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Old 18th May 2011, 02:39   #1
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Default What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Hi Folks,

This might sound a bit odd asking this question now.. But really, what helps the CI engined bulls cruise effortlessly at 40km/hr in 4th gear? It never jerks.. its simply ecstasy indeed.


I couldn't get a proper answer. Is it something with the metallurgy..? Cast Iron vs Aluminum? or the CB Points ignition or the gear ratios or something else? What is it that makes the CI bulls different from the other AVL/UCE engines? What is that 'X' factor?
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Old 18th May 2011, 05:04   #2
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Does the material of the engine matter that much?

I don't know about engines, but in machining most lathe beds and guide-rails are built with high-carbon cast iron due to its self-lubricating properties. The grain structure has carbon in the form of graphite, and wear and tear exposes that graphite on the surfaces continuously - a well machined surface will remain smooth no matter how much it wears out.

Aluminum on the other hand is notorious for high friction surfaces- though I don't think anybody would be using aluminum by itself, and properties of alloys can be very different from those of pure metals.
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Old 18th May 2011, 08:02   #3
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Can you give examples of which CI and AL engine you are referring here?
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Old 18th May 2011, 10:30   #4
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Don't know about CI vs Al, but I read that the older bullets came with a heavy crank & flywheel which resulted in smoother momentum.
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Old 18th May 2011, 11:12   #5
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

I thought no one will ever ask this question.

This ad tells you why.



Note: The parts where the factory and bike are shown are not in slo-mo like the rest of the clip.

Hope that answers.
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Old 18th May 2011, 14:21   #6
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Cast Iron (CI) engines have better low end torque with their heavy crank compared to AVL engines with light crank. So CI engines can cruise comfortably even at low RPM due to their high torque at low RPMs.

Last edited by Rehaan : 19th May 2011 at 15:07. Reason: Mistakenly off-topic post removed :)
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Old 18th May 2011, 14:36   #7
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Cast iron engines had heavy crankshafts compared to AVL and UCE, this helps in slow low rpm cruising. Cast iron engines made till around 78 and after 2003 till the end had heavier crankshafts compared to the model years in between.
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Old 18th May 2011, 17:15   #8
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Quote:
Originally Posted by Ride_4_Fortune View Post
Cast Iron (CI) engines have better low end torque with their heavy crank compared to AVL engines with light crank. ...
Quote:
Originally Posted by Sankar View Post
Cast iron engines had heavy crankshafts compared to AVL and UCE, this helps in slow low rpm cruising...
+1. Inertia does the trick. Downside being slow picking up of speed.
AFAIK, lighter crank can be swapped with heavier ones for that slow speed cruising.

And with older exhausts sound, its bliss cruising on highways!
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Old 18th May 2011, 19:23   #9
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Quote:
Originally Posted by dhanushs View Post
lighter crank can be swapped with heavier ones for that slow speed cruising.
on a UCE engine like the CL500?
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Old 18th May 2011, 20:54   #10
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Default re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Quote:
Originally Posted by aruncheloor View Post
on a UCE engine like the CL500?
Sometime back in Kerala, all the RE authorised service centre folks and mechs (region wise) were invited for a training section on the UCE engine by the manufacturer. "Kozhi Biriyani" was served, and the first point RE folks made was, to never tryout the heavy crank stunt on the UCE.

CI 350's and the CI 500's cranks are different. It wouldn't really make sense to change the crank/add weight on the CI 500. Whatever pickup and top-end the 500 is left with will go for a toss.

Last edited by jeeva : 18th May 2011 at 21:06.
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Old 20th May 2011, 08:40   #11
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Default Re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

It is definitely due to the heavy crank (a flywheel effect!). But do keep in mind that all the CI engined Bulls did NOT have a heavy crank. If Iam right from late 90's to early 2000, the Bull's came with light crank. But coming towards the end of production till 2010, Bull's again starting coming with heavy cranks. The one which I took new in 2009 had a heavy crank.

Heavy Crank :- Takes time to pickup speed (Understand what a flywheel effect is to know better). Once it has the inertia it keeps the momentum going (Newtons 3rd law!). That is why it seems to cruise at low speeds. It kind of becomes a 'hybrid' wherein it gets power from the engine (4th gear) and the inertia from the heavy crank.

