Will a lightweight flywheel damage a starter sprag?

[Howie]

Good points.  If they believed the lighted flywheel would harm the sprag or void your warranty you should have been informed in advance of doing the work, therefore it is their responsibility IMO (since I am not a lawyer do not take this as legal advice).

[Shazaam!]

This is a known problem and an issue that you should consider before you add a lightweight flywheels to a Ducati. Ducati sees a lot of worn-out starter clutches when there is a very light flywheel attached. Why? Often lighter flywheels seem to let the engine kick-back much harder during start-ups and shut-downs, which causes starter clutch damage.



Why You Have a Flywheel

A lightened flywheel is one modification to a STREET bike that will have more negative than positive effects. I left mine stock. Here’s why.

An engine makes the least torque at idle and low rpm, especially when cold (when there are more misfires.) So when you release your clutch a little too fast, the engine torque is too small to overcome the bike's inertia, and it stalls. If you have a flywheel, the stored momentum augments the engine's torque allowing you to use a lower rpm starting-off. Without a flywheel you'd need a higher idle speed, or constantly need to start-off at a higher rpm in order to raise the engine torque output enough to avoid stalling the engine.

As you ride at lower rpm in traffic, you are constantly changing between acceleration and deceleration. Engine torque levels are still fairly low at these speeds, so slack in the drive train needs to be smoothed-out with a flywheel. Otherwise, on-off throttle transitions have a jerky effect, giving a less comfortable ride and causing you to use smaller throttle inputs (which is not always easy to do.)

At higher speeds a flywheel slows the rate at which an engine rpm changes, so cracking the throttle open or closed results in a smoother transition in torque being applied to the drive train and tires. Again, without a flywheel more careful throttle transitions are needed. The key to faster track times is reduced wheel spin so a light flywheel works against you by making it more difficult to modulate wheel spin, even though it helps lap times by producing more acceleration in the straights.

So in effect, a flywheel slows an engine's ability to change rpm producing drive train smoothness and drivability. It also reduces the engine's ability to match it's rpm with the drive train's rpm making it more difficult to downshift without producing wheel-hop.

And, when you miss a shift you’ll be glad you have one ...


Lightweight Flywheel - Pros and Cons

The weight of the stock flywheel is derived by Ducati test riders to provide an overall balance between performance and smooth drive train behavior. That's why Ducati selected the flywheel weight that they did. Did you think that they meant to purposely cripple the bike's performance by using a too-heavy flywheel?

When you remove weight from the flywheel (and to a lesser degree, from the clutch) the effect on the engine’s ability to more quickly spin-up is indistinguishable from increasing your engine’s torque (and consequently horsepower) output.

But only in neutral. In any other gear, there’s little benefit at all.

Obviously, your bike’s ability to accelerate faster through the gears is enhanced by reducing the overall weight of the bike as well as the inertia of rotating components. The crankshaft, pistons and connecting rods, transmission gears, drive chain and sprockets, wheels and tires, clutch and flywheel are all candidates.

However, the overall weight of the bike and rider completely overwhelms any reduction of rotational inertia produced by a lighter flywheel. A two pound lighter flywheel on a 600 pound bike-plus-rider will accelerate only 0.3% faster. F=ma.

Of course every 0.3% helps a racebike. When you reduce weight you’ll get faster acceleration, and faster lap times - IF - you can modulate your wheel spin driving out of corners. Factory racebikes make so much power, for example, that transmitting the power to the road effectively becomes the limiting factor - so heavier flywheels actually become a benefit. Fear the high-side.

The ability of an engine with a lightened flywheel to SPIN-UP more quickly is often pointed-to as a benefit when you bang a downshift and wheel hop is reduced. In this situation, it can be said you have LESS ENGINE BRAKING. However, if your riding “style” makes this an issue, a slipper clutch may be a better alternative than a lightened flywheel; at least that’s what the factory racers think. A lightened flywheel is like a poor man’s slipper clutch in this situation.

