Lotus Elan and Lotus Elan Plus 2 Forums

bill308 wrote:I purchased this T9 conversion from a fellow chatter. It had been stored for a while, but was unused, and with his cooperation I was able to make the shipping arrangements work well.

Bill,
Thanks, this ties together now.
Russ

how about this as a solution? Agreed it would require some chassis mods to fit (access hole from below?) but it would retain the original ratios and gearstick/position and delightful shift quality, the excess weight could be countered by the various alloy gearbox replacement parts (tail housing/bell housing) and would give the required "fifth gear", an added advantage could be an eight speed close ratio box ala TR6. Not sure of the cost, however I'm sure an enterprising soul could duplicate the front case arrangement and modify a Laycock overdrive unit themselves?

Mark

Mark,

Very interesting idea.

Usually, it is difficult to use an overdrive on more than one ratio because you get some strange feeling drops between gears.

But the wide ratio transmission (2.97 1st) is actually relatively amenable to having OD on all gears.

If you could locate an OD with a .834 ratio, this would produce ratios of 2.97, 2.476, 2.01, 1.675, 1.4, 1.167, 1.00, and .834.

If you download the spreadsheet from http://www.lotuselan.net/forums/viewtop ... c&start=14 and enter the above 8 ratios, it calculates a Figure of Merit of 61.4, a surprisingly high figure, and better than most of the 5-speeds being considered.

The Figure of Merit is a mathematical calculation, with a maximum value of 100 and no minimum value. As a frame of reference, the BGH E7 with 2.75, 1.75, 1.26, 1.0, and .82 has a FoM of 84.1. The T5 mentioned by TomR of 2.95, 1.94, 1.34, 1.0, .73 has a FoM of -23.0 (not good at all)

So much for speculation.

David
1968 36/7988

Generally the use of the Over Drive is limited to 3rd & 4th gears as the OD unit can not handle the torque in the lower gears. I have owned a couple of MGs with the OD arrangement and it worked very well. Due to the MG and Triumph markets there is plenty of support for OD units.

This is an avenue of thought that should see some more research.

steveww wrote:Generally the use of the Over Drive is limited to 3rd & 4th gears as the OD unit can not handle the torque in the lower gears. I have owned a couple of MGs with the OD arrangement and it worked very well. Due to the MG and Triumph markets there is plenty of support for OD units.

This is an avenue of thought that should see some more research.

I remember the OD in the MGB.
Very handy location of the switch on the gearknob-making for a natural shift action.
I am wondering about switch options here-gearknob/dash/steering wheel?

robcall wrote:

steveww wrote:Generally the use of the Over Drive is limited to 3rd & 4th gears as the OD unit can not handle the torque in the lower gears. I have owned a couple of MGs with the OD arrangement and it worked very well. Due to the MG and Triumph markets there is plenty of support for OD units.

This is an avenue of thought that should see some more research.

I remember the OD in the MGB.
Very handy location of the switch on the gearknob-making for a natural shift action.
I am wondering about switch options here-gearknob/dash/steering wheel?

Under the or on the throttle pedal would work nicely too, just like the kick down on an automatic transmission. The O.D. unit would take most of the space that thecurrent propellor shaft occupies. The shaft length is 22 inches center of u-joint to u-joint.

http://www.gearvendors.com/index.html

No bolt on applications but lots of info. Torque would not seem to be an issue on these units, at least as far as the Twin Cam is concerned.

An other possible starting point

http://www.overdrive-repairs.co.uk/services.html

Footswitch is a good idea-less obtrusive.
Along those lines I'm thinking of the old fashioned headlight dipswitch foot control.

A bit of Google and I see that an overdrive unit was fitted to Ford Type 3 gearboxes on Scimitar and TVR.

robcall wrote:Footswitch is a good idea-less obtrusive.
Along those lines I'm thinking of the old fashioned headlight dipswitch foot control.

I just ditched the foot dipswitch for the column-mounted variety, in part so I'd have a place to rest my left foot. I would mount an overdrive switch higher on - not the firewall, it's actually the wheel well.

So if we go down this road, someone has to engineer the mating of the OD unit to the rest of the transmission. Does it hook up to the existing tailshaft and housing, or is the housing truncated behind the shift lever and speedo drive? What about the speedo? - it has to know how fast the driveshaft is turning. And so on.

David, I now have a major gear manufacturer showing an interest in producing a set of ratios for the MT75 gearbox. The only major downside I have ever heard on this is the unsuitability of the OEMs set (especially the very short 1st).

What would your dream ratios be (road use primarily, track as a compromise)

Regards


Gerry

I am not familiar with overdrive units so I may be off base.

