Did I screw up my crank?

[mew905]

Disassembled my bike, replaced all the 6202 bearings with SKF ones, drilled the piston sides (where the transfers are, so no port timing change, just two tiny holes), and the crank (pics below). After reassembling I fixed a massive compression leak, replaced all the gaskets with SBP copper gaskets, and installed the new bolts from SBP. Permetex'd the head to run with no gasket again, aaaaaaand it runs like garbage. Sputtering everywhere, no power, every 5-15 seconds you hear a "poof" from the exhaust. Did I screw up the crank job? It's a solid crank, not one of those fancy ones with the removable weight. When I drilled it, I drilled ALL the way through, I didnt just drill one side, flip it, and drill the opposite side. Maybe I need to jet down? Maybe it's being flooded with way too much gas? It runs rougher than before so I'm not sure.

Split crank, brand new industrial SKF bearings


Pre-drilled crank


During drilling


After the job is done

 

[Toadmund]

Why did you drill those holes in your crank?

Sounds like it's badly out of balance.

 

[mew905]

I drilled them to achieve a higher RPM but I'm thinking I'm just running extremely rich now, with how its "chugging" and hearing a"poof" every few seconds I'm thinking its choking on gas, I'll try jetting down, worst case scenario, I'll gut my roommates old motor for his crank, but to my knowledge this should have balanced it more, I dont think I removed more material than most people have from their cranks to balance them.

 

[Toadmund]

How did you go about balancing your crank?
Been reading up on the crank balancing and is much more involved than just drilling a couple holes to lighten your crank.
It involves getting accurate weighs with a decent scale, attaching counterweights, taking off bits of metal at a time, or adding as gravity indicates an off balance on a couple rails.
These motors or any motors run at high RPM's and any change in the crankcase weight even if minute will affect performance dramatically for better or for worse.
Drilling holes also lowers your crankcase compression which will affect performance, some people fill their holes with aluminum slugs, JB weld or whatever.

Maybe your crankshaft balance is so bad it's effecting your fuel delivery, ie. foaming fuel.

 

[2door]

"Did I screw up my crank?"
Yes! Time for a new one. As is stated above, balancing a crankshaft is not something you want to try unless you're well versed on the process. Simply drilling holes to "lighten" the crank won't do it. I also concur that the sever out of balance condition you've created is the reason your engine doesn't run right anymore.

Tom

 

[mew905]

haha ok, ok, It was worth a shot. I only do these things when I have spare parts around. I like to experiment, lets me learn what my boundaries are. It still looks like its far less material removed than alot of people have taken off but, oh well, it was worth it just to get the new bearings in. guess I'll start over with my spare crank.

Just a side note, it lowered crank compression, this I was expecting. lower compression means I need less fuel, correct?

 

[Toadmund]

I would say you would need more fuel?

Anyway, I guess you won't be doing that again, if we made no mistakes we wouldn't learn a thing, but then again why would we have to learn if we didn't make any mistakes?

Never mind

 

[mew905]

Oh I plan on doing it again just not so ridiculously. Every time I try something new (to me), I go to the extremes first, or a rough copy, usually just to get familiar with the parts, what everything does, etc. Afterwards I start over entirely, using engineering precision within my tool's limitations. If you ever see me work on something I've done a thousand times (I'm usually more into electronics, rather than mechanics), I'm notoriously precise.

However I completely understand your statement, toadmund, after all, if we didnt experiment, explore, test our limits, we would still be in caves

 

[Toadmund]

I've ruined a lot of things myself, just last week or so ago I ruined my kit gear extractor by only hand tightening it, I was told to use a wrench and a bit of lube to screw it in by members here, so as not to strip it.
Thanks to excellent advice my brothers gear extractor is still usable after I accomplished my mission, removing the small bevel gear.

I always seem to think I know what I am doing, as a consequence I get put in my place REAL quick, and get my ego adjusted to a manageable level!

 

[mew905]

Thats actually the reason this is the first time ive ever disassembled my motor this far, i kept ruining the pullers. However some lube and running it in and out a few times by hand with a final light turn with a 22mm wrench got it to work rather flawlessly (advice given by people in this forum, thank you very much). If nothing else its a learning experience, going to replace the crank today or tomorrow, i am going to drill this one too, but this time, nowhere near as much, ill weigh it before and after to make sure i only remove a few grams. This is a major stepping stone to my internal supercharger idea, though THAT will be balanced all the way around

 

[Toadmund]

You should really look into proper balancing techniques before you ruin another crankshaft, I'd put a link, but I don't know who knows best on proper procedure. Maybe someone could show us a link with rock solid advice that is easy to understand.
I intend to figure out this static balancing thing because dammit, there will be no more crankshafts going on my bike without being balanced.

I'd at least like to see out my rear view mirror at WOT!

 

[mew905]

problem is alot of the tutorials are vague. Norm posted the best tutorial from what I remember but alot of them require very specialized tools that not everyone has. There has to be a way to do it without spending much money and thats what I love to discover. Seems the average removed is 25g from the piston side of the crank but thats also precisely measured. If I remove just a little bit, say 10g, I should be able to stay well within safe boundaries and still reduce vibrations. But they replace the moved materials with silicone or some other near-weightless material to preserve compression, but noone from what I can tell adds any form of weight to the crank, they use the weights to help find the balance point (again, precisely measured)

Having removed the crank and seen how much it weighs (its like 80% of the weight you feel when you lift that HT motor kit, no joke), those four holes plus the two smaller half-ers in the center probably reduced it by quite a bit more than 25 grams. I'm unsure of what kind of material it is so I dont know its density, if I did, I;d be able to calculate how much I removed yesterday.

