Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Roadster II?

Hi Trevor,

Would you like a technical explanation or the overall effect explanation? Let's see if I can manage both.

Torque, gearbox, final drive gear and power-to-weight determine excitement. Horsepower determines pub bragging rights. (Any large lumbering lorry will out-horsepower the fastest road sports car but they are considerably less fun to drive than a 58bhp Morgan from the 1950s.)

That old Morgan will astonish you because its gearing is right. Sadly Morgan has to borrow its gearing and final drive gear from others and this is not always perfect for the car. However, BTR offers a choice of 3.08, 3.23, 3.45 and 3.78 final ratios and that can be played with.

What do all these axle numbers mean? They tell how many times the propshaft must turn before the rear wheels turn a full circle. The more turns required, the more engine energy squeezed into each wheel rotation.

Transmission gearing works the same way..offering you a range of gears that translate engine rotation into propshaft rotation. To overcome inertia, you need more energy per rotation and that's why first gear and reverse have higher numbers as more rotations of the engine are necessary to move off from a dead stop. As the the car moves and the inertia becomes one of movement, less energy is needed and "higher" gears mean less engine rotation is required turn a full propshaft circle.

So now I got you completely confused! Ok..let's go on.

The key is how many engine rotations are necessary to move the wheels. To find that out multiply the gearbox gear in question by the rear axle ratio. For example, second gear on the old roadster was 2.52 x 3.08 = 7.76. Second gear on the new Roadster is 2.08 x 3.73 = 7.75. So what changed? Nothing..at least for 2nd. Let's look at final drive ratios all down the line.

ROADSTER----OLD------NEW

1st------------13.02----14.44
2nd-------------7.76-----7.75
3rd-------------5.14-----5.07
4th-------------3.76-----3.73
5th-------------3.08-----2.83

As you can see..2nd, 3rd and 4th, are virtually the same. However 1st and 5th are not. This means that the new Roadster, assuming the same engine, would accelerate faster from a standing start and theoretically cruise in 5th at a higher top speed (though it will require more down-shifting on steep hills in that gear). However, the engine did change for a less powerful version (- 8 to 9%). That is almost exactly the difference in the first gears so an acceleration difference will be impossible to detect in that gear.

Outside of the possibility of unrelated supply and/or configuration issues, the lower bhp of the new engines will be mitigated by the more aggressive first gear. The Roadster engines not especially torquey and develop their best high up in the rev range.

On the other hand, final axle ratios are often used with the same gearbox to suit a market situation. For example, the North American Jag e-types of the 60s, competing for sales with Corvettes and Mustangs, were fitted with a more aggressive final ratio to give more them acceleration and excitement...at a sacrifice to top speed.

As an example of moving in the opposite direction, I am lowering the ratio with one of my mogs. It is a VERY light car (appr. 840kilos) and it has far more power than stock (300+ for bhp/torque) with a tuned 4.8. The stock R380 gearing makes 1st gear unnecessary. To rectify that, I am changing the rear axle ratio from 3.45 (which I used with a high revving 3.5) to 3.08. That will bring 1st gear back into play and have the engine work less when cruising.

For more info on your own car and what different axle ratios can do for it..look on the GoMoG Workshop Manual on the Gearbox page
http://www.gomog.com/allmorgan/gearindex.htm Scroll down for a couple of gearbox calculation programs. Fill in the blanks, see what you got and then play with final ratios.

Did this make any sense for you?

Lorne