Technical Stuff: Looking After The Bolts

Those who come to crossbow (horizontal bow) shooting from an archery (vertical bow) background will question what spine, or stiffness, of shaft to use for bolts. After all, the draw weights are higher and there is quite a range. On the face of it this is a sensible question. When you look for an arrow chart to help you will find that you are out of luck.

To explain this, we need to look at a bit of history, and skirt round the mechanics. Crossbows have been available commercially for some time. The Barnett Wildcat was available in the 60’s. It had an alloy prod, and bolts could be made from doweling using a slip-over plastic fletching assembly. The really serious shooters made their own bows. In many cases these would be trackless using a “D” loop on the string and a bolt rest near the prod. These are the early Target bows. Bows had to be tuned using the arrow rest. Bolts were down to the individual to make including piles and nocks.

In the late 70’s/early 80’s the Spirit crossbow made its appearance out of the BARNETT stable. This was a tracked Target bow – that is to say that the shaft rested on the edges of a groove for most of its length. It is around this bow, and similar, that the World Crossbow Shooting Association (WCSA) standard specification for Target bows was written.

The bow was designed to accept 18/64” or 17/64” diameter shafts.

Just for those who don’t know aluminium shafts are designated by two pairs of two digits; e.g., 1914 or 2016. The first two digits (19 or 20 in our examples) give the outer diameter in 64th of an inch. The second two (14 and 16 from the examples) give the wall thickness in 1000th of an inch.

Since commercially manufactured Target bows were in their infancy only a limited range of sizes of piles and nocks were available. That remains the case today. Short shaft length bolts (12 to 18”) are incredibly stiff in relation to the maximum draw weight of 95lbs for a Target bow and the string travel limitation. Add to this the fact that the bolt is supported for most of its length tuning becomes irrelevant. All you can do is change the pile weight. Archer’s paradox simply does not apply.

If you are using a trackless crossbow, it could be argued that spine has some relevance, but that is probably a second order effect.

The use of Sporting Crossbows for various forms of target shooting or hunting has been increasing in many countries – especially the USA where hunting is legal in most states and is considered the norm. This has resulted in the development of a large range of off the shelf (Sport) bows – many with high draw weights intended originally for hunting.

The business is mostly driven from the USA and the industry standard has become a track width set for a 22/64” diameter bolt to give the correct contact between string and nock. Since bows are tracked tuning is, again, irrelevant – you just need the shaft to withstand the acceleration.

Bolts for Sport Crossbows are commercially available. Easton offer three grades of crossbow bolt. All are 22/64” diameter and are available in 20 or 22” lengths. In some cases, they give the wall thickness. Bolts are available from other manufacturers in 16, 18, 20 and 22” lengths, but all in 22/64” diameter. The only variant is the 21/64” diameter shaft marketed by Excalibur (Canada).

In general Target bow bolts are not commercially available as off the shelf items. Some suppliers will make them.

You are liable to find parts or ready-made bolts in the sizes 1714, 1716, 1814, 1816, 2114, 2216 and 2219.

Realistically these break down into 1714 for Target bows and 2114, 2216 and 2219 for Sporting bows.

BOLT MATERIAL

There is then the question of aluminium or carbon shafts. This choice will be down to what form of shooting you about to undertake, and what type of bow.

If you shoot a Target class bow then you will use aluminium shafts. Carbon shafted bolts have been tried with Target crossbows but experience suggests that it is difficult to achieve consistent bolt grouping over a range of distances.

Aluminium shafts can be used on straw butts without problem. For unmarked distances you can use lighter piles to flatten your trajectory. In terms of which particular shaft to use it comes down to the properties of the alloy. The higher the draw weight the more you should pay to get better properties. In some cases, you may also get a better result anyway if you use higher quality alloy – say X7 rather than XX75. Note that X7 and XX78 are made from the same alloy. We suggest that you do not use lesser grades than XX75.

Both aluminium and carbon shafts are available for Sport bows.

In the past we would have advised to use aluminium shafts in preference to carbon. Straw butts are good for aluminium bolts. Butts will hold the bolts and withdrawal is usually reasonably easy. Carbon fibre shafted bolts are normally travelling faster. This gives rise to more friction when the bolt is stopped in a straw butt. More friction means more heat, which melts the latex used to bond the straw and tends to stick the bolts in the straw – hence making it more difficult to remove bolts. The other problem is that the higher rate of deceleration can break the bond between the tip insert and the shaft resulting in the tip and insert continuing to travel after the shaft has stopped. This means that if you use carbon bolts you are much better shooting at foam constructed targets. These tend to be thicker so more so deceleration can be slower.

Aluminium bolts are easier to locate with a metal detector if you miss the boss. With carbon fibre bolts only the pile (or point) is metal so are very difficult to find.

Things have changed because of the advances made in the design and manufacture of Sport style Crossbows.

