Heavy flywheel equals more torque? (2014)

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So chaps ,if you were building a 750 for hill climbing , would you go for standard or light crank?
 
Oldmikew, your question is too simplistic sir.

Which lights crank are you comparing to stock and why? What is you definition of ‘better’?

IF you are asking, which is better for performance for hill climbing, a stock weight crank or lighter one, then I’d also suggest a stock weight crank. BUT even that’s not so simple, it can also be influenced be rider style / preference / prior experience / etc.

IF, however, you are asking, which crank is best for racing IN GENERAL a 40+ year old stock crank of unknown history and condition with a cast flywheel etc... or a new, stronger, Maney crank, that’s also lighter, then I’d suggest the Maney crank.
 
Fast Eddie said:
Oldmikew, your question is too simplistic sir.

Which lights crank are you comparing to stock and why? What is you definition of ‘better’?

IF you are asking, which is better for performance for hill climbing, a stock weight crank or lighter one, then I’d also suggest a stock weight crank. BUT even that’s not so simple, it can also be influenced be rider style / preference / prior experience / etc.

IF, however, you are asking, which crank is best for racing IN GENERAL a 40+ year old stock crank of unknown history and condition with a cast flywheel etc... or a new, stronger, Maney crank, that’s also lighter, then I’d suggest the Maney crank.
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I'd assume all things being equal would be the question not whether or not a 40 year old crank would let go?
I would assume wicking a heavy crank up to say 7000rpm and heading up a hill would be better than a very light crank at 7000 rpm .
But I suppose the light crank could hit 9000 rpm and change the ball game,
I used to have a commando motor in a feather bed frame with a severely lightened /polished crank I would not want another one
 
If I were building a race motor, I think my over riding question would be “will the 40 yr old crank let go”!

Or perhaps, a more valid question is “when will it let go”!!

So I’m not challenging your assumptions baz, I’m suggesting to oldmike that whilst he’s thinking of crank choice, IMHO, he should ensure he is choosing a new crank first and foremost, and then decide upon the weight.

But, if you choose and off the shelf new crank in the U.K. it’s gonna be a lightweight one.
 
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A good example of the heavy/ light flywheel comparison might be the new 1200 WC Triumphs.
The T120 crank is reported to be 7 pounds heavier than the Thruxton 1200 crank. The heavier crank is supposed to smooth the power pulses.
I kind of like the power pulses.
Vibration is non existent on both bikes.

Its not a perfect comparison because the Thrux R is also 60 pounds lighter overall and runs higher compression.
Getting off the T120 and onto the Thrux is like getting out of your Dad's Station Wagon and into your older cousin's 427 Corvette.

Glen
 
Fast Eddie said:
If I were building a race motor, I think my over riding question would be “will the 40 yr old crank let go”!

Or perhaps, a more valid question is “when will it let go”!!

So I’m not challenging your assumptions baz, I’m suggesting to oldmike that whilst he’s thinking of crank choice, IMHO, he should ensure he is choosing a new crank first and foremost, and then decide upon the weight.

But, if you choose and off the shelf new crank in the U.K. it’s gonna be a lightweight one.
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Yep I get where you are coming from , also I didn't know that the the only new cranks that are available are light weight ones ,I suppose also it must depend how deep your pockets are and how quick you want to get up that hill!,
Maybe in short bursts the commando motor would be ok?.
For longer bursts/high sustained revs I suppose you'd be looking at maney cases /crankshaft and a stronger gearbox , maybe a two speed box?
 
One thing I have noticed about the heavy crank in the 850, is it seems to kill throttle response. It doesn't matter what you do the motor tends to spin up at the same rate whether you use low or high overall gearing. So the deal is that once you have the crank spinning high, you keep it there. Between up changes, my revs don't drop much more than 1000 RPM. Down changes with the close box are much smoother. I tried using the standard gearbox for racing and the bike was useless. The four speed close box is excellent every where except off a clutch start when first gear is too high. Five speeds close ratio but with a low first gear would be a good answer for racing. With a road bike, you don't have the same imperatives and a close ratio box might simply be a pain. I am used to Triumph 650s but I really like the heavy crank in the 850 Commando. A good 650 Triumph is hard to beat in an historic race, but an 850 Commando is better. Before I had the 850, I always thought that lowering the overall gearing makes the bike accelerate faster. With the 850 Commando, that is not necessarily the case. If your overall gearing is too low, you depend on the motor spinning up in response to the throttle, so with the heavy crank the bike is slower. I have conscientiously tried to make my motor stall when ridden around a race track by raising the overall gearing , it is almost impossible. Once the heavy crank is spinning, it tends to keep spinning. It is very deceptive.
 
baz said:
Yep I get where you are coming from , also I didn't know that the the only new cranks that are available are light weight ones ,I suppose also it must depend how deep your pockets are and how quick you want to get up that hill!,
Maybe in short bursts the commando motor would be ok?.
For longer bursts/high sustained revs I suppose you'd be looking at maney cases /crankshaft and a stronger gearbox , maybe a two speed box?
Click to expand...

