Drop gears vs Close Ratio vs Final Drive

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Brocky

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Katy TEXAS

5th Dec, 2006 at 04:08:16pm

I know this has come up alot recently so I did a quick spread sheet to have alook at it. I've laid out below the overall reduction and approx. speed in first gear for various set ups. the first 3 lines are for a standard A+ set up and the lines after that are for the minispares clubman set 1st gear for some of the various combinations of avalible drop gear ratios and final drives

http://www.turbominis.co.uk/_photos/random...os/478/3010.jpg

The speed is only approximate and based on an 8" wheel and tyre radius. but gives a relative idea.

I thought it would be quite interesting just to lay this out just to have an idea of the effects of different set up, especially if you are choosing which drops and final drive to go for.

I guess the next question would be is if you know how much your car weighs and how many bannanas and curly wurlys you have can you say if there is sufficient reduction through the transmission or do you just have to try it ?

Edited by Brocky on 5th Dec, 2006.

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SouthPark, Colorado

5th Dec, 2006 at 04:28:25pm

That's hard to read Dave...

:(

I'm guessing you did it in excel? If so, hit print-screen and save it as a jpeg and post that up... :)

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Brocky

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Katy TEXAS

5th Dec, 2006 at 04:29:21pm

On 5th of Dec, 2006 at 04:28pm TurboDave said:
That's hard to read Dave...

:(

I'm guessing you did it in excel? If so, hit print-screen and save it as a jpeg and post that up... :)

Working on it....

johnK

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Norfolk

5th Dec, 2006 at 04:49:08pm

you'll need to produce a "tractive effort or thrust curve" to understand it graphically - which is "tractive effort" up the y axis and road speed along the X axis - you do need an idea of your torque curve for this. Then you can plot your thrust curves through the gears and understand the best set up to suit the engine characteristics. You can then determine if its worth ringing the engine out to max rpm or changing up early - to maximise the thrust avaialble at the road. I may have to try posting a pic of such a curve later - I think this is the sort of thing you are looking for?

alternatively send me your torque curve,tyre size and diff ratio and I'll put yours up here for you!

If Carling made Mini engines
it would probably be like this one!

Brocky

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Katy TEXAS

5th Dec, 2006 at 05:08:09pm

Thanks John,

This is kinda chicken and egg at the moment as I am currently building the engine I want to choose the drop gears and final drive for.... *smiley*

I'm definately going for the clubman gear set with a 2.58 first gear ratio, but wanted to decide which primary gear and input gear to go for in terms of actually being able to get a good take off.

I think the chart helps as changing the input gear can give a little flexiblity to the set up for the least cost if you get it wrong

I will maybe go for the 24 tooth primary and perhaps start with the 24 tooth input. giving 1:1, and then use a 3.21 FD.

This way I can either drop to a 23 tooth input for an 0.985 if I wan to go up or use a 25 tooth input if I want to go down 1.04.

I think the motor will be good for 140 BHP and an equivelent torque, the car wll be full weight and the FD a 3.21. on 13"

Cheers

Brocky *smiley*

Edited by Brocky on 5th Dec, 2006.

modifiedminis

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6th Dec, 2006 at 08:22:24pm

I was running mine on a s/c /r box on 1:1 drops with a 2.7 final drive on 12" wheels.... the initial pull away was crap but once your moving i could get just over 60 out of first gear and it would do 140 mph on the rolling road (not that i would even consider taking it up to that speed on the road)

Hedgemonkey

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Stu from Corwall aka Mr Jazz Piano, Love_Machine, kneegrow

6th Dec, 2006 at 09:16:57pm

On 5th of Dec, 2006 at 04:49pm johnK said:
you'll need to produce a "tractive effort or thrust curve" to understand it graphically - which is "tractive effort" up the y axis and road speed along the X axis - you do need an idea of your torque curve for this. Then you can plot your thrust curves through the gears and understand the best set up to suit the engine characteristics. You can then determine if its worth ringing the engine out to max rpm or changing up early - to maximise the thrust avaialble at the road. I may have to try posting a pic of such a curve later - I think this is the sort of thing you are looking for?

alternatively send me your torque curve,tyre size and diff ratio and I'll put yours up here for you!

IIRC they are called Cascade Curves and a plot is a bit like this. look at where the lines meet. They are like a spread out dyno chart, or a squashed one depending on the gear.

Example of.

The name of the game is having as much area under all the lines as possible :)

Stu
[img]

Edited by Hedgemonkey on 6th Dec, 2006.

Bugger off, I'm getting there.

Brocky

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Katy TEXAS

7th Dec, 2006 at 12:52:07pm

Great info guys *smiley* Thanks

Just wanted to take a little of the guess work out of the selection process.

I'm sure I will have more question soon

Cheers

Brocky

johnK

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Norfolk

7th Dec, 2006 at 02:02:45pm

Brocky - I have yours plotted out - puter problems lately - will put it up here later

cheers

John

If Carling made Mini engines
it would probably be like this one!

Brocky

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Katy TEXAS

7th Dec, 2006 at 02:54:44pm

Great Stuff *smiley*

Brocky

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Katy TEXAS

11th Dec, 2006 at 09:29:15am

Great stuff John, *smiley*

Curves based on the above engine spec

Hey john what torque did you use ?

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johnK

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Norfolk

11th Dec, 2006 at 10:36:13am

Hi Brocky - the gears are run from 2000 rpm to 7000 rpm, I guessed the torque curve, key points - 120 lb/ft at 3000rpm, max torque 135 @5500, max bhp 140 @5500, holding good torque to 7000 were I've used 90 lb/ft. reasonable for a well developed turbo motor? the figures in the chart are at the wheels - a 20% drop from flywheel figs was used. based on the torque curve you can see the additional accelerative force avaialble if you used a 3.44 in place of a 3.2 for not a lot of drop in speed in the gears. If someone has a "real" torque curve of around 140bhp we could input this to compare diff ratios more accurately?

cheers

If Carling made Mini engines
it would probably be like this one!

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