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Ducati Belt tensions, belt life and practical belt maintenance issues

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VinceS:
Some time ago I discovered the Spectroid app, available in both Android and iOS. Makes adjusting the tension of a drive belt to 110Hz +/-5Hz a piece of cake. Just pluck the longest bit of belt and read it on your phone!

Figured I should mention this technique as the earlier way was a bit of stuffing around, and worked almost exactly the same. Anyone can use the app, it is easy. Just run and pluck away...

VinceS:
I was just looking for something else and came across my partly written up story on "part b" of the belt life issue from 2006, and figured I should put it "out there" for the benefit of anybody else that wants to tinker in this area. "Part a" was about how to do the belt tensioning adjustment job properly. "Part b" is about how long the suckers will last if treated well. I had been looking for a credible answer to this latter part as I was doing 20,000+km / year and getting heartily sick of chucking away visually good looking belts. As a result of my various meanderings at the time I have moved to inspecting / adjusting them every 5-7,000km and chucking them away around 50,000km or 4 years old max when they still look pretty good but the risk of a serious problem is definitely increasing!

"Part a" was duly recorded on a fine web site st4s.com which suddenly went poof when gawd knows what happened to the Victorian based owner, so all the info vanished, except I have my original files from that earlier write-up. I remember the free software used was Goldwave (try http://download.cnet.com/GoldWave/3000-2170_4-10001099.html). When you get the hang of it plucking the belt (on the longest section) into a digital dictaphone in my case and playing back on the laptop in the shed was barely 30 secs from pluck to knowing the frequency ready for another adjust and check, so pretty easy when you have it sussed. The frequency shows on the right graph, one is enlarged below to show this, ie where the biggest bump is shows the major harmonic and it is very repeatable. If you have half a clue what you are looking at you will soon make sense of this, I may write it up properly in time, and I may not! Please note the attached files are just what I have, not saying they show the belts properly adjusted just that they demonstrate the process. The correct numbers are in the text below!

When you "get" how to do this with harmonics you will come to realise what a rough technique the standard backyard "allen key thickness" approach is, or the original official Ducati belt push tension meter is for that matter, hence the reason for changing it! there is a relationship between correct tension and life, but you need to explore that for yourself as it is an emotional topic for many and I ain't getting into it!

My earlier (4+ year old) jottings:
OK, I have had a bit of back and forth on this topic and it has yielded some useful information in amongst the miscellaneous stuff. I have pasted extracts of it together in a chronological Q & A format below to capture the more useful bits (and not to duplicate what I've posted further down), my comments added [in square brackets]. I had made it plain these were public questions on behalf of myself and others, so there should be no problem passing the info on:

Q. I originally sent this enquiry to Gates Europe on 14/02/06 (since I understand the belts are manufactured there). They passed it to Melbourne and somebody (Peter?) rang me on 17/02/06 to explain "the situation with Ducati belts". Unfortunately an important customer had me on the mobile ph at the time and I couldn't take the call. The gentleman said he would ring back on Monday 20/02/06 but I have received no further communication [and I knew anybody who was going to explain that wasn't going to provide useful info so I didn't try and chase them up]

A. Thank you for your request for information on the Ducati cam drives. I hope I can help you. [Keith Ashworth, Industrial Business Unit, OE Account Mgr Italy, Tel/Fax: +39 011 62 77 868, Mobile: +39 335 63 72 891]

Q. Ducati's latest info suggests setting belts up to 110Hz +/-5Hz on the 200mm span leg section, but they have no temperature compensation info and it is plainly an issue for actual tensions, can you advise what may be applicable?

A. As you are well aware the all aluminium engine expands when it is warm compared to its ?cold? condition. Since the drive belts have a glass fibre tensile member which extends much less than the aluminium over the same temperature change, the "hot" tension is much higher than the "cold" tension - maybe 3-4 times as much. The installation tension recommended by Ducati accounts for this thermal expansion so that the belt operates correctly when hot, whilst avoiding jumping of teeth which could cause valve collision when the engine starts from cold. [aha, language / crappy q issue!]

Q. Also in looking at a lot of the detailed info posted on the various country [Gates] websites I have become quite curious whether there is an option to adopt a GT2 style belt to increase the life of the base equipment. [a typo caused a non-answer]

Q. Can you give us an insight to what we can do to get better than 20,000km life out of these belts?

A. The 20,000km exchange period is based on test data and many years of experience. It allows for use in different road and ambiental conditions. If you do not want to change the belts at 20,000km then you should carefully examine them to make sure there is no wear in the area between the teeth and that there are no cracks at the tooth roots on either the driven or the driver side. A periodical visual check could allow you to change the belts less frequently but this is obviously more risky, and Gates cannot accept any responsibility for injury or damages caused by this.

