heinrich

Yes.i agree.

Dave928S

Heinrich ... I added a bit more after you quoted my post (in bold) ... "They both have a gap each side, and the gap is necessary for entry and departure of the belt from the pulley because the centre to centre distance of the extreme nose of the teeth is slightly more when the belt is flat on exit and entry, than when its wrapped around the circumference of the pulley".

I added that to highlight that you need the gap to allow belt exit and entry as it changes from flat to conforming to the pulley. No gap would chew the teeth and belt badly ... tight is not always good

Lizard928

Dave,
If the hole was wider, but maintained the same slope on the exit it would not cause much more additional strain. But if the tooth is only contacting at the bottom the stresses on leading edge of the tooth will be substantially higher.

To visualize this picture a T shape, now picture putting the force on the vertical leg parallel the the horizontal leg, with an even force over the leg it will tolerate vast forces. Lets say this was a flat bar weighting 100lbs over 1" at all times remaining perpendicular to the horizontal bar this will be equally distributed over the entire leg, and it will have minimal deflection. If it does deflect, more weight will be transferred to the point where the vertical bar joins the horizontal bar.

Then picture putting force on vertical leg at a 30 deg angle to the horizontal bar, this will result in all the force being put on the furthest point from where the vertical and horizontal bars join. This means all 100lbs is focused in a much smaller point, and it is at the furthest point of strength. This means that the vertical bar will be far more prone to bending to the 30 deg angle where the load will be finally transferred to the joint where the strength is.

With the belt the teeth will flex, but if they have to flex a great deal each time, it will cause a stress fracture on the leading edge from the constant bending. This will be outside the original parameters of the original design and would require a study to determine appropriate belt life expectancy.
The original design prevents pretty much all flexing of the teeth, and every factory belt that I have seen fail due to age/mileage has been the teeth separating at the leading edge, this is also what I base my wear on the belts off of.

I hope that is a little clearer.

jon928se

Colin

I think that Dave understood all that anyway.

Both of you have valid points.

Opening out the tooth profile on either the belt or the pulley will reduce noise and reduce wear on either the sides of the belt tooth near the point of the tooth or near the sides of the tooth on the pulley.

The above will also result in more bending being put on the individual tooth (belt and pulley but pulley doesn't matter because it's much stronger)

What we need to see is pics of the Porsche .04 pulley compared to the .01

I went and had a look at the two old TBs I have that have been running on 100k+ mile old ( original 88and 90) pulleys - Neither belt showed uneven wear on the teeth - pretty even all over that suggests that the original belt tooth profile was matched to the pulleys.

Dave928S

Colin .... I understand what you're illustrating with the analogies you've put, but what I was saying was that with the different pulley profiles, the belts appear to be bedded into the valley of the pulley to the same depth (below the line), and unsupported the same distance in the top of the valley (above the line). I think you're saying they're different?? You can see more unsupported top section in the top example, as you've placed the red belt section, but I think if you bedded it down hard into the valley the contact would be about the same as the bottom example.

When you apply lateral movement to the belt, if each belt is bedded in to the valley the same, the bending moment on the upper section of the tooth at its junction with the belt should be the same.

It looks to me that they're bedded about the same, but if they're not, then I agree that the bending moment would be greater because the contact point would be further away from the belt, and the applied force at an angle not parallel to the belt.

This would be so much easier with a pen and paper/drawing board .. and we really should be comparing the exact geometry of the profiles if we knew what they were.

I think I'll go and have a small double bourbon now ...

Edit: Just saw your post Jon and went and looked at my old pulleys and belts (16V), and the belt and pulleys on the GTS which need changing soon, and saw reasonably even wear too.

Black Sea RD

Carl,

Thank you for bringing another product to the 928 community to keep these cars on the road.

We also know about the sunk costs in R&D work and it is sometimes frustrating to hear negative comments on your efforts. BTDT.

But there are others who do appreciate your efforts and work to keep these beautiful machines on the road.
And that appreciation is what makes it all worth it in the end.

Best regards,

ROG100

Lots of very good information here when we started this discussion back in November 2010

https://rennlist.com/forums/928-foru...la-update.html

M. Requin

I could be wrong but I am pretty sure they would have chosen it for its surface hardness. Heat-treated, it's just about the hardest of all aluminum alloys, although 2024 comes close. And die-cast- not likely (I don't think it can be), forged or machined from billet would be the usual. And the coating on 7075 is as much for corrosion protection as surface hardness for most applications. And forget welding it, break it, it's gone. I got familiar with this stuff using it to make a certain kind of musical instrument, FWIW.

