Singer Owner November/December 2005

Gears, Gear Ratios and Torque

John Dowding

Continued from Singer Owner July/August 2005

Final Drive (Crown Wheel and Pinion) - As stated previously a typical 1st gear ratio is 4 to 1 which although it multiplies engine torque by 4 times is still insufficient to start a vehicle from rest. Therefore a further gear reduction must be introduced into the transmission in the form of a crown wheel and pinion (final drive).

In this example of a low gear ratio, torque is multiplied by 4 times (ignoring friction).

Overall Gear Ratio

This is a typical overall ratio for 1st gear for a Singer. In this example engine torque has been multiplied by 16 times (ignoring friction) and it enables the vehicle to start from rest on a gradient with a full load.

Here are some examples:

i. Singer Hunter

1st = 3.32 to 1       Final drive ratio = 5.125 to 1

2nd = 2.09 to 1      Max. engine torque

3rd = 1.35 to 1      77 lbft @ 2600 rev min-1

Top = 1.00 to 1

Rev = 3.32 to 1

To find the overall gear ratios, each ratio must be multiplied by the final drive ratio.

Overall Gear Ratios

1st = 17.02

2nd = 10.75

3rd = 6.94

Top = 5.125

Rev = 17.02 to 1

\ Maximum torque available in 1st gear at the drive/ half shafts at 2600 rev min-1 engine speed is 17.02 x 77 = 1310.5 lbft (ignoring friction and other losses)

ii. Singer Vogue IV with Overdrive

1st = 3.35 to 1      Final drive ratio = 4.22 to 1

2nd = 2.14 to 1     Overdrive ratio = 0.80 to 1

3rd = 1.39 to 1

Top = 1.00 to 1

Rev = 3.57 to 1

To find the overall gear ratios, each ratio must be multiplied by the final drive ratio.

Overall Gear Ratios

1st = 14.14 to 1

2nd = 9.03 to 1

3rd = 5.86 to 1

Top =1.00 to 1

Rev = 15.06 to 1

The overdrive was available on 3rd and 4th so to find the overall ratios in overdrive 3rd and 4th, multiply all 3 ratios together.

0/d 3rd overall ratio = 3rd gear ratio x final drive ratio x o/d ratio

= 1.39 x 4.22 x 0.80

0/d 3rd overall ratio = 4.69 to 1

O/d 4th overall ratio = 4th gear ratio x final drive ratio x o/d ratio

= 1.00 x 4.22 x 0.80

0/d 4th overall ratio = 3.38 to 1

Because of the o/d 4th ratio of 3.38 to 1 is higher than direct drive 4th ratio, less torque is available at the driving wheels. Anyone who has driven a vehicle with o/drive engaged will know that insufficient torque is available for speeds below 35mph because the drive line will start to judder.

The purpose of an overdrive ratio is to reduce engine speed which in turn results in a reduction in fuel consumption, engine noise and wear.

Engine Braking - Readers will know that changing down to a lower gear when descending a hill helps to prevent the vehicle from accelerating. The driving wheels are trying to rotate the engine faster but they only have an overdrive ration to act through using the final drive diagram used earlier in the text.

The crown wheel is now the driver and the pinion driven.

The same reversal of driver and driven takes place when 3rd, 2nd and 1st are engaged so a further over drive ratio is in use.