car slides to the right or left when i accelerate... why?

hello i am using this car script i got from a tutorial. I modified it to fit my needs such as i added different drive trains(front-wheel drive, rear-wheel drive etc.) to it and fixed a couple bugs i found. but heres my problem when i accelerate depending on the cars drive train it will slide to either the left or the right and i don't know why. any suggestions?

http://answers.unity3d.com/search?q=wheel+collider

It's simply how the physics model is set up in physX, you'll notice that raptor safari doesn't use the built in wheel colliders and made their own ray cast wheels. You can make sure that the vehicle centre of gravity is perfectly aligned and that the wheels are equally spaced, 4 wheel drive cars are more stable. You can also modify your physics on the fly to stabilise the car yourself like lowering the centre of gravity, adjusting wheel torque, increasing angular drag when not turning etc. That's called tuning.

Hi

you can also set Stiffness Factor of sideway friction as per your change log. I am also using Car Tutorial script for my bike game. My bike is rear wheel drive i set 0.083 stiffness Factor for side way friction and it’s work better for me.