Sear Bar/Trigger Pin Relationship

[James Rogers]

In looking at as many fowling pieces as possible I have found the distance on originals from the 18th century seem to average about 5/8".

Has anyone seen any 18 th century originals whether they be rifle, pistol or fowling gun with as close a placement as is promoted in the modern day as the way to to pin a trigger?

Also, if you are holding true to the longer trigger placements based on what you may also see as a trait of certain originals, what steps do you pursue to enhance the performance?

[Dave Faletti]

I determine it by the lock, trigger and the trigger pull I want.  If the trigger is to not slide on the sear bar when the sear releases there is only one location for the trigger pin.  Math and geometry determines it.  Going by typicals will get someone in trouble if their lock has a different sear release force or they use a longer/shorter trigger. Its a good place to start though.

[Jim Kibler]

Dave,

By your description, the only place a trigger should be pinned is in line with the sear pivot point as this is the only place sliding friction between the sear and trigger can be totally eliminated.

My personal viewpoint is that although sear / trigger contact and sliding is something to consider, it is not the only consideration in determining a pivot point location.  If going by this method, distances between sear contact and trigger pivot point may end up excessively long.

To the question James presented regarding longer trigger placements, in order to not have an excessively hard pull, the lock needs to be well designed, setup and tuned.  The longer distance than is typical today greatly shortens creep, but increases pull force required.  A well set-up lock will compensate for this issue.  A second consideration is to pin the trigger high in the stock.  Along a line drawn between the sear contact point and the sear pivot point is good location to shoot for, but some variation doesn't seem to make a great deal of difference.

Jim

[Dave Faletti]

Jim.  Obviously the design and quality of the lock and trigger will make a difference.  My main point is that the trigger pull (and creep even) can be determined in advance.  Compromises can be made and compensated for.  Sliding between sear bar and trigger is not a huge deal but nice to minimize.  If the contact point is low friction, it won't matter.  I often hear  "pin it high".   I just prefer to analyze it more.  It comes out better and more consistent.

[Jim Kibler]

I would be interested in hearing about your process Dave.  Your post has prompted me to write a few simple equations concerning trigger pull force and trigger movement necessary to fire a lock.  I realize this may be completely over the top and likely ridiculous, but here it goes.

Lock characteristics constant
Force to disengage sear = F1
Distance sear must travel at  trigger contact point for sear to disengage=X
Distance from trigger finger contact point to trigger pivot point =Y
Distance from trigger pivot point to sear contact point =Z

so
Y/Z (X)= trigger pull distance for sear to disengage
Z/Y (F1)= trigger pull force to disengage sear


So, for a constant lock, it becomes clear that trigger pull force and trigger pull distance are inversely proportional.

If we have a desired trigger pull, and know F1 and X then we can solve for (Z/Y)

(Z/Y)= (trigger pull force / F1)

Once we know the ration of Z/Y we can set either Z or Y and solve for the other.

So as a simple example, if F1 = 6 pounds and a 3 pound trigger pull is desired the ration of Z/Y would be 1/2.  Or in other words if Y were two inches, Z would be 1 inch.

This simple method does not consider friction and assumes the trigger is pined on a line between the sear pivot point and the point the trigger  bears on the sear.

You must also be able to measure the force to trip the sear (F1) and the distance the sear must travel to disengage (X) on your given lock.

Now, before everyone goes crazy, I agree that this is likely not too practical and for me is just a thought experiment which might give a little better understanding of the mechanics involved.

Jim

[b bogart]

I hope there is not a quiz later on this

[Acer Saccharum]

I hope there is not a quiz later on this

Print Jim's equation out on your palm before you go into the test. Just don't let the nuns see your notes.


While I am not native to the world of relating math to everyday existence, I DO appreciate the longer the distance from the trig pivot to the sear bar, the more effort it will take to trip the lock. Conversely, the shorter that distance, the more creep you will have.

I think one key element that Jim touched on is 'a well set up lock'. A finely tuned lock will dramatically reduce both trigger pull and creep, taking the trigger completely out of the equation.

Yes, reduce the sliding effect as much as possible.

Caseharden the top edge of the trigger bar. Polish both that and the sear bar.

Tom

[flehto]

Seeing the question hasn't been answered, I'll take a stab at it and this is derived only from my own experiences. W/ the majority of locks available today and mostly  used untuned, 5/8" from pivot to sear bar location on the trigger bar, seems too long causing an excessively hard trigger pull. I achieve good results w/ a high pinned trigger and 5/16"-3/8" distance from pivot to sear bar. The resulting TP might be a little on the light side {3lbs} possibly for a fowler but works well on LRs.  I also hi-temp silver solder a piece of high speed tool steel onto  the trigger bar at the sear contact location.......Fred

[James Rogers]

Caseharden the top edge of the trigger bar. Polish both that and the sear bar.

Tom

I had wondered if this would be brought up. I have polished the surfaces in the past but to date have not case treated them. Details.

I have to date also followed the closer pinned relationship basically just from hearing it as the "only" proper way but as a shooter,  I would choose the evils of a longer pinning for a fowling gun. I want to also to be following what seems to be common on originals.

[Dphariss]

Trigger pull quality.