My impression is these units are hydraulically and depend upon input shaft RPM to operate an internal pump. See:

http://nostalgiacars.co.uk/overdrive.htm, and

http://www.gearvendors.com/install.html, specification section

Nostalgia makes clear their unit should only be engaged in top gear so as to ensure sufficient hydraulic pressure.

Not sure, but reading between the lines I get the impression the Gear Vendors unit operation may depend upon locking out transmission neutral to provide pump drive. In any case the Gear Vendors unit with a 6.5" width and 7.25" height would require drive shaft tunnel modification.

The following link provide an interesting overdrive history.

http://wapedia.mobi/en/Overdrive_%28mechanics%29

Also interesting is John Esposito's overdrive article.

http://www.quantumechanics.com/categories.php?op=newindex&catid=11

And a timely related press clipping regarding Gear Vendors:

RE: Only In America - for now.... - frd1 - 11-02-2008 07:29 AM

In a transaction reportedly worth a little more than $2 million, Gear Vendors Inc. purchased all the overdrive production assets of GKN. Gear Vendors will now supply components to more than 2 million production vehicles that came with the GKN, Laycock and DeNormanville overdrives. The transaction involves thousands of parts and equipment. Shipments from GKN?s facility in the United Kingdom to Gear Vendors in California are being carried out now with expected completion date of July/August 2008.

Wow, I didn't realize my ratios were so bad - negative even!

They feel really good to me.

Guess I'll have to look at the spreadsheet now to understand the sensitivity of the FOM. The BGH 37 gears don't look that much different to me except maybe the overdrive 5th but the FOM difference appears very big.

Oh well, at least it fits and works.

Tom

Tom,

Don't feel so bad.

If you download the spreadsheet and enter the prospective T5 ratios, the FoM is -23.0. Column T rows 19-22 are the error terms in the calculation of the FoM. Here, the large value in T21 indicates a problem around the 3-4-5 area. Looking at column L rows 19-22 (speed difference between gears), the speed drop between 4th and 5th jumps out as being large compared to the others. The more gearboxes you look at with this analysis tool, the more experienced you get at identifying problem areas.

You can exclude 5th from being considered in the calculation of the FoM by entering N in cell E22. On doing so, the FoM for 1st-4th is 67.1, a considerable improvement.

Delete E22 and change B23 to .78, another ratio available on the Mustang SVO. The FoM changes to 80.8

Basically, this is saying there is a problem with the 4th-5th spacing, in this case the .73 is too wide and as the gearbox is shifted up through the gears, 4th-5th will "feel" wider than 3rd-4th, etc.

However, there can be a legitimate reason to have a super high 5th (or very close 5th for that matter) If the car is geared for top speed in 4th and the engine has sufficient low end torque to pull the higher 5th up a hill, then this is an appropriate engineering or marketing decision. The help text discusses reasons for this and how to go about designing gearbox ratios with non-conforming top gears, or 1st gears, or both to meet specialized design goals.

And it is a perceptive observation that what appear to be small differences in a gear ratio set produce substantially different FoMs. The average driver shifts what is under the knob and does not intellectualize the ratio drops. We have the opportunity, and the tools, to evaluate choices and pick the one that satisfies what ever criteria we have.

David
1968 36/7988

Gerry,

What are dream ratios? What ever the user wants.

Starting with the CR 4-speed that Gary and I like, a 5-speed extension would be 2.50, 1.67, 1.25, 1.00, and .83. This is an overall ratio of 3.00, a 1st-2nd drop of 50% and a constant speed gap between gears.

Going to a 5% wider overall ratio of 3.15 would produce 2.613, 1.699, 1.259, 1.000, and .829 for a 1st-2nd drop of 53.8% and a constant speed gap between gears. Another possibility is 2.581, 1.714, 1.269, 1.000, and .819 for a 1st-2nd drop of 50.6% and a speed gap that increases by 1 mph with each gear (in the spreadsheet, 48 in B35, 1 in D36, and 3.15 in D38.

A 10% wider overall ratio of 3.30 would produce 2.725, 1.730, 1.267, 1.000, and .826 for a 1st-2nd drop of 57.5% and a constant speed gap between gears. Another possibility is 2.660, 1.761, 1.289, 1.000, and .806 for a 1st-2nd drop of 51.0% and a speed gap that increases by 2 mph with each gear (in the spreadsheet, 46.5 in B35, 2 in D36, and 3.30 in D38.

You can try anything you want. Since the spreadsheet is interactive, many different possibilities can be tried in a short period of time. Enter the tire size, engine RPM, differential ratio, and there are charts and graphs produced for the potential vehicle.

Sure is a lot better than when I started this 50 years ago having to use pencil, paper, and a slide rule.

Of course, a real gearbox needs tooth counts, and for that I would need the tooth counts of the existing MT75 gearbox so I can furnish you with a version of this spreadsheets that spits out the tooth counts for each gear.

Have fun.

David
1968 36/7988