps which provide a specific mechanical advantage over the distance of the head

Imagine having a handful of 2 blade heads that are all 3 inches long but different widths
L x H x BL
3 / 0.5 / 2 = 3 MA (1 inch wide)
3 / 0.75 / 2 = 2 MA (1.5 inch wide)
3 / 1 / 2 = 1.5 MA (2 inch wide)
3 / 1.25 / 2 = 1.2 MA (2.5 inch wide)
3 / 1.5 / 2 = 1 MA (3 inch wide)
3 / 1.75 / 2 = 0.86 MA (3.5 inch wide)
3 / 2 / 2 = 0.75 MA (4 inch wide)

Now, how much force is needed for each of these to accomplish the same level of work?
W / MA = F
1 inch wide = 0.33
1.5 inch wide = 0.5
2 inch wide = 0.66
2.5 inch wide = 0.83
3 inch wide = 1.00
3.5 inch wide = 1.16
4 inch wide = 1.33

So the 1-inch wide head would need 1/3 of the force as the 3-inch wide head to accomplish the same amount of work.

As the bow changes, the recommendation can change. Take 3 bows:

Traditional Bow 700gr@181fps = 50.9ft-lbs, 0.56slugs
Compound Bow 700gr@254fps = 100.26ft-lbs, 0.79slugs
Crossbow 700gr@283fps = 124.5ft-lbs, 0.88slugs

For traditional bows, we might recommend something extremely efficient like a 3 to 1 ratio head. With a mechanical advantage of 3 we are effectively tripling the amount of work we can do with the available force so for comparison sake we can now consider the traditional bow to be 152.7ft-lbs, 1.68 slugs.

For compound bows we might suggest broadheads that are around 2 inches long as there is less chance of bending when dealing with the higher levels of force.

Even though the compound started with double (200%) the kinetic energy and 40 percent more momentum, with a mechanical advantage of only 1 it realistically will have less penetration potential than the traditional bow since it will still be at 100.26ft-lbs and 0.79 slugs.

For crossbow if we use the same 2 inch long and 2 inch wide head as the compound its a similar story. Even though it started with triple the kinetic energy and nearly 60 percent more momentum, it would still have less potential than our traditional bow using the 3 to 1 head.

Is all of that potential truly needed? Not necessarily, but there are many factors that play into it. How much mass the arrow has, whether its a clean hit on the animal or if bone is encountered, how big the animal is (how much penetration is needed for a pass through).

A 2-inch wide head works well on deer size game assuming that major bone isn’t encountered. If a leg bone or shoulder socket is encountered the chances of a pass through greatly reduce unless the arrow is greater than 700gr. So could one set up a huge head that is 3 or 4-inches wide and be successful? Yes, possibly, but it would require very high mass to make it consistent.

With a normal “heavy” crossbow bolt around 500-600gr, we would not want to use anything below a 1 mechanical advantage simply because we want to error on the side of caution and plan for those worst case scenarios.

Table of Contents