## degrees BTDC vs mm BTDC.. ahem.. WARI

if im setting the timing on a bike, it can be referred to as either in mm before top dead center or degrees before TDC, correct? to convert from mm to degrees BTDC, is it as simple as just saying:

2mm before top dead center = 2(360/stroke)

360/stroke gives you the ratio of mms to degrees... multiply by 2mm and that will give you how many degrees?

for example. stroke length = 41mm. 360/41 => 1mm = 8.78 degrees. times 2mm = 17.56 degrees... is that correct? that seems like a bit much.

as i think now i realize it could be 180/stroke? i dunno. am i warm?

the reason i ask for this conversion is because i'd rather set timing in terms of degrees over mm is i feel like i could measure it more accurately.

i mention wari because i know she knows all sorts of special things like this (this is probably pretty basic knowledge for someone who know's their ish)

Thanks,

-Steve

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

steve, draw a circle on some paper the same diameter as the stroke, draw a horizontal line at tangent to the top of the circle, draw another line 2mm below that

connect where the second line intersects the circle to the center

break out ye old protractor and measure the angle

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

mmmhmmm. very clever. i like.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

This is a better drawing....

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

theres always more than one way to skin a cat....

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

To do that with a calculator, divide the mm spec by half the stroke, subtract that result from 1.0, take inverse cosine of that. For example 2 mm and 60 mm should come out to 21 degrees.

I'm not sure if this method is absolutely correct because it doesn't account for the angle of the connecting rod, but when near TDC the connecting rod is approximately parallel to the cylinder bore anyway so that would be a minor factor.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

You can use this.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

Lets try that again.

See attached

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

i think you can make a spark plug hole adapter for a dial indicator

the highest point will be tdc and you just move the piston back to make the needle move 1mm back

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

mk i tried that and it didnt seem work... the pictorial way devised by crackstar, is really helpful but not something you really want to be doing while tearing your bike apart. can't there just be a simple ratio for "for X degrees flywheel is rotated, the piston moves X mm"? or is it because as the piston reaches TDC, and BDC, that ratio changes... im using the .gif at wikipedia to better envision this. pretty sweet moving diagram of a two cycle engine.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

try this

http://www.engineersedge.com/engine_formula_automotive.htm

you want "Piston Travel vs. Crank Rotation"

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

Cool diagram. You will note that even though the crank is turning at a constant speed, the piston slows down and stops as it approaches TDC (thus the term, "dead center"). The ratio of degrees of crank angle to piston position is not constant throughout the cycle. In fact the first degree you turn the crank the piston may not move at all.

Neglecting the possible effect of the connecting rod, it is approximately a sine function. That can be solved by drawing a circle on paper or by using the trig functions on a calculator.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

are u guys talking about those little lawnmower power bicycle things here??

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

yea man i love that diagram. sometimes i find myself staring at it for hours. yes that makes sense now, the piston slows as it reaches TDC. I think those things up top that i suggested would be AVERAGE stroke:rotation ratio?

ahem... what would those trig functions be? For a mechanical engineering major im pretty damn bad at math. that, and i dont always want to think. Im sure that circle trick is the right idea, but you can easily mess shit up and be innaccurate when you do a pencil and paper trick like that.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

steve, 3 posts up. id put it in here but the website wont let me copy text or save the picture. and i dont feel like typing it in again.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

aha, didnt even notice that. thanks a lot hombre.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

Now you have a handle on the position of 2mm you will have to set the timing closer to tdc and if you have bumped up the compression ratio then set it at 1.5mm. stock factory settings are too advanced on most mopeds and mobys especially.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

The rest is just algebra.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

didnt you say you set your timing to 4mm BTDC? Isnt that FURTHER, and MORE advanced than 2mm? maybe i misread at some point.... but if stock timing from factory(too advanced) is 2mm and I need to retard it, shouldnt that mean i need to go more like 1.5-1mm BTDC? Essentially, rotate the timing cam clockwise? Thanks for bearing with me here.

-Steve

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

Mechanical engineering? Go do it in AutoCAD and let the dimension tool find your exact angle....

There has to be a computer with autoCAD on it somwwhere on that campus

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

What is the stroke? I'll do it right now, here at work...

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

we have pro/ENGINEER im sure they're pretty similar. My stroke is 1.64 inches= 41.656 mm... thats true, I could draw circles with tangent lines etc in a matter of seconds. Maybe I'll make a chart of timings in mm and respective degrees, for future reference.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

Here's a way to calculate piston travel versus crankshaft angle. While it is intended to be used to determine port opening/closing event timing (in degrees of crankshaft rotation), it can be used to get what you are looking for.

Just set C and E equal to zero.

If you're just trying to determine a timing point on your flywheel, then this is not the way to go - completely different formula.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

You need to know the dimensions of the connecting rod to find the exact answer. The approximate solution (single cosine or a circle on paper) just uses the stroke length.

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

Sorry for the bad pic - my computer and I don't always get along . . . .

## Re: degrees BTDC vs mm BTDC.. ahem.. WARI

You need to know the dimensions of the connecting rod to find the exact answer. The approximate solution (single cosine or a circle on paper) just uses the stroke length.

For small crank angles, I agree.

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