Conversely, a heavier flywheel will provide more protection for the engine being over-revved in a ham-fisted downshift.

The ability of an engine with a lightened flywheel to SPIN-DOWN more quickly is often pointed-to as a benefit if you want the revs to die as fast as possible when you lift the throttle for a corner. In this situation, it can be said you have MORE ENGINE BRAKING. Under normal street riding conditions we tend to prefer less engine braking so we tolerate a less efficient situation where the motor then has to work harder to put more momentum into the flywheel. When racing, you don’t care about storing momentum, you just want to get around the track as fast as possible.

This enhanced ability of the engine to spin-up and spin-down also makes it less critical to match engine and drive train rpm for smoother shifts. That is, the heavier the flywheel the longer the engine rpm will hang between shifts requiring a need to release the clutch more slowly to avoid lurching forward when quickly engaging the next higher gear.

So, the purpose of the flywheel is to store momentum, reduce vibration and smooth out the loads transmitted to the drive train. It takes energy to first store this momentum, so if the flywheel is lighter it takes less energy and it accelerates up to speed faster. There is an opposite effect when you lift off the throttle and momentum is given up, so the revs drop slower for a heavier flywheel.

Because the engine will spin-up more quickly with a lighter flywheel, when you loose traction, modulating the throttle (especially in the rain) will be more difficult. Especially if your throttle position sensor, idle and CO are not adjusted properly. A light flywheel seem to exacerbate a poorly tuned fuel injection system. If you make 100 horsepower it’s less of a issue. Make 130 hp and it will matter a whole lot more.

The amount of weight removed from the flywheel and inner hub is proportional to this effect, although if you remove most of the weight from the outer rim area, the effect is stronger. Different after-market manufacturers of flywheels offer different weights and geometry. If lightweight is good, ultra-lightweight is better - just doesn’t apply here. Too light a flywheel can make the bike a handful to ride so a two pound flywheel for the street seems to be a good compromise to preserve some ride-ability.

A cold engine runs rough until there’s enough heat to vaporize the fuel, so until then, a lighter flywheel will be less effective in preventing stalls, especially pulling away from a uphill stop. You’ll need to rev the engine a little higher to compensate and you may find it’s a little ornery when running at light loads (3,500 rpm) in the lower gears around town. Often, your idle speed will need to be raised to around 1,300 rpm to help minimize stalling.

You’ll also risk sprag clutch wear or damage. Ducati mechanics see a lot of worn-out starter clutches when there is a very light flywheel attached. Why? Often lighter flywheels seem to let the engine kick-back much harder during start-ups and shut-downs, which causes starter clutch damage.

With all that said, you’ll find plenty of owners that will praise the effects of a lighter flywheel on their bike - but perhaps one that’s different than yours. Not every bike will respond well to a light flywheel, mainly because the flywheel is just part of the overall rotational inertia of the crankshaft, connecting rods and alternator.

So, if you have an old alternator SP/SPS with the lighter crankshaft and titanium connecting rods, the effect of removing 1kg from the end of the crank will be very noticeable, because the total mass is much lower to begin with. A non-SP or 2-valve bike has more crank/rod weight, so the effect is less pronounced.

For example, on a 916, or any model with a single pick-up and older version alternator, you can just remove the flywheel weight and run the starter clutch hub. Inexpensive, and makes them quite responsive.

However, a late-alternator bike with the heavier full-counterweight crankshaft and standard rods has significantly more weight in the alternator assembly and machining the same 1kg from the flywheel will have less of a negative effect, again - due to the higher initial combined mass of the set up.


Lightweight Wheels Instead

Lightweight wheels don't have the stalling and drivability drawbacks of a lighter flywheel. Also, since the wheels have a much greater rotational inertia than a flywheel, weight reduction here results in a much greater improvement in acceleration (and braking) with an added benefit of reduced gyroscopic forces for improved handling. Lighter front rotors have a similar benefit. There’s even a significant difference in tire weights between brands to consider.

