### An evaluation of 5 speeds for the Elan

Posted:

**Fri Nov 17, 2006 1:20 am**A previous post (http://lotuselan.net/forums/viewtopic.php?t=13892) dealt with the question of what differential ratio an Elan would need to achieve maximum speed. Somewhere between 2.89 and 3.37 was the answer, with the conclusion that any four-speed transmission used would be geared too high in 1st gear, so a five or six speed transmission would be necessary.

Which bring up the question, if we had our druthers, just what ratios would be desirable in a five-speed gearbox for an Elan? Gearbox ratio design, just like virtually every thing else in a car, is fraught with a series of often times conflicting requirements. First gear needs to be low enough to start the car up the steepest slope with the heaviest load. Top gear should allow the car to reach its natural top speed, or be an overdrive ratio for enhanced fuel economy and reduced emissions with top speed reached in the previous gear. The number of ratios should be reduced to minimize cost, weight, and size. The maximum ratio gap needs to be appropriate for the engine power band. An engine with a wide torque band can use a wider ratio gap than can a highly tuned engine.

Let?s look at a couple of concrete examples first, just to get an idea about what it is we are talking. The examples will be the two Elan gearboxes. Assume an Elan with 155-13 tires, 3.77 differential, and 7000RPM.

Close ratio

Main Lay Ratio %diff Speed Diff

21...28

32...17..2.510.............48.7

.....................53.4%.........26.0

27...22..1.636.............74.7

.....................33.0%.........24.6

24...26..1.231.............99.3

.....................23.1%.........22.9

21...28..1.000............122.2

Figure of Merit 99.7 (100 max)

Speed difference increment -1.5mph

Wide ratio

Main Lay Ratio %diff Speed Diff

19...30

32...17..2.972.............41.1

.....................47.9%.........19.7

28...22..2.010.............60.8

.....................43.9%.........26.7

23...26..1.397.............87.4

.....................39.7%.........34.7

19...30..1.000...........122.2

Figure of merit 97.5

Speed difference increment 7.5mph

Interestingly, both these boxes have a 1st to 2nd ratio drop of around 50%, about what a sporting box would have. (Sedans often have more than 65%, racecars and motorcycles can be 40% or less.) So the close ratio box has relatively constant speed differences between gears, while the wide ratio box, with roughly the same percentage ratio drop from 1st to 2nd, have increasing speed differences between gears, with the result that each higher gear feels more widely spaced than the previous one.

Let?s look at some of the more commonly available five-speed gearboxes that have been swapped into the Elan. The first is the Austin Maxi based gearbox as originally fitted on the +2.

The Lotus Austin Maxi 5 speed ratios produce

Ratio %diff Speed Diff

3.200............38.2

........59.2%.........22.6

2.010............60.8

........47.0%.........28.6

1.367............89.4

........36.7%.........32.8

1.000..........122.2

........25.0%.........30.5

0.800..........152.7

Figure of Merit -3.4

Speed difference increment 2.8mph

This is a sedan gearbox, with a wide gap between 1st and 2nd (indeed between all gears, in a relative sense), large positive speed difference increment, and poor Figure of Merit (which should be well above 50). Interestingly, an even higher 5th gear of 0.762 would produce a figure of merit of 97.3, a speed difference increment of 5.1, and 160.3mph at 7,000 (126 at 6,000, 137 at 6,500). So 1st is too low, 5th is too high, and all the ratio gaps are too wide, but remember this is a sedan gearbox. Interestingly, the Lotus factory offered this conversion to me, but I turned it down because I regarded the ratios as inappropriate for an Elan. A good decision 30+ years ago, and still a good decision today.

Another popular gearbox conversion is based on the T9 gearbox. Alan Voight makes probably the best conversion, with good shift feel, proper placement of the gearshift lever, and custom bell housing while retaining the standard sedan box and ratios to minimize costs.

