Flywheel weight?

wildbill56

wildbill56

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I'm building a 750 engine with a SS cam, kibbelwhite valves and guides. I'm thinking of using a 850 flywheel, 1 lb. heavier. Will the SS negate any low rpm torque gain?
 
I run a 2S cam in my 850 motor since 1980 works great, but my crank is also balanced at 72% for the Featherbed frame and hard mounts, but of course a bigger cam don't work unless you do other work with the motor, porting and jetting etc etc.
 
You won't get any increased torque output from a heavier or lighter crank. Torque output is a function of what happens in the cylinder and head.

There is a bit of flywheel effect from a heavy crank and this can be useful for avoiding stalling etc, but actual torque output as measured on a dyno will be the same either way, other than a lighter crank gives a wee bit quicker throttle response, so that torque rise on opening the throttle comes just a fraction of a second earlier with a light crank.

Glen
 
If you want more torque then stay with the standard (.330 lift) cam rather than a combat double S (.390/.346 lift). The Combat cam gives a thrilling power surge above 6,000 rpm….I like that.
 
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I do not think you will notice any significant difference with a 1 pound weight increase! But one question, have you checked out the flywheel stud sizing you are going to use on this hybrid?

Just maybe it will have a slightly better idle than the crank you have, and yes the cam change might negate that, or not, depending on the influence of other factors that affect idle, i.e. carburation and timing. (assuming you don't significantly change anything else).

And as a racer I can only comment that road riders spend far too much time worrying about idle settings, and far too much time at idle. Remember that the cam isn't getting lubed until over 2000rpm.

If you wanted a more reliable crank you might choose this steel item from Molnar, but at 7.8 pounds it is much lighter than the 'standard' 11 pounds! (is yours 11, or more?)

Norton Twins Steel Flywheel

Norton Twins Steel Flywheel Commando Atlas Dominator Dommi Racer Dommiracer Domi Domiracer 650 750 850 920 cc 650cc 750cc 850cc 920cc 89 mm 89mm stroke Featherbed Feather Bed
www.tga.co.uk

This is what Steve Maney, who like Molnar is at heart a racer and favours light cranks, says about crank weights:

Standard Norton crankshaft--------------------22-24 pounds
One piece billet crankshaft-------------------------28 pounds
Maney lightweight crankshaft------------------17-18 pounds

'This incredible weight reduction dramatically improves acceleration and reduces stress on the crankcases'.

I have a Maney short stroke crank, so it's probably nearer the 17 than the 18. It is in my race bike, and it spins up great, and personally I love it. Idle is typical lumpy race bike!

However, Billet cranks made by Nourish for Nortons were a popular item when available, and are probably the 28 pound items Maney references.

Other racers have fitted those one piece billet crankshafts that are significantly heavier than the standard items. Guess what? They love those too!
 
I always believed a light crank was better, until I got my 850 motor going quicker. Whatever the torque characteristics of you motor might be, the gearbox affects. You can have a light crank in a motor with a very narrow but strong power band - then you need a gearbox which keeps the revs within the usable rev range. With a light crank, throttle response can be better, but a Commando motor does not need good throttle response to be quick. Once the heavy crank is spinning high, nothing stops it. With close ratio gears behind a Commando motor, you get a leap forward on every up-change. But you cannot shut the throttle to do the change, because then you rely on throttle response to get the revs back. You need to back-off just enough to take the load off the gears.
With a road bike - you do not normally use it in that way.
Your hot cam will still give you some advantage, but you will need to change the gearing to find it.
I sincerely believe that the things which made the biggest improvement to my bike were the gearbox and it's steering geometry.
My motor is nothing unusual.
The bike is actually ridiculous in what it can do. I never believed in it. I can ride it into a corner behind several bikes and simply blast under and past them, while they are all up high on extreme angles of lean, being careful.
 
I've often wondered if a heavier crank provides a little inertia boost that makes a kick start bike easier to start and keep running without a lot of holding the throttle open above idle position when cold.

Are the rod journal centerlines the same on 750 and 850 cranks? I am clueless, since I've never put a hand on an 850 Norton engine.

Apparently, there were 2 SS cams based on that NOC article referenced. Early Domminator/early Commando, and later a Combat SS called a 2S. I have a cam with a pair of S's stamped in the side of one of the lobes. I don't think it is stock though. Sure didn't feel like it was stock at around 5700 RPM and up. If geared up for HWY cruising the bigger cam was not good for short shifting in my experience. For example, riding at 25-30mph and shifting into 3rd gear was lugging the motor.

A heavier 850 crank will probably make the throttle response a little mellower on acceleration from a stop. You might like it, you might not. I have the Molnar crank in my 750. Gotta use common sense and moderation on the throttle in town. It moves forward pretty quick. I also like the natural engine brake with lighter cranks and rotating mass in general.
 
In other words - do what you think might be best with your motor - then adjust the gearing to suit it's characteristics. Do not fit a hot cam and not expect to have to change gearing. A normal Commando has a useless gearbox, which is designed for commuting, not fast riding.
 
Schwany said:
I've often wondered if a heavier crank provides a little inertia boost that makes a kick start bike easier to start and keep running without a lot of holding the throttle open above idle position when cold.

Are the rod journal centerlines the same on 750 and 850 cranks? I am clueless, since I've never put a hand on an 850 Norton engine.

Apparently, there were 2 SS cams based on that NOC article referenced. Early Domminator/early Commando, and later a Combat SS called a 2S. I have a cam with a pair of S's stamped in the side of one of the lobes. I don't think it is stock though. Sure didn't feel like it was stock at around 5700 RPM and up. If geared up for HWY cruising the bigger cam was not good for short shifting in my experience. For example, riding at 25-30mph and shifting into 3rd gear was lugging the motor.