[ducpainter]

Most of what you said made my head hurt.   

This caught my attention though...

Of course every 0.3% helps a racebike. When you reduce weight you’ll get faster acceleration, and faster lap times - IF - you can modulate your wheel spin driving out of corners. Factory racebikes make so much power, for example, that transmitting the power to the road effectively becomes the limiting factor - so heavier flywheels actually become a benefit. Fear the high-side.

[Speeddog]

Never let it be said that Shazaam! is a man of few words. 

Welcome to the asylum. 

[Ducnial]

+1 Shazaam!,

Thanks for explaining the physics in terms we all understand..   

[kuhlka]

This is a known problem and an issue that you should consider before you add a lightweight flywheels to a Ducati. Ducati sees a lot of worn-out starter clutches when there is a very light flywheel attached. Why? Often lighter flywheels seem to let the engine kick-back much harder during start-ups and shut-downs, which causes starter clutch damage.

Why is there any kick-back at all in this system?  My 2003 Triumph Speed Triple has gone through 100x the abuse I've put my 1098 through and its starter is just fine and almost always starts the bike on the 1st or second try unless the battery is low.  Seems to me this 'kick-back' issue is Ducati's fault since as you noted, it does it with the stock flywheel.

Why You Have a Flywheel

A lightened flywheel is one modification to a STREET bike that will have more negative than positive effects. I left mine stock. Here’s why.

An engine makes the least torque at idle and low rpm, especially when cold (when there are more misfires.) So when you release your clutch a little too fast, the engine torque is too small to overcome the bike's inertia, and it stalls. If you have a flywheel, the stored momentum augments the engine's torque allowing you to use a lower rpm starting-off. Without a flywheel you'd need a higher idle speed, or constantly need to start-off at a higher rpm in order to raise the engine torque output enough to avoid stalling the engine.

I had no problems taking off and didn't seem to have to rev the bike much higher than with the stock flywheel to get it to take off from a stop.  HOWEVER, it DID make the entire bike ride smoother at low RPMs and there was virtually zero chugging (mirrors flapping at 3000-3500rpm) compared to stock.  Also, the bike idle'd just fine at the stock idle.  It did stall a few times, but that could have been more related to the fact that the bike had been sitting in the shop waiting to get worked on for over a month or it could have been the flywheel.  No way to know for certain since the sprag crapped out after I shifted into first at a stoplight with my hand off the clutch.  That action probably would have destroyed the sprag on this bike even with the stock flywheel, but DNA found an excuse to deny my claim for the full repair.

As you ride at lower rpm in traffic, you are constantly changing between acceleration and deceleration. Engine torque levels are still fairly low at these speeds, so slack in the drive train needs to be smoothed-out with a flywheel. Otherwise, on-off throttle transitions have a jerky effect, giving a less comfortable ride and causing you to use smaller throttle inputs (which is not always easy to do.)  At higher speeds a flywheel slows the rate at which an engine rpm changes, so cracking the throttle open or closed results in a smoother transition in torque being applied to the drive train and tires. Again, without a flywheel more careful throttle transitions are needed. The key to faster track times is reduced wheel spin so a light flywheel works against you by making it more difficult to modulate wheel spin, even though it helps lap times by producing more acceleration in the straights.

Again, I found the exact opposite to be true with the lighter flywheel for the 3 days I was riding with it.  Lower RPMs were exceptionally smooth compared to stock and throttle inputs were actually EASIER to manage since the entire bike was running smoother.  Also, since the engine was able to spin up and down faster, I was able to click through gears much more smoothly (not that I don't already blip the throttle on downshifts).  Handling was also seemingly improved over stock.  The bike felt a bit more lively and flickable than it already was.

