Forum

[Les H]

Hi John. It certainly does if you are converting a piston position to a crankshaft position or visa versa, which is what the calculator is for. Different con-rod lengths effect the relationship between the two... Try putting in different rod lengths and note the results. It is a nice tool for checking one against the other or finding the alternate measurement if only one is available. I thought you would enjoy using it....that's all.

[Les H]

Hi John....I've checked the con-rod length again on the calculator and it works out that the con-rod length is 172mm long, not 118mm as I had said earlier. I han't noticed that I had entered the bore width of 84mm instead of the stroke of 90mm ....However if the ACTUAL C2C con rod length is different from 172mm long then ONE of the stated Hitchcocks timing figures is possibly wrong...but may well be rounded up. I wonder if Paul(BW)or Norm knows the exact length?

[Norm]

Les,
I've measured half a dozen rods they vary slightly but 174.17 is pretty close. Small ends fairly close, but the big end moves about a bit, bearing in mind these are all old rods. I have 2 of Mr's forged rods with RE13's fitted sitting here, I haven't checked them because I don't want to pull them apart at the moment but I would expect them to be pretty close to this measurement. I can check if anybody is interested

[Les H]

Many thanks Norm...knew you would help. So using the 174.17 figure in the calculator with 8.5mm TDC piston position produces a timing figure of 31.18 degrees advance instead of 32 degrees....so a slight difference but nothing much in real terms. The only bike I've owned that was hyper-critical with ignition timing was a Yamaha RD350 it was the early model with points, they had to be adjusted every few hundred miles to get back the crisp power that would come in with a bang and lift the front wheel off the ground, so when it wasn't possible to do that you knew it was time to adjust the points...Cheers.

[Les H]

Oh my gawd!....must buy some better glasses...really I must....used the piston position calculator again and guess what a figure of 32.04 advance is produced....so quoted figures are spot on or as good as....I'll just go on and try to start my bike with my head and hope it kicks back.....doh!

[John J]

Hi Les, I don't know how you are doing your calculations but they are wrong. If you think about it,for a given crankshaft throw(stroke)the piston must travel the same ammount, up/down, no matter what the con.rod length is. So timing measurements ,whether linear or angular,are independant of con.rod length.

[Les H]

Hi John. Only a con-rod of infinite length on a crankshaft would respond with a true sinusoidal motion at the small end position, so as it is impossible to have anything infinitely long, then it follows that all practical con-rods do NOT follow a sinusoidal wave form. Look at it this way. Imagine the big end at either 3 or 9 o'clock position or at 90 degrees. You might imagine the piston is half way down the stroke but it is not. With the 500 Bullet engine for example, it will have its piston about 52 mm down, not 45mm down or half way on the 90mm stroke (You can do the maths John as at this 90 degree angle it only requires simple trigonometry....try different rod or Hypotenuse lengths and see what you get) However as the stroke is fixed by the crankshaft throw, the remaining 38mm of stroke (90-52) will have to accommodate the other 90 degrees of rotation to get to the BDC position. You can see then that there is not a perfect sinusoidal relationship between the top half of the rotation and the bottom half of the rotation. This angular variation is continuous throughout the entire stroke length, so by changing the con-rod length but keeping the same stroke length you will get different relative positions of the piston and crankshaft angle at every point of one complete rotation. Incidently, the other thing that changes will be the relative height of the swept bore inside the barrel, so a longer rod will mean a taller barrel is required and visa versa. You can change the engine characteristics by changing rod lengths (Ask Paul Henshaw!) because different length rods create different leverage angles for the same crank rotation, this will effect engine breathing, torque even vibration. Bottom line: Different rod lengths create different piston to crank angle relationships.

[John J]

Hi Les, I give up. You seem to think that an engine's cc's changes with con.rod length. I think your mind block is because you think in absolute measurement, where in fact it is all RELATIVE to tdc. Anyone timing to Mr H's measurement;- If you are doing it vertically, then use my earlier posting dimensions.

[John J]

Following my earlier posting. If anyone can calculate the length of a con.rod of an engine of 90mm stroke I will give them £100. This is not a measurement of a Bullet's con rod but of course would apply to any engine no matter its.cc.I'm now closing this posting.

[Les H]

I think we are talking at cross puposes John. It appeared you were saying that different con-rod lengths will NOT affect the relationship between the angular timing and linear (piston) timing after you said: "Hi Les, Con rod length is immeterial, it does not affect any timing measurements, angles or linear". I therefore replied to your statement and tried to demonstrating that different rod lengths do indeed change the relationship of linear to angular.....However if you are simply saying that with a standard engine you can use the factory quoted timing figures given whether they are for the crankshaft in degrees or piston position in mm BTDC.....aren't you just saying something that is completely and utterly obvious?......I only went to the bother to explain in response to you saying "con-rod length is immaterial" after I had shown you the handy calculator tool for checking or creating timing figures!.....Ah well never mind....leave it John...oh by the way, I cannot understand where you think I said engine capacity changes with different con-rod lengths....gawd knows how you came to that conclusion....Best wishes John.