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Ducati L twin belt tension ????


Border raider.
Feb 8, 2022
Member Number
England Scotish border
Talking to a friend today on the phone, he has a 2006 ish ducati ST3 , and it was on the original belt he got the bike new.
He was telling me the story of the job, and researched it as best he could online.
The Belt tension from the sounds of it is crucial, and several online ideas from guitar tunning aps on phones and others he tried and settled on the only idea that he felt worked and that was the old twist the belt 45 degrees and its about right. IDEA.
Well its seemingly all good running great and back together 3k miles in and counting, and no issues apart from the nagging doubt associated with the alleged intricasies of the Ducati belt tension crucial nature.
My advice was strip bodywork off and give it a look see if everything looks ok with no obvious signs of strange wear or potential threats. But Equally i am thinking would it be that obvious and indeed is the Ducati L twin belt tension deal over hyped or not.?
So the point of all this war & peace saga, Well peace of mind from Any of you ducati Knowledgeable on this Job. is He all good as is or any advice to get the job done, the likes of the guitar tuning app deal just seemed a bit non event from what he could tell.
Thoughts opinions advice please.
There are tons of threads on this subject on the Ducati.MS forum. Your friend could spend weeks reading opinions, many from expert and reliable sources. Here's mine (not expert, but been there, done that many times), and I used to have an '06 ST3s. Guitar tuners are typically free downloads, are easy to use and are very accurate. There's also an electronic tensioning tool that comes with Melcodiag (laptop ECU access/tuning software), also free. I bought a $5 USB microphone for use with my laptop that makes the process a little more convenient, but I've heard of people getting good results straight off their iPhones.

The spec for the ST3 is 110 Hz, which is a little tighter than some of the old two valve engines. I believe the 45 degree method your friend used is intended for those older bikes, so he's probably set a little loose. Better a little loose than tight though, so he's probably ok. The other issue is that the twist method is very subjective. How hard do you twist to get to 45 degrees? Real hard? Light pressure? I see potential for wide variations using anything but a tuner (or other electronic method).

The main thing is that your buddy finally changed those 17 year old belts! I think Ducati recommends replacing every two years or 15k miles, whichever comes first. I'm sure Ducati's service interval is conservative, but that's really pushing it! That Desmoquatro is an interference engine. If a belt lets go, you're done.

Just my two cents. Take it for what it's worth.
Thanks for advice, The guitar tune app method was in his words not obvious to him that it was working. (I wasnt there so dont know) All i know was his neighbor a guitarist used the app on his phone and they were confused by it.
He was worried about belt and had not riden it for two years and the bike itself has done some crazy low miles from new and is probably why he got away with it.
He said it would be an easy enough job on say a monster etc, the ST3 it was fiddly for him to get the plastics off he said.
As someone who has ridden and worked on various Ducatis since 1991, I have some opinions on this.

Critical and crucial aren't two words I'd use to describe getting the tension right. Over the time that Ducati has had belt driven cams, how many belt failures have people heard about? Do they happen? Yes. Do they happen OFTEN? Not from what I've seen. Ducati's get neglected all the time. If it were a CRUCIAL aspect of maintanence, they'd be failing more often.

Old school belts were rated for two years. At some point "they" started using kevlar to reinforce the belts and five years became the norm.

There are a lot of methods that have been used over the years to get the correct tension. Everybody has their favorite and with the advent of smart phones, the Hz method is now top of the heap. That's great, and it does eliminate some of the uncertainty. But it's not as foolproof/precise as some will have you believe.

I already mentioned that the belt materials have changed over the years. There isn't just one brand of belt available. Do they ALL use the same materials in the same ratios to get the same stiffnesses and densities? I don't know but my money says no.

If they AREN'T all identical, then how can one frequency be ideal for every brand of belt out there? If you change the density of the belt, you change the frequency it will vibrate at under a given tension. That's why guitar strings aren't all the same size.

Doesn't Ducati give a spec for new belts, and another for belts that are in service? Wouldn't anywhere between these two numbers be ok? I'd agree too tight is worse than too loose. I once bought a 900ss and when I got it home I checked the belts and they were so loose I probably could have slid them off without loosening the tensioner. How long had the guy been riding it like that? Who knows.

Most of the bikes I work on are older air cooled twins. I use the allen wrench past the tensioner trick. Over 30 years have never had an issue with belts.

My take on it is, get it close and you're fine. I'd bet there are some guys who set them by feel and have never had issues. It's not one of the more critical settings in the engine. We can talk theory about it all day but in the real world, there are thousands of bikes running thousands of miles with a wide range of belt tensions, and very few of them ever have a failure related to the tension.

This doesn't mean it doesn't matter. It means find a method and use it. You have to have a baseline, but don't get fanatical about it. A tuner is one way to do it. It's not the ONLY way.

As for the time limit on belts - keep in mind it's statistics, not a bus schedule. Some engineer determined the length of time that there is a 99.9% chance (or whatever limit they deem "good") that the belt won't fail due to age. Let's go with 5 years. Obviously that doesn't mean all belts will fail in their 61st month. What are the chances a belt will fail within six years? More than five but still not huge. I don't know what the degradation rate is but if it were steep, a lot more belts would fail in use.

I go with 5 years for my bikes and always replace them right away on any used bike I buy. But I'm not someone who won't even start a bike if they are seven years old. I've talked with guys like that... and ok, you do you, but that's a bit extreme to me. They aren't really expensive and they're easy to replace, but it's not like a seven year old belt is a time bomb just waiting to go off. 16 is pushing it but it would not surprise me to hear someone is out there riding around on 16 year old belts. And I can assure you there are a lot of people running around on belts over 5 years old. And belts that are "too loose".
I'm new to the Ducati world but thought I would share this specialty tool mfg I found online. HDESA. I bought their Rear Wheel Balance Hub tool and their Front Axle Alignment / Wheel Socket tool and found them to be of good quality. They make tools for servicing the valve train / belts too and I will be purchasing those when the time comes. I'm not affiliated with the company in any way; just sharing my experience...
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