|The 3.89:1 final drive was the original R71
sidecar ratio so it will work on a sidevalve fitted with
an appropriate chair (and the BW40 style isn't it). In
about 1944, the Soviets swapped from the 3.89 to the
4.625 (from the R5) for improved off-road performance.
The original R71 solo ratio was 3.60 and these gear sets
are available though very expensive. A 4.11 or 4.22 final
drive can sometimes be found through the Russian/Eastern
European dealers for a reasonable price and they work
well on a sidecar. The 3.89 ratios are available from
Ural dealers and are of a good quality. Taking time to
mesh the gears properly and shimming correctly is vital
for a long life. Most failures are the result of
incorrect shimming and not quality issues.