Updated spreadsheet including Gerry’s MT75 ratios.
I took Gary’s comments about not giving up the close ratio transmission to heart, as I feel the same way. When my first Elan was consumed by fire, I bought back the pile of pieces to retrieve the CR box.
Interestingly, the general set of ratios in that box has been used through the years and across many manufacturers. It may be close to the “perfect” 4-speed ratio set.
There are some interesting mathematical properties in the ratios. The 1st to 2nd ratio spread is 50% (in round terms) which seems to be a good compromise, not too wide, not too close. Numerologically, it is a 3/2 ratio.
The speed gaps between ratios are relatively constant, which seems to produce a good feeling staging of ratios. So, if it gets 100 mph in top, the speeds in gear are 40, 60, 80, and 100 for a constant 20 mph gap.
Again bringing forth numerology, the gearbox ratios can be generated with a number sequence. In this case, the sequence 5/2, 5/3, 5/4, and 5/5 generates the ratios 2.50, 1.67, 1.25, and 1.00.
This can be extended to produce 5 ratios, 5/2, 5/3, 5/4, 5/5, and 5/6 for 2.50, 1.67, 1.25, 1.00, and .83 for an overdrive 5th. Or 6/2, 6/3, 6/4, 6/5, and 6/6 for 3.00, 2.00, 1.50, 1.20, and 1.00 for direct top.
The corresponding 6-speed would be 6/2, 6/3, 6/4, 6/5, 6/6, and 6/7 for 3.00, 2.00, 1.50, 1.20, 1.00, and .86.
Looking at the canditate 5-speeds in the spreadsheet produces a sinking feeling in me. First gear ratios of 3.3, 3.5 or lower just don’t work well in our light weight cars. In essence, this provides four speeds with a startup gear. As Gary said, it would be better to use a higher diff ratio and retain the benefits of the 4-speed ratios, at a lower cost. The spreadsheet has another sheet with differential ratio possibilities for different ring and pinion teeth, including such possibilities as 36/11 (3.27), 32/10 (3.20), 35/11 (3.182), 31/10 (3.10) and so on.
The other approach would be to cut a new set of gears for the candidate geabox. Several of us have a spreadsheet that can provide optimum tooth numbers closely approximating the 2.50, 1.67, 1.25, 1.00, .83 sequence that would be a logical extension to the current CR 4-speed. It would seem that a shop that can cut gears could take the shafts and gears from a donor gearbox, and cut a new set to our tooth specifications without a lot of agony. This is not rocket science, or the development of a new gearbox.
I will donate the software and time to come up with appropriate tooth counts for a candidate gearbox, and possibly the costs for the prototype gearset. There a several of us here in SoCa who really want a 5-speed but are rather put off with the cost of a Voight-BGH combination. I had been working with a shop which cuts gears for vintage cars, but it looks like they got too busy and the quote they came back with was several times the original estimate so it doesn’t pencil out.
Well, back into my ivory tower.
David
1968 36/7988
20080408 Transmission-Matrix.xls (116 KB)