XRAY - Model racing cars

Hello,

Can anybody tell me if it is suitable to use the hudy spring steel driveshaft in the rear? Based on the size of the shaft it is possible but the manual says not to do this. Why?

are the spring steel versions havier? I supposed based on the price of the parts that the spring steel would be the better ones?

They are heavier. They are better in terms of crash-resistance, and so its better to have them in the front.
In the rear the aluminium parts are ok, and you can save a few gramms if you use them. in case of a crash usually the diff slips and you won't have problems with them.