Question:

The T4 has a very expensive 1,8 m width drum. Why not two smaller and cheaper drums?

Answer:

Because there are a lot of problems with tow drums:

which drum do you want to use, if you test motorcycles? Only one or both drums? For both drums they must have a transmission. For small ATV's the drums need a small distance, competition ATV's need a wide distance. What about the covers for safety? They have to fit in every position.

How do you want to fix a wheel of motorcycle in the wheel clamp for ATV wheels?

Now you have one drum, the middle is for motorcycles and both sides for ATV's. That is much more easier (and cheaper).