Another Rear ???? - BMW Luxury Touring Community
 
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post #1 of 7 Old Oct 9th, 2006, 11:46 am Thread Starter
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Another Rear ????

On Thursday morning my bride and I took off for North GA to meet up with a friend and his wife who live in Blairsville, GA. After a short 30 +/- mile ride on I-20 from Augusta and another 20 +/- ride on some nice secondary roads to Washington GA, we stopped for gas and water. For whatever reason I looked at my rear drive unit and there was redline gear lube over the outside of the housing and on the right edge of my tire (all around the side wall), but none on the rotor or anywhere else. I seems it blew out the air vent on the top of the case and then spilled out onto the tire as we were riding. After the initial shock, I cleaned it off the best I could and checked the play and for the sound of problems. Nothing. The case was no warmer than normal but I thought I noticed some extra "gear wine" when under power - especially over 60 mph. I took it easy the rest of the ride and checked on it often and found no more problem. We were loaded pretty good so I know the gears and bearing were under a greater load so there may be a reason for the noise.

We took a very spirited ride out of Helen after meeting up with our friend for a tour of some nice ( never to be found again ) roads - I checked the rear drive and it was just barely warm - which was fine with me considering we were really pushing it and still fully loaded with luggage. After spending the night in Hiawassee we took a very nice 200 mile return trip with no further problems.

Saturday we rode two up on a short 50 mile ride to meet up with a group for lunch. We were much lighter than the mountain trip and on flat roads, but on our return home the rear drive was so hot I could not hold my hand on it for long. In fact the drive, wheel and tire were hot to the touch - and the front was not hot at all.

What gives?

Sunday morning early - 120 + miles - one up - rear drive about the same temp as friends R1200RT rear drive.

Comments?

Lee Nowell
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post #2 of 7 Old Oct 9th, 2006, 10:28 pm
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What was the ambient temperature durning your various rides? The engine case heat is passed back to the transmission, which intern heats the rear swing arm and rear end. The hot cat unit, just ahead of the rear tire, heats the air flowing over the rear end as well. On a 90 degree day, the rear wheel and tire are hot to the touch after time at speed. A lot more heat gets dissipated to the atmosphere on a 60 degree day.

Also, how many miles since the last time the rear drive lube was serviced. Did your check for goodness involve spinning the wheel to check for spalled balls? Might be a good time to check the drain plug magnet for metal, if you can't blame overfill of the rear and hot weather for fluid out the vent port!

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post #3 of 7 Old Oct 9th, 2006, 10:50 pm
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Quote:
Originally Posted by wilbar00c
What was the ambient temperature durning your various rides? The engine case heat is passed back to the transmission, which intern heats the rear swing arm and rear end. The hot cat unit, just ahead of the rear tire, heats the air flowing over the rear end as well. On a 90 degree day, the rear wheel and tire are hot to the touch after time at speed. A lot more heat gets dissipated to the atmosphere on a 60 degree day.

Also, how many miles since the last time the rear drive lube was serviced. Did your check for goodness involve spinning the wheel to check for spalled balls? Might be a good time to check the drain plug magnet for metal, if you can't blame overfill of the rear and hot weather for fluid out the vent port!
It doesn't take much overfill or atmospheric pressure change to draw a bit of oil out of the vent.

The heat in the final drive is not from the engine or cat, it's from the drive itself. The power transfer is not 100% efficient (98%? 97%? don't know). The lost energy becomes heat. The tire may be picking up some heat from the cat, but most is generated from flexing -- more pronounced when two-up, due to both the weight and the extra power used to move the bike.

Mark Neblett
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post #4 of 7 Old Oct 10th, 2006, 8:59 am Thread Starter
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Weather was very nice - never made it to 80 at any time during the noted rides. It was in the upper 50's when the lube leak appeared.

My friend Jim (in GA) called and said that I might have a hung brake piston. He said the same thing happened to him and heated the rear drive of a RT to the point of being able to smell it.

I spun the wheel several times and felt nothing wrong. Wheel is tight and turns easily - except at one spot - and that seems to be rotor related. I did not get a chance to pull the rear pads to check it - but I think that is the problem. Gear lube (all) is changed every other oil change with is about every 6 - 7000 miles. I checked the lube when I got back and it was at the fill hole and absolutely no metal of any kind of the drain magnet. Always have junk on the transmission magnet.

I have planned to get the replacement rotor just to stop the cow bell but it looks like it may fix something else.
Does this sound valid?

Thanks for the reply.

Lee Nowell
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post #5 of 7 Old Oct 10th, 2006, 12:30 pm
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Exclamation Heat Transfer

Quote:
Originally Posted by mneblett
The heat in the final drive is not from the engine or cat, it's from the drive itself. The power transfer is not 100% efficient (98%? 97%? don't know). The lost energy becomes heat. The tire may be picking up some heat from the cat, but most is generated from flexing -- more pronounced when two-up, due to both the weight and the extra power used to move the bike.
When I get a chance some morning, I'll run the engine, with with a fan blowing rearward, in front of the bike, in the driveway, to see if the rear drive retains it's "cool". I would not have guessed that the ring and pinion would generate all that heat to the rear wheel with the 180F block and hotter exhaust system just ahead, while the front wheel feels cold.

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post #6 of 7 Old Oct 10th, 2006, 12:57 pm
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Quote:
Originally Posted by wilbar00c
When I get a chance some morning, I'll run the engine, with with a fan blowing rearward, in front of the bike, in the driveway, to see if the rear drive retains it's "cool". I would not have guessed that the ring and pinion would generate all that heat to the rear wheel with the 180F block and hotter exhaust system just ahead, while the front wheel feels cold.
Yep -- hard to believe at first blush, but if you do back-of-the-envelope heat transfer calcs, you'll see there's enough waste heat deposited in the final drive to get it up to a steady-state temp in the 140-160F (or more) range, depending on load.

As far as the engine/cat not being the source, think of it this way: if they were the primary source, *everything* in the lower back half of the bike would be getting very,very hot, not just the final drive and tire -- and the things between the engine/cat and the final drive (i.e., closer to the engine) would be even hotter than the drive.

The fact is they're not, because the engine and cat heat is not being deposited there in any great quantity -- by far the greatest amount of heat rejection from the engine is out the sides of the bike from the radiators, and out the end of the exhaust. And while the cat gets hot, it's not rejecting enough heat, at a high enough rate, to significantly raise the temp of all the mass in the tire/wheel, at least not before the wheel itself can reject the heat over its large surface area. I'll also note that the final drive is on the wrong side of the tire relative to the cat, which is toward the left side of the bike; all this adds up to it being very unlikely to even contribute significant heat to the drive, let alone be a primary source.

HTH!

Mark Neblett
Fairfax, VA
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post #7 of 7 Old Oct 13th, 2006, 4:38 pm Thread Starter
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Rotor

Spun the rear wheel and found it binding in one spot a little so I pulled the pads off and no more problem. Rear wheel turns very smoothly. The rear rotor has so much play over about 270% of the rotation I guess that I must have picked up a tree limb or something and caused it to rub the pads too much. Anyway, I called and ordered the EBC replacement and the nice lady said she would send one out when they came off backorder. Thanks for all the previous information. This should fix two problems - I hope.

Lee Nowell
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