And the MT75 ratios produce

Ratio %diff Speed Diff

3.650............33.5

.........85.6%.......28.6

1.967............62.1

.........43.9%...... 27.3

1.367............89.4

.........36.7%.......32.8

1.000..........122.2

.........22.7%.......27.7

0.815..........149.9

Figure of Merit -11.3

Speed difference increment 0.3

Well, what can we say, this is a sedan gearbox. First gear is virtually unusable in an Elan it is so low, the engine bogs when upshifting and overrevs when downshifting. Although it is relatively meaningless, the ratio progression would be improved with one more tooth on third gear counter shaft, bringing the figure of merit close to 80 and improving the ratio progression as the shift progresses from second to third to fourth.

SPComponents (who make the gearbox for Quaife (http://www.quaifeusa.com) who sell it also) approaches the gearbox conversion differently. The input shaft has the right length and splines to bolt up to the TC engine, although they retain the sedan tail shaft. This means the gear lever is too far back in an Elan, and they offer no way to correct the problem. The Seven and Caterham crowd are the usual users of this conversion. The gear sets offered are more oriented to the competition driver, with the following ratio set being the closest to being useable in an Elan.

And the SPC ratios are

Ratio %diff Speed..Diff

2.390............51.1

........55.1%.........28.2

1.541............79.3

........27.3%.........21.3

1.215...........100.5

........21.0%.........21.6

1.000...........122.2

........14.9%.........18.1

0.871...........140.2

Figure of Merit 39.0

Speed difference increment -3.0

The figure of merit of 39.0 is mediocre, and would be improved to above 95 by adding one tooth to second gear countershaft. Right now, the gearbox feels a little wide between first and second, and a little close between second and third. The change would bring the first-second drop down to under 51% and the second-third drop up to over 31% and the shift progression would feel more natural. In SPC?s defense, they designed the gearbox around a first gear ratio of 2.20 and keep the same third and fourth gears, which makes it difficult to get optimum spacing for other first gear ratios. And this answers the unasked question as to whether a one-tooth difference is even noticeable. The answer is yes, it changes the feel of the gearbox quite noticeably.

Another approach is taken by BGH Geartech Ltd. (http://www.bghgeartech.co.uk/html/5_speed.html). They have cut several custom gear sets while retaining the standard tail shaft and input shaft. They claim the gearbox can be bolted to the TC using a Sierra clutch, and claim to be able to bring the gearshift forward. Probably the most interesting ratio set is their E9.

And the BGH Geartech ratios are

Ratio %diff Speed Diff

2.660...........45.9

......52.0%..........23.9

1.750...........69.8

......38.9%..........27.1

1.260...........96.9

......26.0%..........25.2

1.000..........122.2

......22.0%..........26.8

0.820..........149.0

Figure of Merit 72.9

Speed difference increment 0.7

This is a good set. They offer an alternative first gear of 2.750 which has a better figure of merit of 84.1, but a first-second drop of 57.1%, which some may consider acceptable.

There have been several other five speed transplants, and some other vendors claim to be able to convert other five speeds to fit the Elan. I would appreciate anybody having knowledge and details of other approaches to send them to me so I can incorporate the data. Right now, it looks like the optimum approach might be to use Alan Voight?s tail shaft and shift lever, Quaife?s aluminum case, BGH Geartech?s ratio set, and SPC?s input shaft for a light weight, drop in five speed conversion.

So there it is. The mathematics to perform the analysis are not complex. The spreadsheet to put all this together is not the most simple in the world. There is much more information generated than what was presented here. Anybody curious for more information, querulous about the results presented, or wanting to learn more about this arcane area are welcome to request a copy. An additional plaything in the spreadsheet is the ability to design virtual ratio sets around the Lotus four-speed gear shaft dimensions or the Austin Maxi dimensions with optimum ratio sets of four, five, six, or even seven ratios!

Now, I have been designing gear ratio sets for close to fifty years now, originally using a circular slide rule and lots of paper and not knowing how good the ratios were. I wrote my first computer program around these concepts almost forty years ago, on punch cards and FORTRAN. Now with this interactive spreadsheet, it is amazing how much information and understanding can be acquired in a relatively brief amount of time. It is too bad we cannot get a few people together to cast and machine a near optimal gearbox of five or six speeds that would really complement the near perfection of other aspects of our Elans.

