H2

Gentlemen,
What's wrong with the idea of putting thin aluminum or stainless "belt guards" on the 928 cam gears so the belt would hit that and stop before slipping off and trashing the engine? Something kinda shaped like a saucer or plate with a hole in the middle so it could be bolted onto the cam gear? The 968 has this on one of its belt gears. Seems like a good idea to me. Must be a reason why no one hasn't done this. I'd buy a set. What about you engineer/mechanical whiz types?

Ideas on this? What do you think?

Harvey
32V Paranoid

Rod Underwood

I'll expose my ignorance here, but is this the most common mode of failure - slipping off and then breaking? If it is then this would make a lot of sense. If age accompanied by simple tensile failure is the primary mode of failure, then this wouldn't help. I guess any reduction in failure rate would be a godsend, even if only for an unknown percentage of disasters.

Rod

j.kenzie@sbcglobal.net

Harvey,
Brings up the real question of how and why do TB's fail. With all our members we could put together some kind of cross-section or failure analysis. I thought belt breakage or as Ron said tensile failure was most common, probably due to age, heat, etc.
Ron,
Please explain "shark fin antenna" in your signature. I've always thought they look great and wondered if they are AM, FM or cellular.
Sorry to distract from TB theme.
Dave

Garth S

My limited understanding of TB dynamics says that if all stressed rotating elements maintain their axis of rotation absolutely perpendicular to the center plane of TB travel, the belt will track true forever and a day ..... until it dies of old age.
The things that will knock it off course are asymmetric gear wear, water pump bearing collapse, worn pivot bolt/stud carrier arm bushings, bent bolt/stud, badly worn tensioner roller, and excessive TB age. Excessive tension and dry tensioner reservoirs can also fit in there.
Most of these conditions lead to low TB tension alarms prior to failure .... must reflect as to whether the TB slipping off the cam drive cogs is a primary cause of failure ... or an ultimate consequence ... like "too late to close the barn doors once the horses are out".

SteveG

A flange is an interesting thought. When I purchased my shark the date on the TB was unknown, so I dove in. When I removed the covers, in the passenger side cover nested in the bottom were black shavings, resembling a cotton ball in size and texture, could only have come from the belt. The belt was OK, but IIRC it had a 1/16 inch notch in the side of the teeth along a 1 or two foot portion of the belt where it had rubbed the cover, seemed to be an old wound, but it seemed not to be near death. but the cotton ball really bothered me. I replaced the TB and check it every 6 months. I believe it had the proper tooth patter as other threads say an improper one results in wearing of the cam gear as well and they are not worn; possibly an inferior belt. The new one tracks straight. It is a mystery. The shop had cut the tension ground connection -- too lazy to install it properly or buy a new one? They had evidently assured the PO that he didn't need a warning system, as he told me the belt was replaced and the light comes on, but not to worry. I probably shouldn't have driven the car the 3 hours to its new home. If ever there was a case where the tensioner was needed this would be one. So would the added ounces spinning on the end of the cam gear pose any problem? Jaeger, if you build it, I will come, uh, I will be there.

Imo000

This is a bad idea!

There already is a guide like that. It’s on the water pump pulley. The only reason a T-belt would creep off the gears is misalignment of one or more than one component. By misalignment I mean that some rotating parts are not running parallel to each other. This can be caused by worn out bearings, bushings or bent bolts. If you install a guard on the gears the T-belt will still move to the side and the guard will start to wear it out. This will eventually cause the t-belt to be worn off and break. The only real way of stopping the T-belt from migrating is to make absolutely sure that everything is running parallel. Most common part to cause a misalignment is either the water pump bearing , the tensioner roller or tensioner arm bushings.

WallyP

Most common belt failure seems to be shedding teeth...

Rod Underwood

I found a company who just started selling in the US that sells all sorts of shark fin antennas from am/fm to gps. http://skyyear.com/

I purchased one of the very first they had available a month ago after their opening in Las Vegas - I think it was a demonstartion model.. They now have later generation of these models available and I should be receiving a new replacement in a few days. My avatar has the current one illustrated and I can send you additional photos if you would like to see them -well that's a stupid recommendation, you can't see the antenna in that image. If you're interested, let me know and I'll send you a few images. rod@rod-underwood.net .

