You are looking for a set of sprocket.

Which model should I choose for back? Steel or aluminum?

How should I change my ratio to have more takeoff or more top speed?

The advantage of an aluminum sprocket is the weight. In MX racing, the aluminum sprocket is widely used. The disadvantage of the aluminum sprocket is its lifespan. On average, an aluminum sprocket lasts about 15 hours. 

The advantage of a steel sprocket is its durability. The disadvantage of the steel sprocket is its weight. On average, a steel sprocket lasts about 30 to 40 hours with the necessary maintenance. It is therefore advisable to install a good chain so that the lifespan is approximately the same.

For aluminum sprockets I recommend TagMetal sprockets for the quality in relation to the price of the product.
Unfortunately the distributor has ceased all production with the Covid-19. It is for this reason that I put my TagMetal section in liquidation. I am no longer able to order with them.

For steel sprockets, I offer the AFAM SLK sprockets in black color. They offer excellent value for money by design and durability. 

For the front sprocket, there are only steel models but of different quality. 

Do you want to play with your ratios to gain take-off? Very simple, it suffices to choose a front sprocket by decreasing the number of teeth or to choose a rear sprocket by increasing the number of teeth.

You want to gain top speed. Just do the reverse. We add on the front sprocket or decrease the number of teeth on the rear sprocket.

The science isn't exact but the front to rear ratio is about 1 to 3. Which means if I remove one tooth on the front sprocket it's more or less the equivalent of adding 3 teeth. on the rear sprocket.

I have included you on each product sheet the original ratio for each model and year. 

Not sure which is the right sprocket for your motocross? Find the right model via my Parts Finder.