I dont know what the class is but if its more on grammatical errs, here they are.
"Two wheels, a rumbling engine, and an exhaust heard from 1/8 of a mile away" <-- this is a clause, not a complete sentence. Easy fix would be "Two wheels, a rumbling engine, and an exhaust heard from 1/8 of a mile away, what is it?"
Its all mixed up. First you talked about what differs between sport and dirtbikes, then you jumped on the handlebars, then what makes a sportbike, then the parts. Like on the 2nd paragraph, I really cant find that paragraph's topic.. is it the parts ? the idea of a sportbike ? or inexperienced riders ?
Not just because you are comparing 2 diff things, doesnt mean you gotta have 1 par. each. Try to separate your thoughts and topic. Elaborate more, give the reader the feeling of being there, being able to see what you mean.
Your last par shouldve been the first, and your first the last.
As much as possible, refrain from using "IT" and "THE" to start your sentence. When comparing, its better to emphasize the differences in one sentence, like, "on the contrary", "meanwhile", "but", etc.. etc..
For this, I'd give you a C
Focus one paragraph on one thought..
then follow-up the next from the last one...
build it up a lil bit more, the conclude the whole thing with a short, suspense-packed one.
-contracosta times editor