David

1968 36/7988

[email protected]

Which bring up the question, if we had our druthers, just what ratios would be desirable in a five-speed gearbox for an Elan? Gearbox ratio design, just like virtually every thing else in a car, is fraught with a series of often times conflicting requirements. First gear needs to be low enough to start the car up the steepest slope with the heaviest load. Top gear should allow the car to reach its natural top speed, or be an overdrive ratio for enhanced fuel economy and reduced emissions with top speed reached in the previous gear. The number of ratios should be reduced to minimize cost, weight, and size. The maximum ratio gap needs to be appropriate for the engine power band. An engine with a wide torque band can use a wider ratio gap than can a highly tuned engine.

Let?s look at a couple of concrete examples first, just to get an idea about what it is we are talking. The examples will be the two Elan gearboxes. Assume an Elan with 155-13 tires, 3.77 differential, and 7000RPM.

Close ratio

Main Lay Ratio %diff Speed Diff

21...28

32...17..2.510.............48.7

.....................53.4%.........26.0

27...22..1.636.............74.7

.....................33.0%.........24.6

24...26..1.231.............99.3

.....................23.1%.........22.9

21...28..1.000............122.2

Figure of Merit 99.7 (100 max)

Speed difference increment -1.5mph

Wide ratio

Main Lay Ratio %diff Speed Diff

19...30

32...17..2.972.............41.1

.....................47.9%.........19.7

28...22..2.010.............60.8

.....................43.9%.........26.7

23...26..1.397.............87.4

.....................39.7%.........34.7

19...30..1.000...........122.2

Figure of merit 97.5

Speed difference increment 7.5mph

Interestingly, both these boxes have a 1st to 2nd ratio drop of around 50%, about what a sporting box would have. (Sedans often have more than 65%, racecars and motorcycles can be 40% or less.) So the close ratio box has relatively constant speed differences between gears, while the wide ratio box, with roughly the same percentage ratio drop from 1st to 2nd, have increasing speed differences between gears, with the result that each higher gear feels more widely spaced than the previous one.

Let?s look at some of the more commonly available five-speed gearboxes that have been swapped into the Elan. The first is the Austin Maxi based gearbox as originally fitted on the +2.

The Lotus Austin Maxi 5 speed ratios produce

Ratio %diff Speed Diff

3.200............38.2

........59.2%.........22.6

2.010............60.8

........47.0%.........28.6

1.367............89.4

........36.7%.........32.8

1.000..........122.2

........25.0%.........30.5

0.800..........152.7

Figure of Merit -3.4

Speed difference increment 2.8mph

This is a sedan gearbox, with a wide gap between 1st and 2nd (indeed between all gears, in a relative sense), large positive speed difference increment, and poor Figure of Merit (which should be well above 50). Interestingly, an even higher 5th gear of 0.762 would produce a figure of merit of 97.3, a speed difference increment of 5.1, and 160.3mph at 7,000 (126 at 6,000, 137 at 6,500). So 1st is too low, 5th is too high, and all the ratio gaps are too wide, but remember this is a sedan gearbox. Interestingly, the Lotus factory offered this conversion to me, but I turned it down because I regarded the ratios as inappropriate for an Elan. A good decision 30+ years ago, and still a good decision today.

Another popular gearbox conversion is based on the T9 gearbox. Alan Voight makes probably the best conversion, with good shift feel, proper placement of the gearshift lever, and custom bell housing while retaining the standard sedan box and ratios to minimize costs.

And the MT75 ratios produce

Ratio %diff Speed Diff

3.650............33.5

.........85.6%.......28.6

1.967............62.1

.........43.9%...... 27.3

1.367............89.4

.........36.7%.......32.8

1.000..........122.2

.........22.7%.......27.7

0.815..........149.9

Figure of Merit -11.3

Speed difference increment 0.3

Well, what can we say, this is a sedan gearbox. First gear is virtually unusable in an Elan it is so low, the engine bogs when upshifting and overrevs when downshifting. Although it is relatively meaningless, the ratio progression would be improved with one more tooth on third gear counter shaft, bringing the figure of merit close to 80 and improving the ratio progression as the shift progresses from second to third to fourth.