Most of the bows classed as Standard using the WCSA momentum calculation should be fine with aluminium shafts, but you must check.

If you are using the lighter draw weight bows, say up to 125lb, the odds are in favour of being able to use aluminium shafts. You do, however, need to be careful and check what the manufacturer has to say about bolt mass and type.

There are Standard class bows that must be shot with carbon fibre bolts. There are three potential reasons. Firstly, to ensure that the bolt is stiff enough to withstand acceleration so as not to bend when the bow string is released. Secondly, to achieve the correct bolt mass. Some manufactures use carbon fibre bolts that are weighted to increase mass. Finally, the dry fire systems may require a specific nock shape to engage and activate the over-ride. In the latter two cases you will have to use the manufacturer’s bolts rather than cheaper off the shelf bolts.

If you have a Freestyle bow then you should use carbon fibre bolts. You still need to check the manufacturer’s recommendations.

Three examples:

Excalibur Matrix G340 has a draw weight of 230 lbs, a 340 ft/sec bolt speed, and must use a minimum 350 grain bolt. The bolt momentum calculates to be 0.53 lb f/sec (2.28 Newtons/ sec) which makes it a Standard class bow (upper limit 0.55lb f/sec or 2.43 Newtons/ sec). You need to use the manufacturer’s bolts with at least 100 grain point.

Mission Sub 1. There are 4 versions of bow on the Sub 1 platform. One is Standard, and the others are Freestyle. All use a minimum 350 grain weighted bolt and an unusual vee notched nock. These must all be shot using Mission bolts.

Ravin Bows: Only three of the bows offered by Ravin are allowed under WCSA rules. The others all have a bolt momentum over 0.75lbs f/sec (3.32 Newtons/sec) and are therefore not permitted. These bows are all trackless and use a particular shape of knock to clip onto the string and engage with the anti-dryfire system. Without this you cannot shoot these bows, so you must use Ravin bolts.

Of the others the Ravin R9 has a 0.70 lbs ft/sec bolt momentum. Bolts from this bow will pass straight through a new straw buttress, and most foam butts, at 18 metres. Only a block of 3D target foam will stop the bolt so it can be scored.

The point is that you must check carefully what you are buying, and what it requires in terms of bolts and butts.

There is an app you can download from the WCSA web site to allow you to calculate your bow’s bolt momentum and determine if your Sport bow is Standard or Freestyle.

If you do have problems withdrawing bolts, aluminium or carbon, from the butt coat the leading 100 mm with silicon. You will need to do this from time to time as it wears away. Use the liquid silicon that you might use on car tyres to protect them and make them look new. It’s reasonably cheap.

Do not attempt to withdraw carbon bolts bare handed. Carbon bolts may break up on impact. This will not necessarily be obvious. If you attempt to pull a damaged bolt bare handed you may end up with a hand full of carbon fibre shards.

BOLT LENGTH

Bolt length must conform to WCSA Rules of Shooting. For Target bows bolts must be 12 to 18 inches in length. For both Target and Sport class bows the pile, or point of the bolt must be in free space totally clear of the end of the track, or beyond the arrow rest for trackless bows.

If you buy carbon shafts then you will be restricted to the lengths that are available. Beyond conforming to the above, keep the shaft as short as possible. The shaft length should ideally be such that: when the nock is placed against the latched string the pile, or pile and end of the screwed insert, are just beyond the end of the track.

The reasons for this are simple. Firstly, it allows for possible eccentricity in the pile and insert and ensures that the whole shaft rests on the track. Secondly it ensures that the harder pile cannot rub on the track and cause undue wear, especially if there is eccentricity. A longer shaft is more likely to become bent as a result of shooting. Finally, a shorter bolt is lighter and will give a better cast.

However, if you are shooting on “bag” targets, as are sometimes used for Forest rounds, then you may need to consider longer bolts to minimise vane damage.

BOLT WEIGHT

There is not much you can do about weight of the shaft, since you are keeping it as short as possible, or the nock. The rest of the weight is made up of vanes and the pile. Lighter bolts have a better cast. However, unlike arrows for vertical bow, you are using a relatively heavy pile to bring the balance point forward and much nearer to the pile. This is what gives the bolt stability and leads to consistent groups.

For Target bows the pile should be about 225 grains. A lot of work has been done on this by shooters in America. This has shown that a weight around 225 grains gives the best result.

However, you should be wary. Commercially available Target crossbow piles will vary in weight and need to be weighed and trimmed in order to achieve consistent grouping.

In just about every case, Sporting bow shooters will use screw in piles. There are only three weights available – 100, 125, and 150 grains. The advice is that you should use 125 grain piles for target shooting. The choice is yours when it comes to Forest & 3D. The distances are shorter so you can sacrifice a little stability for speed and therefore cast, if you use a lighter pile. This all changes if you are shooting a bow that requires a specific minimum bolt weight. Most usually 100 grain piles are used to meet the minimum weight,

If you are using screw-in piles remember that the rules require the use of the parabolic/bullet shape rather than field points. This is better for butt wear.

