Heavy flywheel equals more torque? (2014)

lcrken said:

my friend Fred Eiker won a lot of road races on his 850 Commando with stock gearbox and un-lightened crank, including the 1986 La Carrera II Mexican road race. He won on high speed tracks like Willow Springs as well as tight, twisty ones like Sears Point. Fred averaged 111.19 mph for the 125 mile race on Mexican roads from San Felipe to Ensenada. And that was before we had all the currently available good stuff, like Maney cases and cylinders, billet cranks, TTI gearboxes, etc, and running on period tires. I

Ken

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That is very impressive.

lcrken said:

You do realize, don't you, that many Commandos with stock "heavy" cranks have been very successfully road raced with stock "wide ratio" 4-speed gearboxes, particularly back in the '70s and '80s? They might not suit your riding style, but they clearly worked well for others. Back when I was still racing Commandos, my friend Fred Eiker won a lot of road races on his 850 Commando with stock gearbox and un-lightened crank, including the 1986 La Carrera II Mexican road race. He won on high speed tracks like Willow Springs as well as tight, twisty ones like Sears Point. Fred averaged 111.19 mph for the 125 mile race on Mexican roads from San Felipe to Ensenada. And that was before we had all the currently available good stuff, like Maney cases and cylinders, billet cranks, TTI gearboxes, etc, and running on period tires. I was lucky, my PR came with a Quaife 5-speed, and it was an advantage over a stock 4-speed, but not as much as you believe. I also raced a MK3 with 920 engine and stock 4-speed for a while, and it never blew up anything. It just had to be ridden a little differently. Would it have been better with a 5-speed? Probably, but it was certainly adequate without it.

Ken

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If you don't know any different, the standard box is OK. But in actual fact it is useless for road-racing, you cannot race-change up with it and get decent acceleration , nor can you downshift smoothly without a lot of effort. I tried racing with the standard box and with the wider ratios, you wait longer for the heavy crank to spin up. After I tried racing with the standard 4 speed box, I bought an American close 4 speeder which has first gear higher than that of Manx. After it lurched off the startline, it was excellent everywhere else - acceleration up through the gears is much faster. I've had guys tell me, you don't need a close box with a torquey motor - guys who have never used a close box. $700 for a close ratio 4 speeder is not expensive, but you run the risk of tie- ing the mainshaft in a knot during a clutch start.

Al, I have come to the conclusion that your Norton gearbox was faulty. And when you replaced it with the race boxes, you not only got the close ratio benefits, but also removed whatever the fault was.

It’s the only explanation for some of your comments.

I do fully agree with you that a good 5 speed close box would be much preferred for racing, but to state that the stock box is ‘useless for road racing’ is just nomesense! How can it be so, when SO many people do exactly that with it?

In fact, for many years, fitting a Norton box to any pre unit Brit bike was seen as a performance upgrade (I disagree, as I preferred the 5 speed Triumph box).

I have ridden my stock Norton box on the track, at a semi reasonable pace, and I abslutoley conform that YES you can “race shift” up (I assume you mean change gear without the clutch?).

And YES you can down shift smoothly.

The box with standard road ratios is, let’s put it this way, only as good as it is for the road. When it comes to racing, most people see the limiting factors of it in standard form and change the ratios, which is easily done, what with the amount of gearbox cogs available to change around

In other words Bernhard, the standard box is not the best choice for racing.

That’s a long way from saying it is completely useless for racing!

A good rider good still get a long way up the pack with a stock box.

But if you want to win, then you probably need to be looking at alternatives.

You can race with a standard box, but you will be slower and more likely to crash. When you are racing, it is essential to get down-changes smooth, particularly in the wet. But if you become fast, it is also important in the dry. While you are picking yourself up off the bitumen, the rest of the guys don't hang around and wait for you. A heap of revs on every down-change doesn't make staying safe any easier. With the standard box, coming up through the gears is slower because there is a bigger drop in revs on every change, so you wait for the crank to wind up. With a close box, you simply back off slightly when you reach max revs to take the load off, then stand on the gear change - the shift takes place with minimum loss of revs and you achieve maximum acceleration.
I might be more aware when my bike is sluggish, than many other people. The heavy crank is excellent when you use a close ratio gear-set. Once that crank is spinning high, it needs to be kept there. I don't think my revs ever drop below 5,500 RPM, once I am mobile.

