Effect of muffler mods on saw safety

[Woodie]

We talk a lot about muffler mods here, and most estimates here say HP increases from 10 - 20%, with the added benefit that it also makes the saw more free-revving. But how does that effect the safety equipment on our saws?

Primarily I'm talking about the inertia brake. Let's say I've got a 4-horse saw (let's say, in particular, that I've got a 4.1-horse Jonny 2159!), and I increase my HP by 20% to near 5. I've still got the inertia brake designed to stop a chain pulled by a 4-horse motor. If I've got that much more grunt behind the rim, is the brake now undersized for the job if I get a kickback? My guess is that it will still stop the chain, but will it increase the stop times so much that I now have a more dangerous saw?

I've got a Jonny, but I'd love to hear opinions from owners of all brands. (Plus that'll give us something to fight about...)

Thanks in advance.

 

[blis]

well, any rational saw manufacturer will desing they'r chain brakes so that they'l stop the chain quickly and then add a safety margin to them, lets say 20-30%...

So, yes, the chain should stop fast as it used to with non-modded muffler...

 

[Lakeside53]

???????

The Engine is only developing full HP if it's pulling in the wood. If not pulling in the wood, not much "hp" to stop - just chain speed...

Having said that.. if you are running your chain FASTER than the recommended max WOT... then there is a potential problem.

 

[Woodie]

???????

The Engine is only developing full HP if it's pulling in the wood. If not pulling in the wood, not much "hp" to stop - just chain speed...

Having said that.. if you are running your chain FASTER than the recommended max WOT... then there is a potential problem.

Okay...help me understand this, Lake. Let's say I've got the bar buried at WOT...I'm now developing max HP. If the tip pinches and the chain hangs, it's going to try to throw itself out of the cut. But at the instant that the bar begins to travel out of the cut, the brake activates. The engine was just developing max HP, but now the chain is essentially free-wheeling. There isn't enough time for me to react to the kickback, so I'm still at WOT.

In the instant that it takes for the bar to throw itself out, has the power level dropped that much that it's only chain speed the brake has to arrest?

I'm not trying to be a smarta$$ here, trust me I'm not, this is driven by a lack of understanding as to how an inertia brake works. Would the inertia brake on a small saw be of the same size/caliber/weight/etc. to function effectively on an 880 or 3120?

Thanks, Lake...I know these are rook questions, but I'm just trying to understand...

 

[Lakeside53]

o.k... I'll try.. Sitting here with the flu and a fever so if I get it wrong, be gentle... and maybe someone else can explain it better.

If you are "buried in the wood", then unless you have a a really small piece of wood, you won't be at WOT.. yes, your throttle is wide open, and the chain is running fast, but the saw isn't pulling full hp - most of the gas is unburnt and spitting out the exhaust. It takes very little "HP" to rev a non loaded chain to WOT. If you really are pulling full HP, then the saw and chain will be running substantially slower. Example - WOT (chain load, not in wood) -14,000rpm. FULL THROTTLE, in decent size wood, saw is pulling something like 9000-10,500 (fictional saw example).

The problem is one of understanding where the energy developed is being absorbed. If you are using a bunch of 'hp', chips are flying... All the brake has to do is stop the chain and the clutch drops out. Yes, there is a "spike" where the chain brake has absorb the HP the saw ties to develop into it, but in milliseconds the saws drops out of it's power band.


Just FYI - All Stihl brake system are similar from the very largest to the smallest saws. Many even use the same parts across a wide range of HP. Yes, the larger saws have to deal with larger chains weights (long bars, big chains) and a higher "spike", but the component size increase is not as great as you may think.

 

[clearance]

There isn't enough time for me to react to the kickback, so I'm still at WOT.

Never mind the brake, you should never put yourself at risk of kickback injury, the brake is a last resort, not the first line of defence, thats your position. Always stand to one side when bucking, always, even if you have to buck lefthanded. Always know where the tip is and figure out where the bar is going to go if it kicks. I think chain brakes are great but brakes and "low kickback" chain give people a false sense of security. Kind of like people who figure a 4x4 will stop faster in the snow and then drive too fast. I don't, but I could run a 100cc saw with no brake and the rakers filed right off and still be safe from kickback. Hope this helps.

 

[twoodward15]

Lake, thanks for the good reply.

 

[Canyon Angler]

Newbie - I think you're confusing two different quantities - Force (which is the amount of pounds needed to stop a spinning chain) and Horsepower (which is Force applied over a Distance divided by Time), which you can only figure over time.

When a chain brake engages, it stops the chain almost instantaneously. When the chain is stopped, there is NO horsepower being produced, since the saw is doing no work at that point, even though the motor is still running.

It doesn't brake the chain like the disc brakes brake your car, converting your momentum into heat because the only momentum in a revving saw is carried by the weight of the spinning chain and clutch, which really weigh very little...which means that there is almost no momentum at all, and hence there is no need to dissipate a lot of heat when the chain brake engages. You know how your brake rotors and drums in a car get red-hot after you go down a long hill? That doesn't happen in a saw, because the only thing the chain brake stops is the chain and clutch.

