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sprocket ratio asssistance 26 Aug 2021 11:13 #854379

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Need some definitive info, so this is to anyone who has or is racing KZ 1000 transmission( 1980 MK2 to exact) I just started racing this year on purpose built bike and every time i go i was getting a little faster, but i seem to have plateaued. the guys i race with are saying the gearing is too tall currently running 15/40 they want me  to go up as high as 15/45. All the research ive done is telling me this should be a good ratio. This person is running a GPZ and that is his setup. im thinking the two are apples and oranges. Motor is stock other than 72mm bore with 10.5:1 pistons. Just want to know if im in the ball park or way off. Just ask if you need want other info. Thank you

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sprocket ratio asssistance 26 Aug 2021 11:59 #854380

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First off, I have no experience racing motorcycles.  :) 

For reference, I think all 73-80 big-fours use the same gear ratios.  There was a factory close-ratio transmission available, but they are hard to find and expensive now.  Here are the ratios (I'm pretty sure the close-ratio numbers are right, but not positive).  As you can see, 1st and 4th are the same, but 2,3,5 are spaced closer to 4th.

Stock             Close-ratio
1: (12/38)       1: (12/38)
2: (16/35)       2: (19/37)
3: (21/35)       3: (22/35)
4: (21/29)       4: (21/29)
5: (23/28)       5: (23/29)

Are you still running the stock cams?  It seems like with the stock cam you don't want to go too short, as most of the power would be closer to the mid range.

One other thought is that is seems like the ideal setup would be very dependent on the track.  A setup using 15/45 puts you at about 104mph at 8500 RPM in 5th gear (depending on your tire OD).  That seems like a good setup for a tight track without a long straight, but not a track that's more open.




 
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sprocket ratio asssistance 26 Aug 2021 12:51 #854381

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Here is a spreadsheet I made that calculates RPM vs Speed.  The green fields should be adjusted for your tire and sprocket setup.  I'm assuming folks will be able to make a copy of this to their own google account?

docs.google.com/spreadsheets/d/1kOp1qNwK...CVI/edit?usp=sharing
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sprocket ratio asssistance 26 Aug 2021 13:34 #854383

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Just to clarify I'm drag racing not road racing. Seem to have topped out at 97 mph just seems like there's more there. Gearing makes my head hurt

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sprocket ratio asssistance 26 Aug 2021 13:58 #854384

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I presume this is on the quarter.
97MPH is slow even for a stock bike.
What are your times at that speed.
The 73 Z1 was timed at around 110mph @12.4 secs with the same gearbox.
Sounds like your gearing is too low and you are topping out early.
You want more like 15/36..

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sprocket ratio asssistance 26 Aug 2021 14:23 #854385

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Yes 1/4 mile racing. Time slip as I remember was in the 13 second range. Bike seems to be still pulling at the finishline. Back and forth between gearing and clutch slipping
 

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sprocket ratio asssistance 26 Aug 2021 18:31 #854394

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First off, I have no experience racing motorcycles.  :) 

For reference, I think all 73-80 big-fours use the same gear ratios.  There was a factory close-ratio transmission available, but they are hard to find and expensive now.  Here are the ratios (I'm pretty sure the close-ratio numbers are right, but not positive).  As you can see, 1st and 4th are the same, but 2,3,5 are spaced closer to 4th.

Stock             Close-ratio
1: (12/38)       1: (12/38)
2: (16/35)       2: (19/37)
3: (21/35)       3: (22/35)
4: (21/29)       4: (21/29)
5: (23/28)       5: (23/29)

Are you still running the stock cams?  It seems like with the stock cam you don't want to go too short, as most of the power would be closer to the mid range.

One other thought is that is seems like the ideal setup would be very dependent on the track.  A setup using 15/45 puts you at about 104mph at 8500 RPM in 5th gear (depending on your tire OD).  That seems like a good setup for a tight track without a long straight, but not a track that's more open.

""I have run that close ratio gear set in my Superbike motors since back in the late '70's. The part number for the "mission assey" was 13254-001.

First gear is way taller than the stock Kz trans. I have the correct ratios listed somewhere, but I ain't going to go dig for it.

