How to lighten a BMW crank.

[michiganbuck]

EG I suppose in your country people only drive on the Wong side of the road.

 

[Clive603]

Seriusly, I wouldn't touch it, that's a specialist machining job for those who have the kit and fixturs etc etc, plus it will have to be balanced.
While Iunderstand your interest and helping a mate tc etc, some jobs are more trouble than they're worth

Another thought, has the crank to be machined been crack tested, we all know how BMWs get treated n the UK and some of the mechanics ar little better - my ex neighbours - the engine shop used to avoid them when ever they could.

Thats a decent point Limy but "jobs more trouble than its worth" is pretty much the story of my life. Right back to when I was properly gainfully(???) employed by MoD as a Scientist / R&D engineer.

Fixture to mill so the sides can be taken off is straightforward, basically four half bearings and caps on posts correctly arranged on a baseplate.

Having slept on it doing the sides of the webs is probably best done by following a template fixed across the lathe bed, flip over for the other side. Pair of round nosed inset tools on left and right offset holders should handle the interrupted cut easily if I take it steady. Setting the template up still needs some thought but nothing apparently insurmountable.

The crank to be done has got several racing seasons under its belt so I figure that if it was gonna break it would have done by now. Especially as lots of balancing metal removal has been done on the front three cylinder cranks suggesting the original forging was way out. Lightening ought to give the centre main bearing an easier life as thats where all the forces resolve. If I were building a racer I'd have looked for a more even crank to start with.

Clive

 

[jim rozen]

"Even if it's dry sump, there's a hefty wall of oil flinging off the crank, which causes a ton of drag. Much more than you would think. "

Story is, if you have a motor with a view port on the side, and watch while it's running, the crank is pretty much obscured by an oil-colored football churning around in there.

 

[Clive603]

Story is, if you have a motor with a view port on the side, and watch while it's running, the crank is pretty much obscured by an oil-colored football churning around in there.

Which is pretty much why splash lubrication works.

 

[GregSY]

It's good to remove the oil from the spinning goo-goo that is the crank/rods pistons. An even higher level of race-car fancy is to use a vacuum pump to let all those parts spin in a vacuum. But like anything....if you remove too much of the oil, the parts of the engine which are directly fed will starve for oil.

 

[Limy Sami]

Thats a decent point Limy but "jobs more trouble than its worth" is pretty much the story of my life. Right back to when I was properly gainfully(???) employed by MoD as a Scientist / R&D engineer.

Fixture to mill so the sides can be taken off is straightforward, basically four half bearings and caps on posts correctly arranged on a baseplate.

Having slept on it doing the sides of the webs is probably best done by following a template fixed across the lathe bed, flip over for the other side. Pair of round nosed inset tools on left and right offset holders should handle the interrupted cut easily if I take it steady. Setting the template up still needs some thought but nothing apparently insurmountable.

The crank to be done has got several racing seasons under its belt so I figure that if it was gonna break it would have done by now. Especially as lots of balancing metal removal has been done on the front three cylinder cranks suggesting the original forging was way out. Lightening ought to give the centre main bearing an easier life as thats where all the forces resolve. If I were building a racer I'd have looked for a more even crank to start with.

Clive

Okay Clive I get your point, but having lived 5 yds from an engine rebuilders for over 20 years with a very good view of their yard etc, the way I've seen so many engine parts handled is nigh unbelievable, ........... and while your ''source '' may have handled it properly - you don't know how many animals have been before him.

From memory I know they had at least 6 cranks turn out to be cracked when theywent on the Prince grinders

 

[EmGo]

From memory I know they had at least 6 cranks turn out to be cracked when they went on the Prince grinders

Definitely want to mag it before you invest any time in an old crank, that's for sure !

 

[Clive603]

Limy

I believe you about the rebuild animals. Over the years a fair bit of such stuff, usually motorcycle not car, has made it down me as the last resort for a miracle fix. Mostly as a final pause on the way to the bin. Harry Potter I'm not!