Stated another way: A saw does almost all of its work in REVS, almost in the same way as a dentist's drill or a Dremel tool. There isn't a whole lot of torque in each rev -- there are just a lot of revs per minute -- and the saw only does a smidgen of work on each rev, but over time, those smidgens of work pile up. Think how easy it is for the dentist to "stall" his drill or his cleaning tool on your teeth -- it doesn't take much force to stop that drill at all. Yet it also cuts through your teeth like butter -- and tooth enamel is harder than glass!

It's almost the same with chainsaws. The only time they're putting out their max horsepower is when: a) they're loaded, b) they're revving fast and c) they're spitting out chips like a snowblower. The instant you stop the chain by applying enough brake to defeat the clutch, the power output drops to zero INSTANTLY. And at that point it doesn't matter whether the motor is a 4hp 2-stroke or a stroked 350 Chevy -- because as soon as the centrifugal clutch gives way, the chain stops and no power is being delivered to the part that matters (the sharp part).

Anyway, hope this helps.

 

[JUDGE1162]

I am not a chainsaw expert, but I do have some input, as a bit of a safety pro (I have worked as a health and safety director). I feel the answers given so far were right on and great, Lakeside as always is very helpful. From what I got from the above posts is that the chainbreak is not stopping the motor so it does not matter what the horsepower is, you are only stopping the chain and as long as you have your saw set that the right Max RPMs that speed of the chain is within manufacture specs and the brakes are made to stop the chain regardless of the size of the motor. That is all well and good and seems reasonable to me.

But here is the problem, you are assuming that everything else on the saw is the same as stock if the muffler has been modified, in some cases that is true but one of the reasons people modify their saw if not only for more power in the cut or to run at lower temps but to use bigger bars, bigger badder chains (heavier, longer, more aggressive). This is where I see a safety problem if a saw is designed from the manufacture to run a max 24 inch bar with .050 gauges, .375 pitch chain. You do your mods (muffler being one, but you port the cylinder, the whole works). Now you have a lot more power to play with, so you up your saw to a 30 inch bar, you throw on .404 pitch 0.063 gauge chain (I don't know if this is possible it is only an example and an extreme on at that) now the weight of the chain you are trying to stop has increase quite a bit, and like Canyon Angler pointed out they are light to start with so a little increase in weight is a big percentage.

Example


24" .375 pitch, .050 gauge chain weighs for this example 16 oz (I have no idea just an example)

30" .404 pitches, .063 gauge chain is 17% longer and 8% thicker so if the 24" is 16 oz, the 30" chain would be 20.5 oz or 28% heavier. If the chains travel at the same speed at max rpm say 12,000 rpms that heavier chain is traveling with 28% more momentum (speed X Mass) so it would take 28% more force to stop it in the same amount of time or 28% longer with the same amount of force, now with the chain break working in milliseconds, I am not sure if that is a big deal or not. But it is something to think about so a modified saw with a chain and bar in the manufacture’s recommended range should be fine, but if you up your power to up your bar length or chain size their could be safety concerns.

 

[Lakeside53]

I'll buy that.

Most guys I know don't go up'ing their chains sizes and bar lengths though - just want more out of what they have, but it is certainly a consideration.

I have to say though, my chain brake rarely gets applied without me doing it manually... In fact I can't remember the last time it "went off", but that's one reason it's there for.

 

[Woodie]

Great explanations guys, and thanks for taking the time to put 'em all down. As always, I appreciate all the good counsel.

BTW, hope you feel better Lakeside!

 

[Kneejerk Bombas]

The big safety issue I think of with modded saws is permanent hearing damage. The saws are just too loud.
Even with ear muffs or plugs, you are still going to suffer hearing loss.

 

[tek9tim]

The big safety issue I think of with modded saws is permanent hearing damage. The saws are just too loud.
Even with ear muffs or plugs, you are still going to suffer hearing loss.

Huh?! Never heard of her!!

Yeah, there are some other things with the volume too, like perhaps someone tries to alert you to an outside hazard, and you can't hear them yelling. (Then again, who's to say you would hear them with a stock saw either) I almost got nailed by a garbage can sized boulder while cutting out an escape route on a fire last year, and if it hadn't been for me letting off the throttle at just the right moment, I would have taken that next step forward without hearing everybody up the hill from me yelling. I would certainly argue that I would not have heard even with a stock saw, but it certainly made me think.

 

[Gypo Logger]

Just to show how tough chain brakes are, especially on the Stihls, I worked adjacent to this woodtick who repeatedly slammed on the chainbreak with the saw running full bore evertime he finished a cutting session. It was totaly crazy and he did it everytime apperantly for years with his 038.
He swore he never burned out the clutch or severed a brakeband.
His saw maintanance was very poor, never chaining sprocket or any of the other basics.

I've also seen where the saw was forceably used with the chainbrake on (036), resulting in the melting of the plastic oil pump parts and the plastic cover over the pump.

Modern day chainbrakes are simply to infalable to self destruct by adding a bit of HP. but it was a good question.
John

 

[Tree Sling'r]

I think the only time I use my chain brake is when I dawg my saw in the wood after a buck while my tape is reeling back in and the bark slips, chunks a part and my saw falls off the log and lands on the brake handle and applies it.