Belt flex under throttle

[drifter17a]

Hi all,
I’ve just finished fitting the timing belt on my RB25, following the standard procedure: rotate the crank twice, release the tensioner, then tighten it down. The belt feels tight, and you can see that clearly in the picture below

I’ve heard some people say it’s actually better for the belt to be slightly loose rather than too tight, as excessive tension can lead to snapping. So that’s in the back of my mind.

What I’m seeing is this:
 • At idle, the belt looks fine.
 • But when cranking and especially when revving (around 3–4,000 RPM), there’s a momentary flap/flex or flicker in the belt, which I’ve tried to capture in the video.

So my question is — based on what you see, is this slight belt movement something you’d consider normal, or am I just being OCD?
Could this amount of movement cause sync issues? Or is it just a harmless bit of flex under load?

 

from what I know belt flap and flex is expected when crank spins up and pull cam with it

Would appreciate any thoughts or similar experiences.

 

 

[GTSBoy]

2 hours ago, drifter17a said:

normal

I didn't even bother watching the video.

[drifter17a]

yeah i know some flex is expected as crank pulls on belt but was getting ocd

[GTSBoy]

Flapping up and down is a consequence of the varying pull on the belt. When the engine speed is suddenly increased, the tension in the belt increases too. When that engine accel changes to decel, the tension is reduced and turns up as a little extra length of beltes between the pulling and dragging pulleys. That extra length flaps up, then down.

There's all sorts of other harmonic stuff going on too.

[MBS206]

1 hour ago, GTSBoy said:

Flapping up and down is a consequence of the varying pull on the belt. When the engine speed is suddenly increased, the tension in the belt increases too. When that engine accel changes to decel, the tension is reduced and turns up as a little extra length of beltes between the pulling and dragging pulleys. That extra length flaps up, then down.

There's all sorts of other harmonic stuff going on too.

For others, what GTSBoy states here should be paid attention. Why?

Well lots of people play with different engines, and they LOVE to change things like remove AC, or steering pumps etc, and it lends to them needing to move the tensioner too.

You want your tensioner, particularly those that are sprung or hydraulically tensioned, to be the first thing after the harmonic balancer, or technically the "last" pulley in the chain. By saying last pulley, I mean look at the direction the crank spins when the engine is running, follow the belt from where the crank is pulling the belt FROM, and keep following that until you're between the last pulley/accessory on the belt and about to reach the crank again, this is the spot where you put the tensioner. This is the area that will always end up with slack.

This is worked out exactly the same way for chains too, as the physics is the same for them.

The crank pulley is where all the force to drag the belt around comes from. You will never ever get rid of the slack that appears, especially under load. The tensioners job is to keep the belt loose enough when stationary that there shouldn't be out of sync movement in slow movement, and then be tight enough when running, that the belt can't jump off any gear and get damaged.

Too tight, bad things happen, too loose, bad things happen.

Have a tensioner (mainly sprung/hydraulic one) in the wrong spot, it can't actually do anything about keeping the tension.