 

[Toadmund]

problem is alot of the tutorials are vague.

I agree 100%, a lot of the tutes leave me with more questions than answers, not written in a way to help someone who's never done it before.

Should be laid out in steps.

The math should be explained.

Some tutes I believe have major errors, each tutorial has different results about final bob weight (some are totals of piston, rings, wrist pin and bearing, clips and small end, some are the total all mathed out and with a weird result that leaves me scratching my head asking, well, WHY???!!!, I have a post out there, all alone with no replies asking about a tute's error (or not, and why?)
OK, it's this one

I find myself searching the web looking for a strait
forward, non complicated method, without all the 'do this, do that, multiply that # by that result from 3 & 4 for 50-60% mumbo jumbo.

Something simple, like weigh this and this and this, put it on rails and if it rolls it's not balanced.
And if I want it smooth at WOT, do this, multiply that (and here is why you multiply that, or divide that because at that point I am stumped and frustrated, I need to know where these #'s are coming from.

And some better pictures too for crying out LOUD!
Hate to complain, but I am.

I know this is really simple stuff, but the tutorials make it seem difficult.

 

[mew905]

I agree 100%, a lot of the tutes leave me with more questions than answers, not written in a way to help someone who's never done it before.

Should be laid out in steps.

The math should be explained.

Some tutes I believe have major errors, each tutorial has different results about final bob weight (some are totals of piston, rings, wrist pin and bearing, clips and small end, some are the total all mathed out and with a weird result that leaves me scratching my head asking, well, WHY???!!!, I have a post out there, all alone with no replies asking about a tute's error (or not, and why?)
OK, it's this one

I find myself searching the web looking for a strait
forward, non complicated method, without all the 'do this, do that, multiply that # by that result from 3 & 4 for 50-60% mumbo jumbo.

Something simple, like weigh this and this and this, put it on rails and if it rolls it's not balanced.
And if I want it smooth at WOT, do this, multiply that (and here is why you multiply that, or divide that because at that point I am stumped and frustrated, I need to know where these #'s are coming from.

And some better pictures too for crying out LOUD!
Hate to complain, but I am.

I know this is really simple stuff, but the tutorials make it seem difficult.

very true, I tend to want very specific, detailed instructions, these guys are trying their best though so you cant really knock them. The fun thing is though that these motors will never be smooth at top speed unless your top speed is limited by aerodynamics. The motor itself is limited by its own vibrations, so lightening the piston side should have, in theory, dramatically reduced vibrations (and thus bring your top speed up until you hit those vibrations again), the more you remove, the higher that sweet spot, unfortunately what I did is it vibrates so much at low RPM's it cant fight it and never speeds up to begin with.

A good tutorial should be: basic tools that everyone should have (basically hand tools, a scale, a drill and the gear puller, maybe use coins as a ballast as suggested in one tutorial). Dremels, lathes and mills are an asset but they can be quite expensive and out of reach of the average builder. Detailed instructions on how to do it (using laymans terms, for example I know what a ballast is, but I have no idea where to place it, I figured on the small rod end). Pictures, step by step, not just a pic every 3-5 steps and being like "look at what I did", without ever showing what it looked like beforehand. Alot of us wont embark on such a journey until we're very familiar with the process, usually because we dont have access to it during the work. As well as clear math at appropriate points, with clearly defined variables. Perhaps maybe even a calculator for those who are good at programming.

 

[mew905]

bump: this is actually the best tutorial I've seen yet

http://motorbicycling.com/showthread.php?t=40568

 

[Toadmund]

Part 2a: The math of STATIC BALANCING and doing it in practice.

...done. When we add our six positions above we get 110 grams.

Now we think. We want to compensate the 6 weights together, let us say with 40% of their true mass. 110g x 0,4 = 44g for the bob weight?
BUT THE CONNECTIONG ROD is hanging there with 100% of its weight.
It should hang with 28g X 0,4 = 11,2g. But it doesn't. From the rod there are hanging 16,8 grams too much. Problem...16,8g too much. So the 44g bob weight is wrong.

Don't laugh at me. You know the solution: Reduce the first calculated bob weight by 16,8 grams. 44g - 16,8g = 27,2 grams is the correct bob weight in this example.

Now I read this about 30 x and it's finally sinking into my ultra-dense black hole material skull, BUT just one thing, the 40 % jazz.

Does that 40% cover the part of operation that is not affected by the friction and drag of reciprocration?
But some guys are going 50-60% which I believe covers the higher RPM's which are less 'draggish because of the increased speed?
Higher RPM smoothness is what I am shooting for, are those numbers for lower or higher RPM targets?

 

[mew905]

those who tune for 57% apparently can rev up to 9500 RPM (which is around 41mph on a 44T sprocket on a 26" tire). my question is wtf is this "6 positions" he keeps talking about?

 

[headtrama]

I spin a stock crank to 9,800 rpm. I have never had a crank problem except for a new engine with a bad bottom end bearing out of the box. If I were you I would play with porting and compression. Invest in a better head and expansion chambers.

 

[mew905]

I spin a stock crank to 9,800 rpm. I have never had a crank problem except for a new engine with a bad bottom end bearing out of the box. If I were you I would play with porting and compression. Invest in a better head and expansion chambers.

My first motor ran 9300 rpm too, 40mph completely stock, problem is build quality. I have a high compression head and gutted exhaust now but theres a vibration barrier at 40mph on a 36t (6500 rpm) on my new one. These motors vary alot. The old crank could run better too. thats why I drilled the crank, I know its capable of much more.

 

[headtrama]

Drilling holes in the crank lowers the secondary compression in the crankcase. You would be better off just truing the crank. You can polish a turd but it's still a turd.