Can anyone explain this belt phenomenon?

[cogent]

Ok, this is weird and has a few people stumped.

I went up to taupo on the weekend for a trackday. Took the rx7, was fine for the first few laps. Started giving it death on my second outing. When I went for my third outing, went to start the car, massive squeeling noise. Erp.

Popped the bonnet, found the waterpump/alternator belt shredded. Whoops, guess I went a bit too hard.

Called it a day and spent a few hours mucking around locating an appropriate belt to replace the broken one with.

the reccomended belt, is a V Rib belt in a 1365 length. The closest we could get was a 1390. figured we could make up the differance by adjusting the alternator up. We got it close enough that it was still quite loose, but not loose enough to come off. We were happy with this to get us back to the motel at least, so we could order in the proper belt.

Started it up and EEK EKK EKK EKK EEK it was squeeling pretty bad, but wasnt slipping off so we went on our way with everyone staring.

About 100m down the road, the squeeling stopped, and we pulled over assuming the belt had snapped. but no! instead it had tightened up perfectly! All slack was gone and the belt was sitting on the pulleys just right. As if it was meant to be there.

We were able to drive back from taupo to wellington with no problems at all, not even a slight squeek.

Can anyone explain why the belt suddenly contracted to fit correctly, despite being fairly over sized?

[TazzieDevil]

Auto Tensioner? I know its on the cam belt, but could be one on the AC belt also?

I have the odd squeel if going hard or on cold occasions, but normally only on tight corners and its for a split second.

[cogent]

lol..cambelt..

I will also add that in order to get it as 'tight' as we could (or should i say with the least amount of slack, since it wasnt really tight) we had all the adjusters at thier maximum and there are no auto tensioners.

The only thing I can think of is that the belt contracted under temperature.

[2LTR Rona]

The only thing I can think of is that the belt contracted under temperature.

that would be my guess right there, pulley on water pump / alternator doing x thousand rev's per minute would be more than enough to generate the required heat / friction to shrink it.

what was the belt like AFTER you got back to Wgtn? was it still tight & not requiring adjustment or had it backed off a wee bit?

[johndoe1025]

normally belts will stretch. so thats very odd.

[nzae86]

Do you have alloy pulley's fitted, as they expand when hot more than steel ,I have this problem on my race car .

[mr30%jr]

lol..cambelt..

rx7...

sounds like something tightening it, wonder why it shred in the first place.

[Dazed&Confused]

The original belt must have been too small?

[Krupkee]

im sure some belts tighten when heated, depends what they are made from, flatmates b16s cambelt is quite loose cold, normal when hot, got told by a engine place that its quite normal.

[neon_spork]

Unlike most other materials rubber will shrink when as it heats up.

A 'fun' experiemnt at home that demonstrates this is to hang a small weight on the end of a rubber band then heat it with a hair dryer. The
length of the rubber band will decrease and the weight will go up.

[z0ton]

V belts and multi rib belts stretch over time though?

[xsspeed]

Stretching over time is likely to be from fatigue not necessarily only caused by heat.

The issue here is that over a short period of time the belt shrunk, most likely due to having been heated up.

[MAGN1T]

You could always take it back off......& measure it.

Steve

[sleeektoy]

[reason for it shredding]alternators/water pumps/tensioner idlers can seize due to sustained high RPM and heat, then have the bearings release once cooled off. Any noise coming from those now that wasnt there before?

[matt dunn]

Or as I found out, if the bottom alt mount is a bit worn it can put the pulley slightly out of alignment.

I had a new belt last about 20 mins running, and another last less than 5 before they shreadded.
replacing the alt mount fixed mine.

I highly doubt that belts would get tighter with heat as they are designed to remain the same length throughout there lifespan.

But weird things can happen..