DECOMP IS A GOOD BRAKE



24 replies to this topic
  • Mike_Hall

Posted September 01, 2000 - 06:48 AM

#21

Very interesting discussion BUT, despite Hick asking the question twice there is still no explanation as to how pulling in the decompression lever can increase braking. It's obvious that less compression equals less engine braking. Can someone tell us all why that's not correct?

  • Hick

Posted September 01, 2000 - 12:00 PM

#22

I can’t answer that but the Jake Brake theory is the (maybe) unintentional answer given earlier in this thread by Mr. Bosch and explained by MotoGreg. That is that braking is increased because, with release pulled, engine does not fire. Without this extra force helping the piston along the braking is increased. I’ve driven a Peterbilt (briefly) with a Jake Brake and can’t believe this is how they work (feels like it about doubles the engine braking force) but MotoGreg seems to know what he’s talking about.

The obvious hole in this theory is that, along with eliminating the power stroke, you eliminate the compression stroke (unlike a Jake Brake) and therefore what you’d assume is the bulk of the braking force (with friction and rotational inertia making up the rest).

My highly unscientific tests of this theory and their results:
I tried rolling in gear down a slight grade (3rd gear, very low rpms) and compared these three trips:

1) In gear
2) In gear w/ release pulled
3) In gear w/ kill button pressed

According to my highly accurate butt, #2 wins the seat-of-the-pants stopping contest. You would think #3 would win if the Jake Brake theory is correct. But the difference was very slight. #2 wins only because the braking force was smoothed out instead of that 4 stroke “every-other” sensation. It actually reminded me a bit of my KX 500’s engine braking without the ugly green plastic. But the Jake Brake theory is really killed by a lack of any noticeable difference between #1 and #3.

Of course, I was pulling very low RPMs, and, despite Clarks prodigious experience I am leery of doing this to my bike. I don’t know how ATKs and XRs comp. release works, and what’s good for a Honda may indeed be bad for blue. Besides, I ride the dez, I don’t need brakes.

But I do use my comp. release to restart after stalls (if I’m still rolling) and will continue to do so, but perhaps more carefully. I sure have learned a lot of stuff on thumpertalk…

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  • Dave_VanBrocklin

Posted September 01, 2000 - 01:36 PM

#23

Her's my 2 cents, I dont think that pulling the compression lever in increases "engine braking". What it does do is smooth out the engine pulse by effectively lowering compression. The engine will be far less likely to stall at low RPM's because it doesn't have to work against that compression. So, you have alot of parasitic drag without quite the danger of stalling the engine, or locking the back wheel up on the compression stroke.By the way, dont compare the diesel engine with a gasoline engine. They are very different,IE diesels produce NO vacuum,have no throttle plates to produce vacuum. They rely on injected fuel in variuos amounts to run,and produce power.Gas engines do not have the same engine braking or power making characteristics. Now that I've totally confused myself, I better go.

  • Dan

Posted September 07, 2000 - 03:03 PM

#24

Originally posted by gbernard:
HAS ANY ONE TRYED USING THER DECOMP LEVER AS A ENGINE BRAKE. TRYED IT LAST WEEKEND AND IT SLOWS DOWN THE BIKE JUST LIKE APPLYING THE REAR BRAKE ONLY ITS MORE CONTROLABLE. A BUDDY WAS HAVING TROUBLE GONING DOWN STEEP HILLS,KEPT LOCKING UP THE REAR(KTM400). SO WE TRYED PULLING IN THE DECOMP LEVER AND IT


WR400 89

It is more work, but I think I would just modify the rear brake or learn to go easy on it. Obviously, a lot of things can happen when you open the comp release. Most of them are bad. When you have ridden Yamahas awhile, you will develop a soft right foot.
Dan

  • Bryan Bosch

Posted September 07, 2000 - 03:58 PM

#25

Dan, what are your suggestions? Unfortunately, I do not have a soft right foot and downhill breaking is pain for me. Can you adjust the break to not be so grabby?




 
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