Why don't big tractor trailer type vehicles have synchronizers in their transmissions?

Question

double clutching ain't no fun.

Answer 1

Synchro's are made of brass and won't hold up very long under the heavy stress and strain of a semi transmission use. They are used in cars to make it easier to shift, take most of the wear and make it cheaper to rebuild or repair rather than replacing expensive machined gears.

Answer 2

If a car transmission lasted 100,000 miles it would be a remarkable success.

If a semi-truck transmission lasted only that long it wouldn't even complete a single year of use.

Answer 3

Unlike an automobile, the truck's transmission does not have gear synchronizers that allow you to smoothly shift between gears with just a single depression of the clutch. In the truck, you have to double-clutch.

For example, here's how you change gears from first to second: assuming you're driving in first gear, you first let up the accelerator, push in the clutch, shift to neutral, then let the clutch out.

Then you push in the clutch again, shift to second, let the clutch out, and press the accelerator to continue driving in second gear.