Timing Belts

[NAKID]

Why is it that Ducati recommends changing the timing belts at 12k miles, but most car manufacturers recommend changing them at 60k-100k?

[zarn02]

the belts are turned over a much smaller wheel, resulting in higher heat and higher stresses on the belt. also, i think most car engines are non-interference, and thus if you break a belt you're just stranded, and not blowing up the top end of the engine.

or that's the tale i've heard passed down.

[A.duc.H.duc.]

Chris,

  In my opinion, mostly it's a "penalty for failure" issue. In your average auto, if a timing belt breaks, all that happens is the motor won't run, the pistons spin around a few times, and the valves stay where they were, no big deal. You'll have to put the new one on making sure you line up the timing marks. In a worst case scenario you could have bits of belt or chain flying around, but that's why they've all got scatter shields over them.

  Ducatis, and other high performance autos, like Ferraris, have "interference" valve design. This means that if the valve is open, and the piston comes up, it's going to smash your valve. Standard motors have clearence so that at TDC the valve can still fully open without obstruction. So, just like Ducati, Ferrari and other such manufacturers will recommend short timing belt change intervals.

 The idea isn't that the belts will break more often, but, since the penalty for failure is so high, change them with an extremely safe margain.

That's my .02 at least.

Justin

[NAKID]

Hmm OK, I see the reason there. But we are talking about 5+ times the change interval.

I'm pretty sure my Nissan 240sx is an interference engine. But, it has a timing chain, not a belt...

[Speeddog]

A friend of my dad's had a Fiat X1/9.
It had a cambelt.
The change intervals were very explicit.
He went past it, and it shucked the belt.
Got the head rebuilt, as it was an interference motor.
Reassembled.
Went past the interval again, and shucked the belt.
Got the head rebuilt, again.
Reassembled, again.
Installed the race-spec belt, and was diligent in replacing the belt on time.
No more problems.

Ducati's interval may be conservative, but beyond that YMMV.

[CETME]

Most automobile ARE interference engines. Very very few are not. The only reasonably modern one I can think of right now is the older Ford 2.3 engines used in the early 90's Mustangs and Rangers.

[Speeddog]

Most automobile ARE interference engines. Very very few are not. The only reasonably modern one I can think of right now is the older Ford 2.3 engines used in the early 90's Mustangs and Rangers.

I can attest to that with FHE, my Merkur with the 2.3 Turbo motor shucked a belt with no harm done (other than to my schedule, and my wallet for the towing).

[ducpainter]

Hmm OK, I see the reason there. But we are talking about 5+ times the change interval.

I'm pretty sure my Nissan 240sx is an interference engine. But, it has a timing chain, not a belt...

If it wasn't for costs...and the fact dry chains were noisy...

we'd still have them and be happy.

[mitt]

I can attest to that with FHE, my Merkur with the 2.3 Turbo motor shucked a belt with no harm done (other than to my schedule, and my wallet for the towing).

+2 - my 86 ranger 4cyl broke a belt middle of no-where nebraska, and no damage done.

mitt

[A.duc.H.duc.]

Most automobile ARE interference engines. Very very few are not. The only reasonably modern one I can think of right now is the older Ford 2.3 engines used in the early 90's Mustangs and Rangers.

I'm pretty sure they aren't, though many of the newer high performance motors may be, I've seen a lot of timing belts break on cars and it's never been a problem other than a delay.

[MadMark]

I'll venture to guess...

Do you think that RPMs have anything to do with belt life?  Guessing that over the life of a Ducati belt it is being subjected to much higher revolutions than most auto's.  Of course I have absolutely zero factual evidence to back that thought up... just a guess.

[Howie]

Back in the '70s when compression was lower interference engines were the exception.  Now it is the opposite.

Here is a good resource for finding out if your car has an interference engine:
http://www.gates.com/part_locator/index.cfm?location_id=3598&go=Interference

Back to our bikes; small sprockets, heat and quick RPM change shorten belt life.

[Speeddog]

Another characteristic of our engines is the uneven firing order, the crank doesn't spin as consistently as you might think.
The variations in crank speed do spike the loading on the belt.

Nearly all automotive engines are more than 4 cylinders, and generally heavy flywheels compared to our bikes, so the cranks turn pretty consistently.
The cams generally serve all cylinders, and all those lobes tend to smooth out the loading.

[hiero]

Another characteristic of our engines is the uneven firing order, the crank doesn't spin as consistently as you might think.
The variations in crank speed do spike the loading on the belt.

Nearly all automotive engines are more than 4 cylinders, and generally heavy flywheels compared to our bikes, so the cranks turn pretty consistently.
The cams generally serve all cylinders, and all those lobes tend to smooth out the loading.

oooh, good info, didn't know that 

[hypurone]

Most automobile ARE interference engines. Very very few are not. The only reasonably modern one I can think of right now is the older Ford 2.3 engines used in the early 90's Mustangs and Rangers.

As a tech for 20 years, I gotta state you are wrong. There are MORE non-interference engines than there are interference ones. Primarily Honda, Nissan and high perf engines from other mfg's were and are interference. That's why I buy Toyota's non-interference. Plus they are back to using chains!    And if ya wanna get totally non-interefence, there is my RX7, no valves, period!!