Shock cooling?

[Testarossa]

Chuck in Indiana may want to chime in here.

When I had the Comanche, I worried about shock cooling every time I crossed the mountains eastbound. From 13,500 over the Continental Divide to 5900 feet at Boulder in about 25 miles -- and a glider tug does a more extreme spiraling dive immediately after a slow full-throttle climb. I managed to nurse the Comanche to 2500 hours before I saw cylinder cracks.

Obviously an air-cooled motorcycle doesn't get that kind of abuse -- a typical climb and descent here is 150 feet per minute at 60 mph over 20 miles, much of the downhill using compression braking against a closed throttle. I've never worried about it. But coming down the canyon yesterday, I wondered whether shock cooling is an issue in the design of motorcycle engines. Thoughts?

[rocker59]

I guess you could monitor it with a couple of CHT gauges, but I have a feeling the issue is negligible in a motorcycle engine.

In your neighborhood, Mount Evans or Pikes Peak would be good places to find out.

[Murray]

Chuck in Indiana may want to chime in here.

When I had the Comanche, I worried about shock cooling every time I crossed the mountains eastbound. From 13,500 over the Continental Divide to 5900 feet at Boulder in about 25 miles -- and a glider tug does a more extreme spiraling dive immediately after a slow full-throttle climb. I managed to nurse the Comanche to 2500 hours before I saw cylinder cracks.

Standard practise with a Pawnee at the local gliding club (rego VH-TUG) with the fixed fine pitch prop was to stand it on its nose keep the engine at around 2000rpm.

With modern Fuel injection and thermostatically controlled oil coolers I doubt its a problem the, while i'll criticise Guzzi for having and archaic engine design foe a true step back in time the current examples of continental and lycoming really take the biscuit.

[Howard R]

I know nothing of airplane engines, but I did once have an uncle with a 454 Chev in a Winnebago motor home.  Those things would eat exhaust manifolds like candy, he said it was a well known problem in the community.  Severe overload going up a mountain pass WFO would have the exhaust manifold glowing red, then hit the peak, all load suddenly removed, coast down the other side much as you describe and the manifold cracks from thermal shock.  Nothing to do with altitude changes.  Probably not as bad on a motorcycle with tubular headers vs. cast iron manifold.

Howard

[Testarossa]

In your neighborhood, Mount Evans or Pikes Peak would be good places to find out.

You're right, it's a negligible problem.  All the high-elevation roads are severely speed-restricted, meaning about 30 to 45mph. This includes Trail Ridge Road. The fastest climb/descent in the state is probably the western approach to the Eisenhower Tunnel (11,600 feet), where the speed limit is 60 mph but left-lane traffic can occasionally hit 85. That would be 250 to 350 fpm.

[Murray]

Dim dark memory an Uncle that did a bit of truck driving back int he day did say some of the early air cooled deutz's were prone to it I think with a motor cycle it would be really hard to achieve. Motorcycle motors are almost always relatively lightly loaded so at full throttle when working their hardest the motor with have a large amount of cooling airflow. In slow traffic on a hot day while the motor will be prone to getting hot due to the outside temp been hot it will always be a slow cool down.

[Triple Jim]

The worst case for an air cooled motorcycle is running hard on a hot day and running into a sudden downpour.  I've never heard of that causing damage, but when it's happened to me I've thought about what might happen.

[wavedog]

When I was a young'un riding two stroke and four stroke dirt bikes never had a problem with shock cooling. My brother and I ran them hard, up and down the mountain, mud and dirt jammed into the cooling fins and splashing thru creeks filled with cold north west water from the snow run off. Not saying it couldn't happen, it just didn't happen to me.

[Triple Jim]

...ran them hard, up and down the mountain, mud and dirt jammed into the cooling fins and splashing thru creeks filled with cold north west water from the snow run off...

I should have said "worst case for an air cooled street motorcycle"   

[rodekyll]

I've run into rain on a hot day and seen the temperature gauge on the ATF cooler drop 40º before my eyes.  The first time I noticed it I thought the gauge had broken.  Then I noticed I could squirt a little mist of water at the cooler when I was at the gas station and it would react almost instantly.  So for my trike design I have incorporated a windshield washer bottle with the nozzles pointed at my radiator.  I'm hoping this shock cooling (didn't know the name for it) is a good thing on account of it took all afternoon to figure out the spritz system.

[normzone]

I am really looking forward to seeing the latest images of that trike.

[rodekyll]

I just got a call from the paint shop verifying the code for the green paint.  I spent four hours last night taping.      Chances are the tank and fairing will be finished today.  I also got carpet in the cargo bed.  It looks better but the screws are kinda low-brow.  I need to find some adhesive that will pucky carpet to powdercoated aluminum.