Ok I'm pulling rank on all's ya all's if ya had the potent engines the listed winners had a 4 spd as Ken Canaga states may not be optimum but certainly adequate to deserve the term Unapporachable. If these guys had the power my P!! had in '68-'69 National drag mid 10 sec 1/4 m champion in its class, gear ratio was least of their concerns - transmission drive train survival was. For anyone to be limited by gear ratios beyond what factory issued directly implies a so so engine power characteristic. PONDER this acrtel and others I found out the hard way a optimalized 45 yr old engine in a 2" built front low stance - could not give WOT in rather tall 4th until well over sixy mph hour. Top end in 1/4 m's approached 150 mph tall gearing for gosh sakes. I first discovered this going 60 mph and decided to pass a few cars in a row so throttled up and leaned a bit for rear to spin free causing me to suddendly flat tracker crossed up slide in oncoming lane, but let off to hook up again and feathered throttle on to get on by adrenalized to the max so gave her WOT over 70 mph to dissapear with some light headness from the G's. * Till severe vibrations started damaging nerves.

That P!! and then Ms Peel Combat on THE Gravel taught me ballistic level handling that is so jerky its like ricochet billards not ice bob sled sluggish smooth lines - stuck on tracks but jumping tracks at will - which requires instant break free power control for the launch and re-hook, which lighter flywheel distinctly assisted. Acrtel seems your Seleey really got the handling physics down but lacks the power even on methanol to attain the orbits the famous winners and my bragging savor more than sex with movie starts. Multiple road orgasms that last way longer. If you can snap throttle in low gears and not need to grit teeth hunkered down white knucked plus a big butt hump to keep bike from flat running out from under ya don't know what ya missing out on yet.

acotrel said:

A heap of revs on every down-change doesn't make staying safe any easier. With the standard box, coming up through the gears is slower because there is a bigger drop in revs on every change, so you wait for the crank to wind up.

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Compelling arguments for a lighter flywheel & lighter crankshaft.

I think it comes down to a choice of directions. A short stroke motor with light internals and a multi-speed close ratio box will give better results, but the usable rev range usually moves upwards and the overall gearing is often lower. The heavy crank works well with high overall gearing and with a five speed close box as long as first gear is low and the rest close and higher. If you already have the heavy crank, the better way to spend your money is probably on the good gearbox before you buy the lighter crank. The other thing is that when you move the usable rev range higher, the small ports in the head are suited to lower revs. If you enlarge them, there is no easy way back if you find you don't like the way the motor performs. The limiting factor with the Commando motor is the valve train. When I use my bike I always change up at 7,000 RPM which effectively becomes 7,500 RPM. With the 850 cam, the valves never float or tangle. If you use long duration cams with gentle lift in a pushrod motor and lighten everything in the valve train, you can get the upper rev limit safely up to about 10,000 RPM, but the power characteristics can make the bike a bastard to ride. Depends a bit on what sorts of race circuit you use the bike on. On Daytona a top end motor might be good. On Brands Hatch, it would probably drive you crazy. My own bike is set up for small tight circuits rather than big flowing power circuits .

At Daytona you would fair better with a lighter crank. At Brands Hatch, you would most definitely fair better with a lighter crank unless the track is wet and the rider is not experienced.

This is similar to Balance Factor, especially among the solid mounted cycles but certainly Isolastics too. There's a general range factory and dealerships aim for but there's also racer shops experimenting on various engine and frame configurations so to them was like Kentucky Fired Chicken or CokeCola secret ingredient to reveal. Here's the real low down based on physics equations with seat of the pants realistic assessment > that flywheel mass ain't much an issue so go with the best cost path and forget about it. Did mention if extra/expensive good power to weight craft, for serious competition then lighter smaller dia. could be an extra bit of advantage. Power pulses dampening to tire hysteria hook up harmonics matter a Lot for great hp/lb craft so more confusion/trial error/failure optimizing with likely small benefit. Unless of course accidentally hit a sweet spot EUREKA.

https://www.w8ji.com/rotating_mass_acceleration.htm

Mz Peel welded nitrided 1/2" less dia, 4.5 lb lighter steel flywheel has two big threaded holes to experiment with adding mass/BF to over 100% from 62% base line for more or less up/dn bouncing on front iso vs more frw/aft oscillations like solid mounted cycles to see if a sweet spot for big block blown power hits grip. I was first shocked at better hook up on extreme ravine escapades that knobby MX and trial bikes in scary bravery testing places 7-8 story drops and climb outs to 45' sane slopes flat tracking trusting crossed up on loose rocks/leaves through trees less than a bike length apart to avoid tumble off bluff edge might not ever get out of... So I again direct attention to the handling affects more than the mild power delivery effects. If ya ain't having to be mild on 1st to second redline shitfs to avoid spin hesitation then stick with heavy crank, other wise go lighter and use throttle skill tire control till fast enough to enjoy hooking up every bit of less resistance to change. Ordinary vaarrRoOOMMM to redline vs BLATT!

