In almost every bike spec chart, you’ll see something describing camshaft layout, and sometimes it looks like gibberish. SOHC, DOHC—what’s it all mean?
First, the basics: virtually every four-stroke internal-combustion engine has a camshaft. Its job is to open and close the engine’s intake and exhaust valves—like a person inhaling and exhaling. The cam can be in two locations: inboard or over-head.
Inboard cams (seen in Harley-Davidsons and old Triumph Twins) need to reach up to the valves with a system of pushrods and rocker arms, all of which reciprocate back and forth. That’s a lot of metal thrashing around, and it means such engines can’t rev as high, and consequently produce less power.
Overhead camshafts eliminate most of these extra parts by putting the cam(s) directly over the valves in the cylinder head. The vast majority of modern motorcycle engines and more and more automotive engines use this design.
SOHC stands for single over-head cam, and it means one shaft (via rockers) opens both the intake and exhaust valves. DOHC is double over-head cam, and means one shaft operates the intake valves while a second controls the exhausts.
DOHC designs allow the highest engine revs, and are typically seen in sportbikes. But they also often complicate valve-adjustment procedures.
SOHC designs allow high-speed engine operation, although not as high as DOHC. (The number of valves per cylinder and their individual weights influence this as well.) But an SOHC layout is often easier to service when it comes time for a valve adjustment.
Do you really need to know all of this just to ride? Maybe not. But it’ll sure help you seem like less of a doofus when you’re talking to those salesman, or to the guy sitting next to you at a biker bar.
In almost every bike spec chart, you’ll see something describing camshaft layout, and sometimes it looks like gibberish. SOHC, DOHC—what’s it all mean?
First, the basics: virtually every four-stroke internal-combustion engine has a camshaft. Its job is to open and close the engine’s intake and exhaust valves—like a person inhaling and exhaling. The cam can be in two locations: inboard or over-head.
Inboard cams (seen in Harley-Davidsons and old Triumph Twins) need to reach up to the valves with a system of pushrods and rocker arms, all of which reciprocate back and forth. That’s a lot of metal thrashing around, and it means such engines can’t rev as high, and consequently produce less power.
Overhead camshafts eliminate most of these extra parts by putting the cam(s) directly over the valves in the cylinder head. The vast majority of modern motorcycle engines and more and more automotive engines use this design.
SOHC stands for single over-head cam, and it means one shaft (via rockers) opens both the intake and exhaust valves. DOHC is double over-head cam, and means one shaft operates the intake valves while a second controls the exhausts.
DOHC designs allow the highest engine revs, and are typically seen in sportbikes. But they also often complicate valve-adjustment procedures.
SOHC designs allow high-speed engine operation, although not as high as DOHC. (The number of valves per cylinder and their individual weights influence this as well.) But an SOHC layout is often easier to service when it comes time for a valve adjustment.
Do you really need to know all of this just to ride? Maybe not. But it’ll sure help you seem like less of a doofus when you’re talking to those salesman, or to the guy sitting next to you at a biker bar.