Selection Of The Transmission Ratio (Fr 125 Mini Max) - Bombardier-Rotax FR 125 MAX FR 125 Junior MAX Installation And Operator's Manual

Operating Instructions FR 125 MAX, FR 125 Junior MAX and FR 125 Mini MAX
Note: To simplify changing of the transmission ratio it is recommended to carry clutch
drums with a prefitted chain sprocket of various numbers of teeth.

4.4. Selection of the transmission ratio (FR 125 Mini MAX)

Because of the special tuning used, the engine performance is very good in the speed range from
6000 to 10000 r.p.m.
Peak performance will be achieved at 8500 r.p.m. but over-revving of up to 11500 r.p.m. is
permitted.
The decline of the engine speed during kart operation will be controlled by the exhaust system.
Above the engine speed of 11000 r.p.m. the effectiveness of the exhaust system decreases,
resulting in a big drop of performance (see diagram 1A). This planned performance drop results in
one intersection of performance graph and graph of driving resistance (see diagram 1A, notice the
intersection at approx. 11500 r.p.m. in this particular case.
Attention: Never run the engine without load. By revving the engine without load, speeds of
above 14000 r.p.m. can be reached and this excessive engine speed will drastically
shorten the life span of some components (conrod, conrod bearings etc).
For approximation and optimizing consult Tables 2 and 3.
Note: The values in the tables refer to a maximum speed of 11500 r.p.m. This engine
speed may not be reached because of too high a drive resistance, therefore use the
stated values for approximation only.
The procedure for selection of the transmission ratio for a certain race track will be explained in the
following example.
The kart is fitted with a random transmission ratio (e.g. number of teeth of sprocket on crankshaft =
13 teeth and number of teeth of chain wheel on rear axle = 82 teeth). In Table 2 find the
corresponding transmission ratio 6,31.
With this transmission, drive a few rounds and note the maximum engine speed reached (e.g.
11000 r.p.m.).
Seite/page 91/108 Ausgabe/edition 09.2003