Any ideas why a belt would squeak at WOT Accelerations and not in Idle ?

My friend drives an S40 with an automatic tensioner (just tensioner, they don't have idlers there).
Everything on the AUX system is looking good. New alternator, new belt. Everything is moving straight... No noise whatsoever in Idle, or even when doing WOT in Idle. There is only a belt squeak when down shifting to 1st+ WOT or WOT accelerations from a standing start.

Toomuchpowa!!!

Originally Posted by Reno

My friend drives an S40 with an automatic tensioner (just tensioner, they don't have idlers there).
Everything on the AUX system is looking good. New alternator, new belt. Everything is moving straight... No noise whatsoever in Idle, or even when doing WOT in Idle. There is only a belt squeak when down shifting to 1st+ WOT or WOT accelerations from a standing start.

Down shifting, in particular, can cause sudden increase (acceleration) in engine RPM. More so than acceleration. The torque from the crankshaft pully is transmitted to the water pump, alternator and power steering pump by friction. Up to a point, that is. The amount of available friction is dependant on the properties and condition of the belt and pulleys and also the angular velocity of the pulleys (which equates to the linear velocity of the belt) at any point in time. The third factor is the magnitude of the force applied to the belt by the tensioner pulley spring. If your belt is squealing under deceleration this means that the acceleration of the crankshaft pulley is not being transmitted to the accessory pulleys. Instead the belt is skidding over the pulley surfaces. Either the belt is worn, affecting the available friction between belt and pullies, or the tensioner spring has weakened. This will inevitably increase wear and bring on the possibility of failure through snapping which can have expensive consequences.

Good explaination Ray, I didn't understand it but it's still good...
Now let's give it five minutes for some of our good friends over the water to suggest replacing every moving part possible, just in case... Right?

Basically when you increase engine RPM, the pulleys require more torque to spin. The belt will slip if you break the maximum static friction force. Anyway, when shifting that hard in first there is probably a large amount of jerk applied to the pulleys. And I mean that in this sense:

Speed is the rate at which you are changing position.
Acceleration is the rate and which you are changing speed.
Jerk is the rate at which you are changing acceleration.

BTW, you can't go "WOT in Idle". Idle is minimum throttle.

Originally Posted by gusmagoo

Good explaination Ray, I didn't understand it but it's still good...
Now let's give it five minutes for some of our good friends over the water to suggest replacing every moving part possible, just in case... Right?

Yes. That seems to be the norm. Personally, I don't know how you can test the voracity of teh tensiioner. In theory you might be able to hook a spring balance onto it and measure the force required to move it over an inch, for example. But that would be too big a pain in the arse. A new tensioner is going to cost you about £40 and a new serpentine belt about £25 maybe less. You might as well change the AC belt while you are there because you have to take it off anyway. You may as well stick a new one on. That's about £15.

Originally Posted by WDE46

Basically when you increase engine RPM, the pulleys require more torque to spin. The belt will slip if you break the maximum static friction force. Anyway, when shifting that hard in first there is probably a large amount of jerk applied to the pulleys. And I mean that in this sense:

Speed is the rate at which you are changing position.
Acceleration is the rate and which you are changing speed.
Jerk is the rate at which you are changing acceleration.

BTW, you can't go "WOT in Idle". Idle is minimum throttle.

Its how that old trick of pulling the table cloth from the table, leaving the plates in place, works.

from my friend

Aha, thanks for the good explanations ! So to recap, it must be a matter of tensioner/belt mechanism ? So it can "be lazy" even when it's looking good in Idle state ? Before I order a new one, Timing belts don't just squeak squeak, right ? The engine would stop before it would actually squeak.. I assume.

Originally Posted by Reno

from my friend

Aha, thanks for the good explanations ! So to recap, it must be a matter of tensioner/belt mechanism ? So it can "be lazy" even when it's looking good in Idle state ? Before I order a new one, Timing belts don't just squeak squeak, right ? The engine would stop before it would actually squeak.. I assume.

Timing belts are toothed so they don't squeak, they jump, and then you bend a valve.