Why Understanding Power Stroke on Your Crossbow Is Vital
July 08, 2015
If you're looking to buy a crossbow,Â be prepared to beÂ inundatedÂ byÂ numerals. There'sÂ the velocity, which isÂ expressed in feet per seconds. Then thereÂ isÂ foot-poundsÂ of energy, overall lengthÂ in inches, axle-to-axle width (cocked andÂ uncocked), draw weight, and power stroke.Â Whew.
Of thoseÂ measurements, the least understoodÂ yet possibly most important, is power stroke.
InÂ its simplest terms, power strokeÂ is the length between the crossbow's string in the rearmost, or cockedÂ position, and it'sÂ uncocked,Â resting point.Â It very roughly equates to draw length in a vertical bow.
WhyÂ is it important? Typically, the longer the power stroke,Â theÂ faster the arrow will fly. The longer the stroke, the moreÂ energy is being imparted to theÂ arrow.
But,Â there is more to it than that, and it's important you know.
Power stroke alone doesn't determine a crossbow's velocity; rather, several interrelated and interdependent features, including cam and limb design and draw weight,Â alsoÂ govern it.
Some companies useÂ power strokeÂ to get speedÂ from their bows. Other insteadÂ designÂ them so the riser, limbÂ orÂ camsÂ give it speed.
RobÂ Dykeman, president of Excalibur Crossbows,Â said that power stroke isÂ "a key component in the crossbow's performance."
Without cams, ExcaliburÂ recurve-style crossbows require changes in power stroke and limb design to increase speed, especially if a shorter overall length is desired.
Take, for example, their new bow, theÂ Micro 335. It hasÂ an overall length of 32Â 1/2-inches and diminutive 9Â 1/2-inch powerÂ stroke.Â Shooters like a shorter bow, but they want speed, too.Â To get speed from such a short stroke,Â the Micro requires a heartyÂ 270-poundÂ draw weight, that is, the amount of resistance of the string when attempting to cock the bow.Â The Micro will shoot a 350-grain arrow at an impressiveÂ 335Â fps.
On the other hand, theÂ ExcaliburÂ Matrix 330 model, achieves nearly the same velocity with aÂ much lighterÂ 220-poundÂ draw weight.
How? The 33.9-inch Matrix 330 has an 11.4-inch power stroke. That slight 1.9-inch increase in lengthÂ means stress can be taken off theÂ limbs — '¯a full 50 pounds less.
Similar comparisons can be observed inÂ TenPointÂ Crossbow Technologies' cam-powered bows, too. For example, with a 15 1/2-inch power stroke and 165-poundÂ draw weight, the 37.4 inch-long Vapor attains 360 fps with a 420-grain bolt. Its cocked andÂ uncockedÂ axle-to-axle measurements are 17 1/2Â inches and 12.6 inches, respectively.
TheÂ TenPointÂ Venom propels a slightly heavier 425-grain bolt to 353 fps. But, due to its 2.8-inch shorter length and 13.6-inch power stroke, its draw weight is 185 lbs. It has 0.10 inch and 0.70 inch wider cocked and non-cocked axle-to-axle widths than its brethren.
Once again, a decrease in power stroke length necessitated an increase in draw weight poundage and a limb design change.
Parallels can be found in the Stryker crossbow line-up, too, which consists of the SolutionÂ and Solution LS models. Clearly illustrating that power stroke alone isn't enough to determine velocity, bothÂ modelsÂ have 35 inches overall lengths and 15Â 1/2-inch power strokes, and yet the latter eclipses the former by a full 40Â fps (350 fps vs. 390 fps).
How?Â Draw weight poundage. TheÂ Solution has a 125-poundÂ draw weightÂ and theÂ Solution LSÂ is 155 pounds.
