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Decompression braking

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Posted by: RakO---------------------

Maybe sounds strange but I do love the decompression braking on a 4 ST but this is more and more difficult to find on the NEW thumpers.
I think that decompression braking helps in certain way to choose your line.

RakO


Posted by: wrooster---------------------

rako,

i've seen this phrase "deompression braking" in a few magazines and i believe that it's incorrect. we need to nip this one in the bud. the correct term is "compression braking", which accurately describes what's going on when the driveshaft/chain/whatever is pushing a four stroke motor through it's cycle rather than the other way around. i'm quite sure that if you ask someone outside the MX scene who has worked on 4 stroke engines for racing applications they will look at you funny when you say "decompression braking".

the braking effect, quite simply, is the result of resistance caused by the compression stroke. the piston is basically compressing air, with no fuel (as the throttle is closed). it's takes energy to compress any fluid. that energy comes from the drivetrain, causing the braking effect. forget for a second that the engine is supposed to make power; with the throttle closed, you have a large air compressor which requires power to be driven. the power to overcome the compression stroke(s) comes from the inertia of the vehicle, slowing it. hence, "compression braking".

nomex is ON.

jim


Posted by: nifty450---------------------

I spent all of the last race season on a 01 yz 426. It was my 1st 4 stroke and after spending some time on it I grew to love compression braking. I dont see why people bash 4 strokes because of it, When used properly I can out brake most 2 stroke riders of the same skill level. Partly because the rearend takes care of its self, giving me more time to focus more on the front brakes which are more important anyway. If more people start seeing compression braking as an advantage instead of a disadvantage they will probably get alot better at using it to their advantage. Good luck.


Posted by: RakO---------------------

[QUOTE]Originally posted by wrooster
[B]rako,

i've seen this phrase "deompression braking" in a few magazines and i believe that it's incorrect. we need to nip this one in the bud. the correct term is "compression braking", which accurately describes what's going on when the driveshaft/chain/whatever is pushing a four stroke motor through it's cycle rather than the other way around. i'm quite sure that if you ask someone outside the MX scene who has worked on 4 stroke engines for racing applications they will look at you funny when you say "decompression braking".............]

Wrooster you are 100% right I always called this COMPRESSION BRAKING but some weeks ago they wrote DECOMPRESSION in a mag.That is why I made this mistake...OK but tell me why the new trend is to eliminite this COMPR .braking ?
Why trying to duplicate the 2ST !!!?
I herad event that some works bike do have a kind of clutch to take away the slightest comp.braking effect. Would they have badder results by keeping this "wonderfull" help (at least in my mind)....

RakO


Posted by: motojunkie---------------------

It seems the manufacturers are trying to appeal to a wider audience. I remember when I got my Thumper it took me about 4-5 rides to get comfortable riding with the compression braking. A lot of guys would have given up by then. Now I can't ride a 2-stroke, I end up overshooting corners. There are a lot of guys that will ride their buddy's Thumper once, and never spend enough time on one to get used to it. I think the manufacturers are going for instant gratification for the 2-stroke converts. I also think that they are doing their customers a disservice by tuning out an advantage.


Posted by: sfc crash---------------------

i love comp breaking, i ride my wr426 alot in my ravine/creek bed, and i find it almost a "crutch" because i don't really have to use the rear break much in that type of riding, especially riding across the grain along the side of the ravine in the mud, let off the gas and comp breaking will bring the rear end around a bit and i can get back on the gas and make pretty smooth up-hill turns. btw, i'm very conscience of comp breaking when approching a jump face.


Posted by: KTM520Man---------------------

I agree completely that it is normally called compression breaking, but there is also significant breaking caused by more than the compression stroke. It takes a lot of work to both pull the air in (drawing a relatively large volume of air through a small open with the throttle closed) and force the air out of the cylinder on exhaust. So, while it is normally called compression breaking decompression breaking isn't completely wrong. I think some of the mags just use it to try and sound like they have superior knowledge. That is my probably less that $.02 worth.


Posted by: Highbeam---------------------

Or did Rako mean pulling the decompression lever while compression braking? The decompression lever is getting harder and harder to find. Using it like a jake brake. Probably not, but that's what I thought he was saying the first time I read it.

I believe the compression braking is mostly due to trying to pull in a charge of air/fuel through the closed throttle plate. A diesel has nearly no compression braking because it has no throttle plate. The compression stroke without fuel is like squishing a balloon, sure you have to compress it but then it decompresses and helps drive the piston. The intake stroke is where it's at, in my opinion.

"Vacuum braking" is my new official term for this. (But you can still call it compression braking)


Posted by: RakO---------------------

Quote:
Originally posted by Highbeam
Or did Rako mean pulling the decompression lever while compression braking? The decompression lever is getting harder and harder to find. Using it like a jake brake. Probably not, but that's what I thought he was saying the first time I read it.

I believe the compression braking is mostly due to trying to pull in a charge of air/fuel through the closed throttle plate. A diesel has nearly no compression braking because it has no throttle plate. The compression stroke without fuel is like squishing a balloon, sure you have to compress it but then it decompresses and helps drive the piston. The intake stroke is where it's at, in my opinion.

"Vacuum braking" is my new official term for this. (But you can still call it compression braking)


The decompression lever braking was uused in the old day of TRIAL races.
IN fact in some really steep technical downhill I am using that technique too.
At slow speed I think there is no danger at all doing this.
But my first concern was for the compr.braking
The last time i rode a klx i realély did enjoy selecting different lines and the trust feeling was so big:
None of the so called modern 4 st keept this unique advantage
On the WR 255 f i really overshooted a lot::::::

Rako



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