Camera settings help???

Ehh, I wouldn't worry too much about it. Your buddies probably don't want their "suck" caught on camera.

i personally find that photos with the most stop action are the most interestng. in other words, if there is a loose sandy berm and your buddies are blowing it up, aim for the fastest shutter speed possible to get the dirt, wheels, etc. frozen in the photo. use a large aperature ( low number) and if possible run manual mode.

the 28-90 will be real nice with this if you can get up close enough as the semi wide angle gives a nice perspective in my opinion.

I use to use the d70 a lot. First there is light. Try to take photos early morning or late afternoon, photos at noon have the harshest light.

Make sure your shutter speed is fast enough, the longer the telephoto lens like at 300 the faster your shutter needs to be, at 300 you should be at a shutter speed of at least 450.

If it is overcast you will probably need to raise your ISO from 200 to 400 or even 800, understanding that higher ISOs produce more noise in your images, highest at 400 for best results if the shutter speed is fast enough. If your shutter speed is slow, you will get a lot of blurry shots.

Try to always crop tightly. If you don't you will get a little tiny motorcycle in a big open space which is usually not really interesting ( can be if you use ultra wide lens).

Try to get several of your friends riding in the shot against each other, a couple riders in the shot makes it more interesting.

If you are unsure how to use manual. Use the P mode and open the aperture to as wide as it will go (use the dial in front of the shutter release buttong to do that), that gives fast shutter speed and less depth of field to help isolate the subject.

If you use the 28-90 get real close, much closer that you think you should. Stand in a corner so the riders are almost on top of you. Try to either get the camera real low to the ground or above the rider, everyone shoots standing up and you see so many pics like that it starts to get old. Try to get different perspectives.

You can go with lower shutter speeds of 60 to 125 and pan but this takes a lot of practice and you will get a lot of blurry shots until you learn how to do it.

The d70 has a flash sync speed of 500 which means you can use a flash in daylight and get some good shoots. Use flash when shooting into the sun (without it try to shoot with your back to the sun) the flash on the d70 is a little weak, if you have an external flash you can get better shots.

Good luck