Posted by Paul Olesen on September 04, 2014
Whenever I’m out and about either riding my motorcycle or participating in racing events occasionally I see things that just make me wonder “why”? One of those moments is when I see someone take a cold bike and fire it up for the first time and bang it off the rev limiter, start riding it immediately, or annoyingly continuously blip the throttle as if it will never idle on its own.
These actions beg the question, “Why is it important to warm up an engine”?
The answer lies in a simple explanation of science and mathematics. Before you quit reading because you may not have been an ace at math and science in high school, just give me a minute to break it down. It is actually really simple.
The whole reason we need to let our engine warm up revolves around the concept of linear thermal expansion. Your engine is made up of a number of different materials. The piston is made from a certain type of aluminum alloy, the cylinder another type of aluminum alloy, the rings cast iron or steel, the valves if you have a four-stroke from steel, stainless, steel, or titanium, and the guides are made from yet another material. Once the engine is started these components begin to heat up from combustion and friction as they slide back and forth. None of these materials are exactly alike, and because of this they will expand when heated or contract when cooled at different rates. This interaction between material and change in temperature is predictable and linear.
Now that we understand that engine components change dimensionally from when the motor is cold to when the motor is warm we can start to see the importance of warming up the engine. When a cold engine is first started the piston heats up and expands first. Heat is transferred from the piston to the rings and then to the cylinder wall. If we rev the engine and generate lots of combustion cycles and increase the frequency of friction too early the piston will grow much faster than the cylinder. If there is not adequate space between piston and cylinder to account for this growth the engine could suffer what is known as a cold seizure and you will have yourself a bad day.
By allowing your engine to warm up before you start riding you allow all the components in the engine to slowly expand and stabilize. Once the engine is warm, changes in the engine part dimensions are less drastic and there is much less risk of damaging the engine.
The picture below shows an engine which was limped home after the coolant started leaking out. As the engine lost its ability to cool down, things began to tighten up. You can see how the piston contacted the cylinder evenly around the bore and created the vertical scuff marks.
Even though this engine didn’t completely seize, you can imagine the severity of scuffing would be much worse for an engine that would seize.
So you are probably wondering, “how do I know when my engine has properly warmed up then?” and, “what exactly do I do to properly warm it up?”
The procedure for warming up the engine is simple.
1. Start the engine using the choke if necessary
2. Once the idle comes up due to the choke turn the choke off
3. Allow the engine to idle with the choke off until the cooling system warms up and the engine comes up to temperature.
Knowing when the engine is ready to ride is a bit subjective. As you begin to pay closer attention to your engine, you will begin to detect when it is ready to ride.
Personally for water cooled engines I like to feel the radiator and use that as an indicator. I place my fingers on the side of the radiator where the coolant is returning from the cylinder head and lightly touch to get an idea of how warm the coolant is. I do this until the radiator is just getting uncomfortable to touch. This typically only takes a few minutes and after that I’m ready to start riding the bike.
For air cooled engines my approach is much the same except I feel the cylinder and head to determine when I think the motor is warm enough to ride without causing any unnecessary wear or damage.
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