If two otherwise identical bikes were travelling down the road, except for light and heavy flywheels, and the heavier flywheel is storing more energy, where/how is that energy dissipated, or transferred to the rear wheel?

Once a heavy flywheel is spun up to speed, isn't the force of the spinning mass helping to propel the bike forward in a way that a light flywheel is not? It's part of the drivetrain, right? So once up to speed, with the greater forces involved from the heavier flywheel, isn't it easing the work of the combustion chambers, by spinning around with a greater 'contained' energy than a light flywheel?

I think I've read every post in this entire thread, but I would like to have read more regarding the discernible differences of light vs. heavy flywheels, for road-use.




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Ok picture a 20 ft dia 2 ton water wheel spining at 20 rpm, for 1200 lbft torque, dropping clutch would snap your neck off on acceleration, to about 12 mph top out on 12-ish hp power. But can grind wheat all day long cheaply. So a heavy flywheel in a cycle would give a bit extra tire chrip on fast up shifts but would feel like a factory 850 strong pull for a short interval on shifts then feel 'sluggish' waiting to spin heavy flywheel and bike mass faster. Its a non issue until reaching extreme racer p0wer/wt configurations and uses, not in sane public use. Norton got it right in crank mass and balance factor for good compromise to stick with unless a skinny hobot maniac on 50 lb lightened 70's hp Commando. Watch the racers only hearing engine sounds, there's another level tire sounds heard above engine/wind but very few get to appreciate-experience that but light flywheel helps get there. If not hp/wt to be scared to snap throttle in lower gears then counter productive to lighten flywheel. If not having to get tires to hot to touch temps before letting hair out ya ain't in hp/wt league to benefit by lighter crank. I remind thee the handling ease is more noticed than bee line sprints, even in legal public use.

Robert_Norton, I would say yes to your hypothesis Robert. That’s why it has been reported that various Land Speed Record bikes had heavier flywheels, exactly for the reasons you describe, keep momentum at WOT when searching for the last fractions of top speed.

But... land speed record bikes aren’t (very) concerned with acceleration or deceleration. Whereas a road race bike, or road bike, spends a lot of it’s time either accelerating or decelerating, trying to do both as quickly as possible (it’s a necessity of racing and it’s where the fun is on the road) and spending VERY little time at sustained top speed. Quite frankly, most road bike users never get their bikes to its real maximum speed.

Fast Eddie said:

Robert_Norton, I would say yes to your hypothesis Robert. That’s why it has been reported that various Land Speed Record bikes had heavier flywheels, exactly for the reasons you describe, keep momentum at WOT when searching for the last fractions of top speed.

But... land speed record bikes aren’t (very) concerned with acceleration or deceleration. Whereas a road race bike, or road bike, spends a lot of it’s time either accelerating or decelerating, trying to do both as quickly as possible (it’s a necessity of racing and it’s where the fun is on the road) and spending VERY little time at sustained top speed. Quite frankly, most road bike users never get their bikes to its real maximum speed.

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It is almost impossible to ride a modern large capacity road bike at anywhere near it's full potential on public roads - unless it is a Harley or a Gold Wing (no potential). Even on country roads, many of the corners are blind. If you were riding a bike with a heavy crank on a hilly piece of road, it would probably cope better a bike with a light crank because as it hit the rises, the inertia of the heavy crank would keep it going. However a bike with a light crank would recover better from severe loss of revs. - Depending on the gearing. On a road race circuit and on public roads, you often ride through gusts of wind. With a land speed record, most are probably done in calm weather, so there would be no advantage in having a heavy crank, and the run up is very long, so no advantage in a light crank.
If I had both the heavy and the light crank, I would choose to use the light one. However I don't think there is much in it after you have changed the gearing to suit the circuit. The only reason I would buy a light Norton crank would be because it was one-piece. I don't like those bolts which hold the three-piece crank together.

Have a look at the videos of the North West 200 or the Ulster GP in Ireland and imagine doing that on uncontrolled roads. The first time you came around a corner and found horse-shit on the road, it would be goodbye cruel world. When you race, all you usually have to do is miss the other guys when they fall off.

Robert_Norton said:

If two otherwise identical bikes were travelling down the road, except for light and heavy flywheels, and the heavier flywheel is storing more energy, where/how is that energy dissipated, or transferred to the rear wheel?