The bit about higher speeds is correct, but I'm not using this bike for the track much and I'm already very conscious of my throttle and braking inputs and the overall 'behavior' of the bike while riding.  If the rear starts to slide, I adjust.  I'm not the least bit hamfisted and looking to race fools in the canyons on public roads, so high-siding isn't really a concern.  If I did a trackday on this bike, I'm sure I would gradually increase my pace rather than trying to WOT out of corners at every exit like a jackass.

And, when you miss a shift you’ll be glad you have one ...

I don't get this statement.  If you miss a shift, the RPMs would flick up or down to where they need to be for the gear with the lightweight flywheel.  I'd be more worried about missing a shift with the stock flywheel as the increased chance of wheel hop on downshifts could be really scary mid-corner if you're 'tarding it up' and just flogging the throttle and shifter mid-turn.

Lightweight Flywheel - Pros and Cons

The weight of the stock flywheel is derived by Ducati test riders to provide an overall balance between performance and smooth drive train behavior. That's why Ducati selected the flywheel weight that they did. Did you think that they meant to purposely cripple the bike's performance by using a too-heavy flywheel?

When you remove weight from the flywheel (and to a lesser degree, from the clutch) the effect on the engine’s ability to more quickly spin-up is indistinguishable from increasing your engine’s torque (and consequently horsepower) output.

But only in neutral. In any other gear, there’s little benefit at all.

Again, wrong.  The acceleration difference was notable.  I wasn't looking for a power increase, just better usability of the available power for my riding style.

Obviously, your bike’s ability to accelerate faster through the gears is enhanced by reducing the overall weight of the bike as well as the inertia of rotating components. The crankshaft, pistons and connecting rods, transmission gears, drive chain and sprockets, wheels and tires, clutch and flywheel are all candidates.

However, the overall weight of the bike and rider completely overwhelms any reduction of rotational inertia produced by a lighter flywheel. A two pound lighter flywheel on a 600 pound bike-plus-rider will accelerate only 0.3% faster. F=ma.

I think your math is a bit off, but regardless, the changes were quite notable.  Have you tried a lightweight flywheel on a 1098?  Also, I did not notice a huge change to engine braking.  I think the rest of the motor's weight and compression was a factor in retaining a good amount of engine braking on the bike.  You'd be pretty hard-pressed to take all of the engine braking away from a big-displacement twin.

Conversely, a heavier flywheel will provide more protection for the engine being over-revved in a ham-fisted downshift.

This is why we have rev-limiters and you shouldn't be over-revving your bike even with the stock flywheel anyway.  Again, I'm not a ham-fisted rider so this doesn't really apply.

So, the purpose of the flywheel is to store momentum, reduce vibration and smooth out the loads transmitted to the drive train. It takes energy to first store this momentum, so if the flywheel is lighter it takes less energy and it accelerates up to speed faster. There is an opposite effect when you lift off the throttle and momentum is given up, so the revs drop slower for a heavier flywheel.

I found the exact opposite in my experience.  The engine seemed to run smoother at idle and it definitely ran smoother through the entire rev range on the road.  Heavy flywheels are put on cars and motorcycles to make the powerband more manageable for the average rider/driver. 

Because the engine will spin-up more quickly with a lighter flywheel, when you loose traction, modulating the throttle (especially in the rain) will be more difficult. Especially if your throttle position sensor, idle and CO are not adjusted properly. A light flywheel seem to exacerbate a poorly tuned fuel injection system. If you make 100 horsepower it’s less of a issue. Make 130 hp and it will matter a whole lot more.

Where did you copy and paste this from?  The 1098 is known to have 140+rwhp.  Also, I've ridden the 1098 in pissing down rain on a trip from Dallas, TX to Toledo, OH and back and it wasn't nearly as horrible as this last statement makes it out to be.  I've also ridden my Speed Triple, a Honda Valkyrie, and several other bikes in rain with no major issues.  You just can't assume you have any traction and you ride accordingly.  Try to drag a knee in the rain and you're gonna die.  Duh.

