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BIKELAND > FORUMS > DRAGBIKE ZONE.com > Thread: Deceleration NEW TOPIC NEW POLL POST REPLY
Thread: Deceleration
aliveagain


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posted January 01, 2007 09:16 AM        
Deceleration

I asked long ago about habits in slowing down after a run.I was just reading about Brock's run at Maxton and he was talking about not using engine braking to slow down.He further stated that by doing so would be a good recipe to snap the cam chain.Has anyone had or heard of that happenning?
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zrexpilot


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posted January 01, 2007 10:28 AM        
I dont know for sure but dont road racers engine break, and they do that for extended period of time.
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aliveagain


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posted January 01, 2007 12:59 PM        
good point!
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entropy


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posted January 01, 2007 02:05 PM        
It would be good to ask Brock directly, he doesn't just "say" stuff, eh? I bet he has a reason for saying what he did.

At the end of the 1/4 i use the engine to brake, then the r brake, THEN the front brake.
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CrotchRocket


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posted January 01, 2007 02:21 PM        
After I go cross the finish line I upshift to 6th, roll off the gas and 2 fingers on the brake lightly to the turn off...
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TurboBlew


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posted January 01, 2007 04:38 PM        
Could be a couple different reasons.. but chopping the throttle at the finish line is not a good idea.
Another reason at Maxton might be the cylinder temps after holding the motor WFO for 40seconds then letting the motor decel the bike under a severe lean condition.

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zrexpilot


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posted January 01, 2007 05:30 PM        
I have been piloting a turbo Bus for a friend, it has a lock-up clutch, there is no way to slow it down without engine breaking, the clutch is locked tight until rpm/s come down.
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'0412R
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8.79@167mph (spinning)
Motor go Kaboom!
on some dope

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bigdtd


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posted January 03, 2007 03:23 PM        
after a long acceleration the motor is turning high rpm and all the stressed parts are loaded one way, suddenly downshfiting or using the motor for braking purposes makes all the stress load travel in the opposite direction. the weakest parts like the camchain would be subject to possible breakage. we all use the motor for braking sometimes but it is not the best thing at the end of a long wide open session. i was racing a buddy on a long quarry road one time and his camchain broke on the big end of a top speed run, the bike was a honda 1100f.
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Y2KZX12R


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posted January 04, 2007 06:01 AM        
An engine should not blow up from deceleration. Nor should it cause any damage.
Infact dynojet tells operators to let the engine revs come down on thier own slowly after a pull to redline and not lull in the clutch and let it drop right to an idle.
The crank and bearings heat up during a high speed run and need an abundant oil supply (volume) to cool the parts back down after such a run.
We all know the oil pump output on the zx12r is minimal at idle on the zx12r. So pulling in the clutch and idling after sustained 11k run would be basically the same as shutting the engine off as far as oil supply. You still need to remove heat for a min or so after the rpms come down.
Most cam chain failures that I have seen are a result of improper (manual) tensioner settings or have high mileage (tens of thousands of miles).

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Y2KZX12R
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CrotchRocket


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posted January 04, 2007 08:17 AM        
Thank you Jim, for posting that great bit of important info for everyone else to read

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entropy


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posted January 04, 2007 08:41 AM        
oiling on decel; good point.

This is a bit off topic...
(but related)

With my billet oil / swing pickup pan i have cured the accel uncovering of the pickup (psi is 80-100psi during launch/60'), but curiously, there is a pretty significant drop in oil pressure (down to 20psi+/-), when i let off the gas at the end of the 1/4. This seems to be mitigated a bit by rolling off the throttle more s;lowly.

I'm guessing that it has to do with a lower oil level in the pan after running the motor WFO thru the 1/4. Perhaps a baffle would help????
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Y2KZX12R


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posted January 04, 2007 11:10 AM        
quote:
With my billet oil / swing pickup pan i have cured the accel uncovering of the pickup (psi is 80-100psi during launch/60'), but curiously, there is a pretty significant drop in oil pressure (down to 20psi+/-), when i let off the gas at the end of the 1/4. This seems to be mitigated a bit by rolling off the throttle more slowly.

I'm guessing that it has to do with a lower oil level in the pan after running the motor WFO thru the 1/4. Perhaps a baffle would help????



Interesting. Oil presure should be a function of engine rpm(mainly).
It could be some sort of baffle problem I guess.....

Karl, I'll call you tonight on the other stuff..... are you in some crazy time zone?
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TRNorBRN6001


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posted January 04, 2007 12:32 PM        
Yep, I think he is in that crazy Mo City time zone! (Central)

With the low presure at the end of the 1/4, You all are saying that most all the oil is up in the engine and just has not had time to travel back down or that it's just being flung around too much and does not return to the pan in a timely manner so the oil pump is going dry for short increments between the oil gathering back to the pan?
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Y2KZX12R


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posted January 04, 2007 06:38 PM        
No, I dont think that.
I think that the engine should be able to maintain redline rpm and keep a constant oil supply without being able to pump the pan dry.

I would say its possable that the g's while slowing down could be causing the oil to go forward in the pan and uncover or partialy uncover the oil pickup tube. ????

Has anyone expierienced this? Or worn out rod bearings in less than 20,000 miles?





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Brock


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posted January 04, 2007 07:24 PM        
I had a guy ask me once.....if you were going to intentionally make a bike jump cam timing, how would you do it?

The answer is simple: I would rev the bike to near-redline rpm/ under full load/ at wide open throttle, then.... I would snap the throttle shut and brake with the engine!

The same goes for breaking the chain.....

Most professionally built (pro stock/pro mod style) engines only last half as long between rebuilds (or failures!) with a slider clutch.

FYI: a slider does not allow the clutch to disengage before a preset rpm....engine braking down to that rpm.

Finally, I had a physics problem in school: Which applies more stress to an engine.... A car pulling a trailer at 60 mph up a hill at wide open throttle/ or the same car coasting without the gas peddle depressed going down the same hill at 60 mph?.... (of course it was the second, or they wouldn't have asked!)

The reason?: when the car is making power, the force of the pistons is subtractive.

We tell everyone: Engines are for speeding up....Brakes are for slowing down.
(Unless you wish rebuild your engine as much as the road race boys!)

The same applies to Dyno's, if the dyno operator slows his $10K model 150 down with your $25K killer engine......he is probably also in the engine repair business!!

Brock
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Brock


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posted January 04, 2007 07:30 PM        
Oh yes,
The problem is less prominent with turbo charged engines because they usually have less static compression AND the turbo is still spinning during decel which also reduces cylinder pressure. Even though the throttle is shut, the pressure in the intake track is far less negative than a normally aspirated engine.

Brock
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zrexpilot


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posted January 04, 2007 08:35 PM        Edited By: zrexpilot on 4 Jan 2007 19:48
correct me if I am wrong but isnt every other stroke a negative one anyways. One piston is being pushed down on combustion but then is being pulled down on the exhaust stroke, which would be pulling on the rod cap. Isnt this all that is happning on decell anyways.


PS I have never blown an engine or done any damage on decell but cant say the same about accell, toasted a few that way..
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'0412R
5.72@131mph
8.79@167mph (spinning)
Motor go Kaboom!
on some dope

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