Q. When adjusting the tension of a drive belt to a nominated frequency (110Hz +/-5Hz for the Ducati timing belt) what allowance should be made for the ambient temperature? We are looking for a guide as to how much to vary the target frequency with variation in ambient temperature while adjusting. We find ourselves making these adjustments in ambient temperatures anywhere from 9 degrees C to 35 degrees C and it appears obvious that the ambient temperature at the time of adjustment will make a significant difference to the all important operating temperature tensions.

A. If the ambient temperature is low (ca. 9deg C) use the minimum tolerance ie 105Hz, whereas if the ambient temp is high, use the upper tolerance ie 115Hz. [well ok then, makes sense!]

Q. We recognise and accept all the proper methodologies required to determine the recommended 20,000km replacement interval. We are suspecting that this recommended interval has become ultra conservative over time as the belts themselves have improved in construction (eg with the use of kevlar) but the recommended intervals have remained the same. Hence we are seeking either a better guide as to what is realistic to expect with modern construction belts as used on small diameter pulleys as in the Ducati engines, on both an absolute quantity of revolutions basis and a total time since the belt itself was manufactured and/or placed into service . Alternatively a clearer condition based maintenance approach and an idea of the inspection intervals that are appropriate given the age of the belt and the condition it was in at last service. The thing is we are routinely discarding belts after 20,000km or so and close inspection reveals no apparent defects of the type you described - the old belts seem to be slightly more flexible than new ones but I would be surprised if the very slight reduction in force required to bend an old belt is representative of a loss of internal element bonding or reinforcement filament fractures to an extent that failure is a remote possibility.

A. The belt construction is chosen during the development period of the engine and is frozen with production start up. The construction is only ever changed if there is an engine upgrade or if the belt change period is altered. This means that the belts which OE manufacturers fit to the serial engines are the most appropriate ones (not necessarily the latest ones or the best ones). Often the change period is not varied with the introduction of a superior belt construction, simply due to the fact that the belt works in more severe conditions - higher temperatures and/or higher dynamic loads/vibrations. Please consider the fact that the belts appear to be in good condition after 20,000km the result of an excellent job done by Ducati and Gates to safeguard the engine and rider (engine seizure resulting from valve collision can put the rider?s safety at risk as I am sure you are well aware).[hard to sort out engineering speak from marketing speak here]

Q. In searching for information on the currently installed PowerGrip HTD timing belts on the Gates website I found many references to it being superceded with the GT2 type, eg http://www.gates.com/part_interchange/htdCrossover.cfm?location_id=3316. To quote "The performance far exceeds HTD and trapezoidal belt capabilities, making PowerGrip GT2 belts the choice for accurate registration, heavy loads and small sprockets. Changeover your current HTD belt to PowerGrip GT2 and see for yourself. No sprocket change is needed for existing HTD type drive systems." This sounds like a pretty attractive idea to me, is it feasible to do for Ducati engine belts as per the details in the original query?

 A. The HTD belts you have seen on our web site are industrial belts made from a CR base compound. They are available in 3,5,8, and14mm pitches. The GT2 belts are again CR based but with a fiber reinforced compound, abrasion resistant jackets, high flex resistant cords and a slightly modified profile. They run in the metric based HTD pulleys. Last year Gates released the new GT3 range of industrial belts again based on CR which outperform the GT2?s. The Ducati belts are based on a very expensive HNBR aramid fiber loaded compound (sometimes the compound is also called HSN) designed to run at very high temperatures, where the CR belts would crack very early on. They have a 9.525mm pitch. I?m sure you will now appreciate that the two belt types are not compatible. [well I am very pleased to hear that Ducati engineers have got their act together, although i remain slightly puzzled how much we have "up our sleeves" in the all important belt life question]

So, I need to write back and thank this fellow. Is there any more q's anybody would especially like to ask? I would note that I am suspecting that we have got about as much as we are reasonably going to get and I am thankful that it was as much as it was.

Like anything you read on the net what you do with it is your business, not mine. I am sure that there are some important attention to detail issues around this topic, so what happens for me may not be what happens for you, please don't go thinking anything you do is any one else's concern but your own!

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