A heavier 850 crank will probably make the throttle response a little mellower on acceleration from a stop. You might like it, you might not. I have the Molnar crank in my 750. Gotta use common sense and moderation on the throttle in town. It moves forward pretty quick. I also like the natural engine brake with lighter cranks and rotating mass in general.
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When you use a motorcycle for fast riding, you usually have a clear idea about the power band you are using, and keep within the rev range. For commuting, you would not have a hot motor. If you have that, you need to learn to live with it.
If I had a normal Commando, I would change the gearbox internal ratios before I did anything else. A Commando 1st gear with the upper 3 as Manx ratios would not be difficult to live with, and much faster -$750 ! A standard 850 Commando motor is fast enough without modification.
 
Schwany said:
....Are the rod journal centerlines the same on 750 and 850 cranks? I am clueless, since I've never put a hand on an 850 Norton engine.....
Click to expand...

Yes. Except for the Mk3 crank and the short stroke crank, all the 750 and 850 cranks have the same critical shaft and journal dimensions, except for some small differences in the timing side main bearing journal between early 750 and later 750/850 cranks. The differences are mainly in the flywheel weights.

Ken
 
Heavy flywheels are for mud pluggers , snow , sidecar road things , touring and so on .
Light flywheels are for Biff wiffle , RD 350s , canyon racers , and quicker pick up . If its not slippery . then they pick YOU up , as its spun up , and you off .

a pound off a 750 , if your young and keen , wouldnt hurt . Putting some on as you are steadies things up . same output essentially . Just differant hook up . INERTIA .
 
I’ve used heavy and light cranks in Commandos, Triumphs twins and triples.

IMO a better / worse debate is futile. It quite simply comes down to rider preference of the different characteristics.

Personally, I prefer the feel of a lighter crank in all of the above mentioned engines. I like the freer and more lively feel they give.

In my current Commando engine rebuild (thanks to Jim Schmidt) I am reducing reciprocating weight of the pistons, pins, rods ends, etc by OVER HALF A POUND (it’s not a stock engine) and am therefore now working on fitting a flywheel with a similar reduction, whilst achieving the desired balance factor. I am rather intrigued to see the difference of that amount of weight reduction in something going up-stop-down-stop 120 times per second !
 
I would never go to a 850 crank counterweight. In a configuration with 9:1 compression with stock Commando cam, a 750 responds much better to throttle than an 850.
My experience is that 10:1 compression with better carbs and free flowing exhaust are what a Commando responds favorably to. Sadly, Dunstall silencers aren't widely available anymore. Maybe the replicas are as good at reducing back pressure.
I would more so up your compression to 10: 1 via pistons or shaving the head .040 and shorting your pushrods if you mill the head. If you're going to retain Amal carbs, go to 32mm and use '74 850 manifolds to bolt to your pre Combat head. Leave the intake ports stock, it flows quite nicely with bigger carbs with the 850 manifolds. Cam is you choice, stock Commando is quite good all around street cam. Others are less in the mid range but more power in upper revs.
 
Hepolite PowerMax pistons were good for upping the compression. I don’t believe they are made any longer, but someone likely has a box of new ones stashed.
 
When I first started racing my Seeley 850, I wondered what the hell I had struck. It is very different to any Triumph twin. I could not do anything with it while it had the standard ratio gearbox. It used to die in the bum on up-changes. The secret is to keep it spinning high and to not lose revs on the up-changes. With slow taper needles in the carbs, throttle response becomes better. The size of the needle jets is critical. I have also found that it's power characteristic is very deceptive - I never knew what overall gearing I could operate with, until I started raising it. Lowering the gearing does not make the bike accelerate faster..
When I raced my 500cc short stroke Triton, lowering the gearing would make it accelerate like a rocket, then it would run out of steam towards the ends of the straights. The 850 Commando motor does not do that - it is fast everywhere.
One of my mates has a fast 650 Triumph - he never revs it over 6,300 RPM, and pulls high gearing. It is tuned for torque. I have ridden it. It is neutral handling and quick towards the ends of the straights. But I think it had a wide ratio box when I rode it, and now has a 5 speed Triumph box. Closer ratios make a significant difference.
 
With the close ratio gearbox, my motor operates between 5,500 RPM and 7.300 RPM. With a normal gear box, that lower number .can become 4,500 RPM - that 1000 RPM difference is where better throttle response is needed. With the bigger gap between the gears, the carbs become open faster, and you use the tapered part of the needles more. The slight richness causes the motor to respond slower. It could be fixed with fuel injection. With my bike, II feed the throttle on, as I would do with a two-stroke. Also using methanol halves the tuning errors because the jets are bigger.
 
wildbill56 said:
I'm building a 750 engine with a SS cam, kibbelwhite valves and guides. I'm thinking of using a 850 flywheel, 1 lb. heavier. Will the SS negate any low rpm torque gain?
Click to expand...
I suggest it does not matter much what cam you use. When you change it you cannot expect it to work as well or better then the one you replaced, until you optimise everything which works in conjunction with it.
I did not choose a cam for my 850 motor. It is 1950s technology, so I went to a cam grinder who ground cams for race bikes in the 1950s, and asked for his best Norton grind. It is probably 650SS. I then adjusted cam timings , jetting and exhaust system by riding the bike on a race circuit.
A bog standard 850 motor , optimised in the same way , would probably be just as quick, except for the silly crank balance factor.
 
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