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[Fot] Gear ratios?

Subject: [Fot] Gear ratios?
From: levilevi at comcast.net (Bud R)
Date: Thu, 11 Jan 2018 11:34:09 -0700
References: <9940DDBC-84D3-4515-AF63-7453C94627AC@comcast.net> <1862346519.1114456.1515676762409@mail.yahoo.com> <709903065.344317.1515681083676@connect.xfinity.com>
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)
>> 
>> _______________________________________________
>> fot at autox.team.net
>> 
>> http://www.fot-racing.com
>> 
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>> at yahoo.com
>> 
>> 
>> 
> 
>  
> 
>> _______________________________________________
>> fot at autox.team.net
>> 
>> http://www.fot-racing.com
>> 
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