How the Timing Belt Tensioner Actually Moves

[j2k4]

Agreed with Larry - the crank is the driver, so it only stands to reason any slack - due to stretch or not - would manifest immediately after the crank pulley where the tensioner is.

[sergechronos]

I would've assumed as much, except aren't idler pulleys usually for slack side in applications? Yet our slack side is supposed to be the side with the tensioner (and tensioner pulley) and our idler pulley is on the opposite side of the crank cog.

If you're correct, then the only thing I could think is not so much the slack itself but vibrations causing long runs of the belt to potentially whip laterally off and potentially come at least partially off the cam cogs (which would likely lead to skipped timing or a shredded belt.) While maybe on a steady increase in RPM or a slow increase it doesn't matter much, but I would be curious about fluctuating loads (like it would see with actual driving and shifting) possibly inducing far greater vibration than the roughly 10 mil that we could see on the indicator just from the steady climb. Not sure how accurately it could be emulated with this test rig though.

The increased rigidity of the kevlar belt may reduce that possibility (heck, might even reduce the vibrations) so that it is less of a concern. Although, of course, just pure speculation on my part until I get the belt ordered and shipped to Maddog and it gets tested. Curious why the hydraulic on the DOHC and the spring on the SOHC. Are we seeing that much more slack/vibration/whatever than the SOHC?

[j2k4]

I'd be interested to find answers to both the Kevlar and SOHC questions...

[J. Fast]

J Fast if the belt isn't stretching what do you think is causing the movement? I'm open to ideas.

The engine and valvetrain are constantly accelerating and decelerating. If one were to draw a belt force diagram between each gear in the timing assembly ie. timing cog, cam gears, water pump, tensioner etc. and also take into consideration the belt spans in 720° of crank rotation at no place on the belt in between cogs is the tension and compression equal. Due to this phenomena there will always be dynamic slack or run-out. As the magnitude of the engine throw increases so does the differential of tension and compression which is transmitted on the tangental plane of the gears as acceleration and deceleration.

In plain English... as the engine throw magnitude increases the belt is being pushed faster than it's being pulled. The hydraulic tensioners job is to ensure the slack is maintained through dynamic run-out.

On another note, I do not believe a Kevlar belt will change the dynamic run-out. If anything at all I think the slack run-out will be equal, but the rate of slack pickup will be faster due to a higher belt spring rate. Will be interesting to see the outcome... Also makes me wonder if the tensioner pulley was elliptical if the slack would be the same... Mehh. That's just my brain throwing ideas out there.

Cool thread. Haven't had one of those around here for awhile...

[sergechronos]

Given that the timing belts use fiberglass reinforcing cords, I'm not sure how much they might actually stretch as they age (and 30 mils worth of change as everything heats up isn't exactly super significant given the length of the belt as a whole.)

This leads me to believe, and correct me if I'm wrong (as it often happens), but I'm not sure now that the t-belt changes are due to stretch, but are instead due to the aging of the rubber allowing it to become more brittle. Possibility of teeth becoming damaged as it ages (especially exposed to the heat it sees) likely increases quickly, or even just brittleness of the rubber as a whole, might make it hard to keep contoured properly to the cogs under high load situations? Throw in possibility for the tensioner to weaken as it ages, and lower tension perhaps leading to damaging the brittle teeth as well from poor engagement?

I believe that the SOHC spring is just used when setting the initial tension and then it basically becomes a fixed tensioner?

Update: Solid tensioner and kevlar belt order placed with shipping to Maddog for testing, so we should have some more information in the next little bit.

[futurevr4man]

I hope one of the old fogies chimes in soon, but I'm quite sure that I have seen a comparison pic of an old belt and a new belt, and the old used one was noticeably longer. Like inches longer. But I may be totally off my rocker on that as well.

As far as brittleness, yeah it happens, but it seems like you're comparing rubber that you run into every day with the rubber a tbelt is made of. The tbelt rubber is much more resistant to heat stress, and wearing as well. But I do agree that it gets more brittle over time, it's one of the main reasons you change it as preventative maintenance (and of course because we have interference engines).

I would like someone to confirm or deny if the belt stretches. I guess I could google it but I'm on my phone...

I am curious if the tensioner weakens over time. If so, how long? Do temps affect it... dramatically even? How much would it weaken? The way that tensioner is made, it doesn't push out with much force, but it resists moving inward very hard.

[mb3000]

.....................

Just as a note, synchronous belts with fiberglass tensile cords do not stretch more than .001-.003" over the lifetime of the belt. All automotive synchronous belts, from the stock Mitsubishi belt, to the Gates aftermarket belt, to the Gates racing belt all have fiberglass tensile members, and will not stretch. The kevlar portion of the racing belt comes from the kevlar fibers they put into the rubber compound to strengthen the teeth. It makes the teeth stiffer and stronger and less prone to sheering off. Being an engineer at Gates, I don't see any reasons why a Gates Racing Belt would require more tension than the standard belt.

Also, I would almost always advise against using a solid tensioner. It's not because the belt stretches, but as the engine heats and cools, the geometry of the drive system changes, and the forces on the belt change. By not having something to take up this slack and differences in changes, it may adversely affect the belt.

[familyMAN]

^Thanks! Was trying to find that post but couldn't.

Question, even if belt can't stretch (much) can't the teeth give and that end up creating slack?

[KeithMac]

Would be interesting to see the stock belt at 2500 rpm while it's on the test rig?.

I and a few others have strange fueling issues at 2500 so it would be great to see if there were any odd harmonics going on with the belt..

In your vid the belt threw (sp) a "wobbler" but couldn't work out what rpm it was..

[J. Fast]

Question, even if belt can't stretch (much) can't the teeth give and that end up creating slack?

Seriously... I thought you were smarter than this? This subject is way over your head Engler.