You’ll also risk sprag clutch wear or damage. Ducati mechanics see a lot of worn-out starter clutches when there is a very light flywheel attached. Why? Often lighter flywheels seem to let the engine kick-back much harder during start-ups and shut-downs, which causes starter clutch damage.

I'm still waiting for better evidence than the half-ass explanation I was given.  Does Ducati have documented evidence which shows empirically that lightweight flywheels destroy starter sprags?  Do they have proof their starting system design isn't flawed to begin with and a lightweight flywheel only makes this much more obvious?...

So, if you have an old alternator SP/SPS with the lighter crankshaft and titanium connecting rods, the effect of removing 1kg from the end of the crank will be very noticeable, because the total mass is much lower to begin with. A non-SP or 2-valve bike has more crank/rod weight, so the effect is less pronounced.

Again, where did you cut'n'paste this from?  We're talking about a 1098, not an older 2-valve bike.  The last bit about lightweight wheels has nothing to do with the topic at hand; "Will a lightweight flywheel damage a starter sprag?"

[Capo]

"Conversely, a heavier flywheel will provide more protection for the engine being over-revved in a ham-fisted downshift."

"This is why we have rev-limiters and you shouldn't be over-revving your bike even with the stock flywheel anyway.  Again, I'm not a ham-fisted rider so this doesn't really apply."

You miss the point,  down shift one or more ratios than intended and the rear wheel will overdrive the engine, the rev limiter would have no effect.

BTW I have the DP lightweight flywheel fitted to my S4R and can only report positive results over the stock item.

Having read many of Shazzam's dissertations over the years, if any cutting and pasting is going on it would be from his posts, he is a well respected engineer in Ducati circles and is privy to information not readily accessible to the rest of us.

Many points in his post are illustrative and serve to further the understanding of the operation and the factors affecting it.

[Norm]

I've been watching this thread with interest. I think the physics of how a flywheel works is pretty easy to understand (as well as any other rotating engine part). I wouldn't recommend one for a bike that is used for touring or commuter duty, but they have a big advantage in a sport riding arena. They let the motor spin up and down quicker and give the rider a better throttle response and a better "connection" to the motor. Most of us aren't nearly as good of riders that we think we are and anytime you give the rider more control or adjustability, it can lead to worse results as well as better ones.
Overall, I think a lighter flywheel makes a big improvement (I include one on every bike) and haven't seen any evidence of sprag problems or any form of "kick back". It isn't, nor should be considered, a substitute for a slipper clutch.

[Langanobob]

Having read many of Shazzam's dissertations over the years, if any cutting and pasting is going on it would be from his posts, he is a well respected engineer in Ducati circles and is privy to information not readily accessible to the rest of us.

I think the OP is probably justifiably frustrated with his starter sprag problem and some of it's understandably showing through in his post.  This is a very interesting and educational thread, and I hope we can overlook a few transgressions and keep it from getting personal.

It would also be interesting to get Nichol's Mfg view on some of this, especially the sprag clutch issue.  If I have some time later today I'll email them.

[Capo]

+1 I noticed that my sprag lay in pieces after the engine being stipped by one of the junior guys at the shop.
Guess we will be having a conversation over it when the bill is produced.

[JetTest]

In reference to the original post: No offence intended, honest question, how did you knock it into gear at a traffic light? Been riding for nearly 40 years, dirt bikes and street bikes, never done that, and don't think I know anyone who has, and when I started riding there was no standard control layout. Some were left shift, some right, some 5 up, some 1 up 4 down, etc. Is it that common to happen?

[kuhlka]

In reference to the original post: No offence intended, honest question, how did you knock it into gear at a traffic light? Been riding for nearly 40 years, dirt bikes and street bikes, never done that, and don't think I know anyone who has, and when I started riding there was no standard control layout. Some were left shift, some right, some 5 up, some 1 up 4 down, etc. Is it that common to happen?