While less draw weight in general makes it easier to cock a crossbow, there are so many effectiveÂ cocking aidsÂ on the market now, such as theÂ Mission RSD,Â TenPointÂ ACUdrawÂ andÂ Barnett Crank Cocking Device, among others, reduce the effort needed to cock the heavier poundage crossbows. As such,Â heacyÂ draw weights areÂ mostly a moot pointÂ to most hunters.
But the power stroke numeral is different compared to the draw-weight numeral. It's not a moot point at all, and in fact, it's often a big part of how a shooter rates a crossbow. A long power stroke can make the bow feel "nose heavy."
PhilÂ Bednar, director of marketing forÂ TenPointÂ Crossbow Technologies,Â said the main disadvantage of a long power stroke is the possibility of having a long, poorly balanced, and unwieldy crossbow.
Increased weight is probable, too. To counter this, companies such asÂ TenPoint, Parker, and Carbon Express, among others have created bull-pupÂ variants in which the barrel resides closer to the butt of the stock.
"The bull-pup trigger design enables us to create shorter crossbows but maintain the longer power stroke," saidÂ Bednar.Â "We are able to market short, narrow, light, well-balanced, and fast crossbows due to the bull-pup trigger design that we started in 2013."
Four such models are found in theÂ TenPointÂ line, as well as two ofÂ Darton'sÂ new offerings.
REVERSE LIMB ADVANTAGES
Another way to maximize, or simply preserve, power-stroke length in a compact crossbow platform is to utilize reverse-draw technology.Â By moving the riser from the front of the barrel to the back, and putting the cams on the front of the bow, you can increase power stroke without increase the "nose heavy" feel of a long-stroke traditional crossbow.
Take the Barnett Vengeance, for example. Despite its relatively short 33 3/4Â inches overall length,Â its reserve-draw designÂ enables it to feature a full 18-inch power stroke, which isÂ one ofÂ the longestÂ on the market.
As such, with modest 140-poundÂ draw weight, the Vengeance propels a 400-grain bolt to 365 fps.
Similar in design,Â the 35 3/4-inch-long Horton Storm RDXÂ attains 370 fps from a 16Â 1/2-inch power stroke and 165-poundÂ draw weight.
Short power-stroke length, on the other hand, can negatively affect performance unless it's accounted for in the crossbow's other speed-influencing components. It can also affect the legality of the crossbow for hunting use.
In addition to minimum (and maximum) draw weights, some states and provinces have minimum lengths for power stroke and arrows; therefore, it's prudent to check the regulations of the area you'll be hunting before purchasing (or arriving) with a crossbow.
Most adult crossbows have minimum power-stroke lengths of 11 inches or 12 inches, but as mentioned elsewhere, there are those that are shorter. The 9Â 1/2-inch power stroke of the Excalibur Micro 335 is a prime example. On average, youth models will have shorter power strokes. AsÂ BednarÂ explained, "For a youth model, performance is not nearly as important as physical size-weight and safety."
This all begs the question, "Does power stroke length come first when designing a new crossbow?" According toÂ Dykeman, "We start off by determining the consumer needs for function, performance, fit, and feel, and then design the power stroke around those features to meet the criteria. For this reason, almost every Excalibur uses a different power stroke."
This parallels the trend in Barnett's extensive line, too.
Bednar, on the other hand, reported, "It depends on what we are trying to accomplish. We have several different stroke lengths on our 13 crossbow models."
Parker Crossbows are similar in they have relatively few power-stroke lengths across the entire line-up.
In the end, power stroke is but one element in an interdependent system that helps dictate crossbow speed; cam and limb design and draw weightÂ areÂ equally important. And, by itself that measurement means relatively little, unless one is ensuring a crossbow's legality for hunting in a specific state or province.
According toÂ Bednar, more important considerations than power stroke are, "Fit and feel, physical weight, length, axle-to-axle (or limb tip-to-tip) width, and speed. Moreover, I encourage buyers not to pay attention to the specs, but rather go to their local shop and shoulder and shoot all the models they're interested in."
Now that's sage advice!