As soon as I receive the new one, I will be clear coating it to match the car color and will post it..

Rod

Dave Howerdel

I disagree. The most common belt failure is lack of maintenance.

I have owned many 928's in the last 20yrs and put alot of miles on them.

I have never has a belt failure. A 45000 mi replacement schedule, proper retensioning, and a monthly visual inspection (oil on the belt, sprocket wear, belt creeping) has shown me that the system works superbly if properly maintained.

99% of failures I have seen on 928's and 944's would have been prevented if the owner took the time to maintain the cars. The other 1% (defective parts, installation error) does not cause me concern as it can minimized by purchasing parts of reputable quality and using a qualified shop or turning your own wrenches.

Many do not do the job right. They scrimp on replacing bolts, nuts, rollers, cam sprockets, rebuilding tensioners. That's why I do a tb job when I purchase a car regardless of whether it has documentation that it's been done. I factor it into the price of car before purchase.

IMHO, belt problems become more prevelant as "unqualified" owners purchase these cars thinking they can move into Porsche ownership with $4000.00 and $50.00 a year on maintenance.

Bill Ball

We've had several failure reported recently that involved the tensioner arm bushing, allowing the tensioner wheel to cant to one side and mess up the belt. A number of people here also have personal experience with seized water pumps destroying the belt. Don Hanson's belt had a clean break in it. Yes, as Dave says, these are almost all maintenance-related in the sense that the owners probably had fair warning. The water pumps may have failed prematurely, but I'm pretty sure there were some sounds of agony coming from the motor well before the belt failure. Don's was probably a defective belt - my guess is that it was creased in storage before installation. Regardless, trying to keep a failing belt on the cam gears is fruitless, although the belt rubbing on the plate might be another new audible warning system.

JEC_31

I agree.

I feel that more effective failure-preventative modifications would involve:

A) Upgrading the bushings, bolts, (and bearings?) that are critical for planar alignment to higher-strength and more wear-resistant materials.

B) A re-designed tensioner with far lower parts count and simpler, more robust design.

C) Timing Chain. Kidding! Just kidding!

The week after Christmas I plan to tear into the timing belt replacement on a '92 Honda Accord. That belt has 140,000 miles on it and hasn't bent my valves yet. I can't wait to steal some ideas from their design.

- Josh

Big Dave

I would imagine that a plate would act like a slicer at high rpm and be able to sheer the side of the belt off just as easily if the belt is tracking off center.

Dave Howerdel

Replacing the parts as should be done when doing the tb makes uprading unnecessary. alot of people/shops replace the belt/WP and call it a day and expect bearings/bushings/previously torqued bolts, etc to last forever.

The tensioner when rebuilt is of more than adequate design. Actually almost an optimum design considering it keeps tension and dampens vibrations.

JEC_31

Good point on the vibration dampening, and of course more frequent replacement solves most durability issues (ask a Ferrari owner!)...

- but I personally can't quite bring myself that close to the word "optimum" with that complex oil-filled bi-metallic washer setup. That thing drove me crazy.

It's method is to apply increasing force according to rising heat in one front corner of the block.
I would rather cut out the middleman and apply increasing force according to the actual growing of the belt.

Dave Howerdel

When you consider the engine expands as it warms up, a rubber belt and a mechanical manually adjusted tensioner with a built in dampener is a perfect design to maximize the longevity of the belt while still maintaining a safety margin when it comes to belt failure.

I cannot think of a better design that would allow for a safer 45000 or better change interval on a 928 motor.

A system that applied increasing force as the belt grew would not be advantageous in this instance due to the following:
1) It may increase the force beyond the "stretch" point of the belt whereas it would fail.
2) It would only be able to compensate so much whereas if the belt did not fail, it would not be tensioned properly.
3) By compensating for the slack belt it would mask other problems in the systems that may be causing poor tension.
4) You take the person out of the equation.

The way it is now, the warning light comes on and
1) Loose connection, false warning
2) Loose belt.
3) Warning circuit inop.
No matter what it is the proper thing is to diagnose and fix it. How is this system flawed?

Even an error on system with built in redundency would have to be diagnosed to see if the error is in the system or the just the warning component.