FLETCHING

There is no valid ballistic based reason to have vanes on a bolt. The situation is simply that the rules require that vanes be fitted.

By virtue of the fact that they are there vanes will impart some spin to the bolt. Theoretically this will give some added stability. This is however marginal.

Do not be tempted to spiral fletch as archers do. Helical, or spiral, fletching can result in the vane catching in the track, and will throw the bolt off course.

Taller profile vanes have more effect but are heavier. In addition, taller vanes will increase drag and slow the bolt more rapidly, so use small low-profile vanes.

For target bows 25 or 35 mm long low-profile vanes seem to give the best results. However, most standard archery vanes have a profile that is too tall. Get your vanes from a specialist supplier, or hunt through your local archery supplier’s stock for something that is suitable. Otherwise, be prepared to make a jig and spend hours trimming vanes to size.

Sporting bolts are frequently fitted with larger vanes. Those supplied with sport bows may be 5 inches long. Don’t use them. There is too much possibility of them catching in the track groove. Most shooters seem to use 3, or 4, inch standard archery vanes. There is no reason why smaller vanes should not be used.

The variety of vane colours available and their combination does usually make it easier to determine who each bolt belongs to.

ALUMINIUM SHAFTS

If you are using aluminium shafts you need to consider how the tubing is made.

The aluminium is extruded through a die over a target pin. The shaft then goes through a number of straightening processes. This means that the shaft appears to be uniform and straight.

The problem is that all this work is done on the outside. In the extruding process the target pin can wander. Therefore, the specified wall thickness is only nominal. The variations will be small. But if you plug the ends of your un-built bolt and set it to float in a large tub of water you will find that it will tend to always float the same way up. One side will be ever so slightly heavier and turn to the bottom. The lighter side, top side as floated, should carry the cock vane and thus be placed in the track; so that when the string applies a load the shaft is deflected down into the track, remembering that the string should strike slightly above the shaft axis.

From the practical point of view there is no sense in attempting this. It’s fiddly and difficult to do, and it would not guarantee that your bolts would group together. In addition, the effect is reduced for longer shafts because the target pin “wander” evens out.

Assuming that you shoot all bolts in exactly the same way they will hopefully group reasonably. The trouble is that you will not be able to make exactly the same shot each time. So, was the bad shot you, the bow, the bolt, or a gust of wind? And are you shooting three bolts that group together?

If one bolt starts going off in the same direction the problem is the bolt. Otherwise, during a shoot you can’t tell unless you know a lot about that bolt. You need to test your bolts, whatever the shaft material. 

BOLT INSPECTION

Inspect your bolts regularly, before, during and after shooting. You should check for the following:

  1. Loose vanes.
  2. Damaged vanes.
  3. Damaged piles – an indicator that the shaft might have become bent.
  4. Piles that have moved: i.e. there is a gap between the pile, or insert, shoulder and the shaft, or piles that have unscrewed.
  5. Damaged nocks.
  6. Scratches and gouges in the shaft.

Loose and damaged vanes can throw off the shot. The rules also require that you have the same number of vanes on each bolt, so if a vane comes off the bolt is unusable.

If a nock has a chip removed directly opposite the cock vane, then there is the possibility that the string may “jump the bolt”.  Replace damaged nocks or in an emergency, rotate the nock so that the damage is adjacent to the cock vane.

Carefully check any scratches and gouges in aluminium shafts – use a magnifying glass. The scratch may hide a fracture of the shaft wall. This weakens the shaft and can throw off your shot. More important is the fact that the shaft could break when the string is released. There is no telling how dangerous this could be.

Between ends make sure that your bolts are clean. Straw boss construction includes latex which is used to help bond the straw into braids. This can be transferred to the bolts, possibly along with bits of straw, and will change how the bolt sits on the track. It is essential that all of this debris be removed.

STRAIGHTENING ALUMINIUM BOLTS

Much depends on the grade of alloy you are using for your shafts.

There is definitely value in straightening XX75 shafts. In relative terms the alloy is soft and can easily be bent and, therefore, straightened.

X7 and XX78 shafts are a different matter. The alloy is much tougher and its brittle. The name X7 came originally from the number of straighten processes the shaft was put through by the manufacturer. You cannot reproduce those sorts of conditions. If you do straighten shafts made of this alloy it may very well be that that the first shot taken will undo all your work.

If you can see an obvious bend in a shaft of this tougher alloy then the shaft is probably scrap. There is some possibility that with XX75 shafts you can recover the situation.

Straightening of bolts must be done before you start the bench test process.

Checking straightness at a competition is just a matter of something to do when not shooting. It’s already too late.