[Fot] Gear ratios?

Bud R levilevi at comcast.net
Thu Jan 11 11:34:09 MST 2018 

Thanks guys for the help,

Doing things backwards seems to be my specialty lately. 

The tranny is back in the car so I can’t redo the revolutions directly on the tranny. I do have the rear end on jack stands putting on some new brake lines so can I pull the spark plugs and just rotate the rear wheel while counting the crank pulley revolutions to get the ratios for each gear?

I did count the teeth on the gears, but I didn’t measure the diameters because I didn’t think to do that.

1st = 30 teeth
2nd = 26 teeth
3rd = 22 teeth
4th = 19 teeth 
Reverse = 33 teeth

Pics below.

The ratios I pulled from the mintylamb gearspeed calculator showed for a GT6 gearbox (for KC/KF/KE gearboxes) are:
1st 2.646
2nd 1.779
3rd 1.254
4th 1.0

These are real close to the ratios shown for a TR7 4 speed gearbox. I have a refurbished (by TRF) TR7 4 speed awaiting an adapter to my Spitfire MK III bell housing. So if/when I need to replace the GT6 tran my plan is to use the TR7 4 speed as it is supposed to be stronger. But I’d like to know what ratios I have now before that decision point is reached.
1st 2.66
2nd 1.78
3rd 1.26
4th 1.0

I used a 3.89 welded diff initially with this gearbox but I switched to a welded 4.11 which is now in the car.

Thanks again.
Bud 

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)

On Jan 11, 2018, at 7:31 AM, Ron KRANTZ <rkrantz77 at comcast.net> wrote:

> 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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>> 
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>> 
> 
>  
> 
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