Topic: gearing mystery - SOLVED!

[turboguzzi]

Now, this is puzzling

turns out that one of the circuits I race on provides trap speeds at the end of the main straight. Quite useful, I am not the faster 500-4 out there, but also not the last, just  a few mph between me and the fastest.  

You enter this medium length, 700m, straight (less than 1/2 mile) doing 80kmh/50mph and the trap shows around 150kmh/90mph for the 500's towards the end. The trap is set up for modern 600-1000 that brake earlier (they reach 140...) so it might be off by a few mph from max speed for us on slower bikes who brake later, but that is not the issue.


From the driver's seat, I know that at that point I am pulling 5th @ at 10-10.5K, so gearing is pretty much spot on. An avon's AM23 circumference is about 2meters/80" so at that speed wheel is turning at about 1250 rpm. That means that I am running an overall 8:1 reduction crank to wheel (10,000/1250=8). my notes show that I am using a final 16/36 ratio (2.25:1)

Now's the strange part:

crank to primary shaft reduction, the manual says 2:1

primary shaft to clutch basket in a 500-4 according to brent (no data on manual) 64:23=2.78

5th gear 0.9:1

final reduction (chain) 2.25

So:

2 X 2.78 X 0.9 X 2.25 = 11.25! that's 30% shorter from my wheel rpm calc showed earlier (8:1) !

What am I missing here? Is there some secret reduction in this box that i should know of? Or maybe the crank to primary is not 2:1 at all but rather 1.4:1? That would bring theory and practice back in line. Motor is closed, so cant count teeth.

To remind you, its  a 500-4, k1 or k2 dont remember now, certainly not k3.

Not that I am depending on this, got my gearing sorted out after three seasons, just puzzled, any clues will be welcome.

TG

[bwaller]

Well TG, the crankshaft sprocket is 24T and the primary shaft gear is 28T.

This is from 550 parts and correct me if I'm wrong but I think these parts are shared between the 500/550.

[turboguzzi]

Well TG, the crankshaft sprocket is 24T and the primary shaft gear is 28T.

This is from 550 parts and correct me if I'm wrong but I think these parts are shared between the 500/550.

Still it doesnt add up.

with 24:28=1.16 I get

1.16 X 2.78 X .9 X 2.25 = 6.53

That would mean 180kmh @ 10,000 rpm. Closer but still 25% off, this time on the long side....  

just for the science, will turn today the motor 6.5 turns and see if the wheel does indeed one full turn  

Still, it makes you wonder if the guy that wrote the specs in the manual had some extra strong Sake at lunch break.... the 2:1 primary ratio doesnt relate to anything in the trans!

last options:

1. rev counter is off big time and I need to check it. My two times on the dyno say it aint so and that would mean also that engine is turning way too slow by 25%, not likely.

It's electronic, so hard to think what went wrong....

2. Speed trap radar in track is off by 25% or positioned so far ahead that it takes reads  when I am still 30 kmh away from my top speed. Seeing how slowly you gain speed in four and fifth, this is hard to believe too.

Still puzzled, I wasnt kidding when i said its a mystery!

TG

[bwaller]

The primary reduction listed in the 550 manual is 3.063:1...that should clear it all up. 

[turboguzzi]

The primary reduction listed in the 550 manual is 3.063:1...that should clear it all up. 

Not really, it brings DOWN the overall gear ratio to 6.18, so pushes speed even higher.

there must be something I am missing here.....

Not critical, I know what gearing works just strange that these internal ratios dont show anywhere.

[754]

The 2 or 3 shafts in the trans, willl have a further reduction/overdrive, unless the input and output gears are same tooth count..

[bwaller]

I was joking on the "clear it all up" comment. I mentioned to you I played with all this on the 500/550/650's before striking into the gearbox and all it did was give me a headache! 

[kos]

And just when you think you have it all figured out....you will have start taking into consideration the growth of rear tyre circumference and how that messes up your true rear wheel speed. They grow more than you think!

KOS

[paulages]

i thought i had written all these numbers down somewhere, but this is the closest I could find:

http://forums.sohc4.net/index.php?topic=14672.msg146019#msg146019

i can't remember if the primary ratio as listed includes crank to primary shaft ratio plus primary gear to clutch basket... maybe that's why you got a 2:1 ratio?

[turboguzzi]

Ok, lets see if it gets better with the new info, according to brent the 500's clutch reduction is 23:64 (can anyone else confirm this tooth count in a 500?)

So:

crank- P.Shaft 28:24 = 1.16

P. Shaft - Clutch 23:64 = 2.78

5th spd = 0.9

final reduction 16:38 = 2.375 ( Cant find my notes, but I think I was 16:36 at that track, so just trying to see where does it get me)

1.16 X 2.78 X 0.9 X 2.375 = 6.89

10,000 rpm / 6.89 = 1451 rear wheel rpm = 172 km/h = 110 mph. Still high by about 15 mph than the radar reading but closer to reality.

Will count motor revolutions per one wheel turn pretty soon.

   

[turboguzzi]

Ooops, mistery solved....

while talking to a friend this afternoon, he mentioned he thinks the speed trap is not on the longer back straight at all, but on the start finish line on shorter main straight, where you never get out of fourth gear.... 

So now with 4th gear ratio at 1.036 and final 16:36 like i remembered, it all falls neatly into place

1.16 X 2.78 X 1.036 X 2.25 = 7.5 @ 4th speed

10,000 / 7.5 = 1333 wheel rpm =  155kmh, almost spot on, and I am not even sure I am hitting 10K at that point, maybe just 9500 before braking and going down two gears, so the 150kmh radar reading is quite precise actually....

What it also means is that we do get up to a nice 180kmh / 113mph in the not so long 1/2 mile back straight. all of the sudden, 120mph +  at Vallelunga with its mile long straight sounds pretty real too.

Sorry to have bothered you all, but now we really know what the primary ratio is in a 500!

TG

   

[Hannibal Smith]

I have a stock 550 (1974) gearbox opened up, anybody need some counts before I button her up?