Light Crank:- 4-S Electra's (and Light crank CI350's) came with lighter crank and hence 0-90kmph was a breeze on those. But they can't even think of low speed cruising.

The pre 1964 British models of CI's came with heavy to very heavy cranks. I knw those Bull's pulling at even below 30kmph at 4th gear and the 'beat' at that point is just mesmerizing!

Note: In a light crank CI Bull, crank weight can be increased.

Warning:
Though having said so, the Bullet Guru's in team-bhp have termed it 'lugging' and have warned that it could seriously damage the gearbox, put undue stress on the engine and even strain the chain and sprockets.

Last edited by sam_sant2005 : 20th May 2011 at 08:43.
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Old 20th May 2011, 09:06   #12
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Default Re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Quote:
Originally Posted by sam_sant2005 View Post
It is definitely due to the heavy crank (a flywheel effect!). But do keep in mind that all the CI engined Bulls did NOT have a heavy crank. If Iam right from late 90's to early 2000, the Bull's came with light crank. But coming towards the end of production till 2010, Bull's again starting coming with heavy cranks. The one which I took new in 2009 had a heavy crank.
Bullets till 78 (iirc) came with heavy crank. From then onwards bullets had lighter cranks till 2002. From 2003 onwards they went back to the heavy crank configuration till production stopped.

I had discssed this in a very old post of mine, it has more proceise information reg this, crank weight too, forgot most of it.
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Old 20th May 2011, 09:37   #13
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Default Re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Quote:
Originally Posted by Sankar View Post
Bullets till 78 (iirc) came with heavy crank. From then onwards bullets had lighter cranks till 2002. From 2003 onwards they went back to the heavy crank configuration till production stopped.

I had discssed this in a very old post of mine, it has more proceise information reg this, crank weight too, forgot most of it.

Hey guys,

just help me with this:

Once the vehicle is moving and the clutch is engaged, wouldn't the weight of the vehicle (and moment of inertia of the rear wheel) act as a "crank/flywheel weight" too?

I guess it'll reduced by the effective gear ratio (or rpm ratio between the engine and the wheels) before adding to the flywheel's own moment of inertia, but I don't know whether this one will be insignificant.
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Old 20th May 2011, 12:09   #14
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Default Re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

The two engines are very different.
Not just because of the material of contruction - Al vs Fe.

But also the shape and internal dynamics during the Otto cycle.
And there is difference in the valve seats, as well as exhaust ports also.
And then the carbs employed - in both cases are different.

All these differences are not because of difference in material only - but because of difference in engine.

This is the reason why both have such a difference in torque, fuel consumption and sound.

If you check the specs - the peak torque of old CI engine was at around 2500 RPM.
The peak torque of the new Al engine is about 3500 RPM.

There is a huge difference of 1000RPM. Thus the CI engine used to produce peak torque earlier in its life. If you manage to get the torque / RPM graphs - you will realize that at 500-1000RPM the old CI engine had enough torque to keep the bike up and running, whereas the new one manages the same torque only after 1000RPM.

Heavier flywheel does not change the torque characteristics of the engine. It only determines how nimble will be the RPM changes. (Light flywheel = lower moment of inertia = less torque required to change the RPM)


As a simpler analogy - its like difference between a diesel engine and a petrol engine. Both have different torque / rpm characteristic - even though they may have exact same cubic capacity.

Second, The flywheel and crankshaft is dictated by the engine characteristic, and not the other way round.

Last edited by alpha1 : 20th May 2011 at 12:12.
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Old 20th May 2011, 14:47   #15
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Default Re: What makes the Cast Iron (CI) engined Bullets cruise effortlessly at low rpms?

Maybe not. But it does change the way how an engine behave in the real world.
In my Bullet (1970) i have run with the OE heavy crank and a lighter 1980's light crank. Heavy crank stores more energy than light crank. Heavy crank pulls the bike from slow speed, lighter crank required dropping a gear. Heavy crank took time to reach higher speeds, lighter crank reached those speeds faster. Heavy crank sustained the high speeds when let off the throttle, lighter crank dropped speed faster when let off the throttle.
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