Best practice with these motors for classic saloon racing is to grab one that has done 50,000 to 75,000 miles with only routine servicing. Crack test the crank when the bottom end comes apart then tune and go racing. Jody generally won't touch an engine that has evidence of being stripped by a normal mechanic, and some "race prep" folks too.

I guess its similar to the old BMW turbocharged F1 motors which were always built from used blocks that had done significant miles.

Clive

 

[jim rozen]

Motorbikes, but: Pridmore won two superbike championships on BMW bikes. The first one was originally set up with a specially lightened crank. The motor ate the crank, they put in a stock one (R90S) and raced with that instead, and won. I think the stock bike made about 50 hp, in race tune it was 100 or so.

 

[Dave W]

Intrigued by the idea of using a grinder to shift most of the metal quickly. Not something I'd risk. It seems very hard to ensure equal amounts of metal are taken off each web. Setting up in the lathe afterwards to clean up and equalise things looks to be difficult too as the initial grading will surely have taken out everything that can be used for reference.

Most inline six cranks have four counterweights. Some have none. Your BMW crank has twelve. That's done primarily for NVH purposes. Each throw may only be partially countweighted to start with; without cutting up a spare crank and spinning a single throw on the balance machine (which I've done with some 4-cylinder cranks) you can't tell.

The nifty thing about an inline six (or V12) is that the balance percentage doesn't really matter; as long as it's reasonably even end-to-end it will run smoothly.

 

[EmGo]

EG I suppose in your country people only drive on the Wong side of the road.

No, but we have a problem with streets turning into deer

(Sorry, words for street and deer sound the same, classic awful china pun, makes everyone groan, best I could come up with to counter buck's terrible joke)

 

[technocrat]

"Even if it's dry sump, there's a hefty wall of oil flinging off the crank, which causes a ton of drag. Much more than you would think. "

Story is, if you have a motor with a view port on the side, and watch while it's running, the crank is pretty much obscured by an oil-colored football churning around in there.

yes, but if were possible to lubricate the engine with a permanant non-liquid lube, the engine would likely overheat. You could argue that in a race motor, the primary role of oil is cooling and lube is probably secondary. Crank splash and/or squirters take a lot of the heat from the piston via the back side. Change the oil dynamics and you change the cooling.

 

[Limy Sami]

I have no problem with BMW cars as such - I like the engineering, as I do Audi and Mercedes, it's the drivers of all 3 I have problems with, they don't know what and indicators for, and believe neither the laws of physics, the road and some would say humanity, apply to them.

A friend proposed they're Germany's revenge for losing 2 wars

 

[michiganbuck]

I suspect the correct amount of splash is needed for the cylinder walls, likely the wrist pins are a roller-bearing type. I guess a lathe turned, and then a precision ground crank would be best and the rod and main journals ground last after stress relieving.
I have thought of building a crank with roller bearings on the mains and rod journals but never had the resources or equipment to do it. Thinking of a hard press-in shell bushing with roller clearance to the crank (or just tightly set in the mains and assembled rods) , and inserting the rollers through a hole, and then a precision length stop screw inserted the exact length to but the rollers. This so not needing a press-together crank.

 

[jim rozen]

BMW motorbikes with pressed-up cranks and roller bearing bottom ends have clearances around a couple of tenths to work well.

 

[EmGo]

BMW motorbikes with pressed-up cranks and roller bearing bottom ends have clearances around a couple of tenths to work well.

They don't work well. No roller bearing crank works well at high rpm. (Jeff Bratton made a career out of doing nothing but grand prix two-stroke cranks, $$$, and they only had rollers because they had to, being two-strokes and all). There is too much inertia involved; speeding up and slowing down the rollers every revolution is hell on the cages and rollers skidding and races scuffing ... it's something that sounds good but works bad. XR's that will run all season at 8,000 last two races at 9. There was only one guy I can think of rebuilding the roller bearing hirth porsche cranks and he charged a fortune (rightfully so). It's not a good idea.

