Pocketbike Racer

There are 5 tracks in the game. You need to complete a lap in under 1:30 on each track. Go to Ultimate Time Trial. Preferably use a 150cc bike, as they are the fastest. If you have control problems, it can be done with a 100cc bike. Use your boost (B button) on any straight sections you can, but be aware that the race will end when your boost bar (on the right side of the screen) is empty. Driving between cones will refill your boost incrementally. Once you've gotten a lap under the 1:30 time, boost away the meter to end the race. Repeat for each track.

The construction zone will probably be the hardest one to do, as there are many tight turns. Use the power slide (A button) to set up corners and use the boost when you can.

To help on the construction site, please follow JD Buck's shortcut (taken from the Xbox forums http://forums.xbox.com/8423891/ShowPost.aspx). It help me a lot :

"After the 2nd long jump, there's an open area that veers to the right with a large backhoe and a pile of dirt. Just past this is a wooden path that a sharp 90 degree turn to the left. After you clear the turn there is a drop off almost right in front of you. The path normally takes you around it. Drop down. You'll see a 'wrong way' warning for just a second. You'll see arrows telling you to go right. There you are, you're back on the main course and got to skip a big step. With a 50cc and very little boost, you can clear this in about 1:24."

To get to Ultimate Time Trial
Single Player > Free Play >Map Selection (Level) > Event: Ultimate Time Trial
I struggled to find it. So decided it might help someone else.

Tracks:
Parking Lot
Backyard Track
The King's Garden
Under Construction
Fantasy Ranch

Achieve 1 (single) lap under 1:30:00. Added extra emphasis here because I thought it was total time and kept retrying Parking Lot in a 3 lap trial. Same with Backyard Track. Kept retrying the race
Now for the reason you're here.

Under Contruction:
I used 150cc bike (Little Dipper) an achieved a time 1:20:00. With falls 1:25:00 at the most. There is a secret hole other people have commented on. There is one. Guide over. Jk. After taking 2 90's left turn you enter a wide open platform. LANDMARK: RED TOWER. Take the long jump over to get to the wooden under construction platform. Upon leaving this wooden corner, drive straight down to the open hole ahead of you. You'll drop down and see, a backhoe and a dirt mound. If you are driving down a dirt mound and see a backhoe in which you can drive underneath. You've driven too far. Furthermore, if you are still having trouble finding it. Go to standard race with 10 laps to explore. Tried following these other guides, still took me 20 minutes to find exactly what they were trying to describe. All in all, shortcut is around 0:50:00-0:55:00 [50-55 seconds into the level] Another hint, Press(A) to power slide 180 turns.
Achievement Unlocked upon crossing the finish line. Very wordy guide, other guides were too vague for me to follow. Hope this helps.