Mike Frye

I'd like to second this. I've been involved in startups and know that it's a really tough balance, especially when it's basically all on spec. Until the product is proven you don't know if you've forgotten something or even if the market will buy into the project.

Carl doesn't just make this crap up in a vacuum and turn it loose on the public. He races his 928. He designs the mods for that purpose and then uses them in his own stuff. The fact that he is willing to make those products available to the rest of us is a great asset and yet there are some on here who look for his posts and start looking for everything from a typo to something that just looks different from the norm.

Seriously.

A discussion of the merits and possible flaws is valid and warranted, but some clearly have an agenda and it's kind of childish.

In the end we're going to see whether it's a good product or not because undoubtedly Carl will be using these in his race car and it will get as much testing in a wide variety of conditions as any of our cars and far more than most of them (no plans to race my car to the top of Pike's Peak or go for a land speed record in my immediate future, how about you?).

Thanks for all of your design work Carl. I'm looking forward to the results of your 'real world' use of these parts.

Fabio421

Just a quick mention for those planning on re-annodizing their old gears. There was a thread here on Rennlist back around 2005 or so where someone had sent their gears out for re-annodizing. The electrolosis destroyed the gears. They looked as if they had been eaten away by acid. MAJOR parts of the gear were gone. I'll try to find the thread and link it here. I believe the consensus was that maybe the gear wasn't 100% aluminum.

Here it is <-----

M. Requin

I would like to add one more point. The discussion of the profile of the cam sprockets is interesting, but (and I mean this in a technical way) naive. If you take a look at gear design, or better, chain and sprocket design, the tooth profile is not necessarily intuitive. Some interesting mechanical analysis goes into the relative motion of those mating surfaces. For example, the slightly wider profile of the top of the sprocket opening may be there to prevent the belt from flexing in the wrong way as is enters/leaves the sprocket. I am not saying that is correct, just giving an illustration of how a completely 180 degree interpretation can be made from the same facts. Roller chain looks like it ought to jump off the sprocket in some apps, if you look at it that way, but it is designed like that for a reason. I used to have some good discussions of this with engineers at Morse Chain when I lived in Ithaca, NY. They were developing product for Honda at the time, and I was the local Honda MC dealer. Got to meet some of the Japanese engineers, too. Interesting times.

And I'll my appreciation of Carl's work, too, and of all the others who contribute to, as Mr Merlin says, keeping 'em flying.

Carl Fausett

Yes I am certain I worked off the last/latest superceded part number from Porsche. Again, I called my manufacturer to have the laser profiling company send back my sample. I will post pics of it with belt wrap next to mine when it arrives.

Cosmo - as to price. I already beat Porsche's price by almost $50 and am hearing "not good enough". And Roger's talking about a "group buy" to get maybe $370 retail for the OEM part. Don't know if that will ever come to pass.

So why don't I let a few of these out at $320 each so you guys can fit them, test them, play with them, and report back?

That's my Rennlist offer: if you want to try one or more, they are $320 each. I'll hold that price for the first 10 I sell.

Carl Fausett

As am I. Thats why I'm going to send out 10 at $320 so that we can get some drive time on them. I have already fitted them to camshafts here and I know they are perfect. It's a brand new part and Io dont have an engine running with them yet. Let's get that fixed.

heinrich

To be very honest with you Carl, my tack on it is that for me to spend $600 - $800 on nothing but two little rollers in an old engine, it would have to come with a warrantee that says "If my product ever wears below 1/zzz mm, I will replace it free of charge".

If not, and the Porsche part is available for the same price, then IMHO we are dealing with unknowns in both cases.

Even more to the point: If the original gears, which let's face it were designed to be a wear item, a consumptible to be replaced with your next belt change, were available still today, I wager that 99% of all owners would buy those at 1/5th the price of anything available today.

Carl Fausett

I completely agree. A quick peak into the Bosch Automotive Handbook in the belt and sprocket section will make your head swim on loads and vectors of belt drive units. I would not presume to engineer this sprocket and the tooth profile. Thats why I bought a brand new one and had it sent out for laser profiling. So there would be no question that we got the tooth shape and crown profiles dead-on.