Once a heavy flywheel is spun up to speed, isn't the force of the spinning mass helping to propel the bike forward in a way that a light flywheel is not? It's part of the drivetrain, right? So once up to speed, with the greater forces involved from the heavier flywheel, isn't it easing the work of the combustion chambers, by spinning around with a greater 'contained' energy than a light flywheel?

I think I've read every post in this entire thread, but I would like to have read more regarding the discernible differences of light vs. heavy flywheels, for road-use.

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I think the matter of "where torque goes" has been asked and answered already.

Dances with Shrapnel said:

I am not implying torque is lost when using a heavy flywheel, it's just being consumed for accelerating other things like the extra rotational and translational mass of a heavier flywheel - basic engineering dynamics. With a lighter flywheel, energy ends up being conserved in a faster moving motorcycle rather than a flywheel.

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I suspect the assertion that a heavier flywheel is more desirable for land speed applications may have everything to do with traction management as most land speed challenges are not on asphalt. Same goes for dirt track applications.

With a lighter flywheel, you will likely have greater drive line hysteresis loss but I doubt it is significant when compared to a heavier flywheel (within reason). Furthermore, there may even be a trade off of hysteresis loss between the engine and drive line.

I think Jim Schmidt mentioned the effect of riding style determining the preference for a light or heavy crank. When I ride a bike, I adjust my style to suit the machine to get the best out of it. With the heavy crank and the wide ratio box, the motor was too slow coming up through the rev range and too hard to be smooth coming down through the gears. The close box fixed those two problems, but with a lighter crank I might not have had them. There is another difference - with a light crank, it is easier to be jerky. If the motor is vicious, it can cause anxiety in the rider. In dirt racing, there is always a balance to be maintained between slide and drive. It also happens on the bitumen, but under much more extreme circumstances. In comparison with one of my earlier race bikes, the methanol-fuelled Seeley 850 is an absolute soda to ride really fast. It is so easy to ride that I suspect it might be waiting to bite me, so I never do anything sudden - only ever smooth.
With motorcycles, it is extremely easy to build yourself a race bike which will scare you shitless. A Commando with short stroke, light crank and full race cams and twin exhausts with big megaphones could be very nasty. The adrenalin rush might not be worth it. From experience, I know that my bike is fast enough for anybody and there is no anxiety associated with riding it.

acotrel said:

A Commando with short stroke, light crank and full race cams and twin exhausts with big megaphones could be very nasty.

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From experience sans commando and big megaphones, the 750 Seeley with a 75mm stroke is a dream of a bike to ride and race. Smooth power throughout. Its power and torque start like a hopped up long stroke Norton and keeps on pulling and producing through +8,500 rpm. Absolutely nothing nasty about it. It's a dream but I said that already.

Sheeze dudes all the smoothness in the road racing world all goes to shit on cow turds, falling riders, animals, fuel spills, wind gusts, engine/drive train sezure or sudden 'hydroplaning' on moist or powder dry loose salt. There is no smooth way to recover an upset which exactly same as slightly over doing a turn faster than the rest but require some jerking brake/throttle/fork action to pull off ahead.

Acrtel I've ridden against late E-start 850's in Ozarks Mts. both with my 2000 SV650 and hot to trot Ms Peel to agree with you a decent 850 on just sighting rates was teeth gritting to hang with cargo and all, literally stuck on rails with locomotive drive out of apexes, up to 70's mph in 15-35 mph marked curves. I'd had to risk life bike and others to apply WOT power for a pass - which scared snot out of me to contemplate so just stayed on toes letting 850 stretch lead so easy after apexes. The article I posted tended to agree with acrtel's thoughts, decently heavy flywheel benefit all but the extreme dragsters or stunters which would include un-paved hill climbers and me on Peel. I will sell off or part out Peel and keep factory Trixie Combat as so easy to operate and heavier crank more stable crossing THE Gravel and maintaining constant speed in wind gusts as a old fart likes but no way could contest with sports bikes but to hang with them not far behind up to 120 mph for a mile or so. Alas must consider hanging behind 1000+cc moderns means they get to pass cars with plenty or time to spare while I had to match or best their acceleration over the ton to get by car in time after 5 of em passed first. Need full out Combat factory racer performance to top end but we all slowed sanely for sharp turns to bunch back up like most race events have happen.

Stick with stock or heavy-ish cranks for most security which often wins races and pleases sane thrill seeks and leave the light weight flywheels to extremists like Maney 18.5 lb 1/3rd ligther cranks please.