Angle BTDC - ignition setting

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john.bebb
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Re: Angle BTDC - ignition setting

Postby john.bebb » Mon Jul 20, 2020 2:28 pm

An explanation of the Conrod.
I am somewhat bemused that persons who seemingly may consider themselves to be (motorcycle) “mechanics” appear to have so little understanding of true mechanics in the real world.

A conrod is a rigid linkage lever that transfers linear motion into angular (rotational) motion.
It is a momentum arm of fixed length with a pivot at each end.
The big end pivots on the big end journal on the crank web and the small end pivots at the piston.
Thus the big end is restricted to rotary motion and the small end is restricted to linear motion.

The Stroke of a reciprocating engine is defined by the “throw” of the crank; -
i.e. the offset on the crank web of the big end journal from the axis of the crankshaft.

Example; - big end journal offset from crankshaft axis = 3” therefore stroke = 6”.
Using conrod = 8”; -
@ TDC small end is 8”+ 3” = 11” from crankshaft axis
& @ BDC small end is 8” - 3” = 5” from crankshaft axis
Displacement = TDC – BDC = 11 – 5 = 6” – i.e. the stroke, which is precisely what we would expect…

Using conrod = 9”; -
@ TDC small end is 9”+ 3” = 12” from crankshaft axis
& @ BDC small end is 9” - 3” = 6” from crankshaft axis
Displacement = TDC – BDC = 12 – 6 = 6” – EXACTLY the SAME value as previous

Using conrod = 7”; -
@ TDC small end is 7”+ 3” = 10” from crankshaft axis
& @ BDC small end is 7” - 3” = 4” from crankshaft axis
Displacement = TDC – BDC = 10 – 4 = 6” – again, EXACTLY the SAME value.

The linear displacement of the small end with respect to angular motion of the big end is determined only by the throw of the crank. This is the same for ANY length of conrod.

The length of conrod simply defines the distance of the centre of displacement from the crank axis.
There is no change to the range of displacement as such is solely determined by the crank throw.

The relationship between the angular displacement of the big end and the linear displacement of the small end is determined solely by the throw of the crank.

Conrod length is irrelevant to such discussion – it merely defines the centre of linear displacement wrt to the crank axis.

To any who can demonstrate mathematically that the physical length of a conrod has any effect on such relationship will have (apparently) succeeded in breaking Newton’s Laws of Motion.
They would then be welcome to claim ownership of my ’49 Mk1 Square…

You do NOT need to know the length of the conrod - simply use the calculator to which I gave a link earlier in this subject string.

Cheers, John

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cmfalco
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Re: Angle BTDC - ignition setting

Postby cmfalco » Mon Jul 20, 2020 6:55 pm

john.bebb wrote:You do NOT need to know the length of the conrod - simply use the calculator to which I gave a link earlier in this subject string.
You are oh so completely wrong. But, I don't want to pay the $2000 it would cost to ship your bike to the U.S. so I won't go through the simple geometry to show you why the length matters.

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Re: Angle BTDC - ignition setting

Postby david.anderson » Tue Jul 21, 2020 11:00 am

I would suggest that anyone who sets the NH ignition timing at 18mm before tdc and expects to get a 38 degree advance will be extremely disappointed.
The NH rod is 7” c/c
The rod stroke ratio does have an effect on ignition timing, on how hot the cam appears and on the torque the engine delivers. An alteration to the length of the conrod results in the piston being a different distance from tdc over normal ignition timing ranges. The displacement at top and bottom of the stroke 0 and 180 degrees is the same but the displacement at 38 btdc will not be the same if the conrod length is altered.
As a so called mechanic I have played around with different rod/stroke ratios ( and have adjusted the deck height accordingly) in the VH and was surprised at how much I had to reduce my ignition timing with the longer rod. I was also surprised that the cam appeared much hotter and had lost a lot of low down torque with the longer rod. Longer rod stroke ratios are often used in hot engines in vintage racing by those in the know. A longer conrod results in the piston travelling slower at the top and bottom of the stroke, ie there is a longer dwell time. The longer dwell time enables better high rev cylinder filling and reduced ignition timing. The 18.000 rpm formula 1 engines were running rod stroke ratios of around 2.8 to 1 whereas modern cars are closer to a 1.7 average, although modern high performance motorcycles are often around 2 to 1 or higher.
Ariel seem to run a rod stroke ratio of about 2 to 1 throughout the range.
This site gives an insight into the real world effect of conrod length. http://victorylibrary.com/mopar/rod-tech-c.htm
David

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Re: Angle BTDC - ignition setting

Postby jj.palmer » Tue Jul 21, 2020 4:55 pm

One method of proving mathematically whether the con rod length effects the BTDC angle calculation is to carry out two calculations, keeping all things constant except the con rod length, the con rod length is different in both calcs. If the BTDC angle stays the same in both calcs. then it proves that the con rod length has nil effect but if the BTDC angle changes then it proves that the con rod length has effect and is therefore critical.

Calculating the BTDC angle is simple trigonometrical mathematics using the cosine rule, school boy/girls stuff.

See attached calculations 1 and 2, by changing the con rod length from 200 mm in calc 1 to 300 mm in calc 2 it has changed the BTDC angle from 41.566 degrees to 42.783 degrees thereby proving that the con rod length is critical to such calculations.

The fundamentals are that if you change the length of one side of a triangle then it will change the angles within that triangle.

Cmfalco is absolutely correct, we cannot obviously share the bike so I suggest that we toss a coin for it.

John P.
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cmfalco
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Re: Angle BTDC - ignition setting

Postby cmfalco » Thu Jul 23, 2020 4:11 am

jj.palmer wrote:Cmfalco is absolutely correct, we cannot obviously share the bike so I suggest that we toss a coin for it.
Even if we figured out a way to share the bike, we'd still have to toss a coin to decide who had to set the timing...

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Re: Angle BTDC - ignition setting

Postby jj.palmer » Thu Jul 23, 2020 3:43 pm

I suppose a way out of our dilemma is to ask John.bebb if he would kindly take back ownership of our bike ;).

John P.

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Re: Angle BTDC - ignition setting

Postby JohnnyBeckett » Thu Jul 23, 2020 6:39 pm

hi what has been done to the engine that you can not set the timing like we all do i set my 350 timing with the piston 1/2" before top dead center and the bike runs well :?: :?:


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