First gear in the close ratio box is closer to a stock gearbox second gear!!!! My first gear is good for almost 60MPH in my Superbike. I ran like 15/34 gearing most of the time. When I took it to the drag strip I ran a few more teeth on the rear. With a 1015cc motor it would run anywhere from 11.20 to 10.80 at like 128 to 131MPH.  That was doing a road race start and no powershifting!!!

If the bike has a close ration trans, you can't run the stock kickstart gear. Unless your motor has one of the rare Andrews Products kickstarter gears they made for the close ratio trans. Most folks just left out the whole kickstart shaft and gear when running the Close ratio trans. ""








 

1972 H2 750 Cafe Racer built in 1974.
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1979 Kz1000 MK. II Old AMA/WERA Superbike.
1986 RG500G 2 stroke terror.
1986 GSXR750RG The one with the clutch that rattles!

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sprocket ratio asssistance 26 Aug 2021 19:31 #854396

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Yes 1/4 mile racing. Time slip as I remember was in the 13 second range. Bike seems to be still pulling at the finishline. Back and forth between gearing and clutch slipping
 


The clutch might be an issue.

For comparison, in '93 I ran a 13.9 1/4 on a 1981 GPz550; the bike was geared fairly short (don't remember tooth counts) and my technique was crap, but she had a new clutch.
1979 KZ400 Gifted to a couple of nephews
1967 Yamaha YCS1 Bonanza
1980 KZ440B
1981 Yamaha XT250H
1981 KZ440 LTD project bike
1981 GPz550
2013 Yamaha FZ6R

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sprocket ratio asssistance 27 Aug 2021 04:50 #854401

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Those close ratio numbers are correct except for first gear.  The numbers are 40/16 for a 2.5 ratio.  I have one that caused me a lot of grief in an engine rebuild until I realized what I had.  Today it sits on my shelf but I used it for a year before changing it out last winter.  It's a very tall first gear compared to a standard tranny.  You can ride around at 30 mph in first and not be over 4000 rpm.  Overall, in 5th gear there is a difference of about 500 rpm at highway speed.

I would be prepared to sell the close ratio tranny if someone wanted it.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.
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sprocket ratio asssistance 27 Aug 2021 05:13 #854403

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Maybe Gearing Commander will help?

www.gearingcommander.com/

Good Racin'
slmjim & Z1BEBE
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sprocket ratio asssistance 27 Aug 2021 05:42 #854404

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Southpaw - I'd be looking for that missing horsepower before doing any gearing changes.  I agree with zed1015 - your mph is down.  I'd expect an easy 105 - 107 mph trap speed on a bone stock KZ1000, much less one with a 1075 kit.  How many passes have you made and how consistent was that 97 mph trap speed?  If your clutch is toast, I'd expect quite a bit of variation in speed at the top end.  Are you in a full tuck position once you have launched and making your way down the track?  If not, you'll lose a few mph.

Just some general thoughts on dialing in gearing for the 1/4 mi.  1)  Engine horsepower pretty much dictates top end speed, regardless of gearing (within reason).  2)  Your E.T. will be greatly influenced by gearing, launch technique (60ft time), available traction, weather conditions.  3)  With your exisiting combination, whatever it might be, try to get at least a half dozen consistent passes to establish a baseline.  4)  An ideally geared dragbike will be at it's redline, in high gear, as it crosses, or even slightly before it crosses, the finish line.   5) Dragracing a motorcycle only looks easy to the casual observer...

Hope this helps.  

Mike




 
Presently - 1980 KZ1000B4 LTD w/mild ported head, Megacycle cams, 1015cc Wiseco, Falicon Supercrank, R&D undercut trans
In a former life - KZ-based dragbike - CrMo car tire chassis, 1395cc, Hahn Racecraft turbo system, VP C16, Orient Express 3-spd auto, 7.80's @ 165-170.

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sprocket ratio asssistance 27 Aug 2021 09:17 #854411

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Dragbike Mike,
 Ive got about 3 passes in the 90+ mph range. I'm as green as one can get at this stage, as soon as the light turns green my brain turns to mush. I have been leaning more toward the clutch, it was a new trac king clutch with extra plate 9 passes ago. Think im going to pull it. I bought sprockets but my gut just keeps telling its not gearing. doing my best to learn to tuck down after launch. Have MUCH to learn. Thank you for your insight

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