Its not the trigger,  ITS THE LOCK that is tuned. IT gives the light pull. IT gives the crisp break. If the lock is not right playing games with the trigger is just playing games.
Sure the trigger needs to be properly done as well but the distance from trigger pin to sear arm contact can be 3/4" plus IF the lock is right and the trigger pinned at the correct height. 
The sear must break from the notch, not slide out like pulling a rock down a gravel road on a rope.
Even massive Military type locks can have safe trigger pulls in the 8-10 ounce range without excessively close pin to sear arm distances or worry about sliding on the trigger.
The sear and tumbler need to be hard enough to maintain their shape, the angles have to be right at the sear nose and the notch and the surfaces must be smooth.
The sear spring is NOT to hold the sear in position in the notch, its to PLACE THE SEAR IN THE NOTCH. The sear should stay in with no spring pressure. It should be of moderate strength and be snappy. Too light and flys may not work properly if so equipped.

Dan

[James Rogers]

Dan,
It looks like you have echoed the same and with some detail included. Thanks for that. I think the point about the bar though is a good detail to include with all that work into the lock and appears to be an added consideration since the lock was mentioned as the main Target. Taint gonna make much difference with a $#@* lock though.  

[Acer Saccharum]

If the trigger pivot is the same distance as the sear bar to sear pivot, the pull will be about the same as pushing directly on the sear.

Some stuff I did years ago, take with grain of salt. Something between layout B and C will get your gun set up fine. Consider,too, the depth of your stock, where the sideplate is, where the trigger pin will come out on the other side. In C, the trigger pin is right under the sear pivot screw, leaving not much wood to support the trigger pivot pin.

Sear bar slides about 1/8" on the trigger plate.


In this scenario, the sear bar slides about 1/16 on the trigger plate, not bad.


In the below C, there is no sliding between sear bar and trigger plate. But in practice, may be hard to attain perfect alignment.

[Dphariss]

Acer is fundamentally correct.
The important thing about the high trigger pin is the longer arm on the lever the finger presses on.
Like the short arm from pin to sear it can create excess trigger movement if the lock is not properly adjusted.

If the sear notch, sear nose and spring are properly adjusted when the sear moves it should move all the way. IE break. If it does there will be no feeling of the sear moving on the trigger or the sear moving in the notch. Like the rock on the gravel road.
But this is not as easy as it might sound. Getting it this way and able to stand 3 blows from a dead blow or light rawhide mallet in the barrel or stock takes some work sometimes. The neophyte may end up buying more lock parts before its over. Not is it absolutely necessary to achieve the "ideal". If the trigger feels like to breaks clean at the desired pull weight then there is no reason for further adjustment. But some shooters can feel things in a trigger than others may not.

When working on rifles that have set in concrete dimensions for example playing with the trigger pin is not an option. Gotta fix the lock. I worked on quite a few of these.

Dan

[Jim Kibler]

That's good stuff Tom.  Although from a purely mechanical standpoint, pinning the trigger at the sear pivot (diagram C) may be best, sometimes this distance is quite large.  For example on a Chambers round faced lock it mearsures around .850".  If the distance from the point your finger contacts the trigger vertical to the trigger pivot is the same as this (.850" in this case), the trigger pull weight and travel required will basically be equal to that required to set the lock off by pushing directly on the sear.  This weight required to trip the lock by pushing directly on the sear is sometimes a little much even if the lock is setup as well as possible.  In order to decrease the trigger pull required this ratio must be increased given the lock is kept constant:

(distance from finger contact point to pivot point)/(distance from sear contact point to pivot point)

In order to increase this ratio two things can be done.  Either the distance from the finger contact point to pivot point (numerator) can be increased or the distance from the sear contact poin to pivot point (denominator) can be decreased.  Now it must be noted that by increasing this ratio (increasing leverage and decreasing required trigger pull) the trigger travel necessary to trip the lock will increase.  There is always a trade-off.

So, you can only do so much with a trigger given a constant lock condition.  With this being the case, as mentioned previously, the benefits of a well set-up lock become apparent. Perhaps this would be a good subject for another thread.

Jim

[James Rogers]

A keeper thread for sure. Thanks gentlemen!

[Dave Faletti]

Obviously the lock is the most important factor.  Any build suggestions I make assume the lock is already as good as it will be. Trigger location won't improve the feel but the combination of the lock and trigger leverage does determine the trigger pull not just the lock.

@Jim   Looks like we both run the numbers the same way.  Hopefully a lock breaks clean for the trigger movement to release isn't noticed.  Agreed if creep is noticed it is in inverse proportion to trigger pull force.  My focus is having the trigger pull the amount I want with the least friction in the trigger components.  I think we are all in agreement the lock is the limiting factor.  I do look at the amount of trigger travel to break and have enough room in the trigger guard for some extra overtravel if needed.

@Acer.   Nice pics.  That shows it well.  I probably should modify my statement about trigger/sear bar engagement.  I try for least slippage.  For the locks I have used      it would take a long trigger to have a light pull, so the trigger pin is between the sear pivot and bar.

@James.  I hope we answered your question at least in a round about way.

[Pete G.]

I like to put of a crown on top of the trigger bar and harden. This reduces the contact area and any sliding friction.