Knife-edge a race crank?

[andyb6]

Rick,

Did the same amount of material come off of where the rod attaches as for the bob weight area for that same rod? I have heard some SCCA Alfa racers say that the 88.5mm stroke cranks could actually use a touch MORE weight on the bob weights (opposite from rod) because of our long stroke, long rod, heavy high domed piston. I have no experience in this, I am just posing theory.
It looks as though the crank was turned on a lathe and if I am picturing it correctly the bob weights had more area that could be removed and therefore more mass than the other (rod end) doing the opposite, actually making the spinning crank think the pistons and rods are heavier than the opposing bob weight.
Any thoughts on this?
Will this make the crank want to wiggle like a snake at high RPM?

[jrclem]

This is not a first time experiment... cranks with this modification are all ready proven in race winning engines. It does look scary though. If you think about it the dynamic loads associated with the combustion dwarf the static balance loads. The crank has to have brute strength to go racing at all. The rod journals on this 50+ year old design are three or more times the size (and weight) of a modern (i.e. Honda) engine design.
I'll report back here with real world results.

[Little Italian]

Why do you knife-edge a crankshaft?

[PSk]

Quote:

Originally Posted by Little Italian Why do you knife-edge a crankshaft?

[velocedoc]

JrClem: I have a crankshaft identical to yours that is going in to my 2 liter. I will be interested in hearing how your crankshaft works out.

Spins up faster = better acceleration?

If you knife edge a crank, add lightened pistons, lightened flywheel, I would imagine that you can really feel the engine want to rev up faster.

[endel]

We used a knife -edge crank in our 280-Z GT2 car for a few years. It proved to be a factor in more performance, as other mechanical factors remained the same. We ran National events at Lime Rock, Summit Point, Nelson Ledges, Watkins Glen, Loudon and Bridgehampton. The knife-edge crank was an expensive modification for our car. I can't offer the technical physics behind the performance increase, but it was notable for sure. We beat Paul Newman's Zcar at Lime Rock.

[FMG_V6_btb]

The technical physics was explained in my last post.

When you are driving and push the gas pedal, the engine torque is used for two things:

1- accelerate the rotating masses -> T=I*A
2- accelerate the car -> F=m*a

The lower the rotating masses inertia (I) the lower the engine torque needed to rev-up these rotating masses, as a consequence of this, more torque is available to accelerate the car. And thats why with the same engine power (without modifications in the fluid dymamics neither in the thermodinamics of the engine) you feel the car accelerates better (and it does).

I remember that some years ago I drove a '75 Corvette, and whith the car stopped, when I pushed the gas pedal and the engine rev-up, the entire car roll in the opposite direction of the engine, that was because the high crank and flywheel inertia. The car roll was produced by the reaction torque of the engine in its mounts (the inertia torque above explained).

The same physics can be applied to the wheel rims, a friend of me own a 156 JTD (turbo Diesel), and he changed the original 15' steel rims with its 185/65/15 pneumatics, for a 17' aluminium rims with 215/45/17, that last rims weighted at least 2 kg more than the original steel ones (!!!), and I suppose the pneus were also heavier, as result of this the car acceleration was around 1 sec. worse in 0 to 100 km/h than with the original wheels.

Also, I must to point here, that for the above mentioned reasons, inertial test benches to measure the power are not valid, are only good for before-after comparison.

Paco.

[AllthingsZagato]

Knife edging does improve the aero performance, particularly when you are running a dry sump and have sealed off each crank chamber, but you also need to knife edge the faces of the crank 'web' not just the leading edge. You also are trying to ensure as little oil as possible is carried around on the crank.

[velocedoc]

If you are running a knife edge crank shouldn't you also install a plate between the crank and the sump to limit oil splash? Who makes a sump plate for an alfa?

[genericwood]

Chris, Jim Steck makes a pretty good "windage tray" for Alfas. I believe he makes the ones that Spruell sells also? As a side benefit, it probably adds at least a little stiffness to the main caps.

Erik

Quote:

Originally Posted by velocedoc If you are running a knife edge crank shouldn't you also install a plate between the crank and the sump to limit oil splash? Who makes a sump plate for an alfa?

[jrclem]

As promised, here are the results of the knife edge crank:
VARA @ California Speedway last weekend, B Sedan Flag Race:
1 33 > Rick Clemente < 2:01.965 5 85.008 -
2 680 Fabrizio Rimicci 2:02.838 5 84.404 -
3 682 Gianfranco Masdea 2:08.206 6 80.870 -
4 029 Vincent Torres 2:14.388 7 77.150 -
5 85 Bob Wass 2:12.404 6 78.306 -
6 204 Jeff Thraen 1 Lap 2:04.811 8 83.070 -
7 49 Steve Link 2:04.359 2 83.372 -

The engine is down about 10 HP on the dyno, but does spinn RIGHT NOW!. We'll find the missing HP next week (cams...)

[super-racer]

These results are due to much more than a knife edge crank! The Old Master drove one hell of a race and gave me a little lesson. Great result Rick.

[bianchi1]

..jrclem.. i came by in the dark of night.. and stold your 10 horsepower.... if you put 4 dozen choc.chip cookies in a plain brown bag.. and drop it of in the location , that i will tell you later.. and remember..no cops..or i will give the horsepower to msiert...and .. we know what he will do with it.. don't we????( insert a evil laugh)

[jrclem]

If I can figure out how to upload a U-Tube movie... you will all see an amazing display of mistake-free driving in both the qualifying race and the flag race at Fontana ... as displayed by the Whippersnapper Fabrizio.
His Super is VERY well prepared, and is likely the fastest Gulia Super/ti example on earth... That said, the wheel-wells do not allow enough room for the 205-55-14 Hoosier's that I run... AND I was running the "Auto-Cross" gummy compound! (Good for two sessions). So, even though I made a horrible start (with a missed shift!!!) and went from 3rd on the grid to 8th at turn 3 on lap 1...(YIKES!) After I settled down (two more missed shifts, what the HELL IS WRONG with me??? Old Age?) I was able to hack my way back and be in a position to take advantage of lapped traffic to get close to Fabrizio on the last lap. From there on in it was just a matter of old age and treachery over-coming youth and talent (and more $$Bucks$$$ spent on tires etc!) and a last ditch bonzi shot for a win. What a fabulous race! You can't possibly run that hard against anyone that you don't have absolute faith in... and that goes BOTH ways. Hat's off to Fabrizio! GREAT RACE!!!

[George Willet]

Nadar: Don't!! George