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Dan Cyr
Joined: 08 May 2006
Posts: 39
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 Posted: Tue May 09, 2006 9:41 am Post subject: |
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I personally haven't changed from 15/27 in at least 2-3 yrs. I usually pick a top S2000 driver to watch and if they shift to 3rd - I'll change to 16/27 (if I'm not too lazey).
It is 10x worse to be overgeared than undergeared (unless you're worried for saftey of motor buzzing it for several seconds in top gear). |
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Scott Nagler
Joined: 07 Mar 2006
Posts: 44
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| Posted: Mon Jul 10, 2006 2:53 am Post subject: |
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well we've had a few autoxs lately where I was topped out in 6th gear after the 3rd turn leading into a long straight... door slammers were creeping up on me cause I had no more speed. I'm using 15/26 on a built YZ125... don't know the rpms or speed I'm going. I built a cool lil excel file to calculate each gear for different sprocket combos... I'm gonna borrow a GPS to discover my current top speed and go from there... I suspect its around 65.
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Still a noob, but can't wait to be "on the pipe" |
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Larry MacLeod
Joined: 28 Sep 2005
Posts: 299
Location: United States, Michigan, Ypsilanti
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| Posted: Mon Jul 10, 2006 3:21 pm Post subject: |
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| Scott Nagler wrote: | | well we've had a few autoxs lately where I was topped out in 6th gear after the 3rd turn leading into a long straight... door slammers were creeping up on me cause I had no more speed. I'm using 15/26 on a built YZ125... don't know the rpms or speed I'm going. I built a cool lil excel file to calculate each gear for different sprocket combos... I'm gonna borrow a GPS to discover my current top speed and go from there... I suspect its around 65. |
65 mph in a Honda CR125 is a shorter gear than anything I own. I bet the shortest I run tops out about 70mph. I usually gear between 70 ( 15f/28r) and 84mph (16/26), depending on course. I can't find my gear chart, but its something like that. I'm sure someone will correct me if I'm wrong on those numbers.
LM
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F125 #196
Old Birel/ hodgepodge Honda CR125
Lefty Funk
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Brian Garfield
Joined: 04 Apr 2004
Posts: 667
Location: United States, Maryland, SKCA Racing!
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| Posted: Mon Jul 10, 2006 4:46 pm Post subject: |
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| Larry MacLeod wrote: | 65 mph in a Honda CR125 is a shorter gear than anything I own. I bet the shortest I run tops out about 70mph. I usually gear between 70 ( 15f/28r) and 84mph (16/26), depending on course. I can't find my gear chart, but its something like that. I'm sure someone will correct me if I'm wrong on those numbers.
LM |
Not according to Secrets of Speed and MY speedometer. Your 15/28 IS 65mph and a 16/26 is 75mph.
I run a 15/26 on mine and I haven't topped out yet. I still haven't fixed my setup yet so I could even change that, but I've had other more serious problems to worry about...
Brian |
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Ryan D Thompson
Joined: 22 Aug 2005
Posts: 199
Location: United States, Georgia,
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| Posted: Mon Jul 10, 2006 4:57 pm Post subject: |
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| It also depends on your top RPM. 15/28 should be 69.5 at 12500 rpm, 72.9 at 13,100. At least thats what my gear calculator says. I've been running a 15/27 and I hit 75mph with it regularly. |
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Larry Andrews
Joined: 13 May 2002
Posts: 2852
Location: United States, California, SC Mtns
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| Posted: Mon Jul 10, 2006 6:29 pm Post subject: |
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Y'all don't need to gear for 80+?!? 
-la |
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Steve Elzinga
Joined: 24 Aug 2004
Posts: 69
Location: United States, Michigan,
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| Posted: Tue Jul 11, 2006 6:32 am Post subject: |
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| Brian Garfield wrote: |
Not according to Secrets of Speed and MY speedometer. Your 15/28 IS 65mph and a 16/26 is 75mph.
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Assuming a 35" rear tire circumference 16/26 equates to 80.9 at 12,500 rpm and 84.2 at 13,000 rpm. A 15/28 would be 70.5 and 73.3 for the same two rpm ranges. Even at 12,000 rpm at 15/28 is 67.6 mph and 16/26 is 77.7 mph. I think your book is either assuming a lower rpm or a smaller tire. I believe a rear MG green tire which Larry uses is approximately 35". |
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Steve Elzinga
Joined: 24 Aug 2004
Posts: 69
Location: United States, Michigan,
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| Posted: Tue Jul 11, 2006 6:48 pm Post subject: |
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| Well I got home and measured a tire and a MG green is about 34" in circumference rather than 35", so that makes the numbers about 2 mph slower for each gear and rpm range. The range Larry posted would be correct if your max rpm was about 13,500, for 12,500 the 16/28 would be about 78.6 mph and the 15/28 would be about 68.4 mph. For some reason I thought the rear tires were 35" from memory. |
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Scott Nagler
Joined: 07 Mar 2006
Posts: 44
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| Posted: Sun Jul 16, 2006 7:37 pm Post subject: |
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if you want I can upload that .xls I made.... shows speed in each gear based on sprockets, rpm, tire circum, and primary ratio (internal crank to tranny ratio)... I based mine on 13000 rpm, 3.3pr, 33.5" circum, and 1.1 6th gear... I need a 15/24 to run 71mph... I dunno how fast I'm going now but I can tell ya I feel like I coulda held another 5-10mph.... I hate being topped out.
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