I was sitting at the light and went to put my foot down and my pant-leg caught the shifter peg.  Total stupid accident.  You've never accidentally bumped the shifter into gear at a stop in 40 years of riding?  Maybe you've just been really lucky and developed exceptionally good habits?  I've seen plenty of people have their bikes click into gear at stops, while coasting in neutral to a stop (thinking the bike couldn't ever just be in a false neutral...), go over a bump in neutral and have their bike click into gear (my wife somehow managed this one in a parking lot).  I think I've accidentally clicked into gear maybe 5 times or so across the last 20ish years of riding, maybe more, but its still pretty rare.  That doesn't mean it should completely frag a starter sprag though.  This is the first bike I've been on which has had anything starter-related go out.  It doesn't help that Ducati has multiple warranty service notices on various parts of the early 1098 starting system.  I've got a nice stack of paper to take into whatever dealership I end up with to make damn sure everything is up to date and operating properly.

I meant no offense to Shazaam when I mentioned cut'n'paste.  I just felt some of the points could have been better applied to the 1098 and the topic at hand.  In general, most of those points fit, especially for cars not designed for a lightweight flywheel.  I'm just annoyed at the shop who did the install (told me the lightweight flywheel would cause no damage then flip-flopped when DNA said no) and DNA because the law requires they provide proof the flywheel caused the damage, but I don't have the time or money right now to start a he-said-she-said small claims court filing from halfway across the country and end up putting more into that than I've already spent on the install/uninstall/repair.

Oddly enough, now the bike seems to be starting normally although the start-up is still pretty clicky/noisy compared to what I remember before this whole bs situation.  Hopefully things will stay right. 

So here is another question;  How many of you would renew or buy an extended Ducati warranty if you'd gone through this situation?  Mine runs out in June.

[JetTest]

My bike is never in neutral if it is moving, only stopped and I never coast, to a stop or otherwise. You have less control and it allows more false neutrals. When stopped, my left foot stays on the peg incase I have to move if a car approaching from behind appears to not be stopping in time. That aside, I agree you have every reason to be pissed at the shop that gave you terrible service and stole your flywheel, aswell as DNA for not honoring your warranty ans supporting their product.

[beethoven]

Laws are fine and dandy but unless you yourself are a lawyer, its going to cost you more money and time to settle the matter legally than to just explore your options and settle accordingly. Experiences with DNA (Ducati North America) have gone both ways so but most of the time they are reasonable.

1) Another Dealer
2) Appeal to DNA directly
3) Get a lawyer to write a letter to the dealership
4) Become a greasemonkey and do it yourself.

Although I will never own a 1098 with a light flywheel I am interested in Ducatis response to these warranty issues. Did you communicate directly with DNA as suggested earlier. If so what was their response to you. It seems from other posts DNA response is sometimes more favorable than the dealer.

[kuhlka]

I sent an email and attempted to call several times but didn't get through to anyone important.  The few 'friend of a friend' connections I had were for the wrong region (east coast, and I was in Texas at the time).  If I run into another situation like this in the future, now that I'm no longer hopping around dealing with grad school and career moves, I'll be doing everything I can to get my voice heard by DNA or any other manufacturers I might run into BS with. 

Every time I take the bike out, I'm reminded why I fell in love with it in the first place.  Its just frustrating when I see my money burning off for nothing.  Not that I have a ton of cash having just finished grad school and moved to southern California (living expenses...  $$$$$). 

I'm thinking once the warranty runs out, I'll be doing all work myself unless it is motor-related.  I can't afford to buy broken engine parts if I don't assemble something juuuuust right, so I'll leave that to the pros.  I also shudder to think of how many specialty tools I'd have to buy to work on that motor.  There seem to be plenty of competent Ducati mechanics around here, so I should be good on that front.  I've done the rest myself on multiple bikes including a custom GSXR front end swap on my Triumph.