I believe that in order to decrease the jump in power, from a 125cc at 17 and 18, to 47bhp at 19, there should be an intermediate cc at 18, therefore I propose that at 18, a 125cc rider should be able to take another test, in order to ride a 250cc motorbike.
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Most riders jump from a 125cc to a restricted 600cc, which is way too much of a jump. 15bhp to 47bhp is more than enough to knock experienced 125cc riders off, if they're not careful. A smaller jump to 47bhp would increase safety for those who are willing to ride a 250cc at 18