SPComponents (who make the gearbox for Quaife (http://www.quaifeusa.com) who sell it also) approaches the gearbox conversion differently. The input shaft has the right length and splines to bolt up to the TC engine, although they retain the sedan tail shaft. This means the gear lever is too far back in an Elan, and they offer no way to correct the problem. The Seven and Caterham crowd are the usual users of this conversion. The gear sets offered are more oriented to the competition driver, with the following ratio set being the closest to being useable in an Elan.

And the SPC ratios are

Ratio %diff Speed..Diff

2.390............51.1

........55.1%.........28.2

1.541............79.3

........27.3%.........21.3

1.215...........100.5

........21.0%.........21.6

1.000...........122.2

........14.9%.........18.1

0.871...........140.2

Figure of Merit 39.0

Speed difference increment -3.0

The figure of merit of 39.0 is mediocre, and would be improved to above 95 by adding one tooth to second gear countershaft. Right now, the gearbox feels a little wide between first and second, and a little close between second and third. The change would bring the first-second drop down to under 51% and the second-third drop up to over 31% and the shift progression would feel more natural. In SPC?s defense, they designed the gearbox around a first gear ratio of 2.20 and keep the same third and fourth gears, which makes it difficult to get optimum spacing for other first gear ratios. And this answers the unasked question as to whether a one-tooth difference is even noticeable. The answer is yes, it changes the feel of the gearbox quite noticeably.

Another approach is taken by BGH Geartech Ltd. (http://www.bghgeartech.co.uk/html/5_speed.html). They have cut several custom gear sets while retaining the standard tail shaft and input shaft. They claim the gearbox can be bolted to the TC using a Sierra clutch, and claim to be able to bring the gearshift forward. Probably the most interesting ratio set is their E9.

And the BGH Geartech ratios are

Ratio %diff Speed Diff

2.660...........45.9

......52.0%..........23.9

1.750...........69.8

......38.9%..........27.1

1.260...........96.9

......26.0%..........25.2

1.000..........122.2

......22.0%..........26.8

0.820..........149.0

Figure of Merit 72.9

Speed difference increment 0.7

This is a good set. They offer an alternative first gear of 2.750 which has a better figure of merit of 84.1, but a first-second drop of 57.1%, which some may consider acceptable.

There have been several other five speed transplants, and some other vendors claim to be able to convert other five speeds to fit the Elan. I would appreciate anybody having knowledge and details of other approaches to send them to me so I can incorporate the data. Right now, it looks like the optimum approach might be to use Alan Voight?s tail shaft and shift lever, Quaife?s aluminum case, BGH Geartech?s ratio set, and SPC?s input shaft for a light weight, drop in five speed conversion.

So there it is. The mathematics to perform the analysis are not complex. The spreadsheet to put all this together is not the most simple in the world. There is much more information generated than what was presented here. Anybody curious for more information, querulous about the results presented, or wanting to learn more about this arcane area are welcome to request a copy. An additional plaything in the spreadsheet is the ability to design virtual ratio sets around the Lotus four-speed gear shaft dimensions or the Austin Maxi dimensions with optimum ratio sets of four, five, six, or even seven ratios!

Now, I have been designing gear ratio sets for close to fifty years now, originally using a circular slide rule and lots of paper and not knowing how good the ratios were. I wrote my first computer program around these concepts almost forty years ago, on punch cards and FORTRAN. Now with this interactive spreadsheet, it is amazing how much information and understanding can be acquired in a relatively brief amount of time. It is too bad we cannot get a few people together to cast and machine a near optimal gearbox of five or six speeds that would really complement the near perfection of other aspects of our Elans.

David

1968 36/7988

[email protected]