How close are those to max length
Depends on many details what makes up the ability for the shaft to be at a determined length, every truck,car and boat are different.
Example in a performance setting fairly modified Dodge SRT V-10 is mph limited with a 5" AL shaft to 110 mph or 130 mph depending on final gear ratio at it's length being long for that shaft at those requirements, for no mph limits it needs a 4.5" CF shaft at that length.
There is no max length per se, length for a shaft is dictated by power, trans, weight, rpm, final ratio, from that you can figure what diameter can handle the power and speed, from there you can determine if it will fit and what needs to be remedied for it to work, and then cost.
Another example, had a truck making 3000 ft lbs, there wasn't enough room for 6", 5" was kinda on the line, CF can do it in less diameter than AL, the price between the 2 is nearly 2 times that of the AL though.
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