I think that to answer those questions Mike needs a good, straight, talk with his nephew whilst looking deep into his eyes!

If he’s a mechanically sympathetic kinda rider, I think you’re right that the stock motor would be fine for a long time given the short burst nature of the events.

But if he’s a Blood n Snot, Death or Glory, young hot shot ‘Shrapnel Maker’ then, well, perhaps not.

Come on Mike... fill us in... so we can start spending your money...!!
 
I must admit I do like the sound of building a commando hill climber myself
 
Fast Eddie said:
Oldmikew, your question is too simplistic sir.

Which lights crank are you comparing to stock and why? What is you definition of ‘better’?

IF you are asking, which is better for performance for hill climbing, a stock weight crank or lighter one, then I’d also suggest a stock weight crank. BUT even that’s not so simple, it can also be influenced be rider style / preference / prior experience / etc.
Click to expand...

Eddie , by better I simply was thinking in terms of performance. If there is some sort of straight before the first bend then a stock crank will catapult the bike off the line in a shorter time than a light one, however a light crank will have better engine braking which might mean the bike will handle better under heavy braking making it possible toget power on sooner coming out of say a hairpin like at Olivers Mount. So swings and roundabouts ?
 
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baz said:
Yep I get where you are coming from , also I didn't know that the the only new cranks that are available are light weight ones ,I suppose also it must depend how deep your pockets are and how quick you want to get up that hill!,
Maybe in short bursts the commando motor would be ok?.
For longer bursts/high sustained revs I suppose you'd be looking at maney cases /crankshaft and a stronger gearbox , maybe a two speed box?
Click to expand...

Well this is the interesting question , a hill climb is much shorter than a road race , and I might have been prepared to chance it , but I once put a rod through the cases of a BSA Super rocket at 80mph , not an experiance to be repeated. So whatever the crank it will not be the original 47,000mile 45 year old one
 
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acotrel said:
One thing I have noticed about the heavy crank in the 850, is it seems to kill throttle response. It doesn't matter what you do the motor tends to spin up at the same rate whether you use low or high overall gearing. So the deal is that once you have the crank spinning high, you keep it there. Between up changes, my revs don't drop much more than 1000 RPM. Down changes with the close box are much smoother. I tried using the standard gearbox for racing and the bike was useless. The four speed close box is excellent every where except off a clutch start when first gear is too high. Five speeds close ratio but with a low first gear would be a good answer for racing. With a road bike, you don't have the same imperatives and a close ratio box might simply be a pain. I am used to Triumph 650s but I really like the heavy crank in the 850 Commando. A good 650 Triumph is hard to beat in an historic race, but an 850 Commando is better. Before I had the 850, I always thought that lowering the overall gearing makes the bike accelerate faster. With the 850 Commando, that is not necessarily the case. If your overall gearing is too low, you depend on the motor spinning up in response to the throttle, so with the heavy crank the bike is slower. I have conscientiously tried to make my motor stall when ridden around a race track by raising the overall gearing , it is almost impossible. Once the heavy crank is spinning, it tends to keep spinning. It is very deceptive.
Click to expand...

To keep revs within a 1000 rpm band on most hill climbs would be difficult even with a veryclose ratio box ,so good point
 
Fast Eddie said:
I think that to answer those questions Mike needs a good, straight, talk with his nephew whilst looking deep into his eyes!

If he’s a mechanically sympathetic kinda rider, I think you’re right that the stock motor would be fine for a long time given the short burst nature of the events.

But if he’s a Blood n Snot, Death or Glory, young hot shot ‘Shrapnel Maker’ then, well, perhaps not.

Come on Mike... fill us in... so we can start spending your money...!!
Click to expand...

Hmm it might be alright but then he has never ridden Brit stuff, so in anger be quite easy to blow up a stock Commando.

so best to build a seperate race motor and leave the original engine alone .
 
Fast Eddie said:
You don’t need to... we’ll help Mike build his...

Much easier to spend someone else’s money !!
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Lol , ok so I would have thought Steve Maney cases and crank beyond that I am not sure. I had not thought of camshaft whip
is this the price one pays for having a stiffer valve train ? And If Jim Comnez is working on three bearing cams it might be better to wait and see. Am taken by Jseng's point that if you have a cam profile and followers that do not bounce at up to 9000 rpm you donot have to do 9000 rpm to get the benefit.. Its the rate at which the engine develops its power that I am thinking of
The Uk class is upto 750cc so might be better to use Iron barrels
 
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