Bud & Bob,
Your going at the ratios backwards. Mark the output flange and tail housing
and the input shaft. Turn the OUTPUT flange 1 revolution while counting the
input shaft revs. The number of input revs is the ratio. example, 1 output
rev has input turning 4 revs = 4.00:1 ratio.
Not ALL transmissions have a 1:1 4th gear ratio. I build a racing trans that
4th gear can be overdrive, underdrive or 1:1. Spitfires do not have a good
selection of differential ratios which is why I designed and built my trans. I
can also give you any 1st, 2nd, 3rd gear ratio you want. I use Hewland/Webster
gears and best of all it is a bolt in trans for any Spit.
Ron Krantz
ReK Racing
> On January 11, 2018 at 8:19 AM Robert Lang via Fot <fot at autox.team.net>
> wrote:
>
> Hi Bud,
>
> Those numbers look fishy. 4th gear should be 1:1, e.g. turning the input
> shaft one turn should yield one turn of motion at the output flange. The
> lower gears would turn the output flange less than one turn. A 1.33:1 overall
> third gear ratio would see the output flange turn 3/4 of a turn per one turn
> on the input shaft. Similarly, a 1.75:1 2nd gear would result in slightly
> more than half a turn (.57 to be exact) of the output flange per turn of the
> input shaft. Note that 1.33 and 1.75 are random numbers shown to illustrate
> and not actual gear ratios. I think the GT6 used 1.78 and 1.25 for 2nd and
> 3rd. At least the Mk3 GT had those ratios.
>
> What you reported seems to be the opposite of my example.
>
> It is possible that you are seeing some sort of "sticktion" where gooey
> oil in the gearbox is coming into play. But the key is that 4th should always
> be 1:1 unless there's an overdrive and that O/D is engaged.
>
> But the only way to know for sure of what the ratios are is to count
> teeth and "do the math".
>
> Can you remove the top cover and count the teeth on any of the gears on
> the main shaft? That should make it easier to identify what gear clusters are
> in there. You probably only need to count the teeth on the main shaft but I
> know squat about GT6 gearbox internals other than they are more Spitfire-like
> than TR-like so the lay shaft gear ratios may be fixed as they are on the
> Spitfire. The point being that there are only so many ratios available that
> would fit in the gearbox, so it should be fairly easy to figure out what you
> have there.
>
> Regards,
> Bob Lang
>
>
>
>
>
> On Wednesday, January 10, 2018 9:09 PM, Bud R via Fot <fot at
> autox.team.net> wrote:
>
>
> Listerati,
>
> I?ve been trying to figure out what gear ratios I have in the gearbox
> that came with my race car but don?t know how to do the proper math. The PO
> told me it was a close ratio GT6 gearbox but one never knows until you actual
> check.
>
> I at least took some ?data? while it was out so I?m hoping someone can
> tell me how to figure out the gear ratios and if it is indeed close ratio GT6
> gears or whatever it might be.
>
> When the input shaft was turned one full rotation the output shaft turned
> the following amount.
>
> 1st gear it turned 1/4 turn
> 2nd gear it turned 1 1/8 turns
> 3rd gear it turned 1 5/8 turns
> 4th gear it turned 1 3/4 turns
>
> Any help on how I should do the math is appreciated.
>
> Also does anyone know what happened to the minty lamb gearspeed website?
> It does not appear to be up any longer and I used to play with it a lot. Very
> useful site that now appears gone. Waaahhhh.
>
> Thanks
> Bud Rolofson
>
> Extreme Parts Racing (more than just a haircut)
>
> 71TR6 CC57365 (Good 6)
> 71 Spitfire MK IV Race Car #3
> 69 Spitfire MK III (back up FE engine/dinghy car)
> 93 Minnie Winnie Race Support Vehicle
> 77 Z-50A Hardly Davidson Honda Mini-Trail Bike (Triumph Pit Bike)
>
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