gear at end of crank? - MVAgusta.net
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post #1 of 12 (permalink) Old 05-23-2010, 12:57 AM Thread Starter
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gear at end of crank?

I have a tech question maybe you guys can assist with. I changed the sprocket on my '04 Brutale today and at the end of the cranke (which the sprocket slide over) there is a gear like bit which spins along with the crank. It doesn't drive anything, so I thought it might be a something to do with sending a signal to the tach, but this seems not it. Long story short, I messed my little gear up, and I am wondering what the function might be and is it safe to ride the bike w/o it until I can source a new one. Ideas?
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post #2 of 12 (permalink) Old 05-23-2010, 02:03 AM
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It's called a phonic wheel.
I believe it's purpose is to determine the speed of the bike.
Torque the bolt that holds it in place to 8 Nm and use Loctite 243.
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post #3 of 12 (permalink) Old 05-23-2010, 06:58 AM
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It generates the signal in the crack position sensor....this determines ignition and fuel injector timing as well as rpm signal for the tach. If you damage it the wrong way you may have problems with the EFI system and running problems (like incorrect ignition timing). replace it.

I used to be fast....now I just dream about it.
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post #4 of 12 (permalink) Old 05-23-2010, 07:00 AM Thread Starter
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Phonic wheel sounds right - I knew it wasn't a gear exactly, but looks enough like one to describe it - and it does seem to have something to do with indicating engine speed, which is why I thought it may have been for the tach. A quick ride up the block though shows the tach and speedo still function w/o it, so I am just nervous it has some more intricate function (timing? engine cpu management?) and I don't want to harm the bike but have a nice ride planned for Sunday that I don't want to cancel. Hmm... whaddya think?
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post #5 of 12 (permalink) Old 05-23-2010, 07:03 AM Thread Starter
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@esq'z me - I was posting as you posted so didn't see your reply until after I sent mine. Sorry! I actually stripped out the nut, so the wheel itself is fine, but currently unattached. The bike seemed to run okay w/o it - but it sounds like it will start to run very poorly if I run for more than a few miles, eh?
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post #6 of 12 (permalink) Old 05-23-2010, 07:48 AM
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The phonic wheel on the end of the transmission main shaft is part number 800088343 and its sole purpose is to calculate vehicle speed. You can operate the bike without fear of engine damage. However, replace the part promptly for safe operation.

The "other" phonic wheel part #800086726, the more critical one is carefully tucked into a cast cavity at the end of the engine crankshaft, and this is the component that determines crank angle and part of the engine/fuel management system.

Safety first,
Dick
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post #7 of 12 (permalink) Old 05-23-2010, 07:52 AM Thread Starter
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Dick - thanks! yes, part 36 is what is currently sitting on my bench, and part 37 is what I foolishly stripped. Hmm... part 38 may be buggered as well since that is what 37 seems to thread into. I'll check it out. I wonder how this determines vehicle speed though, since it would seem to only show engine revolution speed.
One more question... where did you find this excellent drawing? I looked online last night trying to find a parts PDF or manual and came up with nothing.
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post #8 of 12 (permalink) Old 05-23-2010, 06:41 PM
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Ahhhh ... I was confused by your use of the term "crank" like in crankshaft....you meant transmission shaft...

I was wondering why you had the timing cover off if you were replacing the front sprocket???

I used to be fast....now I just dream about it.
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post #9 of 12 (permalink) Old 05-23-2010, 07:04 PM Thread Starter
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Yeah - my knowledge of modern bike innards is nil, so apologies for the bad description. The bike ran wonderfully all day, and I was only lacking the speedo. As a side note, the new sprockets and chain transformed the bike - pretty amazing what it can do. I'll order the new phonic wheel bolt tomorrow.
Thanks everyone!
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post #10 of 12 (permalink) Old 05-23-2010, 07:26 PM
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Its all math my boy!

Hello AgoFan: The phonic wheel is attached to the transmission output shaft. The crafty engineers at MV Agusta know the transmission ratio between the engine speed and the transmission output speed.

Since this is controlled by a set of gears, the ratio never changes.

They also know the "standard" transmission sprocket size, they know the "standard" rear wheel sprocket, and they know the "standard" rear wheel diameter, with the "specified" tyre.

Knowing this info, one simply "counts" the teeth on the phonic wheel, and uses a tiny bit of the on-board computing power in the electronic speedometer or some other tiny black box built into the bike. MV knows that X number of "teeth" that go by the speed sensor in Y period of time = Z MPH / KPH. Its all math!

So while you are eyeing the girls as you drive thru town, the computer calculates the speed and displays it on your dash. As you watch the gentle sway of "her" hips, in "real time" the computer has been loafing along at the speed of light and has completed the speedo calculation about a bazillion times.

The drawback to attaching the speed sensor to the output shaft of the transmission becomes obvious when one decides to make ANY change to the final drive ratio, be it a smaller/larger transmission sprocket, a larger/smaller rear wheel sprocket, a different tyre, or a different rear wheel.

A better strategy is to attach the speed sensor to the rear wheel. This way, IF one changes the sprocket sizes, BUT retains the same rear wheel diameter, the computer will accurately calculate the ground speed. This is how speed is "sensed" on a 1090RR.

The drawing I posted is a simple cut and paste from a readily available MV Agusta parts (spares) list for your model bike. This spares list along with the Factory workshop manuals, there are two, a chassis manual and an engine manual can be found for free in several places on the internet.

A person is strongly encouraged to obtain these "tools" and review the spares list and workshop manual BEFORE doing any work. Fewer parts will be broken, the bike will remain safe and reliable, and the joy of ownership will increase.

best wishes,
Dick
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