OzVMX Forum

Clubroom => Tech Talk => Topic started by: TM BILL on July 24, 2012, 07:56:58 pm

Title: Gear ratio replacment options opinions sought
Post by: TM BILL on July 24, 2012, 07:56:58 pm

Due to a lack of availability of the correct gears ( 3rd gear ) i need to change the ratios slightly in an 250cc air cooled , reed valved ,twinshock MXer

My question is ? from the following ratios would you change just 3rd gear or 1st , 2nd and 3rd gears  ???


  Gear            Original             Ratio                      Replacements         Ratio                  Option 3             Ratio 
   1st              30 - 14            2.14                             28 - 12              2.33                   39 - 12               3.25
   2nd             29 - 17            1.71                             26 - 15              1.73                   37 - 15               2.47
   3rd             29 - 21             1.38                             24 - 17              1.41                   33 - 18               1.83
   4th             22 - 19             1.16                             22 - 19              1.16                   28 - 23               1.22
   5th             20 - 20             1.00                             20 - 20             1.00                    22 - 28               0.79 

Primary ratio 59 - 22             2.68

Final   drive   50 - 14             3.57 


Obviously option 3 is more trials trail bike gearing 

Any opinions     

Title: Re: Gear ratio replacment options opinions sought
Post by: Davey Crocket on July 24, 2012, 08:55:58 pm

Whats it for fatboy?...the dreaded A5 3rd gear?

Title: Re: Gear ratio replacment options opinions sought
Post by: Nathan S on July 24, 2012, 08:57:55 pm

Option 3 is pooh.
Option 2 looks pretty good - don't reckon you'd notice the difference between it and the stock ratios - maybe the slightly larger jump from 1st to 2nd would be noticable sometimes, but I'd be amazed if you ever felt the difference 2nd~3rd or 3rd~4th.

Title: Re: Gear ratio replacment options opinions sought
Post by: Lozza on July 24, 2012, 09:19:01 pm

Option 2 as Nathan said there is only a few revs in it.  Rpm drop is about 2000 from 1st to 2nd and about 1000-1200 rpm from then on. 1:1 top is the way to go

Title: Re: Gear ratio replacment options opinions sought
Post by: TM BILL on July 25, 2012, 02:43:04 am

Thanks chaps , yes Joan KX 250 A5 1979

Original ratios are 1979 KX 250 A5

Optional ratios are 1974 KX 250 , 1975 KX 250A 1978 KX 250 A4

Option 3  is KT 250

All will fit into cases A5 cases and gears interchange on shafts , output shaft is same across all models  :)

Title: Re: Gear ratio replacment options opinions sought
Post by: Davey Crocket on July 25, 2012, 07:41:36 am

JohnnyO just put A4 stuff in his Bill, I think Simon Healy got them made on his ones.

Title: Re: Gear ratio replacment options opinions sought
Post by: JC on July 25, 2012, 11:14:33 am

Bill,
To answer yr original q'n, based on those ratios

on A5,
2nd-3rd drops 19.3%
3rd-4th drops 16%

on A3/A4
2nd-3rd drops 18.5%
3rd-4th drops 17.7%

If you replace 3rd gear only on A5 (w A3/A4 gear)
2nd-3rd drops 17.8%
3rd-4th drops 17.7%

So you could probably just change 3rd gear. I doubt you'd notice much diff.

Title: Re: Gear ratio replacment options opinions sought
Post by: TM BILL on July 25, 2012, 02:30:40 pm

Thanks JC thats very helpfull  :) i will look at the spare alterternative clusters i have and report back

Bill