(Roz - Kenny Augustine did a lot of the work on those Udo Geitl Bimmers ... they were only successful because no one else was pumping money into that class then. And oh yeah, they were pretty serious cheaters. But then, so was everyoe else Cook Neilson won the class the year earlier and he was two guys doing it in their spare time. Paul Ritter did well - don't know if he won the class but did win several races - right about then, his owner was just one guy. A well-off guy but still, not exactly a team of trained professionals. A year or two later, not a chance. Gotta make hay while the sun shines

You might get a kick out of this, I think posted before but maybe not, go to factorypro dot com, down the left side (it's a frames website ) find "stories" then the one about richard schlacter. Same class, maybe a year or two later ?

Really, lightening a crank is not a big thing. Friend (won 500 production class a couple years in a row) was a glass guy. He lightened and polished his crank on the belt sander. A glass belt sander is like a 6' tall mass of spooge with free-floating belts and a row of mucky round things you support the part on. They run wet and glass makes a mess. Lots of hand work but no big deal if you're coordinated. Jerry quit after he crashed trying to pass everyone on the outside in turn one at Ontario, so the bike was not slow.

 

[michiganbuck]

I suspect that a pressed-together, a shorter crank might be easier to manufacture and maintain accuracy then a longer crank like a 4 Cylinder or 6 cylinder.
But likely getting into other Topics with this.
lightening the crank..would taking more stock from the Od be an asset, or that reduce the needed splash for libe to cylinder walls?
Agree that I am a novice in the field of race engines.

 

[EmGo]

I suspect that a pressed-together, a shorter crank might be easier to manufacture and maintain accuracy then a longer crank like a 4 Cylinder or 6 cylinder.

It's not about the construction. XR's are the simplest possible, two forged flywheels with integral shafts, a pin and a rodset. The problem is that the speed of the rod bearing is not constant as the crank revolves. In fact, the rod bearing speeds up and slows down continuously as the crank rotates. Look at an animation of the rod as the crank revolves and you can see what's going on.

That's okay at slow speeds but when you wick it up, the inertia becomes tremendous. Cages have to be steel (but that makes them heavier which is going opposite to the way you want), you want light rollers but that makes them skid easier and you need more if they are smaller; everything you do to make it better actually makes it worse, the whole setup is just bad at high rpm. Plain bearings kick butt for this.

 

[jim rozen]

When AMOL was running in dumont, nj, they had a bunch of things that Udo Geitl worked on. Was cool to see those. BMW used pressed-up cranks with roller bearing bottom ends up until 1970, when they *finally* got into a more modern plain bearing bottom end with an honest-to-god high pressure oil pump. Before that it was all whacky stuff with slingers and ball bearing crank bearings. At one time you could get parts for that stuff supposedly because there was some big-deal manufacturer in europe that had a jones for vintage ducatis, same deal with those bikes.

 

[Limy Sami]

It's not about the construction. XR's are the simplest possible, two forged flywheels with integral shafts, a pin and a rodset. The problem is that the speed of the rod bearing is not constant as the crank revolves. In fact, the rod bearing speeds up and slows down continuously as the crank rotates. Look at an animation of the rod as the crank revolves and you can see what's going on.

That's okay at slow speeds but when you wick it up, the inertia becomes tremendous. Cages have to be steel (but that makes them heavier which is going opposite to the way you want), you want light rollers but that makes them skid easier and you need more if they are smaller; everything you do to make it better actually makes it worse, the whole setup is just bad at high rpm. Plain bearings kick butt for this.

Which was why the racing 2 strokes of the 70s (Yam TZ , Kawa etc ) ate cranks, (and they would only stand so many rebuilds before they'd fall to bits and scrap an entire engine)