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Timing Belt Parallelism

Discussion in 'Technical Q&A' started by troy_wood, Feb 1, 2009.

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  1. troy_wood

    troy_wood Formula 3

    Apr 28, 2007
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    Troy Wood
    #1 troy_wood, Feb 1, 2009
    Last edited by a moderator: Sep 7, 2017
    I've seen FBB use the term 'Parallelism' to refer to how true a timing belt runs through its phase around all the bearing held components. Having recently just installed a new belt I thought I'd investigate to see how parallel mine was running.

    After rotating the crank 30-40 times to seat the belt I brought the Number 1 cylinder to TDC. Here are what my images look like:

    Image 1 : 1-4 bank
    Image 2 : 5-8 bank

    After rotating the crank 6 more times:

    Image 3 : 1-4 bank
    Image 4 : 5-8 bank

    You can see a variance (belt-to-cam cog leading edge) of about 1.6mm between the 2 banks. After 6 cycles of the crank the relationship difference changes sides. The final 2 images show this observable difference:

    Image 5 : 1-4 bank
    Image 6 : 5-8 bank

    I realize once the belt is actually running it will seat itself but I am curious if this 'wondering' belt is okay. Maybe this doesn't even refer to the concept of parallelism...
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  2. smg2

    smg2 F1 World Champ
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    #2 smg2, Feb 1, 2009
    all belts walk it is normal, that's why there are flanges on the pulleys. some styles use a crown on the pulley and the belt will center itself to the crown, but not for timing belts.
     
  3. troy_wood

    troy_wood Formula 3

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    #3 troy_wood, Feb 1, 2009
    Last edited: Feb 1, 2009
    I was hoping this is normal. The reason I am concearned is because our 348's have pulley-fence separation issues which have been known to destroy belts. I am a bit suspicious that the fence separation is being caused by belts that 'ride' these fences.

    Its also interesting to note that not all Ferrari's have pulley fences. I wonder if belt 'walking' or 'wondering' is more pronounced on the cars that have the fences versus the ones that don't?
     
  4. FandLcars

    FandLcars F1 Rookie

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    #4 FandLcars, Feb 2, 2009
    I'm thinking the same thing, Troy. Seems if the belts wander, they should not stay on one fence for long and certainly not push on it with much force. Otherwise would seem to be a gear alignment problem, bearing wear problem, or questionable belt quality (if that's possible). So it seems the fence is just a safety factor if everything else is Ok. I didn't rotate mine a lot during install, but enough that it seems these do wander a bit. If the belt stays against a fence, I think there's probably an alignment problem of some sort.
     
  5. windsock

    windsock Formula 3
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    #5 windsock, Feb 3, 2009
    All timing belt or cogged belt drives have at least one pulley with a fence. Whether more than one pully has a fence is generally determined by the length of belt and the run. Longer belts require multiple pulleys with fences.
    Many cars use the crank pulley as the out fence.
     
  6. troy_wood

    troy_wood Formula 3

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    #6 troy_wood, Feb 4, 2009
    Really interesting. This explains the heavily fenced 348 engine - a long, single (versus double) belt that runs around many bearing held components. 348 owners probably should devout a bit more attention to their timing systems.

    Every component that belt runs over has a bearing - when bearings start to wear, belts start to wander stressing pulley fences. When fences fail, belts fail which leads to engine failier. I don't know whether this is correct or not but it seems logical.
     

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